GB2174379A

GB2174379A - Process for recovering carbon dioxide

Info

Publication number: GB2174379A
Application number: GB08607530A
Authority: GB
Inventors: Melvyn Duckett; David Ian Limb
Original assignee: Costain Petrocarbon Ltd
Current assignee: Costain Petrocarbon Ltd
Priority date: 1985-03-27
Filing date: 1986-03-26
Publication date: 1986-11-05
Also published as: GB8607530D0; GB2174379B; GB8508002D0

Abstract

A process for recovering carbon dioxide from a gas mixture containing it in a concentration of at least about 40%, wherein the gas mixture is distilled at superatmospheric pressure and subambient temperature to yield substantially pure carbon dioxide as a liquid bottoms product, the distillation overhead is then warmed and subjected to membrane separation to yield a carbon dioxide-rich permeate which is returned to the distillation column.

Description

SPECIFICATION Process for recovering carbon dioxide This invention relates to the recovery of high purity carbon dioxide from gas mixtures and is particularly concerned with the treatment of gas mixtures in which the carbon dioxide is present in high concentrations, and the associated constituents are mainly light hydrocarbons such as methane and ethane.

Processes for the recovery, purification and liquefaction of carbon dioxide from carbon dioxide rich streams e.g. fermentation gas and off gases from chemical processes such as ethylene oxide production or ammonia production, are well known. These processes normally carry out the final purification (involving removal of light gases such as hydrogen, nitrogen, oxygen, methane and carbon monoxide) either by cooling and partial condensation, thus producing a liquid enriched in carbon dioxide and a tail gas lean in carbon dioxide, or by cooling and distillation if a purer carbon dioxide product e.g. greater than 99% pure, by volume, is required.

In a process developed for the recovery of high purity carbon dioxide from a gas mixture the gas mixture is compressed, preferably to a pressure of approximately 15 to 25 bar, and dried by adsorption or other conventional means (all pressures are given in bar absolute). After removal of undesirable impurities such as sulphur containing compounds, the mixture is cooled and separated by distillation, yielding high purity carbon dioxide as a bottoms product. However, the overhead product of the distillation column will always contain a significant proportion of carbon dioxide, generally at least 60 percent, which is normally wasted.

The high percentage of carbon dioxide in the overhead stream is due to the equilibrium conditions prevailing in mixtures with light hydroarbons which, at the usual operating pressures of around 20 bar and condenser temperatures around -30"C, lead to vapour concentrations around 60 to 70% CO2. This situation is not substantially improved by operation at lower temperatures, which may lead to azeotrope formation and/or solidification of carbon dioxide. As a result, particularly when the feed gas contains a significant amount of non-condensibles or inerts, such as nitrogen, oxygen or methane, it is not possible to approach 100% recovery of carbon dioxide. For example with only 3 mol percent of non-condensibles or inerts present the CO2 recovery would be limited to about 95 percent.

A gas separation method that has recently found commercial success is the membrane separation process whch involves passing the mixture to be separated at superatmospheric pressure over a semi-permeable membrane across which a pressure drop is maintained and through which one or more of the components of the gas mixture is/are selectively permeable.

This technique has already been proposed for the removal of carbon dioxide as an undesirable impurity from a gas stream, and also for the separation of carbon dioxide from an oil well stream for recycle to the oil well to assist recovery of oil. In neither case, however, is the purity of the separated carbon dioxide stream a matter of importance, nor is high recovery achieved.

It has now been found that the recovery of high purity CO2 (e.g. 99% by volume or purer) from gas mixtures in which a high concentration of carbon dioxide is present can be raised to an economically desirable level, e.g. 99 percent or more by employing in a particular manner a combination of distillation and membrane separation using a suitable membrane.

According to the present invention there is provided a process for the recovery of carbon dioxide from a gas mixture containing it in a high concentration, the process comprising supplying the gas mixture at superatmospheric pressure and substantially free of contaminants which would solidify under the process conditions, distilling said gas mixture at sub-ambient temperature in a distillation column and recovering substantially pure carbon dioxide as a liquid bottoms product of the distillation, warming the overhead stream from the distillation to a temperature suitable for membrane separation, passing the overhead stream at superatmospheric pressure over a membrane which is selectively permeable for carbon dioxide and thereafter recycling carbon dioxide-rich permeate for recompression and feeding to the distillation column.

By high concentration is meant a concentration of carbon dioxide of at least 40% by volume.

However, the lower limit of carbon dioxide concentration which can be employed may depend on the nature of the other components of the gas mixture. For example, to avoid azeotrope formation when ethylene is present the carbon dioxide will normally be present in a concentration greater than about 70%.

By a temperature suitable for membrane separation, is meant a temperature at which the physical properties of the membrane are such that it is mechanically stable and a sufficient fluid flux can be maintained across the membrane. Thus the temperature should not be so low that the membrane would be embrittled and consequently unable to withstand the pressure differential across it, nor so high that the membrane would be softened and distort to an unacceptable extent. Suitably the overhead stream is warmed to a temperature in the range 0 to 50"C, preferably 0 to 30"C and most preferably to around ambient temperature.

In a preferred embodiment, the overhead stream from the distillation column is warmed by indirect heat exchange with a refrigerant employed in closed cycle to provide cold for the column reflux. In such circumstances design features of the refrigeration cycle and an appropriate choice of refrigerant can advantageously lead to the overhead stream being warmed to ambient temperature.

The process of the invention enables the achievement of a high recovery of high purity carbon dioxide from gas streams containing carbon dioxide in high concentrations. For example a recovery of above about 90% and most preferably above about 95% carbon dioxide can be achieved. Under suitable conditions the recovery of carbon dioxide can be improved to 99% or more of the CO2 in the feed gas. For gas streams containing lower concentrations of CO2, e.g.

below about 40%, separation is usually carried out for the purpose of removing the CO2 as an undesirable impurity, rather than for recovering CO2. Examples of gas streams containing carbon dioxide in high concentrations from which it may be desired to separate carbon dioxide are certain natural gases and gas effluents from petrochemical works and other gas sweetening operations. In such gas mixtures the carbon dioxide may be in admixture with, for example, methane, ethane and/or ethylene.

Any suitable membrane may be employed but it is preferred to use those wherein the carbon dioxide permeability is at least 10 times that of the gas or gases from which it is to be separated under the chosen separation conditions. Examples of suitable membranes are those formed from polysulphone or cellulose acetate.

The invention will now be described in more detail with reference to one preferred embodiment and with the aid of the accompanying drawing, which is a flow diagram of a recovery plant suitable for carrying out the process of the invention.

Referring to the drawing, 1 is a compressor, 2 is an after cooler, 3 is a purification unit of conventional kind, 4 is a drier, 5 is a distillation column, 6 is a reboiler, 7 is a reflux condenser, 8 is a refrigerant compressor, 9 is a refrigerant condenser, 10 is a heat exchanger, and 11 is a refrigerant expansion valve, and 12 is a membrane separation unit.

The carbon dioxide-containing feed gas mixture is provided through pipeline 20 usually at a pressure in the range 0.7 to 2.0 bar, compressed in compressor 1 usually to a pressure in the range 10 to 30 bar, and then cooled in after cooler 2 to remove the heat of compression. The gas mixture is then supplied via line 21 to purification unit 3 where sulphur-containing impurities are removed by known means such as absorption or catalytic reaction. When such a sulphur removal step is carried out at high temperature in a catalyst bed, then this may precede the after-cooler 2. After purification the gas mixture is passed through line 22 to drier 4 which is of conventional form using beds of adsorbent. This removes trace amounts of water which might otherwise solidify under the process conditions.The dried gas mixture is then supplied via line 24 to distillation reboiler 6 where it gives up heat to provide reboil for the distillation and is cooled to the desired distillation temperature usually -3 to --40"C. The thus-cooled feed is then fed to an intermediate point in the distillation column, which generally operates with a bottom temperature in the range -40 to -12"C and an overhead temperature in the range --50 to -200C. The distillation generally operates at a pressure which is slightly, e.g. 1 to 3 bar, below the pressure of gas leaving compressor 1, e.g. a pressure in the range 10 to 25 bar.High purity carbon dioxide e.g. 99% or more pure, is recovered from the bottom of the column in line 26 and the tail gas is recovered in line 27 warmed in heat exchanger 10, usually to a temperature of 0 to 500C and fed to membrane separation unit 11. There the tail gas is separated into a carbon dioxide-rich permeate which is recycled via line 32 to feed line 20 and a stream depleted in carbon dioxide which is recovered through line 30. Using this process a high recovery of high purity carbon dioxide can be obtained.

In the illustrated embodiment the cold for production of liquid CO2 and for the distillation column reflux is provuded by a closed refrigeration cycle for which in this case the refrigerant is ammonia. In the refrigeration cycle, warm liquid refrigerant from the condenser, 9 passes via line 36 to exchanger 10 where it is sub-cooled before expanding through valve 11 and passing via line 34 to reflux condenser 7, where it is evaporated by indirect heat exchange with the carbon dioxide containing gas mixture. The refrigerant is then compressed by refrigerant compressor 8, cooled and condensed in refrigerant condenser 9.

The tail gas from the distillation must be warmed before contact with the membrane separation unit.

It is suitably warmed to ambient temperature, that is to a temperature in the range 0 to 50"C, preferably 0 to 300C, and most preferably ambient temperature, to avoid potential problems associated with mechanical instability of the membrane at low temperatures. It is convenient to do this by heat exchange with a condensed refrigerant as described above in which refrigerant condensation has conveniently been effected by heat exchange with air or cooling water provided at ambient temperature. Another possibility is to warm the tail gas by heat exchange with the carbon dioxide containing gas mixture before said mixture is fed to the reboiler 6. In either case the tail gas will preferably be warmed to about ambient temperature before contact with the membrane separation unit.

Example The details of temperature, pressure and composition of the gas streams in various parts of the process described above with reference to the drawing are set out in Tables 1 and 2 below.

The carbon dioxide recovery in this case is 99.6%.

Table 1 Line Temp Pressure ( C) (bar) 20 +30 1.0 21 +30 17 22 +30 16.6 24 +35 16.3 26 -29 14.5 27 -35 14.3 28 +20 14.2 32 +15 1.1 34 -40 0.72 36 +40 15.5 Table 2 Flows in- kg mol/hr (dry basis) Component CH4 C2H6 CO2 Total Line 20 0.12 1.73 152.53 154.38 24 0.13 1.82 157.53 159.48 26 - - 151.94 151.94 28 0.13 1.82 5.59 7.54 30 0.12 1.73 0.59 2.44 32 0.01 0.09 5.00 5.10

Claims

1. A process for the recovery of carbon dioxide from a gas mixture containing it in a high concentration, the process comprising supplying the gas mixture at superatmospheric pressure and substantially free of contaminants which would solidify under the process conditions, distilling said gas mixture at sub-ambient temperature in a distillation column and recovering substantially pure carbon dioxide as a liquid bottoms product of the distillation, warming the overhead stream from the distillation to a temperature suitable for membrane separation, passing the overhead stream at superatmospheric pressure over a membrane which is selectively permeable for carbon dioxide and thereafter recycling carbon dioxide-rich permeate for recompression and feeding to the distillation column.

2. A process as claimed in claim 1 in which the overhead stream from the distillation column is warmed to a temperature in the range 0 to 50"C before being fed to said membrane.

3. A process as claimed in claim 1 in which the overhead stream from the distillation column is warmed to a temperature in the range 0 to 30 C before being fed to said membrane.

4. A process as claimed in any one of claims 1 to 3 in which the overhead stream from the distillation column is warmed before being fed to said membrane by indirect heat exchange with a refrigerant employed in closed cycle to provide cold for the column reflux.

5. A process as claimed in any one of claims 1 to 4 in which more tham 90% by volume of the carbon dioxide in the gas mixture is recovered as a liquid bottoms product of the distillation.

6. A process as claimed in any one of claims 1 to 4 in which more than 95% by volume of the carbon dioxide in the gas mixture is recovered as a liquid bottoms product of the distillation.

7. A process as claimed in any one of claims 1 to 6 in which the carbon dioxide is recovered from a gas mixture containing it in a concentration greater than 40%.

8. A process as claimed in claim 1 substantially as illustrated in the drawing.

9. A process as claimed in claim 1 substantially as described herein with reference to the Example.