WO2008153379A1

WO2008153379A1 - Method and device for separating co2 from a flue gas or synthesis gas mixture from fossil and biomass-fuelled processes

Info

Publication number: WO2008153379A1
Application number: PCT/NL2008/050323
Authority: WO
Inventors: Raichel Elton Taciano Leito; Jozef Johannes Hubertus Brouwers
Original assignee: Romico Hold A.V.V.
Priority date: 2007-05-29
Filing date: 2008-05-28
Publication date: 2008-12-18
Also published as: NL2000665C2

Abstract

The invention relates to a method for separating CO2 from a flue gas or synthesis gas mixture from fossil and biomass-fuelled processes. The method comprises the processing steps of providing a CO2-containing gas mixture at a starting pressure and a starting temperature, cooling the gas mixture to a final temperature and a final pressure at which at least one of the fractions present is present at least partially in the liquid phase in the gas mixture, subjecting the thus resulting medium mixture to a volume force, and discharging at least the CO2 as one of the separated fractions. The invention also comprises a device for separating CO2 from the gas mixture.

Description

Method and device for separating CO2 from a flue gas or synthesis gas mixture from fossil and biomass-fuelled processes

The invention relates to a method and device for separating CO₂ and optionally other substances from a flue gas or synthesis gas mixture from fossil and biomass-fuelled processes.

Flue gases and synthesis gases are created in respectively the combustion and the chemical conversion of fossil and biomass fuels. Synthesis gases are increasingly applied in, among others, the chemical industry, for instance as power supply. In the combustion and/or chemical conversion of fossil and biomass fuels there is often a relatively high emission of harmful gases, such as for instance the greenhouse gas CO₂. Through ratification of the Kyoto Protocol additional obligations have been imposed on many countries to reduce the emission of greenhouse gases, and CO₂ in particular. It is to be expected that the threat of climate change will result in even more stringent requirements in respect of this emission. For this and other reasons the energy market is in a state of great turmoil at the moment. Coal is thus for instance gaining importance again as a fossil fuel. Because of the competitive cost price of coal compared to other fossil fuels, growth economies such as China and India are at the moment making massive investment in the construction of new coal-fired power plants.

The use of fossil fuels has drawbacks. The flue gases of classic coal-fired power plants contain high concentrations of SOχ, NOχ, soot particles and dust particles. Furthermore, large quantities of CO₂ are generally released in the combustion of fossil fuels. Technologies have meanwhile become available for the removal of SOχ, NO_x, soot particles and dust particles. The so-called Integrated Gasification Combined Cycle (IGCC) technology has thus been developed, wherein the coal is not combusted but is converted into so-called synthesis gas at high pressure and temperature via a gasification process. The synthesis gas comprises on average about 30 mol% H₂, 65 mol% CO, 3 mol% N₂, 1 mol% H₂O and 1 mol% CO₂. The contaminants are removed from the synthesis gas by cooling it to a temperature in the order of magnitude of 10O⁰C, wherein steam is produced. The sulphur, nitrogen and soot particles are then removed with the usual low-temperature techniques. The separation OfCO₂ from flue gases and/or synthesis gases has heretofore been found to be time-consuming. In the above described IGCC for instance a decarbonization of the synthesis gas is performed. CO from the synthesis gas is converted with steam via a so-called CO-shift reaction into a gas mixture comprising H₂ and CO₂ (wherein several mol% N₂ may also be present). A gas mixture with a high concentration of CO₂ is thus obtained. A gas mixture of 40-50 mol% H₂, 40-50 mol% CO₂ and several mol% N₂ is typical. A physical adsorption technique is then applied thereto using membranes. Physical separation techniques are less economic, certainly when processing large volumes of gas mixture. It has moreover been found that when membrane technology is used the total CO₂ recovery is generally no higher than between 50% and 75% of the quantity present. The recovered CO₂ is further often impure, wherein more than 5 mol% of other molecules are present in the recovered CO₂ mixture. The membrane technology is also expensive, among other reasons because it consumes a great deal of energy. In addition, operation usually takes place in multiple steps in the known membrane separation, wherein the gas mixture for purifying must be guided through a cascade of membranes in order to obtain a fraction with high purity.

The present invention has for its object to provide a method and device for separating CO? from a gas mixture which has an increased selectivity for the CO₂ to be separated, and which also enables a rapid separation.

The invention provides for this purpose a method as according to the preamble, which is further characterized in that it comprises the processing steps of:

A) providing a CO₂-containing flue gas or synthesis gas mixture from fossil and biomass-fuelled processes at a starting pressure and a starting temperature,

B) cooling the gas mixture to a final temperature and a final pressure at which at least one of the fractions is present at least partially in the liquid phase in the gas mixture,

C) subjecting the thus resulting medium mixture to a volume force, and

D) discharging at least the CO₂ as one of the separated fractions.

Surprisingly, a higher selectivity for the CO₂ fraction is achieved relative to the known method by ensuring according to the invention that at least one of the fractions is present at least partially, and preferably substantially wholly, in the liquid phase in the gas mixture, and then subjecting this mixture to a volume force. In other words, the CO₂ is separated in a purer form than heretofore known. It has been found that with the method according to the invention a separated CO₂ mixture can be obtained with a CO₂ content of at least 90 mol%, preferably at least 95 mol%, and most preferably at least 99 mol%.

It is noted that a method for separating a medium mixture into fractions is known from, among others, WO 2005/11811OA. In the known method a medium mixture is provided which is cooled to a final temperature and a final pressure at which at least one of the fractions present is present at least partially in the liquid phase in the medium mixture. The thus resulting medium mixture is subjected to a volume force, wherein separation occurs. The separated fractions are then discharged. WO 2005/1 1811OA describes an example in which a contaminated natural gas is purified of, among other gases, CO₂. It cannot be inferred from WO 2005/1181 1OA that the method described therein is specifically suitable for separating CO₂ from flue gas or synthesis gas mixtures from fossil and biomass-fuelled processes. Such gases do after all originally contain no CO?, or only a small quantity thereof.

It is advantageous to characterize the method according to the invention in that the gas mixture is cooled to a final temperature at which the CO₂ is present at least partially, and more preferably substantially wholly, in the liquid phase in the gas mixture.

Because the separated CO₂ fraction is liquid and can moreover have a high degree of purity, this fraction can be readily pumped over relatively large distances. The pumping of the liquid CO₂ fraction can take place with little energy loss, wherein the temperature is moreover easily held at the desired low level. A further advantage of the method according to the present invention is that the energy consumption thereof is exceptionally low.

In a further preferred embodiment of the invented method a Cθ₂-containing flue gas or synthesis gas mixture from fossil and biomass-fuelled processes is provided by converting CO from a flue gas or synthesis gas by means of a CO-shift reaction with steam. A gas mixture is thus obtained with a high concentration of CO₂, from which the CO₂ can be separated with high efficiency. According to the invention the present invention can further be applied for separating other substances (in addition to CO₂) possibly present in the gas mixture. In a number of industrial processes a gas mixture is thus obtained of at least CO₂ and the notorious hydrogen cyanide gas (HCN). Such a mixture can be separated into at least two fractions (CO₂ and HCN) using the invented method and device. Gas mixtures of CO₂ and HCN, or cyanides in general, occur for instance in (pyro)metallurgical processes in which metal oxides are purified in the melt through heating with coke or coal. At least a part of all contaminants can enter the atmosphere, for instance via the cooling tower. In addition to said gases, metallurgical deposition can also comprise heavy metals such as for instance cadmium, mercury, lead and zinc. Existing gas treatment methods such as the known scrubbing of contaminants (wet gas scrubbing) are often found to be time- consuming and economically not sufficiently cost-effective. If the method is applied to such an HCN-containing gas mixture, the method according to the invention is then preferably characterized in that the gas mixture is cooled to a final temperature at which the HCN is present substantially in the liquid phase in the gas mixture. The CO₂ present in the gas mixture will preferably be in the gas phase at this final temperature. It is not therefore necessary according to the invention for the CO₂ to be in the gas phase during step C) of the method, this can also be a different fraction of the gas mixture.

The temperature decrease to the final temperature can for instance be achieved by feeding the gas to active or passive cooling means. Although not essential according to the invention, it is advantageous when the temperature is reduced isobarically, therefore at almost constant pressure. Because the temperature is reduced according to the invention to below the temperature at which at least a part of at least one of the fractions present in the gas mixture is liquid, the gas mixture as a whole will undergo a phase separation. By cooling the gas mixture before starting the separation at least one of the fractions present in the gas mixture, and preferably the CO₂ fraction, will undergo a phase change from gas to liquid, wherein possible other constituents remain in the gaseous phase. A medium mixture is in this way created with a gaseous matrix incorporating liquid droplets. It has been found that CO₂ can be can be separated with increased selectivity from such a mixture.

The method according to the invention is preferably characterized in that prior to step B) the gas mixture is brought in a step E) to a pressure which is increased relative to the starting pressure, and the gas mixture is cooled in step B) to the final temperature by being subjected to adiabatic and/or isentropic expansion. The expansion can be performed according to the invention in any per se known expansion means suitable for the purpose. By means of expansion the temperature of a medium can be decreased within a very short period of time. Expansion can optionally be realized by applying an expansion cooler of the "Joule Thomson" type. The gas mixture is cooled isenthalpically in such an expansion cooler, whereby the pressure can be decreased relatively independently of the temperature. Another possibility is for the cooling to be brought about by a cooling medium, which is expanded for instance in a separate circulation system so as to be thus brought to the desired low temperature level. The expansion is preferably performed isentropically (or adiabatically) using a turbine. Pressure and temperature are decreased together in such a cooling. The advantage of working with a separate cooling medium, compared to expansion of the medium to be separated, is for instance that this separate cooling medium can be optimized for the desired cooling action. The combination of the described phase change of at least one of the fractions present in the gas mixture, preferably the CO₂ fraction, and the subsequent subjecting of the thus obtained medium mixture to a volume force (for instance by rotation) provides for a very advantageous separation of the mixture into at least two fractions, one of which is CO₂.

In a preferred embodiment of the method according to the invention the cooling and/or the expansion is performed such that the final temperature is a maximum of 50⁰C higher than the temperature which at the final pressure corresponds to the transition of the at least one fraction to the solid phase. The final temperature is more preferably a maximum of 20⁰C higher than the temperature which at the final pressure corresponds to the transition of the at least one fraction to the solid phase. As already indicated above, it is recommended that the at least one fraction is the CO₂ fraction. In such a preferred embodiment wherein the at least one fraction is the CO₂ fraction, it is advantageous that the final temperature lies between -80⁰C and -20⁰C, and the final pressure between 10 and 80 Bar. The choice of the final temperature is still more preferably between -60⁰C and -4O⁰C, and the final pressure between 20 and 60 Bar, and most preferably between -50⁰C and -40⁰C and between 30 and 60 Bar. In a preferred embodiment wherein the at least one fraction is the HCN fraction, it is advantageous that the final temperature lies between -20⁰C and 50⁰C and the final pressure between 1 and 60 Bar. In this preferred method the choice of the final temperature is most preferably between 0⁰C and 4O⁰C, and the final pressure between 5 and 30 Bar.

A phase diagram of the gas mixture (a diagram of the pressure against the temperature) is generally characterized by a range where the fractions of the gas mixture form one phase (the mixing range) and a more or less closed range where at least a part of the fractions form a distinct phase (demixing range). A gaseous range, a liquid range and a solid range are generally further distinguished, wherein the gaseous range is located on average at high pressure and temperature and the solid range, conversely, at low pressure and temperature. A number of lines demarcate these ranges, in particular a liquid line which indicates the boundary between combinations of pressure and temperature under which (in addition to other phases) a liquid phase also occurs, and a solid line which indicates the boundary between combinations of pressure and temperature under which (in addition to other phases) a solid phase also occurs. The temperature which at the final pressure corresponds to the transition of the at least one fraction to substantially solid phase is thus the temperature as according to the intersection of the final pressure line and the solid line for the relevant phase.

By selecting the cooling and/or expansion such that the end point (the combination of obtained final pressure and final temperature) on the phase diagram is located as closely as possible to the solid line of the at least one fraction, a separation of the CO₂ is obtained with further improved selectivity. It thus becomes even possible in principle to apply any separation technique, even those which do not provide high selectivity per se, such as for instance separation by means of gravitational force and/or a cyclone. The separation efficiency of the rotation means is increased according to the present invention by bringing at least one fraction, and preferably the CO₂ fraction, at least partially and preferably substantially into liquid form before the gas mixture reaches the rotation means. This phase change takes place by changing the temperature (heating or cooling subject to the conditions) and/or the pressure of the gas mixture. It is thus simpler to separate the fractions of the gas mixture from each other by means of rotation (as a result of the increased difference in centripetal forces exerted on the fraction). It is noted here that the separation of the fractions is understood to mean at least partial separation of two fractions such that a significant difference in the average mass density of the two fractions results; a complete (100%) separation will be difficult to realize in practice. As a consequence of the rotation of the mixture, now with increased differences in the mass density of the fractions for separating, the lighter fraction will migrate at least substantially to the inner side of the rotation and the heavier fraction (the liquid fraction) will migrate at least substantially to the outer side of the rotation.

In addition to the increased selectivity for CO₂ compared to the prior art, the present invention relates to a separation which moreover increases the possible uses of at least one of the fractions compared to the gas mixture. Even after separation, this usable ("cleaned") fraction may still comprise a part of another undesired fraction (be contaminated with another fraction), although this other fraction will be significantly smaller than the presence of this undesired fraction in the original mixture. When applying the method for separating CO₂ from for instance flue gases and/or synthesis gases, the usable fraction will comprise an H₂- containing gas mixture. This H₂- containing gas mixture, which has a high hydrogen content due to the high selectivity of the method according to the invention, can if desired be converted into electricity in a coal gasification unit and/or be sold as end product, as feedstock for a chemical plant or for instance for automotive applications. A further advantage of the method according to the present invention is that the energy consumption thereof is exceptionally low, generally in the order of magnitude of 3000 J/mol. This energy loss amounts to only l%o of the enthalpic heating value of the H₂-containing gas mixture (in the order of 3.10⁶ J/mol).

The successive steps of the method according to the invention result in an unexpectedly high separation efficiency without bulky equipment being required for this purpose (i.e. the device can be given a very compact form) and wherein the medium need only be treated for a short period. A device can be given an even smaller form (with a smaller volume) if the medium mixture is carried under higher pressure through the device.

In a preferred variant of the method according to the invention the medium mixture is subjected to gravitational force during processing step C). Such a separation technique is very simple and requires little energy and investment. In a further preferred variant the medium mixture is subjected to a centrifugal force during processing step C) by feeding the medium mixture to rotation means. The rotation means can for instance be formed by at least one cyclone (vortex), or alternatively by an assembly of a plurality of cyclones. In the case of a cyclone it is possible to give the rotation means a stationary form and to set only the medium into rotation. The application of a plurality of (smaller) cyclones has an advantage relative to a single cyclone which is comparable to the advantage of a rotating assembly of feed channels. Baffles can optionally be placed in a cyclone, for instance for the purpose of causing a determined fraction to condense on the baffles and for controlling the cyclone.

A particularly favourable method according to the invention is characterized in that during processing step C) the medium mixture is set into rotating flow in rotation means provided for this purpose and comprising a rotating assembly of feed channels. Such rotary separators have the advantage that the average distance of the medium from a wall (in radial direction) remains limited, whereby a desired degree of separation can be achieved in a relatively short time (which corresponds to a relatively limited length of the rotary separator in axial direction). The operation of such a rotating assembly of feed channels is further influenced positively if a preferably laminar flow of the medium is maintained in the channels. Conversely, it is also possible for the medium to be carried through the channels with turbulent flow. The flow speeds to be applied can be varied or optimized according to the situation. A particularly suitable rotary separator of the present type is described for instance in EP 0286160A, the content of which is expressly incorporated in the present application.

In particular preference the method according to the invention is applied by subjecting the medium mixture during processing step C) to gravitational force and/or to a centrifugal force, and/or setting the medium mixture into rotating flow in rotation means provided for this purpose and comprising a rotating assembly of feed channels. The combination of separating techniques can result in a further increased selectivity of the CO₂ to be separated from the gas mixture. In a further preferred embodiment of the method the separated gaseous fraction is further purified by being guided downstream of the rotation means through at least one membrane (physical adsorption) and/or through a washer (chemical absorption). This has the additional advantage that the selectivity is further improved, particularly also because physical adsorption and/or chemical absorption separation consumes less energy the lower the CO₂ content in the gas mixture becomes. The method according to the invention can be performed with a relatively small throughflow device since the separate processing steps can be carried out within a very short period of time, for instance individually in less than 1 second, usually in less than 0.1 second or even in less than 10 or less than 5 milliseconds. This makes lengthy processes, with associated devices which are dimensioned such that they can contain large volumes, unnecessary.

The method according to the invention is applied particularly for the purpose of separating CO₂ from a flue gas and/or synthesis gas of a fossil and/or biomass-fuelled process. The method is then characterized in that a flue gas and/or synthesis gas of a fossil and/or biomass-fuelled process is provided during processing step A).

According to the invention the CO₂ can be separated from the gas mixture with high selectivity. This particularly favourable effect is achieved, among other ways, by bringing at least one of the fractions present in the gas mixture, and preferably the CO₂ fraction, at least partially into liquid form prior to the actual separation, whereby a phase separation occurs in the gas mixture. The method according to the invention has the additional advantage that the liquid fraction can be discharged in step D) by being pumped. If this liquid fraction is the CO₂ fraction, the separated CO₂ can then be transported in very simple manner to a location where it can be stored. This is preferably a location on the seabed. The separated CO₂ can in this way also be carried to an underground reservoir of porous rock. Storage on the seabed can make a significant contribution to the solution of the greenhouse problem. It is noted that a location on the seabed is understood to mean any location deep enough below sea level to allow for instance CO₂ to easily dissolve in the seawater.

The invention also relates to a device for separating CO₂ from a gas mixture, the operation and advantages of which have already been elucidated at length above in respect of the method. The device comprises: - rotation means for rotating the flowing gas mixture to be separated, a cooling and/or expansion means, connecting to the rotation means upstream in flow direction of the gas mixture, for causing transition in physical manner of at least one of the fractions present in the gas mixture to the liquid phase, a feed for the medium mixture to be separated connecting to the cooling and/or expansion means, and pump means connecting to the rotation means for discharging the separated liquid phase.

The rotation means are preferably formed by a rotating assembly of feed channels and/or by at least one cyclone. It is further advantageous to characterize the device in that the pump means connect to a location on the seabed and/or underground reservoir of porous rock.

With the device according to the invention it is possible in selective and inexpensive manner to in any case separate CO₂ from a gas mixture which preferably also contains H₂ and/or HCN.

The present invention will be further elucidated on the basis of the non-limitative exemplary embodiments shown in the following figures. Herein: figure 1 shows a schematic view of a device according to the invention, and figure 2 shows an example of a part of a phase diagram of a first gas mixture to be separated with the method according to the invention; and figure 3 shows an example of a part of a phase diagram of a second gas mixture to be separated with the method according to the invention.

Figure 2 shows a phase diagram of a contaminated flue gas which can be cleaned with the invented method. This is more particularly the phase diagram of a 50/40/10 mol% H₂/Cθ₂/N₂ mixture. The y-axis shows the pressure 100, while the temperature 200 is shown along the x-axis. The phase diagram comprises a range (designated with G) where the fractions of the gas mixture form one gas phase (the mixing range) and which extends at relatively high temperatures. Two ranges are further visible (designated with G+L and G+S+L), where at least a part of the fractions form a distinct phase (demixing ranges). In range G the medium mixture is gaseous, in range G+L a mixture is present of liquid and gas, wherein in the present case CO₂ is in the liquid phase and the rest of the components in the gaseous phase. Present in range G+S+L is a mixture of gas, liquid and solid. A number of lines demarcate the relevant ranges, in particular a dew point line 1 10 which indicates the boundary between combinations of pressure 100 and temperature 200 below which (in addition to other phases) a liquid phase L also occurs, and a solid line 120 which indicates the boundary between combinations of pressure 100 and temperature 200 below which (in addition to other phases) a solid phase VS also occurs. It will be apparent that the phase diagram shown in figure 2 is given only by way of example, and that the method is likewise applicable for separating CO₂- containing gas mixtures with more fractions, and therefore a more complicated phase diagram.

Figure 3 thus shows a phase diagram of a synthesis gas which can be cleaned with the invented method. This is more particularly the phase diagram of a 50/50 mol%

HCN/CO₂ gas mixture. The y-axis shows the pressure 100, while the temperature 200 is shown along the x-axis. The phase diagram further comprises a range (designated with G or L) where the fractions of the gas mixture form one phase (the mixing range) and a more or less closed range (designated with G+L, L+S and G+L+S) where at least a part of the fractions form a distinct phase (demixing range). In range G the gas mixture is gaseous, in range L the gas mixture is liquid. In range G+L a mixture is present of liquid and gas, wherein in the present case HCN is in the liquid phase and CO₂ in the gaseous phase. Present in range G+S+L is a mixture of gas, liquid and solid. A number of lines demarcate the relevant ranges, in particular a dew point line 110 which indicates the boundary between combinations of pressure 100 and temperature 200 below which (in addition to other phases) a liquid phase L also occurs, and a solid line 120 which indicates the boundary between combinations of pressure 100 and temperature 200 below which (in addition to other phases) a solid phase S also occurs. The phase diagram shows a critical point 140, a concept generally known to the skilled person, at which the gaseous phase and liquid phase are in equilibrium with each other. The critical temperature is indicated in the phase diagram of figure 3 with T_cnt.

Referring to figure 1 , a device 1 is shown for cleaning a contaminated gas such as for instance the above stated CCVcontaining flue gas, in which device 1 the method according to the invention can be performed. In a first preferred variant of the method according to the invention the contaminated gas is supplied as according to arrow Pi by a feed 2 under a pressure of between 100 and 300 Bar (usually a typical pressure of about 250 Bar) and at a temperature of for instance more than 100⁰C. The gas supplied as according to arrow Pi is optionally cooled in a heat exchanger 3, for instance by means of cooling into the atmosphere. The cooling is such that the gas mixture is brought to a temperature which keeps the mixture gaseous. The gas is preferably cooled at almost constant pressure. The thus obtained gas mixture flows from heat exchanger 3 as according to arrow P₂ to a throttle valve 4. The gas mixture supplied as according to arrow P₂ is expanded by means of throttle valve 4, preferably in isentropic manner, to a lower pressure of between for instance 5 and 20 Bar. This isentropic pressure and temperature decrease is indicated in figures 2 and 3 by means of line 130. As a result of the sudden fall in pressure the temperature of the gas mixture will fall back to a final temperature T_e (and a corresponding final pressure p_e) such that a part of the CO₂ fraction present in the gas mixture changes phase. For the 50/40/10 mol% H₂/CO₂/N₂ mixture the CO₂ fraction will become liquid. For the 50/50 mol% HCN/CO₂ mixture the HCN fraction will become liquid. According to the invention the final temperature T_e is preferably relatively close to, for instance a maximum of 50⁰C higher than, the temperature which corresponds at the final pressure p_e with the transition of at least one of the fractions of the gas mixture to the solid phase which, referring to figures 2 and 3, is the temperature Ts. For the 50/40/10 mol% H₂/CO₂/N₂ gas mixture the CO₂ fraction will become solid at T_s, for the 50/50 mol% HCN/CO₂ mixture the HCN fraction. By bringing the gas mixture to temperature T_e a medium mixture 5 is created with a gaseous matrix incorporating CO₂ or HCN liquid droplets. This gas/liquid droplet mixture 5 is carried through channels 6 of a rotor 7 whereby, as a result of the rotation R of rotor 7, the CO₂ or HCN liquid droplets condense against the sides of channels 6 of rotor 7 which are directed toward a rotation shaft 8. The condensed liquid droplets are collected in a basin 10 which can be emptied by means of activating a pump 11 such that the liquid fraction is discharged as according to arrow P₃. This discharge must preferably take place in cooled manner, at least for CO₂, in order to keep the CO₂ liquid. The gaseous phase (H₂ or CO₂) leaves rotor 7 on the side remote from throttle valve 4 as a gas flow 12. The gas flow 12 with the CO₂ at least largely removed and containing substantially H₂ is extracted and leaves device I as according to arrow P₄ as cleaned gas. For the 50/50 mol% HCN/CO₂ mixture the gas flow substantially comprises CO₂, with HCN at least partly removed. If desired, the gas/liquid droplet mixture 5 created by the method according to the invention can be separated beforehand by being subjected to gravitational force. Referring to figure 2, it is also possible according to a second preferred embodiment of the method according to the invention for the gas to be cooled at almost constant pressure to temperature T_e, as indicated by line 140. According to a third preferred embodiment of the method according to the invention, it is also possible for the gas to be cooled to final temperature T₆ by means of a Joule Thompson condenser, as indicated by line 150.

Using the above described first preferred variant of the method according to the invention a 50/40/10 mol% gas mixture of respectively H₂, CO₂ and N₂ was separated into a gas flow and a liquid flow at a final temperature T_e of -55°C and a final pressure P_e of 45 Bar. Following separation the gas flow contained 66.6 mol% H₂, 20.3 mol% CO₂ and 13.1 mol% N₂. Following the separation the liquid flow contained 0.5 mol% H₂, 98.9 mol% CO₂ and 0.6 mol% N₂. The molar fraction of gas/liquid amounted to about 0.75/0.25. An almost purely CO₂ liquid flow is therefore obtained with the method. The residue gas flow contains only about 20 mol% CO₂. The total quantity of CO₂ recovered from the original gas mixture amounts to about 62%. The above obtained gas flow was then fed once again to the device shown in figure 1 and subjected for a second time to the method according to the invention. Before the separation the gas flow contained 66.6 mol% H₂, 20.3 mol% CO₂ and 13,1 mol% N₂, and after the separation 70.2 mol% H₂, 16.1 mol% CO₂ and 13.7 mol% N₂. After the second separation the liquid flow contained 0.9 mol% H₂, 98.2 mol% CO₂ and 0.9 mol% N₂. The molar fraction of gas/liquid amounted this time to about 0.95/0.05. The quantity of CO₂ recovered from the original gas mixture in the second step amounts to about 25%. The total quantity of CO₂ recovered from the original gas mixture thus amounts to about 75% for the two steps together. This is higher than has heretofore been usual, certainly taking into account the limited amount of steps (two).

Using the above described first preferred variant of the method according to the invention a 50/50 mol% gas mixture of respectively HCN and CO₂ was separated into a gas flow and a liquid flow at a final temperature T_e of 25⁰C and a final pressure p_e of 10 Bar. At this final temperature and pressure the CO₂ is in the gaseous phase, while the HCN is substantially in the liquid phase. Following the separation the gas flow contained 89.7 mol% CO₂ and 10.3 mol% HCN. Following the separation the liquid flow contained 91.0 mol% HCN and 8.9 mol% CO₂. The molar fraction of gas/liquid amounted to about 0.51/0.49. It is thus possible with the invented method to separate a large part of the HCN and the CO₂ in a single process step, wherein each fraction has about 90 mol% purity.

Claims

1. Method for separating CO₂ from a flue gas or synthesis gas mixture from fossil and biomass-fuelled processes, comprising the processing steps of: A) providing a CC^-containing flue gas or synthesis gas mixture from fossil and biomass-fueiled processes at a starting pressure and a starting temperature, B) cooling the gas mixture to a final temperature and a final pressure at which at least one of the fractions present is present at least partially in the liquid phase in the gas mixture, C) subjecting the thus resulting medium mixture to a volume force, and D) discharging at least the CO2 as one of the separated fractions.

2. Method as claimed in claim 1 , characterized in that the gas mixture is cooled to a final temperature at which the CO₂ is present substantially in the liquid phase in the gas mixture.

3. Method as claimed in claim 1, characterized in that the gas mixture also contains HCN and is cooled to a final temperature at which the HCN is present substantially in the liquid phase in the gas mixture.

4. Method as claimed in any of the foregoing claims, characterized in that the gas mixture is cooled substantially isobarically in step B).

5. Method as claimed in any of the foregoing claims, characterized in that prior to step B) the gas mixture is brought in a step E) to a pressure which is increased relative to the starting pressure, and the gas mixture is cooled in step B) to the final temperature by being subjected to adiabatic and/or isentropic expansion.

6. Method as claimed in any of the foregoing claims, characterized in that the final temperature is a maximum of 5O⁰C higher than the temperature which at the final pressure corresponds to the transition of a mixture fraction to the solid phase.

7. Method as claimed in claim 6, characterized in that the final temperature is a maximum of 2O⁰C higher than the temperature which at the final pressure corresponds to the transition of a mixture fraction to the solid phase.

8. Method as claimed in claim 2, characterized in that the final temperature lies between -60⁰C and -40⁰C, and the final pressure between 20 and 60 Bar.

9. Method as claimed in claim 3, characterized in that the final temperature lies between 0⁰C and 4O⁰C, and the final pressure between 5 and 30 Bar.

10. Method as claimed in any of the foregoing claims, characterized in that the medium mixture is subjected to gravitational force during processing step C).

1 1. Method as claimed in any of the foregoing claims, characterized in that the medium mixture is subjected to a centrifugal force during processing step C).

12. Method as claimed in any of the foregoing claims, characterized in that during processing step C) the medium mixture is set into rotating flow in rotation means provided for this purpose and comprising a rotating assembly of feed channels.

13. Method as claimed in any of the foregoing claims, characterized in that during processing step C) the medium mixture is subjected to gravitational force and/or to a centrifugal force, and/or the medium mixture is set into rotating flow in rotation means provided for this purpose and comprising a rotating assembly of feed channels.

14. Method as claimed in any of the foregoing claims, characterized in that the CCh-containing flue gas or synthesis gas mixture from fossil and biomass-fuelled processes is provided by converting CO from a flue gas or synthesis gas by means of a CO-shift reaction with steam.

15. Method as claimed in any of the foregoing claims, characterized in that the liquid fraction is CO₂ and this fraction is discharged in step D) by being pumped to a location on the seabed and/or an underground reservoir of porous rock.

16. Device for separating CO₂ from a flue gas or synthesis gas mixture from fossil and biomass-fuelled processes, comprising: rotation means for rotating the flowing gas mixture to be separated, a cooling and/or expansion means, connecting to the rotation means upstream in flow direction of the gas mixture, for causing transition in physical manner of at least one of the fractions present in the gas mixture to the liquid phase, a feed for the medium mixture to be separated connecting to the cooling and/or expansion means, and pump means connecting to the rotation means for discharging the separated liquid phase.

17. Device as claimed in claim 16, characterized in that the rotation means are formed by a rotating assembly of feed channels.

18. Device as claimed in claim 16 or 17, characterized in that the rotation means are formed by at least one cyclone.

19. Device as claimed in any of the claims 16-18, characterized in that the pump means also comprise a connection suitable for connecting to a location on the seabed and/or an underground reservoir of porous rock.

20. Use of a device as claimed in any of the claims 16-19 for separating CO₂ from a flue gas or synthesis gas mixture from fossil and biomass-fuelled processes.