METHOD FOR RECOVERY OF CO2 FROM CHEMICAL SOLVENTS AND INTEGRATION
WITH SUBSEQUENT PROCESSES
TECHNICAL FIELDThe present invention concerns a method for removal of carbon dioxide (CO2) from a chemical solvent such as mono-ethanolamine (MEA). In particular, the method enables recovery of captured
CO2 from chemical solvents with low energy consumption, at low temperatures, compared to established processes. The method may reduce release of CO2 to the atmosphere.
BACKGROUND ARTAn established method for removal of CO2 (and other acid gases such as H2S) from a gas stream relies on the use of chemical solvents (such as mono-ethanolamine MEA) to absorb the acid gas.
The solvent is regenerated, and the acid gas recovered by desorption. The known method typically involves an absorber unit and a regenerator unit to which other equipment may be added, as needed. In the absorber, the amine solvent flows down countercurrent to - and in direct contact with - the gas stream, from which the CO2 is absorbed. The solvent, enriched with the acid gas, is then routed into a regeneration unit, in which the acid gas is stripped from the solvent. The solvent may be recycled to the absorber. In this method, an acid gas stream with a high concentration of
CO2 (close to pure) may be produced. Typically, the CO2 is subsequently compressed or liquefied for convenient transport, storage and further use.
The known method is characterized by a high energy consumption, typically 3-4 GJ per ton of
CO2 recovered, inherent to the energy required to reverse the chemical absorption reaction between solvent and acid gas. The energy must be provided at a sufficiently high temperature level, and the regenerator is typically operated at a temperature of 120-130°C. In addition, the known method needs considerable power for pumping and compression.
If the conventional chemical absorption/desorption method is used for removal of CO2 from flue gas, the high energy demand may even increase the amount of flue gas from which the CO2 must be captured. The energy consumption of an integrated chemical absorber/stripper system producing pure and pressurized CO2 can be reduced by for instance optimizing the type of chemical solvent, its concentration, the operating conditions and also by heat integration.
Obtaining high purity CO2 at increased pressure generally requires high stripper temperatures.
It is an aim of the present invention to provide a method for removal of CO2 from a chemical solvent with a low consumption of energy and at low temperature, at lest compared to the known methods.
GENERAL DISCLOSURE OF THE INVENTIONThe present invention thereto provides a method as claimed in claim 1. A method is provided for stripping of carbon dioxide (CO2) from a sorbent liquid, the method comprising: a) providing an enriched sorbent liquid, such as an amine solvent into which CO2 has been absorbed, in a transportable and storable containment; b) providing a stripping gas. comprising hydrogen or nitrogen, for removing the absorbed
CO2 from the enriched sorbent liquid; c) stripping the absorbed CO2 from the enriched sorbent liquid by the stripping gas so as to obtain a regenerated lean sorbent liquid and a CO2 containing gas; d) using the CO2 containing gas and the stripping gas as feedstock for subsequent chemical conversion or fuel producing steps: or e) recycling the stripping gas via a gas membrane separation step to yield a high purity
CO2 stream.
The invented method needs a relatively low consumption of energy and may be carried out at a relatively low temperature. The recovered CO2 is obtained as a mixture with the stripping gas. i.e. with hydrogen or nitrogen. In one embodiment the CO2 containing gas mixture may be supplied to subsequent chemical conversion processes to produce synthetic fuels or chemicals for instance.
Input to the method or process is so-called “rich” solvent, by which is denoted a chemical sorbent loaded with carbon dioxide (CO2). The solvent is for example (but not limited to) a mixture of mono-ethanolamine (MEA) and water. The CO2 may be captured from for example (but not limited to) flue gas produced by combustion processes. The “lean” solvent, defined as the chemical sorbent from which CO2 has been stripped. is discharged from the process and may be re-used for capture of CO2, thereby closing the solvent cycle.
The sorbent liquid may be contained and transported in exchangeable containments, such as containers. These can also be stored. One advantage of the invention is that capture of CO2 and recovery/re-use of the CO2 can be uncoupled with regard to the rate of CO2 capture, rate of recovery, capture time and capture location, and recovery time and location.
Particularly preferred is to use renewable hydrogen to strip the CO2 from the solvent. The mixture of carbon dioxide CO2 and hydrogen H2 discharged from the desorption process may be supplied to subsequent users, at suitable conditions for e.g. conversion to synthetic fuels or chemicals.
Alternatively, nitrogen N2 is used to strip the CO2 from the solvent, and the nitrogen may then be recycled via a gas membrane separation step, where a mixture of a highly concentrated CO2 stream and trace percentages of nitrogen is discharged from the desorption process.
A method according to an embodiment is provided wherein the stripping gas is hydrogen and the
CO2 containing gas, together with the hydrogen, is supplied to a chemical conversion process comprising at least one of - CO2 conversion to methanol CO2 + 3 H2 -> CH30H + 2 H20; - CO2 conversion to methane by a Sabatier process CO2 + 4 H2 => CH4 + 2 H20; - CO2 conversion to CO by a reverse water gas shift reaction CO2 + H2 -> CO + H20; and - CO2 based Fischer-Tropsch synthesis to produce C2+ products such as (iso)paraffins: nCO2 + (3n+1)H2 -> CnH2n+2 + 2nH20.
A method as claimed in a preferred embodiment comprises obtaining the hydrogen stripping gas from electrolysis of water. Such electrolysis is known per se and splits water into oxygen and hydrogen under the action of electricity.
Preferably. an embodiment of the method is provided wherein the electrolysis of water uses electricity provided from renewable sources, such as wind, hydrological and solar sources.
According to this embodiment, the CO? is stripped from the rich solvent by the hydrogen in a stripper. The stripper discharges a gaseous mixture of CO2 and hydrogen, the CO2 containing gas.
The CO2 containing gas is suitable as a feedstock for a range of conversion processes to produce fuels and chemicals, as listed above. The invention contributes to the re-use of CO2 produced by combustion of fossil fuels. The invention can also be part of. and improve efficiency of, a closed
CO2 cycle which may consist of combustion of synthetic fuels, capture of the CO2. conversion of the CO2 with hydrogen to produce new synthetic fuels. The net input to the cycle is energy from sustainable sources.
Conversion of CO2 by hydrogenation is challenging since the CO2 molecule is thermally very stable. This typically results in high temperatures and pressures required for the conversion, as well asa need for advanced catalysts, and low conversions and/or selectivity. Still, considerable progress is and has been made in the conversion of CO2. into products with a single carbon atom in its molecules (methanol, methane. carbon monoxide, formic acid), but also longer chain molecules (olefins, (iso}paraffins).
As already listed above, examples of such processes comprise but are not limited to: - CO2 conversion to methanol (CH3OH) has already been industrialized in Reykjavik, Iceland using heterogeneous catalysis and geothermal energy CO2 + 3 H2 -> CH30H + 2 H20. - Carbon dioxide conversion to methane (synthetic natural gas) with hydrogen using the
Sabatier process. This process according to CO2 + 4 H2 > CH4 + 2 H2O requires a catalyst, temperatures in range 300-400°C and pressures of order 30 bar. Recent developments have demonstrated that the hydrogenation of CO2 to methane can also be achieved at near atmospheric pressure and low temperatures, with conversion of CO2 and selectivity towards CH4 close to thermodynamic equilibrium levels, with suitable types of catalyst. - Hydrogenation of CO2 to CO can be achieved by the “reverse” water gas shift reaction: CO2 + H2 -> CO + H20. So-called syngas. a mixture of CO and H2, is the feed for a wide range of well-developed conversion processes to produce chemicals, such as fertilizers.
- Fischer-Tropsch synthesis based hydrogenation of CO2 to produce C2+ products such as light olefins and (iso)paraffins: nCO2 + (3n+1)H2 -> CnH2n+2 + 2nH20.
Yet another useful embodiment relates to a method wherein the CO2 containing gas is further separated from the stripping gas by a membrane system, and discharging from said membrane system a substantially pure CO2 stream and a substantially pure stripping gas, and recycling said substantially pure stripping gas to the stripper. This embodiment is particularly preferred for end- users requiring substantially pure CO2, whereto the membrane separation system is integrated in the method. The stripping gas. for instance hydrogen, is separated and remains in a closed loop by
IO re-circulation to the stripper.
Alternatively, the invented method uses nitrogen. This can be produced by (for example, but not limited to} gas separation of air by membranes. In this embodiment, the CO2 is stripped from the rich solvent by the nitrogen in a stripper. The stripper discharges a gaseous mixture of CO2 and nitrogen, the CO2 containing gas. This gaseous mixture of CO2 and nitrogen discharged from the stripper is in an embodiment carried through a membrane unit. Therein, the stripping gas is separated from the CO2 where after the pure stripping gas may be recycled to the stripper. Make- up stripping gas (hydrogen, preferably nitrogen) may be added to compensate for eventual losses of stripping gas. A substantially pure CO2 stream is discharged from the membrane unit for supply to users that require high purity CO2. This high purity CO2 is suitable as a feedstock for a large range of product applications or carbon sequestering applications.
The invented method is particularly advantageous in that it may consume a limited amount of energy only, relative to known methods.
In a useful embodiment of the invention therefore. a method is provided wherein the stripping step c) is performed at a temperature below 100°C, more preferably below 70°C, even more preferably below 40°C, and most preferably at ambient temperature. This save considerable energy, yet vields a CO2 containing gas in an efficient manner.
Pressures in the stripping step may also be chosen relatively low. In a useful embodiment, a method is provided wherein the stripping step ¢) is performed at ambient pressure.
The stripping step may be carried out by using conventional stripping units, known to a person skilled in the art. Preferably however, the stripping step c) is carried out with a stripper unit without a reboiler and/or condenser unit. This increases efficiency even further.
BRIEF DESCRIPTION OF THE FIGURESThe above description, as well as other objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of presently preferred, but nonetheless illustrative embodiments and arrangements, when taken in conjunction with the accompanying figures. In the figures:
Fig. 1 is a schematic representation of a method of CO2 desorption from a “rich” liquid sorbent according to an embodiment of the invention; and
Fig. 2 is a schematic representation of a method of CO2 desorption from a “rich” liquid sorbent according to another embodiment of the invention.
DETAILED DISCLOSURE OF THE INVENTIONOne element of the present invention is the use of rich sorbent as a means to transport and store captured CO2. Capture and recovery of the carbon dioxide CO2 may be separated in rate, place and time. An advantage of the invention is that this may considerably simplify size, weight and operation of equipment at the CO2 capture location. Further, it may reduce costs at the CO2 capture location which can be for example (but not limited to) off-shore on a ship or on-shore at a combustion facility. Recovery and subsequent use of CO2 may be carried out at the most suitable rate, location and time according to the demand of the subsequent user. Users may comprise chemical processes. for example (but not limited to) conversion to synthetic natural gas, methanol or carbon monoxide.
A scheme of an embodiment of a system by which the method may be carried out is shown in Fig. 1. The shown system in short provides a sorbent liquid in an exchangeable container. The sorbent is carried through a stripper and contacted with renewable hydrogen gas. Heating the sorbent upstream the stripper is optional. The “lean” liquid sorbent is discharged into another exchangeable container. Renewable hydrogen is provided by electrolysis of water. The gaseous mixture of CO2 and hydrogen discharged from the stripper is further conditioned, e.g. compressed, in a subsequent step (6) and then supplied to a CO2 hydrogenation process (7) for conversion into synthetic fuels or chemicals (stream S7).
The system in particular comprises a stripper (1), a liquid container (2) provided with “rich” sorbent liquid, a container (3) provided with “lean” sorbent liquid, an optional solvent heater (4), a water electrolysis system (5), a feed preparation unit (6) and a CO2 conversion process (7).
Suitable sorbents are for example (but not limited to) MEA, DEA or Piperazine. The “rich” solvent (stream S1) is carried through the optional heater (4) and made to flow through the stripper (1).
The conditions in the stripper (1) with regard to gas-liquid contact, temperature and pressure are selected such that part or most of the CO2 present in the solvent (S1} is transferred to the stripping gas (53). In an embodiment, temperature and pressure are both ambient. The stripping gas (stream 83) in the embodiment shown is renewable hydrogen obtained from a water electrolysis system (5). The system uses water (stream S8) and electric power (stream S9) from renewable sources such as (but not limited to) wind and solar sources. A mixture of hydrogen and CO2 is discharged from the stripper (stream S35). The “lean” solvent (stream S2) 1s discharged from the stripper (1) to the container (3) with lean solvent.
As will be obvious to persons skilled in the art, the stripper (1) may be fitted with packing material for improvement of gas-liquid contact area and mass transfer coefficients, at the same time keeping pressure drop as low as possible. The stripper (1) may further be fitted with provisions to avoid liquid carry-over with the gas stream (S5) leaving at the top. The stripper (1) is preferably operated with counter-current flow of stripping gas and sorbent liquid. The optional sorbent heater (4) supplies energy to account for the endothermic dissociation reaction releasing CO2.
One element of the present invention is that the stripping can be done at low (near ambient) pressure which in combination with the low partial pressure of CO2 in the stripping gas enhances release of CO2 from the chemical solvent. The stripping temperature may be well below the temperatures of up to 120-130°C of conventional strippers. This has the advantage that low temperature heat sources can be utilized to provide the energy for the endothermic desorption reaction. A further advantage is that solvent degradation, contamination and evaporation is minimized.
The gaseous stripping medium, a mixture of hydrogen and CO2, leaves the stripper as stream S5.
The mixture of hydrogen and CO2 can be conditioned (H2/CO?2 ratio trim, washing. contaminant removal, compression) in step (6) before supply to a subsequent chemical conversion process.
The mixture of hydrogen and CO? is then supplied to a chemical conversion process (7) in which the gas mixture is converted to for example (but not limited to) carbon monoxide, methanol or synthetic natural gas (and water as a by-product).
By using hydrogen obtained from electrolysis of water, employing electricity from sustainable sources such as (but not limited to) wind and solar, the stripping process can be integrated into a closed cycle for combustion of carbon based synthetic fuels; with subsequent CO2 capture, recovery and conversion to synthetic fuels using hydrogen. The energy efficiency of the CO2 recovery process contributes to minimizing the carbon footprint of such a cycle. The hydrogen introduced into the cycle provides per effect the energy input, and is obtained from sustainable sources.
Another embodiment of a system for recovery of CO2 from chemical solvents is shown in Fig. 2.
In short, a sorbent liquid is provided in an exchangeable container. The sorbent is carried through a stripper and contacted with recycled stripping gas, which is hydrogen or nitrogen. Heating the sorbent upstream the stripper is optional. The “lean” liquid sorbent is discharged into another exchangeable container. The gaseous mixture of CO2 and stripping gas discharged from the stripper is carried through a membrane unit. The stripping gas is separated from the CO2 and recycled to the stripper. Make-up stripping gas hydrogen or Nitrogen can be added to compensate for eventual losses. CO2 is discharged from the membrane unit for supply to users that require high purity CO2.
The system in particular comprises a stripper (1), a liquid container (2) provided with “rich” sorbent liquid, a container (3) provided with “lean” sorbent liquid, an optional solvent heater (4), a gas circulation blower (9) and a membrane system (8). As with the embodiment shown in figure 1, suitable sorbents are for example (but not limited to) MEA, DEA or Piperazine. The “rich” solvent (stream SI) is carried through the optional heater (4) and made to flow through the stripper (1).
The conditions in the stripper (1) with regard to gas-liquid contact, temperature and pressure are such that part or most of the CO2 present in the rich solvent (stream S1) is transferred to the stripping gas (stream S3). The “lean” solvent (stream S2) is discharged from the stripper (1) to the container with lean solvent (3). A mixture of the stripping gas and the recovered CO2 is discharged at the top of the column (stream S35), and carried by blower (9) through the membrane system (8).
The membrane system removes the stripping gas from the gas mixture; and the stripping gas (stream S11) is recycled to the stripper. The substantially pure CO2 (stream S10) is discharged to the subsequent user. A small make-up stream of stripping gas (stream S12) can be supplied to the stripping gas circuit to compensate for losses.
The arrangement of Fig. 2 enables recovery of captured CO2 in a pure form, with minimized energy consumption. The heat supplied to stimulate CO2 release from the solvent is at low temperature level. Hydrogen or nitrogen can be used as the stripping gas. The stripping gas can be effectively separated by a membrane system and operated in a closed loop as a carrier for the recovered CO2.
Pending the requirements of the end-user of the substantially pure CO2 (stream S10), and costs/availability of nitrogen gas in comparison to hydrogen, and costs of the membrane system, nitrogen can be an attractive stripping medium.