DE102012015340A1 - Process for the cryogenic separation of methane from synthesis gas, in particular oxo gas - Google Patents

Abstract

The invention relates to a method for separating methane from a synthesis gas stream, in particular oxogas stream (21), comprising the steps: condensing the synthesis gas stream (21) so that a methane-depleted gaseous synthesis gas fraction (22) and a methane-enriched CO-rich condensate (23) are formed , Separating the condensate (23), and separating the condensate (23) in a column (35) into a CH4-rich, liquid stream (24) and a methane-depleted, CO-rich, gaseous stream (25), withdrawing the CO rich stream (25) from the top of the column (35), compressing the CO-rich stream (25), and mixing the CO-rich stream (25) with the methane-depleted synthesis gas fraction (26).

Description

The invention relates to a method for the separation of methane from a synthesis gas, in particular from an oxo gas.

Such a method for the separation of methane from carbon monoxide-containing synthesis gases is z. In DE 20 30 740 B2 described. Thereafter, a gas mixture of hydrogen, carbon monoxide and methane is cooled at a pressure of about 8 bar in one or more condensation stages to the liquefaction temperature of carbon monoxide. The resulting condensate, mainly consisting of liquid carbon monoxide, is then used as a detergent in a countercurrent column to purify the gas mixture described above, with methane, inter alia, being washed out by the liquid carbon monoxide. A disadvantage of this method is that part of the carbon monoxide is discarded together with the methane and thus lost.

On this basis, the present invention seeks to provide an alternative method for reducing the methane content of a synthesis gas stream, in particular Oxogasstromes to achieve greater product purity (H ₂ + CO) by applying a simple cryogenic process available. An oxo gas is a synthesis gas which has a predefined H ₂ / CO ratio, in particular close to 1 (see Table 1).

This problem is solved by a method having the features of claim 1.

Thereafter, it is provided in the inventive method that a synthesis gas stream (feed), in particular in the form of a Oxogasstromes, is partially condensed, so that a methane-depleted (gaseous) synthesis gas fraction (mainly comprising CO and H ₂ ) and a methane-enriched, CO-rich condensate ( comparatively low H ₂ content), the condensate being separated off and separated in a column into a CH ₄ -rich liquid stream (bottom stream) and a correspondingly methane-depleted, CO-rich gaseous stream (top stream), and then the CO-rich stream is withdrawn from the top of the column, compressed and mixed with the methane-depleted synthesis gas fraction (oxogas fraction) to form a synthesis gas product stream (eg, defined methane-depleted oxo gas of defined purity with respect to H ₂ and CO).

In this way, the process advantageously makes it possible to achieve a higher oxogas purity (H ₂ + CO) with simultaneously high H ₂ and CO yields.

Preferably, in the method according to the invention, the synthesis gas stream for partial condensation is cooled in a first and a subsequent second heat exchanger against existing process streams. The two separate heat exchangers can also be designed as a uniform (first) heat exchanger.

The resulting condensate is thereafter preferably separated in a separator from the methane-depleted synthesis gas fraction, preferably at a temperature in a range of -150 ° C to -170 ° C, in particular about -165 ° C, said syngas fraction, in particular from the top of the separator is withdrawn and then warmed in the two heat exchangers.

The produced methane-enriched, CO-rich condensate is preferably expanded for further separation to the pressure of the column connected to the bottom of the separator and passed over the second heat exchanger, where it partially evaporated and placed in said column (CO / CH4 separation column), in the condensate is preferably separated by distillation into said CH ₄ -rich liquid stream (bottom stream) and said methane-depleted, CO-rich gaseous stream (top stream), in particular at a pressure in the range of 10 bar to 70 bar, in particular 10 bar to 20 bar, in particular in about 14.5 bar.

To realize a reboiler (circulation evaporator) of the column, a CH ₄ -rich liquid stream (in particular from a bottom of the column or from a bottom of the column) is withdrawn from this, passed into the second heat exchanger, evaporated there and added back into the column. The reboiler is thus integrated in the second heat exchanger, but can alternatively be realized in a separate heat exchanger.

For cooling the process streams to be cooled, in particular the synthesis gas stream used (partial condensation), z. B. nitrogen can be used, in particular a liquid first N ₂ stream introduced into the two heat exchangers and warmed there against the said process streams, in particular can be evaporated.

By means of such a refrigerant, in particular, a top condenser of the column can be operated, which serves to condense the column reflux in the top of the column. In this case, a liquid second N ₂ stream in a line forming the top condenser (tube coil) are passed through the top of the column and thereby vaporized, said second N ₂ stream preferably for further heating the first N ₂ stream upstream of the second heat exchanger is added. Alternatively, instead of an integrated tube coil, a plate heat exchanger or an external capacitor can also be used as top condenser.

The CH ₄ -rich, liquid stream (bottom product of the column) is preferably withdrawn from the bottom of the column, warmed in the two heat exchangers and delivered in particular at a plant boundary. The CH ₄ -rich material stream can be burned, for example, as fuel.

The CO-rich, withdrawn from the top of the column stream is finally preferably heated in the two heat exchangers and fed into a subsequent compressor in which the CO-rich material stream is compressed and then mixed with the methane-depleted synthesis gas fraction (gas phase from the separator).

The thus advantageously prepared mixture (synthesis gas product stream) is a synthesis gas (oxo gas) with a defined reduced CH ₄ content and a defined purity (H ₂ + CO).

In the present case, the two heat exchangers, the separator and the column are preferably arranged in a cold box or constitute components of a cold box which surrounds those components in an insulating manner in order to limit cooling losses.

According to a variant of the process according to the invention, part of the synthesis gas stream (feed) can be led past the said cold box (thus not passing through the separation described above) and mixed with the methane-depleted synthesis gas product stream (oxogas product stream) coming from the cold box. If the CH ₄ specification of the abovementioned end product is to be kept, more CH ₄ must be removed during the separation process (see below).

Furthermore, the nitrogen used for cooling can also be moved in a closed circuit and then z. B. are not introduced by a plant boundary.

Another idea of the invention is to provide a device or plant for the separation of methane from a synthesis gas, in particular oxo gas, in particular for use in carrying out the method according to the invention.

Thereafter, such a device (plant) has a first and a second heat exchanger for condensing a synthesis gas stream (Oxogasstromes), which may also be formed as a uniform heat exchanger (see above), so that a methane-depleted (gaseous) synthesis gas fraction and a methane-enriched CO rich condensate arise, wherein the heat exchangers are connected to a separator, which is designed for separating the condensate, wherein a bottom of the separator is connected to an inlet of a column, so that the condensate from the bottom of the separator is introduced into the column, wherein the column for separating the condensate into a CH ₄ -rich liquid stream (bottom stream) and a methane-depleted, CO-rich, gaseous stream (top stream) is formed, wherein a head of the column is connected via the two heat exchangers with a compressor, so that the CO-rich material flow for heating the CO-r oak stream can be guided through the heat exchanger in the compressor and there compressible, and wherein the compressor opens into an outgoing from the head of the separator line so that the compressed CO-rich stream is miscible with the methane-depleted synthesis gas fraction, said conduit is adapted to to pass the methane-depleted synthesis gas fraction originating from the top of the separator over the two heat exchangers for heating.

Preferably, the two heat exchangers, the separator and the column are arranged in a cold box (see above).

Furthermore, the column preferably has a reboiler (circulation evaporator), which is preferably formed by the second heat exchanger or integrated into it. The reboiler has, in particular, a line, via which a CH ₄ -rich liquid stream can be withdrawn from the column and introduced into the second heat exchanger, and a further line for the return of the components of the CH ₄ -rich material stream evaporated in the heat exchanger into the column ,

Furthermore, the column preferably has a top condenser, in particular for condensing the column reflux, wherein that head condenser preferably has a tube coil arranged in the top of the column for receiving a refrigerant stream (eg nitrogen). The device may be designed, in particular, to divide a refrigerant stream brought from the boundary of the plant into a first partial refrigerant flow (N ₂ flow) and a second partial refrigerant flow (N ₂ flow), the device (system) being designed in particular to accommodate the first Refrigerant partial flow to evaporate in the two heat exchangers for cooling for cooling process streams, and wherein the device is in particular adapted to lead the second partial refrigerant flow through said line (coil) of the top condenser, evaporate there and for further heating in the first partial refrigerant flow (z B. upstream of the second heat exchanger).

Finally, the device according to the invention (system) can be designed to pass a portion of the synthesis gas stream via a bypass line to the cold box and to mix it with the synthesis gas product stream (oxogas product stream) coming from the cold box.

Further details and advantages of the invention will be explained by the following description of an embodiment and its variants with reference to the figure.

It shows:

1 a schematic representation of the method according to the invention or a corresponding system.

According to 1 For the cryogenic separation of CH ₄ from a Oxogas to be purified Oxogasstrom 21 in an adsorber 31 if necessary, freed of MeOH and CO ₂ and dried (see Table 1) and then into a cold box 37 directed, which receives the apparatus required for cryogenic separation and isolated against cold losses. In the cold box 37 becomes the oxo gas stream 21 first in a first and a subsequent second heat exchanger 32 and 33 cooled against cold process streams and partially condensed. The two separate heat exchangers 32 . 33 can also act as a unified first heat exchanger 32 . 33 be educated.

A resulting two-phase flow becomes a downstream separator 34 fed and at about -165 ° C in a methane-depleted oxogas fraction (gas phase) 22 , which consists mainly of H ₂ and CO, and in a methane-enriched CO-rich condensate (small liquid phase) 23 separated, which consists mainly of CO and has only a small H ₂ content.

The gas phase 22 is then over head from the separator 34 subtracted, in the two heat exchangers 32 and 33 warmed up and leaves the cold box 37 at high pressure. The liquid phase 23 is, however, from the bottom of the separator 34 deducted, on the column pressure of a subsequent CO / CH4 separation column 35 relaxed, in the second heat exchanger 33 partially evaporated and then in said column 35 fed. In that column 35 CO and CH _{4 are} separated by distillation at a pressure of preferably about 14.5 bar. A reboiler 38 the column 35 is in the second heat exchanger 33 integrated. A top condenser 39 the column 35 is as a coil in the head of the column 35 integrated and is preferably with external cold by evaporation of liquid N ₂ 29a operated.

The bottoms product 24 the CO / CH ₄ column 35 is a CH ₄ -rich material stream 24 , which is relaxed, in the two heat exchangers 33 and 32 is evaporated and warmed up and at the plant boundary 14 is discharged (eg as fuel).

At the head of the column 35 becomes a CO-rich material stream 25 subtracted with a defined CH ₄ content. This is in the two heat exchangers 33 and 32 warmed up and at the highest possible pressure from the cold box 37 to a suction side of a compressor 36 led, in which said material flow 25 compressed and then with the warmed gas phase 26 from the separator 34 is mixed. The mixture 27 represents a Oxogasproduktstrom with a defined reduced CH ₄ content and a defined purity (H ₂ + CO). Table 1: Example of the composition and parameters of the oxogas stream 21 used (downstream of the adsorber 31) composition in mol%: Hydrogen (H ₂ ) 49,20 Carbon monoxide (CO) 48.27 Methane (CH ₄ ) 1.16 Nitrogen (N ₂ ) 1.02 Argon (Ar) 0.35 Carbon dioxide (CO ₂ ) 10 ppm Methanol (MeOH) 50 ppm temperature about 40 ° C print about 27 bar

The refrigeration requirement of the process described above is preferred by liquid N ₂ 28 . 29a covered by the plant boundary, which is divided into a first and a second N ₂ stream (partial stream) 28 . 29a can be shared. The first N ₂ stream 28 is doing in the heat exchangers 33 and 32 evaporated and warmed, the other, second N ₂ stream (partial flow) 29a is in the top condenser 39 the column 35 evaporated and then the first N2 stream 28 added for further heating. The combined warmed, gaseous N ₂ stream 29b can be delivered to the atmosphere.

The advantage of the method according to the invention is, in particular, that the majority of the oxogas stream used (use) 21 leaves the process at high pressure (synthesis gas fraction 26 ). The pressure loss via the adsorber station 31 and the cold box 37 is about 1.5 bar. It just has to be so much use 21 in the separator 34 condensed, in the column 35 be purified and then compressed, as necessary, to the required product purity H2 + CO in the Oxogasproduktstrom 27 to reach. In the example described above, only about 7% of the use 21 condensed. The achieved CO yield of the process is 99.5%. The rejected residual gas amount is only about 0.6% of the amount used. The pressure in the column 35 can be chosen in particular so that the necessary compression of the overhead product 25 can be carried out in one stage. The other impurities in the feed gas 21 , N ₂ and argon, remain in the oxo gas.

The adsorbers 31 can be regenerated with N ₂ or synthesis gas.

In a variant of the method according to the invention is provided, a portion of the feed gas 21 in the bypass around the cold box 37 and then with the depleted in CH ₄ oxo gas (Oxogasproduktstrom 27 ) from the cold box 37 to mix. In this case, more CH ₄ must be in the separator 34 condensed and in the column 35 are separated to the same CH ₄ specification in the product stream 27 to achieve. The external cooling demand increases, the column 35 gets bigger and the required power of the compressor 36 is increasing. In return, the heat exchangers 32 and 33 , as well as the separator 34 smaller.

In a further modification of the method according to the invention is provided, instead of liquid N ₂ from the plant boundary a refrigeration cycle z. B. with N2 to provide the required cold for the separation process described above.

As already explained above, the process according to the invention is not restricted to oxo gas as feedstock but can also be used for the CH ₄ separation of other synthesis gases. Furthermore, the described process is not limited to the above-specified use pressure. LIST OF REFERENCE NUMBERS 100 Device (plant) for methane separation 14 Methane-rich bottom stream for combustion, plant limit 21 Synthesis gas stream (insert) 22 methane-depleted synthesis gas fraction 23 methane-enriched CO-rich condensate 24 Sumpfstrom (methane rich) 25 Overhead current (CO-rich) 26 warmed methane-depleted synthesis gas fraction 27 Synthesis gas product stream 28 Liquid nitrogen 29a Liquid nitrogen 29b Evaporated nitrogen 31 adsorber 32 First heat exchanger 33 second heat exchanger 34 separators 35 column 36 compressor 37 Cold box 38 reboiler 39 top condenser

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 2030740 B2 [0002]

Claims (11)

Process for the separation of methane from a synthesis gas, in particular oxo gas, comprising the steps of: - condensing a synthesis gas stream ( 21 ), so that a methane-depleted synthesis gas fraction ( 22 ) and a methane-enriched CO-rich condensate ( 23 ), - separating the condensate ( 23 ), - separating the condensate ( 23 ) in a column ( 35 ) in a CH ₄ -rich, liquid stream ( 24 ) and a methane-depleted, CO-rich gaseous stream ( 25 ), - removal of the CO-rich material stream ( 25 ) from the top of the column ( 35 ), - compacting the CO-rich material stream ( 25 ), and - mixing the CO-rich material stream ( 25 ) with the methane-depleted synthesis gas fraction ( 26 ) for producing a synthesis gas product stream ( 27 ). Process according to Claim 1, characterized in that the synthesis gas stream ( 21 ) for condensing in at least a first and in particular in a subsequent second heat exchanger ( 32 . 33 ) is cooled against at least one process stream. Method according to claim 1 or 2, characterized in that the condensate ( 23 ) in a separator ( 34 ) from the syngas fraction ( 22 ) is deposited, in particular at a temperature in a range of -150 ° C to -170 ° C, especially -165 ° C. Process according to Claims 2 and 3, characterized in that the synthesis gas fraction ( 22 ) from the head of the separator ( 34 ) and in the at least one first heat exchanger ( 32 . 33 ) or in the first and the second heat exchanger ( 32 . 33 ) is warmed up. Method according to claim 2 or any one of claims 3 to 4 as far as dependent on claim 2, characterized in that said condensate ( 23 ) to the pressure of the column ( 35 ), in which at least one first heat exchanger ( 32 . 33 ) or in the second heat exchanger ( 33 ) is partially evaporated, and in said column ( 35 ) for separating the condensate ( 23 ), in particular the condensate ( 23 ) in the column ( 35 ) is separated by distillation, preferably at a pressure in the range of 10 bar to 70 bar, preferably 10 bar to 30 bar, in particular 14.5 bar. Process according to Claim 2 or according to one of Claims 3 to 5 as far as dependent on Claim 2, characterized in that from the column ( 35 ) a CH ₄ -rich liquid stream ( 24 ), in which at least one first heat exchanger ( 32 . 33 ) or in the second heat exchanger ( 33 ) is evaporated and in the column ( 35 ) is returned. Method according to claim 2 or according to one of claims 3 to 6 as far as dependent on claim 2, characterized in that for cooling process streams to be cooled, in particular of the synthesis gas stream ( 21 ), a liquid first N ₂ stream ( 28 ) in the at least one first heat exchanger ( 32 . 33 ) or in the first and the second heat exchanger ( 32 . 33 ) is introduced and vaporized and warmed there. Method according to one of the preceding claims, characterized in that for condensing CH ₄ in the top of the column ( 35 ) a liquid second N ₂ stream ( 29a ) in a pipe, in particular in the form of a pipe coil ( 39 ), through the head of the column ( 35 ) is conducted and evaporated, in particular the second N ₂ stream ( 29a ) for further heating the first N ₂ stream ( 28 ) is added. Process according to claim 2 or any of claims 3 to 8 as far as dependent on claim 2, characterized in that the CH ₄ -rich liquid stream ( 24 ) from the bottom of the column ( 35 ), in which at least one first heat exchanger ( 32 . 33 ) or in the first and the second heat exchanger ( 32 . 33 ) and, in particular, at a plant boundary ( 14 ) is delivered. Process according to claim 2 or any of claims 3 to 9 as far as dependent on claim 2, characterized in that the CO-rich, from the top of the column ( 35 ) withdrawn stream ( 25 ) in the at least one first heat exchanger ( 32 . 33 ) or in the first and the second heat exchanger ( 32 . 33 ) and heated in a compressor ( 36 ), in which the CO-rich material flow ( 25 ) and then with the methane-depleted synthesis gas fraction ( 26 ) for producing said synthesis gas product stream ( 27 ) is mixed. Method according to one of the preceding claims, characterized in that a part of the synthesis gas stream ( 21 ) with the synthesis gas product stream ( 27 ) is mixed.

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