Imperial Chemical Industries Limited

Alwyn Pinto

John B Johnson

Peter J Davidson

Antony P Limbach

Stanley A Ward

Anthony John Connell

Ian C Jeffery

Synthesis gas production comprising primary catalytic steam reforming a first stream of desulphurised hydrocarbon feedstock, optionally followed by secondary reforming using an oxygen-containing gas, and then cooling; adiabatically low temperature steam reforming a second stream of the feedstock, preferably adding a hydrogen-containing gas, and then subjecting the product to partial oxidation with an oxygen-containing gas, and then cooling; and mixing the cooled products. For methanol production, the partial oxidation step pressure may be greater than the primary reforming pressure, and the hydrogen-containing gas is taken from the methanol synthesis loop: if the partial oxidation step is non-catalytic and in the absence of steam, the pre-reforming stage can be omitted. Methanol can be synthesised from the reformed first and/or second streams in an auxiliary synthesis stage at an intermediate pressure before the relevant stream is added to the synthesis loop.

Synthesis gas production

The endothermic reaction of a hydrocarbon feedstock with steam and/or carbon dioxide is carried out using a double-tube reactor. In one preferred form of the reactor the catalyst is present as a coating on the outside of the inner tube. In another form inner tubes are mounted in one tube-plate and the outer tubes in a second tube plate and the tube plates are disposed across a cylindrical shell so as to define a heat exchange zone (provided with a heat source) a reactants feed zone and a products offtake zone. The heat source is preferably a burner, to be fed with the product of the endothermic reaction, followed by a secondary reforming catalyst. The apparatus makes possible processes for making raw hydrogen-containing gases with advantageous internal heat recovery.

Steam reforming

In an ammonia synthesis process a nitrogen-hydrogen gas is reacted partially over a catalyst at a pressure in the range 30.degree.-120.degree. C., ammonia is separated as liquid after cooling the reacted synthesis gas and unreacted synthesis gas is recycled. The liquid ammonia is evaporated in heat exchange with reacted synthesis gas to provide the required cooling effect. Such heat exchange is effected using a heat exchange surface (as in a plate-fin heat exchanger) of at least 1.5 m.sup.2 per kg mol per hour of ammonia to be condensed, with cold-end temperature approach of less than 80.degree. C. and a hot-end temperature approach of less than 5.degree. C. whereby exploit the heat effect of, inter alia, the non-ideality of ammonia to provide product gaseous ammonia at a convenient pressure with minimal mechanical refrigeration.

Ammonia synthesis plant

Steam reforming process

Ammonia synthesis gas is prepared by primary and secondary reforming followed by shift and carbon oxides removal. So that the process can use naphtha and methane, e.g. natural gas, as alternative feedstocks, at least the initial part of the primary reforming catalyst is alkalized. When using a methane feedstock, the primary reformer heating and outlet temperature are reduced compared to when using naphtha, and the amount of air fed to the secondary reformer is increased, so that, after shift, the hydrogen to nitrogen molar ratio of the gas stream is lower and is such that the gas contains an excess of nitrogen. This excess of nitrogen is subsequently removed, e.g. from the ammonia synthesis loop by a cryogenic hydrogen-recovery unit treating a loop purge stream and recovering hydrogen for recycle to the loop.

Ammonia synthesis gas

Ammonia synthesis gas is made from a raw gas comprising hydrogen, carbon dioxide and medium boiling point gases including nitrogen in excess of the proportion required an ammonia synthesis gas, by a pressure swing adsorption process characterized by feeding to the absorbent a raw gas in which hydrogen and total medium boiling point gases are present in a volume ratio in the range 1.25 to 2.5, and the medium boiling point gases comprise nitrogen to the extent of at least 90% v/v on the total of such gases. Preferred ways of making the raw gas, of ensuring purity of the product gas and of recovering useful heat are described.

Ammonia synthesis

Production of a nitrogen gas stream from the waste gas from a pressure adsorption process separating a raw gas stream containing nitrogen, carbon dioxide, and combustible gas including hydrogen, into a hydrogen containing product stream and a waste gas stream containing carbon dioxide, some combustible gas, and at least some of the nitrogen, by catalytically combusting the waste gas stream with an oxygen containing gas and removing water from the combusted waste gas stream. Carbon dioxide is removed from the waste gas stream before and/or after catalytic combustion. To reduce the residual oxygen content of the combusted waste gas stream, a small amount of hydrogen, e.g. a portion of the hydrogen containing product from the pressure swing adsorption, may be added to the combusted waste gas stream and the mixture catalytically reacted.

Nitrogen production

A catalytic partial oxidation or secondary reforming process wherein a flame is established at a burner upstream of the catalyst is started-up by catalytic combustion of a suitable combustible material to produce a hot gas stream and directly, or indirectly, using the heat from the gas stream to heat the reactants fed to the burner to the autoignition temperature. After a flame has been established at the burner, production of the hot gas stream by combustion of the combustible material is discontinued.

Catalytic process

A reactor suitable for synthesis of ammonia or methanol comprises a catalyst bed 36 equipped with heat exchange tubes 32 supported by means 28-31 or 40, 42 providing catalyst fillable space between the tubes over substantially their whole length. Preferably catalyst particles between the tubes and in an uncooled bed 36 downstream thereof constitute a single body of catalyst. The reactor is especially suitable for synthesis at low pressures such that the heat transfer area provided by the tubes is relatively low.

Synthesis process and reactor

A PSA based process for the production of technical hydrogen wherein the feed to the PSA contains nitrogen and carbon monoxide with a hydrogen/(carbon monoxide+nitrogen) volume ratio in the range 1.2 to 4 and a nitrogen/carbon monoxide volume ratio of at least 10. The adsorption step in the PSA stage is stopped when the integrated nitrogen content of the unadsorbed product is 1 to 10% volume. The raw gas fed to the PSA is preferably made by primary reforming a hydrocarbon feedstock with steam followed by secondary reforming followed by shift and water removal: the heat required for primary reforming is preferably supplied by the secondary reformer outlet gas. The PSA waste gas is preferably combusted in a gas turbine driving the secondary reformer air compressor.

Technical hydrogen

A raw ammonia synthesis gas, preferably containing an excess of nitrogen, is converted to synthesis ready ammonia synthesis gas by two pressure swing adsorption (PSA) stages operated in succession. If the raw gas contains carbon monoxide this is chemically removed, e.g. by methanation, between the PSA stages. In the first PSA stage, CO.sub.2 is removed and waste gas from the second PSA stage is preferably fed back to the first PSA stage for sweeping, purging or repressurizing. In a preferred system the first PSA cycle includes a sweep with CO.sub.2 -rich gas prior to depressurization and purge with second PSA waste gas. In the second PSA stage inert medium boiling gases, and any unwanted nitrogen are removed as the waste gas.

Gas separation

The endothermic reaction of a hydrocarbon feedstock with steam and/or carbon dioxide is carried out using a double-tube reactor. In one preferred form of the reactor the catalyst is present as a coating on the outside of the inner tube. In another form inner tubes are mounted in one tube-plate and the other tubes in a second tube plate and the tube plates are disposed across a cylindrical shell so as to define a heat exchange zone (provided with a heat source) a reactants feed zone and a products offtake zone. The heat source is preferably a burner, to be fed with the product of the endothermic reaction, followed by a secondary reforming catalyst. The apparatus makes possible processes for making raw hydrogen-containing gases with advantageous internal heat recovery.

Energy is recovered from low calorific value gas by means of a gas turbine. Heat is recovered from the turbine exhaust by indirect heat exchange with water. The resultant hot water is used to saturate the fuel and, optionally, also the combustion air, and/or to aid regeneration of an absorbent used in a wet process to remove carbon dioxide from the gas prior to combustion. Preferably the low calorific value gas is waste gas from a PSA process used to remove impurities in the production of a hydrogen-containing gas stream from a raw gas made by primary/secondary steam reforming a hydrocarbon feedstock at superatmospheric pressure and the turbine drives the secondary reformer air compressor.

Energy recovery

A carbonaceous feedstock, e.g. coal or heavy oil, having a low hydrogen to carbon atomic ratio is partially oxidized using air, or oxygen enriched or depleted air, to form a crude gas which is subjected to catalytic shift and excess steam removal. Carbon dioxide and nitrogen are then removed from the resultant raw gas, which has a hydrogen to nitrogen, plus carbon monoxide, volume ratio in the range 0.5 to 1.5 and a nitrogen content at least 10 times the carbon monoxide content, in a pressure swing adsorption process thereby producing a product gas stream containing at least 50% hydrogen and 0.5 to 40% nitrogen, by volume. The waste gas from the pressure swing adsorption may be catalytically combusted to power a turbine driving the compressor for the air or the crude gas.

Hydrogen streams

Production of ammonia synthesis gas by partial oxidation of a hydrocarbon feedstock, using air, oxygen enriched air, or oxygen depleted air, in admixture with steam, followed by shift and the removal of the excess of nitrogen, and also impurities such as carbon oxides and methane, by pressure swing adsorption (PSA). The PSA waste gas is combusted and used to heat the air and/or steam prior to the use thereof in the partial oxidation.

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Middlesbrough, United Kingdom

Alwyn Pinto