GB2270309A - Recovery of excess nitrogen from ammonia synthesis feed gas - Google Patents

Abstract

A process for the separation of excess nitrogen from an ammonia synthesis feed gas mixture having a hydrogen to nitrogen ratio of less than 3:1. The feed gas 110 is cooled and partially condensed to form an uncondensed gas stream 112 having a hydrogen to nitrogen ratio of substantially 3:1. The liquid condensate, 113 which contains the excess nitrogen, is evaporated in indirect counter-current heat exchange 101 with the cooling feed gas. Refrigeration for the process is provided by work expanding evaporated condensate and passing the expanded gas in indirect counter-current heat exchange with the feed gas to cool it. <IMAGE>

Description

RECOVERY OF EXCESS NITROGEN FROM AMMONIA SYNTHESIS FEED GAS This invention relates to the removal of excess nitrogen from ammonia synthesis feed gas.

In one widely employed method, ammonia synthesis feed gas is formed from a hydrocarbon source such as natural gas. In a typical procedure, the hydrocarbon source, which is usually but not necessarily natural gas, is subjected to primary reforming, in which the hydrocarbon is partially converted to produce hydrogen by steam reforming, followed by secondary reforming with air which introduces the required nitrogen. Carbon monoxide formed during reforming is then converted to the dioxide and hydrogen by a shift reaction with steam and thereafter the gas is usually treated to-stit1 further reduce or remove any further impurities such as residual oxides of carbon, methane and argon.

Ammonia synthesis, as commonly practised, involves cycling compressed synthesis gas round a loop in which partial conversion occurs, ammonia product is removed e.g. by condensation, unreacted synthesis gas is recompressed and make up synthesis feed gas is added as required.

Depending on the source of the synthesis gas and despite the purification procedures referred to above, this gas will also generally contain small amounts of impurities such as methane, argon and helium and in order to maintain these at a constant level in the synthesis loop it is also necessary to remove a purge gas at an appropriate rate. In general, the purge gas is treated to recover hydrogen and nitrogen values.

In our modification, which is favored for economic reasons, the crude gas from the reforming deliberately contains nitrogen in excess of the stoichiometric amount of 1 mole nitrogen to 3 moles hydrogen required for ammonia synthesis and it is desirable to remove the excess nitrogen before effecting the synthesis. Much of the methane and argon values in the gas will be removed with the nitrogen but small quantities still remain.

Several methods are employed to recover the excess nitrogen from the crude gas. In one procedure, which is generally considered to be more appropriate to smaller plants, e.g. around 500 tons per day or less, the excess nitrogen is removed by pressure swing adsorption. For larger plants, however, cryogenic separation is favoured.

In one embodiment, the gas stream is cooled and partially condensed, the excess nitrogen is separated as condensate and all the required refrigeration is provided b Joule Thomson expansion of the condensate.

However, this procedure is only suitable where the ratio of hydrogen to nitrogen in the untreated gas is sufficiently low, e.g. about 1.2:1 or less. Such low ratios are generally only appropriate where the hydrocarbon feedstock has a high carbon:hydrogen ratio.

In an alternative procedure, which is suitable for higher hydrogen:nitrogen ratios the crude gas is cooled expanded through a turbine to provide refrigeration for the process, and then further cooled and partially condensed. The mixture of liquid and gas so formed in then fed to a column from which the desired nitrogen/hydrogen mixture is recovered overhead and the excess nitrogen is removed from the bottom as liquid, expanded and supplied as refrigerant to the overhead condenser for the column. As the feed is expanded, the desired nitrogen/hydrogen mixture is recovered at a pressure below that of the feed gas and therefore requires additional energy to compress it to synthesis pressure.

Both these alternative cryogenic procedures have been known for over ten years.

We have now found an improved cryogenic process for the separation of nitrogen which reduces br avoids the disadvantages which accompany the use of these two alternatives.

According to the present invention, there is provided a process for the separation of excess nitrogen from an ammonia synthesis feed gas mixture wherein the ratio of hydrogen to nitrogen is below 3:1 and generally between about 1.1:1 or 1.2:1 and 2.3:1. The process comprises cooling and partially condensing the crude feed gas to form an uncondensed gas stream wherein the ratio of hydrogen to nitrogen is substantially 3:1, separating from said uncondensed gas stream a liquid condensate stream containing the excess nitrogen, evaporating said condensate in indirect counter-current heat exchange with the cooling crude feed gas and providing refrigeration for the process by work expanding the evaporated condensate and passing the expanded gas in indirect counter-current heat exchange with the feed gas to cool it.The uncondensed gas stream is preferably also passed in counter-current indirect heat exchange with the feed gas to utilise the cold in it.

The amount of refrigeration required from work expansion will depend on the amount of excess nitrogen in the crude feed gas. Where applicable, the pressure of the liquid condensate may be reduced by Joule Thomson expansion to an intermediate pressure before being evaporated and expanded in the turbine expander, thereby optimising the amount of refrigeration provided by the expander.

The amount of evaporated condensate which is expanded through the turbine may also be adjusted by providing a by-pass round the expander.

The evaporated condensate by-passing the expander is let down through a valve to the same pressure as the gas leaving the exhaust of the expander.

In a modification of the process, which is suitable at relatively low ratios of hydrogen and nitrogen e.g. 1.6:1 and below, typically 1.1:1 to 1.6:1, the refrigeration provided by work expansion may be provided instead by evaporation of liquid ammonia recovered from the ammonia synthesis. A combination of both forms of refrigeration may also be used, if desired.

The invention will now be described in more detail with reference to preferred embodiments thereof and with the aid of the accompanying drawings in which Figure 1 is a generalised flow diagram of a process for the production of ammonia starting from a hydrocarbon source, Figure 2 is a flow diagram of one embodiment of the present invention and Figure 3 is a flow diagram of an alternative embodiment of the present invention.

Referring to Figure 1, a hydrocarbon stream, such as natural gas, provided through line 1, is reformed with steam provided through line 2 in steam reformer 10 and the product is supplied through line 3 to secondary reformer 20 to which air is supplied through line 4. The product, comprising hydrogen, nitrogen and oxides of carbon is passed via line 5 to shift reactor system 30 where carbon monoxide therein is reacted with steam provided through line 6 to form carbon dioxide which is then removed in plant 8. The substantially carbon dioxide-free gas is then passed to methanator 50 wherein any remaining carbon oxides are converted to methane.The resultant gas stream, comprising nitrogen, hydrogen, small amounts of argon (derived from air) and methane (derived from reforming and methanation) is passed via line 9 to cryogenic separator 60 where nitrogen in excess of the stoichiometric ratio of 3 moles hydrogen to 1 mole nitrogen for ammonia synthesis is removed via line 11 together with most of the methane and argon. The remaining gas is passed via line 12 to ammonia synthesis reactor loop generally designated as 70, from which ammonia and a purge gas are recovered. If desired, the purge gas may be recycled to be combined with the feed to the cryogenic separator.

Referring to Figure 2, in one embodiment of the invention the cryogenic separator comprises a heat exchanger 101, gas/liquid separator 102 turbine expander 103 and valve 104. The feed to the separator, comprising hydrogen and nitrogen in a molar ratio of between 1.1:1 and 3:1, and also some methane and argon, is supplied to the separator through line 110 and iscooledand partially condensed in heat exchanger 101. The mixture of condensate and uncondensed gas is passed through line 111 to gas/liquid separator 102 from which a gas stream containing hydrogen and nitrogen in a molar ratio of substantially 3:1 and trace amounts of methane and argon is recovered overhead in line 112 and a condensate stream comprising nitrogen, methane, argon and a small amount of hydrogen is recovered in line 113.Cold in the gas in line 112 is used to assist cooling of the incoming feed gas by passing it back in counter-current heat exchange with the feed gas through heat exchanger 101. The product is recovered in line 114 and passed as make-up feed gas to the ammonia synthesis reactor loop (see Figure 1).

The condensate in line 113 is passed through valve 104 where it may, if desired, be expanded to an intermediate pressure by Joule-Thomson expansion, then passed back via line 115 to the heat exchanger where it is evaporated in counter-current heat exchange with the feed gas and thereafter passed via line 116 to turbine expander 103 wherein it is expanded and then passed back again via line 117 through heat exchanger 101 in indirect counter-current heat exchange relationship with the incoming feed gas to provide refrigeration for the process and recovered through line 118. Further or alternative control of the amount of refrigeration provided by expansion through the turbine may be provided by arranging for a controlled portion of the evaporated condensate to by-pass the turbine through line 119.

The procedure just described may be employed for a gas wherein the ratio of hydrogen to nitrogen is anywhere in the range between about 1.1:1 and 3:1. However, at lower ratios in that range, e.g. below 1.6:1, an alternative procedure may be employed which may be more economical.

This alternative arrangement is illustrated in Figure 3 wherein the lines and apparatus components common with those of the embodiment illustrated in Figure 2 are accorded the same reference numerals. In this embodiment the turbine 103 is not required, the evaporated condensate recovered in line 116 from heat exchanger 101 is removed from the plant and refrigeration is provided in part by evaporating liquid ammonia, supplied from the ammonia synthesis reactor loop (see Figure 1) in heat exchanger 101 through line 120 and in part by Joule Thomson expansion of condensate through valve 104.

By way of example, an ammonia synthesis feed gas having the composition shown in Table I is provided to the separator of Figure 2 through line 110 at a flow rate of 2865.8 kgmol/h, a pressure of 36.3 bar a and a temperature of 13 C. A gas stream suitable for use as an ammonia synthesis feed gas and having the composition shown in the Table is recovered in line 114 at a flow rate of 2370.9 kgmol/h, a pressure of 35.0 bar a and a temperature of 10 C and a waste gas having the composition shown in the Table is removed through line 118 at a flow rate of 494.9 kgmol/h, a pressure of 2.0 bar a and a temperature of 10 C.

Ammonia Synthesis Gas Feed Hydrogen Product Off Gas Composition: molX Composition: mol% Composition: mol% Hydrogen 62.98 Hydrogen 74.48 Hydrogen 7 &commat; 7.89 Nitrogen 35.01 Nitrogen 24.88 Nitrogen 83.54 Argon 0.68 Argon 0.36 Argon 2.21 Methane 1.33 Methane 0.28 Methane 6.36 The relevant temperatures and pressures of the various streams are shown in Table II below.

Stream Temperature ("C) Pressure (bar a) 110 13 36.3 111 -177 113 -177 114 10 35.0 115 183 6.0 116 -165 6.0 117 -177 2.0 118 10 2.0 It will be seen that as in the process of the invention, the required nitrogen/hydrogen mixture is recovered at about the same pressure as the feed gas, less energy is required to compress it to ammonia synthesis pressure than in the prior art arrangement where the feed gas is expanded. Also, a simpler apparatus is used. It will also be seen that the process of the present invention may be used with a wider range of hydrogen:nitrogen ratios than is possible with the prior art alternative which employs only Joule-Thomson expansion.

It is also believed that the process of the present invention offers an economically viable alternative to pressure swing adsorption in the smaller plants, e.g. 500 tons per day or less, even though the use of pressure swing adsorption may obviate the need for a separate unit for removal of carbon dioxide from the feed gas to the ammonia synthesis loop.

Claims (4)

1. A process for the separation of excess nitrogen from an ammonia synthesis feed gas mixture wherein the ratio of hydrogen to nitrogen is below 3:1, said process comprising cooling and partially condensing the crude feed gas to form an uncondensed gas stream wherein the ratio of hydrogen to nitrogen is substantially 3:1, separating from said uncondensed gas stream a liquid condensate stream containing the excess nitrogen, evaporating said condensate in indirect counter-current heat exchange with the cooling crude feed gas, and providing refrigeration for the process by work expanding evaporated condensate and passing the expanded gas in indirect counter-current heat exchange with the feed gas to cool it.

2. A process as claimed in claim 1 wherein the uncondensed gas stream is passed in counter-current indirect heat exchange with the feed gas.

3. A process as claimed in claim 1 or claim 2 wherein the condensate is expanded to an intermediate pressure by Joule Thomson expansion prior to evaporation.

4. A modification of process as claimed in claim 1 or claim 2 wherein the uncondensed gas stream is supplied as synthesis gas to an ammonia synthesis, the evaporated condensate is recovered from the system and refrigeration is provided in part by evaporation of liquid ammonia formed in the ammonia synthesis and in part by Joule Thomson expansion of the condensate prior to evaporating it in indirect counter-current heat exchange relationship with the cooling crude feed gas.