UREA PROCESS WITH CONTROLLED EXCESS OF CO2 AND/OR NH3

Abstract

A process for producing UREA, said process comprising the steps of:purification of a hydrocarbon feed gas removing Sulphur and/or chloride components if present, reforming the hydrocarbon feed gas in a reforming step where the steam/carbon ratio is less than 2.6 thereby obtaining a synthesis gas comprising CH4, CO, CO2, H2 and H2O, optionally adding H2O to the synthesis gas from the reforming step maintaining an overall steam/carbon less than 2.6, shifting the synthesis gas in a shift section comprising one or more shift steps preferably in series, optionally washing the synthesis gas leaving the shift section with water, removing CO2 from the synthesis gas from the shift section in a CO2 removal step to obtain a synthesis gas with less than 500 ppm CO2, preferably less than 20 ppm CO2 and a CO2 product gas, removing residual H2O and/or CO2 from the synthesis gas preferably in an absorbent step, removing CH4, CO, Ar and/or He preferably in a nitrogen wash unit and adding stoichiometric nitrogen to produce NH3 to the synthesis gas, synthesizing NH3 to obtain a NH3 product, adding at least part of the product CO2 and at least part of the NH3 product to a UREA synthesis step to make a UREA product, Wherein the amount of excess CO2 and/or NH3 is controlled by adjusting the steam/carbon in the reforming step and/or the H2O addition upstream the shift step and/or adjusting the inlet temperature to at least one of the one or more shift steps.

Claims

1. A process for producing urea, said process comprising the steps of: optionally purification of a hydrocarbon feed gas removing sulphur and/or chloride components if present, reforming the hydrocarbon feed gas in a reforming step where the steam/carbon ratio is less than 2.6 thereby obtaining a synthesis gas comprising CH.sub.4, CO, CO.sub.2, H.sub.2 and H.sub.2O, optionally adding H.sub.2O to the synthesis gas from the reforming step maintaining an overall steam/carbon ratio less than 2.6, shifting the synthesis gas in a shift section comprising one or more shift steps preferably in series, optionally washing the synthesis gas leaving the shift section with water, removing CO.sub.2 from the synthesis gas from the shift section in a CO.sub.2 removal step to obtain a synthesis gas with less than 500 ppm CO.sub.2, preferably less than 20 ppm CO.sub.2 and a CO.sub.2 product gas, removing residual H.sub.2O and/or CO.sub.2 from the synthesis gas preferably in an absorbent step, removing CH.sub.4, CO, Ar and/or He preferably in a nitrogen wash unit and adding stoichiometric nitrogen to the synthesis gas to produce NH.sub.3, synthesizing NH.sub.3 to obtain a NH.sub.3 product, adding at least part of the product CO.sub.2 and at least part of the NH.sub.3 product to a urea synthesis step to make a urea product, wherein the amount of excess CO.sub.2 and/or NH.sub.3 is controlled by adjusting the steam/carbon in the reforming step and/or H.sub.2O addition upstream and/or in between the shift steps and/or adjusting the inlet temperature to at least one of the one or more shift steps.

2. Process according to claim 1, wherein a process condensate originating from cooling and washing the synthesis gas leaving the shift and/or the washing section is sent 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 wherein at least part of this steam stream is used as the H.sub.2O added upstream the shift section and/or between shift steps in the shift section.

3. Process according to claim 1, wherein the one or 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.

4. Process according to claim 3, wherein the temperature in the high temperature shift step(s) is 300-600 C.

5. Process according to claim 3, wherein the promoted zinc-aluminum oxide based HT shift catalyst comprises Zn and Al in 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.

6. Process according to claim 1, wherein the steam/carbon ratio in the reforming step is 2.6-0.1.

7. Process according to claim 1, wherein the reforming takes place in an autothermal reformer (ATR).

8. 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.

9. Process according to claim 1, further comprising a prereforming step.

10. Process according to claim 1, wherein the steam/carbon ratio in the reforming step is adjusted thereby obtaining a selected CO.sub.2 content in the synthesis gas stream entering the CO.sub.2 removal step.

11. Process according to claim 1, wherein the shift section comprises one or more high temperature shift steps, one and more medium temperature shift steps and/or one or more low temperature shift steps thereby obtaining a selected CO.sub.2 content in the synthesis gas stream entering the CO.sub.2 removal step.

12. Process according to claim 1, wherein steam added to the synthesis gas before one or more of the shift steps in the shift section is adjusted thereby obtaining a selected CO.sub.2 content in the synthesis gas stream entering the CO.sub.2 removal step.

13. Process according to claim 1, wherein the inlet temperature to one or more of the one or more shift steps is adjusted, thereby obtaining a selected CO.sub.2 content in the synthesis gas stream entering the CO.sub.2 removal step.

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

15. Process according to claim 1, wherein the ammonia process loop is an inert free loop.

16. A plant arranged to carry out the processes according to claim 1.

Description

[0087] The invention is further explained by examples 1 and 2 together with the accompanying figures. Examples and figures are provided to exemplify the invention and are not to be construed as limiting to the invention.

[0088] FIG. 1 shows a schematic flow diagram of an embodiment of the present invention. The numbers given in the figure corresponds to the positions given in the tables relating to the two examples.

[0089] FIG. 2 shows details of an embodiment of the feed purification and SynCOR steps of FIG. 1. Feed 1 e.g. natural gas and hydrogen is heated in a fired heater 2 where after it is treated 3 by hydrogenation and/or sulfur removal. The hydrogenated/sulfur reduced stream may be heated again and further processed by pre-reforming. The stream obtained by the pre-reforming is then reformed in a SynCOR step comprising an auto thermal reforming (ATR) step 5. The SynCOR step may be provided with oxygen from an air separation unit (ASU) 6. Nitrogen may be led from the ASU by 7. Steam may be added via 8 and fuel may be supplied for heater by 9.

EXAMPLE 1 FULL UREA PRODUCTION

[0090] The below positions refer to the inlet of the units.

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 Pos. 8 Pos. 9 Temp., C. 380 465 360 330 73 20 46 45 381 Pressure, 47.9 42.4 37.1 37.1 34.3 33.6 185 51.5 45.9 kg/cm.sup.2 g Flow, Nm.sup.3/h 137,123 211,944 465,894 608,589 471,813 356,204 440,547 139,546 89,309 Position Pos. 10 Pos. 11 Pos. 12 Pos. 13 Pos. 14 Pos. 15 Pos. 16 Temp., C. 256 380/172 50 32 50 20 Pressure, 43.8 45.9/4.6 0.5 8.2 0.5 32.2 kg/cm.sup.2 g Flow, Nm.sup.3/h 97,924 9,731/26,591 0 0 119,576 219,658 109,675

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 Pos. 9 Ar 0.07 0.05 0.07 0.09 C.sub.2H.sub.6 0.06 0.04 CH.sub.4 90.83 58.76 1.25 0.96 1.24 1.63 CO 21.41 16.39 1.35 1.75 CO.sub.2 4.10 3.18 23.78 20 ppm 0.30 H.sub.2 3.25 2.11 52.43 40.15 71.52 94.20 75.00 0.03 N.sub.2 5.86 3.79 1.73 1.32 1.70 2.26 25.00 CH.sub.3OH 0.12 0.02 0.53 NH.sub.3 H.sub.2O 35.30 19.00 37.83 0.32 0.07 99.14 100.00 Comp., Mole % Pos. 9 Pos. 10 Pos. 11 Pos. 12 Pos. 13 Pos. 14 Pos. 15 Pos. 16 Ar C.sub.2H.sub.6 CH.sub.4 0.01 0.01 CO 0.01 0.01 CO.sub.2 0.21 91.72 91.72 H.sub.2 0.05 0.18 0.18 0.08 N.sub.2 0.06 CH.sub.3OH 0.76 0.06 0.06 NH.sub.3 99.81 99.86 H.sub.2O 100.00 98.98 100.00 8.02 0.19 8.02 CH.sub.4N.sub.2O 100.00

EXAMPLE 2 REDUCED UREA PRODUCTION

[0091] The below positions refer to the inlet of the units.

TABLE-US-00003 TABLE 3 Flows and operating conditions for FIG. 1 Position Pos. 1 Pos. 2 Pos. 3 Pos. 4 Pos. 5 Pos. 6 Pos. 7 Pos. 8 Pos. 9 Temp., C. 380 465 340 325 68 20 46 45 381 Pressure, 47.9 42.4 37.1 37.1 34.3 33.6 185 51.5 45.9 kg/cm.sup.2 g Flow, Nm.sup.3/h 147,025 227,249 498,691 545,746 481,494 382,357 440,547 67,158 42,981 Position Pos. 10 Pos. 11 Pos. 12 Pos. 13 Pos. 14 Pos. 15 Pos. 16 Temp., C. 256 380 50 32 50 20 Pressure, 43.8 45.9 0.5 8.2 0.5 32.2 kg/cm.sup.2 g Flow, Nm.sup.3/h 47,055 82,715 0 31,119 102,655 188,554 94,145

TABLE-US-00004 TABLE 4 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 Pos. 9 Ar 0.07 0.07 0.07 0.09 C.sub.2H.sub.6 0.06 0.04 CH.sub.4 90.83 58.76 1.27 1.16 1.31 1.64 CO 21.46 19.61 6.46 8.10 CO.sub.2 4.08 3.75 19.97 20 ppm 0.22 H.sub.2 3.25 2.11 52.49 47.97 70.10 87.84 75.00 0.03 N.sub.2 5.86 3.79 1.73 1.57 1.80 2.26 25.00 CH.sub.3OH 0.06 0.01 0.48 NH.sub.3 H.sub.2O 35.30 18.90 25.81 0.28 0.07 99.27 100.00 Comp., Mole % Pos. 9 Pos. 10 Pos. 11 Pos. 12 Pos. 13 Pos. 14 Pos. 15 Pos. 16 Ar C.sub.2H.sub.6 CH.sub.4 0.01 0.01 CO 0.01 0.01 CO.sub.2 0.15 91.71 91.71 H.sub.2 0.05 0.20 0.20 0.08 N.sub.2 0.06 CH.sub.3OH 0.68 0.05 0.05 NH.sub.3 99.81 99.86 H.sub.2O 100.00 99.12 100.00 8.02 0.19 8.02 CH.sub.4N.sub.2O 100.00
I.e. For Example 2 this Means:

[0092] Prereformer: Tin/Tout: 465/427 C. (T=1 C.)

[0093] Steam/carbon ratio, S/C=0.6 inlet the prereformer

ATR:

[0094] The process gas enters the ATR at 650 C. and the temperature of the oxygen is around 230 C.

[0095] Steam/carbon ratio, S/C=0.7 as per definition in the description

[0096] 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.

Shift Section:

[0097] 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

[0098] The shift section consists of two high temperature shift step

[0099] HT(1): Tin/Tout: 325/449 C. (T=124 C.)

[0100] HT(2): Tin/Tout: 340/368 C. (T=28 C.)

[0101] 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.

[0102] 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.

[0103] The methanol content exit the shift section is 368 Nm.sup.3/h.

Synthesis Gas Wash

[0104] After the shift section the synthesis gas is cooled and washed with water The methanol content in the synthesis gas, position 5 leaving the synthesis gas wash is 47.7 Nm.sup.3/h after separation of the process condensate and washing water

Process Condensate Stripper

[0105] 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 8 to 0.7 Nm.sup.3/h in in the stripped process condensate leaving the process condensate stripper.

[0106] The stripper steam, position 10 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.

CO.SUB.2 .Removal Section

[0107] 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 and/or 12.

N.SUB.2 .Wash Section

[0108] 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.

Syngas Compressor:

[0109] 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.

Inert Free Loop:

[0110] The loop can be defined as inert when no purge gas system is required.

[0111] 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.

[0112] 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.

[0113] If required the level of inert gas in the loop can be reduced by an intermittent purge of a small gas stream.

[0114] 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.

[0115] 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 12 and/or 14, is less than what is obtained in typical known processes in operation today and enables reduction to lower levels should this be required.

[0116] In example 1 109,675 Nm.sup.3/h CO.sub.2 reacts with 219,350 Nm.sup.3/h NH.sub.3 to produce 109,675 Nm.sup.3/h CH.sub.4N.sub.2O.sub.2 corresponding to full urea production.

2NH.sub.3+CO.sub.2.Math.NH.sub.2CONH.sub.2+H.sub.2O+heat(1)

[0117] In example 2 with reduced urea production 94,145 Nm.sup.3/h CO.sub.2 is produced and reacts with 188,290 Nm.sup.3/h NH.sub.3 to produce 94,145 Nm.sup.3/h CH.sub.4N.sub.2O.sub.2. In this case 31,060 Nm.sup.3/h excess NH3 is sent to storage.

[0118] The two examples show the option of adjusting and thus tailor design the CO.sub.2 production by steam addition and shift inlet temperature adjustment. If reduced urea production is requested less steam can be added and 2HTS shift combination may be used. If full urea production is requested additional steam is added and HTS+MTS shift configuration may be used. Even an over stoichiometric amount of CO.sub.2 can be produced if requested by adding more steam and adjusting the shift inlet temperatures to the shift stages appropriately.

[0119] In conventional ammonia/urea plants the CO.sub.2/NH.sub.3 ratio is defined by the feed gas composition. This means that excess CO.sub.2 is typically vented if reduced urea production is requested. If full urea production is requested in conventional ammonia/urea plants part of the synthesis gas is used as fuel instead of ammonia production to adjust the CO.sub.2/NH.sub.3 ratio. This means a larger front end is required.