Ammonia process using advanced shift process

Abstract

A process for producing an ammonia synthesis gas, said process comprising the steps of: —Reforming a hydrocarbon feed in a reforming step thereby obtaining a synthesis gas comprising CH.sub.4, CO, CO.sub.2, H.sub.2 and H.sub.2O, —Shifting the synthesis gas in one in or more shift steps in series, —Optionally wash the synthesis gas leaving the shift section with water, —Sending the process condensate originating from cooling and washing the synthesis gas leaving the shift section to a process condensate stripper wherein the dissolved shift byproducts and dissolved gases are stripped out of the process condensate using steam resulting in a steam stream containing more than 99% of the dissolved methanol in process condensate. —Adding all or part of said steam stream from the process condensate stripper to the synthesis gas downstream the reforming step, prior to the last shift step, wherein —The steam/carbon ratio in the reforming step and the shift step is less than 2.6.

Claims

1. A process for producing an ammonia synthesis gas, said process comprising the steps of: reforming a hydrocarbon feed in a reforming section thereby obtaining a synthesis gas comprising CH.sub.4, CO, CO.sub.2, H.sub.2 and H.sub.2O; shifting the synthesis gas in a shift section comprising one or more shift steps in series; optionally washing the synthesis gas leaving the shift section with water; sending a process condensate originating from cooling and washing the synthesis gas leaving the shift section to a process condensate stripper, wherein dissolved shift byproducts and dissolved gases are stripped out of the process condensate using steam resulting in a steam stream; and adding at least part of said steam stream from the process condensate stripper to the synthesis gas downstream the reforming section, upstream the last shift step, wherein the steam/carbon ratio in the reforming step and the shift steps is less than 2.6.

2. The process according to claim 1 wherein the one and more shift steps are one or more high temperature (HT) shift steps, using a promoted zinc-aluminum oxide based catalyst, with possibility for cooling and/or steam addition in between.

3. The process according to claim 2 wherein the temperature in the high temperature shift step(s) is 300-600° C.

4. The process according to claim 1, wherein the promoted zinc-aluminum oxide based HT shift catalyst comprises a Zn/Al molar ratio in the range 0.5 to 1.0 and a content of alkali metal in the range 0.4 to 8.0 wt % and a copper content in the range 0-10% based on the weight of oxidized catalyst.

5. The process according to claim 1, wherein the steam/carbon ratio in the reforming step is 2.6-0.1.

6. The process according to claim 1, wherein the reforming takes place in an autothermal reformer (ATR).

7. The process according to claim 1, wherein the space velocity in the ATR is less than 20,000 Nm.sup.3 C/m.sup.3/h.

8. The process according to claim 1, further comprising a prereforming step.

9. The process according to claim 1, comprising one or more additional shift steps comprising one or more high temperature shift steps, one and more medium temperature shift steps and/or one or more low temperature shift steps.

10. The process according to claim 1, wherein steam is optionally added to the synthesis gas before the one or more additional shift steps downstream the high temperature shift step.

11. The process according to claim 1, further comprising a CO.sub.2 removal step removing CO.sub.2 from the synthesis gas down to a level less than 400 vppm CO.sub.2.

12. The process according to claim 1, further comprising a N.sub.2 wash step.

13. The process according to claim 1, wherein the steam stream contains more than 90% of the methanol dissolved in process condensate.

14. The process for producing ammonia according to claim 1, wherein the ammonia process loop is an inert free loop.

Description

EXAMPLE

(1) The below positions refer to the inlet of the units.

(2) TABLE-US-00001 TABLE 1 Flows and operating conditions for FIG. 1 Position Pos. 1 Pos. 2 Pos. 3 Pos. 4 Pos. 5 Pos. 6 Pos. 7 Temp., ° C. 465 650 340 325 180 68 20 Pressure, kg/cm.sup.2 g 42.4 40.2 37.1 37.1 35.8 34.3 33.7 Flow, Nm.sup.3/h 227,249 231,433 498,691 545,746 545,649 481,494 382,357 Position Pos. 8/9 Pos. 10 Pos. 11 Pos. 12 Pos. 13 Pos. 14 Temp., ° C. 15 −32 45 44.9 66 50 Pressure, kg/cm.sup.2 g 32.7/185 8.2 51.5 255 42.8 0.51 Flow, Nm.sup.3/h 447,373 219,762 67,158 47,055 102,651

(3) TABLE-US-00002 TABLE 2 Stream compositions, Positions refer to FIG. 1 Comp., Mole % Pos. 1 Pos. 2 Pos. 3 Pos. 4 Pos. 5 Pos. 6 Pos. 7 Pos. 8/9 Pos. 10 Pos. 11 Pos. 12 Pos. 13 Pos. 14 Ar 0.07 0.07 0.07 0.07 0.09 C.sub.2H.sub.6 0.04 CH.sub.4 58.76 56.87 1.27 1.16 1.16 1.31 1.64 0.01 CO 0.02 21.46 3.75 5.70 6.46 8.10 0.02 CO.sub.2 0.89 4.08 19.61 17.65 19.97 20 ppm 0.22 0.15 0.12 91.71 H.sub.2 2.11 5.63 52.49 47.97 61.86 70.10 87.84  75.00 0.03 0.05 0.20 N.sub.2 3.77 3.70 1.72 1.57 1.57 1.78 2.24 25.00 CH.sub.3OH 0.0586 0.0675 0.0099 0.4772 0.6795 0.0011 0.0466 NH.sub.3 100.00 H.sub.2O 35.30 32.87 18.90 25.81 11.92 0.28 0.07 99.27 99.11 99.88 8.02

(4) Prereformer:

(5) Tin/Tout: 465/427° C. (ΔT=−1° C.)

(6) Steam/carbon ratio, S/C=0.6 inlet the prereformer

(7) ATR:

(8) The process gas enters the ATR at 650° C. and the temperature of the oxygen is around 230° C.

(9) Steam/carbon ratio, S/C=0.7 as per definition in the description

(10) The process gas leaves the reforming section at about 1050° C. through a refractory lined outlet section and transfer line to the waste heat boilers in the process gas cooling section.

(11) Shift Section:

(12) Steam containing shift reaction byproducts is added to the synthesis gas inlet the high temperature shift changing the steam/carbon ratio to 1.0 and the methanol content from 0 to 320 Nm.sup.3/h, position 4

(13) The shift section consists of two high temperature shift step

(14) HT(1): Tin/Tout: 325/449° C. (ΔT=124° C.)

(15) HT(2): Tin/Tout: 340/368° C. (ΔT=28° C.)

(16) After reforming, about 26.5 vol % CO is present in the gas (dry basis). In the first high temperature shift converter the CO content is reduced to approximately 9.8 vol %, and the temperature increases from 325° C. to 449° C. The heat content of the effluent from the high temperature CO converter is recovered in a waste heat boiler and in a boiler feed water preheater.

(17) The process gas is thereby cooled to 340° C. and passed on to the second high temperature shift converter in which the CO content is reduced to approximately 6.5 vol %, while the temperature increases to 368° C.

(18) The methanol content exit the shift section is 368 Nm.sup.3/h, position 5

(19) Synthesis Gas Wash

(20) After the shift section the synthesis gas is cooled and washed with water

(21) The methanol content in the synthesis gas, position 6, leaving the synthesis gas wash is 47.7 Nm.sup.3/h after separation of the process condensate and washing water

(22) Process Condensate Stripper

(23) The process condensate and washing water is stripped with steam reducing the methanol in the process condensate and wash water from 320.5 Nm.sup.3/h, position 11 to 0.7 Nm.sup.3/h in position 13.

(24) The stripper steam, position 12, leaving the condensate stripper contains 320 Nm.sup.3/h of methanol, which, in this example, all is added to the shift section again as described above.

(25) CO.sub.2 Removal Section

(26) The CO.sub.2 content in the outlet stream from shift section is reduced to 20 ppm. All Methanol in the synthesis gas going to the CO.sub.2 removal section will leave this section with the CO.sub.2 product stream, position 14.

(27) N.sub.2 Wash Section

(28) First step in this section is a quantitatively removal of CO.sub.2 and H.sub.2O in a molecular sieve dryer. Next step is a N.sub.2 liquid wash removing components other than H.sub.2 and N.sub.2 down to ppm level. The third step is to adjust the H.sub.2/N.sub.2 ratio to approximate 3 using gaseous nitrogen.

(29) Syngas Compressor:

(30) The synthesis gas is compressed from 33.7 to 185.8 kg/cm.sup.2g in the centrifugal type two-casing synthesis gas compressor. Part of the last casing forms the recirculation compressor in the synthesis loop.

(31) Inert Free Loop:

(32) The loop can be defined as inert when no purge gas system is required.

(33) The small amounts of inert gases entering the loop with the make-up synthesis gas will accumulate in the loop until the amount of inert gases dissolved in the liquid ammonia exit the let-down vessel equals the amount entering the loop. Off gas from the let-down vessel is recycled back to the synthesis gas compressor.

(34) The recycled inert level is dependent on the level of inerts dissolved in the liquid ammonia leaving the ammonia separator and the let-down vessel.

(35) If required the level of inert gas in the loop can be reduced by an intermittent purge of a small gas stream.

(36) In this example the inert level in the purified gas leaving the N.sub.2 wash is 17 ppm Ar, in the make-up gas 53 ppm Ar (after addition of the off gas recycle stream from the let-down vessel) and 0.30% Ar inlet the converter.

CONCLUSION

(37) The example shows that the process described in the present invention reduces the methanol byproduct formation by 320/(320+48.4)*100=86.9%. Furthermore, the presented process ensures a content of methanol in the CO.sub.2 stream, position 14, is less than what is obtained in typical known processes in operation today and enables reduction to lower levels should this be required.