Process for making a synthesis gas by reforming of a hydrocarbon and including recovery of carbon dioxide at high temperature

Abstract

Process for making a hydrogen-containing synthesis gas (105) from a hydrocarbon feedstock (101), comprising the reforming of said hydrocarbon feedstock and purification of raw synthesis gas, said purification comprising shift conversion of carbon monoxide into carbon dioxide and subsequent absorption of carbon dioxide into an absorbing medium (7a, 14), resulting in a stream of a CO2-rich medium (5), and regeneration of said medium with recovery of CO2 absorbed therein, wherein said raw synthesis gas (102) is produced by the reforming step at a pressure of at least 45 bar, said regeneration of the CO2-loaded medium includes a step of chemical regeneration and the CO2-loaded medium has a temperature of at least 150° C. during said chemical regeneration.

Claims

1. A process for making a hydrogen-containing synthesis gas from a hydrocarbon feedstock, comprising the reforming of said hydrocarbon feedstock into a raw synthesis gas and purification of said raw synthesis gas, said purification comprising shift conversion of carbon monoxide into carbon dioxide and carbon dioxide removal, said removal of carbon dioxide from the synthesis gas including absorption of carbon dioxide into an absorbing medium, resulting in a stream of a CO2-loaded medium, and regeneration of said medium with recovery of CO2 absorbed therein, wherein: said raw synthesis gas is produced by said reforming step at a pressure of at least 45 bar, said regeneration of the CO2-loaded medium includes a step of chemical regeneration wherein the CO2-loaded medium receives a heat input from a heat source, the CO2-loaded medium, during said chemical regeneration process, has a temperature of at least 160° C., wherein said heat source comprises at least one of the following: an effluent of a shift converter; a feed stream of a shift converter; and a cooling medium which circulates in an isothermal shift converter; wherein said heat source for regeneration of the CO2-loaded medium is a heat source stream having a dew point of at least 190° C.

2. The process according to claim 1, wherein the steam-to-carbon ratio in the reforming of said hydrocarbon feedstock is 2.9 or greater.

3. The process according to claim 2, wherein the steam-to-carbon ratio in the reforming of said hydrocarbon feedstock is 3.3 or greater.

4. The process according to claim 1, wherein the regeneration of CO2-loaded medium comprises: a first stage of flashing of the CO2-loaded medium from an input pressure to a predetermined flashing pressure, which results in a first amount of physically released carbon dioxide and a semi-lean medium; a second stage of heat stripping of at least a portion of said semi-lean medium, which includes transferring said heat input to the medium, and results in the production of a second amount of chemically released carbon dioxide and a lean medium, said stage of heat stripping being carried out at a stripping pressure; the second amount of carbon dioxide being at least 40% of the total amount of carbon dioxide.

5. The process according to claim 4, said stripping pressure of the semi-lean medium being at least 3 bar or higher.

6. The process according to claim 5, said stripping pressure being at least 5 bar or higher.

7. The process according to claim 6, said stripping pressure being in the range 5 to 10 bar.

8. The process according to claim 4, said second amount of carbon dioxide being greater than said first amount of carbon dioxide.

9. The process according to claim 4, wherein the flashing pressure, to which the CO2-rich medium is flashed and the semi-lean medium is obtained, is greater than or equal to said stripping pressure.

10. The process according to claim 1, wherein the absorbing medium is an aqueous solution.

11. The process according to claim 10, wherein the absorbing medium is an amine solution.

12. The process according to claim 1, said shift converter being a low-temperature shift converter or a medium-temperature shift converter.

13. The process according to claim 1, wherein the reforming of the hydrocarbon feedstock into said raw synthesis includes steam reforming and secondary reforming, or auto-thermal reforming.

14. The process according to claim 1, wherein the hydrogen-containing synthesis gas is suitable for synthesis of ammonia.

15. The process according to claim 1, wherein at least part of the recovered CO2 is compressed for sequestration or for a further use.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block scheme of reforming a hydrocarbon feedstock and production of a hydrogen-containing synthesis gas.

(2) FIG. 2 is a scheme of a CO2 recovery section according to an embodiment of the invention.

(3) FIG. 3 is a diagram showing the cooling of a shift converter effluent at conventional reforming pressure of about 30 bar, which is typically used in the prior art as the heat source of the stripping of the semi-lean solution.

(4) FIG. 4 is similar to FIG. 3, showing a diagram showing the cooling of a shift converter effluent at a higher pressure, which can be used according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) FIG. 1 illustrates a block scheme of a front-end for making a synthesis gas according to an embodiment of the invention.

(6) Block 100 denotes a reforming section, where a natural gas feedstock 101 is converted into a raw syngas 102, which is purified in a purification section 150 to obtain a product synthesis gas 105.

(7) The purification section 150 comprises a shift section 110 providing a shifted gas 103, a carbon dioxide recovery section 120 providing a CO2-depleted gas 104 and optionally a methanation section 130.

(8) The shift section 110 may comprise one or more shift converters, for example a high-temperature or medium-temperature converter followed by a low-temperature converter.

(9) The front-end usually comprises a number of heat exchangers, e.g. to remove heat form the hot effluent 102 before admission to the shift converter, which are not shown in FIG. 1.

(10) The reforming process in block 100 is operated at a high pressure of at least 45 bar. Accordingly, the shifted gas 103 is at a similar pressure, apart from pressure losses through the shift converter and heat exchangers.

(11) FIG. 2 illustrates a scheme of the CO2 recovery section 120. Said section 120 comprises an absorbing section embodied with an absorber column 1 and a regeneration section embodied with a tower 2 comprising a depressurization zone 3 and a stripping zone 4. The depressurization zone 3 is located above the stripping zone 4.

(12) The CO2 contained in gas 103 is absorbed in the absorber column 1 which produces a CO2-rich solution 5 (loaded solution). The tower 2 separates the CO2 contained in the loaded solution 5 and provides a stream of partially regenerated absorbing solution (semi-lean solution) 7 and a stream of fully regenerated solution (lean solution) 14. The separated CO2 is exported with a first CO2 stream 11 from the depressurization zone 3 and a second CO2 stream 23 from the stripping zone 4.

(13) More in detail, the syngas 103 is supplied to the bottom of the absorber column 1 as stream 103a after a passage in a reboiler 16 of the tower 2. In a lower portion 1b of the column 1, the syngas 103a is contacted with a portion 7a of the semi-lean solution 7 coming from the regeneration tower 2 and, as a consequence, part of the carbon dioxide is absorbed. Then the partially purified syngas passes through the upper portion 1a of the column 1 contacting the lean solution 14 for further CO2 removal (polishing). The CO2-depleted syngas 104 is released from top of the column 1.

(14) The absorption in the column 1 takes place at the high pressure of the gas 103a which, as stated above, is substantially the same pressure as reforming. The loaded solution 5 collected at the bottom of the column 1 is fed to the zone 3 of the tower 2 where it is depressurized to an intermediate pressure, preferably 5 to 10 bar.

(15) Some of the CO2 contained in the loaded solution 5 is released during this step of depressurization, resulting in a gaseous stream 6 containing carbon dioxide, water vapour and small amounts of amine, and the semi-lean solution 7.

(16) The carbon dioxide containing stream 6 is withdrawn from the upper portion of the depressurization zone 3 and passed through a reflux condenser 8 wherein water vapour and amine are condensed. The resulting two-phase stream 9 is passed to a phase separator 10 wherein it is separated into the above mentioned first CO2 gas 11 and into a condensate 12 which essentially comprises water and amine. Said condensate 12 returns to the depressurization zone 3.

(17) A first portion 7a of the semi-lean solution 7 is recycled via pump 13 to the absorber column 1, namely to the lower portion 1b.

(18) A second portion 7b of the semi-lean solution 7 is preheated by the lean solution 14 in a heat exchanger 15 and sent to the stripping zone 4.

(19) The stripping zone is held at an elevated temperature by reboiler 16. A portion 14a of lean solution withdrawn from bottom of the tower 2 enters the reboiler 16, wherein it is partially or completely vaporized, and the vapours so obtained are returned to the stripping zone 4 to drive the stripping process. The heat source of said reboiler 16 is the gas 103. The gas 103 leaves the reboiler 16 as stream 103a and enters the column 1 as shown in FIG. 2. A further heat source (e.g. steam) can be provided if necessary.

(20) A stream of carbon dioxide 19 saturated with water is withdrawn from the top of the stripping zone 4. Said stream 19 passes through a condenser 20 and a separator 21. The separated condensed water 22 is refluxed into the stripping zone 4 and the second CO2 gas 23 is obtained.

(21) The lean solution 14 leaving the bottom of said stripping zone 4 is cooled by the heat exchanger 15 and is recycled to the upper portion 1a of the absorber column 1 via pump 17 and cooler 18.

(22) The majority of the carbon dioxide contained in the loaded solution 5 is removed during the stripping of the semi-lean solution 7b. The stripping of the solution 7b is promoted by the heat recovered from gas 103 (through the reboiler 16) and can be termed heat stripping. Then, the process which releases the CO2 stream 19 is essentially a chemical process. For example about 80% of the total amount of CO2 originally contained in the solution 5 is represented by the chemically-removed CO2 stream 19.

Comparative Example

(23) A comparison of FIGS. 3 and 4 shows the better efficiency of the invention in recovering the heat content of the shift converter effluent, for its use as the heat source of the stripping process of the semi-lean solution. The curves are for the same urea production capacity.

(24) FIG. 3 shows the typical curve of cooling of said effluent, at a low pressure of about 1.7 bar. The horizontal axis denotes the temperature (° C.) and the vertical axis shows the % of heat flow (MW).

(25) The curve shows a typical profile of heat flow when cooling the syngas from an inlet temperature of 210° C. to an outlet temperature of 130° C., which are the common conditions. FIG. 3 may represent for example the cooling of the gas 103 in the boiler 16.

(26) The dew point D is about at 165° C. Above the dew point (portion A of the curve) cooling of the gas results in only a small amount of heat exchanged. For example cooling from 210 to 165° C. results in a transfer of less than 20% of the total heat flow which can be theoretically transferred from 200 to 130° C. The large majority of heat is transferred below the dew point (portion B of the curve) i.e. when cooling the syngas from 165 to 130° C.

(27) The outlet temperature of the syngas is dictated by the temperature of the bottom liquid in the tower 2, which ultimately depends on the pressure since the bottom liquid is saturated. Hence the prior art does not allow to increase the pressure of stripping, since it would result in a higher outlet temperature of the syngas and, consequently, would reduce the heat input available to the stripping of the solution.

(28) FIG. 4 shows an embodiment of the invention wherein, thanks to the higher reforming pressure, the dew point D of the shifted gas is about 200° C.

(29) Accordingly, a larger amount of heat is available at high temperature, in particular more than 60% of the total heat flow is transferred above 170° C. Hence the stripping pressure (and then the pressure of delivery of the CO2) can be increased without affecting the ability to recover heat form the shifted gas.