Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS AND APPARATUS FOR THE TREATMENT OF C02-CONTAINING GAS
USING CARBONIC ANHYDRASE
FIELD OF THE INVENTION
The present invention relates generally to processes for the treatment of gas
effluents
with a view to cleaning or purifying such effluents. More particularly, it
relates to a
process and an apparatus using a spray absorber for the biocatalytic treatment
of gases.
BACKGROUND OF THE INVENTION
Contemporary industrial activities generate gaseous effluents containing a
multitude of
chemical compounds and contaminants which interfere with the equilibrium of
elements
in nature and affect the environment at different levels. Acid rain, the
greenhouse effect,
smog and the deterioration of the ozone layer are examples that speak volumes
about
this problem. Reduction of noxious emissions is therefore not surprisingly the
subject of
more and more legislation and regulation. Industrial activities and
applications which
must contend with stricter environmental regulatory standards in order to
expect any
long-term commercial viability will turn more and more to biological and
environmentally
safe methods. Consequently, there is a real need for new apparatuses and
methods
aimed at the treatment of gaseous waste or effluents.
There already exists a vast array of technologies aimed at the separation and
recovery
of individual or mixed gases and a number of different biological methods is
known to
treat gaseous waste or effluents: bacterial degradation (JP 2000-287679; JP
2000-
236870), fermentation by anaerobic bacteria (WO 98/00558), photosynthesis
through
either plants (CA 2,029,101 Al; JP 04-190782) or microorganisms (JP 03-
216180).
Among the more popular are those gained through the harnessing of biological
processes such as peat biofilters sprinkled with a flora of microorganisms in
an aqueous
phase, or biofilter columns comprising immobilized resident microorganisms
(Deshusses
et al. (1996) Biotechnol. Bioeng. 49, 587-598). Although such biofilters have
contributed
to technological advances within the field of
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gaseous waste biopurification, the main drawbacks associated with their use
are
their difficult maintenance and upkeep, lack of versatility, as well as time
consuming
bacterial acclimation and response to perturbation periods (Deshusses et al.).
A number of biological sanitation/purification methods and products is known
to use
enzymatic processes, coupled or not to filtration membranes (US 5,250,305;
US 4,033,822; JP 63-129987). However, these are neither intended nor adequate
for
the cleansing of gaseous waste or effluents. The main reason for this is that,
in such
systems, contaminants are generally already in solution (US 5,130,237;
US 4,033,822; US 4,758,417; US 5,250,305; WO 97/19196; JP 63-129987).
Efficient
enzymatic conversion and treatability itself of gaseous waste or effluents in
liquids
therefore depend on adequate and sufficient dissolution of the gaseous phase
in the
liquid phase. However, the adequate dissolution of gaseous waste or effluents
into
liquids for enzymatic conversion poses a real problem which constitutes the
first of a
series of important limitations which compound the problem of further
technological
advances in the field of gas biopurification.
Although triphasic Gas-Liquid-Solid (GLS) reactors are commonly used in a
large
variety of industrial applications, their utilization remains quite limited in
the area of
biochemical gas treatment (US 6,245,304; US 4,743,545). Also known in the
prior art
are the GLS bioprocesses abundantly reported in the literature. A majority of
these
concerns wastewater treatment (JP 09057289). These GLS processes are
characterized in that the gaseous intake serves the sole purpose of satisfying
the
specific metabolic requirements of the particular organism selected for the
wastewater treatment process. Such GLS treatment processes are therefore not
aimed at reducing gaseous emissions.
As previously mentioned, these systems are neither intended nor adequate for
the
treatment of gaseous waste or effluents. An additional problem associated with
the
use of these systems is the non retention of the solid phase within the
reactor.
Biocatalysts are in fact washed right out of the reactors along with the
liquid phase.
Different concepts are, nonetheless, based on this principle for the reduction
of
gaseous emissions, namely carbon dioxide. Certain bioreactors allow the uptake
of
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CO2 by photosynthetic organisms (JP 03-216180) and similar processes bind CO2
through algae (CA 2,232,707; JP 08-116965; JP 04-190782; JP 04-075537).
However, the biocatalyst retention problem remains largely unaddressed and
constitutes another serious limitation, along with gaseous effluent
dissolution, to
further technological advancements.
The main argument against the use of ultrafiltration membranes to solve this
biocatalyst retention problem is their propensity to clogging. Clogging
renders them
unattractive and so, their use is rather limited for the retention of
catalysts within
reactors. However, a photobioreactor for medical applications as an artificial
lung
(WO 92/00380; US 5,614,378) and an oxygen recovery system (US 4,602,987;
US 4,761,209) are notable exceptions making use of carbonic anhydrase and an
ultrafiltration unit.
The patent applications held by the assignee, C02 Solution Inc., via Les
Systemes
Envirobio Inc. (EP 0 991 462; WO 98/55210; CA 2,291,785) propose a packed
column for the treatment of carbon dioxide using immobilized carbonic
anhydrase.
Carbonic anhydrase is a readily available and highly reactive enzyme that is
used in
other systems for the reduction of carbon dioxide emissions (US 4,602,987;
US 4,743,545; US 5,614,378; US 6,257,335). In the system described by
Trachtenberg for the carbonic anhydrase treatment of gaseous effluents
(US 6,143,556; CA 2,222,030), biocatalyst retention occurs through a porous
wall or
through enzyme immobilization. However, important drawbacks are associated
with
the use of enzyme immobilization, as will be discussed below.
Other major drawbacks are associated with the use of enzymatic systems. One of
these stems from systems where enzymatic activity is specifically and locally
concentrated. This is the case with systems where enzymes are immobilized at a
particular site or on a specific part of an apparatus. Examples in point of
such
systems are those where enzymes are immobilized on a filtration membrane
(JP 60014900008A2; US 4,033,822; US 5,130,237; US 5,250,305; JP 54-132291;
JP 63-129987; JP 02-109986; DE 3,937,892) or even, at a gas-liquid phase
boundary (WO 96/40414; US 6,143,556). The limited surface contact area
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obtainable between the dissolved gas substrate, the liquid and the enzyme
active
site poses an important problem. Hence, these systems generate significantly
greater waste of input material, such as expensive purified enzymes, because
the
contact surface with the gaseous phase is far from optimal and limits
productive
reaction rates. Therefore, as mentioned previously, overcoming the contact
surface
area difficulty should yield further technological advances.
Other examples of prior art apparatuses or methods for the treatment of gas or
liquid
effluents are given in the following documents: CA 2,160,311; CA 2,238,323;
CA 2,259,492; CA 2,268,641; JP 2000-236870; JP 2000-287679; JP 2000-202239;
US 4,758,417; US 5,593,886; US 5,807,722; US 6,136,577; and US 6,245,304.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an apparatus that is distinct
from and
overcomes several disadvantages of the prior art bioreactor for the treatment
of gas
effluent, as will be discussed in detail below.
Another object is to provide a process for the biocatalytic treatment of
gases, which
is more efficient with respect to the reaction rate of the reactants.
In accordance with the present invention, that object is achieved with a
process
characterized in that it comprises a step of contacting a gas phase,
containing a
particular gas to be treated, to spray droplets of liquid phase containing
biocatalysts.
More particularly, the present invention proposes a process for the
biocatalytic
treatment of gases, comprising the steps of:
a) contacting a gas phase, containing a gas to be treated, to a liquid phase
containing a reactant capable of absorbing and/or chemically reacting with the
gas,
thereby producing a spent liquid phase containing at least one reaction
product and a
treated gas phase substantially free of the gas, such step being performed in
the
presence of biocatalysts suitable for catalyzing the chemical reaction between
the
gas and the reactant, the process being characterized in that:
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- it comprises, prior to step a) of contacting, the step of mixing the
biocatalysts to the liquid phase; and
- the step of contacting comprises the step of spraying droplets of the
liquid phase containing the biocatalysts.
5 The liquid phase may be aqueous or non aqueous and the gas to be treated is
usually soluble in the liquid. Contact between the gas and liquid phases
results in the
absorption of the gas to be treated and thus to its extraction from the gas
phase.
The use of droplets of liquid containing the biocatalysts, sprayed into the
reaction
chamber, enables to increase the gas-liquid interface, thereby allowing a high
mass
transfer rate of the gas to be treated from the gas phase to the liquid phase.
These
conditions of high mass transfer enable to transform the gas with a maximum
reaction rate.
Then, the dissolved gas is transformed in presence of appropriate biocatalysts
into
one or more products. The role of biocatalysts is to accelerate the
transformation
reaction of the dissolved gas in the liquid phase environment. In addition to
biocatalysts, the liquid phase may contain reactants required for the
transformation of
the dissolved gas or for a reaction with one or more reaction products of the
dissolved gas. Depending on the reactants and biocatalysts present in the
liquid
phase, products are in a soluble or solid form. Reaction products are
preferably
further treated to give useful products to be used in other applications or to
be
disposed of.
Absorption and biocatalytic transformation of the gas take place in the
reaction
chamber of a spray absorber bioreactor. The gas phase, containing the selected
gas
to be treated, is fed into the reaction chamber where it is contacted to spray
droplets
of a liquid phase containing the biocatalysts. Droplets are obtained using
atomizers.
The gas phase in contact to the liquid phase has one or more components
absorbed
in the liquid phase. Then, the dissolved gas is transformed because of
biocatalysts
activity and presence of reactants, if required. The gas phase is almost free
of one or
more components and exits the reaction chamber purified. The liquid phase
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containing the biocatalysts and reaction products (dissolved and/or solid)
exits from
the reaction chamber. In accordance with a preferred aspect of the invention,
the gas
is further treated to remove droplets in suspension or to decrease its
humidity.
Hence, the present invention is also directed to a biocatalytic treatment unit
for the
biocatalytic treatment of gases, comprising:
- a bioreactor comprising a reaction chamber having a liquid inlet for
receiving
a liquid, a gas inlet for receiving a gas to be treated, a liquid outlet for
discharging a
spent liquid and a gas outlet for releasing a treated gas;
the treatment unit being characterized in that it comprises;
a mixing unit for mixing biocatalysts to the liquid;
means for conveying the liquid from the mixing unit to the liquid inlet of the
reaction chamber; and
the liquid inlet comprises at least one atomizer mounted within the reaction
chamber to spray droplets of liquid containing the biocatalyst.
Biocatalysts are usually very costly. Therefore, it is preferable to recycle
the spent
liquid phase containing biocatalysts and reactants. However, recycling
requires that
reaction products be removed from the liquid phase. Therefore, in accordance
with a
preferred aspect of the invention, the process further comprises the steps of:
b) removing from the spent liquid phase the at least one reaction product,
thereby obtaining a recycled liquid phase free of the reaction product and
containing
the biocatalysts; and
c) recycling the recycled liquid phase to step a) of contacting.
In accordance with that aspect of the invention, the treatment unit further
comprises
a separation unit in fluid communication with the liquid outlet for removing
the
reaction products contained in the spent liquid. The separation unit has a
first liquid
outlet for discharging a liquid fraction substantially free of the reaction
products and a
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second outlet for discharging the liquid fraction containing the reaction
products.
Conveying means are provided for conveying the fraction substantially free of
the
reaction products to the mixing unit.
Removal assures that a maximum mass transfer rate of the gas from the gas
phase
to the liquid phase is achieved at each pass of the liquid phase in the
reaction
chamber of the spray absorber bioreactor. Removal of dissolved reaction
products is
preferably obtained using membrane processes such as ultrafiltration,
microfiltration
processes and/or ion exchange and/or adsorption processes. Removal may also be
obtained by first precipitating the reaction products with appropriate
reactant (s) and
then by removing the particles using separation processes. Solid particles
originating
from solid reaction products or subsequently precipitated products are
preferably
removed by using separation processes such as settling, filtration or
expression.
Moreover, agents facilitating removal of particles, such as coagulants or
flocculants
or filter aids, may be added to the liquid effluent prior to particle removal
units or to
the liquid phase entering the spray absorber bioreactor.
A biocatalyst is a biological entity, which can transform a substrate in one
or more
products. The biocatalysts used in the process are preferably enzymes,
cellular
organelles (mitochondrion, membranes), animal, vegetal or human cells. The
biocatalysts can be used free or immobilized. Immobilization is preferably the
result
of fixation to a solid support, entrapment inside a solid matrix.
Immobilization can
also be obtained by using intermolecular binding of biocatalysts molecules or
structures. In all these cases, the solid support, the solid matrix and the
intermolecular binding must be in the form of micro particles sufficiently
small to pass
through the atomizer with the liquid phase.
Depending on patterns of spray, droplet size, uniformity of spray, turndown
ratio
and/or power consumption, available atomizers may fall in three categories:
pressure
nozzles, two-fluid nozzles or rotary devices. However, any other type of
atomizer may
be used.
Membrane processes for removal of dissolved products or solid particles may
include
the use of flat or tubular membranes. Those membranes are preferably involved
in
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different modules such as plate-and-frame, spiral-wound, tubular capillary and
hollow
fiber module. The operation of those modules may be dead-end or cross-flow (co-
current, countercurrent, cross-flow with perfect permeate mixing and perfect
mixing).
Those filtration units may be used in a single-stage or in a multi-stage
process in a
single-pass system or a recirculation system.
The present invention also provides a process for the biocatalytic treatment
of a C02-
containing gas, comprising the steps of:
a) contacting the C02-containing gas phase, containing CO2 gas to be treated,
to
a liquid phase containing carbonic anhydrase capable of catalyzing the
hydration
reaction of CO2 into bicarbonate and hydrogen ions, thereby producing a spent
liquid phase containing the bicarbonate and hydrogen ions and a treated gas
phase substantially free of the CO2, the process being characterized in that:
it comprises, prior to step a) of contacting, the step of mixing the carbonic
anhydrase to the liquid phase; and
the step of contacting comprises the step of spraying droplets of the liquid
phase
containing the carbonic anhydrase.
The present invention also provides a carbonic anhydrase treatment unit for
the
treatment of C02-containing gas, comprising:
a bioreactor comprising a reaction chamber having a liquid inlet for receiving
a
liquid, a gas inlet for receiving the C02-containing gas to be treated, a
liquid
outlet for discharging a spent liquid and a gas outlet for releasing a treated
gas;
the treatment unit being characterized in that it comprises;
a mixing unit for mixing carbonic anhydrase to said liquid;
means for conveying the liquid from the mixing unit to the liquid inlet of the
reaction chamber; and
the liquid inlet comprises at least one atomizer mounted within the reaction
chamber to spray droplets of liquid containing the carbonic anhydrase.
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The present invention also provides a process for enzymatic treatment of a C02-
containing gas, comprising:
mixing enzymatic particles comprising carbonic anhydrase with a liquid
solution to
form a sprayable mixture;
spraying the sprayable mixture to form droplets thereof comprising the
enzymatic
particles;
contacting the C02-containing gas with the droplets such that the carbonic
anhydrase catalyzes the hydration reaction of CO2 into bicarbonate and
hydrogen
ions, thereby producing a spent liquid phase containing the bicarbonate and
hydrogen ions and a treated gas phase substantially free of the 002.
The present invention also provides a process for treatment of a fluid by
enzymatic
catalysis of reaction (I) with carbonic anhydrase, wherein the reaction (I) is
as follows:
CO2 +H20 arbo HCO3+H+ (I)
the process comprising:
feeding the fluid into a reaction zone in the presence of enzymatic particles
comprising carbonic anhydrase, the enzymatic particles being sized to be mixed
and sprayed with a liquid solution thereby forming droplets comprising the
enzymatic particles;
allowing the reaction (I) to occur within the droplets in the reaction zone,
to
produce a gas stream and a liquid stream; and
releasing the gas stream and the liquid stream from the reaction zone. In one
embodiment, the process as described above, wherein the fluid is a C02-
containing gas; the process comprises spraying the droplets comprising the
enzymatic particles into the reaction zone to contact the CO2-containg gas so
as
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to dissolve CO2 from the C02-containing effluent gas into the droplets; the
reaction (I) is a forward reaction catalyzing the hydration of dissolved C02
into the
bicarbonate and hydrogen ions; and the gas stream is a treated gas phase
substantially free of the CO2 and the liquid stream is a spent liquid phase
containing the bicarbonate and hydrogen ions.
The present invention further provides a carbonic anhydrase treatment unit for
the
treatment of C02-containing gas, comprising:
a bioreactor comprising a reaction chamber having a liquid inlet for receiving
a
liquid, a gas inlet for receiving the C02-containing gas to be treated, a
liquid
outlet for discharging a spent liquid and a gas outlet for releasing a treated
gas;
a mixing unit for mixing enzymatic particles comprising carbonic anhydrase
with
the liquid to form a sprayable mixture;
means for conveying the sprayable mixture from the mixing unit to the liquid
inlet
of the reaction chamber; and
the liquid inlet comprises at least one atomizer mounted within the reaction
chamber to spray droplets of sprayable mixture containing the enzymatic
particles
into the reaction chamber.
The present invention further provides a carbonic anhydrase treatment unit for
the
treatment of a fluid by enzymatic catalysis of reaction (I) with carbonic
anhydrase,
wherein the reaction (I) is as follows:
C02 +H20 erbonic HCO3 +H+ (1)
the treatment unit comprising:
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a bioreactor comprising a reaction chamber for accommodating the reaction (I),
a
liquid inlet, a liquid outlet for discharging a liquid stream and a gas outlet
for
releasing a gas stream;
a mixing unit for mixing enzymatic particles comprising carbonic anhydrase
with a
liquid solution thereby forming a sprayable mixture;
means for conveying the sprayable mixture from the mixing unit to a liquid
inlet of
the reaction chamber; and
the liquid inlet comprises at least one atomizer mounted within the reaction
chamber to
spray droplets of the sprayable mixture containing the enzymatic particles
into the
reaction chamber such that the reaction (I) to occur within the droplets. In
one
embodiment the carbonic anhydrase treatment unit as described above, wherein
the
fluid is a C02-containing gas; the bioreactor comprises a gas inlet for
feeding the C02-
containing gas into the reaction chamber; the droplets comprising the
enzymatic
particles; the reaction (I) is a forward reaction catalyzing the hydration of
dissolved CO2
into the bicarbonate and hydrogen ions; and the gas is a treated gas phase
substantially
free of the C02 and the liquid stream is a spent liquid phase containing the
bicarbonate
and hydrogen ions.
The present invention also provides a carbonic anhydrase treatment unit
comprising a
reaction chamber having a liquid inlet for receiving a liquid containing micro-
particles
comprising carbonic anhydrase, a gas inlet for receiving a C02-containing gas
to be
treated, a liquid outlet for discharging a spent liquid and a gas outlet for
releasing a
treated gas, the liquid inlet being sized and configured to feed the liquid
and micro-
particles into the reaction chamber.
The present invention also provides a process for enzymatic treatment of a C02-
containing gas, comprising:
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contacting the C02-containing gas with an aqueous liquid solution comprising a
buffer compound in presence of carbonic anhydrase that catalyzes the hydration
reaction of CO2 into bicarbonate and hydrogen ions, thereby producing a liquid
phase containing the bicarbonate and hydrogen ions and a treated CO2 depleted
gas phase.
The present invention also provides a process for enzymatic treatment of a C02-
containing gas, comprising:
spraying an aqueous liquid solution into a reaction chamber;
contacting the C02-containing gas with the aqueous liquid solution within the
reaction chamber in presence of carbonic anhydrase that catalyzes the
hydration
reaction of CO2 into bicarbonate and hydrogen ions, thereby producing a liquid
phase containing the bicarbonate and hydrogen ions and a treated CO2 depleted
gas phase comprising mist of the aqueous liquid solution; and
removing the mist from the CO2 depleted gas phase to produce a mist depleted
gas.
The present invention also provides a process for enzymatic treatment of a C02-
containing gas, comprising:
spraying an aqueous liquid solution into a reaction chamber; and
contacting the C02-containing gas with an aqueous liquid solution in presence
of
carbonic anhydrase that is entrapped inside a matrix, the carbonic anhydrase
catalyzing the hydration reaction of C02 into bicarbonate and hydrogen ions,
thereby producing a liquid phase containing the bicarbonate and hydrogen ions
and a treated CO2 depleted gas phase.
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The present invention also provides a process for enzymatic treatment of a C02-
containing gas, comprising:
spraying an aqueous liquid solution into a reaction chamber; and
contacting the C02-containing gas with an aqueous liquid solution in presence
of
carbonic anhydrase, the carbonic anhydrase catalyzing the hydration reaction
of
CO2 into bicarbonate and hydrogen ions, thereby producing a loaded liquid
phase
containing the bicarbonate and hydrogen ions and a treated C02 depleted gas
phase;
releasing the loaded liquid phase from the reaction chamber;
precipitating solid particles in the loaded liquid phase;
removing the precipitated solid particles from the loaded liquid phase to
produce a
precipitate removed liquid; and
recycling the precipitate removed liquid back into the reaction chamber as the
aqueous liquid solution.
The present invention also provides a method of reducing the ratio of liquid
flow rate to
gas flow rate in a carbonic anhydrase C02 absorption bioreactor and decreasing
an
amount of carbonic anhydrase for achieving a enzymatically catalyzed C02
removal
rate, by spraying an aqueous liquid solution comprising carbonic anhydrase
into the
bioreactor.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic representation of a spray absorber bioreactor
according to a
preferred embodiment of the present invention.
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8f
Figure 2 is a schematic flow chart of a second preferred embodiment of the
process
according to the present invention.
Figures 3a and 3b are schematic flow charts of a third preferred embodiment of
the
process according to the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to figure 1, a gas phase containing CO2 is fed to a reaction chamber
(1) of a
spray absorber bioreactor. The gas phase is preferably fed at the bottom (2)
of the
reaction chamber (1) of the spray absorber bioreactor. The aqueous liquid
phase
containing biocatalysts is preferably fed at the top of the reaction chamber
(3) through
atomizers (4) where the liquid phase forms droplets. Additional atomizers may
be found
at the side (4) of the reaction chamber. In this preferred embodiment, the gas
phase
flows upward and contact spray droplets of the liquid phase. During the
contact, CO2 is
absorbed and then transformed into bicarbonate and hydrogen ions. This
transformation
is catalyzed by a biocatalyst accelerating CO2 transformation. The biocatalyst
is
preferably the enzyme carbonic anhydrase but may be any biological catalyst
enabling
CO2 transformation. CO2 transformation reaction is the following:
C02 +H20 e`b `` a"''yd`I HCO;+H+ Equation 1
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This reaction is natural. It is at the basis of CO2 transportation and removal
phenomenon in the human body and in most living beings. Reaction products are
ions in solution in the aqueous liquid phase (6). The treated gas phase (5)
exits at
the top of the spray absorber bioreactor and is almost free of CO2. However,
droplets
of the liquid phase may remain in suspension in the purified gas exiting the
bioreactor (5). An additional equipment, such as a mist eliminator, is thus
preferably
added to the spray absorber bioreactor to remove those droplets. Both gas and
liquid
phases are preferably further treated for proper disposal or reuse.
It is well known that under specific conditions, bicarbonate and/or carbonate
ions
may react with some cations (Na+, Cat+, Mgt+, Bat+) to precipitate. For
example, if
the liquid phase contains calcium ions and the pH is around 9,0, bicarbonate
and
carbonate ions are present because of natural physicochemical equilibrium
reactions. These carbonate ions may react with calcium ions (see Eq. 2 below),
for
example, by adding Ca(OH)2 to the solution, thus leading to the formation of
solid
particles of calcium carbonate in the liquid phase. Consequently, bicarbonate
ions
transform into carbonate ions, because of equilibrium between both species.
Those
newly formed carbonate ions will then lead to the formation of calcium
carbonate. In
this case, an agitation device is preferably required to facilitate discharge
of solid
particles. Moreover, the bottom of the bioreactor preferably has a conical
shape.
C032" + Ca 2+ = CaCO3 Equation 2
Biocatalysts are usually costly material. Therefore, it is interesting to
recycle the
liquid phase containing the biocatalysts. Figure 2 shows a process implying
steps
described previously in figure 1. However, for the recycling of the liquid
phase,
additional considerations have to be made. CO2 removal process is based on
absorption, thus removal of the reaction products (HC03 and H+) is required in
order
to maintain optimal conditions for CO2 mass transfer. In this case,
bicarbonate and
hydrogen ions are soluble. Bicarbonate ions are preferably removed in a
product
removal process (11) such as membrane separation processes (ultrafiltration,
nanofiltration), ion exchange or adsorption units separately or in
combination.
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Referring now to figure 2, the liquid phase containing biocatalysts and rich
in reaction
products (8) is fed to a product removal process (11). Two effluents (12,10)
are
generated: one (12) rich in reaction product, bicarbonate ions and one (10)
containing a very low level of bicarbonate ions. This latter liquid phase is
preferably
5 enriched in fresh liquid phase (9) or biocatalysts to compensate for
possible loss of
those two components. Acid or alkali may also be added to the liquid phase in
order
to control pH of the liquid phase. pH control may also be done inside the
product
removal process. In the present application, alkali such as NaOH might be
added to
the liquid phase to neutralize protons produced during the biological
transformation
10 of CO2 (Equation 1). The resultant liquid phase (7) enters at the top of
the bioreactor.
The effluent (12) is preferably further treated for proper disposal or use in
another
process.
Turning now to figures 3a and 3b, when solid particles are present in the
liquid phase
(14) because of a precipitation reaction (Eq. 2), solid particles have to be
removed
for recycling of the liquid phase. The particles removal process (15) may
consist of
one or more separation units, as shown in figure 3a. Separation is preferably
performed by settling (by gravity, centrifugal force, heavy media, flotation,
magnetic
force) and/or filtration (on screens or on filters (by gravity, pressure,
vacuum or
centrifugal force)) and/or expression (batch presses or continuous presses
(screw
presses, rolls or belt presses)). Moreover, agents facilitating removal of
particles
such as coagulants or flocculants or filter aids may be added to the liquid
effluent
(16) prior to or in the particle removal process or to the liquid phase
entering the
spray absorber bioreactor (13).The liquid phase (18) exiting the particles
removal
process is preferably supplemented in fresh liquid phase and/or biocatalysts
(9).
Moreover, acid or alkali may also be added to liquid phase for pH control. The
liquid
phase (13) is then fed to the bioreactor. Solid particles (17) obtained may
further be
treated or be disposed of.
The separation unit may, in a particular case, be integrated to the
bioreactor. Figure
3b shows a process where separation by gravity is integrated to the
bioreactor. In
this particular case, an agitation device is preferably added at the bottom of
the
bioreactor and the bottom of the bioreactor preferably has a conical shape.
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Example
An experiment was conducted to validate the concept of a spray absorber for
removal of CO2. The process diagram of the spray absorber for the test was
similar
to the one shown in figure 1. The spray absorber consists of a column having a
7,5 cm diameter and a 70 cm height. A pressure nozzle atomizer was mounted
within
the reaction chamber. Five litres of a 12 mM Tris solution with 20 mg carbonic
anhydrase per litre of solution were pumped into the spray absorber at a flow
rate of
1,5 I/min. Carbonic anhydrase was used free within the Tris Solution. The Tris
solution is a buffer consisting of 2-amino-2-hydroxymethyl-1,3-propanediol.
The
solution was used in a closed loop operation, until the Tris solution was
saturated
with dissolved C02.The gas flow rate was 6,0 g/min at a CO2 concentration of
52000 ppm. Gas and solution were at room temperature. The pressure inside. the
spray absorber was set at 5 psig.
The results obtained showed that the CO2 contained in the gas phase was
removed
at a rate of 2,3 x 10"3 mol of CO2 /min.
These results were compared with the ones obtained from experiments conducted
with a conventional bioreactor using a reaction chamber filled with carbonic
anhydrase immobilized on rashig supports. The following table provides a
comparison of these results.
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Parameters Spray absorber Prior art packed
according to the bioreactor
invention
Concentration of CO2 in 52000 pm 50000 ppm
the gas phase
Liquid flow rate 1,5 I/min 0,5 I/min
Biocatalyst Carbonic anhydrase free Carbonic anhydrase
in liquid immobilized in the
bioreactor
Gas flow rate 6,0 g/min 1,5 g/min
Ratio of liquid flow rate to 0,25 I/g 0,5 I/g
gas flow rate
Mass of enzyme within the Less than 100 mg* 275 mg
reactor
Removal rate of CO2 2,3 x 10 mol of CO2 /min 1,3 x 10 moI of CO2 /min
* Five liters of Tris solution contains 100 mg. However, for the absoption of
C02, only
a portion of the enzyme ends in the reaction chamber.
These results show that the process according to the invention is surprisingly
more
efficient than the process known in the prior art, since 1) at all times, the
enzyme
participating in the removal of CO2 is less, therefore the removal of CO2
requires less
enzyme, and 2) even though the ratio of liquid flow rate to gas flow rate is
lower with
the process according to the invention, the removal rate of CO2 is greater.
Indeed, it
is well known for a person skilled in the art that in a process for absorbing
a gas, if
that ratio decreases, the performance of the process also decreases. In other
words,
the performance of the packed bioreactor would have been even less if the
ratio of
liquid flow rate to gas flow rate used had been 0,25 I/g, as for the
experiments with
the spray absorber of the invention.
Although the present invention has been explained hereinabove by way of
preferred
embodiments thereof, it should be understood that the invention is not limited
to
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13
these precise embodiments and that various changes and modifications may be
effected therein without departing from the scope or spirit of the invention.
