Urea process with controlled excess of CO
A process for producing urea with controlled excess of CO 2 and/or NH 3 . The process includes the steps of: reforming the hydrocarbon feed gas, thereby obtaining a synthesis gas comprising CH 4 , CO, CO 2 , H 2 and H 2 O, shifting the synthesis gas, removing CO 2 from the synthesis gas, removing residual H 2 O and/or CO 2 from the synthesis gas, removing CH 4 , CO, Ar and/or He, and adding stoichiometric nitrogen to produce NH 3 to the synthesis gas, synthesizing NH 3 to obtain a NH 3 product, and adding at least part of the product CO 2 and at least part of the NH 3 product to a urea synthesis step to make a urea product. The amount of excess CO 2 and/or NH 3 is controlled by adjusting the steam/carbon in the reforming step and/or the H 2 O addition upstream the shift step and/or adjusting the inlet temperature to at least one or more shift steps.
1. A process for producing urea, said process comprising the steps of:
optionally purifying a hydrocarbon feed gas to remove sulfur and/or chloride components if present,
reforming the hydrocarbon feed gas in a reforming step with a steam/carbon molar ratio of 2-0.2, thereby obtaining a synthesis gas comprising CH 4 , CO, CO 2 , H 2 and H 2 O,
optionally adding H 2 O to the synthesis gas from the reforming step to maintain an overall steam/carbon molar ratio less than 2,
shifting the synthesis gas in a shift section comprising one or more shift steps in series utilizing a zinc-aluminum oxide based catalyst, and operating the shift section at a steam/carbon molar ratio of less than 2.6,
optionally cooling and/or adding H 2 O to the synthesis gas between the shift steps,
optionally washing the synthesis gas leaving the shift section with water,
removing CO 2 from the synthesis gas from the shift section in a CO 2 removal step to obtain a synthesis gas with less than 20 ppm CO 2 and a CO 2 product gas,
removing residual H 2 O and/or CO 2 from the synthesis gas in an absorbent step,
removing CH 4 , CO, Ar and/or He from the synthesis gas in a nitrogen wash unit and adding stoichiometric nitrogen to the synthesis gas to produce NH 3 synthesis gas,
synthesizing NH 3 from the NH 3 synthesis gas to obtain a NH 3 product,
adding at least part of the product CO 2 and at least part of the NH 3 product to a urea synthesis step to make a urea product,
wherein the amount of excess CO 2 and/or NH 3 is controlled by adjusting the steam/carbon molar ratio in the reforming step and/or adjusting H 2 O 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, derived from the cooling of the synthesis gas between the shift steps and including methanol formed by the shift steps, is sent to a process condensate stripper, wherein dissolved shift byproducts and dissolved gases, including the methanol formed by the shift steps, 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 2 O 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.
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/A 1 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 wt%.
6. Process according to claim 1 , wherein the steam/carbon molar ratio in the reforming step is 1.5-0.3.
7. Process according to claim 1 , wherein the reforming takes place in an autothermal reformer (ATR).
8. Process according to claim 7 , wherein the space velocity in the ATR is less than 20,000 Nm 3 C/m 3 /h.
9. Process according to claim 1 , further comprising a prereforming step.
10. Process according to claim 1 , wherein the steam/carbon molar ratio in the reforming step is adjusted.
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 2 content in the synthesis gas stream entering the CO 2 removal step.
12. Process according to claim 1 , wherein steam is added to the synthesis gas before one or more of the shift steps in the shift section, thereby obtaining a selected CO 2 content in the synthesis gas stream entering the CO 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 2 content in the synthesis gas stream entering the CO 2 removal step.
14. Process according to claim 2 , wherein the steam stream contains more than 90% of the methanol dissolved in the process condensate.