U.S. patent application number 10/855034 was filed with the patent office on 2004-12-30 for process for manufacturing high purity methacrylic acid.
Invention is credited to DeCourcy, Michael Stanley, Elder, James Edward, Juliette, Jamie Jerrick John.
Application Number | 20040267050 10/855034 |
Document ID | / |
Family ID | 33435272 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040267050 |
Kind Code |
A1 |
DeCourcy, Michael Stanley ;
et al. |
December 30, 2004 |
Process for manufacturing high purity methacrylic acid
Abstract
A process is provided herein for the high yield production of
high purity glacial methacrylic acid ("HPMAA") that is
substantially pure, specifically 99% pure or greater, with water
content of 0.05% or less and low levels of other impurities,
including HIBA, acrylic acid, MOMPA, methacrolein and others. This
improved process comprises providing a crude MAA stream and
purifying the crude methacrylic acid stream in a series of
successive distillation steps involving two distillation columns.
The inventive process is capable producing high purity methacrylic
acid product that is especially suitable for the production of
specialty MAA polymers.
Inventors: |
DeCourcy, Michael Stanley;
(Houston, TX) ; Elder, James Edward; (Houston,
TX) ; Juliette, Jamie Jerrick John; (Houston,
TX) |
Correspondence
Address: |
ROHM AND HAAS COMPANY
PATENT DEPARTMENT
100 INDEPENDENCE MALL WEST
PHILADELPHIA
PA
19106-2399
US
|
Family ID: |
33435272 |
Appl. No.: |
10/855034 |
Filed: |
May 27, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60483703 |
Jun 30, 2003 |
|
|
|
Current U.S.
Class: |
562/600 |
Current CPC
Class: |
C07C 51/44 20130101;
C07C 51/50 20130101; C07C 51/487 20130101; C07C 51/44 20130101;
C07C 57/04 20130101; C07C 51/487 20130101; C07C 57/04 20130101;
C07C 51/50 20130101; C07C 57/04 20130101 |
Class at
Publication: |
562/600 |
International
Class: |
C07C 051/42 |
Claims
What is claimed is:
1. A process for the preparation of high purity methacrylic acid,
said process comprising: (i) providing a first distillation column
and a second distillation column; (ii) feeding a crude methacrylic
acid stream to said first distillation column, said crude
methacrylic acid stream comprising methacrylic acid, light ends and
heavy ends; (iii) distilling said crude methacrylic acid stream in
said first distillation column to form a first overhead vapor
stream and a first bottom liquid stream; (iv) feeding said first
bottom liquid stream to said second distillation column; (v)
distilling said first bottom liquid stream in said second
distillation column to form a second overhead vapor stream and a
second bottom liquid stream; and (vi) drawing a product stream from
a middle section of said second distillation column.
2. The process for the preparation of high purity methacrylic acid
according to claim 1, wherein said first overhead vapor stream
comprises substantially acetone, water, and other light ends and
wherein said first bottom liquid stream substantially comprises
methacrylic acid, HIBA, and other heavy ends.
3. The process for the preparation of high purity methacrylic acid
according to claim 1, wherein said second overhead vapor stream
comprises substantially acetone, water, and methacrylic acid and
wherein said second bottom liquid stream comprises substantially
HIBA.
4. A process for the preparation of high purity methacrylic acid,
said process comprising: (i) providing a first distillation column
and a second distillation column; (ii) feeding a crude methacrylic
acid to said first distillation column, said crude methacrylic acid
stream comprising methacrylic acid, light ends and heavy ends;
(iii) distilling said crude methacrylic acid stream in said first
distillation column to form a first overhead vapor stream and a
first bottom liquid stream; (iv) condensing said first overhead
vapor stream to produce a reflux stream and a feed stream; (v)
directing said feed stream to said second distillation column; (vi)
distilling said feed stream in said second distillation column to
form a second overhead vapor stream and a second bottom liquid
stream; and (vii) drawing a product stream from a middle section of
said second distillation column.
5. The process for the preparation of high purity methacrylic acid
according to claim 4, wherein said first overhead vapor stream
comprises substantially methacrylic acid, acetone, water, and other
light ends and wherein said first bottom liquid stream
substantially comprises HIBA and other heavy ends.
6. The process for the preparation of high purity methacrylic acid
according to claim 4, wherein said second overhead vapor stream
comprises substantially acetone, and water and wherein said second
bottom liquid stream comprises substantially heavy ends.
7. A high purity methacrylic acid comprising: at least 99% by
weight of methacrylic acid; water, present at up to 0.05% by
weight; up to 0.1% by weight methacrolein; and up to 0.9% by weight
of HIBA;
8. The high purity methacrylic acid of claim 7 further comprising:
up to 0.1% by weight of 3-methacryloxy-2-methylpropionic acid and
methyl methacrylate; and and up to 0.075% by weight
methacrylamide.
9. The high purity methacrylic acid of claim 7 further comprising:
up to 0.1% by weight of acetic acid; up to 0.08% by weight of
acetone; up to 0.04% by weight of acrylic acid; and up to 0.02% by
weight of propionic acid.
10. A method of preventing HIBA decomposition within a stream
containing methacrylic acid and an acid-catalyst comprising: adding
a stabilizing agent to said methacrylic acid containing stream,
said stabilizing agent being capable of neutralizing the
acid-catalyst.
11. The method of claim 10 further comprising removing the
neutralized acid-catalyst from said methacrylic acid containing
stream by filtration or distillation.
12. The method of claim 10, wherein said stabilizing agents are
selected from the group consisting of ammonia, caustic, magnesium
hydroxide, and magnesium oxide.
13. A method of removing HIBA from a methacrylic acid containing
stream comprising: adding a Lewis acid component to said
methacrylic acid containing stream to create a mixture; heating
said mixture such that the HIBA within said methacrylic acid
containing stream is decomposed into decomposition products; and
distilling said decomposition products from said methacrylic acid
containing stream.
14. The method of claim 13 wherein said Lewis acid component is
selected from the group consisting of sulfuric acid, phosphoric
acid, a superacid, and an ion exchange resin having acidic
functionality.
15. A method of preventing HIBA decomposition within a stream
containing methacrylic acid and an acid-catalyst comprising:
contacting said methacrylic acid containing stream with an ion
exchange resin.
16. A method of purifying a HIBA-containing MAA stream in a
purification process which utilizes a distillation column,
comprising: maintaining residence time of said MAA stream at a
bottom section of the distillation column up to 48 hours, whereby
HIBA decomposition is minimized.
17. A method of purifying a HIBA-containing MAA stream in a
purification process which utilizes a distillation column,
comprising: maintaining temperature of said MAA stream at a bottom
section of the distillation column at up to 145.degree. C., whereby
HIBA decomposition is minimized.
18. A method of purifying a HIBA-containing MAA stream in a
purification process which utilizes a distillation column,
comprising: maintaining total acid-catalyst concentration of said
MAA stream at a bottom section of the distillation column at up to
2.5 percent by weight, whereby HIBA decomposition is minimized.
19. The method of claim 18, wherein said acid-catalyst is selected
from the group consisting of sulfuric acid and compounds capable of
providing sulfate ions.
Description
[0001] The present invention is related to a process for the
production of substantially pure methacrylic acid having at least
99 weight % methacrylic acid, 0.05 weight % or less water, and
specified reduced amounts of other impurities compared to other
grades of methacrylic acid. More specifically, the substantially
pure methacrylic acid produced by the process of the present
invention is suitable for use as a feedstock for certain specialty
polymers.
[0002] Methacrylic acid ("MAA") is used in a wide variety of
applications. Typical end-use applications include: acrylic plastic
sheeting; molding resins; polyvinyl chloride modifiers; processing
aids; acrylic lacquers; floor polishes; sealants; auto transmission
fluids; crankcase oil modifiers; automotive coatings; ion exchange
resins; cement modifiers; water treatment polymers; electronic
adhesives; metal coatings; and acrylic fibers. MAA is especially
prized in these applications and others because of the hardness it
imparts to the products in which it is used. MAA also enhances
chemical stability and light stability, as well as ultraviolet
radiation resistance, of the products in which it is used.
Therefore, MAA is often used in applications requiring resins of
excellent transparency, strength, and outdoor durability. The MAA
market is extremely cost-sensitive; thus, any improvement in
process yield, however slight, can result in significant market
advantage.
[0003] Unpurified MAA generally has impurity levels of about 5% or
greater and is herein referred to as "crude methacrylic acid" or
"crude MAA." Impurities in MAA may comprise one or more of: water,
acetic acid, acrylic acid, acetone, methacrolein, acrolein,
isobutyric acid, 2-hydroxyisobutyric acid (HIBA), mesityl oxide,
3-methacryloxy-2-methylpr- opionic acid ("MOMPA", also known as
methacrylic acid dimer), methylmethacrylate, propionic acid, and
methacrylamide. The presence of specific impurities in the
unpurified crude methacrylic acid is dependent at least in part on
the production process employed to produce the original crude
methacrylic acid.
[0004] For example, if the crude methacrylic acid is derived from
the reaction of acetone cyanohydrin and sulfuric acid to form
methacrylamide (MAM) intermediate, the MAM intermediate then
further reacted with water to form methacrylic acid, (referred to
herein as an "ACH process"), impurities such as HIBA,
methacrylamide, and water would typically be present.
[0005] Alternatively, if the crude methacrylic acid is derived from
the catalytic gas phase oxidation of one or more raw material gases
selected from the list including butylene, isobutylene, butane,
iso-butane, t-butyl alcohol, or methacrolein (referred to herein as
a "C-4 process"), impurities such as methacrolein, acrolein, and
water would typically be present.
[0006] Impurities present in MAA can have a negative impact on some
of the fundamental properties of the end-use applications of MAA.
Accordingly, MAA that has a very low percentage by weight of
impurities is very desirable.
[0007] Purified MAA having an impurity level of less than 5% by
weight is referred to herein as glacial methacrylic acid ("GMAA").
Commercially available GMAA product typically has a methacrylic
acid content of about 98.5 to 99.5 wt %, a water content of about
0.2 to 0.3 wt %, and a product color of about 20 to 25 APHA (i.e.,
the color measurement standard established by the American Public
Health Association) (see, for example, "Standard Specification for
Glacial Methacrylic Acid", D3845-96 (2000), ASTM and "Glacial
Methacrylic Acid" Technical Information, TI/ED 1655 e February
2002, BASF Aktiengesellschaft).
[0008] It will be apparent to one of ordinary skill in the art,
however, that although not identified in product specifications,
commercial GMAA must additionally contain impurities other than
water, such unspecified impurities being responsible for the high
product color (i.e., APHA value) of commercial GMAA. These
unspecified impurities may include compounds such as those
described above, which are commonly found in unpurified crude MAA.
Because processes for producing GMAA do not consistently control
impurities other than water, it is not surprising that the
concentration of such impurities is not identified in the product
specifications for commercial GMAA.
[0009] Purified MAA having an impurity level of not more than 1% by
weight wherein no more than 0.05% by weight is water is referred to
herein as high purity glacial methacrylic acid ("HPMAA"). Although
HPMAA is a desirable product, it is costly to produce.
[0010] A three-column distillation process for producing HPMAA from
crude methacrylic acid is described in co-pending U.S. application
U.S. Ser. No. 10/420,273 filed Apr. 22, 2003. Producers of HPMAA
would greatly welcome improvements in the production of HPMAA that
could further lower capital and operating costs. Therefore, there
is an unaddressed need for a method to produce HPMAA at a reduced
cost for the manufacturer.
[0011] With its low water content, HPMAA is suitable as a feedstock
in the manufacture of specialty polymers comprising MAA, herein
referred to as "specialty MAA polymers". Representative examples of
specialty MAA polymers and their methods of manufacture are
provided, in U.S. Pat. No. 5,002,979; US 2003/0028071; US
2002/0156220; U.S. Pat. No. 4,351,931; U.S. Pat. No. 5,973,046; and
U.S. Pat. No. 3,264,272.
[0012] In many specialty MAA polymer processes, the presence of
water is particularly detrimental for the formation and composition
of the polymer end product. Therefore it is common for specialty
MAA polymer producers to treat the raw material monomers to remove
any residual traces of water. For example, in anionic
polymerizations, water can act as a chain terminator and in
polymerization processes based on diimine catalysts (e.g., U.S.
Pat. No. 6,310,163) water may interfere with the catalyst itself.
Common treatment methods include passing the monomer through
neutral alumina or drying the monomer over molecular sieves. Such
additional purification steps, however, add unwanted cost and
complexity to specialty MAA polymer production. This cost is
increased in cases where the monomer contains a high concentration
of water. It will be evident, therefore, that any reduction in the
water level of the raw material monomer, even if the water content
cannot be reduced to zero, will beneficially impact the polymer
production process by reducing the extent and associated cost of
such treatment steps. Impurities, other than water, that may be
present in methacrylic acid, including GMAA, can also have a
detrimental effect on the production of specialty MAA polymers.
[0013] For example, organic impurities that may be present in MAA,
such as acetic acid and aldehydes (e.g., methacrolein), can act as
undesired chain transfer agents during specialty MAA polymer
preparation. Other organic impurities, such as the dimer of MAA
(MOMPA) and methylmethacrylate, may become incorporated into the
polymer molecule, leading to undesired changes in the polymer's
properties.
[0014] Of the potential organic impurities in methacrylic acid the
presence of HIBA is particularly problematic to the preparation of
specialty polymers. The HIBA dimer can cyclize to produce
3,3,6,6-tetramethyl-1,4-dioxane-2,5-dione (also known as
tetramethylglycollide). Incorporation of tetramethylglycollide into
a polymer chain lowers the resistance of the final specialty
polymer to acids and bases--a result which is particularly
undesirable in ethylene-MAA ("E-MAA") copolymers. The dimerization
and cyclization of HIBA also generates water in-situ that further
complicates the polymer production process.
[0015] HIBA in methacrylic acid may also undergo acid catalyzed
decomposition, during specialty MAA polymer preparation processes,
to form decomposition products, including water, acetone, and
carbon monoxide, thereby altering the structure and molecular
weight of the polymer end product. This water-forming reaction
would be especially problematic, for example, in a process where
methacrylic acid is used as the solvent for a strong acid catalyst
(e.g., U.S. Pat. No. 5,948,874).
[0016] Thus it will be evident to one of ordinary skill in the art
that impurities present in methacrylic acid can have significant
negative impacts on specialty MAA polymer properties such as melt
index, transparency, or resistance to acidity. In some cases, the
presence of impurities may make the polymer unsuitable for a
specific application, such as in-vivo use of a non-bioreactive or
ocular-compatible polymer. In other cases, the impact of undesired
impurities may be evident only after further processing, such as
during or after compounding of the specialty MAA polymer with other
materials.
[0017] Accordingly, there is also an unaddressed need to produce a
high purity methacrylic acid product that is especially suitable
for the production of specialty MAA polymers. Given that even minor
reductions in product methacrylic acid impurity levels can have
significant positive effects on the quality and production cost of
specialty MAA polymers, producers of specialty MAA polymers would
greatly welcome the advent of an improved process for purifying
crude Methacrylic acid, capable of producing methacrylic acid
product with impurity levels even lower than typical HPMAA
specifications.
[0018] The purification process of the present invention may be
used to remove undesirable impurities from crude methacrylic acid,
thereby economically producing HPMAA, as well as grades of MAA with
minimal impurity levels that are especially desirable for use in
specialty MAA polymer production.
[0019] Objects, features and advantages of the present invention
will be apparent from the following description of some embodiments
of the invention. These embodiments are given for the purpose of
disclosure and may be considered in conjunction with the
accompanying drawings. A more complete understanding of the present
embodiments and advantages thereof may be acquired by referring to
the following description taken in conjunction with the
accompanying drawings, in which like reference numbers indicate
like features, and wherein:
[0020] FIG. 1 is a schematic flow diagram of one embodiment of the
process for producing HPMAA in accordance with the present
invention.
[0021] FIG. 2 is a schematic diagram of one embodiment of a side
draw configuration useful in connection with the process for
producing HPMAA of the present invention.
[0022] FIG. 3 is a schematic flow diagram of an alternative
embodiment of the process for producing HPMAA in accordance with
the present invention.
DETAILED DESCRIPTION
[0023] The present invention provides a novel and economical
process for producing HPMAA. The present invention involves the
purification of crude MAA, or other commercial grades of MAA, to
HPMAA via the use of distillation columns to remove various
impurities (generally, light impurities, heavy impurities, and
water) from an MAA stream such that the resultant product is
substantially purified methacrylic acid. More particularly, the
substantially purified methacrylic acid, HPMAA, is at least 99%
pure MAA having not more than 0.05 wt % water, based on the total
weight of the HPMAA. The improved process of the present invention
comprises a two-column distillation system in which capital
investment is reduced and operating complexity is minimized. The
operating cost is therefore also reduced. The improved process of
the present invention is also capable of operation in a manner that
will produce methacrylic acid that exceeds the purity
specifications for typical HPMAA, thereby providing methacrylic
acid that is especially beneficial for use in the preparation of
specialty MAA polymers.
[0024] As previously noted, eliminating impurities from MAA to
produce HPMAA enhances the function of the MAA as a precursor to
end use applications of specialty MAA polymers. Some of the
characteristics attributable to HPMAA include low color (APHA
value), resistance to the effects of acidic or basic substances,
and the ability to control the reaction temperature of polymers.
HPMAA also should not interfere with the function or operation of
catalysts used during specialty MAA polymer production. Many
end-use applications of specialty MAA polymers, such as films and
contact lenses, also require uniformity of color or a high-degree
of transparency. Low color MAA is also preferred when making E-MAA
copolymers, especially when the objective is to produce ionomers,
where transparency is often a key feature. Additionally, because
color generally increases with increasing impurity levels, the
color of the methacrylic acid product is indicative of its relative
purity. For this reason, specialty MAA polymer producers would find
it desirable to utilize raw material methacrylic acid with low
color, that is, the product color measures not more than 20 APHA,
for example, less than 15 APHA, or less than 10 APHA. The product
color measurement may be as low as 5 APHA, or even less. The
process of the present invention produces product methacrylic acid
with low color and low-levels of impurities such as, but not
limited to, acetic acid, acrylic acid, aldehydes, HIBA, MOMPA,
methylmetacrylate, acetone, and water.
[0025] For clarity, the following definitions are used herein:
"top" is the vapor space existing at the extreme top of a
distillation column; "bottom" is the liquid sump existing at the
extreme bottom of a distillation column; "upper section" is the
approximate uppermost 1/3 of the distillation column which is below
and adjacent to the "top"; "lower section" is the approximate
lowermost 1/3 of the distillation column which is above and
adjacent to the "bottom"; "middle section" is the approximate 1/3
of the distillation column intermediate the "upper section" and the
"lower section"; "line" is a fluidic connection for transporting
vapor and/or liquid into a unit, out of a unit or between two or
more units, and may include such common peripherals as valves,
condensers, flow meters, etc.
[0026] One embodiment of the present invention is illustrated in
FIG. 1 and utilizes two distillation columns. In this embodiment, a
crude MAA stream 100 is provided to the first of the two
distillation columns, HPMAA light ends column 110. Crude MAA stream
100 can be any stream comprising methacrylic acid that requires
purification, including, for example, a crude MAA stream
originating from an ACH-process or a crude MAA stream originating
from a C-4 process, or a partially purified, commercial grade MAA
stream originating from another purification process. In light ends
column 110, light impurities produced in the crude MAA production
process are separated from the MAA, and are removed from light ends
column 110 as light ends overhead stream 105. The exact composition
of light ends overhead stream 105 varies depending on the specific
source of the crude MAA. With regard to the process shown in FIG.
1, the light ends overhead stream 105 comprises one or more
components including, but not limited to, acetone, water, acetic
acid, acrylic acid, methacrolein, and acrolein. HPMAA light ends
column 110 also includes column ancillaries (not shown), wherein
the term "column ancillaries" means any and all secondary equipment
and associated piping that is connected to a column, such as vacuum
equipment, reboilers, condensers, pumps, and process lines
including but not limited to feed lines, bottoms lines, overhead
lines, vent lines, inhibitor addition lines, oxygen addition lines,
reflux lines, and rundown lines, such as are well known by persons
of ordinary skill in the art.
[0027] HPMAA light ends column 110 and its column ancillaries are
preferably constructed of materials resistant to corrosion.
Suitable materials of construction resistant to corrosive effects
include but are not limited to: 300 series stainless steel, 904L,
6-moly stainless steel, HASTELLOY.RTM. (e.g., B, B-2, B-3, C-22,
and C-276), tantalum, and zirconium. In some embodiments, the
manufacturer may reduce construction costs by utilizing covered
base metals. "Covered base metal" materials are materials that are
generally thought not to be corrosion resistant, such as carbon
steel, combined with a covering thereon, which is capable of
resisting corrosion such as glass, epoxy, elastomer, fluoropolymer
(e.g., TEFLON.RTM.), or one of the above-listed corrosion resistant
metals. Covered base metals are constructed by placing a covering
capable of resisting corrosion over, and optionally bonding the
covering to, the base metal. The covering prevents contact between
the base metal and the process stream. Covered base-metal
construction is especially preferred for large-diameter piping (3.8
cm or larger nominal diameter) and for heat exchanger tubes in high
fluid-velocity service (fluid velocity of 0.15 meter/second or
more) and other components, where significant metal thickness (3 mm
or more metal thickness) may be used to provide structural
strength. The materials described above such as 300 series
stainless steel, 904L, 6-moly stainless steel, HASTELLOY.RTM.
(e.g., B, B-2, B-3, C-22, and C-276), tantalum, zirconium, and
covered base-metal materials are hereinafter referred to,
collectively, and in the alternative, as "corrosion resistant
material."
[0028] Internal components such as trays or packing may be used in
HPMAA light ends column 110, if desired. Such internal components,
if present, may be made from the same materials as the column
itself or may be constructed from one or more different materials;
for example, the upper portion of the column may contain 300 series
stainless steel packing, while the lower portion of the column
contains HASTELLOY.RTM. B-2 packing. Trays may be used in HPMAA
light ends column 110. Perforated plate trays are effective, since
they have been found to be particularly resistant to MAA polymer
accumulation. By the term "perforated plate trays" as used herein
is meant any tray comprising a planar portion with a plurality of
holes through said planar portion. Optional tray features,
including but not limited to weirs, downcomers, baffles,
distributors, valves, bubblecaps, and drain holes, may also be
present. Examples of perforated plate trays include sieve trays,
dual flow trays, and combination valve and perforation trays. It
has been determined that two theoretical separation stages are
desired within the HPMAA light ends column 110. If trays are used,
it is preferable that two to ten perforated plate trays be
used.
[0029] HPMAA light ends column 110 may be operated under a vacuum
to minimize the temperature at the bottom of the column. For
example, in a particular embodiment, the pressure at the bottom of
the column is maintained from 40 mmHg to 80 mmHg, allowing the
bottom of the column to be operated at temperatures ranging from
70.degree. C. to 110.degree. C.
[0030] At least one heat exchanger may be used as the heating
apparatus for HPMAA light ends column 110. Desuperheated steam is
an effective heat source for these exchangers. If a reboiler is
used as the heat exchanger, it may be internal or external to the
distillation column. Vortex breakers are also useful in the bottom
of HPMAA light ends column 110.
[0031] It is oftentimes useful to add water-soluble or
organic-soluble polymerization inhibitor to HPMAA light ends column
110 to inhibit polymerization of methacrylic acid. Suitable
examples include but are not limited to: Hydroquinone (HQ);
4-methoxyphenol (MEHQ); 4-ethoxyphenol; 4-propoxyphenol;
4-butoxyphenol; 4-heptoxyphenol; hydroquinone monobenzylether;
1,2-dihydroxybenzene; 2-methoxyphenol; 2,5-dichlorohydroquinone;
2,5-di-tert-butylhydroquinone; 2-acetylhydroquinone; hydroquinone
monobenzoate; 1,4-dimercaptobenzene; 1,2-dimercaptobenzene;
2,3,5-trimethylhydroquinone; 4-aminophenol; 2-aminophenol;
2-N,N-dimethylaminophenol; 2-mercaptophenol; 4-mercaptophenol;
catechol monobutylether; 4-ethylaminophenol;
2,3-dihydroxyacetophenone; pyrogallol-1,2-dimethylether;
2-methylthiophenol; t-butyl catechol; di-tert-butylnitroxide;
di-tert-amyInitroxide; 2,2,6,6-tetramethyl-piperidinyloxy;
4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy;
4-oxo-2,2,6,6-tetramethyl-p- iperidinyloxy;
4-dimethylamino-2,2,6,6-tetramethyl-piperidinyloxy;
4-amino-2,2,6,6-tetramethyl-piperidinyloxy;
4-ethanoloxy-2,2,6,6-tetramet- hyl-piperidinyloxy;
2,2,5,5-tetramethyl-pyrrolidinyloxy;
3-amino-2,2,5,5-tetramethyl-pyrrolidinyloxy;
2,2,5,5-tetramethyl-1-oxa-3-- azacyclopentyl-3-oxy;
2,2,5,5-tetramethyl-3-pyrrolinyl-1-oxy-3-carboxylic acid;
2,2,3,3,5,5,6,6-octamethyl-1,4 -diazacyclohexyl-1,4-dioxy; salts of
4-nitrosophenolate; 2-nitrosophenol; 4-nitrosophenol; copper
dimethyldithiocarbamate; copper diethyldithiocarbamate; copper
dibutyldithiocarbamate; copper salicylate; methylene blue; iron;
phenothiazine (PTZ); 3-oxophenothiazine; 5-oxophenothiazine;
phenothiazine dimer; 1,4-benzenediamine;
N-(1,4-dimethylpentyl)-N'-phenyl- -1,4-benzenediamine;
N-(1,3-dimethylbutyl)-N'-phenyl-1,4-benzenediamine; N-nitrosophenyl
hydroxylamine and salts thereof; nitric oxide; nitrosobenzene;
p-benzoquinone; or isomers thereof; mixtures of two or more
thereof; mixtures of one or more of the above with molecular
oxygen.
[0032] The inhibitor(s) may be used alone or combined with suitable
diluents. Preferred diluents include, but are not limited to, MAA,
water, and organic mixtures comprising acetone. Hydroquinone ("HQ")
inhibitor is especially effective for use in HPMAA light ends
column 110, and it may be added directly, or with a diluent in one
or more locations throughout HPMAA light ends column 110 and its
ancillaries. If used, the inhibitor may be added at a rate of 1 kg
to 10 kg of HQ per 10,000 kg of HPMAA light ends column feed, for
example, 1.3 kg to 8 kg of HQ per 10,000 kg of HPMAA light ends
column feed. Another example would be 1.5 kg to 5 kg of HQ per
10,000 kg of HPMAA light ends column feed.
[0033] When phenolic inhibitors, such as HQ and MeHQ, are used,
oxygen-containing gas may be added to the distillation column to
enhance the effectiveness of the inhibitor. The term
"oxygen-containing gas," as used herein, refers to any gas
comprising from 0.01% up to 100% oxygen. Oxygen-containing gas may
be added in one or more locations throughout HPMAA light ends
column 110 and its column ancillaries. Operating temperatures and
pressures impact the flammability limits and oxygen solubility
within the purification system, and these properties must be taken
into account when determining the appropriate oxygen concentration
to be used for the oxygen-containing gas. Considerations of such
factors are within the ability of one of ordinary skill in the art,
and either pure oxygen or atmospheric air may be commonly employed.
Surprisingly, we have found that there is an important factor
affecting the efficacy of inhibition within the purification
systems not previously considered with respect to oxygen
addition--that is the avoidance of high oxygen concentrations
within the MAA-containing stream itself. When oxygen concentrations
are large relative to inhibitor concentrations, oxygen can actually
increase the rate of polymerization by promoting the formation of
monomer radicals. For this reason, it is not recommended that
oxygen-containing gas be added to HPMAA light ends column 110 when
no inhibitor is present. Further, when oxygen-containing gas and
inhibitors are added to the purification system, the
oxygen-containing gas may be added in a prescribed ratio relative
to the inhibitor addition rate. The optimal oxygen to inhibitor
ratio will vary with respect to the inhibitor used.
[0034] When HQ is the selected inhibitor, the ratio of the
oxygen-containing gas feed to the HQ inhibitor feed added to the
purification system may be maintained at 0.65 moles to 10 moles of
O.sub.2/mole of HQ, for example, at 1 mole to 8.5 moles of
O.sub.2/mole of HQ or at 1.5 moles to 6 moles of O.sub.2/mole of
HQ.
[0035] When MEHQ is the selected inhibitor, the ratio of
oxygen-containing gas feed to the MEHQ inhibitor feed added to the
purification system may be maintained at 1 moles to 11.5 moles of
O.sub.2/mole of MEHQ, for example, at 1.5 moles to 9 moles of
O.sub.2/mole of MEHQ, or at 2 moles to 6 moles of O.sub.2/mole of
MEHQ.
[0036] Light ends overhead stream 105 may be recycled for use
elsewhere (e.g., in the MAA process) or may be routed to an acetone
recovery vessel. To minimize condensation polymerization, vapor
spaces on HPMAA light ends column 110 and its ancillaries,
including condensers and interconnecting vapor lines, may be
maintained at a temperature above the dew point of MAA. Insulation
and electric or steam tracing are effective for this purpose.
[0037] If light ends overhead stream 105 is condensed in one or
more condensers (not shown) after removal from HPMAA light ends
column 110, coolant having a temperature above 16.degree. C. may be
used in the condenser to avoid freezing MAA in the stream. In a
particular embodiment, tempered water in the range of 16.degree. C.
to 35.degree. C. is used for the condenser coolant. In one
embodiment, a portion of the condensate may be re-circulated
through a recirculation line (not shown) back to the condenser(s)
and optionally to the vapor inlet line of the condenser(s), to
minimize fouling and improve condenser efficiency. The condensate
may flow freely out of the re-circulation line or may be applied to
the tubesheet, condenser interior surfaces, and/or inlet vapor line
interior walls. If inhibitor is added to the condenser(s), it may
be added into this condensate re-circulation stream to improve the
distribution of the inhibitor. In another embodiment, at least a
portion of this condensate re-circulation stream may pass through
an apparatus capable of spraying the condensate on the interior
surfaces of HPMAA light ends column 110 and/or its ancillaries to
wash off polymerizable condensates.
[0038] In another embodiment, an optional partial-condenser
arrangement (not shown) is utilized, wherein light ends overhead
stream 105 is divided into two or more streams, including at least
one MAA/water stream and one water/acetone stream. In this way, the
MAA/water stream can be recycled directly into an MAA process and
the water/acetone stream can be routed to another process such as
an acetone recovery operation, a scrubber, or a flare.
[0039] Drawn from the bottom section of the HPMAA light ends column
110 is the bottoms stream 115 that typically contains MAA and heavy
ends, such as HIBA, and is generally substantially free of acetone
and water. Bottoms stream 115 is fed to the second impurity removal
apparatus, HPMAA heavy ends column 120. In HPMAA heavy ends column
120, the heavy impurities produced in the crude MAA production
process are separated from the MAA. The heavy impurities include,
but are not limited to, one or more of isobutyric acid, HIBA,
mesityl oxide, MOMPA, methylmethacrylate, propionic acid,
methyacrylamide, sulfates, and PTZ. HPMAA heavy ends column 120 and
its column ancillaries are preferably constructed of corrosion
resistant material, as described above for HPMAA light ends column
110.
[0040] Internal components such as trays or packing may be used in
HPMAA heavy ends column 120, if desired. Internals, if present, may
be made from the same materials as the column itself or may be
constructed from one or more different materials; for example, the
upper portion of HPMAA heavy ends column 120 may contain 300 series
stainless steel trays, while the lower portion of the column may
contain 904L trays. Trays may be used in HPMAA heavy ends column
120, for example, perforated plate trays, since they have been
found to be particularly resistant to MAA polymer accumulation. It
has been determined that at least five theoretical separation
stages are desired within the HPMAA heavy ends column 120. If trays
are used, it is preferable that five to fifteen perforated plate
trays are used. It is further possible that the HPMAA heavy ends
column 120 may be operated under a vacuum to minimize the bottoms
temperature. For example, the pressure at the bottom of the HPMAA
heavy ends column 120 should be maintained at 50 mmHg to 100 mmHg,
allowing the bottom of the HPMAA heavy ends column 120 to be
operated at temperatures ranging from 80.degree. C. to 120.degree.
C.
[0041] At least one heat exchanger (not shown) may be used as the
heating apparatus for HPMAA heavy ends column 120. Desuperheated
steam is preferred as the heat exchanger's heat source. If a
reboiler is used as the heat exchanger, it may be internal or
external to the column. Vortex breakers are also useful in the
bottom of HPMAA heavy ends column 120.
[0042] Heavy ends, such as HIBA and other impurities, are removed
from the bottom of HPMAA heavy ends column 120 via heavy ends
column bottoms stream 130. The HPMAA heavy ends column bottoms
stream 130 may be disposed of, but optionally some components of
this stream may be recovered and used as a fuel. Optionally,
bottoms stream 130 may be further processed in an independent
stripping system (not shown) to recover residual MAA. In one
embodiment of an independent stripping system (not shown), bottoms
stream 130 may be heated in one or more glass-lined stripping
vessels with live steam (steam that comes into direct contact with
the MAA-containing heavies column bottoms stream). The stripping
vessels may be operated at sub-atmospheric pressure to maximize the
recovery of MAA. The recovered MAA may be recycled back, for
example, into a MAA process or into a methacrylic acid-ester
process. In one embodiment, the heavies column bottoms are further
processed in a spent sulfuric acid unit, either alone or in
combination with spent acid streams selected from the list
including ACH-based MAA process acid residues, ACH-based MMA
process acid residues, and acrylic ester process acid residues, to
recover sulfur.
[0043] To minimize condensation polymerization, vapor spaces on
HPMAA heavy ends column 120 and its ancillaries, including
condensers and interconnecting vapor lines, may be maintained at a
temperature above the dew point of MAA. Insulation and electric or
steam tracing are effective for this purpose.
[0044] The HPMAA heavy ends column overhead stream 125 contains a
significant amount of MAA as well as water, acetone, and other
light ends. Overhead stream 125 may be at least partially condensed
in one or more condensers (not shown) to remove at least a portion
of the light ends. If overhead stream 125 is so condensed, tempered
water may be used in the condenser(s) to avoid freezing the MAA in
the stream. To maintain the required purity of the product stream
135, it is often necessary to return a portion of the condensate
back to HPMAA heavy ends column 120 via heavy ends reflux stream
155. The fraction of condensate returned may vary from 0% to 100%,
depending on the operating conditions of HPMAA heavy ends column
120 and the MAA purity level desired in the product stream 135. In
a particular embodiment, a portion of the condensate may be
re-circulated back to the condenser(s) and optionally to the
condenser inlet line, to minimize fouling and improve condenser
efficiency. The condensate may flow freely out of the
re-circulation line or may be applied to the tubesheet, condenser
interior surfaces, and/or inlet vapor line interior walls. If
inhibitor is added to the condenser(s), it may be added into this
condensate re-circulation stream to improve the distribution of the
inhibitor. In another embodiment, at least a portion of this
condensate re-circulation stream may pass through an apparatus
capable of spraying the condensate on the interior surfaces of
HPMAA heavy ends column 120 and/or its ancillaries to wash off
polymerizable condensates. The remaining condensate, comprising MAA
and light end impurities, is then returned to HPMAA light ends
column 110 as heavy ends column recycle stream 165.
[0045] Removal of the product stream 135 from the side of the
second distillation column of the process, i.e., from the HPMAA
heavy ends column 120 as shown in FIG. 1, instead of from the top
of the column 120, allows for improved operation of HPMAA heavy
ends column 120 and for the production of a higher purity of HPMAA.
This configuration is referred to herein as a "side-draw"
configuration. Increased purity of HPMAA can be achieved by using a
side draw configuration due to the tray composition in the middle
section of the HPMAA heavy ends column 120 being lower in light
ends impurities than the trays at the top of the column.
[0046] Additionally, because the highest temperature occurs at the
bottom of HPMAA heavy ends column 120, polymers or other
undesirable insoluble impurities that may form are less likely to
be present in a side-draw stream versus HPMAA product drawn
directly from the bottom of the column. While optional filtration
of product stream 135 through means known in the art may be used,
the use of the side-draw configuration in this embodiment may
reduce the level of potential insoluble impurities in the HPMAA
product stream. Thus, the need for filtration may be minimized or
possibly eliminated and the cost of operation reduced. It is noted
that the side-draw configuration may instead be applied to the
second distillation column of a two-column purification system, in
accordance with the present invention, even where the second column
is not the heavy ends column, but is a light ends column. This will
be clarified hereinafter in connection with the description of the
embodiment shown in FIG. 3.
[0047] FIG. 2 is a schematic sectional diagram representing one
embodiment of a side draw configuration adapted for use with heavy
ends removal column 120 of FIG. 1. This side draw configuration may
be used with perforated plate distillation trays, such as sieve
trays, but is also suitable for use with trays of other types as
well. As shown in FIG. 2, a piping connection 200 is made with the
sidewall of the distillation column 202. With regard to the
embodiment of FIG. 1, the distillation column 202 of FIG. 2 is
analogous to the heavy ends removal column 120. The piping
connection shown in FIG. 2 allows liquid accumulation in downcomer
210 to be drawn through opening 205. Inlet weir 225 restricts flow
out of downcomer 210 onto side draw tray 220, thereby maintaining
sufficient liquid head in downcomer 210 to create liquid outflow
via opening 205. This liquid outflow is side draw product stream
135. The appropriate height of weir 225 is dependent on a number of
variables including: the height 215 of liquid within downcomer 210
(also known as "downcomer backup"), the pressure above side draw
tray 220, the downcomer clearance 230, the desired flow rate of
side draw product stream 135, and the diameter of opening 205.
Selection of an appropriate height for inlet weir 225 is within the
ability of one of ordinary skill in the art of distillation design.
In one possible arrangement, for example, the height 215 of liquid
within downcomer 210 is 11 inches (27.94 cm), the pressure above
side draw tray 220 is 45 mmHg, downcomer clearance 230 is 3/4-inch
(1.91 cm), the desired flow rate of side draw product stream 135 is
approximately 11,000 kg/hr, the diameter of opening 205 is 3 inches
(7.62 cm), and the height of weir 225 is 6 inches (15.24 cm).
[0048] It is oftentimes useful to add one or more inhibitors, such
as those listed above, to HPMAA heavy ends column 120 with or
without a diluent. Inhibitor may be added in one or more locations
throughout HPMAA heavy ends column 120 and its ancillaries.
[0049] If HQ inhibitor is used, the inhibitor may be added
(optionally with a diluent) at a rate of 1 kg to 10 kg of HQ per
10,000 kg of HPMAA heavy ends column 120 feed, for example, 1.3 kg
to 8 kg of HQ per 10,000 kg of HPMAA heavy ends column 120 feed or
1.5 kg to 5 kg of HQ per 10,000 kg of HPMAA heavy ends column 120
feed.
[0050] MEHQ is a particularly effective inhibitor and may be added
directly, or with a diluent such as MAA, in one or more locations
throughout HPMAA heavy ends column 120 and its ancillaries. If
used, MEHQ may be added at a rate of 1 kg to 15 kg of MEHQ per
10,000 kg of HPMAA heavy ends column feed stream 115, for example,
1.5 kg to 12 kg of MEHQ per 10,000 kg of HPMAA heavy ends column
feed, or 2 kg to 9 kg of MEHQ per 10,000 kg of HPMAA heavy ends
column feed.
[0051] As described above, when phenolic inhibitors, such as HQ and
MEHQ, are used, oxygen-containing gas may be added to the
distillation column to enhance the effectiveness of the inhibitor.
Oxygen-containing gas may be added in one or more locations
throughout HPMAA heavy ends column 120 and its column ancillaries.
Operating conditions and concerns and recommended
oxygen-to-inhibitor ratios for HPMAA heavy ends column 120 are
identical to those described in connection with HPMAA light ends
column 110.
[0052] Side-draw removal of the HPMAA product stream increases the
number of inhibitor options for use in HPMAA heavy ends column 120.
This is due to the fact that inhibitors are generally heavy
components that exit distillation column 120 through the bottoms
stream. Thus, in an embodiment wherein the product stream is drawn
from the bottom of column 120, any added inhibitor would exit along
with it. By way of contrast, when the product is drawn from the
side of the column as shown in FIG. 1, all of the inhibitor is not
drawn off with the product. Thus, in the embodiment illustrated in
FIG. 1, a wide variety of inhibitors may be supplied directly to
the HPMAA heavy ends column 120 itself and may even include
inhibitors that are undesired in the product HPMAA, provided they
are added to the column at locations below the point where the side
draw stream is removed.
[0053] Accordingly, in some embodiments, PTZ may be added directly
to the lower section of HPMAA heavy ends column 120 to minimize
polymer formation in the bottoms of the column. If used, PTZ may be
added (optionally with a diluent) at a rate of 0.005 kg to 8 kg of
PTZ per 10,000 kg of HPMAA heavy ends column 120 feed, for example,
0.01 kg to 5 kg of PTZ per 10,000 kg of HPMAA heavy ends column 120
feed, or 0.05 kg to 1 kg of PTZ per 10,000 kg of HPMAA heavy ends
column 120 feed.
[0054] HPMAA product stream 135 may be cooled before storage to
inhibit polymerization. As with all polymerizable monomers, it is
beneficial for the product methacrylic acid to comprise
polymerization inhibitors in order to prevent polymerization in
shipment and storage. Any of the inhibitors previously listed as
suitable for use in the distillation columns may be employed for
this purpose. However, when MAA is used in specialty polymer
production, MeHQ is most commonly utilized as the inhibitor, and is
typically employed at a concentration of between 200 ppm and 300
ppm. In some embodiments of the present invention, a variable
amount of MeHQ inhibitor may be added directly to the HPMAA product
stream 135 to ensure that the HPMAA product stream inhibitor
concentration remains within final product specifications.
[0055] Often, methacrylic acid may not have to be freed of MeHQ
inhibitor prior to use by polymer manufacturers in polymerization
reactions. This applies particularly to the preparation of
copolymers where MAA is less than 50 wt % of the monomer mixture,
based on the total weight of the mixture. In such instances,
somewhat larger quantities of initiator may be employed than with
uninhibited monomer. Alternatively, because MeHQ is a phenolic
inhibitor, some specialty MAA polymer manufacturers may instead
choose to perform their polymerization reactions in the absence of
oxygen, thereby rendering the MeHQ substantially inactive in the
reaction system. If, however, it is not practical to adjust
initiator levels or exclude oxygen from the reaction system, known
methods such as alkaline wash, treatment with a suitable
ion-exchange resins, or distillation of the methacrylic acid
product may be employed to remove the MeHQ inhibitor prior to
use.
[0056] Examples of the HPMAA product stream produced by the present
invention include a composition wherein the assay of the MAA is at
least 99 wt %, water is present at a concentration of up to 500 ppm
(0.05 wt % H.sub.2O), the methacrolein content of the MAA is up to
1000 ppm (0.1 wt % methacrolein), and the HIBA content of the MAA
is up to 9,000 ppm (0.9 wt % HIBA), all weight percents being based
upon the total weight of the HPMAA product stream.
[0057] Another example of the HPMAA product stream of the present
invention includes a composition wherein the assay of the HPMAA is
at least 99 wt %, water is present at a concentration of up to 300
ppm (0.03 wt % H.sub.2O), the methacrolein content of the HPMAA is
up to 500 ppm (0.05 wt % methacrolein), and the HIBA content of the
HPMAA is up to 5,000 ppm (0.5 wt % HIBA), all weight percents being
based upon the total weight of the HPMAA product stream.
[0058] Yet another example of the HPMAA product stream of the
present invention includes a composition wherein the assay of the
HPMAA is at least 99 wt %, water is present at a concentration of
up to 200 ppm (0.02 wt % H.sub.2O), the methacrolein content of the
HPMAA is up to 200 ppm (0.02 wt % methacrolein), and the HIBA
content of the HPMAA is up to 1,500 ppm (0.15 wt % HIBA), all
weight percents being based upon the total weight of the HPMAA
product stream.
[0059] An additional example of a product HPMAA stream produced by
the present invention includes a composition wherein the assay of
the HPMAA is at least 99 wt %, water is present at a concentration
of up to 150 ppm (0.015 wt % H.sub.2O), the methacrolein content of
the HPMAA is up to 50 ppm (0.005 wt % methacrolein), and the HIBA
content of the HPMAA is up to 1,000 ppm (0.10 wt % HIBA), all
weight percents being based upon the total weight of the HPMAA
product stream.
[0060] An example of the HPMAA product stream produced by the
present invention further includes a composition wherein the
collective MOMPA and MMA content of the HPMAA is up to 1000 ppm
(0.1 wt % of both MOMPA and MMA), and the methacrylamide content of
the HPMAA is up to 750 ppm (0.075 wt % MAM), all weight percents
being based upon the total weight of the HPMAA product stream.
[0061] Another example of the HPMAA product stream of the present
invention further includes a composition wherein the collective
MOMPA and MMA content of the HPMAA is up to 500 ppm (0.05 wt % of
both MOMPA and MMA), and the methacrylamide content of the HPMAA is
up to 500 ppm (0.05 wt % MAM), all weight percents being based upon
the total weight of the HPMAA product stream.
[0062] Yet another example of the HPMAA product stream of the
present invention further includes a composition wherein the
collective MOMPA and MMA content of the HPMAA is up to 200 ppm
(0.02 wt % of both MOMPA and MMA), and the methacrylamide content
of the HPMAA is up to 100 ppm (0.01 wt % MAM), all weight percents
being based upon the total weight of the HPMAA product stream.
[0063] A yet additional example of a product HPMAA stream produced
by the present invention further includes a composition wherein the
collective MOMPA and MMA content of the HPMAA is up to 100 ppm
(0.01 wt % of both MOMPA and MMA), and the methacrylamide content
of the HPMAA is up to 25 ppm (0.0025 wt % MAM), all weight percents
being based upon the total weight of the HPMAA product stream.
[0064] An example of the HPMAA product stream produced by the
present invention further includes a composition wherein the acetic
acid content of the HPMAA product stream is up to 1000 ppm (0.1 wt
% acetic acid), the acetone content is up to 800 ppm (0.08 wt %
acetone), the acrylic acid content is up to 400 ppm (0.04 wt %
acrylic acid), and the propionic acid content is up to 200 ppm
(0.02 wt % propionic acid), all weight percents being based upon
the total weight of the HPMAA product stream.
[0065] Another example of the HPMAA product stream of the present
invention further includes a composition wherein the acetic acid
content of the HPMAA product stream is up to 750 ppm (0.075 wt %
acetic acid), the acetone content is up to 400 ppm (0.04 wt %
acetone), the acrylic acid content is up to 200 ppm (0.02 wt %
acrylic acid), and the propionic acid content is up to 150 ppm
(0.015 wt % propionic acid), all weight percents being based upon
the total weight of the HPMAA product stream.
[0066] Yet another example of the HPMAA product stream of the
present invention further includes a composition wherein the acetic
acid content of the HPMAA product stream is up to 200 ppm (0.02 wt
% acetic acid), the acetone content is up to 50 ppm (0.005 wt %
acetone), the acrylic acid content is up to 25 ppm (0.0025 wt %
acrylic acid), and the propionic acid is up to 100 ppm (0.01 wt %
propionic acid), all weight percents being based upon the total
weight of the HPMAA product stream.
[0067] An additional example of a product HPMAA stream produced by
the present invention further includes a composition wherein the
acetic acid content of the HPMAA product stream is up to 100 ppm
(0.01 wt % acetic acid), the acetone content is up to 25 ppm
(0.0025 wt % acetone), the acrylic acid content is up to 10 ppm
(0.001 wt % acrylic acid), and the propionic acid content is up to
50 ppm (0.005 wt % propionic acid), all weight percents being based
upon the total weight of the HPMAA product stream.
[0068] Another embodiment of an improved HPMAA purification system
is shown in FIG. 3. This embodiment utilizes a "reverse-order"
configuration to purify crude MAA stream 300 into an HPMAA product
stream in accordance with the present invention. Crude MAA stream
300 is initially fed to the first impurity removal apparatus, HPMAA
heavies column 310. In HPMAA heavies column 310, heavy ends,
including HIBA, are removed from the bottom of the column via line
315. As will be recognized by persons of ordinary skill in the art,
early removal of heavies in this embodiment minimizes HIBA
decomposition to water and light ends in the next column.
[0069] HPMAA heavies column 310 and its column ancillaries are
preferably constructed of corrosion resistant material, as
previously described for HPMAA light ends column 110. Internal
components such as trays or packing may be used in HPMAA heavies
column 310, if desired. Internals, if present, may be made from the
same materials as the column itself or may be constructed from one
or more different materials. Trays may be used in HPMAA heavies
column 310, for example, perforated plate trays, since they have
been found to be particularly resistant to MAA polymer
accumulation. It has been determined that at least five theoretical
separation stages are desired within the HPMAA heavies column 310.
If trays are used, it is preferable that five to fifteen perforated
plate trays are used. HPMAA heavies column 310 may be operated
under a vacuum to minimize the temperature of the bottom of the
column. For example, in a particular embodiment, the pressure at
the bottom of the HPMAA heavies column 310 is maintained at 50 mmHg
to 90 mmHg, allowing the bottom of the HPMAA heavies column 310 to
be operated at 85.degree. C. to 125.degree. C. At least one heat
exchanger (not shown) may be used as the heating apparatus for
HPMAA heavies column 310. Desuperheated steam may be used
effectively as the heat exchanger's heat source. If a reboiler is
used as the heat exchanger, it may be internal or external to the
column. Vortex breakers are also useful in the bottom of HPMAA
heavies column 310.
[0070] While some inhibitors may still be present in the crude MAA
from prior processing, it is oftentimes useful to add inhibitors
such as those listed above, with or without diluents, to HPMAA
heavies column 310. HQ inhibitor is especially effective and may be
added directly, or with a diluent such as water, in one or more
locations throughout HPMAA heavies column 310 and its ancillaries.
If used, the inhibitor may be added at a rate of 1 kg to 10 kg of
HQ per 10,000 kg of feed to the HPMAA heavies column 310, for
example, at a rate of 1.3 kg to 8 kg of HQ per 10,000 kg of feed to
the HPMAA heavies column 310, or 1.5 kg to 5 kg of HQ per 10,000 kg
of feed to the HPMAA heavies column 310.
[0071] As described above, when phenolic inhibitors, such as HQ and
MEHQ, are used, oxygen-containing gas may be added to the
distillation column to enhance the effectiveness of the inhibitor.
Oxygen-containing gas may be added in one or more locations
throughout HPMAA heavies column 310 and its column ancillaries.
Operating conditions and concerns and recommended
oxygen-to-inhibitor ratios for HPMAA heavies column 310 are
substantially identical to those described in connection with HPMAA
light ends column 110.
[0072] HIBA, and other heavy ends impurities are removed from the
bottom of the HPMAA heavies column 310 via stream 315 and it may be
disposed of or recovered for use as fuel. Optionally, the heavies
column bottoms can be further processed in an independent stripping
system (not shown) to recover residual MAA. In one embodiment of an
independent stripping system (not shown), the heavies column
bottoms are heated in one or more glass-lined stripping vessels
with live steam. The stripping vessels may be operated at
sub-atmospheric pressure to maximize the recovery of MAA. In one
embodiment, the heavies column bottoms are further processed in a
spent sulfuric acid unit, either alone or in combination with spent
acid streams selected from the list including ACH-based MAA process
acid residues, ACH-based MMA process acid residues, and acrylic
ester process acid residues, to recover sulfur.
[0073] The HPMAA heavies column overhead stream 305 contains a
significant amount of MAA as well as water, acetone, other light
ends, and trace amounts of heavy ends. HPMAA heavies column
overhead stream 305 may be at least partially condensed in one or
more condensers (not shown). Further, the condensed stream may be
split into a reflux stream 355 and a feed stream 365.
[0074] To maintain the required purity of lights column feed stream
365, it is often necessary to return a portion of the condensate
back to the heavies column 310 via reflux stream 355; the fraction
of condensate returned may vary from 0% to 100%, depending on the
operating conditions of HPMAA heavies column 310 and the purity
level desired for feed stream 365. The remaining condensate is then
transferred via feed stream 365 to HPMAA lights column 320.
Tempered water may be used in the heavies column condenser(s) (not
shown) to avoid freezing MAA in the stream. To minimize
condensation polymerization, vapor spaces on HPMAA heavies column
310, and its ancillaries including condensers and interconnecting
vapor lines, may be maintained at a temperature above the dewpoint
of MAA. Insulation and electric or steam tracing are effective for
this purpose.
[0075] In a particular embodiment, a portion of the condensate may
be recirculated back to the condenser (not shown), and optionally
to the condenser inlet line, to minimize fouling and improve
condenser efficiency. The condensate may flow freely out of the
recirculation line or may be applied to the tubesheet, condenser
interior surfaces, and/or inlet vapor line interior walls. If
inhibitor is added to the condenser, it may be added to this
condensate recirculation stream to improve the distribution of the
inhibitor. In a particular embodiment, at least a portion of this
condensate recirculation stream may pass through an apparatus
capable of spraying the condensate on the interior surfaces of
HPMAA heavies column 310 and/or its ancillaries to wash off
polymerizable condensates.
[0076] HPMAA lights column 320 removes water, acetone, and other
light impurities, where present, from the MAA via stream 325. A
partial-condenser arrangement (not shown) may be used, wherein
stream 325 is at least partially condensed into a liquid. If stream
325 is so condensed, tempered water may be used as a coolant to
avoid freezing MAA in the stream 325. To minimize condensation
polymerization, vapor spaces on HPMAA lights column 320, and its
ancillaries including condensers and interconnecting vapor lines,
may be maintained at a temperature above the dewpoint of MAA
Insulation and electric or steam tracing are effective for this
purpose. In a particular embodiment, a portion of the condensate
may be recirculated back to the condenser and optionally to the
condenser inlet line, to minimize fouling and improve condenser
efficiency. The condensate may flow freely out of the recirculation
line or may be applied to the tubesheet, condenser interior
surfaces, and/or inlet vapor line interior walls. If inhibitor is
added to the condenser, it may be added to this condensate
recirculation stream to improve the distribution of the inhibitor.
In an particular embodiment, at least a portion of this condensate
recirculation stream may pass through an apparatus capable of
spraying the condensate on the interior surfaces of HPMAA lights
column 320 and/or its ancillaries to wash off polymerizable
condensates.
[0077] HPMAA lights column 320 and its column ancillaries are
preferably constructed of corrosion resistant material, as
previously described for HPMAA lights column 110. Internal
components such as trays or packing may be used in HPMAA lights
column 320, if desired. Internals, if present, may be made from the
same materials as the column itself or may be constructed from one
or more different materials. Perforated plate trays are especially
effective, since they have been found to be particularly resistant
to MAA polymer accumulation. It has been determined that at least
two theoretical separation stages are desired within the HPMAA
lights column 320. If trays are used, it is preferable that two to
ten perforated plate trays are used. HPMAA lights column 320 may be
operated under a vacuum (i.e., below atmospheric pressure) to
minimize bottoms temperature. In a particular embodiment, the
pressure at the bottom of HPMAA lights column 320 is maintained at
60 mmHg to 100 mmHg, allowing the bottom of HPMAA lights column 320
to be operated at 75.degree. C. to 115.degree. C.
[0078] At least one heat exchanger (not shown) may be used as the
heating apparatus for the HPMAA lights column 320. Desuperheated
steam is effective as the heat exchanger's heat source. If a
reboiler is used as the heat exchanger, it may be internal or
external to the column. Vortex breakers are also useful in the
bottom of HPMAA lights column 320.
[0079] HPMAA product stream 350 exits HPMAA lights column 320 from
the side of the column 320 having purity levels greater than or
equal to 99% and containing less than 0.05% water. Suitable
connecting means, such as those previously described and depicted
in FIG. 2, are used to connect the piping of stream 350 to the side
of column 320. Similar to the side draw of the Heavy Ends Removal
Column 120, details of the side draw of the HPMAA Lights Column 320
of FIG. 3 are illustrated in FIG. 2. With respect to the reverse
order process of FIG. 3, the distillation column 202 represented in
FIG. 2 is the HPMAA Lights Column 320. Likewise the stream exiting
the opening 205 is the product stream 350. Product stream 350 may
be cooled before storage to inhibit polymerization.
[0080] While optional filtration of product stream 350 may be used
as described in the previous embodiment depicted in FIG. 1, the use
of the side-draw configuration in this embodiment may reduce the
level of potential impurities in the HPMAA product stream. Thus,
the need for filtration may be minimized and the cost of operation
reduced.
[0081] As shown in FIG. 3, heavy end impurities, which may
accumulate in the bottom of HPMAA lights column 320, are removed
via stream 330 and recycled back to HPMAA heavies column 310. This
recycle step allows the MAA present in stream 330 to be recovered
in column 310. Any heavy ends and undesirable impurities present in
stream 330 will exit HPMAA heavies column 310 with the other heavy
end components in stream 315.
[0082] It is oftentimes useful to add inhibitors, such as those
listed above, to HPMAA lights column 320, optionally with a
diluent. Inhibitor may be added in one or more locations throughout
HPMAA lights column 320 and its ancillaries.
[0083] The side-draw removal of the HPMAA product stream 350
increases the number of inhibitor options for use in HPMAA lights
column 320. This is due to the fact that inhibitors are generally
heavy components that exit a distillation column through its
bottoms. Thus, when the product stream is drawn from the bottom of
the column any inhibitor added to the column exits along with it.
By way of contrast, when the product is drawn from the side of the
column, any inhibitor added to the column below the product removal
point--e.g., below the side-draw tray--is not drawn off with the
product; rather, the inhibitor drops to the bottom of the column
for removal. Thus, in the reverse-order embodiment illustrated in
FIG. 3, a wide variety of inhibitors may be employed in HPMAA
lights column 320 and different inhibitors may be added at
different locations in the HPMAA lights column 320, if desired.
[0084] PTZ is particularly useful for minimizing polymer formation
in the bottoms of HPMAA Lights Column 320. If used, PTZ may be
added (optionally with a diluent) at a point on the HPMAA Lights
Column 320 below the side-draw tray and at a rate of 0.005 kg to 8
kg of PTZ per 10,000 kg of HPMAA lights column 320 feed, for
example, 0.01 kg to 5 kg of PTZ per 10,000 kg of HPMAA lights
column 320 feed, or 0.05 kg to 1 kg of PTZ per 10,000 kg of HPMAA
lights column 320 feed.
[0085] If HQ inhibitor is used, it may be added (optionally with a
diluent) at a rate of 1 kg to 10 kg of HQ per 10,000 kg of HPMAA
lights column 320 feed, for example, 1.3 kg to 8 kg of HQ per
10,000 kg of HPMAA lights column 320 feed, or 1.5 kg to 5 kg of HQ
per 10,000 kg of HPMAA lights column 320 feed.
[0086] MEHQ inhibitor may also be added to HPMAA lights column 320
in this embodiment and may be added directly, or with a diluent
such as MAA, throughout HPMAA lights column 320 and its
ancillaries. In general, satisfactory performance will be achieved
in HPMAA lights column 320 if the MEHQ addition rates for HPMAA
Heavy Ends column 120, as described above, are utilized.
Optionally, a variable amount of MEHQ inhibitor may be added
directly to the HPMAA product stream 350 to ensure that the HPMAA
product stream inhibitor concentration is within final product
specifications.
[0087] As described above, when phenolic inhibitors, such as HQ and
MEHQ, are used, oxygen-containing gas may be added to the
distillation column to enhance the effectiveness of the inhibitor.
Oxygen-containing gas may be added in one or more locations
throughout HPMAA lights column 320 and its column ancillaries.
Operating conditions and concerns and recommended
oxygen-to-inhibitor ratios for HPMAA lights column 320 are
substantially identical to those described in connection with HPMAA
lights column 110.
[0088] In addition to the aforementioned difficulties of producing
specialty MAA polymers from methacrylic acid product comprising
significant amounts of HIBA, it should be noted that the presence
of HIBA in crude methacrylic acid could also have detrimental
effects in purification processes, including the purification
process of the present invention as well as others. Specifically,
HIBA that is present in crude methacrylic acid may thermally
decompose during purification to produce decomposition products
including acetone, carbon monoxide, and water. Such in-situ
decomposition-product formation is counterproductive to the
objective of water and acetone removal and may in some cases
prevent the attainment of desired low-impurity levels of the
product MAA.
[0089] It has been determined that decomposition of HIBA during
purification is promoted by acid-catalysts, such as sulfuric acid.
One or more of such acid-catalysts may be present in the crude
methacrylic acid as a result of prior processing. For example,
crude methacrylic acid derived from an ACH-process may contain
residual sulfuric acid or other compounds capable of providing
sulfate (SO4.sup.-2) ions.
[0090] The extent of HIBA decomposition is dependent on several
process variables including acid-catalyst concentration in the
bottom of the column, column bottoms temperature, and bottoms
residence time. By controlling one or more of these variables the
extent of HIBA decomposition during purification may be minimized,
thereby improving operation of the purification process by reducing
the water load on the purification system.
[0091] By operating MAA purification distillation columns at less
than atmospheric pressure (as measured in the vapor space below
Tray 1), it is possible to maintain the column bottoms temperatures
at less than 150.degree. C. For minimization of HIBA decomposition
in an MAA purification column, the temperature at the bottom of
column may be maintained at not more than 145.degree. C., for
example not more than 120.degree. C. or not more than 100.degree.
C. The temperature at the bottom of the column may even be
maintained at not more than 90.degree. C.
[0092] Maintaining low bottoms residence time in MAA purification
columns also serves to minimize HIBA decomposition. As used herein,
the term "bottoms residence time" refers to the average time a
given unit-volume of material remains within the bottom sump of the
column and its ancillaries (including but not limited to the
reboiler and the interconnecting circulation piping between the
sump and the reboiler). Generally, the bottoms residence time can
be determined by calculating, at steady state operating conditions,
the total process fluid volume in the bottom (sump) of the column,
the reboiler, and associated bottoms piping, and then dividing this
total process fluid volume by the volumetric flow rate (removal
rate) of the bottoms stream. While the maximum residence time is
ultimately determined by the physical dimensions of the
purification equipment (distillation column diameter and the like),
the actual bottoms residence time employed may be further
controlled through the adjustment of process operating variables
such as the sump operating level, boil-up rate, column feed rate,
and column bottoms removal rate. It has been determined that HIBA
decomposition may be minimized when bottoms residence time is
maintained at not more than 48 hours, for example not more than 24
hours, or not more than 15 hours, or even not more than 5
hours.
[0093] The decomposition of HIBA in purification may also be
minimized by maintaining the total acid-catalyst concentration in
the bottom of a distillation column (as measured in the bottoms
stream) at not more than 2.5 wt % (25,000 ppm), for example not
more than 1.5 wt % (15,000 ppm), or not more than 0.5 wt % (5,000
ppm), based on the total weight of the bottoms stream. This may be
achieved by maintaining low acid-catalyst concentration in the
crude MAA feed to purification, or by controlling bottoms
composition through the adjustment of column operating variables
such as MAA vaporization rate and bottoms residence time. Optional
treatment methods, such as treatment with stabilizing agents
(described below), may also be employed to neutralize at least a
portion of the acid-catalyst and thereby lower the acid-catalyst
concentration.
[0094] In addition to the methods described above, the problem of
thermal decomposition of HIBA may be avoided, at least in part, by
minimizing the HIBA content in the crude methacrylic acid prior to
purification. In the case of crude methacrylic acid derived from
ACH-processes, the formation of HIBA, and thus the resulting final
HIBA content in crude methacrylic acid, may be minimized through
optimization of various crude MAA production steps. For example, by
utilizing essentially anhydrous conditions in the hydrolysis
process-step, HIBA formation can be minimized. Such anhydrous
conditions may be achieved through the use of dry acetone
cyanohydrin, dry sulfuric acid (e.g., oleum), and SO.sub.3
addition, either alone or in combination, in the hydrolysis step of
the MAA production process to minimize HIBA formation.
Additionally, the use of high temperatures, long residence times,
and plug-flow conditions, either alone or in combination, will
improve conversion of HIBA to MAM during the thermal conversion
(aka: "cracking") step of the MAA production process.
[0095] It will be obvious to one of ordinary skill in the art that
the HIBA content of crude methacrylic acid may also be minimized
through the use of processes that do not form MAA via the HIBA
intermediate. For example, C-4 based methacrylic acid processes
typically utilize methacrolein as an intermediate and can therefore
be reasonably expected to not contain a significant level of HIBA
impurity in the resultant crude MAA product. Purification of such
`low-HIBA` crude methacrylic acid streams in the above-described
distillation process to obtain low-HIBA content methacrylic acid is
within the scope of the present invention.
[0096] In addition to minimizing the HIBA content in the crude
methacrylic acid prior to purification, optional HIBA treatment
methods may also be employed during the purification process, for
example, in combination with the distillation method of the present
invention, to further improve the quality and reduce the impurity
levels of product methacrylic acid. These HIBA treatment methods
include stabilization of HIBA to prevent thermal decomposition
during purification and elimination of HIBA prior to
purification.
[0097] Such optional treatment methods may be used alone or in
combination with the above-described crude MAA production steps to
minimize HIBA content in the crude MAA product. By employing these
optional HIBA treatment methods, the quality of said optionally
treated product methacrylic acid could be further improved.
[0098] As previously stated, it has been discovered that the
thermal decomposition reaction of HIBA is catalyzed by the presence
of acid-catalyst, such as sulfuric acid. By neutralizing at least a
portion of these acid-catalysts prior to purification of the crude
methacrylic acid stream, decomposition of HIBA may be substantially
avoided.
[0099] In one embodiment of this HIBA treatment method, treatment
of the crude methacrylic acid with stabilizing agents, such as for
example, magnesium oxide, magnesium hydroxide, caustic, or ammonia,
serves to neutralize the acid-catalyst. Salts formed by
neutralization of the acid-catalyst are then removed from the
treated methacrylic acid through known filtration methods, thereby
stabilizing the HIBA and allowing thermal decomposition to be
substantially avoided.
[0100] In a particular embodiment, wherein sulfuric acid is the
acid-catalyst present in the crude MAA, magnesium oxide is used as
the stabilizing agent However, in this embodiment, the
substantially insoluble magnesium sulfate salts resulting from the
neutralization are not filtered from the crude methacrylic acid
following treatment. The treated crude methacrylic acid is then
purified in the distillation process of the present invention. The
heavy ends stream from the distillation process comprises magnesium
sulfate (MgSO4) and is further processed in a spent acid recovery
unit (not shown). The presence of magnesium sulfate in the spent
acid recovery unit is beneficial in the prevention of corrosion of
the process apparatus.
[0101] In an alternative embodiment, the crude methacrylic acid
stream is contacted with a suitable ion exchange resin (IER) to
remove the acid-catalyst, for example a suitable IER having a basic
functionality. Contact may be achieved by passing the crude MAA
stream through an IER bed or by adding IER directly into the crude
MAA stream, followed by filtration after treatment is complete.
Suitable IER's having basic functionality include, but are not
limited to, one or more selected from the list including
AMBERLYST.RTM. A-21 or AMBERLYST.RTM. A-26 (both available from
Rohm and Haas Company of Philadelphia, Pa.). Using these methods,
HIBA is stabilized and the decomposition of HIBA is substantially
avoided in purification.
[0102] In comparison to the stabilization of HIBA, the destruction
of HIBA prior to purification is a HIBA treatment method that
minimizes thermal decomposition of HIBA in the purification process
and reduces the concentration of HIBA in the crude MAA stream. The
HIBA destruction treatment method comprises contacting the crude
MAA stream with one or more Lewis acid components that are, for
example, strong Lewis acids. The Lewis acid components can include
sulfuric acid, phosphoric acid, "superacid" (e.g., TiOSO4-H2SO4 or
ZrOSO4-H2SO4), an ion exchange resin with acidic functionality
(such as, but not limited to, AMBERLYST.RTM. 15 available from Rohm
and Haas Company of Philadelphia, Pa.), and combinations thereof.
After the addition of the Lewis acids the mixture is then heated,
for example above 90.degree. C., to substantially decompose the
HIBA to water, acetone, and carbon monoxide prior to final
purification. Such HIBA decomposition products are removed as light
ends in the purification process of the present invention described
hereinabove, resulting in an MAA product with low levels of water
and HIBA impurities.
[0103] In one embodiment of HIBA reduction, methacrylic acid
comprising HIBA is combined with 20% H2SO4, heated to at least
100.degree. C., and optionally mixed. The heating of this mixture
may be conducted at atmospheric pressure. The HIBA is substantially
decomposed and the resulting light ends are then removed by
distillation according to the purification method of the present
invention.
[0104] It will be apparent to one of ordinary skill in the art,
with the benefit of the disclosure of the present invention, that
these optional HIBA treatment methods may also be employed alone,
or in combination with other methacrylic acid purification
processes to produce a variety of MAA products, including glacial
methacrylic acid, HPMAA, and HPMAA with low HIBA content. These
optional HIBA treatment methods may also be successfully applied to
other methacrylic acid purification processes including, but not
limited to, the three-column distillation process described in
co-pending U.S. application U.S. Ser. No. 10/420,273 filed Apr. 22,
2003, the solvent extraction process described in U.S. Pat. No.
3,414,485, as well as crystallization processes and hybrid
distillation/crystallization purification processes.
EXAMPLE 1
[0105] A two column side-draw purification apparatus of the type
shown in FIG. 1 and FIG. 2 was used to produce HPMAA in accordance
with the purification process of the present invention. In this
example, lights ends column 110 comprised five sieve trays and
heavy ends column 120 comprised ten sieve trays. The position of
each tray within each column is hereinafter referenced by a
numerical reference, with the bottom-most tray, for example, being
referred to as "Tray 1". The sieve trays in light ends column 110
have 1/2" (1.27 cm) diameter holes and a 13% open area. All light
ends column trays are installed with a 33-inch (83.82 cm) spacing
between them. The sieve trays in heavy ends column 120 have 1/2"
(1.27 cm) diameter holes and a 10.2% open area. Trays in the heavy
ends column 120 are installed with variable spacing between them:
Trays 1 through 4 are installed 24 inches (60.96 cm) apart from one
another, Tray 5 is 36 inches (91.44 cm) above Tray 4, and Trays 5
through 10 are installed 33 inches (83.82 cm) apart from one
another. Heavy ends column Tray 7 is a sidedraw tray, of the type
illustrated in FIG. 2 and described in detail hereinabove. In this
example, both columns were operated at less than atmospheric
pressure. Approximately 7,250 kg/hr of crude MAA feed stream 100
was supplied to light ends column 110. Crude MAA feed stream 100
was derived from an ACH-process and comprised 95.05 wt % MAA, 2.00
wt % water, 0.8 wt % HIBA, 0.25 wt % methacrylamide, 0.90 wt %
acetone, and 1.00 wt % MOMPA, based on the total weight of the
crude MAA stream. The crude MAA stream was co-fed with heavy ends
column recycle stream 165 to Tray 5 (top) of light ends column 110.
Aqueous HQ inhibitor was provided to light ends column 110;
atmospheric air was also supplied to the bottom section of the
column 110 at a rate sufficient to ensure the efficacy of the HQ
inhibitor. Sufficient steam was supplied to the light ends column
reboiler (not shown) to maintain a bottoms temperature of
90.degree. C. at 58 mmHg bottoms pressure. Approximately 900 kg/hr
of light ends stream 105, comprising water and acetone, was removed
from light ends column 110. Light ends bottoms stream 115,
comprising greater than 98 wt % MAA and not more than 1.0 wt %
H2SO4, based on the total weight of the light ends bottoms stream,
was withdrawn from light ends column 110 and fed to the sump area
(below Tray 1) of heavy ends column 120.
[0106] A solution of MeHQ inhibitor in MAA was provided to heavy
ends column 120. Atmospheric air was also supplied to the bottom
section of the heavy ends column 120 at a rate sufficient to ensure
the efficacy of the MeHQ inhibitor. Sufficient steam was supplied
to the heavy ends column reboiler (not shown) to maintain a bottoms
temperature of 100.degree. C. at 75 mmHg bottoms pressure in the
heavy ends column 120. Overhead stream 125 was withdrawn from the
upper section of heavy ends column 120, condensed, and partially
refluxed. Approximately 2,800 kg/hr of MAA-containing heavy ends
column recycle stream 165 was returned to light ends column 110.
Approximately 1,550 kg/hr of bottoms stream 130, comprising MOMPA,
MAM, and HIBA, was removed from heavy ends column 120. The bottom
of the heavy ends column 120 had an internal diameter of 2.1m and
provided a working volume of 4 m.sup.3. The reboiler and associated
bottoms piping (not shown) provided an additional 8.3 m.sup.3 of
volume, for a total process fluid volume of 12.3 m.sup.3. At the
bottoms removal rate of 1,550 kg/hr, the resulting bottoms
residence time was calculated to be approximately 8 hours.
[0107] Approximately 5,080 kg/hr of side-draw HPMAA product stream
135 was removed from Tray 7 of heavy ends column 120. The side-draw
HPMAA product stream 135 comprised 99.860 wt % MAA, 0.108 wt %
HIBA, 0.007 wt % MOMPA, 0.002 wt % (20 ppm) acetic acid, 0.001 wt
%(7 ppm) MMA, 0.004 wt % acetone, and 0.013 wt % water, based on
the total weight of the side-draw HPMAA product stream 135.
Methacrylamide, acrylic acid, and methacrolein concentrations in
the side-draw HPMAA product stream 135 were below detectable limits
and product color measurements varied between 2 and 7 APHA. This
improved, low-impurity grade of HPMAA is especially desirable for
use in the production specialty MAA polymers, including
ethelyene-MAA copolymers and ionomers.
[0108] It will be apparent to one of ordinary skill in the relevant
art that numerous variations and modifications may be successfully
made to the foregoing example of the process of the present
invention. It will be recognized, for example, that an increase in
the total number of trays in heavy ends column 120 will, at a
constant production rate, tend to further increase the purity level
of the HPMAA produced. Such changes are within the scope of the
present invention.
[0109] Further, it will be recognized that changes in the flow rate
of recycle stream 165 and/or reflux stream 155 might also be made
to increase purity or throughput. For example, by increasing the
flow rate of recycle stream 165, the purity of the product MAA may
be increased, however, the throughput (i.e., the production rate of
product MAA stream 135) will be simultaneously decreased.
Alternatively, by decreasing the flow rate of recycle stream 165,
the purity of the product MAA may be reduced, but the production
rate of product methacrylic acid will be increased. From an
economic standpoint, it will be appreciated that lower production
rates through the same process equipment will generally increase
total manufacturing cost, while higher rates will generally lower
total manufacturing cost. Therefore, it is possible using the
process of the present invention to balance the factors of total
manufacturing cost against product purity to achieve an optimum
solution for a given specialty MAA polymer application. Such
optimization of purity versus manufacturing cost--and the required
adjustment of associated operating parameters, such as for example
steam flow to the reboiler and inhibitor addition rates--is within
the ability of one of ordinary skill in the art, given the benefit
of the foregoing description of the process and product of the
present invention.
[0110] The present invention described herein, therefore, is
well-adapted to carry out the objects and attain the ends and
advantages mentioned, as well as others inherent therein. While
several embodiments of the invention have been provided
hereinabove, numerous changes in the details of procedures,
processes, operating conditions and apparatus may be made without
departing from the intent and scope of the present invention. For
example, the purification process of the present invention can be
successfully applied to any industrial process involving a crude
MAA stream as a part of the process. These and other similar
modifications will readily suggest themselves to those skilled in
the art, and are intended to be encompassed within the spirit and
scope of the present invention.
* * * * *