U.S. patent application number 15/989268 was filed with the patent office on 2018-12-20 for process including a carbonation step.
The applicant listed for this patent is T&L Sugars Limited. Invention is credited to Anthony Baiada, Robert Jansen, John Kerr, Matthew Shue, Gordon Walker.
Application Number | 20180363073 15/989268 |
Document ID | / |
Family ID | 49355823 |
Filed Date | 2018-12-20 |
United States Patent
Application |
20180363073 |
Kind Code |
A1 |
Kerr; John ; et al. |
December 20, 2018 |
PROCESS INCLUDING A CARBONATION STEP
Abstract
The invention relates to a process for the removal of
contaminants from a liquor, the process comprising: introducing a
metal or ammonium hydroxide into the liquor; introducing the liquor
into a reaction vessel; bubbling a carbon dioxide gas comprising at
least 25% carbon dioxide through the liquor within the reaction
vessel; and separating the precipitate formed by the carbonation of
the metal hydroxide from the liquor, the precipitate comprising at
least some of the contaminants from the liquor; wherein, on
average, the liquor is resident within the reaction vessel for a
period of no more than about 60 minutes. The invention also relates
to a process for the removal of contaminants from a liquor, the
process comprising: introducing a metal or ammonium hydroxide into
the liquor and bubbling a carbon dioxide gas comprising at least
25% carbon dioxide through the liquor to fom1 a precipitate by
carbonation in a period of no more than about 60 minutes. The
carbonation processes may be included in sugar refining or water
softening and/or decontamination processes. A use of a carbon
dioxide gas comprising at least 25% carbon dioxide in a carbonation
process for removing contaminants from a hydroxide-treated liquor
is also provided, wherein the process forms a precipitate in period
of no more than about 60 minutes.
Inventors: |
Kerr; John; (London, GB)
; Baiada; Anthony; (London, GB) ; Jansen;
Robert; (London, GB) ; Shue; Matthew; (London,
GB) ; Walker; Gordon; (London, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
T&L Sugars Limited |
London |
|
GB |
|
|
Family ID: |
49355823 |
Appl. No.: |
15/989268 |
Filed: |
May 25, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14913179 |
Feb 19, 2016 |
9982315 |
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PCT/GB2014/052584 |
Aug 22, 2014 |
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15989268 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C02F 2103/32 20130101;
C02F 1/5245 20130101; C02F 2101/308 20130101; Y02A 20/156 20180101;
C02F 1/004 20130101; Y02A 20/152 20180101; C13B 20/06 20130101;
C02F 1/5236 20130101; C02F 1/68 20130101; C02F 2209/06 20130101;
C02F 5/06 20130101 |
International
Class: |
C13B 20/06 20110101
C13B020/06; C02F 1/00 20060101 C02F001/00; C02F 1/52 20060101
C02F001/52; C02F 5/06 20060101 C02F005/06 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 23, 2013 |
GB |
1315092.5 |
Claims
1. A process for the removal of contaminants from a liquor, the
process comprising: introducing a metal or ammonium hydroxide into
the liquor; introducing the liquor into a reaction vessel; bubbling
a carbon dioxide gas comprising at least 50% to 100% carbon dioxide
through the liquor within the reaction vessel; and separating the
precipitate formed by the carbonatation of the metal or ammonium
hydroxide from the liquor, the precipitate comprising at least some
of the contaminants from the liquor; wherein, on average, the
residence time is no more than about 60 minutes and wherein the
size of the precipitate is controlled by altering the pH of the
liquor in the reaction vessel.
2. A process as claimed in claim 1, wherein the size of the
precipitate is further controlled by altering the residence time of
the liquor in the reaction vessel.
3. (canceled)
4. (canceled)
5. A process as claimed in claim 1, wherein flow rate of carbon
dioxide gas into the reaction vessel is controlled by monitoring
the pH of the liquor.
6. (canceled)
7. A process as claimed in claim 1, wherein the pH of the liquor
within the reaction vessel is from about 3 to about 12.
8. (canceled)
9. A process as claimed in claim 1, wherein the residence time is
controlled by one or more of: (i) the flow rate of the liquor into
the reaction vessel; (ii) the working volume of the reaction vessel
and its associated pipework; and (iii) the amount of liquor being
recycled into the reaction vessel.
10. A process as claimed in claim 1, wherein the pH of the liquor
in the reaction vessel is controlled by recycling a portion of the
liquor into the reaction vessel.
11. A process as claimed in claim 1, wherein the precipitate is
separated from the liquor when it reaches a size of at least about
5 .mu.m.
12. (canceled)
13. A process as claimed in claim 1, wherein the flow rate of the
liquor into the reaction vessel is up to about 120 m.sup.3/h.
14. A process as claimed in claim 1, wherein the carbon dioxide gas
comprises at least about 99% carbon dioxide.
15. A process as claimed in claim 1, wherein the carbon dioxide gas
bubbled into the liquor is recycled and reintroduced into the
reaction vessel, optionally in combination with fresh carbon
dioxide gas.
16. A process as claimed in claim 15, wherein at least 85% of the
carbon dioxide bubbled through the liquor is either used in the
carbonatation reaction or is recycled.
17. A process as claimed in claim 1, wherein the metal hydroxide is
calcium hydroxide.
18. A process as claimed in claim 1, wherein the precipitate formed
by the carbonatation of the metal hydroxide is separated from the
liquor by filtration.
19. A process for the removal of contaminants from a liquor, the
process comprising: introducing a metal or ammonium hydroxide into
the liquor and bubbling a carbon dioxide gas comprising from 50% to
100% carbon dioxide through the liquor to form a precipitate by
carbonatation in a period of no more than about 60 minutes wherein
the size of the precipitate is controlled by altering the pH of the
liquor in the reaction vessel.
20. (canceled)
21. A process as claimed in either claim 19, wherein the size of
the precipitate is further controlled by altering the residence
time of the liquor in the reaction vessel.
22. A process as claimed in either claim 19, wherein the
carbonatation process takes place in a single reaction vessel.
23. A sugar refining process comprising a process as claimed in
claim 1.
24. A water softening or decontamination process comprising a
process as claimed in claim 1.
25. Use of a carbon dioxide gas comprising at least 50% to 100%
carbon dioxide in a carbonatation process for removing contaminants
from a hydroxide-treated liquor, wherein the process forms a
precipitate in a period of no more than about 60 minutes and
wherein the size of the precipitate is further controlled by
altering the pH of the liquor in the reaction vessel.
26. Use of a carbon dioxide gas as claimed in claim 25, wherein the
size of the precipitate is further controlled by altering the
residence time of the liquor in the reaction vessel.
Description
FIELD
[0001] The invention relates to improvements to processes including
a carbonation step.
BACKGROUND
[0002] Carbonation is used in a variety of different processes to
remove impurities such as, but not limited to, unwanted ions or
high molecular weight compounds from liquids. The processes
generally involve the addition of a metal or ammonium hydroxide
whose carbonate is as least partially insoluble under the
conditions employed. Carbon dioxide (CO.sub.2) is also added,
resulting in the formation of an insoluble carbonate as a
precipitate which may be separated from the liquid, for example by
filtration.
SUMMARY
[0003] In some embodiments, according to a first aspect of the
invention, a process for the removal of contaminants from a liquor
is provided, the process comprising: introducing a metal or
ammonium hydroxide into the liquor; introducing the liquor into a
reaction vessel; bubbling a carbon dioxide gas comprising at least
25% carbon dioxide through the liquor within the reaction vessel;
and separating the precipitate formed by the carbonation of the
metal or ammonium hydroxide from the liquor, the precipitate
comprising at least some of the contaminants from the liquor;
wherein, on average, the residence time is no more than about 60
minutes and wherein the size of the precipitate may be at least
partially controlled or controllable by altering (a) the residence
time of the liquor in the reaction vessel or (b) the pH profile of
the liquor in the reaction vessel.
[0004] In some embodiments, the size of the precipitate may be at
least partially controlled or controllable by altering (a) the
residence time of the liquor in the reaction vessel and (b) the pH
profile of the liquor in the reaction vessel.
[0005] In some embodiments, on average, the residence time is no
more than about 30 minutes, or the residence time is from about 20
to about 25 minutes.
[0006] In some embodiments, the flow rate of carbon dioxide gas
into the reaction vessel may be controlled or controllable by
monitoring the pH of the liquor.
[0007] In certain embodiments, the pH of the liquor introduced into
the reaction vessel may be from about 10.5 to about 11.
[0008] In some embodiments, the pH of the liquor within the
reaction vessel may be from about 3 to about 12.
[0009] In some embodiments, the pH of the liquor exiting the vessel
may be from about 8.1 to about 8.3.
[0010] In some embodiments, wherein the residence time is
controlled by one or more of: (i) the flow rate of the liquor into
the reaction vessel; (ii) the working volume of the reaction vessel
and its associated pipework; and (iii) the amount of liquor being
recycled into the reaction vessel.
[0011] In some embodiments, the pH of the liquor in the reaction
vessel may be at least partially controlled or controllable by
recycling a portion of the liquor into the reaction vessel.
[0012] In some embodiments, the precipitate may be separated from
the liquor when it reaches a size of at least about 5 .mu.m, or
from about 5 .mu.m to about 60 .mu.m.
[0013] In some embodiments, the flow rate of the liquor into the
reaction vessel may be up to about 120 m.sup.3/h.
[0014] In some embodiments, the carbon dioxide gas comprises from
about 50% to about 100% carbon dioxide, or comprises at least about
99% carbon dioxide.
[0015] In some embodiments, the carbon dioxide gas bubbled into the
liquor is recycled and reintroduced into the reaction vessel,
optionally in combination with fresh carbon dioxide gas.
[0016] In some embodiments, at least 85% of the carbon dioxide
bubbled through the liquor is either used in the carbonation
reaction or is recycled.
[0017] In some embodiments, the metal hydroxide is calcium
hydroxide.
[0018] In some embodiments, the precipitate formed by the
carbonation of the metal hydroxide is separated from the liquor by
filtration.
[0019] In some embodiments, according to a second aspect of the
invention, a process for the removal of contaminants from a liquor
is provided, the process comprising: introducing a metal or
ammonium hydroxide into the liquor and bubbling a carbon dioxide
gas comprising at least 25% carbon dioxide through the liquor to
form a precipitate by carbonation in a period of no more than about
60 minutes and wherein the size of the precipitate may be at least
partially controlled or controllable by altering (a) the residence
time of the liquor in the reaction vessel or (b) the pH profile of
the liquor in the reaction vessel.
[0020] In some embodiments, the precipitate is formed in a period
of no more than about 30 minutes.
[0021] In some embodiments, the carbonation process may take place
in a single reaction vessel.
[0022] In some embodiments, according to a third aspect of the
invention, a sugar refining process is provided, comprising a
process according to the first or second aspects.
[0023] In some embodiments, according to a fourth aspect of the
invention, a water softening or decontamination process is
provided, comprising a process according to the first or second
aspects.
[0024] In some embodiments, according to a fifth aspect of the
invention, a use of a carbon dioxide gas comprising at least 25%
carbon dioxide in a carbonation process for removing contaminants
from a hydroxide-treated liquor is provided, wherein the process
forms a precipitate in a period of no more than about 60 minutes
and wherein the size of the precipitate may be at least partially
controlled or controllable by altering (a) the residence time of
the liquor in the reaction vessel or (b) the pH profile of the
liquor in the reaction vessel.
BRIEF DESCRIPTION OF DRAWINGS
[0025] For the purposes of example only, embodiments of the
invention are described below with reference to the accompanying
drawings, in which:
[0026] FIG. 1 is a schematic illustration of a carbonation process
for a sugar liquor. FIGS. 2 to 5 are tables of data which are
referred to in the Examples.
DETAILED DESCRIPTION
[0027] Carbonation is a chemical reaction in which a hydroxide
reacts with carbon dioxide and forms an insoluble carbonate. For
example, the hydroxide may be calcium hydroxide, so that calcium
carbonate is formed:
Ca(OH).sub.2+CO.sub.2.fwdarw.CaCO.sub.3+H.sub.2O
[0028] Carbonation is used in a variety of different ways as part
of a variety of different processes. In some of these processes,
the carbonation is used to remove undesirable constituents or
contaminants.
[0029] For example, carbonation may be used in sucrose refining. In
such processes, calcium hydroxide, which is also commonly referred
to as "lime", is added to a color red sucrose syrup, often in the
form of an aqueous calcium hydroxide suspension (known as limewater
and often formed by adding calcium oxide to water). The mixture is
agitated and CO.sub.2 is bubbled into the mixture. This causes
calcium carbonate to form and precipitate out of the solution.
During the precipitation and subsequent flocculation that the
calcium carbonate u undergoes, color bodies in the syrup are bound
up and trapped in the precipitate:
Ca(OH).sub.2+CO.sub.2+free color bodies.fwdarw.Ca(CO.sub.3) with
bound color bodies+H.sub.20
[0030] By subsequently filtering the resulting suspension to remove
the calcium carbonate and bound color bodies through a suitable
filter medium, color bodies can be readily removed from the syrup,
forming a partially decolorized liquor for further processing.
[0031] In practice, the source of the carbon dioxide is often the
flue gases from boilers used on site. These gases can contain up to
10-15% CO.sub.2.
[0032] The standard carbonation processes suffer from a number of
issues.
[0033] Firstly, the use of CO.sub.2 is inefficient. Typically only
25-35% of the CO.sub.2 fed into the reaction vessel is converted
into calcium carbonate. The rest is usually vented to atmosphere.
Systems have been developed for improving the standard carbonation
process (e.g., improved apparatus for introducing the CO.sub.2 into
the reaction vessel, such as Richter tubes) but these improvements
still only give 35-45% CO.sub.2 efficiency.
[0034] Secondly, the standard processes can be quite variable and
difficult to control.
[0035] Thirdly, the standard processes can be slow. Consequently,
it may be necessary to use larger reaction vessels to achieve the
desired rate of throughput of the liquor and/or to conduct the
reaction in a series of stages in separate vessels. Typically, the
carbonation process used in sugar refining is completed in two or
more steps, each of which can take approximately 40 to 45 minutes.
Therefore, the whole carbonation process will generally take at
least about 1.5 hours.
[0036] In light of the foregoing, embodiments of the invention seek
to address, at least partially, one or more of the issues outlined
above. In particular, certain embodiments of the invention seek to
achieve one or more of the following improvements: [0037] 1. To
increase the efficiency of CO.sub.2 usage to at least about 70%.
[0038] 2. To provide a process that can be closely controlled and
is more robust. [0039] 3. To provide a process that can be
completed much more quickly, resulting in the possibility of using
smaller tanks and/or shortened residence times. [0040] 4. To
provide a process that may be carried out in a single step or stage
and/or in a single reaction vessel.
[0041] The improved carbonation processes disclosed and claimed
herein may be used in processes in which carbonation is currently
used, to gain one or more of the above mentioned advantages. In
addition, the improved carbonation processes may make the use of a
carbonation process feasible or more attractive (for example, from
a commercial or practical perspective), so that it may replace
alternative reactions or processing steps.
[0042] Thus, for example and by no means intended to be limiting,
the carbonation processes according to the present invention may be
used as part of sugar refining processes and other processes where
the novel process makes carbonation more viable. As discussed
above, it is already known to use carbonation in sugar and High
Intensity Sweetener refining processes.
[0043] In addition, the carbonation processes according to the
present invention may also be used in the treatment of waste
streams from various industries, for the removal of undesired
constituents and contaminants. For example, the waste streams may
be waste water (both in the water industries and other industries)
and the carbonation processes of the invention may be used to
remove undesired constituents or contaminants from the water.
[0044] In one embodiment, the waste water is, for example, the
regenerate water produced when an ion exchange resin is
regenerated. Ion-exchange resins are widely used in different
separation, purification, and decontamination processes. The most
common examples are water softening and water purification, juice
purification and in the manufacture of sugar. An ion exchange resin
works by exchanging sodium for contaminants in the liquid being
filtered/treated. In the case of water softening, the contaminants
might be calcium and magnesium ions, in the case of ion exchange
resins
used in sugar processing, they might be color bodies and
non-colored components.
[0045] When an ion exchange resin requires regeneration, this is
achieved by washing the resin with a salt solution, such as brine
(a sodium chloride solution). This reverses the reaction, releasing
the trapped contaminants in exchange for the sodium in the brine.
The resulting ion exchange regenerate material may have high
concentrations of salt and contaminants and this regenerate may be
treated using a carbonation process according to the present
invention to remove those contaminants and produce water with
significantly reduced total dissolved solids and a precipitate
containing trapped contaminants.
[0046] The carbonation processes described herein remove u
undesired constituents, also referred to interchangeably herein as
contaminants, from a starting material or liquor. The term liquor
as used herein means a liquid (optionally in the form of a solution
or suspension) which includes contaminants. In some embodiments,
the liquor may be a liquid that has been produced or used in a
process.
[0047] Water softening is a well known industrial process which may
benefit from the novel carbonation process disclosed herein. "Hard"
water includes multivalent cations such as calcium (Ca.sup.2+) and
magnesium (Mg.sup.2+) which are to be removed to produce softened
water. Removal of the multivalent cations is typically achieved by
adding sodium hydroxide or sodium carbonate. This results in the
precipitation of the less soluble (divalent) carbonates, for
example by either or both of the following reactions:
Mg.sup.2++CO.sub.3(.sup.2-).fwdarw.MgCO.sub.3
Ca.sup.2++CO.sub.3(.sup.2-).fwdarw.CaCO.sub.3
[0048] As particular examples which should not be interpreted as
being limited, the liquor may be, for example, a liquid which has
been produced as part of the sugar refining process. Particularly,
it may be the liquid resulting from the affinition step.
Alternatively, the liquor may be waste water, in the form of a
solution or suspension. For example, the waste water may result
from a rinsing or cleaning step, including, for instance, an ion
exchange regenerate material which may contain contaminants from
the regeneration of the ion exchange resin. As used herein, the
term "ion exchange regenerate material" refers to any material
exiting the resin during the regeneration process. Alternatively,
the liquor may be hard water, which is water containing multivalent
cations as undesired constituents or contaminants which are to be
removed.
[0049] At least some of the improvements associated with the
process of the invention as listed above may be achieved by
utilizing a carbon dioxide gas having a high concentration of
CO.sub.2, meaning that the concentration of the CO.sub.2 added to
the reaction vessel is from about 25 to about 100% pure CO.sub.2.
In some preferred embodiments, the CO.sub.2 concentration is at
least about 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%,
85%, 90%, 95%, 97%, 98%, 99%, 99.5%, 99.6%, 99.7%, 99.8%, 99.9% or
at least about 99.95% pure CO.sub.2, or the gas may be 100% pure
CO.sub.2.
[0050] One consequence of the use of a high concentration of
CO.sub.2 is that it has a significant effect on the speed of the
carbonation reaction. The metal carbonate precipitate forms very
quickly and this has an effect on the size and shape of the
particles of precipitate and on the size of the floes formed. The
precipitate formed in an uncontrolled carbonation process using
high concentration CO.sub.2 is fine and the precipitate particles
may be elongate in shape, rendering the particles of precipitate
difficult to separate from the liquor by filtration. The whole
carbonation process is rendered useless if the precipitate cannot
be separated from the liquor. If the separation requires special
processes and equipment, this can add significantly to the time the
process takes and the cost, reducing the efficiency of the process
to remove contaminants.
[0051] Generally, the precipitate may be separated from the liquor
using various techniques such as filtration, decantation,
centrifugation and other such methods. Often, the carbonation
process will be associated with a particular separation technique
and it may be preferred to continue with the existing technique
where an existing carbonation process is being improved using the
invention. In the case of sugar refining, the carbonation process
will often involve a filtration step to remove the precipitate and
it may be desirable to retain this mode of separation, as the
equipment will be in place.
[0052] Filters that might be considered suitable for the filtration
step are well known in the art. Suitable filters include, for
example, Putsch presses, Gaudfrin filters and candle filters.
[0053] In some embodiments, the precipitate may be removed from the
liquor continuously using filtration. Alternatively, it may, in
some embodiments, be preferable to allow the precipitate to build
up on the surface of the filtration medium so as to form a bed of
precipitate. This bed may then act to improve the filtration
characteristics of the filter.
[0054] In some embodiments, the processes of the present invention
therefore include measures to control the precipitate formation and
to increase the size of the particles of precipitate and/or of the
floes formed so that they may be separated from the liquor using
standard filtration processes.
[0055] The high concentration CO.sub.2 added to the liquor
accelerates the formation of particles of carbonate precipitate,
but the growth of the crystals and flocculation can be controlled
to result in larger particles that may be filtered.
[0056] The so-called "residence time" is the time that the average
metal ion in the hydroxide-treated liquor would spend in the
reaction vessel. Put another way, it is the average period of time
over which the metal or similar (e.g., ammonium) cations will be
exposed to the anionic carbon dioxide species in solution and/or
the carbon dioxide itself so that the carbonation reaction may take
place. During the course of this residence time, the cation will be
converted into a carbonate species which then forms a crystal. This
crystal then grows in size and flocculates with color bodies and
other crystals to form the larger carbonate "floes" that are then
separated out of solution, for example by filtration. The residence
time may alternatively be defined as being the average period of
time for which the liquor is resident within the reaction
vessel.
[0057] Thus, the particle size of the precipitate can be controlled
by the residence time of the liquor in the reaction vessel. This
may be adjusted by a number of factors, one being a recycling of a
proportion of the liquor exiting the reaction vessel, so that this
is fed back into the reaction vessel. The percentage of the liquor
being recycled can be varied to control the residence time and the
size of the calcium carbonate precipitate.
[0058] In some embodiments, the nature of the precipitate, for
example its mass, density, shape and particle-size distribution may
be controlled or controllable by recycling a portion of the liquor
into the reaction vessel.
[0059] The size distribution of the precipitate will be affected by
a number of factors, such as the size and shape of the vessel, the
location at which the CO.sub.2 and pre-carbonated liquor are
introduced into the vessel and how efficiently the liquor is mixed
within the vessel.
[0060] Mixing of the liquor may be achieved using any suitable
method. For example, the vessel may be equipped with an agitation
means to mix the contents of the vessel. In some embodiments, the
agitation means may be any means suitable for mixing the contents
of the vessel, such as an impeller, turbine or paddle. Additionally
or alternatively, mixing may be effected by the bubbling of the
CO.sub.2 through the liquor.
[0061] While it is important that the liquor is well-mixed, care
must be taken to ensure that the size of the precipitate is not
reduced by the mixing technique. For example, vigorous stirring of
the liquor with an impeller may well cause the precipitate that has
already formed to break-up. Thus, in some embodiments, it is
important that the mixing technique is gentle enough to ensure that
the size of the precipitate is not reduced, while at the same time
providing a well-mixed liquor.
[0062] Mixing of the liquor may also be achieved by continuously
withdrawing the liquor from the vessel and reintroducing it into
the vessel at a different location. Such recycling or recirculation
can be a much more efficient way of mixing a liquor which comprises
a large proportion of particulate material (for example, viscous or
slurry-like liquors) and is difficult to mix using conventional
means. In addition, recirculation does not subject the liquor to
high shear forces and the like, and so the precipitate does not
fragment.
[0063] The liquor may be withdrawn from and returned to the vessel
at any suitable point. As previously discussed, the size
distribution of the precipitate may vary throughout the volume of
liquor in the vessel. Thus consideration may be given to the
locations at which the liquor is withdrawn and reintroduced into
the vessel. For example, in some embodiments, it may be
advantageous to withdraw the liquor from the base of the vessel and
reintroduce it into a different (for example upper) region of the
vessel.
[0064] In certain embodiments, the liquor may be reintroduced
directly into the vessel. Alternatively, or in addition, the
recycled liquor may first be combined with the pre-carbonated
liquor prior to reintroduction into the vessel.
[0065] Recycling portions of the liquor may also help to seed the
mixture. Without wishing to be bound by theory, it has been found
that the recycling process influences the particle size, shape and
particle-size distribution.
[0066] Such recycling also allows the residence time to be
accurately controlled without having to increase the size of the
reaction vessel, i.e., the working volume of the vessel and its
associated pipework, or the use of multiple vessels in series,
which would be alternative (or additional) ways to increase the
residence time.
[0067] In some embodiments, the carbonation takes place in a single
reaction vessel. There may be a number of advantages associated
with performing the carbonation in a single reaction vessel, such
as a smaller apparatus footprint, greater efficiency and lower
overall running costs. Alternatively, the carbonation may take
place in multiple reaction vessels and/or over a series of steps.
In some embodiments, the concentration of CO.sub.2 used for each
step may be kept the same. Alternatively, different concentrations
of CO.sub.2 may be used for each carbonation step.
[0068] In some embodiments, the residence time, that is the average
period of time for which the liquor is resident within the reaction
vessel, is no more than about 60 minutes. In some embodiments, the
residence time is no more than about 45 minutes or no more than
about 30 minutes.
[0069] In some embodiments, the residence time may be selected to
provide a suitable or tailored size of precipitate and/or floes. In
some embodiments, the process may involve a residence time of from
about 20 to about 25 minutes. In other embodiments, especially
where the generation of a fine precipitate is not considered a
significant disadvantage, for example, where the precipitate is
separated from the liquor by a means other than filtration, the
residence time may be as low as from about 1 minute to about 30
minutes.
[0070] In some embodiments, the residence time may be from about 1,
2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or
20 minutes. In some embodiments, the residence time may be up to
about 60, 55, 50, 45, 40, 35, 30, 28, 26, 25, 24, 23, 22, 21, 20,
19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6 or 5
minutes.
[0071] A further factor which will affect the residence time and
can therefore be adjusted to control the size of the particles of
precipitate is the liquor flow rate into the reaction vessel (and
therefore the flow rate out of the vessel). The flow rate will
depend on the size of the vessel and the nature of the process
involving the carbonation step.
[0072] In some specific embodiments, which may in particular relate
to a carbonation process being used in the refining of sugar, the
liquor including the metal hydroxide, such as calcium hydroxide
(formed by adding calcium oxide to water), may be pumped into the
reaction vessel at a flow rate of up to about 50, 55, 60, 65, 70,
75, 80, 85, 90, 95, 100, 105, 110, 115 or 120 m.sup.3/h. In some
embodiments, the liquor may be pumped into the reaction vessel at a
flow rate of from about 120, 115, 110, 105, 100, 95, 90, 85, 80,
75, 70, 65, 60, 55 or so m.sup.3/h. In certain embodiments, the
liquor may be pumped into the reaction vessel at a flow rate of
about 70, 75, 80, 85, 90, 95, 100 or 105 m.sup.3/h. In some
embodiments, the flow rate of the liquor into the reaction vessel
may be about 76, or about 120 m.sup.3/hr. The precise flow rate may
be adjusted to control the residence time in the vessel and to
match the rate at which the other stages of the refining process
are carried out.
[0073] In some embodiments, the metal hydroxide may be added to the
liquor before bubbling the carbon dioxide gas through the liquor.
In other embodiments, the metal hydroxide may be added to the
liquor while the carbon dioxide gas is being bubbled through the
liquor.
[0074] In some embodiments, the metal hydroxide is introduced by
adding a metal oxide (optionally in water) to the liquor.
[0075] The sequence of the steps of introducing the metal hydroxide
and CO.sub.2 into the liquor will determine the pH changes in the
liquor and a balance may need to be struck between ability to
control the reaction and avoidance of large pH fluctuations. One
possible disadvantage associated with pH changes is that they can
trigger unwanted side reactions.
[0076] However, conversely introducing the metal hydroxide and
CO.sub.2 simultaneously can make the overall reaction harder to
control. If, for example, the metal or ammonium hydroxide is added
before the CO.sub.2 is bubbled through the liquor, the amount of
hydroxide added may be adjusted to achieve a pH of a target value
or within a target range.
[0077] The solubility of the metal or ammonium hydroxide in the
liquor must also be considered and, in some embodiments, it may be
desirable to take steps to ensure that the hydroxide is evenly
dispersed within the liquor. This may involve making adjustments to
the temperate re or pH, and/or stirring or the like. Similarly, in
some embodiments it may be desirable to ensure that the CO.sub.2
becomes evenly distributed throughout the liquor.
[0078] An increased flow rate of liquor compared to that used in
conventional carbonation processes can be accommodated as a result
of the use of higher concentration CO.sub.2 and the associated
faster rate of the carbonation reaction. This can mean that more
liquor may be treated in the same period of time, or a smaller
reaction vessel may be used to provide other benefits.
[0079] Considering the speed of the reaction, a faster carbonation
reaction may be advantageous, provided that the contaminants, such
as the "color bodies" in a sugar liquor, have time to diffuse to,
and stick to, the surface of the carbonate crystal as it forms. The
rate of the carbonation reaction will depend upon the concentration
of the carbon ions in the liquor and this depends not only on the
concentration of the carbon dioxide gas being bubbled into the
liquor but also on the rate at which the CO.sub.2 is absorbed.
[0080] In some embodiments, the CO.sub.2 may be injected into the
liquor at a pressure which ensures the efficient dissolution of
CO.sub.2 into the aqueous phase via small bubbles. The pressure
exerted on the bubbles of CO.sub.2 influences the size of the
bubbles and so, in some embodiments, the size of the bubbles may be
controlled by adjusting the pressure. For example, the head
pressure in the liquor will contribute to the pressure exerted on
the bubbles and therefore will affect the size of the bubbles.
[0081] The high concentration carbon dioxide gas may be provided
and fed into the reaction vessel for the carbonation process from a
site bulk storage tank. Such bulk storage tanks can, for example,
hold 49 tons of food grade CO.sub.2 and may form part of a site
CO.sub.2 plant. In some embodiments, the CO.sub.2 is warmed to
25.degree. C. with steam and vaporized to 6.o barge, then piped to
the reaction vessel.
[0082] In some other embodiments, the CO.sub.2 may be generated on
site, rather than being bought. Potentially suitable sources of
CO.sub.2 include, for example: (i) calcination of CaCO.sub.3 on
site; and (ii) recovery of pure CO.sub.2 from process off-gases
(e.g., flue gas, anaerobic digesters, bioethanol fermentation
streams, etc.).
[0083] In certain embodiments, the CaCO.sub.3 cake produced as a
result of the carbonation may be converted to CO.sub.2 for use in
the carbonation step. As is well known, conversion of the
CaCO.sub.3 into CO.sub.2 may be achieved by calcining it.
[0084] It may, in some embodiments, be advantageous to introduce
the CO.sub.2 into the reaction vessel so as to encourage an even
distribution of the gas throughout the vessel. In some embodiments,
the CO.sub.2 enters the vessel via a set of laterals located in the
bottom of the vessel which distribute the CO.sub.2 evenly.
[0085] In some embodiments, any non-dissolved and/or unreacted
CO.sub.2 is not vented to atmosphere. Instead, the vessel headspace
is piped to a gas blower which recompresses the CO.sub.2 enabling
it to be redistributed through the laterals. As the CO.sub.2 is
consumed (dissolved and reacted), the headspace pressure of the
vessel decreases. Pressure is therefore maintained by supplying
fresh CO.sub.2 from the CO.sub.2 header via a pressure control
valve. In some embodiments, the headspace pressure is controlled to
50 mbar above atmospheric.
[0086] In some embodiments, it may be necessary to ensure that the
bubbles of CO.sub.2 are not so small that they have a tendency to
form a foam or "mousse". This may be disadvantageous in some
carbonation processes. For example, where the precipitate is
separated from the liquor by filtration, the presence of a foam or
mousse may hinder the filtration step or may interfere with the
recycling of the CO.sub.2 from the headspace of the reaction
vessel.
[0087] If the bubbles are too big, the surface area:volume ratio
may be too small to provide the desired rapid and/or effective
absorption of CO.sub.2 into the aqueous phase.
[0088] Economically and operationally, it may be attractive to use
a reaction vessel that is not above 1 bar in pressure re (and not u
under vacuum either) as this is then classified as "a non-pressure
vessel". In some embodiments, it may be key for the carbonate
floe/particle shape and size to be adequate for acceptable
filterability and this may be achieved by adjusting the pressure,
as discussed in more detail in the examples below.
[0089] In some embodiments, the size of the precipitate considered
to be suitable for acceptable filterability may be from about 1, 2,
3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, or 70
.mu.m. In preferred embodiments, the precipitate is separated from
the liquor when it reaches a size of from about 5 .mu.m, or from
about 5 to 60 .mu.m.
[0090] Traditional carbonation vessels using low concentration
CO.sub.2 have a tendency to accumulate a thick layer of calcium
carbonate on the gas distributors. This poses a problem in that
they must be periodically removed from the vessel and cleaned.
Moreover, the gas distributors or laterals may be fixed to the
internal wall of the vessel, thus requiring a worker to enter the
vessel and remove the laterals for inspection and cleaning. Such
operations are labor intensive and expensive.
[0091] Surprisingly, it has been found that the use of high
concentration and/or purified CO.sub.2 leads to reduced formation
of scale around the gas distributors. Without wishing to be bound
by any particular theory, it is speculated that this may be due to
a local reduction in pH.
[0092] The CO.sub.2 adsorption efficiency may be affected by the
rate at which the CO.sub.2 is introduced into the vessel. For
example, increasing the CO.sub.2 flow rate into the reaction vessel
may increase the amount of CO.sub.2 that is dissolved. The CO.sub.2
gas dissolving rate may also be influenced by a number of other
parameters such as, for example, the size of the CO.sub.2 bubbles
and the overall pH profile of the liquor in the vessel. It is
important, to ensure that the CO.sub.2 flow rate and bubble size is
well controlled, not only to optimize the color removal and
filtration processes, but also to avoid any adverse foaming
issues.
[0093] In some embodiments, the flow rate of CO.sub.2 into the
reaction vessel via the laterals may be from about 500, 600, 700,
800, 900, 1000, 1250, 1500, 1750 or 2000 kg/h. In some embodiments,
the flow rate of CO.sub.2 into the reaction vessel via the laterals
may be up to about 2000, 1750, 1500, 1250, 1000, 900, 800, 700, 600
or 500 kg/h. In preferred embodiments, the flow rate of CO.sub.2
into the reaction vessel may be from about 500 to about 2000 kg/h,
from about 600 to about 1500 kg/h, or from about 700 to 900
kg/h.
[0094] In some embodiments of the invention, in order to enhance
the efficiency of the use of the CO.sub.2, the non-dissolved and/or
unreacted CO.sub.2 is recycled rather than being vented. CO2 is
bubbled into the liquor in the reaction vessel. The CO.sub.2 which
is not absorbed into the liquor and/or which does not react with
the hydroxide in the carbonation reaction will be captured in the
reaction vessel and fed back into the system. This recycling of the
CO.sub.2 enables CO.sub.2 efficiencies of at least 85%, at least
about 90%, at least about 95%, at least about 98% or at least about
99% to be achieved.
[0095] In some embodiments, fresh CO.sub.2 being fed into the
reaction vessel is combined with recycled CO.sub.2 which has passed
through the reaction vessel without being utilized/absorbed.
Alternatively, or in addition, the fresh CO.sub.2 and recycled
CO.sub.2 may be fed into the reaction vessel via separate
conduits.
[0096] There are a number of factors in play in carbonation
processes and these all have to be taken into consideration for a
typical carbonation process and for the processes of the present
invention. For example, pH control is important in all carbonation
processes, including those of the present invention.
[0097] In some embodiments, the size of the precipitate formed as a
result of the carbonation process may be affected by the pH of the
liquor in the reaction vessel. In some embodiments, the size of the
precipitate may be at least partially controlled or controllable by
altering the pH of the liquor.
[0098] In some embodiments, the pH of the liquor may vary
throughout the reaction vessel (i.e., the liquor will have a "pH
profile"). In some embodiments, the pH profile of the liquor may be
altered by recycling a portion of the liquor into the reaction
vessel. For example, the liquor may be withdrawn from a region of
the vessel where the pH is low, and reintroduced into a region of
the vessel where the pH is high. Such recycling may affect the pH
profile of the liquor in the reaction vessel and hence the size of
the precipitate.
[0099] In some embodiments, the pH of the liquor in the vessel may
be from about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 or 13 or 14. In
some embodiments, the pH of the liquor in the vessel may be up to
about 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, and 1. In certain
embodiments, the pH of the liquor in the reaction vessel may be in
the range of from about 2 to about 13, from about 3 to about 12, or
from about 4 to about 11.
[0100] In some embodiments, the target pH at the end of the
carbonation process is around 8.1-8.3. There are advantages
associated with having the pH as close to neutral as possible, but
below a pH of about 8 the CaCO.sub.3 may re-dissolve in the form of
calcium hydrogen carbonate (also known as calcium bicarbonate).
This may present, for example, the problems of scaling in the
process or downstream of the process.
[0101] The addition of the hydroxide to the liquor will increase
the pH and it may, in some embodiments, be important to quickly
reduce the pH to the target range. This may be achieved by the
addition of the CO.sub.2. The amount of CO.sub.2 required to adjust
the pH will be dependent upon the amount of lime added to the
liquor.
[0102] While the desired pH ranges and need to control the pH of
the processes are the same as in traditional carbonation processes,
the processes of the invention may, in some embodiments, offer
advantages over some known processes because (i) the carbon dioxide
gas has a consistent and reliable CO.sub.2 concentration which will
not exhibit as much variation as the flue gases used in some known
processes; and (ii) the entire carbonation process is carried out
in one vessel so there is a greater chance of controlling the pH
more closely with less equipment.
[0103] The basic carbonation process discussed herein and as
applied to the processing of sugar is illustrated in FIG. 1.
[0104] The starting material is typically the affinition liquor,
containing about 65% dissolved solids. These solids include
contaminants such as color bodies. The lime added is typically
added to the liquor in the form of "milk of lime", an aqueous
suspension of calcium hydroxide.
[0105] In some embodiments, the aqueous suspension of calcium
hydroxide may have a concentration of approximately 10% CaO. This
may be a suitable form in which to add the metal hydroxide to a
sugar liquor which is a viscous liquor. Where the liquor is
essentially contaminated water, the amount of metal hydroxide
(i.e., the resultant concentration in the liquor) is likely to be
more important than the concentration of the suspension being
added.
[0106] In some embodiments, the concentration of the metal
hydroxide in the liquor may be between about 0.5 and about
1.5%.
[0107] The lime is typically added to the sugar liquor prior to the
liquor being "gassed" with the CO.sub.2 stream. At this point, the
liquor stream may have a pH of from 10.5 to 11. The liquor stream
may also be heated at this point (for example, from 70 to
80.degree. C.), and the combination of the high temperature and
high pH is undesirable as sucrose decomposes rapidly under these
conditions. It is therefore important to reduce the pH of the
liquor as soon as possible. This may be done by gassing the liquor
with CO.sub.2.
[0108] Following the addition of the lime to the flow of liquor,
the limed liquor is introduced into the reaction vessel. In some
embodiments, the liquor and lime may be introduced separately into
the vessel, but this can make the process harder to control.
[0109] Carbon dioxide gas is fed into the reaction vessel. The
CO.sub.2 is absorbed into the limed liquor via bubbles. The carbon
dioxide gas preferably has a CO.sub.2 concentration of from 50-100%
CO.sub.2. In some embodiments, the CO.sub.2 concentration is at
least 60%, 70%, 80%, 90%, 95%, 97%, 98%, 99%, 99.5% or at least
99.95%.
[0110] As discussed above, the CO.sub.2 being fed into the liquor
in the reaction vessel may include "fresh" CO.sub.2 and recycled
CO.sub.2 and the ratio of this mixture may be reduced as far as
possible to maximize the recycling, while retaining the desired
CO.sub.2 concentration. In some embodiments, the ratio of "fresh"
to recycled CO.sub.2 may be from about 1:1 to about 1:3, and in
some embodiments it may be from about 1:2 to about 1:3, or about
1:2 to about 1:2.5.
[0111] As the CO2 bubbles move upwards through the limed liquor in
the reaction vessel, the dissolved CO2 reacts with the calcium
hydroxide, forming calcium carbonate which quickly precipitates out
of the solution. The concentration of the CO2 in the liquor affects
the speed at which the carbonation reaction proceeds and the faster
the reaction, the smaller the particles of precipitate.
[0112] The filter process used to separate the precipitate from the
treated liquor may be a standard or traditional filtration process,
as this part of the process does not need to be adjusted or
adapted.
[0113] The mean grain size and size distribution of the calcium
carbonate precipitate as well as the shape of the "grains"
influences the filterability of the calcium carbonate cake. For
filtration, it may be preferable in some embodiments for the
precipitate to have a shape which is approximately spherical. This
is in contrast to the crystals formed by the most rapid
precipitation reactions seen using high concentration CO.sub.2,
which tend to be elongate or needle-like in shape. The shape of the
precipitate is discussed below in the Examples and in the data
presented in Tables 1 to 5 of the Figures.
[0114] In some embodiments, the average size of the precipitate
considered to be suitable for standard or traditional filtration
processes may be at least about 5 .mu.m to about 30 .mu.m (for
example, see FIG. 2-4). It should be remembered and well understood
that the particle size distribution is also critical in determining
the filterability of the precipitate. Without wishing to be bound
by theory, it is thought that a precipitate having a broad particle
size distribution may result in a densely packed filter-cake,
through which the
passage of liquor to be filtered may be impeded (i.e., the filter
may become blocked).
[0115] Thus, in some embodiments, it may be important to ensure
that the precipitate has a narrow particle-size distribution. It
may, in some embodiments, be desirable to vary a number of factors
such as, but not limited to, the residence time of the liquor in
the reaction vessel, the concentration of CO.sub.2 gas introduced
into the vessel and the pH profile of the liquor in the vessel in
order to obtain a precipitate having the desired particle-size
distribution.
[0116] The quantity of impurities (e.g., color bodies) removed from
the liquor and the characteristics (e.g., size, particle-size
distribution and shape) of the precipitate may give an indication
of the overall effectiveness of the carbonation process. In some
embodiments, it may be particularly desirable to remove as much of
the color and other impurities from the liquor as possible, while
ensuring that the precipitate has characteristics (e.g., size,
size-distribution and shape) which are considered desirable for
good filtration.
[0117] In some embodiments, it may be desirable to achieve both
adequate impurity (e.g., color removal) and good filterability of
the liquor. In some embodiments, the impurity removal and size of
the precipitate may be affected and/or controlled by the residence
time and pH profile of the liquor in the reaction vessel.
[0118] Using the process and equipment illustrated in FIG. 1 in
treating sugar liquor, in some embodiments from about 40% to about
50% decolorization can be achieved.
[0119] Early experimental work showed that if the CO.sub.2 gas
pressure was too high, even though the decolorization of the sugar
worked well, the calcium carbonate particles were too fine and the
resulting "carbonated liquor" could not be filtered using standard
equipment. Hence, significant development work was done to optimize
the CO.sub.2 gas pressure and to achieve filterability that was as
good as (or better than) the standard process.
[0120] Firstly, by recycling the carbonated liquor, it was
determined that a combination of good decolorization and good
filterability could be achieved. This lengthened the residence time
in a manner that offers good control and allows for adjustment.
[0121] A further advantage was that the rate of the carbonation
step was increased to such an extent that the required residence
time in the tank was dramatically reduced compared to the residence
time required using the conventional carbonation process, allowing
the whole carbonation process to be completed in one tank (i.e., as
a one step process rather than requiring a second tank to repeat
the exposure of the liquor to the carbon dioxide.
[0122] Apart from sucrose refining, the process of the present
invention may be applied to the removal of color bodies from any
carbohydrate or related streams--including carbonation of
non-aqueous or mixed solvent streams (for example, sucralose
streams). It could also be utilized in a new process looking at the
removal of divalent species from other ion exchange processes. As
well as removing color bodies, the carbonation process of the
present application may also remove a wide range of other organic
and inorganic impurities such as, but not limited to, higher
molecular weight waxes, gums and other materials that may be
considered to be impurities.
[0123] The carbonation processes described herein may be applied to
water softening processes, using the following steps: [0124] 1.
Raise the pH of the solution to be treated by adding a hydroxide
base (e.g., NaOH or Ca(OH); [0125] 2. Precipitate the divalent ions
by bubbling in purified CO.sub.2; and [0126] 3. Separation of the
precipitated calcium carbonate, which may be done using techniques
such as filtration, decantation, centrifugation and other such
methods.
Example 1--Initial Laboratory Tests at Relatively High CO.sub.2
Pressure
[0127] Initial work focused on the use of CO.sub.2 under "high
pressure" conditions (2-4 barge). These reactions were conducted in
a Parr reactor. While these experiments "worked" in terms of making
calcium carbonate crystals quickly (<5 mins), with reasonable
color removal, the filterability of these crystals was poor and the
color removal was relatively low, compared with the color removal
seen on the commercial scale (see Table 1 in FIG. 2).
[0128] Optical microscopy showed that the particles were
non-spherical (elongated) and relatively small compared to standard
carbonate floes. It is generally believed that elongated ("needle
shaped") crystals are not good for filterability.
[0129] It appeared that the crystallization process was occurring
too fast, resulting in crystals/floes that are too small and not of
the right shape for good filterability. However, these results were
encouraging, in that they demonstrated that a one-stage carbonation
process could result in fast calcium carbonate formation with
reasonable color removal. It was noted that pH control, temperature
and residence time were key parameters that needed to be
controlled.
Example 2--Further Laboratory Tests at Reduced CO.sub.2
Pressure
[0130] Given the observations in Example 1, further laboratory
tests were completed under relatively low pressures (inlet CO.sub.2
pressure 1-2 barge, outlet pressure ca. 1 bara). The results are
given in Table 2a (see FIG. 3). Further tests were completed at a
larger scale (up to 4 liters), with the results given in Table 2b
(see FIG. 4). These results--as well as those in Table 1--show that
a one-stage process can result in color removal and filterability
close to that achieved on the main plant.
Example 3--Pilot Plant Tests at Reduced CO.sub.2 Pressure
[0131] A larger scale glass pilot plant carbonation vessel was set
up to investigate parameters that could only be checked on a larger
scale (effect of liquid height, etc.). The results from these tests
are shown in Table 3 (see FIG. 5).
[0132] From this pilot plant work, the following interim
conclusions were drawn: [0133] a. Good filterability and color
removal can be achieved with an apparent residence time of about
30-35 mins (true residence time of about 20-25 mins, with 30% gas
voidage); [0134] b. A liquid column height of 3-3.5 meters gives
good control of the CO.sub.2 gas uptake; [0135] c. The uptake of
CO.sub.2 in the pilot rig was approximately 50%; [0136] d.
Temperature control is critical--the optimum temp is between
70.degree. C. and 80.degree. C. (balance between viscosity and
in-situ color development), in the proposed process the inlet and
outlet syrup temperatures are the same; [0137] e. Control of the
syrup outlet pH is key and the process may be controlled to
establish a final pH through close control of the CO.sub.2 and
syrup flows.
* * * * *