Process for production of ammonia and derivatives, in particular urea

Abstract

A process for producing ammonia and a derivative of ammonia from a natural gas feed comprising conversion of natural gas into a make-up synthesis gas; synthesis of ammonia; use of said ammonia to produce said derivative of ammonia, wherein a portion of the natural gas feed is used to fuel a gas engine; power produced by said gas engine; is transferred to at least one power user of the process, such as a compressor; heat is re-covered from exhaust gas of said gas engine; and at least part of said heat may be recovered as low-grade heat available at a temperature not greater than 200 C., to provide process heating to at least one thermal user of the process, such as CO2 removal unit or absorption chiller; a corresponding plant and method of modernization are also disclosed.

Claims

1. A process for producing ammonia and a derivative of ammonia from a natural gas feed (NG) comprising: converting of the natural gas feed into synthesis gas in a front-end section; synthesizing ammonia from said synthesis gas in a synthesis loop; using at least part of said ammonia to produce said derivative of ammonia, said process being carried out with power users requiring a mechanical power for operation, and thermal users requiring a heat input for operation; wherein: a portion of said natural gas feed is used to fuel a reciprocating gas engine; power produced by said gas engine is used to cover at least partially the power demand of said power users; heat is recovered from an exhaust gas of said gas engine, and at least part of said heat is recovered, to provide heat to at least one of said thermal users, wherein the heat recovered from said exhaust gas of said gas engine is at least in part low grade heat and the low grade heat is transferred to at least one of said thermal users via a heat exchange medium, said medium being heated by indirect heat exchange with the exhaust gas to a temperature which is not greater than 200 C.

2. The process according to claim 1, wherein said power is transferred from said gas engine to at least one of said power users in an electrical or mechanical form.

3. The process according to claim 1, wherein said conversion of natural gas into synthesis gas in said front-end section is carried out by steam reforming with a global steam-to-carbon molar ratio not greater than 2.7.

4. The process according to claim 1, wherein said heat exchange medium is water or oil, natural gas or other fuel, process air or combustion air.

5. The process according to claim 1, wherein: a first portion of heat recovered from exhaust of said gas engine is used in a heat recovery steam generator to produce steam and said steam is expanded in a backpressure or extraction steam turbine producing further mechanical power, thus forming a combined cycle, and a second portion of heat recovered from the exhaust gas of said gas engine is used to provide said low-grade heat.

6. The process according to claim 5, wherein a steam flow taken from said backpressure or extraction steam turbine is used to provide heating to at least one of said thermal users.

7. The process according to claim 1, wherein converting the natural gas feed into synthesis gas comprises feeding the natural gas feed to a primary steam reformer and a secondary reformer; an autothermal reformer; or a partial oxidation reactor, obtaining a raw synthesis gas, and purifying the raw synthesis gas, wherein the purification comprises at least a shift reaction and removal of carbon dioxide from the shifted gas.

8. The process according to claim 7, wherein the shift reaction comprises a high temperature shift (HTS) on an iron-based catalyst, or a medium temperature shift (MTS) on a copper-based catalyst.

9. The process according to claim 7, wherein the removal of carbon dioxide is carried out by absorption using any of the following absorbents: amines, or activated amines, or potassium carbonates.

10. The process according to claim 1, wherein said derivative of ammonia is urea.

11. The process according to claim 1, in which said power users are at least one of a compressor, a fan or a pump.

12. The process according to claim 1, further comprising a fired reformer or other fired heater, wherein said exhaust gas from said gas engine is used to supply in part a combustion medium for a fired reformer or other fired heater.

13. The process according to claim 1, wherein part of the fuel for the gas engine is a waste fuel stream.

14. A plant for producing ammonia and a derivative of ammonia, said plant comprising: a front-end section for generation of ammonia make-up synthesis gas; a synthesis loop for synthesis of ammonia from said make-up synthesis gas; a section for the conversion of at least part of the synthesized ammonia into said derivative; power users requiring a mechanical power for operation, and at least one thermal user requiring a heat input for operation; wherein said plant further comprises: at least one reciprocating gas engine, the power produced by said gas engine being transferred to at least one of said power users, heat recovery section for recovery of low-grade heat from exhaust gas of said gas engine, wherein the low-grade heat is transferred to at least one of said thermal users via indirect heat exchange with a heat exchange medium, wherein the heat exchange medium is heated by the exhaust gas to a temperature not greater than 200 C.

15. A method of modernizing a plant for producing ammonia and a derivative of ammonia, comprising the steps of: providing a plant that comprises: a front-end section for generation of ammonia make-up synthesis gas; an ammonia synthesis loop for synthesis of ammonia from said make-up synthesis gas; an ammonia conversion section for the conversion of at least part of the synthesized ammonia into said derivative; and power users and thermal users; providing to the plant at least one reciprocating gas engine and a suitable power transfer section to transfer the power produced by said engine to at least one of said power users, and providing to the plant a heat recovery section for recovery of low-grade heat from exhaust gas of said gas engine, wherein the low-grade heat is transferred to at least one of said thermal users via indirect heat exchange with a heat exchange medium, wherein the heat exchange medium is heated by the exhaust gas to a temperature not greater than 200 C.

16. The method according to claim 15, wherein the power transfer section comprises: an electrical motor and an electrical generator coupled to said gas engine.

17. The method according to claim 16, wherein at least one thermal user is one of the following: a reboiler of a CO.sub.2 rich solution in a CO.sub.2 removal unit, a reboiler of an absorption refrigeration chiller, a reboiler of an ammonia-rich aqueous ammonia solution distillation system, a preheater of a natural gas or fuel gas, or a preheater of process air or a preheater of combustion air.

18. The method according to claim 15, further comprising the step of reducing a global steam-to-carbon ratio of the front-end section to a value lower than the original.

19. The process according to claim 1, wherein said conversion of natural gas into synthesis gas in said front-end section is carried out by steam reforming with a global steam-to-carbon molar ratio in the range 2.3 to 2.6.

20. The method according to claim 18, wherein said global steam-to-carbon ratio of the front-end section is in a range of 2.3 to 2.6.

21. The process according to claim 13, wherein part of said waste fuel stream is a purge gas taken from said ammonia synthesis loop.

22. The process according to claim 1, wherein said at least one of said thermal users is a CO.sub.2 removal unit, an absorption refrigeration chiller or an ammonia distillation system.

Description

(1) The invention will be further elucidated by the following description of an embodiment thereof, given by way of non-limiting example with reference to the attached FIG. 1.

DETAILED DESCRIPTION

(2) FIG. 1 illustrates a scheme of a process for ammonia synthesis from natural gas, according to a preferred embodiment of the invention.

(3) Block 1 denotes an ammonia-urea plant comprising: an ammonia synthesis section, comprising a front end section and a high pressure synthesis loop, and a urea plant where some or all of the ammonia is reacted with carbon dioxide to produce urea.

(4) Said front end section comprises preferably a steam reforming section and a purification section. Said steam reforming section comprises for example a primary steam reformer and a secondary reformer. Said purification section may include shift converters of CO to CO2, a CO2 removal unit and, optionally, a methanator.

(5) The ammonia-urea plant 1 comprises a number of power users 2 and thermal users 3. Said power users (PU) include gas compressors, fans and pumps. Thermal users (TU) typically use steam as a source of heat and include for example the CO2 removal unit where heat is needed for regeneration of a CO2 removal solution.

(6) A portion 15 of the available natural gas feed NG is used to fire a reciprocating gas engine 6, including a plurality of cylinder-piston assemblies.

(7) The power produced by said gas engine 6 is transferred to one or more of the PUs (line 7) in electrical or mechanical form, that is via conversion into electrical energy or direct mechanical coupling.

(8) For example, in a first embodiment a PU such as a pump is powered by an electric motor powered at least in part by electrical energy produced by a generator driven by said gas engine 6; in a second embodiment said PU is mechanically coupled to said gas engine.

(9) The power produced by gas engine(s) 6 hence will replace one or more of the steam turbines of the prior art.

(10) Exhaust gas flow 8 discharged by said gas engine 6 is fed to a heat recovery unit 9. Said recovery unit 9 produces a low-grade steam 10 by evaporating a feed water 14. Said steam 10 has a temperature not greater than 200 C., preferably in the range 150-200 C., and is used in at least one of the TUs 3 of the ammonia section 1. The cooled exhaust gas leaves the recovery unit 9 at line flow 11.

(11) A particularly preferred use for low-grade steam 10 is regeneration of CO2 removal solution in the CO2 removal unit of the purification section. Removal of carbon dioxide is preferably carried out with any of the following methods: amines, or activated amines, or potassium carbonate.

(12) Since the gas engine exhaust gas 8 is usually at a higher temperature (e.g. 350-400 C.), the heat recovery unit may also provide an additional amount of mechanical or electrical power, as indicated by line 13, for example via a heat recovery steam generator (HRSG) and a backpressure or extraction steam turbine.

(13) In a preferred embodiment, the global steam-to-carbon ratio in the front-end section of the plant 1 is regulated at a low value of less than 2.7, preferably in the range 2-2.6 and more preferably in the range 2.3-2.6. As stated above, the reduction of said ratio has a positive and synergistic effect with the provision of the gas engine(s) 6 and of the heat recovery unit 9.

(14) The global steam-to-carbon ratio can be reduced in conjunction with one or more of the following: by installing a pre-reformer upstream the primary reformer; bypassing a portion of natural gas (typically more than 10% of the reformer feed) around the steam reformer tubes and sending it directly to the secondary reformer.

(15) In some embodiments, the ammonia-urea plant 1 comprises a hydrogen recovery unit (HRU). The tail gas 12 of said HRU may be used as fuel in the gas engine(s) 6 as shown in FIG. 1. For a revamp, this is very convenient compared to the recycle in the steam reformer, because it avoids the otherwise typically necessary modification of the steam reformer burners. This is possible even if the tail gas of the HRU is at low pressure, such as for instance from a cryogenic HRU or a PSA.

(16) Additional steam 4 for the thermal users 3 can be optionally provided by a gas-fired auxiliary boiler 5.

(17) Further preferred aspects of the invention are the following. Energy can be saved by installing a means for recovering reactants (H2 and N2) from the synthesis loop purge, while effectively rejecting the inerts (Ar and especially CH4). Such means may include a membrane, or on adsorbents, or preferably a cryogenic HRU which recovers most of the reactants at a pressure preferably of at least 60 bar and preferably more than 100 bar.

(18) Both reducing the S/C ratio alone and installing a purge gas recovery HRU alone provides some energy saving, but there is synergy in applying both solutions together.

(19) In fact, a lower S/C ratio reduces the methane conversion in the reforming process, increasing the residual methane concentration in the make-up gas and ultimately in the synthesis loop. This offsets saving in process steam consumption. However, coupling an HRU with a lower S/C ratio eliminates the drawbacks of the latter, i.e. the increased methane concentration in the synthesis loop, while retaining the benefits of both: reduced firing, less inerts in the synthesis loop, H2 and N2 recovered at high pressure.

(20) Depending on the selected S/C ratio, either a high temperature (HTS) or a medium temperature (MTS) shift may be deployed. A HTS allows recover of a higher level heat, hence ensuring a higher overall efficiency and less gas consumption. However, HTS can be used only down to a global S/C ratio of about 2.6-2.7. In some cases it may be useful to reduce the S/C ratio to lower values, hence MTS is then required. The MTS can be adiabatic or isothermal. Isothermal MTS means that the shift converter contains a heat exchanger adapted to keep the temperature of the shift converter product gas within a desired range. Adiabatic MTS can be used when the amount of heat released in the shift converter is limited, for example when the oxidant in the secondary reformer is air and the concentration of CO inlet to the shift is not too high.