GB2126573A - Ammonia process - Google Patents

Abstract

A process for the manufacture of ammonia from hydrocarbons and an oxygen-containing gas comprising the steps of A, heating the hydrocarbon in a fired heater, B, optionally adding steam, C reacting the hydrocarbon with an oxygen-containing gas, D, cooling the product from such reaction and shifting at least some of the carbon monoxide in the gas to carbon dioxide, E, further cooling the gas, F, passing the gas to a separation unit so as to give an ammonia synthesis gas and another stream, which other stream is used as fuel, G, methanating the synthesis gas and passing the methanated gas to an ammonia synthesis unit. By means of the present invention the expensive primary reformer of conventional processes can be eliminated, together with some of its associated steam raising equipment. <IMAGE>

Description

SPECIFICATION Ammonia process This invention relates to a process for the production of ammonia from feed such as natural gas.

Atypical modern ammonia flowsheet comprises desulphurisation, primary reforming, secondary reforming, high temperature and low temperature carbon monoxide shift, carbon dioxide removal, methanation, compression and an ammonia synthesis loop.

Such a flowsheet is very expensive on capital cost and the present invention seeks to reduce the capital cost of plants to make ammonia. Furthermore this invention seeks to eliminate the expensive water purification equipment associated with steam systems. One of the largest items of capital cost is the primary reformer and its associated steam raising sections. An advantage of this invention is the eiiminatic a of the primary reformer and such steam raising equipment.

This invention seeks to provide a process for the synthesis of ammonia from hydrocarbons which comprises heating compressed air, or an oxygencontaining gas, and natural gas feed in a fired heater, preferably but not necessarily passing the natural gas feed through a saturator, passing both streams into a reactor which may or may not contain a catalystwherein most of the natural gas feed reacts with the oxygen contained in the aforesaid stream, and with steam, if present, and, within the said reactor or immediately thereafter, quenching the reacted stream by means of water in order to reduce the temperature of the gases and to saturate the said reacted stream with water so as to provide water to react with carbon monoxide contained therein in a downstream carbon monoxide shift unit.The water which is heated by the quench may be circulated to the natural gas feed saturator.

The removal of the primary reformer from the flowsheet means that the pressure constraint associated wich such reformers may be eliminated, which is an added advantage.

The quench system need only be supplied with soft water.

The carbon monoxide shift section preferably comprises only a high temperature shift unit and may utilise a start-up combuster placed in series upstream of the catalytic bed. The effluent from the shift section may then pass to a CO2 removal section which, because of a downstream PSA or cryogenic separation device, does not necessarily have to remove carbon dioxide down to a level normally associated with a flowsheet such as is found in a typical modern ammonia process.

If a PSA unit is utilised downstream of the CO2 removal section it may, but not necessarily, be fed with the total gas flow. Depending upon the com position of the effluent from the carbon dioxide removal section it may be possible to by-pass some of the gas around the PSA unit and thereby reduce its size. The calorific value of the purge from the PSA unit may be controlled by adjusting the conditions in both the secondary reformer and the carbon monoxide shift section. This purge gas may be used to fuel the fired heater or a gas turbine our a boiler to raise stream, e.g. for use in an associated urea plant.The flow from the PSA bed having been re-combined with its by-pass, if any, is next methanated in order to reduce the concentration of oxides of carbon prior to the gas being, if necessary, compressed and fed to the synthesis loop.

Inerts, methane and argon, which build up in the loop may be totally or partially either used as fuel in a similar manner to that described for the purge from the PSA unit or may be re-cycled back to the reformer or PSA units, as appropriate.

Alternatively, the effluent from the carbon dioxide removal system may be methanated and may then be fed to a cryogenic removal system which may utilise an open nitrogen recycle system for refrigeration and may substantially separate the methane from the nitrogen in order to raise the calorific value of the methane-rich stream in order to achieve high flame temperature in, e.g. the fired heater.

The advantage of using a fired heater instead of a primary reformer may be emphasised by the fact that the temperature of the gases leaving the fired heater are of the order of 450"C whilst those leaving a typical primary reformer would be of the order of 800"C. Furthermore the flow through a primary reformer would be considerably larger because of the large amount of steam added to the feedstock; and finally the tubes of a fired heater do not contain catalyst whose volume in the primary reformer has to be contained by metal under extreme conditions of temperature.

In the description of the preferred embodiment of the invention which follows it must be understood that because of the great number of possible ways in which heat can be recovered and re-used within this ammonia process, the preferred embodiment is preferred in the sense of the sequence of main process stages and fired heaters. Because each possible location is likely to have a different feedstock value and different cost of capital it is not possible to give a fully detailed description including heat recovery. This will be a familiar problem to those experienced in the art.

The following preferred embodiment description should be read in conjunction with Figure 1. All flows are kg mols/hr, pressures in kg/cm2A (Ata), and temperatures in C.

Natural gas (1), comprising CH4 196.6, C2H610.4, pressure 35 Ata, 40", enters the plant and from which a fuel gas stream (2) totalling 19.05 kgmois/hr is withdrawn. The natural gas feed is combined with re-cycle gas, CH4 1.0, H2 6.2, N2 2.1, and is then pre-heated in a direct fired heater (3) to 400 . It is then de-sulphurised in (4) and passes to a saturator (5) which is fed with steam and water and is also indirectly heated such that a stream (6) comprising CH4179.51, C2H6 9.44, H2 6.2, N2 2.1 and steam 570, leaves the saturator and is then further heated to 450 . A stream of air (7) 610.4 is compressed in compressor (8) and heated in the fired heater (3) to 450"C and the air and the natural gas feed are then both fed to a reformer (9) in which the oxygen in the air reacts with the natural gas feed to give an exit product at 30 Ata, 8350C, comprising CH4 3.3, H2 444.6 N2 478.7, Ar 5.7, CO 96.5, CO2 97.8 H20 521.7.

This reforming effluent stream (10) is cooled by raising steam at 38 Ata in boiler (11) and is then further cooled (12) by giving up its heat to raise steam in the saturator. The stream then enters a carbon monoxide shift reaction vessel (13) at 370"C and leaves (13) at 420"C, its composition is Cm43.3, H2 515.3, N2 478.7, Ar 5.7, CO 25.8, C02 168.5, H20 451.0. Following (13) the stream is cooled (14) such that it enters a boiler feed water heater (15) at 310"C.

The boiler feed water stream (16) is heated from 105 . The gases are then further cooled (17) using cooling water and enter the knock-out pot (18) from where they are fed to a pressure swing adsorption unit (19). The feed to the PSA unit after cooling contains 2.7 kgmol/hr of water. The purge gas stream (20) (in combination with the fuel stream (2)), comprising CH4 3.3, H2 139.1, N2 354.3, Ar 5.7, CO2 24.8, CO2 168.5, H20 2.7, is used as fuel both for the boiler (21) and for the fired heater (3).

The syngas (22) from the PSA unit comprises H2 376.2, N2 124.4, CO 1.0. It is pre-heated (23/14) to a temperature of 320 and its residual carbon oxides are then converted to methane in the methanation reator (24). The effluent from the methanation reactor (25) comprises CH4 1.0, H2 373.2, N2 124.4, H20 1.0. It is then cooled in (23/26) passes through the knock-out pot (27) and is then compressed (28) and fed into an ammonia synthesis section (29) which gives the product ammonia (30) 244.67 which is equivalent to 100 metric tonnes/day of ammonia.

Purge from the ammonia synthesis loop is re-cycled (31), this comprises CH41.0, H2 6.2, N2 2.1.

Claims (7)

1. A process for the manufacture of ammonia from hydrocarbons and an oxygen-containing gas comprising the steps of A, heating the hydrocarbon in a fired heater, B, optionally adding steam, C, reacting the hydrocarbon with an oxygen-containing gas, D, cooling the product from such reaction and shifting at least some of the carbon monoxide in the gas to carbon dioxide, E, further cooling the gas, F, passing the gas to a separation unit so as to give an ammonia synthesis gas and another stream, which other stream is used as fuel, G, methanating the synthesis gas and passing the methanated gas to an ammonia synthesis unit.

2. A process as claimed in Claim 1 wherein a separation unit utilises molecular sieves to effect the separation.

3. A process as claimed in Claim 1 wherein the separation unit is a cryogenic separation device.

4. A process as claimed in Claim 3 wherein the cryogenic device separates the fuel stream into the relatively methane-rich stream and a relatively nitrogen-rich stream.

5. A process as claimed in any one of the preceding Claims wherein part of the feed to the separation unit is by-passed around the separation unit.

6. A process as claimed in any one of the preceding Claims wherein a carbon dioxide removal unit is placed immediately upstream of the separation unit.

7. A process for the manufacture of ammonia substantially as hereinbefore described with reference to the accompanying drawing.