Abstract:
Process and apparatus for the combined production of ammonia synthesis gas and pure hydrogen. There is withdrawn (at 21) a flow of synthesis gas which is first treated in a permeation unit (24) with the return (at 28) of the non-permeated gas, while the permeate (32)is treated by adsorption (34) (35) (36), which permits producing (in 57) very pure hydrogen. The process has a high energy efficiency.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the combined production of synthetic ammonia and pure hydrogen, from a hydrocarbon base gas, particularly methane. To this end, the sequence of the following operations is followed: 
     a) a reforming in at least two steps, of which a last step is with introduction of air, elaborating from said hydrocarbon base gas and reforming air, a composite gaseous mixture incorporating essentially nitrogen and hydrogen, with small proportions of carbon dioxide, water vapor and carbon monoxide, and traces of argon and methane; 
     b) substantial elimination of the carbon monoxide by conversion to carbon dioxide, leaving only traces of carbon monoxide; 
     c) a process for the elimination of carbon dioxide, leaving only traces of carbon dioxide; 
     d) substantial methanization of the carbon monoxide reducing to a very low level the traces of carbon monoxide, and producing methane; 
     e) if desired drying to eliminate water vapor; 
     all so as to produce a gas for the synthesis of ammonia (NH 3 ) constituted of hydrogen (H 2 ) (content of the order of 74-75), nitrogen (N 2 ) (content of the order of 24-25%), argon (Ar) and methane (CH 4 ) (combined content of the order of 1%). 
     BACKGROUND OF THE INVENTION 
     There has sometimes been used an installation for the production of ammonia, which treats extremely high flow rates of synthesis gas, to withdraw a moderate flow rate of synthesis gas so as to elaborate hydrogen and, for this purpose, while compressing in several compression stages the synthesis gas to send it to an ammonia synthesis reactor to produce the flow of synthetic ammonia, there is withdrawn, between two compression stages, a relatively low flow rate of withdrawn synthesis gas under medium pressure, the hydrogen is substantially separated from the other constituents: nitrogen, argon, methane, to produce a flow of hydrogen as pure as possible, said separation taking place generally by permeation delivering substantially under said medium pressure a stream consisting essentially of hydrogen and nitrogen with substantially all the argon and all the methane, which stream is returned to the ammonia synthesis gas under medium pressure and a permeation gas under lower pressure consisting essentially of hydrogen with a small content of nitrogen and traces of argon, methane and as the case may be water vapor. There has also been proposed effecting said separation by adsorption on a molecular sieve according to the process with cyclic pressure variation, called &#34;pressure swing adsorption&#34; or PSA, delivering on the one hand, under medium pressure, a production gas consisting essentially of hydrogen with traces of argon and traces of nitrogen, methane and as the case may be water vapor, on the other hand, under a lower pressure, a residual gas with higher content of hydrogen, a substantial content of nitrogen, and low contents of argon, methane and as the case may be water vapor. 
     With these processes, one is confronted either with poor quality hydrogen product, or poor energy use, because the nitrogen, which is found in the residual low pressure gas, must be recompressed at the withdrawal pressure of the synthesis gas. On the other hand, and this is particularly true in the adsorption process, this process leads to poor performance of the hydrogen production installation because of recycling of substantial quantities of argon which is hardly adsorbable. 
     SUMMARY OF THE INVENTION 
     The present invention has for its object a process for the production of high purity nitrogen from an ammonia synthesis gas, while limiting the quantities of nitrogen and hydrogen at low pressure, and which must be either recompressed or purged to the exterior, and these objects are achieved according to the invention in that the production stage of pure hydrogen comprises at least two substeps of purification, namely: 
     an initial substep of permeation delivering under substantially said medium pressure an effluent consisting essentially of hydrogen and nitrogen with substantially all the argon and all the methane, which effluent is sent to the ammonia synthesis gas; and a permeation gas under lower pressure consisting essentially of hydrogen with a low content of nitrogen and traces of argon, methane and as the case may be water vapor; 
     a final substep for adsorption of the permeation gas by adsorption on molecular sieves, according to the cyclic pressure variation process called &#34;pressure swing adsorption&#34; or PSA delivering, on the one hand under said lower pressure, a production gas consisting essentially of hydrogen with traces of argon of the order of or less than 100 ppm, and traces of nitrogen, of methane, and as the case may be of water vapor, each with a content of the order of, or less than, 1 ppm, on the other hand, under an even lower pressure, a residual gas with elevated content of hydrogen, a substantial content of nitrogen, and low contents of argon, methane and as the case may be water vapor. 
     In this way, by preliminary separation by permeation, there is produced at a pressure near the withdrawal pressure a gaseous mixture with a high content of hydrogen and nitrogen, which is easily sent to the synthesis gas, with minimum energy consumption. 
     This subsequent adsorption unit then need only treat a gaseous mixture with very high hydrogen content and containing only a small amount of nitrogen, which facilitates the operation. Moreover, the residual gas from the adsorption, which is again rich in hydrogen, can be recycled to the permeation gas, before treatment by adsorption. 
     The invention also relates to an installation for the combined production of synthetic ammonia and pure hydrogen, practicing the above process. 
     BRIEF DESCRIPTION OF THE DRAWING 
     The characteristics and advantages of the invention will become apparent from the description which follows, by way of example, with reference to the accompanying drawing, which is a diagram of an installation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the drawing, an installation comprises a primary reforming unit 1, supplied at 2 with natural gas (essentially methane) and at 3 with water vapor, producing a gaseous mixture (H 2 , CH 4 , CO, CO 2 , H 2  O) which is then sent via conduit 4 to a secondary reforming unit 5 into which is admitted air at 6, and from which leaves at 7 a gaseous mixture (N 2 , H 2 , CO, CO 2 , H 2  O) with traces of methane CH 4  and argon Ar, which is then treated by washing at 8, eliminating essentially the CO, such that the gaseous mixture at 9 will comprise, in addition to N 2  and H 2 , only low proportions of CO 2  and H 2  O and traces of CH 4 , Ar and CO. In 10 the substantial elimination of CO 2  is effected, such that the mixture then comprises only, in addition to N 2 , H 2 , H 2  O, traces of CH 4 , Ar, CO and CO 2 . In 11, the methanization of CO takes place, such that at 12 will remain only small traces of CO, CO 2 , Ar and CH 4  (Ar+CH 4  of the order of 1%). There is obtained a synthesis gas with H 2  of the order of 74%, N 2  of the order of 25%, Ar+CH 4  +H 2  O of the order of 1%. To improve the H 2  :N 2  ratio, adjusting means can be included for the reforming operation, as described hereinafter. If desired, water vapor may be eliminated by drying at 13. 
     This mixture 14 is sent at 15 to a compressor with several stages 16 and 17, then to the synthesis reactor 18 producing a flow of ammonia NH 3 . 
     Between the two compression stages 16 and 17, at an upstream point 21, there is withdrawn a moderate flow of synthesis mixture, which is conducted via 22 to an inlet 23 of a permeation unit 24 with a membrane 25, having an outlet 26 for the gas that did not permeate, therefore practically at the same medium pressure as at the inlet 23, and an outlet 27 for permeation gas under lower pressure. The unpermeated gas emerging at 26 is sent via 28 to a recycling point 29, situated downstream of the withdrawal point 21. There is provided either a compressor 30 in the conduit 28, or a pressure reducing device 31 between the withdrawal point 21 and the recycling point 29, so as to ensure the gaseous flow through the permeation unit 24. 
     The permeation gas emerging at 27 is sent via 32 to a supply conduit 33 under pressure (in fact the &#34;low&#34; pressure of the permeation gas of the permeation unit 24) of an adsorption unit with several adsorbers (three adsorbers are shown here by way of example at 34, 35, 36) each having an &#34;inlet&#34; 37, 38, 39 connected on the one hand by valves 41, 42, 43 to the supply conduit 33, on the other hand, by valves 44, 45, 46 to a residual gas conduit 47, itself connected, on the one hand, to a purge conduit 48 by a valve 49, and on the other hand to a recycling conduit 51 by a valve 52 and a compressor 53. 
     A downstream &#34;outlet&#34; 54, 55, 56 (in the co-current direction f) opens on the one hand, via valves 62, 63, 64 into a production conduit 57, on the other hand via valves 58, 59, 60 into a conduit for gas elution and final repressurization 61, connected to the production conduit 57. 
     The described installation therefore treats first of all a portion of the gaseous synthesis mixture conducted via 22 into the permeation unit permitting returning via 28 a gas of high content of N 2  and H 2  substantially under the medium withdrawal pressure. The permeation gas under low pressure in 33 has a very high content, of the order of 95%, of H 2 , the rest being substantially nitrogen (1 to 2%) and traces of argon, methane and water vapor. At the inlet (37, 38, 39) of the adsorption unit (34, 35, 36) operating in cyclic fashion, one of the adsorbers (34) is in isobaric production in the circulation direction f (called co-current), the second (35) is first in depressurization (in co-current equilibrium via 59, 60 then in countercurrent for purge via 45, 47, 49, 48), then in countercurrent elution (via 59, 45) and recycling via 51, 52, 53, while the third adsorber 36 is in the course of repressurization (pressure balancing with 35, via 59 and 60, then finally with the production gas via 60). It should be noted that there has been described a simplified circuit and that there could be used a cycle substantially more elaborate with a greater number of adsorbers, which permit delivering (at 57) a very pure hydrogen production gas (of 99.99%) with traces of nitrogen (N 2 ) less than 1 ppm, argon (Ar) less than 100 ppm, methane (CH 4 ) less than 1 ppm and water vapor (H 2  O) less than 1 ppm. The gas purged at 48 has high contents of H 2  and N 2  and low contents of Ar, CH 4 , H 2  O. What is recycled at 51, 52, 53, and which arises from the elution of an adsorber under low pressure, has generally a higher content of hydrogen. 
     By way of example, there are to be noted hereafter the results obtained according to the process of the invention, with a process that does not differentiate the residual gases of final decompression and elution. 
     Preferably, the adjustment parameters of the reforming operation are modified so as to increase the ratio H 2  :N 2  at the inlet of the compression stages 14, so as to reach the optimum ratio in the synthesis reactor. In a preferred manner, the ratio H 2  :N 2  at the inlet of the compression stages 14 is greater than 3:1. 
     
         __________________________________________________________________________  Gas to be       PSA  Product  treated       Return            Permeation                  Inlet                       H.sub.2                            Recycling                                  Purge  (via 22)       (via 28)            (via 32)                  (via 33)                       (via 57)                            (via 51)                                  (via 48)__________________________________________________________________________Pressure  75   74   28    28   27   1.5   1.5(bar abs)Compositionvol.H.sub.2  73.4%       42.4%            98.4% 94.6%                       99.99%                            78.7% 78.7%N.sub.2  25.0%       54.3%            1.4%  4.8% &lt;1 ppm                            18.8% 18.8%Ar     0.4% 0.8% 0.1%  0.1%  &lt;100 pp                            0.4%  0.4%CH.sub.4  1.1% 2.4% &lt;500 ppm                  0.1% &lt;1 ppm                            0.5%  0.5%H.sub.2 O  0.1% 0.1% 0.1%  0.4% &lt;1 ppm                            1.5%  1.5%Flow rate  18000       8056 9944  12310                       9200 2366  744Nm.sup.3 /h__________________________________________________________________________