U.S. patent application number 12/003576 was filed with the patent office on 2009-07-02 for ionic liquid catalyst alkylation using a loop reactor.
This patent application is currently assigned to Chevron U.S.A. Inc.. Invention is credited to Moinuddin Ahmed, Bong-Kyu Chang, Abdenour Kemoun, Huping Luo, Krishniah Parimi, Hye-Kyung Timken.
Application Number | 20090171133 12/003576 |
Document ID | / |
Family ID | 40799291 |
Filed Date | 2009-07-02 |
United States Patent
Application |
20090171133 |
Kind Code |
A1 |
Luo; Huping ; et
al. |
July 2, 2009 |
Ionic liquid catalyst alkylation using a loop reactor
Abstract
Provided is a process for producing low volatility, high quality
gasoline blending components which comprises recirculation of at
least a portion of a recovered stream comprising primarily
isoparaffins. Recirculation of the stream allows for an enhanced
I/O ratio and a more cost effective process.
Inventors: |
Luo; Huping; (Richmond,
CA) ; Kemoun; Abdenour; (Pleasant Hill, CA) ;
Parimi; Krishniah; (Alamo, CA) ; Ahmed;
Moinuddin; (Hercules, CA) ; Chang; Bong-Kyu;
(San Rafael, CA) ; Timken; Hye-Kyung; (Albany,
CA) |
Correspondence
Address: |
CROWELL & MORING LLP;INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Assignee: |
Chevron U.S.A. Inc.
San Ramon
CA
|
Family ID: |
40799291 |
Appl. No.: |
12/003576 |
Filed: |
December 28, 2007 |
Current U.S.
Class: |
585/14 |
Current CPC
Class: |
C10G 2300/1081 20130101;
C10G 2400/02 20130101; C10G 2300/1088 20130101; B01J 4/002
20130101; C10G 29/205 20130101; C10G 50/00 20130101; B01J
2219/00047 20130101; B01J 2219/0011 20130101; C10G 2300/1092
20130101; B01J 19/2465 20130101 |
Class at
Publication: |
585/14 |
International
Class: |
C10L 1/04 20060101
C10L001/04 |
Claims
1. A process for the production of low volatility, high quality
gasoline blending components comprising: (a) providing at least one
olefin feed stream comprising olefins; (b) providing at least one
isoparaffin feed stream comprising isoparaffins; (c) contacting the
at least one olefin feed stream with the at least one isoparaffin
feed stream in the presence of an ionic liquid catalyst in an
alkylation zone under alkylation conditions to provide at least one
product stream; and (d) recirculating to the alkylation zone a
stream comprised of primarily isoparaffin.
2. The process according to claim 1, wherein the olefin feed stream
comprise at least one olefin selected from the group consisting of
ethylene, propylene, butene, pentene, and mixtures thereof.
3. The process according to claim 1, wherein the isoparaffin feed
stream comprise at least one isoparaffin selected from the group
consisting of isobutane, isopentane, and mixtures thereof.
4. The process according to claim 1, further comprising: passing a
product stream through at least one heat exchanger; and removing
heat from the product stream.
5. The process according to claim 1, wherein the stream comprised
of primarily isoparaffin is separated from effluent obtained from
the contacting in step (c).
6. The process according to claim 1, wherein the stream comprised
primarily of isoparaffin is condensed from vaporous overhead in a
horizontal reactor in which the contacting in step (c) occurs.
7. The process according to claim 1, wherein the ionic liquid
catalyst is selected from the group consisting of: a
chloroaluminate ionic liquid catalyst comprising a hydrocarbyl
substituted pyridinium halide or a hydrocarbyl substituted
imidazolium halide of the general formulas A and B, respectively; a
chloroaluminate ionic liquid catalyst comprising an alkyl
substituted pyridinium halide or an alkyl substituted imidazolium
halide of the general formulas A and B, respectively; ##STR00003##
and mixtures thereof, where R.dbd.H, methyl, ethyl, propyl, butyl,
pentyl or hexyl group and X is a haloaluminate and preferably a
chloroaluminate, and R.sub.1 and R.sub.2.dbd.H, methyl, ethyl,
propyl, butyl, pentyl, or hexyl group and where R.sub.1 and R.sub.2
may or may not be the same.
8. The process according to claim 7, wherein the ionic liquid
catalyst is selected from the group consisting of
1-butyl-4-methyl-pyridinium chloroaluminate (BMP),
1-butyl-pyridinium chloroaluminate (BP),
1-butyl-3-methyl-imidazolium chloroaluminate (BMIM), 1-H-pyridinium
chloroaluminate (HP), and N-butylpyridinium chloroaluminate.
9. The process according to claim 7, wherein the catalyst further
comprises an HCI co-catalyst.
10. The process according to claim 1, wherein there are multiple
injections of olefin into the alkylation zone.
11. The process according to claim 1, wherein there are multiple
injections of isoparaffin into the alkylation zone.
12. The process according to claim 1, wherein the I/O ratio of
newly added reactants injected into the alkylation zone is in the
range of from 6:1 to 10:1.
Description
FIELD OF ART
[0001] The present invention relates to a process for producing low
volatility, high quality gasoline blending components which
recirculates at least a portion of a recovered stream comprising
isoparaffins to the process. More particularly, the present
invention relates to an alkylation process utilizing an ionic
liquid catalyst that produces a product comprising gasoline
blending components and recirculates at least a portion of a
recovered stream comprising isoparaffins to the alkylation
process.
BACKGROUND
[0002] Modern refineries employ many upgrading units such as fluid
catalytic cracking (FCC), hydrocracking (HCR), alkylation, and
paraffin isomerization. As a result, these refineries produce a
significant amount of isopentane. Historically, isopentane was a
desirable blending component for gasoline having a high octane (92
RON), although it exhibited high volatility (20.4 Reid vapor
pressure (RVP)). As environmental laws began to place more
stringent restrictions on gasoline volatility, the use of
isopentane in gasoline was limited because of its high volatility.
As a consequence, the problem of finding uses for by-product
isopentane became serious, especially during the hot summer season.
Moreover, as more gasoline compositions contain ethanol instead of
MTBE as their oxygenate component, more isopentane had to be kept
out of the gasoline pool in order to meet the gasoline volatility
specification. So, the gasoline volatility issue became even more
serious, further limiting the usefulness of isopentane as a
gasoline blending component.
[0003] An alkylation process, which is disclosed in U.S. Patent
Application Publication 2006/0131209, was developed that is capable
of converting the undesirable, excess isopentane into desirable and
much more valuable low-RVP gasoline blending components. The
contents of U.S. Patent Application Publication 2006/0131209 are
incorporated by reference herein. This alkylation process involves
contacting isoparaffins, preferably isopentane, with olefins,
preferably ethylene, in the presence of an ionic liquid catalyst to
produce the low-RVP gasoline blending components. This process
eliminates the need to store or otherwise use isopentane and
eliminates concerns associated with such storage and usage.
Furthermore, the ionic liquid catalyst can also be used with
conventional alkylation feed components (e.g. isobutane, propylene,
butene, and pentene).
[0004] The ionic liquid catalyst distinguishes this novel
alkylation process from conventional processes for converting light
paraffins and light olefins to more lucrative products.
Conventional processes include the alkylation of paraffins with
olefins, and polymerization of olefins. For example, one of the
most extensively used processes in the field is the alkylation of
isobutane with C.sub.3-C.sub.5 olefins to make gasoline cuts with
high octane number. However, this and all conventional processes
employ sulfuric acid and hydrofluoric acid catalysts.
[0005] Numerous disadvantages are associated with sulfuric acid and
hydrofluoric acid catalysts. Extremely large amounts of acid are
necessary to initially fill the reactor. The sulfuric acid plant
also requires a huge amount of daily withdrawal of spent acid for
off-site regeneration. Then the spent sulfuric acid must be
incinerated to recover SO.sub.2/SO.sub.3 and fresh acid is
prepared. While an HF alkylation plant has on-site regeneration
capability and daily make-up of HF is orders of magnitude less, HF
forms aerosol. Aerosol formation presents a potentially significant
environmental risk and makes the HF alkylation process less safe
than the H.sub.2SO.sub.4 alkylation process. Modern HF processes
often require additional safety measures such as water spray and
catalyst additive for aerosol reduction to minimize the potential
hazards. Thus, the ionic liquid catalyst alkylation process
fulfills the need for safer and more environmentally-friendly
catalyst systems.
[0006] Benefits of the ionic liquid catalyst alkylation process
include the following:
[0007] (1) substantial reduction in capital expenditure as compared
to sulfuric acid and hydrofluoric acid alkylation plants;
[0008] (2) Substantial reduction in operating expenditures as
compared to sulfuric acid alkylation plants;
[0009] (3) substantial reduction in catalyst inventory volume
(potentially by 90%)
[0010] (4) a substantially reduced catalyst make-up rate
(potentially by 98% compared to sulfuric acid plants)
[0011] (5) a higher gasoline yield
[0012] (6) comparable or better product quality (Octane number,
RVP, T50)
[0013] (7) significant environment, health and safety
advantages;
[0014] (8) expansion of alkylation feeds to include isopentane and
ethylene; and
[0015] (9) higher activity and selectivity of the catalyst.
[0016] Ionic liquid catalysts specifically useful in the alkylation
process described in U.S. Patent Application Publication
2006/0131209 are disclosed in U.S. Patent Application Publication
2006/0135839, which is also incorporated by reference herein. Such
catalysts are chloroaluminate liquid catalysts comprising an alkyl
substituted pyridium halide or an alkyl substituted imidazolium
halide of the general formulas A and B, respectively. Such
catalysts further include chloroaluminate liquid catalysts
comprising a hydrocarbyl substituted pyridium halide or a
hydrocarbyl substituted imidazolium halide of the general formulas
A and B, respectively.
##STR00001##
where R.dbd.H, methyl, ethyl, propyl, butyl, pentyl or hexyl group
and X is a haloaluminate and preferably chloroaluminate, and
R.sub.1 and R.sub.2.dbd.H, methyl, ethyl, propyl, butyl, pentyl, or
hexyl group and where R.sub.1 and R.sub.2 may or may not be the
same. Preferred catalysts include 1-butyl-4-methyl-pyridinium
chloroaluminate (BMP), 1-butyl-pyridinium chloroaluminate (BP),
1-butyl-3-methyl-imidazolium chloroaluminate (BMIM) and
1-H-pyridinium chloroaluminate (HP).
[0017] However, the ionic liquid catalyst has unique properties,
which requires that the ionic liquid catalyst alkylation process be
further developed and modified to achieve superior gasoline
blending component products, improved process operability and
reliability, and reduced operating costs, etc. More particularly,
the ionic liquid catalyst alkylation process requires uniform
mixing of the hydrocarbon and catalyst, sufficient interfacial
contact between the hydrocarbons and catalyst, good temperature and
pressure control, and a high isoparaffin to olefin (I/O) ratio. In
addition, alkylation by means of the ionic liquid catalyst is an
exothermic reaction requiring the removal of heat generated. Thus,
it would be beneficial to the industry if an improved alkylation
process for converting isoparaffins and olefins in the presence of
an ionic liquid catalyst was available.
[0018] One technique that has been used in general alkylation
processes is the recycling of effluent. For example, ExxonMobil's
auto refrigeration process described at page 243 of Petroleum
Refining--Technology and Economics (3rd edition) by James Gary and
Glenn Handwerk involves recycling catalyst and isobutane to the
reactor where alkylation between the olefins and isobutane takes
place. U.S. Pat. No. 5,347,064 describes an isoparaffin-olefin
alkylation process wherein recycled isobutane is added to a series
of alkylation reaction stages. U.S. Pat. No. 4,225,742 discloses an
HF alkylation process of isoparaffins with olefins wherein an
alkane stream substantially free of alkylate (the product) and
comprising principally normal C.sub.3 and C.sub.4 paraffin
hydrocarbons is recycled to the reaction zone. However, the
industry continues to strive for improved, more efficient processes
in order to lower the cost of products, and in particular when
using an ionic liquid catalyst.
SUMMARY
[0019] Provided is a process for producing low volatility, high
quality gasoline blending components incorporating recirculation of
at least a portion of a recovered stream comprising primarily
isoparaffins. Either all of the product or only a mere portion of
the isoparaffins may be recirculated. In any case, the process
includes the following steps:
[0020] (a) providing at least one olefin feed stream comprising
olefins;
[0021] (b) providing at least one isoparaffin feed stream
comprising isoparaffins;
[0022] (c) contacting the at least one olefin feed stream with the
at least one isoparaffin feed stream in the presence of an ionic
liquid catalyst in an alkylation zone under alkylation conditions
to provide at least one product stream, and
[0023] (d) recirculating to the alkylation zone a stream comprised
primarily of isoparaffin.
[0024] Among other factors, recirculation of a stream comprised
primarily of isoparaffin has been found to provide a more efficient
and cost effective alkylation process when using an ionic liquid
catalyst. Most importantly, the recirculation of a stream comprised
primarily of isoparaffin reactant allows the reaction in the
presence of an ionic liquid catalyst to maintain a high effective
I/O ratio, which minimizes undesired side reactions. One can also
use a lower quality of feed while maintaining the high I/O ratio
within the reaction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a schematic illustration of a first embodiment of
present invention having an external loop for recirculation of
primarily isoparaffin.
[0026] FIG. 2 is a schematic illustration of a second embodiment of
the present invention using a horizontal reactor with recycled
vapor of primarily isoparaffin.
DETAILED DESCRIPTION
[0027] The present invention provides a process for the production
of low volatility, high quality gasoline blending components.
According to the broadest aspect of the present invention, the
process involves recirculating a portion of at least one recovered
stream from an alkylation reaction comprised primarily of
isoparaffin back to the alkylation reaction.
[0028] As used herein, the term alkylation reaction refers to the
reaction that occurs between olefins and isoparaffins. The term
"isoparaffin" means any branched-chain saturated hydrocarbon
compound, i.e. a branched-chain alkane with a chemical formula of
C.sub.nH.sub.2n+2. Examples of isoparaffins are isobutane and
isopentane. The term "olefin" means any unsaturated hydrocarbon
compound having at least one carbon-to-carbon double bond, i.e. an
alkene with a chemical formula of C.sub.nH.sub.2n. Examples of
olefins include ethylene, propylene, butene, and so on. The olefins
can comprise at least one olefin selected from the following
olefins: ethylene, propylene, butene, pentene, and mixtures of
these. The isoparaffins can comprise at least one isoparaffin
selected from the following isoparaffins: isobutane, isopentane,
and mixtures of these.
[0029] According to one aspect, the process begins by providing at
least one olefin feed stream comprising olefins and at least one
isoparaffin feed stream comprising isoparaffins. The at least one
olefin feed stream and the at least one isoparaffin feed stream
contact one another in the presence of an ionic liquid catalyst
within at least one alkylation zone under alkylation conditions.
The term "alkylation zone" refers to the physical area in which the
alkylation between olefins and isoparaffins occurs. Interaction
between the olefins and the isoparaffins under the influence of the
catalyst provides at least one product stream comprising the
gasoline blending components. The at least one alkylation zone may
be a single alkylation zone or a plurality of separate and distinct
alkylation zones.
[0030] The process thereafter requires that at least a portion of a
recovered stream comprised primarily of isoparaffin is recirculated
to the alkylation zone. By primarily isoparaffin is meant a stream
of at least 50 volume % isoparaffin, and in another embodiment at
least 70 volume %, and in yet another embodiment at least 90 volume
%.
[0031] Referring to FIG. 1, a process is depicted which uses an
external loop for recirculating a stream comprised of primarily
isoparaffin. The hydrocarbon feeds 1, comprised of a isoparaffin
feed and an olefin feed mixed together, is split and injected at
three different points, 4, 5 and 6, into the alkylation
zone/reactor 7. Effluent 8 from the reactor generally comprises
isoparaffin, catalyst and reaction product. Essentially all of the
olefin is reacted, as the I/O (isoparaffin/olefin) ratio is
maintained as high as practical in order to insure complete
reaction. At the beginning of the reaction process the I/O ratio is
generally around 10:1 as injected into the reactor 7. However, the
effective ratio in the reactor, as the reaction occurs, can be
generally 1,000:1, or 10,000:1, or even higher, as almost all of
the olefin is reacted and substantially only isoparaffin remains of
the reactants.
[0032] The effluent 8 is then pumped via pump 9 through a heat
exchange 10 in order to remove reaction heat and help control the
temperature in the reactor 7. Some part of the effluent can be
separated and removed 11, while the remaining portion 12 comprised
primarily of isoparaffin, is recirculated to the reactor 7.
Additional catalyst 13 can be added to the recirculated stream.
[0033] By recirculating the stream of primarily isoparaffin, one
can achieve an effective high I/O ratio and insure product quality
by employing a lower I/O ratio in the feed, which is more cost
effective. The recirculated isoparaffin allows the charged I/O
ratio to remain high while the ratio of newly added isoparaffin and
olefin can be lower, for example 8:1, or even 6:1. This results in
a tremendous savings in isoparaffin cost.
[0034] Another embodiment is shown in FIG. 2, using a horizontal
reactor. Isoparaffin 21 is injected into the reactor 22 at a first
nozzle 23. Catalyst 24 is also injected at nozzle 23. Olefin 25 is
injected into the reactor at multiple olefin injection points 26,
which increases the internal I/O ratio and provides improved mixing
inside the reactor. The horizontal reactor is generally run at low
pressure so that reaction heat is removed by isoparaffin
evaporation. The generated vapor provides extra mixing inside the
reactor, and the isoparaffin vapor is removed at 27 and fully
condensed in a condenser 28 and recycled 29 back to the reactor 22.
Product is removed at 30.
[0035] It should be appreciated that the olefins and isoparaffins
need not exist in separate olefin feed stream(s) and the
isoparaffin feed stream(s). Rather, the olefins and isoparaffins
can be mixed or otherwise combined to form one or more hydrocarbon
feed stream(s). Thus, at least one hydrocarbon feed stream can
comprise the at least one olefin feed stream and the at least one
isoparaffin feed stream.
[0036] Alkylation is a exothermic reaction. Thus, it is necessary
to remove heat from the at least one alkylation zone by some means
in order to maintain the desired reaction temperature or
temperature range. A variety of methods are available for removing
such reaction heat and maintaining control of the reaction
temperature in the alkylation zone. One method of cooling the at
least one alkylation zone involves passing the at least one product
stream (or part of the at least one product stream) through at
least one heat exchanger. This method is illustrated and discussed
above in relation to FIG. 1. Another method of cooling the at least
one alkylation zone involves evaporation. In this method, as
depicted in FIG. 2, reaction heat is removed instantly by
isoparaffin evaporation within the alkylation. Other conventional
methods, such as cooling jackets, can also be used as are known in
the art.
[0037] The non-recirculated portion of a product stream(s) may be
treated by any known separation technique in order to separate the
gasoline blending components from the other constituents in the
product stream(s). Generally, the catalyst and hydrocarbon phase,
which comprises unreacted isoparaffins and the gasoline blending
components, are first separated. Next, the gasoline blending
components are separated from the remainder of the hydrocarbon
phase. A variety of feasible separation methods are known in the
art. An example of a useful method of separating the gasoline
blending components from hydrocarbon phase is distillation.
[0038] The present process employs an ionic liquid catalyst. Ionic
liquid catalysts are well known in the art.
[0039] The process can employ a catalytic composition comprising at
least one aluminum halide and at least one quaternary ammonium
halide and/or at least one amine halohydrate. An example of an
aluminum halide which can be used in accordance with the invention
is aluminum chloride. Quaternary ammonium halides which can be used
in accordance with the invention are described in U.S. Pat. No.
5,750,455, which is incorporated by reference herein, which also
teaches a method for the preparation of the catalyst. An exemplary
ionic liquid catalyst is N-butylpyridinium chloroaluminate
(C.sub.5H.sub.5NC.sub.4H.sub.9Al.sub.2Cl.sub.7).
[0040] The ionic liquid catalyst can also be a pyridinium or
imidazolium-based chloroaluminate ionic liquid. These ionic liquid
have been found to be much more effective in the alkylation of
isopentane and isobutane with ethylene than aliphatic ammonium
chloroaluminate ionic liquid (such as tributyl-methyl-ammonium
chloroaluminate). The ionic liquid catalyst can be a
chloroaluminate ionic liquid catalyst comprising a hydrocarbyl
substituted pyridinium halide or a hydrocarbyl substituted
imidazolium halide. Alternatively, the ionic liquid catalyst can be
a chloroaluminate ionic liquid catalyst comprising an alkyl
substituted pyridinium halide or an alkyl substituted imidazolium
halide. More specifically, the ionic liquid catalyst may be
selected from the group consisting of:
[0041] a chloroaluminate ionic liquid catalyst comprising a
hydrocarbyl substituted pyridinium halide mixed in with aluminum
trichloride or a hydrocarbyl substituted imidazolium and aluminum
trichloride preferably in 1 molar equivalent hydrocarbyl
substituted pyridinium halide or hydrocarbyl substituted
imidazolium halide to 2 molar equivalents aluminum trichloride of
the general formulas A and B, respectively;
[0042] a chloroaluminate ionic liquid catalyst comprising an alkyl
substituted pyridinium chloride and aluminum trichloride or an
alkyl substituted imidazolium chloride and aluminum trichloride
preferably in 1 molar alkyl substituted pyridinium chloride or
alkyl substituted imidazolium chloride to 2 molar equivalents of
aluminum trichloride of the general formulas A and B,
respectively;
##STR00002##
and mixtures thereof, where R.dbd.H, methyl, ethyl, propyl, butyl,
pentyl or hexyl group and X is a haloaluminate and preferably a
chloroaluminate, and R.sub.1 and R.sub.2.dbd.H, methyl, ethyl,
propyl, butyl, pentyl, or hexyl group and where R.sub.1 and R.sub.2
may or may not be the same.
[0043] Preferably the ionic liquid catalyst is selected from the
group consisting of 1-butyl-4-methyl-pyridinium chloroaluminate
(BMP), 1-butyl-pyridinium chloroaluminate (BP),
1-butyl-3-methyl-imidazolium chloroaluminate (BMIM), 1-H-pyridinium
chloroaluminate (HP), and N-butylpyridinium chloroaluminate
(C.sub.5H.sub.5NC.sub.4H.sub.9Al.sub.2Cl.sub.7).
[0044] A metal halide may be employed as a co-catalyst to modify
the catalyst activity and selectivity. Commonly used halides for
such purposes include NaCl, LiCl, KCl, BeCl.sub.2, CaCl.sub.2,
BaCl.sub.2, SiCl.sub.2, MgCl.sub.2, PbCl.sub.2, CuCl, ZrCl.sub.4,
and AgCl as published by Roebuck and Evering (Ind. Eng. Chem. Prod.
Res. Develop., Vol. 9, 77, 1970). Preferred metal halides are CuCl,
AgCl, PbCl.sub.2, LiCl, and ZrCl.sub.4.
[0045] HCl or any Broensted acid may be employed as an effective
co-catalyst to enhance the activity of the catalyst by boosting the
overall acidity of the ionic liquid-based catalyst. The use of such
co-catalysts and ionic liquid catalysts that are useful in
practicing the present invention are disclosed in U.S. Published
Patent Application Nos. 2003/0060359 and 2004/0077914. Other
co-catalysts that may be used to enhance the catalytic activity of
the ionic liquid catalyst include IVB metal compounds preferably
IVB metal halides such as TiCl.sub.3, TiCl.sub.4, TiBr.sub.3,
TiBr.sub.4, ZrCl.sub.4, ZrBr.sub.4, HfC.sub.4, and HfBr.sub.4 as
described by Hirschauer et al. in U.S. Pat. No. 6,028,024.
[0046] Alkylation conditions are maintained in the at least one
alkylation zone. The molar ratio between the isoparaffin and the
olefin is in the range of 1 to 100, for example, advantageously in
the range 2 to 50, preferably in the range 2 to 20. Catalyst volume
in the reactor is in the range of 2 vol % to 70 vol %, preferably
in the range of 5 vol % to 50 vol %. The reaction temperature can
be in the range -40.degree. C. to 150.degree. C., preferably in the
range -20.degree. C. to 100.degree. C. The pressure can be in the
range from atmospheric pressure to 8000 kPa, preferably sufficient
to keep the reactants in the liquid phase. Residence time of
reactants in the at least one alkylation zone is in the range of a
few seconds to hours, preferably 0.5 min to 60 min.
[0047] Typical alkylation conditions may include a catalyst volume
in the at least one alkylation zone of from 5 vol % to 50 vol %, a
temperature of from -10.degree. C. to 100.degree. C., a pressure of
from 300 kPa to 2500 kPa, an isoparaffin to olefin molar ratio of 2
to 10 and a residence time of 1 minute to 1 hour.
[0048] Although the present invention has been described in
connection with preferred embodiments thereof, it will be
appreciated by those skilled in the art that additions, deletions,
modifications, and substitutions not specifically described may be
made without departing from the spirit and scope of the invention
as defined in the appended claims.
* * * * *