METHOD AND DEVICE FOR DEHYDRATING A CO2 CONTAINING GAS

Abstract

Proposed is a method for dehydrating a CO2 containing gas (1) by cooling the gas (1) and separating the condensed water from the gas (1), wherein the gas is contacted with liquid CO2 to condense water contained in the gas (1) and the condensate is separated from the remaining gas. Further, a device for dehydrating a CO2 containing gas is proposed, comprising a gas feeding system for feeding the gas which has to be dehydrated, wherein the device comprises a CO2 feeding system for feeding liquid CO2, a contacting device (C) for contacting the gas and the liquid CO2 for cooling the gas (1) to condense the water contained in the gas (1), and which comprises a first separator (4) for separating the condensate from the remaining gas.

Description

The present invention relates to a method for dehydrating a CO₂ containing gas by cooling the gas and separating the condensed water from the gas, wherein the gas is contacted with liquid CO₂ to condense water contained in the gas and the condensate is separated from the remaining gas. Further, a device for dehydrating a CO₂ containing gas is proposed comprising a gas feeding system for feeding the gas which has to be dehydrated, wherein the device comprises a CO₂ feeding system for feeding liquid CO₂, a device for contacting the gas and the liquid CO₂ for cooling the gas to condense the water contained in the gas and comprising a first separator for separating the condensate from the remaining gas.

According to the invention the CO₂ containing gases to be dehydrated comprise natural gas, as well as gases containing other than hydrocarbonic gases, such as methane, ethane, propane, butane, pentane etc, and also hydrogen sulfide, nitrogen, oxygen etc.

It is known to dehydrate carbonic acid gas by consistently compressing the gas in two compressors (see patent CA 325811, 1932). After that each compressor gas is cooled in air chillers and directed to a trap in which condensed moisture is removed from the gas.

The gas from the trap is directed to an adsorber where in a process of adsorption of water vapour definitive carbonic gas dehydration occurs.

Drawbacks of the way discussed above are huge dimensions of the installations realizing this way caused by presence and costs of adsorber, and also the necessity of an energy supply for the regeneration of the absorber.

A further way of dehydrating gases is described in DE 20 65 941 C2, where the gas is cooled in a heat exchanger using a cold gas and injection of ammonia.

DE 2014776 teaches the use of CO₂ as a cooling element in a heat exchanger for cooling natural gas.

A further known method is the dehydration of high pressure gas enriched with carbonic acid gas (compare JP 63074908, 1988), in which the gas containing CO₂ is cooled by means of an air chiller, wherein the cooled gas extends in a throttle valve. The water formed after expansion is separated from the gas in a separator. The obtained gas is compressed in a compressor, cooled in an air chiller and supplied to a final gas consumer.

A serious drawback of the described way is the impossibility of achieving deep-cut gas dehydration. This is due to the impossibility of obtaining low temperatures of the gas in the course of its expansion caused by the possible formation of hydrates at low temperatures.

It is therefore the object of the present invention to provide a method and a corresponding device for an effective and high degree dehydration of CO₂ containing gases.

The problem of the present invention is solved by providing a method for dehydrating a CO₂ containing gas wherein the gas is mixed with liquid CO₂ for cooling the gas to condense water contained in the gas and the condensate is separated from the remaining gas. That is, the gas is directly contacted with liquid CO₂ in order to condense water contained in the gas.

The separation of condensed water, of possible hydrates and of water dissolved in liquid CO₂ may be carried out in a separator.

By cooling with liquid CO₂ very low temperatures are achievable and the condensation process may be carried out in a very efficient way. A further advantage of the present method is that condensation of the water vapour to water can be realized with very low costs.

In a preferred embodiment of the present invention the gas which is obtained after contacting with liquid CO₂ and which comprises gaseous CO₂ is expanded to condense remaining water in this gas and the obtained liquid phase comprising liquid CO₂, water and, optionally, hydrates is separated from the dehydrated gas. The dehydrated gas will be derived.

Due to the strong cooling of the gas phase by mixing with fluid CO₂, the expansion can be realized with a very small energy input. Further, hydrate crystals optionally formed in the course of expansion are dissolved in liquid CO₂ which is also formed at expansion. This way the efficiency of the gas dehydration process is increased.

In an especially preferred embodiment of the invention the obtained liquid phase comprising liquid CO₂, water and, optionally, hydrates is partially or completely used for mixing with a gas to be dehydrated. The liquid phase may be separated in a separator.

That means, the liquid CO₂ produced as a by-product during a cooling process is used for cooling the gas in another cooling process of the present invention in order to dehydrate the gas. Additionally, expansion of the gas to be dehydrated below temperature of hydrate formation allows separation of the formed hydrates because they are dissolved in the liquid CO₂. also generated in the expansion process.

Preferably, the liquid CO₂ which is mixed with the gas to be dehydrated has a temperature within a range from −60° C. to 20° C. The most preferred range is from −40° C. to −20° C.

The gas which is mixed with the liquid CO₂ has preferably a temperature within a range from −40° C. to 50° C. The most preferred range is from 30° C. to 50° C.

In an especially preferred embodiment, the gas has a temperature of about 40° C. and the liquid CO2 has a temperature of about −27° C. The pressure of the gas is preferably within a range from 4000 to 6000 kPa, more preferred at about 5000 kPa.

The temperature of the gas may preferably be lowered by expansion below 0° C. and most preferred to at most −20° C. The pressure of the gas is preferably lowered by expansion below 7000 kPa and most preferred to at most 2000 kPa.

In an especially preferred embodiment before the expansion the gas phase has a temperature of about 15° C. and a pressure of about 5000 kPa. After cooling by expansion the gas phase has a temperature of at most −20° C. and a pressure of at most 2000 kPa.

The obtained liquid phase comprising liquid CO₂ and water may have a pressure within a range from 100 kPa to 7000 kPA and a temperature within a range from −63° C. to 20° C. In the most preferred embodiment the obtained liquid phase comprising liquid CO₂ and water may have a pressure within a range from 1200 kPa to 1600 kPa and a temperature within a range from −40° C. to −20° C.

Preferably the pressure of the liquid phase comprising liquid CO₂ and water is 1400 kPa and the temperature is −30° C.

The liquid phase comprising liquid CO₂ and water is preferably pumped up to a pressure of about 5000 kPa, wherein temperature increases up to about −27° C.

The liquid phase comprising liquid CO₂ and water and having the high pressure is mixed with the next load of gas, where applicable in a continuous process.

According to the present invention there is also provided a device for dehydrating a CO₂ containing gas comprising a gas feeding system for feeding gas which has to be dehydrated, wherein the device comprises a CO₂ feeding system for feeding liquid CO₂, a contacting device for contacting the gas and the liquid CO₂ for cooling the gas so that water comprised in the gas is partially or completely condensed, and comprises a first separator for separating the condensate from the remaining gas.

The contacting device may be a mixing device for mixing the gas and the liquid CO₂. In some cases the contacting device may be a vessel or a tower.

Preferably, the device comprises an expansion device for cooling the remaining gas which comprises gaseous CO₂ and for obtaining a liquid phase containing liquid CO₂ and water and, optionally hydrates, as well as for obtaining dehydrated gas.

For separating the liquid phase containing liquid CO₂ and water from the dehydrated gas, the device comprises a second separator, wherein the second separator is fluidically connected to the contacting device.

The fluidic connection is arranged in order to feed the liquid phase containing liquid CO₂ and water into the contacting device for cooling the gas.

Preferably, the expansion device is a turbine or a so called turbo expander.

In that case, the device may comprise a a compressor for application of pressure on the gas, wherein the turbine is mechanically connected to the compressor in such a manner that the torque generated by the turbine is usable or is used for driving the compressor.

As an alternative, torque of the turbine is used for driving a generator, wherein electrical energy generated by the generator is used for driving the compressor and/or other aggregates.

The device according to the invention can be a part of a system for extracting CO₂ from gases, in particular from natural gases.

A further subject-matter of the present invention is the use of the device for dehydrating a CO₂ containing gas according to the invention for dehydrating a CO₂ containing gas, in particular for dehydrating natural gas.

That is, subject-matter of the present invention is the use of the device according to the invention for performing the method according to the invention.

The present invention provides a method for the dehydration of gas containing CO₂, based on reception of a bi-phased mix at its expansion and extraction from a mixture of liquid phase in a separator. According to the invention, preferably crude gas is cooled and contacted or mixed with liquid CO₂ with water dissolved in it. The received mixture is divided into a gas phase and a liquid phase containing water. The gas phase is expanded, obtaining the liquid containing liquid CO₂ and water, wherein liquid is partially or completely routed to mixture with the crude gas, thus expansion is possible at a temperature below the temperature of hydrate formation.

In the course of expansion, gas can be passed through a throttle valve or through a turbine, namely in a so called turbo-expander.

In case of gas expansion in a turbo expander it is possible to compress the dry or crude gas in a compressor by means of the torque generated by the turbo expander. That is, the shaft of the turbo expander may be mechanically connected to the compressor.

The gas can also be expanded in a swirling stream in the channel of a cyclonic separator or in the channel of a vortex tube.

Expansion process can occur, at least, in two stages, and in one of the stages expansion occurs in a cyclonic separator and/or in the throttle valve.

The bi-phase mixture received after expansion can be separated to the dry gas and to a bi-phase stream from which it is possible to extract liquid in an additional separator. The liquid phase of the additional separator is compressed, cooled and mixed with crude gas.

In the following the present invention is explained on the basis of examples shown in the enclosed drawings.

FIG. 1 shows in a scheme a device according to the invention in a first embodiment.

FIG. 2 shows in a scheme a device according to the invention in a second embodiment.

FIG. 3 shows in a scheme a device according to the invention in a third embodiment.

FIG. 4 shows in a scheme a device according to the invention in a fourth embodiment.

FIG. 5 shows in a scheme a device according to the invention in a fifth embodiment.

FIG. 6 shows in a scheme a device according to the invention in a sixth embodiment.

FIG. 7 shows in a scheme a device according to the invention in a seventh embodiment.

FIG. 8 shows in a scheme a device according to the invention in a eighth embodiment.

In FIG. 1 a principal diagram shows the installation of a device for realizing a proposed method of low-temperature dehydration of the gas containing CO2. That is, FIG. 1 shows an installation scheme realizing the method according to the invention, wherein gas expansion is carried out in a Joule-Thompson valve.

Crude gas 1 containing CO₂ is cooled by contacting or mixing it with a stream 2 of liquid CO₂ with water dissolved in it.

That is, the crude gas 1 directly contacts the stream 2 containing liquid CO₂ for cooling the gas in order to condense water contained in the crude gas 1.

The obtained mixture 3 is divided in a separator 4 into a gas phase 5 and a liquid phase 6 containing water. Stream 6 may contain water as in a free form, as in the dissolved form, as well as in the form of hydrates. That is, stream 6 may include water as a liquid, liquid CO₂ and/or other condensed components contained in the crude gas 1, gas phase 5 is expanded in a throttle valve 7, obtaining a bi-phase mixture 8 from which in a separator 9 liquid 10 containing liquid CO₂ and water is separated. By means of a pump 11 this liquid or a part of it is conducted to be mixed with crude gas 1. The dried gas 12 is subjected to further processing if necessary, so that the dehydrated gas may be straightly supplied to gas consumer.

Gas expansion in the throttle valve 7 occurs at a temperature below the temperature of hydrates formation.

In order to define the temperature of hydrates formation in a stream after a throttle valve or other expansion devices, widely known software programs, such as HYSYS etc. can be used. The condition of achievement of the stream temperature lower than the temperature of hydrates formation is provided by choice of sufficient degree of gas expansion.

In the described device, gas dehydration is carried out by means of gas cooling during its expansion in the throttle valve 7. Liquid containing liquid CO₂ and water formed in that process is directed to crude gas 1. Water is separated from the liquid phase formed after mixture of crude gas 1 and liquid 2 containing liquid CO₂ and water. It is worth noticing that by means of contacting or mixing of crude gas 1 with a liquid 2 containing liquid CO₂, gas is strongly cooled by means of evaporation of liquid CO₂.

Expansion of gas to temperature below the temperature of hydrates formation allows to increase the efficiency of the gas dehydration process according to the invention. If hydrate crystals are formed in the course of expansion, these hydrate crystals will be dissolved in liquid CO₂ which is also formed during expansion.

In certain cases, when in a stream there is free water and the temperature of a stream 3 is below the temperature of hydrates formation, hydrates of water are separated in the separator 4, and then destroyed by their heating, or by injection of hydrates formation inhibitors (methanol, glycol, etc).

In order to increase gas dehydration, it is possible to carry out a stronger pre-cooling of gas by means of turbo expander as shown in FIG. 2, which is a preferred embodiment of the present invention. That is, in FIG. 2 gas expansion is carried out by means of a turbo expander 13. The turbo expander 13 in that embodiment is installed instead of the throttle valve 7 shown in FIG. 1, wherein a shaft of the compressor 11 may be connected to the shaft of the turbo expander 13.

In FIG. 2 the compressor 14 is installed in the drained gas. However, the compressor 11 could be located either in the drained gas stream, or in the crude gas stream.

Parameters of the streams in the system shown in FIG. 2 are listed in table 1 below.

In the case under consideration, crude gas 1 consists basically of carbonic acid gas. Preferably, inlet pressure of crude gas is 6000 kPa and the temperature of the crude gas is 40° C. Target pressure of the dry gas is 3000 kPa.

As shown in FIG. 3, the compressor 14 is installed in the crude gas stream 1. In order to decrease the temperature of crude gas after the compressor 14, it is expedient to install an additional air cooling device 18. Preferably, also in that embodiment a shaft of the compressor 11 is connected to the shaft of the turbo expander.

The device for dehydration, using the proposed method, can be part of a complex unit for CO₂ extraction from gas.

In certain cases in order to reduce costs and operational expenses, a cyclonic separator or a vortex tube can be installed instead of the turbo expander. In these cases, a lower pressure is achievable in the course of gas expansion in the channel of a cyclonic separator or a vortex tube and it is possible to reach lower temperatures of gas, and thus to increase efficiency of gas dehydration process.

A further possible example of a device and a method according to the invention having a cyclonic separator or vortex tube for gas expansion is shown in FIG. 4. Position 20 designates a cyclonic separator or a vortex tube.

In certain cases it is not possible to qualitatively separate the gas stream from a liquid in a vortex tube or a cyclonic separator. Then, the bi-phase stream 21 routing from a cyclonic separator or a vortex tube is directed to a separator 9 where liquid 10 is separated and directed to mixing with crude gas as shown in FIG. 5. That is, in that embodiment a two-level expansion of gas is carried out.

For a possibility to control parameters of the dehydration process in a wide range, expansion can be carried out at least in two stages. In one of the stages expansion is conducted in a cyclonic separator and/or in a throttle valve.

One of the variants for the realization of the invention with a two-stage expansion of gas is shown in FIG. 6. Here the bi-phase stream flowing from a cyclonic separator or a vortex tube is directed to an additional separator where separation of the liquid, directed to mixture with crude gas, takes place. In this variant first gas expands in turbo expander 13, and then in the channel of a cyclonic separator or a vortex tube 20.

In FIG. 7 a method and a device are shown in which the bi-phase mix obtained after expansion is separated from the dry gas. The liquid is extracted in an additional separator from a bi-phase stream. Gas from an additional separator is compressed, cooled and mixed up with crude gas. That is, the bi-phase mixture obtained after expansion is separated from the drained gas and a bi-phase stream from which liquid in an additional separator is extracted. In FIG. 7 an example for realization of such a case is shown.

After gas expansion in turbo expander 13, gas additionally expands in the channel of a vortex tube (or a cyclonic separator) 20. Before the gas is conducted to the vortex tube (for example to a cyclonic separator), it is possible to separate condensate 29 from gas. The bi-phase stream from the vortex tube is routed to an additional separator 9, the liquid from this additional separator is routed to the crude gas, and the gas is further compressed in the compressor 14, cooled in the air cooling device 18 and mixed with crude gas.

For a stronger cooling of gas, and accordingly lowering the dew-point of the water of the drained gas, crude gas, and/or a mix formed after mixture of crude gas with a liquid containing water in the dissolved form may be cooled.

In FIG. 8 an example for realization of such a cooling is shown. Here crude gas formed after mixture of crude gas with a liquid containing water in the dissolved form is cooled. That is, crude gas in the given variant is cooled consistently in the air cooling device 18 and in the recuperative heat exchanger 24.

In all alternatives of the method according to the invention there is the possibility of adding liquid containing free water to crude gas before its expansion is realized, wherein this liquid can be additionally cooled.

Following table 1 exhibits the operability and the results achieved with the method of the invention, especially the results which are achievable with a device according to FIG. 2.

	TABLE 1
	1	2	3	5	6	8	10	12	15
temperatur, ° C.	40.00	−27.86	15.46	15.46	15.46	−30.23	−30.23	−30.23	31.52
pressure (kPa)	5066.25	5066.25	5066.25	5066.25	5066.25	1400.00	1400.00	1400.00	2985.85
mass flow	300410	52937	353347	337894	15453	337894	52930	284965	284965
(kg/h)
water dissolved in	410	159	325	160	165	160	159	1	1
liquid CO2
(kg/h)
“free” water	0	0	244	0	244	0	0	0	0
(kg/h)
content of CO2, in	0.9768	0.9921	0.9791	0.9811	0.9350	0.9811	0.9921	0.9791	0.9791
mol
content of N2, in	0.0199	0.0006	0.0170	0.0177	0.0028	0.0177	0.0006	0.0209	0.0209
mol
content H2O, in mol	0.0033	0.0073	0.0039	0.0011	0.0622	0.0011	0.0073	0.0000*	0.0000*
*value 0.0000 means that the water content is less than 0.0001

LIST OF REFERENCE SIGNS

• contacting device c
• crude gas 1
• stream of liquid CO₂ with water dissolved in it 2
• mixture of crude gas and liquid CO₂ 3
• first separator 4
• gas phase 5
• liquid phase containing water 6
• throttle valve 7
• bi-phase mixture 8
• second separator 9
• liquid phase containing liquid CO₂ and water 10
• pump 11
• dried gas 12
• turbo expander, turbine 13
• compressor 14
• dried gas 15
• gas phase 16
• additional separator 17
• air cooling device 18
• cooled gas 19
• cyclonic separator or a vortex tube 20
• bi-phase stream 21
• purified gas 22
• mixture of purified gas 22 and gas phase 16 23
• heat exchanger 24
• cooled gas 25
• gas after compression 26
• dried gas 27
• gas phase 28
• condensate 29
• condensate after Joule-Thompson valve 30
• Joule-Thompson valve 31

Claims

1. A method for dehydrating a CO2 containing gas by

cooling the gas (1) and separating the condensed water from the gas (1),

wherein the gas (1) is contacted with liquid CO2 for cooling the gas (1) to condense water contained in the gas (1) and the condensate is separated from the remaining gas, and

wherein the remaining gas which comprises gaseous CO2 is expanded to condense water contained in the gas, and the thus obtained liquid phase comprising liquid CO2 water (10) and, optionally, hydrates is separated from the dehydrated gas.

2. (canceled)

3. The method according to claim 1, characterized in that the obtained liquid phase (10) is at least partially used for contacting with the gas to be dehydrated.

4. The method according to claim 1, wherein the liquid CO2 to be mixed with the gas has a temperature within a range of −60° C. to 20° C.

5. The method according to claim 1, wherein the gas to be mixed with the liquid CO2 has a temperature within a range of −40° C. to 50° C.

6. The method according to claim 1, wherein the temperature of the gas is lowered by expansion below 0° C.

7. The method according to claim 1, wherein the pressure of the gas is lowered by expansion below 7000 kPa.

8. The method according to claim 1, wherein the obtained liquid phase comprising liquid CO2, water (10) and, optionally, hydrates has a pressure of 100 kPa to 7000 kPa and a temperature of −63° C. to 20° C.

9. A device for dehydrating a CO2 containing gas, comprising

a gas feeding system for feeding the gas (1) to be dehydrated,

a CO2 feeding system for feeding liquid CO2,

a contacting device (C) for contacting the gas (1) and the liquid CO2 for cooling the gas (1) to condense water contained in the gas (1),

a first separator (4) for separating the condensate from the remaining gas, and

an expansion device for cooling the remaining gas which comprises gaseous CO2 and for obtaining a liquid phase comprising liquid CO2, water (10) and, optionally, hydrates.

10. (canceled)

11. Device for dehydrating a CO2 containing gas according to claim 9, wherein the expansion device is a turbine (13).

12. Device for dehydrating a CO2 containing gas according to claim 9, wherein the device comprises a second separator (9) for separating the liquid phase (10) from the dehydrated gas, wherein the second separator (9) is fluidicly connected to the contacting device (C).

13. Device for dehydrating a CO2 containing gas according to claim 11, wherein the device comprises a compressor (14) for application of pressure on the dehydrated gas (12), wherein the turbine (13) is mechanically connected to the compressor (14) in such a manner that the torque generated by the turbine (13) is usable or is used for driving the compressor (14).

14. A method comprising using a device for dehydrating a CO2 containing gas according to claim 9 for dehydrating a CO2 containing gas (1), wherein the CO2 containing gas is natural gas.