Method for producing hydrogen by reforming hydrocarbons using steam, combined with carbon dioxide capture and steam production

Abstract

A method for producing hydrogen by reforming hydrocarbons using steam, combined with carbon dioxide capture and steam production, which involves mixing the hydrocarbons to be reformed with steam in order to produce a feedstock for reforming, generating a syngas; the syngas produced is cooled, enriched with H2 and CO2, and then cooled; the condensates of the method are separated from the syngas in order to be used in the method, the saturated syngas being treated by adsorption with pressure modulation so as to produce hydrogen and a gaseous effluent containing CO.sub.2 that is captured in a CPU unit. The condensates from the cooling of the syngas at the outlet of the shift reactor are used in the method for producing impure steam supplying the mixing point; the CPU unit also produces CPU condensates that are recycled to be treated jointly with the condensates of the method.

Claims

1. A process for producing hydrogen by reforming hydrocarbons using steam, combined with carbon dioxide capture and steam production, the process comprising the steps of: a) mixing the hydrocarbons to be reformed with steam so as to produce a feedstock for reforming; b) reforming the feedstock under conditions effective to produce a synthesis gas comprising hydrogen, carbon monoxide and also excess steam, carbon dioxide, residual methane, water and impurities from feedstock; c) cooling the synthesis gas; d) generating a synthesis gas enriched with H.sub.2 and CO.sub.2 by steam conversion of the CO contained in the cooled synthesis gas resulting from step c); e) cooling the synthesis gas enriched with H.sub.2 and CO.sub.2 resulting from step d) by indirect heat exchange to a temperature at least below or equal to 60 C., so as to produce a saturated synthesis gas and a first condensates; f) separating the first condensates from the saturated synthesis gas resulting from e); g) treating the synthesis gas resulting directly or indirectly from step f) in a unit for hydrogen purification by pressure swing adsorption, so as to produce a stream of hydrogen having a purity of at least 99% and a stream of offgas containing at least 40% of CO.sub.2; h) treating the stream of PSA offgas resulting from step g) in a cryogenic purification unit (CPU) unit producing at least one stream of purified CO.sub.2 and at least one stream of non-condensables; i) treating all or part of the first condensates resulting from the process, wherein step i) further comprises the steps of: k1) reducing the pressure of the first condensates to a pressure of between 1 and 5 bar(a); k2) passing the first condensates through a deaerator in order to eliminate a part of the gases dissolved in the first condensates; k3) pumping the first condensates at a pressure between 15 and 90 bar(a); k4) preheating the first condensates under pressure at a temperature between 150 and 290 C.; k5) vaporizing the first condensates under pressure at a temperature of between 200 and 300 C., so as to produce a stream of impure steam; and k6) recycling the impure steam to step a) so as to be mixed with the hydrocarbons to be reformed; and j) producing highly pure steam intended for export, wherein step j) further comprises the steps of: v1) feeding a steam circuit associated with the process with demineralized water alone or mixed with the first condensates; v2) preheating the water; v3) vaporizing the preheated water against the synthesis gas originating from the reforming in a waste heat boiler so as to produce a stream of steam; and v4) recycling a part of the steam to step a) so as to be mixed with the hydrocarbons to be reformed, wherein treatment step h) also produces at least one stream of second condensates, and the second condensates are recovered and recycled into the process upstream of step k5) of vaporizing the first condensates.

2. The process as claimed in claim 1, wherein the second condensates are mixed with the first condensates before the deaeration of the first condensates.

3. The process as claimed in claim 1, wherein the second condensates are preheated and then mixed with the first condensates before the deaeration of the first condensates.

4. The process as claimed in claim 1, wherein the second condensates are mixed with the first condensates after the deaeration but before the pumping of the first condensates.

5. The process as claimed in claim 1, wherein the second condensates are mixed with the first condensates after the deaeration and the pumping but before the preheating of the first condensates.

6. The process as claimed in claim 1, wherein the second condensates are mixed with the first condensates after the preheating of the first condensates.

7. The process as claimed in claim 1, wherein the second condensates are preheated and then mixed with the first condensates after the preheating of the first condensates.

8. The process as claimed in claim 1, further comprising the steps of washing the PSA offgas stream with water, thereby producing an impurity-loaded stream; and recycling the impurity-loaded liquid stream into the process upstream of step k5) of vaporizing the first condensates.

9. The process as claimed in claim 1, wherein only a part of the demineralized water of step v1) is mixed with the first condensates.

10. The process as claimed in claim 1, wherein all the demineralized water of step v1) is mixed with the first condensates.

11. A facility for producing hydrogen by reforming hydrocarbons using steam, combined with carbon dioxide capture and steam production, the facility comprising: (i) a mixing point configured to mix the hydrocarbons to be reformed with steam so as to produce the feedstock for the reforming; (ii) a reformer configured to produce a synthesis gas comprising essentially hydrogen, carbon monoxide and also excess steam, carbon dioxide, residual methane, water and impurities from the mixture resulting from step a); (iii) at least one heat exchanger configured to cool the synthesis gas; (iv) at least one shift reactor for steam conversion of the CO contained in the cooled synthesis gas resulting from the exchanger (iii); (v) at least one heat exchanger configured to cool the synthesis gas enriched with H.sub.2 and CO.sub.2 produced by the shift reactor (iv) by indirect heat exchange to a temperature at least below or equal to 60 C., so as to produce a saturated synthesis gas and the first condensates; (vi) a separator configured to separate the first condensates from the saturated synthesis gas resulting from the exchanger (v); (vii) a unit for hydrogen purification by pressure swing adsorption for treating the synthesis gas resulting directly or indirectly from step (vi) and producing a stream of hydrogen having a purity of at least 99% and a stream of offgas containing at least 40% of CO.sub.2; (viii) a cryogenic purification unit (CPU) unit configured to treat the PSA offgas stream produced by the PSA unit (vii) and producing at least one stream of purified CO.sub.2 and at least one stream of non-condensables, and also means for treating all or part of the first condensates from the separator (vi) and comprising at least (ix) a valve configured to reduce the pressure of the first condensates to a pressure of between 1 and 5 bar(a); (x) a deaerator configured to eliminate a part of the gases dissolved in the first condensates; (xi) a pump configured to pump the first condensates at a pressure of between 15 and 90 bar(a); (xii) a heat exchanger configured to preheat the first condensates under pressure at a temperature of between 150 and 290 C.; (xiii) a heat exchanger configured to vaporize the first condensates under pressure at a temperature of between 200 and 300 C., so as to produce a stream of impure steam, wherein the heat exchanger is in fluid communication with the mixing point such that the mixing point is configured to receive the stream of impure steam from the heat exchanger; (xiv) a steam production circuit configured to produce steam, and at least: (xv) a conduit configured to feed a circuit of steam associated with the process with demineralized water alone or mixed with the first condensates; (xvi) a heat exchanger configured to preheat the water; (xvii) a heat exchanger configured to vaporize the preheated demineralized water against the synthesis gas originating from the reforming in a waste heat boiler so as to produce a stream of steam; and (xviii) a recycling conduit configured to recycle a part of the steam to step a) so as to be mixed with the hydrocarbons to be reformed; wherein since the CPU unit also produces at least one stream of second condensates, the facility also comprises means for recovering and recycling the second condensates so as to be treated together with the first condensates.

12. The facility as claimed in claim 11, further comprising means for mixing the second condensates with the first condensates placed before the deaerator of the first condensates.

13. The facility as claimed in claim 11, further comprising means for preheating the second condensates, means for mixing the preheated condensates C2 with the first condensates placed before the deaerator of the first condensates.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

(2) The FIGURE shows an embodiment of the present invention.

DETAILED DESCRIPTION

(3) Other characteristics and advantages of the invention will emerge on reading the description which follows. One embodiment of the invention is represented therein in the single figure, and described hereinafter by way of nonlimiting example.

(4) The figure thus presents a scheme for a process for producing hydrogen according to the invention with separate productions of steams of distinct qualities in two distinct circuits.

(5) A stream 1 of natural gas (GN) is sent to the mixing point 2 so as to be mixed there with two streams of steam 22 and 27 produced in the process. The mode of production of these two streams of steam will be described subsequently. The stream 3 resulting from the mixing constitutes the feedstock feeding a steam methane reforming (SMR) module 4, the gas produced 5 is a synthesis gas comprising essentially hydrogen H.sub.2 and CO, but also CO.sub.2, CH.sub.4, water and impurities; the gas 5 is cooled by heat exchange with water, first in the waste heat boiler 6 where the water is vaporized and then via a cooling module 6b comprising two heat exchangers in parallel, then feeds the conversion module 7, thus producing a synthesis gas enriched with H.sub.2 and CO.sub.2 and depleted of CO compared with the gas 5 produced by reforming, and also containing additional impurities generated during the conversionof about 65% to 85% of H.sub.2, 11% to 22% of CO.sub.2, 0.5% to 6% of unconverted CO and 3% to 10% of CH.sub.4.

(6) The synthesis gas is then cooled in 8 by heat exchange against water, in particular against demineralized water 18, to a temperature below or equal to 60 C., more generally below or equal to 40 C., allowing partial condensation of the water present in the synthesis gas and also of certain condensable impurities. The saturated synthesis gas 9 is separated from the liquid fraction 10 consisting of the impure process condensates C1 (i.e. loaded with impurities entrained with the water).

(7) The synthesis gas 9 is then sent at 11 to the H.sub.2 PSA unit which produces at least one gaseous stream of highly pure (greater than 99%) hydrogen, and also a residual gas 12 which for its part contains all the carbon dioxide, the vast majority of the unconverted methane and of the unconverted carbon monoxide, and a large part of the nitrogen and hydrogen, the quality of which depends on the yield from the H.sub.2 PSA.

(8) The residual gas 12 is sent at 13 to a CPU unit for separation of the CO.sub.2, the CPU unit producing at least one stream 14 of CO.sub.2, a gaseous stream 15 rich in hydrogen H.sub.2 which is recycled so as to feed the H.sub.2 PSA in order to recover the hydrogen contained and to thus improve the overall hydrogen yield of the facility, a stream 16 of non-condensables comprising methane, hydrogen, carbon monoxide, non-condensed carbon dioxide, nitrogen and water, constituting the CPU offgas which is recycled as reforming fuel to the reforming module 4, and a liquid stream 17 made up of the CPU condensates C2; produced in the CPU unit, at least partly at the compression/drying stage which precedes the first CO.sub.2 condensation step, these CPU condensates (condensates C2) contain predominantly water and also dissolved impurities, for instance methanol, aqueous ammonia and amines. By application of the process of the invention, the condensates C2 are recycled so as to be treated with the process condensates C1.

(9) More specifically, the stream 10 of the process condensates C1, the stream 17 of the CPU condensates C2 and an additional stream of water 19 taken from the external supply of demineralized water 18 and heated by heat exchange with the synthesis gas in the heat exchanger 8 are combined so as to form a stream 20 containing water and the impurities contained in the two streams of condensates C1 and C2. The resulting stream 20 is treated in the deaerator 21 then heated by heat exchange against the synthesis gas in one of the two heat exchangers of the synthesis gas cooling module 6b and vaporized against the flue gases F in the convection zone of the reforming module 4, thus forming the stream of impure steam 22. This impure steam containing the impurities contained in the condensates C1 and C2 feeds the mixing point 2.

(10) Alongside this circuit for producing dirty steam, the process of the FIGURE makes available highly pure superheated steam for the specific needs of the process and for the production of export steam H.sub.2O(v). This second steam production circuit, distinguished from the impure steam circuit, operates in the following way. The second part 23 of water taken from the external supply of demineralized water 18 is heated against the synthesis gas in the exchanger 8, treated in the deaerator 24, then heated by heat exchange against the synthesis gas in the second heat exchanger of the module 6b, and vaporized in the boiler 6, thus forming the stream of steam 25. This stream of highly pure steam passes through the convection zone of the reformer where it recovers additional heat from the flue gases F; it is separated into two parts, one of them constitutes the superheated highly pure export steam 26, the second forms the stream of steam 27 sent to the mixing point 2.

(11) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(12) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(13) Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of comprising). Comprising as used herein may be replaced by the more limited transitional terms consisting essentially of and consisting of unless otherwise indicated herein.

(14) Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

(15) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(16) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

(17) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.