PROCESS FOR HIGH-YIELD PRODUCTION OF HYDROGEN FROM A SYNTHESIS GAS, AND DEBOTTLENECKING OF AN EXISTING UNIT

Abstract

Process for debottlenecking a plant that produces hydrogen including reforming of hydrocarbons, then conversion of CO, purification of hydrogen by PSA-H2 for the production of a high-pressure gaseous stream of ultra-pure hydrogen with associated production of a low-pressure residue, the two major constituents of which are carbon dioxide and hydrogen, the debottlenecking of the plant is carried out by installing, level with the PSA residue, an EHS electrochemical cell for supplying, from the PSA residue, hydrogen and a hydrogen-depleted residue, the additional hydrogen stream recovered in the EHS cell is compressed and sent to the inlet of the PSA unit thus increasing the hydrogen production of the plant while keeping the purity of the hydrogen produced by the PSA unchanged. The invention also relates to a process and a plant for producing hydrogen having an optimized hydrogen yield.

Claims

1.-11. (canceled)

12. A method for debottlenecking a hydrogen production plant comprising; a module for generating a synthesis gas by reforming from light hydrocarbons, a shift module for enrichment in hydrogen and carbon dioxide by conversion of the carbon monoxide contained in the synthesis gas with water vapor, and a PSA-H.sub.2 unit for the purification of hydrogen and the production of a high-pressure gas stream of ultrapure hydrogen with associated production of a low-pressure gaseous waste, the two major constituents of which are carbon dioxide and hydrogen, the method comprising: installing an electrochemical hydrogen purification cell on the PSA low-pressure gaseous waste thereby separating hydrogen and a hydrogen-depleted waste from the PSA waste, recovering the hydrogen to form an additional hydrogen stream which is compressed to a pressure of between 8 and 25 bar and sending at least a portion of the compressed hydrogen stream to the inlet of the PSA unit to increase the hydrogen production of the plant while keeping the purity of the hydrogen produced by the PSA unchanged.

13. The debottlenecking method as claimed in claim 1, wherein, in the event of production of excess hydrogen, the operation of the electrochemical hydrogen purification cell is interrupted so as to optimize the power consumption of the plant

14. A hydrogen production process comprising the steps of: a) generating, by reforming, a synthesis gas from a light hydrocarbon feedstock, b) enriching the synthesis gas with hydrogen and carbon dioxide by steam conversion of the carbon monoxide to give carbon dioxide, c) purifying the enriched synthesis gas for the production of a high-pressure gas stream of ultrapure hydrogen by pressure swing adsorption with associated production of a low-pressure gaseous PSA waste, the two major constituents of which are carbon dioxide and hydrogen, d) supplying an electrochemical cell with at least part of the low-pressure PSA waste in order to recover additional hydrogen from the PSA waste, and a hydrogen-depleted waste, e) compressing the additional hydrogen recovered to a pressure of between 8 bar and 25 barg, and f) recycling all or part of the compressed recovered additional hydrogen in the process upstream of the PSA unit to supply the PSA so as to increase the production yield of very high purity hydrogen of the plant.

15. The process as claimed in claim 14, wherein step e) of compressing the additional hydrogen recovered is carried out at least in part by the electrochemical cell.

16. The process as claimed in claim 14, wherein at least one portion of the additional hydrogen recovered at the outlet of the electrochemical cell is used to desulfurize the light hydrocarbon feedstock prior to step a).

17. The process as claimed in claim 14, wherein at least part of the hydrogen-depleted waste leaving the electrochemical cell is recovered to produce carbon dioxide.

18. The process as claimed in claim 14, wherein at least one portion of the hydrogen-depleted waste leaving the electrochemical cell is used as reforming fuel.

19. A plant for producing hydrogen from a light hydrocarbon feed stream having an optimized yield comprising: a module for generating, by reforming, a synthesis gas from said light hydrocarbon feed stream; a module for steam conversion of the carbon monoxide to give carbon dioxide, for enriching the synthesis gas with hydrogen and carbon dioxide; a PSA-H.sub.2 unit for purifying the hydrogen contained in the synthesis gas with production of an outgoing high-pressure gas stream of ultrapure hydrogen and associated production of a low-pressure outgoing gaseous PSA waste, the two main constituents of which are carbon dioxide and hydrogen; an electrochemical cell capable of being supplied by the low-pressure gaseous PSA waste and capable of separating hydrogen present in the PSA waste from the other constituents so as to produce a hydrogen stream and a hydrogen-depleted waste; a means for compressing the hydrogen stream separated by the EHS cell; a means for treating and/or a means for using the EHS cell waste; and a means for discharging, conveying and supplying the various streams used.

20. The plant as claimed in claim 19, wherein the electrochemical cell is configured to carry out, at least in part, the compression of the additional hydrogen recovered.

21. The plant as claimed in claim 19, further comprising means for compressing and transferring at least a portion of the additional hydrogen recovered at the outlet of the electrochemical cell to a module for desulfurization of the light hydrocarbon feedstock.

22. The plant as claimed in claim 19, further comprising means for using the hydrogen-depleted waste leaving the electrochemical cell as a fuel for the reforming and/or for producing carbon dioxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The invention will be better understood by virtue of the following description given with reference to the appended figures, among which:

[0075] FIG. 1 is a block diagram of a conventional hydrogen production plant;

[0076] FIG. 2 is a block diagram of a hydrogen production plant of the same type, but debottlenecked in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0077] According to the conventional diagram of FIG. 1, a hydrocarbon feedstock 1 intended to produce the hydrogen is subjected to a(n) (optional) compression step 2a, then to a desulfurization step 2b and to a(n) (optional) preforming step 2c, before being mixedafter preheating, not shownwith water vapor at the mixing point 3 and then introduced into a steam reforming reactor 4 where it is reformed at high temperature by means of external heat supplied by burners 5 installed in the walls of the reactor 4it being possible for the burners to be installed in the side walls, installed in a terraced manner, in the crown or in the floor depending on the manufacturersto produce a synthesis gas 6, a mixture containing for the most part the hydrogen and carbon oxidesmainly CO.

[0078] The synthesis gas 6, also known as syngas, is produced at high temperature (of the order of 600 C.-800 C.) and high pressure, it is then enriched in H.sub.2 and CO.sub.2 in a shift reactor 7 by conversion of the CO by the excess water vapor present in the syngas to produce the hydrogen-enriched syngas 8.

[0079] After cooling in 9a and 9b to room temperature and with separation of the condensates 10, the syngas enriched in H.sub.2 and CO.sub.2 and cooled 11 supplies a PSA unit 12.

[0080] In terms of small production, for example for a hydrogen flow rate of less than 2000 Nm.sup.3 of H.sub.2, the H.sub.2 yield of the PSA is of the order of 78-80% for 4 adsorbers; and as reported in table 1, it increases in the case of large-sized plants reaching 88-89% for 10 adsorbers for plants producing 50 000 Nm.sup.3/h or more.

[0081] The PSA unit 12 produces ultrapure hydrogen 13 under pressure, and also a low-pressure gaseous waste 14 which combines all the components present in addition to hydrogen in the syngas 11 supplying the PSA, i.e. the very predominant CO.sub.2, but also CO, residual CH.sub.4, water vapor, nitrogen, but also alongside these gases, hydrogen in a proportion that is greater, the smaller the plant is.

[0082] The hydrogen produced 13 passes (optionally) into a production buffer tank (not referenced) in order to smooth out the pressure and flow rate variations related to the PSA cycles. A buffer capacity 14 is installed on the PSA waste gas to smooth out variations in pressure, flow rate and composition of the waste gas that could affect the correct operation of the reforming furnace burners.

[0083] The waste gas is used as fuel gas, especially for heating the reformer, owing to its hydrogen and methane contents.

[0084] The diagram does not reproduce the complexity of the plant; among the elements of the overall processnot necessary for the understanding of the inventiononly some are present (referenced or not): heat exchanger 9b between the syngas 8 and water with recovery of the condensates 10 upstream of the PSA, supply and preheating of the combustion air, supply of water to the plant with heating in the convection chamber of the reformer against the flue gases and in the exchanger 9b against the syngas etc.

[0085] The material balance of the hydrogen recovery for a plant of conventional type such as the one from FIG. 1 on the basis of a PSA with 4 adsorbers is presented in tables 2A, 2B and 2C below.

TABLE-US-00002 TABLE 2A compositions in mol % Fluid reference 11 14 13 Components % % % Hydrogen H.sub.2 76.4 39.2 >99.99 Nitrogen N 00.2 00.6 <100 ppm Methane CH.sub.4 03.5 08.9 <10 ppm Carbon monoxide CO 02.0 05.1 <10 ppm Carbon dioxide CO.sub.2 17.7 45.5 <10 ppm Water H.sub.2O 00.3 00.7 <10 ppm Total 100.0 100.0 100.0

TABLE-US-00003 TABLE 2B Parameters (Temperature, Pressure, Flow Rates) Temperature C. 35 35 35 Pressure in barg 21 0.01 20 Flow rate Nm.sup.3/h 1000 389.12 610.88

TABLE-US-00004 TABLE 2C How Rates (Nm.sup.3/h) Hydrogen H.sub.2 763.60 152.72 610.88 Nitrogen N 002.40 002.40 Methane CH.sub.4 034.70 034.70 Carbon monoxide CO 019.70 019.70 Carbon dioxide CO.sub.2 176.90 176.90 Water H.sub.2O 002.70 002.70

[0086] Overall, the hydrogen efficiency of this conventional plant is that of the PSA, it is therefore 80% (=H.sub.2 flow rate of stream 13/H.sub.2 flow rate of stream 11).

[0087] The diagram of FIG. 2 represents a plant deduced from that of FIG. 1, but which has been debottlenecked in accordance with the invention. The elements of FIG. 1 that are in FIG. 2 bear the same references, in particular all the fluids and means participating in the generation of the synthesis gas upstream of the purification of hydrogen.

[0088] Thus, the hydrocarbon feedstock 1 is here also compressed, desulfurized and prereformed in 2a, 2b, 2c before being mixed with the water vapor at the mixing point 3 and then introduced into the steam reforming reactor 4 where it is reformed at high temperature by means of external heat supplied by burners 5 to produce the synthesis gas (or syngas) 6.

[0089] The syngas at high temperature and high pressure is enriched in H.sub.2 and CO.sub.2 in a shift reactor 7 by reaction between water vapor and the CO present in the syngas.

[0090] After cooling to room temperature and separation of the condensates, the syngas 11 enriched in H.sub.2 and CO.sub.2 is sent to the PSA unit.

[0091] The PSA unit 12 produces very high purity hydrogen 13 under pressure, and also the low-pressure gaseous PSA waste 14.

[0092] The hydrogen produced 13 passes (optionally) in a production buffer tank (not referenced) in order to smooth out the pressure and flow rate variations related to the PSA cycles. A buffer capacity 14 is installed on the PSA waste gas in order to smooth out variations in pressure, flow rate and composition of the PSA waste gas.

[0093] In accordance with the invention, the waste gas 14 supplies an electrochemical purification cell 15 which operates in the following manner: the electrochemical cell separates the constituents of the waste 14 from the hydrogen and thus produces hydrogen 16 and a second gas stream 20 containing essentially all of the gases present in the PSA waste 14 with only a few percent of hydrogen. This second gas stream 20 (identified as EHS cell waste) isin the exampleused as a fuel gas for heating the reformer. Other uses known per se are possible depending on the circumstances and requirements. The hydrogen 16 is compressed in 17, the gas thus compressed 18 is combined with the syngas 11 to form a new feed gas 19 for the PSA 12.

[0094] The material balance of the hydrogen recovery for a plant of conventional type such as the one from FIG. 1 is presented in tables 3A, 3B and 3C below which present a (new) material balance calculated for the debottlenecked unit:

TABLE-US-00005 TABLE 3A compositions in % Fluid reference 11 19 14 20 16 18 13 Components % % % % % % % H.sub.2 76.4 79.1 37.6 3.0 98.40 98.40 >99.99 N 0.2 0.6 1.0 00.05 00.05 <100 ppm CH.sub.4 3.5 9.1 14.2 00.05 00.05 <10 ppm CO 2.0 5.2 8.1 00.05 00.05 <10 ppm CO.sub.2 17.7 46.2 72.6 00.05 00.05 <10 ppm H.sub.2O 0.3 1.2 1.1 01.40 00.30 <10 ppm Total 100.0 100.0 100.0 100.0 100.00 98.90 100

TABLE-US-00006 TABLE 3B Parameters (Temperature, Pressure, Flow Rates) Temper- 35 35 35 35 35 35 35 ature Pressure 21 0.01 0.01 0.01 15 21 20 in barg Flow 1000 1139.1 382.70 243.60 139.10 139.10 756.40 rate Nm.sup.3/h

TABLE-US-00007 TABLE 3C (Nm.sup.3/h) H.sub.2 763.60 900.47 144.08 7.20 136.87 136.87 756.40 N 2.40 2.47 2.47 2.40 0.07 0.07 CH.sub.4 34.70 34.77 34.77 34.70 0.07 0.07 CO 19.70 19.77 19.77 19.70 0.07 0.07 CO.sub.2 176.90 176.97 176.97 176.90 0.07 0.07 H.sub.2O 2.70 4.65 4.65 2.70 1.95 1.95
in which the estimated compression power is 51.64 kW, the estimated EHS power is 23.24 kW.

[0095] The overall hydrogen efficiency is 99% (Table 3C: fluid 13 values/fluid 11 values) with an EHS hydrogen efficiency of 95% (Table 3C: fluid 16 values/fluid 14 values), and a PSA hydrogen efficiency of 84% (Table 3C: stream 13 values/stream 19 values).

[0096] In the example presented here, for the same flow rate as in the conventional version without EHS, the flow rate of hydrogen produced (with identical purity) thus changes from 610 Nm.sup.3/h to 756 Nm.sup.3/h, an increase of 24% for a maximum additional electricity requirement of 75 kW.

[0097] This additional electricity requirement can be advantageously reduced (to around 40 kW) by combining the electrochemical purification step and the compression step in the same electrochemical cell.

[0098] The separation of hydrogen by proton exchange membrane PEMcarried out in the EHS cellsapplied to the separation of hydrogen contained in the PSA gaseous waste functions in the following manner: the PSA gaseous waste, available at a temperature of the order of room temperature and at a pressure of 300 to 500 mbar above atmospheric pressure supplies an electrochemical cell which contains catalyst-covered electrodes on either side of a membrane. When the electric current passes into the electrodes, the PEM membrane used in the EHS cell allows the hydrogenin H.sub.3O.sup.+ formto pass selectively through the membrane, so that pure hydrogen is recovered from the other side.

[0099] The reactions involved are:

[0100] At the anode: H.sub.2=>H.sup.+ e

[0101] At the cathode: H.sup.+ e=> H.sub.2

[0102] Ultimately, the balance is: H.sub.2=>H.sub.2, hydrogen being transferred from the anode compartment to the cathode compartment.

[0103] The electrochemical potential is:

[00001]$E^{\mathrm{cathode}}-E^{\mathrm{anode}}=-\frac{R.Math.T}{2.Math.F}.Math.\ln \left(\frac{P_{H_2}^{\mathrm{Cathode}}}{P_{H_2}^{\mathrm{Anode}}}\right)$

[0104] At the same time, the membrane thereby creates a second stream containing the other compounds of the PSA waste, which cannot pass through the membrane which rejects them; they form the rejected stream. This rejected streamthe stream 20 of FIG. 2 and the exampleis therefore the waste gas of the EHS within the meaning of the invention. It could, depending on its composition, be actually rejected, or treated and/or reused in other processes, or used as fuel in the reforming furnace as shown in the example presented.

[0105] As for the hydrogen thus recovered at the outlet of the EHS cell, it does not have a sufficient purity to be added to the hydrogen produced by the PSA, the quality of which it would greatly degrade, after compression. On the other hand it is perfectly suitable for being recycled to feed the PSA. It should be noted that the hydrogen can also be simultaneously compressed.

[0106] Among the advantages of the invention, mention will be made of: [0107] increase in the hydrogen production of an existing plant in proportions much greater than those that can be achieved by conventional debottlenecking means; [0108] no modifications of the plant requiring expensive work; [0109] preserving the purity of the hydrogen gas produced; [0110] in the case of a new plant, adopting the solution of the invention during construction, production of very high purity hydrogen with a maximum hydrogen yield, oversizing of the other equipment (SMR notably) can then be avoided; [0111] possibility of recovering the second fluid produced by the EHS cell (stream 20 in FIG. 2) lean in H.sub.2 and rich in CO.sub.2 in order to produce CO.sub.2 if this is upgradable or in order to capture CO.sub.2 if need be.

[0112] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.