WO2013159215A1

WO2013159215A1 - Co2 capture using low concentration ammonia based absorption solutions in presence of enzymes

Info

Publication number: WO2013159215A1
Application number: PCT/CA2013/050201
Authority: WO
Inventors: Albert Rikus Johannes ARENDSEN; Geert Frederik Versteeg
Original assignee: Co2 Solutions Inc.
Priority date: 2012-04-24
Filing date: 2013-03-14
Publication date: 2013-10-31

Abstract

Methods and processes related to C0₂ capture with low concentration ammonia base absorption solutions. Operating conditions may be determined so as to operate within an operation window enabling a maximal range of the C0₂ capture; and the ammonia based absorption solution may be enhanced by adding at least one enzyme or analogues thereof for accelerating the hydration of C0₂ from the C0₂ containing-gas into the absorption solution for reducing the size of the C0₂ capture equipment within the operation window for example. Operating conditions may include a C0₂ partial pressure in the absorption solution, an absorption temperature, a lean C0₂ liquid loading range of the absorption solution, an ammonia concentration range in the absorption solution and an absorption solution flow rate. Processes and methods may include selecting the absorption solution flow rate in accordance with a maximum rich C0₂ liquid loading of the absorption solution.

Description

C0₂ CAPTURE USING LOW CONCENTRATION AMMONIA BASED ABSORPTION SOLUTIONS IN PRESENCE OF ENZYMES

FIELD OF THE INVENTION

The present invention generally relates to the field of gaseous C0₂ capture. More particularly, the present invention concerns processes for C0₂ capture using low concentration ammonia based absorption solutions in combination with enzymes.

BACKGROUND OF THE INVENTION

Warnings from the world's scientific community combined with greater public awareness and concern over the issue of global climate change has prompted increased momentum towards global regulations aimed at reducing man-made greenhouse gas (GHGs) emissions, most notably carbon dioxide (C0₂). Ultimately, a significant cut in North American and global C0₂ emissions will require reductions from large power generation and industrial point-sources of fossil fuel-based emissions. According to the International Energy Agency's (IEA) GHG Program, as of 2008 there were approximately 8,200 such point-sources worldwide generating 14.7 billion tons of C0₂, representing nearly half of all global anthropogenic C0₂ emissions. Carbon Capture and Sequestration (CCS) provides a solution to reducing emissions from these sources.

The CCS process involves selective removals of C0₂ from a C0₂-containing flue gas, and production of a highly concentrated C0₂ gas stream which is then compressed and transported to a geologic sequestration site. This site may be a depleted oil field or a saline aquifer. Sequestration as mineral carbonates is an alternate way to sequester C0₂ that is in the development phase. Captured C0₂ can also be used for enhanced oil recovery, for injection into greenhouses, for chemical reactions and production, and for other useful applications.

Technologies for C0₂ capture from post-combustion flue gases and other gas streams are based primarily on the use of an aqueous alkanolamine based solution which is circulated through two main distinct units: an absorption tower coupled to a desorption or stripping tower.

Most aqueous solutions currently in use in this regard are alkanolamine-based solutions. However, an alternative approach is based on the use of aqueous ammonia, known as the "chilled ammonia" process. This process has been described in several publications including patent application No. PCT/US2005/012794. The chilled ammonia process (CAP) has been described as providing a relatively low cost means for capturing/removing C0₂ from a gas stream, including a post combustion flue gas stream. US patent No. 7,862,788 (hereinafter referred to as Gal et al) describe a promoter enhanced chilled ammonia based system for removal of C0₂ from a flue gas stream. Gal et al describe a CAP process in which a promoter is used to help accelerate certain chemical reactions that occur between C0₂ and ammoniated ionic solutions. The promoter may be an amine such as piperazine or an enzyme or enzyme system.

CAP installations have used relatively low temperatures, e.g. between 0-20°C or between 0-10°C, along with relatively high ammonia concentrations. The low temperature minimizes ammonia losses to evaporation and the high ammonia concentration enables adequate absorption of C0₂ from the flue gas. Conventional wisdom has indeed suggested that decreasing NH₃ concentration would result in a proportional increase of the overall solvent circulation rate in the absorption tower for a given C0₂ capture rate. Increasing solvent circulation rate within the system would, in turn, increase the overall equipment and operational cost significantly. For instance, a lower NH₃ concentration can induce an increase of solvent circulation rate to maintain the C0₂ loading capacity, thereby inducing a significant increase in energy consumption and equipment size.

It can also be desirable in a C0₂ capture process to minimize or control the formation of solids which can precipitate out of the absorption solution once it has become highly loaded with C0₂ depending on the solubility properties of the given solution and operating conditions of the process. At low temperatures, for instance, there can be a greater tendency towards solids precipitation. The operational window of a CAP system can thus be limited by the formation of solids in the absorption tower.

While it is an advance in the field of C0₂ capture, CAP and known variants thereof still have a variety of limitations and drawbacks.

There is thus a need for a technology that overcomes at least some of the limitations or drawbacks of what is known in this field of C0₂ capture especially as relates to ammonia based capture processes. SUMMARY OF THE INVENTION

The present invention responds to the above need by providing a processes and methods for capturing carbon dioxide using ammonia based absorption solutions.

In one aspect, there is provided a method for reducing a size of C0₂ capture equipment for capturing C0₂ from a C0₂ containing gas with an ammonia based absorption solution at low ammonia concentrations. The method includes:

determining operating conditions so as to operate within an operation window enabling a maximal range of the C0₂ capture; and

enhancing the ammonia based absorption solution by adding at least one enzyme or analog thereof for accelerating the hydration of C0₂ from the C0₂ containing gas into the absorption solution, thereby reducing the size of the C0₂ capture equipment within the operation window.

In an optional aspect, the operation window is defined by operating conditions comprising an absorption temperature, a lean C0₂ liquid loading range of the absorption solution, an ammonia concentration range in the absorption solution and an absorption solution flow rate.

In another optional aspect, the method may include managing an ammonium bicarbonate saturation percentage of the absorption solution for increasing or maximizing a rich C0₂ liquid loading in the absorption solution.

In another optional aspect, the method may include managing a C0₂ partial pressure in the absorption solution for increasing or maximizing the rich C0₂ liquid loading in the absorption solution.

In another aspect, there is provided a process for removing C0₂ from a C0₂ containing gas. The process includes:

determining an operation window enabling operation near or at an elevated or maximized rich C0₂ liquid loading, the operation window being defined by an operating ammonia concentration range and an operating absorption solution flow rate range, at a given absorption temperature and a given lean C0₂ liquid loading range; providing an ammonia based absorption solution having an ammonia concentration selected in the operating ammonia concentration range and having a C0₂ liquid loading selected in the lean C0₂ liquid loading range;

contacting the ammonia based absorption solution with the C0₂ containing gas in an absorber reactor, the absorption solution having a flow rate selected in the operating absorption solution flow rate range;

operating the absorber reactor in the operation window in presence of at least one enzyme or analogue thereof sufficient to accelerate the hydration reaction of C0₂ into the absorption solution and thereby reduce equipment sizing within the operation window;

generating an ion-rich solution comprising hydrogen ions and bicarbonate ions and releasing the same from the absorber reactor near or at the elevated or maximized rich C0₂ liquid loading; and

generating a C0₂ depleted gas stream and releasing the same from the absorber reactor.

In another aspect, there is provided a method of using low ammonia concentrations in an ammonia based C0₂ capture process. The method includes:

determining an operation window for the C0₂ capture process enabling capture near or at an elevated or maximized rich C0₂ liquid loading, the operation window being defined by an operating ammonia concentration range and an operating absorption solution flow rate range, at a given absorption temperature and a given lean C0₂ liquid loading range;

selecting a low concentration of ammonia in the operating ammonia concentration range and a lean C0₂ liquid loading in the lean C0₂ liquid loading range of the operation window; and

providing a reduced or minimized absorption solution flow rate in the absorption solution flow rate range of the operation window in accordance with the low concentration of ammonia.

In an optional aspect, the method may include using at least one enzyme or analogue thereof for accelerating the hydration reaction of C0₂ into the absorption solution and thereby reduce equipment sizing within the operation window. In another aspect, there is provided a method for maximizing absorption of C0₂ from a C0₂-containing gas using an ammonia based absorption solution. The method includes: determining a maximum rich C0₂ liquid loading in the ammonia based absorption solution in accordance with given operating conditions;

selecting an absorption solution flow rate in accordance with the maximum rich C0₂ liquid loading; and

circulating the ammonia based absorption solution at the absorption solution flow rate for contacting the C0₂ containing gas and absorbing the C0₂ there from according to the maximum rich C0₂ liquid loading.

In an optional aspect, the selecting of the absorption solution flow rate may be performed according to the following equation:

. _ 1

9NH₃ ^~ Γ ( _ \ ^WNEREIN> Ν_Η^ is the absorption solution flow rate in m³/kmol;

° ;_Cft max '^{s tne} maximum rich C0₂ liquid loading; a_lean is the lean C0₂ liquid loading of the absorption solution; and

C_m is the NH₃ concentration in the absorption solution in kmol/m³.

In another aspect, there is provided a process for capturing C0₂ from a C0₂ containing gas with an ammonia based C0₂ capture system. The process includes:

using an ammonia based absorption solution with low ammonia concentration for capturing C0₂ from the C0₂ containing gas; and

controlling or providing operating conditions so as to operate within an operation window where the C0₂ capture is in a maximal range.

In an optional aspect, the process may include controlling or reducing an absorption solution flow rate for operating near or at a maximum C0₂ capture at low ammonia concentrations.

In another optional aspect, the process may include controlling the ammonia concentration, an absorption temperature, a lean C0₂ liquid loading or an absorption solution flow rate or a combination thereof, to reduce or avoid formation of precipitated solids in the absorption solution.

In another aspect, there is provided an ammonia based absorption solution for use in a C0₂ capture system, the absorption solution having an ammonia concentration sufficiently low as to be enhanced by the presence of an enzyme while reaching an enhanced or maximized C0₂ liquid loading.

In an optional aspect, the lean C0₂ liquid loading may be between about 0.1 and about 0.5, optionally between about 0.2 and about 0.45, and further optionally between 0.35 and 0.4.

In another optional aspect, the absorption temperature may be between about 0°C and about 50°C, optionally between about 10°C and about 30°C, and further optionally between about 15°C and about 25°C.

In another optional aspect, the ammonia concentration may be between about 1 kmol/m³ and about 8 kmol/m³. Optionally, the ammonia concentration may be between about 2 kmol/m³ and about 5 kmol/m³ when the absorption temperature is about 10°C. Further optionally, the ammonia concentration may be between about 2 kmol/m³ and about 3 kmol/m³. Alternatively, the ammonia concentration may be between about 4 kmol/m³ and about 8 kmol/m³ when the absorption temperature is about 25°C. Optionally, the ammonia concentration may be between about 5 kmol/m³ and about 7 kmol/m³. Optionally, the ammonia concentration may be about 6 kmol/m³.

In another optional aspect, the absorption solution flow rate may be between about 0.5 and about 3 m³/kmol, optionally between about 1 m³/kmol and about 2 m³/kmol.

In another optional aspect, the enzyme may comprise a recombinant enzyme, a variant enzyme or a naturally occurring enzyme or a combination thereof. Optionally, the enzyme may be derived from archeal source enzyme, bacterial source enzyme or fungal source enzyme or a combination thereof. Optionally, the enzyme may include carbonic anhydrase or analogues thereof.

In another optional aspect, the operation window may be defined for the given absorption temperature and the given lean C0₂ liquid loading by:

the operating ammonia concentration range between about 1 kmol/m³ and 8 kmol/m³; and the operating absorption solution flow rate range wherein the absorption solution flow rate is within 0.5 m³/kmol above or below a corresponding solvent flow rate at the given lean C0₂ liquid loading.

the operating ammonia concentration range wherein the ammonia concentration is within 1.5 kmol/m³ above or below a transitional ammonia concentration; and the operating absorption solution flow rate range wherein the absorption solution flow rate is within 0.5 m³/kmol above or below a corresponding solvent flow rate at the given lean C0₂ liquid loading.

In another optional aspect, methods and processes may include determining the transitional ammonia concentration at a transition point for the given absorption temperature and the given lean C0₂ liquid loading.

In another optional aspect, the operation window may be pre-determined, calculated, estimated and/or determined during start-up, turndown or regular operation.

In another optional aspect, the operation window may be based on models, simulations, experiments or representations or a combination thereof, as represented in the appended figs for example.

While the invention will be described in conjunction with example embodiments, it will be understood that it is not intended to limit the scope of the invention to such embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included as defined by the present description. The objects, advantages and other features of the present invention will become more apparent and be better understood upon reading of the following non-restrictive description of the invention, given with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the processes and methods according to the present invention are represented in the following figures.

Fig. 1 is a process flow diagram of a C0₂ capture process including absorption and desorption units according to an optional embodiment of the present invention. Fig. 2 is a graph of rich C0₂ liquid loading versus NH₃ concentration at a temperature of 10C with iso-lines of the logarithm of C0₂ partial pressure according to an optional embodiment of the present invention.

Fig. 3 is a graph of rich C0₂ liquid loading versus NH₃ concentration at a temperature of 25'C with iso-lines of NH ₄HC0₃ saturation according to an optional embodiment of the present invention.

Fig. 4 is a graph of NH₃ concentration versus rich C0₂ liquid loading at a temperature of 10C with iso-lines of the logarithm of C0₂ partial pressure according to an optional embodiment of the present invention.

Fig. 5 is a graph of NH₃ concentration versus rich C0₂ liquid loading at a temperature of 10C with iso-lines of NH ₄HC0₃ saturation according to an optional embodiment of the present invention.

Fig. 6 is a graph of NH₃ concentration versus rich C0₂ liquid loading at a temperature of 10C with iso-lines of the logarithm of NH ₃ partial pressure according to an optional embodiment of the present invention.

Fig. 7 is a graph of rich and lean C0₂ liquid loadings versus NH₃ concentration at a temperature of 25'C, a C0₂ partial pressure of 1 bar and a NH₄HC0₃ saturation of 100%, according to an optional embodiment of the present invention.

Fig. 8 is a graph of absorption solution flow rate versus the NH₃ concentration at a temperature of 25'C for several lean CO ₂ liquid loadings at constant C0₂ capture with operation windows according to an optional embodiment of the present invention.

Fig. 9 is a graph of NH₃ concentration versus absorption solution flow rate at a temperature of 100 for several lean C0₂ liquid loadings at constant C0₂ capture with operation windows according to an optional embodiment of the present invention.

Fig. 10 is a graph of NH₃ concentration versus absorption solution flow rate at a temperature of 25'C for several lean C0₂ liquid loadings at constant C0₂ capture according to an optional embodiment of the present invention.

Fig. 1 1 is a graph of NH₃ concentration versus absorption solution flow rate at a temperature of I OC for a lean C0₂ liquid loading of 0.4 at constant C0₂ capture with an operation window according to an optional embodiment of the present invention. Fig. 12 a graph of NH₃ concentration versus absorption solution flow rate at a temperature of ^0 for several lean C0₂ liquid loadings at constant C0₂ capture with operation windows according to an optional embodiment of the present invention.

Fig. 13 is a process flow diagram of a C0₂ capture process including absorption and desorption units according to an optional embodiment of the present invention.

While the invention will be described in conjunction with example embodiments, it will be understood that it is not intended to limit the scope of the invention to these embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included as defined by the appended claims. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to techniques for absorbing C0₂ from a C0₂-containing gas with an ammonia based absorption solution in combination with an enzyme.

Gaseous carbon dioxide (C0₂) is absorbed by the ammonia (NH₃) based absorption solution, thereby forming ammonium bicarbonate [NH₄ ⁺ ; HC0₃ ^"] in aqueous solution, according to the following equation (1):

C0₂ + H₂0 + NH₃ →· NH₄HC0₃ (1)

The formation of ammonium bicarbonate in solution involves the hydration of gaseous C0₂ into bicarbonate ions (HC0₃ ^") and hydrogen ions (H⁺), which may be catalyzed by an enzyme. In aqueous solution, equation (1) may therefore be written as follows:

HC0₃ ^" + H⁺ + NH₃ NH₄ ⁺ + HC0₃ ^" (2)

At solubility equilibrium, ammonium bicarbonate is in chemical equilibrium between its solid state NH HC0_{3 (S)} and its solute [NH ⁺ ; HC0₃ ^"] _(aq). The degree of saturation of the absorption solution may be expressed in percentage of the NH C0₃ solute (also referred as solubility product) at a given absorption solution composition. At solubility equilibrium, rates of dissolution and precipitation are equal to one another and the absorption solution is said to be saturated. As the dissolution of ammonium bicarbonate is endothermic, a rise of temperature increases the solubility (according to Le Chatelier's principle).

According to one optional aspect, the process comprises feeding the absorption solution into an absorber reactor, to contact the C0₂-containing gas so as to dissolve C0₂ from the C0₂-containing gas into the absorption solution in presence of the enzyme, thereby catalyzing the hydration of dissolved C0₂ into bicarbonate ions and hydrogen ions, and producing a gas stream and a liquid stream. The gas stream is a C0₂-depleted gas and the liquid stream is an ion-rich solution comprising bicarbonate ions and ammonium ions.

Referring to Fig. 1 , the overall C0₂ capture process 10 may include an absorption unit 12 and a desorption unit 14. The absorption unit 12 may include the absorber reactor 16 which receives the C0₂-containing gas 18 that can come from a variety of sources. In one aspect, the C0₂-containing gas 18 is an effluent gas such as power plant flue gas, industrial exhaust gas, aluminum refining flue gas, aluminum smelting off-gas, steel production flue gas, chemical production flue gas, combustion gas from in-situ oil sands production, etc. In another optional aspect, the C0₂-containing gas 18 is a naturally occurring gas such as ambient air. The absorber reactor 16 also receives the absorption solution 20. In the absorber reactor 16, the conversion of C0₂ into bicarbonate and hydrogen ions takes place in the presence of at least one enzyme or analogue thereof, thereby producing the C0₂-depleted gas 22 and the ion-rich solution 24. Preferably, the absorber reactor 16 is a direct-contact type reactor, such as a packed tower or spray scrubber or otherwise, allowing the gas and liquid phases to contact and mix together. The ion-rich solution 24 may be pumped by a pump 26 to downstream parts of the process, such as heat exchangers, desorption units, regeneration towers and the like. Part of the ion-rich solution 24 may be recycled back to the absorber reactor 16 via an ion-rich solution return line, which can improve mixing of the bottoms of the absorber reactor to avoid accumulation of precipitates and reactor deadzones, as the case may be. The absorber 16 may also have other recycle or return lines, as desired, depending on operating conditions and reactor design.

Optionally, the process may be operated according to process operating conditions including an absorption temperature, a NH₃ concentration in the ammonia based absorption solution, an absorption solution flow rate, a rich C0₂ liquid loading and a lean C0₂ liquid loading. It should be understood that the lean C0₂ liquid loading is defined as the ratio of C0₂ molarity over NH₃ molarity in the absorption solution entering the absorber, also referred to herein after as ai_ean- The rich C0₂ liquid loading is defined as the ratio of C0₂ molarity over NH₃ molarity in the ion-rich solution exiting the absorber, also referred to herein after as a_rich. Experiments and simulations have been conducted and have shown that the partial pressure of C0₂ at equilibrium in the ammonia based absorption is a function of the absorption temperature, the NH₃ concentration and the rich C0₂ liquid loading. Fig. 2 is a graph providing iso-lines of C0₂ partial pressure as a function of the NH₃ concentration and the rich C0₂ liquid loading at an absorption temperature of 25°C. For a given NH₃ concentration of the absorption solution, each iso-line provides the corresponding maximum rich C0₂ liquid loading at 25'C. For example, when the C0₂ partial pressure is 0.1 bar (10,000 Pa), the corresponding iso-line of C0₂ partial pressure is the iso-line with a value of 4. At a NH₃ concentration of 2.9 kmol/m³, the maximum rich C0₂ liquid loading is 0.7 at 25 .

Experiments and simulations have also shown that the NH₄HC0₃ saturation at equilibrium is a function of the absorption temperature, the NH₃ concentration and the rich C0₂ liquid loading. Fig. 3 is a graph providing iso-lines of NH HC0₃ saturation percentage as a function of the NH₃ concentration and rich C0₂ liquid loading at an absorption temperature of 25'C. For a given NH ₃ concentration of the absorption solution, each iso-line provides the corresponding maximum rich C0₂ liquid loading at 25'C. For example, when the saturation percentage i n the absorption solution is 50%, the corresponding iso-line has a value of 50. At a NH₃ concentration of 2.9 kmol/m³, the maximum rich C0₂ liquid loading is 0.62 at 25^tC.

Figs. 4 to 6 are graphs respectively providing iso-lines of C0₂ partial pressure, iso-lines of NH HC0₃ saturation and iso-lines of NH₃ partial pressure as a function of the NH₃ concentration and the rich C0₂ liquid loading at an absorption temperature of 10°C.

The maximum rich C0₂ liquid loading, a_rich, may be limited either by the C0₂ partial pressure in the absorption solution or the NH HC0₃ saturation at equilibrium depending on the NH₃ concentration. Fig. 7 is a graph based on a combination of the graphs of Figs. 2 and 3 for a C0₂ partial pressure of 1 bar and a NH HC0₃ saturation of 100%. Indeed, Fig. 7 provides the maximum a_rich at 25'C as a function of NH ₃ concentration based on the combination of a given C0₂ partial pressure (one iso-line of Fig. 2) and a given NH HC0₃ saturation at equilibrium (one iso-line of Fig. 3). Experiments showed that the NH₃ concentration has a transition value under which the maximum a_rich is limited by the given C0₂ partial pressure; and above which the maximum a_rich is limited by the given NH HC0₃ saturation. For example, Fig. 7 shows that under a transition value NH₃ concentration of 6.3 kmol/m³, the maximum a_rich is limited by the given C0₂ partial pressure and above 6.3 kmol/m³ the maximum a_rich is limited by the given NH4HCO₃ saturation. Fig. 7 also provides a series of operation lines for several lean C0₂ liquid loading (ai_ean)- The difference between the maximum rich and lean C0₂ liquid loading (a_riCh,max - ai_ean) varies according to the NH₃ concentration due to either C0₂ pressure limitation or NH4HCO₃ saturation limitation.

According to an optional aspect, the process may comprise controlling the absorption solution flow rate for operating near or at a maximum rich C0₂ liquid loading of the ion- rich solution at the exit of the absorber. The required absorption solution flow rate may be determined according to the following equation (1).

Ν_Η^ is the absorption solution flow rate in m³/kmol;

or_{ncft max} is the maximum rich C0₂ liquid loading;

a_lean is the lean C0₂ liquid loading of the absorption solution entering the absorber; and C_m is the NH₃ concentration in the absorption solution in kmol/m³.

Fig. 8 is a graphical representation of equation (1) showing the required absorption solution flow rate for attaining the maximum rich C0₂ liquid loading as a function of the NH₃ concentration for 5 different lean C0₂ liquid loadings ranging from 0.1 to 0.5. Each of the five curves includes a transition point T at which the maximum rich C0₂ liquid loading is the same for a given C0₂ partial pressure and a given NH4HCO₃ saturation. For example, in Fig. 7, the NH₃ concentration at the transition point T for each lean C0₂ liquid loading is 6.3 kmol/m³. At a NH₃ concentration lower than 6.3 kmol/m³, the required absorption solution flow rate is limited by the C0₂ partial pressure. At a NH₃ concentration higher than 6.3 kmol/m³, the required absorption solution flow rate is limited by the NH4HCO₃ saturation.

According to an optional aspect, the process may comprise controlling or providing process operating conditions according to an operation window enabling to maximize the rich C0₂ liquid loading, also referred to hereinafter as maximum C0₂ capture. Additionally, the operation window may be chosen so as to maximize the rich C0₂ liquid loading at low NH₃ concentrations. Optionally, the operation window may be chosen so as to maximize the rich C0₂ liquid loading at low NH₃ concentrations at which the maximum rich C0₂ liquid loading is limited by a given C0₂ partial pressure. Optionally, the operation window may be chosen so as to maximize the rich C0₂ liquid loading at low NH₃ concentrations at which the maximum rich C0₂ liquid loading is limited by a given NH₄HC0₃ saturation. Optionally, the operation window may be chosen so as to maximize the rich C0₂ liquid loading at low NH₃ concentrations at which the maximum rich C0₂ liquid loading is limited either by a given C0₂ partial pressure or a given NH₄HC0₃ saturation.

For a given absorption temperature, a given C0₂ partial pressure, a given NH₄HC0₃ saturation and a given lean C0₂ liquid loading, the operation window may be determined by an operating NH₃ concentration range and an operating absorption solution flow rate range enabling to maximize the rich C0₂ liquid loading. The operation NH₃ concentration range may include low NH₃ concentrations. Additionally, the operation window may also be defined for a given absorption temperature, a given C0₂ partial pressure, a given NH₄HC0₃ saturation and a given range of lean C0₂ liquid loading. The operation window defined for a given lean C0₂ liquid loading is referred to as W_a in the Figs.

Referring to Fig. 8, NH₃ concentration may be lowered without causing a significant increase of the solvent flow rate required to attain or come closer a maximum rich C0₂ liquid loading. For example, for an ai_ean of 0.3 at the entrance of the absorber, a variation of the NH₃ concentration from 6.3 kmol/m³ to 4 kmol/m³ implies a slight increase of the absorption solution flow rate from 0.5 m³/kmol to 0.7 m³/kmol so as to maintain a maximum rich C0₂ liquid loading. Optionally, the operation window may be defined for enabling to operate at low NH₃ concentrations without increasing the absorption solution flow rate above a targeted threshold.

Optionally, the NH₃ concentration may be at most 8 kmol/m³. The NH₃ concentration may be at most 6 kmol/m³ range between about 1 to about 8 kmol/m³. The NH₃ concentration may range from about 2 to about 4 kmol/m³. The lean C0₂ liquid loading may range from about 0.1 to about 0.5. The lean C0₂ liquid loading may range from about 0.2 to about 0.45. Optionally, the lean C0₂ liquid loading may be about 0.4. The absorption solution flow rate may range from about 0.5 to about 3 m³/kmol.

According to an optional aspect, for a given absorption temperature, the operation window may include a plurality of individual operation windows for each optional operating range of NH₃ concentration and absorption solution flow rate defined above. The operation window may also be defined to operate with a ratio of the operating NH₃ concentration range over the operating absorption solution flow rate range between 1 and 10. The operating NH₃ concentration range may or may not include the transition value of NH₃ concentration depending on the limitation imposed to the process, i.e. C0₂ partial pressure limitation, NH₄HC0₃ saturation limitation or a combination thereof. For example, in Fig. 8, the operating NH₃ concentration range of the operation window W₀.i does not include the transition value of NH₃ concentration of 6.3 kmol/m³. The maximizing of the rich C0₂ liquid loading for a lean C0₂ liquid loading of 0.1 at the entrance of the absorber is performed under a C0₂ partial pressure limitation only. At higher NH₃ concentrations, for example 8 kmol/m³, maximizing the rich C0₂ liquid loading would imply operating at an absorption solution flow rate of 0.25 m³/kmol, which is an inoperable absorption solution flow rate. For the same lean C0₂ liquid loading of 0.1 , operating within the operating window W₀.i at lower NH₃ concentrations, for example 3 kmol/m³, enables to maximize the rich C0₂ liquid loading with an acceptable value of flow rate of 0.6 m³/kmol.

According to another optional aspect, the process may include contacting the C0₂ containing gas with the ammonia based absorption solution in presence of a biocatalyst, such as an enzyme. The enzyme may include carbonic anhydrase and analogues thereof. The presence of a biocatalyst allows catalyzing the hydration of C0₂ into hydrogen ions and bicarbonate ions in the absorption solution. While operating within the operating window at low NH₃ concentrations, the size of the absorber may have to be adapted in accordance with the required absorption solution flow rate for maximizing the rich C0₂ liquid loading. When the required absorption solution flow rate is decreased, the size of the absorber may have to be increased accordingly. The enzyme may therefore be advantageously used so as to reduce the size of the absorber within the chosen operating window.

It should be noted that carbonic anhydrase and analogues thereof may include naturally occurring, modified or evolved carbonic anhydrase enzymes; and analogues thereof may be variants or non-biological small molecules that are naturally occurring or synthesized to achieve or mimic the effect of the enzyme.

According to another optional aspect, the process may include controlling solid formation of NH₄HC0₃ within the absorber. Referring to Figs. 9 and 10, the comparison of the two sets of curves shows that an increase of the absorption temperature (e.g. from 10Ό to 25Ό) induces an increase of the transition value of NH₃ concentration and a widening of the operating window W_a. As the NH₃ concentration is sufficiently low to benefit from the use of an enzyme, the absorption temperature may be chosen accordingly so as to preserve the activity of the enzyme. The absorption temperature may be further chosen so as to minimize the ammonia losses by evaporation and maximize the solubility of NH₄HC0₃. More particularly, the absorption temperature may range from about 0Ό to about 50Ό. Optionally, the absorption temperature may range from about 10Ό to about 30Ό. Alternatively, the absorption temperature may be 15Ό.

According to another optional aspect, the operation window may be defined with a varying operating absorption solution flow rate range. As better seen in Fig. 11 , for an operating NH₃ concentration range of 1 to 5 kmol/m³, the operating absorption solution flow rate may vary. For example, this latter range may be from 1.2 to 1.6 m³/kmol for 1 kmol/m³, and from 1 to 1.4 m³/kmol for 5 kmol/m³. This optional embodiment of the operation window may enable to operate in accordance with an operating absorption solution flow rate range which is within 0.2 m³/kmol above or below the corresponding absorption solution flow rate at a constant C0₂ capture. The operating absorption solution flow rate range could be within 0.5 m³/kmol to 0.1 m³/kmol above or below the corresponding absorption solution flow rate at a constant C0₂ capture, so as to operate near or at the maximum rich C0₂ liquid loading.

According to another optional aspect, a total operation window may include a plurality of operation windows in accordance with the plurality of lean C0₂ liquid loadings that are expected at the entrance of the absorber.

Referring to Fig. 12, several total operation windows W_T1, W_T2 and W_T3 are drawn around a transitional operation line L. The windows become smaller as they approach the transitional operation line L. One may choose to operate within a wider or smaller window depending on the selected operating ranges for the NH₃ concentration and the absorption solution flow rate. One may further choose to operate along the transitional line where the maximization of the rich C0₂ liquid loading is obtained for the same operating NH₃ concentration whatever the lean C0₂ liquid loading is, under C0₂ partial pressure limitation or NH HC0₃ saturation limitation. As seen in Fig. 1 1 , each total operation window of Fig. 12 may have a shape which varies so as to follow the trend of the curves according to the lean C0₂ liquid loading range. For a lean C0₂ liquid loading ranging from 0.30 to 0.45, W_T3 is defined by an operating absorption solution flow rate range which is within 0.50 m³/kmol above or below the corresponding absorption solution flow rate at a constant lean C0₂ liquid loading, and by an operating NH₃ concentration range which is within 1.5 kmol/m³ above or below the transition value of NH₃ concentration on the line L. W_T2 is defined by an operating solvent flow rate range which is within 0.35 m³/kmol above or below the corresponding absorption solution flow rate at a constant lean C0₂ liquid loading, and by an operating NH₃ concentration range which is within 1 kmol/m³ above or below the transition value of NH₃ concentration on the line L. W_T1 is defined by an operating absorption solution flow rate range which is within 0.15 m³/kmol above or below the corresponding absorption solution flow rate at a constant lean C0₂ liquid loading, and by an operating NH₃ concentration range which is within 0.50 kmol/m³ above or below the transition value of NH₃ concentration on the line L. The operation line L is defined by an operating absorption solution flow rate range which is within 0.05 m³/kmol above or below the transition value of the absorption solution flow rate. For instance for an operating NH₃ concentration of around 2.6 kmol/m³, the absorption solution flow rate at ai_ean = 0.45 is 1.10 m³/kmol and the corresponding upper limit of the operating absorption solution flow rate range is 1.60 m³/kmol for W_T3, 1.45 m³/kmol for W_T2, 1.25 m³/kmol for W_Ti and 1.15 m³/kmol for L. The process may be operated within an operation window which enables to provide or manage the absorption solution flow rate in accordance to the transitional value of NH₃ concentration at a given absorption temperature and a given range of lean C0₂ liquid loading. Optionally, the absorption temperature may be minimized so as to consequently lower the transitional value of NH₃ concentration and therefore operate at low NH₃ concentration.

According to another optional aspect, the process further includes regenerating the captured C0₂ as a C0₂ gas which can be separated for sequestration, storage or various uses.

Referring back to Fig. 1 , the ion-rich solution 24 may then be fed to the desorption unit 14, in which it can be regenerated. The ion-rich solution 24 is preferably heated, which may be done by one or more heat exchanger 32, to favor the desorption process. The heat exchanger may use heat contained in one or more downstream process streams in order to heat the ion-rich solution 32, e.g. ion-depleted solution 42. The heated ion-rich solution 34 is fed into a desorption reactor 36. In the desorption unit, the at least one enzyme or analogue thereof may be present within the ion-rich solution 34, allowing the at least one enzyme to flow with the ion-rich solution 34 while promoting the conversion of the bicarbonate ions into C0₂ gas 38 and generating an ion-depleted solution 40. The at least one enzyme could also be fixed or immobilized within reactors or particles passing within and/or through the reactors. Alternatively, the enzymes could also be removed from the ion-rich stream prior to feeding it to the desorption reactor 36. The process also includes releasing the C0₂ gas 38 and the ion-depleted solution 40 from the desorption unit 14 and, preferably, sending a recycled ion-depleted solution 42 to make up at least part of the absorption solution 20. A make-up stream 50 introducing the at least one enzyme further makes up the absorption solution 20. The ion-depleted solution 42 is preferably cooled prior to re-injection into the absorption unit, which may be done by the heat exchanger 32. The desorption reactor 36 may also include various recycle or return streams (not illustrated) as desired. The desorption unit 14 may also include one or more reboilers each of which takes a fraction of the liquid flowing through a corresponding one of the desorption reactors and heats it to generate steam that will create a driving force such that C0₂ will be further released from the solution.

According to another optional aspect, the process further includes managing a NH₄HC0₃ solids content in the ion rich solution for optimizing desorption. Referring to Fig. 13, the reduction of the size of the desorption unit 14 may result from the separation, in a separation unit 52, of the ion-rich solution 24 into a pure liquid stream 54 and a solid/liquid slurry 56. The pure liquid stream 54 has a relatively low C0₂ content and the solid/liquid slurry 56 has a high C0₂ content. As the volume of the solid/liquid slurry is smaller than the volume of the ion-rich solution 24, the size of the desorption reactor 36 is reduced (comparing to the process according to Fig. 1). This can result in lower energy costs without affecting the advantages of having high NH₃ levels in the desorption reactor, and therefore a low lean C0₂ liquid loading. Operating in the operation window as defined above enables to provide a controlled or known amount of NH HC0₃ solids in the desorption unit, for optimizing the size of this latter unit.

Embodiments or aspects of the above-mentioned processes and methods can provide several advantages, some of which are as follows; decrease of ammonia slip or loss in the absorber reactor; ability to operate the process at higher temperatures; optimization of the stripping performance based on the net energy consumption as an optimization parameter; better control of solids formation; less complicated process set-up and less process units.

It should be understood that any one of the above mentioned aspects of each process, method and solution may be combined with any other of the aspects thereof, unless two aspects clearly cannot be combined due to their mutually exclusivity. For example, the various operational steps of the processes described herein-above and/or in the appended Figures, may be combined with any of the method descriptions appearing herein and/or in accordance with the appended claims.

Claims

1. A method for reducing a size of C0₂ capture equipment for capturing C0₂ from a C0₂-containing gas with an ammonia based absorption solution at low ammonia concentrations, the method comprising:

enhancing the ammonia based absorption solution by adding at least one enzyme or analogues thereof for accelerating the hydration of C0₂ from the C0₂ containing-gas into the absorption solution, thereby reducing the size of the C0₂ capture equipment within the operation window.

2. The method of claim 1 , wherein the operation window is defined by operating conditions comprising an absorption temperature, a lean C0₂ liquid loading range of the absorption solution, an ammonia concentration range in the absorption solution and an absorption solution flow rate.

3. The method of claim 2, wherein the lean C0₂ liquid loading is between about 0.1 and about 0.5.

4. The method of claim 3, wherein the lean C0₂ liquid loading is between about 0.2 and about 0.45.

5. The method of claim 4, wherein the lean C0₂ liquid loading is between about 0.35 and about 0.4.

6. The method of any one of claims 2 to 5, wherein the absorption temperature is between about 0°C and about 50°C.

7. The method of claim 6, wherein the absorption temperature is between about 10°C and about 30°C.

8. The method of claim 7, wherein the absorption temperature is between about 15°C and about 25°C.

9. The method of any one of claims 2 to 8, wherein the ammonia concentration is between about 1 kmol/m³ and about 8 kmol/m³.

10. The method of claim 9, wherein the ammonia concentration is between about 2 kmol/m³ and about 5 kmol/m³ when the absorption temperature is about 10°C.

1 1. The method of claim 10, wherein the ammonia concentration is between about 2 kmol/m³ and about 3 kmol/m³.

12. The method of claim 9, wherein the ammonia concentration is between about 4 kmol/m³ and about 8 kmol/m³ when the absorption temperature is about 25°C.

13. The method of claim 12, wherein the ammonia concentration is between about 5 kmol/m³ and about 7 kmol/m³.

14. The method of claim 13, wherein the ammonia concentration is about 6 kmol/m³.

15. The method of any one of claims 2 to 14, wherein the absorption solution flow rate is between about 0.5 and about 3 m³/kmol.

16. The method of claim 15, wherein the absorption solution flow rate is between about 1 m³/kmol and about 2 m³/kmol.

17. The method of any one of claims 2 to 16, comprising managing an ammonium bicarbonate saturation percentage of the absorption solution for increasing or maximizing a rich C0₂ liquid loading in the absorption solution.

18. The method of claim 17, comprising managing a C0₂ partial pressure in the absorption solution for increasing or maximizing the rich C0₂ liquid loading in the absorption solution.

19. A process for removing C0₂ from a C0₂ containing gas, the process comprising: determining an operation window enabling operation near or at an elevated or maximized rich C0₂ liquid loading, the operation window being defined by an operating ammonia concentration range and an operating absorption solution flow rate range, at a given absorption temperature and a given lean C0₂ liquid loading range;

providing an ammonia based absorption solution having an ammonia concentration selected in the operating ammonia concentration range and having a C0₂ liquid loading selected in the lean C0₂ liquid loading range;

contacting the ammonia based absorption solution with the C0₂ containing gas in an absorber reactor, the absorption solution having a flow rate selected in the operating absorption solution flow rate range; operating the absorber reactor in the operation window in presence of at least one enzyme or analogue thereof sufficient to accelerate the hydration reaction of C0₂ into the absorption solution and thereby reduce equipment sizing within the operation window;

20. The process of claim 19, wherein the enzyme comprises a recombinant enzyme, a variant enzyme or a naturally occurring enzyme or a combination thereof.

21. The process of claim 20, wherein the enzyme is derived from archeal source enzyme, bacterial source enzyme or fungal source enzyme or a combination thereof.

22. The process of claim 20 or 21 , wherein the enzyme is carbonic anhydrase.

23. The process of any one of claims 19 to 21 , wherein the operation window is defined for the given absorption temperature and the given lean C0₂ liquid loading by:

the operating ammonia concentration range between about 1 kmol/m³ and 8 kmol/m³; and

the operating absorption solution flow rate range wherein the absorption solution flow rate is within 0.5 m³/kmol above or below a corresponding solvent flow rate at the given lean C0₂ liquid loading.

24. The process of any one of claims 19 to 21 , wherein the operation window is defined for the given absorption temperature and the given lean C0₂ liquid loading by:

25. The process of claim 24, comprising determining the transitional ammonia concentration at a transition point for the given absorption temperature and the given lean C0₂ liquid loading.

26. The process of any one of claims 1 to 25, wherein the operation window is predetermined, calculated, estimated and/or determined during start-up, turndown or regular operation.

27. The process of claim 26, wherein the operation window is based on models, simulations, experiments or representations or a combination thereof.

28. A method of using low ammonia concentrations in an ammonia based C0₂ capture process, the method comprising:

29. The method of claim 28, comprising using at least one enzyme or analogue thereof for accelerating the hydration reaction of C0₂ into the absorption solution and thereby reduce equipment sizing within the operation window.

30. The method of claim 28 or 29, wherein the enzyme comprises a recombinant enzyme, a variant enzyme or a naturally occurring enzyme or a combination thereof.

31. The method of claim 30, wherein the enzyme is derived from archeal source enzyme, bacterial source enzyme or fungal source enzyme or a combination thereof.

32. The method of claim 30 or 31 , wherein the enzyme is carbonic anhydrase.

33. The method of any one of claims 28 to 32, wherein the operation window is defined for the given absorption temperature and the given lean C0₂ liquid loading by:

34. The method of any one of claims 28 to 32, wherein the operation window is defined for the given absorption temperature and the given lean C0₂ liquid loading by:

35. The method of claim 34, comprising determining the transitional ammonia concentration at a transition point for the given absorption temperature and the given lean C0₂ liquid loading.

36. The method of any one of claims 28 to 35, wherein the operation window is predetermined, calculated, estimated and/or determined during start-up, turndown or regular operation.

37. The process or method of claim 25, wherein the operation window is based on models, simulations, experiments or representations or a combination thereof.

38. A method for maximizing absorption of C0₂ from a C0₂-containing gas using an ammonia based absorption solution, the method comprising:

determining a maximum rich C0₂ liquid loading in the ammonia based absorption solution in accordance with given operating conditions;

selecting an absorption solution flow rate in accordance with the maximum rich C0₂ liquid loading; and circulating the ammonia based absorption solution at the absorption solution flow rate for contacting the C0₂-containing gas and absorbing the C0₂ there from according to the maximum rich C0₂ liquid loading.

39. The method of claim 38, wherein the selecting of the absorption solution flow rate is performed according to the following equation:

. _ 1

9NH₃ ^~ Γ ( _ \ wherein, Ν_Ηί is the absorption solution flow rate in m³/kmol;

cx_{rich max} is the maximum rich C0₂ liquid loading;

o_lean is the lean C0₂ liquid loading of the absorption solution; and

C_m is the NH₃ concentration in the absorption solution in kmol/m³.

40. The method of claim 38 or 39, wherein the given operating conditions comprise a C0₂ partial pressure in the absorption solution, an absorption temperature, a lean C0₂ liquid loading range of the absorption solution, an ammonia concentration range in the absorption solution and an absorption solution flow rate.

41. The method of claim 40, wherein the lean C0₂ liquid loading is between about 0.1 and about 0.5.

42. The method of claim 41 , wherein the lean C0₂ liquid loading is between about 0.2 and about 0.45.

43. The method of claim 42, wherein the lean C0₂ liquid loading is between about 0.35 and about 0.4.

44. The method of any one of claims 40 to 43, wherein the absorption temperature is between about 0°C and about 50°C.

45. The method of claim 44, wherein the absorption temperature is between about 10°C and about 30°C.

46. The method of claim 45, wherein the absorption temperature is between about 15°C and about 25°C.

47. The method of any one of claims 40 to 46, wherein the ammonia concentration is between about 1 kmol/m³ and about 8 kmol/m³.

48. The method of claim 47, wherein the ammonia concentration is between about 2 kmol/m³ and about 5 kmol/m³ when the absorption temperature is about 10°C.

49. The method of claim 48, wherein the ammonia concentration is between about 2 kmol/m³ and about 3 kmol/m³.

50. The method of claim 47, wherein the ammonia concentration is between about 4 kmol/m³ and about 8 kmol/m³ when the absorption temperature is about 25°C.

51. The method of claim 50, wherein the ammonia concentration is between about 5 kmol/m³ and about 7 kmol/m³.

52. The method of claim 51 , wherein the ammonia concentration is about 6 kmol/m³.

53. The method of any one of claims 40 to 52, wherein the absorption solution flow rate is between about 0.5 and about 3 m³/kmol.

54. The method of claim 53, wherein the absorption solution flow rate is between about 1 m³/kmol and about 2 m³/kmol.

55. The method of any one of claims 40 to 54, comprising managing an ammonium bicarbonate saturation percentage of the absorption solution for reaching the maximum rich C0₂ liquid loading in the absorption solution.

56. The method of any one of claims 40 to 55, comprising managing a C0₂ partial pressure in the absorption solution for reaching the maximum rich C0₂ liquid loading in the absorption solution.

57. A process for capturing C0₂ from a C0₂ containing gas with an ammonia based C0₂ capture system, the process comprising:

using an ammonia based absorption solution with low ammonia concentration for capturing C0₂ from the C0₂-containing gas; and

58. The process of claim 57, comprising controlling or reducing an absorption solution flow rate for operating near or at a maximum C0₂ capture at low ammonia concentrations.

59. The process of claim 57 or 58, comprising controlling the ammonia concentration, an absorption temperature, a lean C0₂ liquid loading or an absorption solution flow rate or a combination thereof, to reduce or avoid formation of precipitated solids in the absorption solution.

60. An ammonia based absorption solution for use in a C0₂ capture system, the absorption solution having an ammonia concentration sufficiently low as to be enhanced by the presence of an enzyme while reaching an enhanced or maximized C0₂ liquid loading.

61. The ammonia based absorption solution of claim 60, wherein the enzyme comprises a recombinant enzyme, a variant enzyme or a naturally occurring enzyme or a combination thereof.

62. The ammonia based absorption solution of claim 61 , wherein the enzyme is derived from archeal source enzyme, bacterial source enzyme or fungal source enzyme or a combination thereof.

63. The ammonia based absorption solution of claim 61 or 62, wherein the enzyme is carbonic anhydrase.