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 the feedstock for the reforming, generating a syngas; the syngas is then cooled, and enriched with H.sub.2 and CO.sub.2; 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 CO.sub.2-purifying unit. The condensates from the cooling of the syngas at the outlet of the shift reactor are used in the method for producing highly pure steam supplying the mixing point and for exportation; the CPU 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 highly pure 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 the 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 with demineralized water to a temperature of about ambient temperature, so as to produce a saturated synthesis gas and first condensates; f) separating the first condensates from the saturated synthesis gas resulting from step 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) producing at least one stream of purified CO.sub.2 and at least one stream of non-condensables; and i) treating all or part of the first condensates resulting from the process, wherein step i) further comprises the steps of: k1) pumping the first condensates at a pressure between 15 and 90 bar(a); k2) heating the first condensates under pressure at a temperature between 150 and 290 C.; k3) steam stripping of the compressed and heated first condensatesin a stripperusing a first portion of a highly pure steam so as to produce an impure steam stream at the top of the column and a stream of purified condensates at the bottom of the column; k4) recycling the impure steam to step a) so as to be mixed with the hydrocarbons to be reformed; k5) recycling the purified condensates into the demineralized water in step e); j) producing highly pure steam intended for export, wherein step j) further comprises the steps of: v1) feeding a highly pure steam circuit with the demineralized water; v2) preheating the demineralized water circulating in the circuit by indirect heat exchange with at least one fluid from the process to form a preheated pure water; v3) vaporizing the preheated pure water against the synthesis gas originating from the reforming in a waste heat boiler to form the highly pure steam; v4) using the first portion of the highly pure steam for step k3); v5) recycling a second portion of the highly pure steam to step a) so as to be mixed with the hydrocarbons to be reformed; v6) making a third portion of the highly pure steam available for export, wherein the treatment step h) also produces at least one stream of second condensates, and are recovered and recycled into the process upstream of step k3) of steam stripping, so as to be treated together with the first condensates.

2. The process as claimed in claim 1, wherein the first condensates and the second condensates are introduced separately into the stripper, the introduction of the stream of the most impure condensates being carried out above that of the stream of the purer condensates.

3. The process as claimed in claim 2, wherein the second condensates are preheated in a dedicated preheater before introduction into the stripper.

4. The process as claimed in claim 2, wherein the second condensates are pumped before introduction into the stripper.

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

6. The process as claimed in claim 1, wherein the second condensates are pumped and then mixed with the first condensates before the pumping of the first condensates.

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

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

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

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

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

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

13. The process as claimed in claim 1, wherein step h) further produces a liquid stream comprising CPU condensates, wherein the process further comprises the steps of: withdrawing the liquid stream from the CPU; and recycling the liquid stream comprising CPU condensates into the process upstream of step k3) of steam distillation, so as to be treated together with the first condensates and the second 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) The single FIGURE thus illustrates a process in which 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 23 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 heat exchanger 6b, 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 conversionthus containing 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.

(4) The synthesis gas is then cooled in 8 by heat exchange against water, in particular against demineralized water 20, 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 condensate C1 (i.e. loaded with impurities entrained with the water).

(5) 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, a large part of the nitrogen and hydrogen the quality of which depends on the yield from the H.sub.2 PSA.

(6) 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 enriched with 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 liquid stream 10 of the process condensates C1.

(7) The process condensate C1 form the stream 10; augmented with the CPU condensates C2which form the stream 17they are subjected, in the stripper 18, to steam stripping countercurrently to a stream of highly pure steam 19 taken from highly pure steam circulating in an associated highly pure steam production circuit; the stream 22 of steam feeding the mixing point 2 is also taken from the associated highly pure steam production circuit.

(8) The associated highly pure steam production circuit operates in the following way. The watersteam or liquidcirculates at a pressure between 20 and 60 bar(a). An external demineralized water supply 20 is provided, to which are added the purified condensates 21 recovered at the outlet of the stripper 18; the resulting stream of pure water is heated by heat exchange with the syngas in the heat exchangers 8 and 6b, and vaporized in the waste heat boiler 6. The saturated highly pure steam leaving the boiler is then separated into two parts; the first part constitutes the stream of highly pure steam 19 which is sent to the stripper, the second part of the saturated highly pure steam leaving the boiler 6 passes through the convection zone of the reformer where it recovers additional heat from the flue gases; it is again separated into two parts, one of them constitutes the superheated highly pure export steam 24, the second forms the stream of steam 22 sent to the mixing point 2. The stream 23 of steam feeding the mixing point 2 is for its part made up of the impure steam recovered at the top of the stripper 18 which contains the impurities, entrained in the steam, originating from the process condensate C1 and the CPU condensates C2.

(9) 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.

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

(11) 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.

(12) 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.

(13) 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.

(14) 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.

(15) 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.