PROCESS FOR PRODUCING SYNTHESIS GAS

Abstract

A process for producing hydrocarbons is disclosed in which a first feed substream and a second feed substream are obtained from a hydrocarbonaceous feed stream, of which the first feed substream is converted by means of partial oxidation or autothermal reforming to a first synthesis gas stream and the second feed substream is converted by means of steam reforming to a second synthesis gas stream and subsequently combined with the first synthesis gas stream to give a third synthesis gas stream, of which at least a first portion is converted by Fischer-Tropsch synthesis to a crude product stream comprising hydrocarbons of different chain lengths, from which light hydrocarbons are separated in a tail gas, in order to recycle them and use them in the partial oxidation or autothermal reforming. The characteristic feature here is that unsaturated hydrocarbons are separated from at least a portion of the tail gas.

Claims

1. A process for producing hydrocarbons, in which a first feed substream and a second feed substream are obtained from a hydrocarbonaceous feed stream, of which the first feed substream is converted by means of partial oxidation or autothermal reforming to a first synthesis gas stream and the second feed substream is converted by means of steam reforming to a second synthesis gas stream and subsequently combined with the first synthesis gas stream to give a third synthesis gas stream, of which at least a first portion of the third synthesis gas stream is converted by Fischer-Tropsch synthesis to a crude product stream comprising hydrocarbons of different chain lengths, from which light hydrocarbons are separated in a tail gas, in order to recycle them and use them in the partial oxidation or autothermal reforming, characterized in that unsaturated hydrocarbons are separated from at least a portion of the tail gas in order to obtain a stream which is substantially free of unsaturated hydrocarbons as a feed for the partial oxidation or autothermal reforming.

2. The process according to claim 1, characterized in that the unsaturated hydrocarbons separated from the at least one portion of the tail gas are used for underfiring within the process or are released for a credit as a product for physical or thermal utilization.

3. The process according to claim 1, characterized in that the ratio of the volume flow rates of the first feed substream and the second feed substream is set in order to set the ratio of hydrogen and carbon monoxide in the third synthesis gas stream.

4. The process according to claim 1, characterized in that the ratio of the volume flow rates of the first synthesis gas and the second synthesis gas before they are combined to give the third synthesis gas stream is altered by branching off one or more substreams under closed-loop control, in order to set the ratio of hydrogen and carbon monoxide in the third synthesis gas stream.

5. The process according to claim 3, characterized in that the ratio of hydrogen to carbon monoxide in the third synthesis gas stream is set to a value in the range from 1.5 to 2.5.

6. The process according to claim 1, characterized in that the third synthesis gas stream is cooled in a cooling unit with water to raise steam which is subsequently used to generate electrical energy, and the steam, after superheating in the waste heat system of the steam reformer, is expanded in a steam turbine coupled to an electrical generator.

7. The process according to claim 1, characterized in that the third synthesis gas stream is divided into a first synthesis gas substream and a second synthesis gas substream, and the first synthesis gas substream is used as feed for the Fischer-Tropsch synthesis, while the second synthesis gas substream is subjected to a water-gas shift reaction in order to reduce the CO content in the second synthesis gas substream and to increase the hydrogen content.

8. The process according to claim 7, characterized in that the second synthesis gas substream, after the water-gas shift reaction, is subjected to a pressure swing adsorption to obtain a hydrogen-rich stream and a purge gas stream laden with removed gas components.

9. The process according to claim 8, characterized in that the hydrogen-rich stream is used for hydrogenation of heavy hydrocarbons obtained in the Fischer-Tropsch synthesis and/or for desulphurization of the feed stream or one or both feed substreams.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention is to be elucidated in detail hereinafter with reference to a working example shown schematically in the FIGURE.

[0023] The FIGURE shows a preferred configuration of the process according to the invention

DETAILED DESCRIPTION OF THE INVENTION

[0024] The feed stream 1, which is natural gas, for example, is guided through an adsorber unit A for desulphurization, in order to remove sulphur compounds present and hydrogen cyanide down to contents below 1 ppm. Subsequently, the desulphurized feed stream 2 is divided into two feed substreams 3 and 4, it being possible to individually adjust the respective volume flow rates of these substreams, in order more particularly to set the ratio of hydrogen to carbon monoxide in the third synthesis gas stream 5 (see below).

[0025] The first feed substream 3 is combined with the stream 20, consisting predominantly of saturated hydrocarbons and carbon monoxide, carbon dioxide and hydrogen, and preheated together therewith in the preheater unit H, before both streams are introduced into the reactor R as feed stream 6 and reacted with an oxidizing agent 7 to give a first synthesis gas stream 8. If the reactor R is a PDX reactor, the preheating can be effected up to temperatures of 450° C., while 600° C. is even possible when the reactor R is an ATR reactor. The oxidizing agent 7 used is preferably oxygen of technical grade purity, which is obtained, for example, by cryogenic air fractionation. The oxygen can also be obtained in another way, for instance in a membrane method or by pressure swing adsorption.

[0026] The second feed substream 4 is subjected to steam reforming D, for which it is mixed with steam and/or carbon dioxide 9 and converted to a second synthesis gas stream 10 in the reactor tubes of the steam reformer D, in which a suitable catalyst is disposed, at a temperature between 700° C. and 950° C. and a pressure in the range from 15 bar to 45 bar.

[0027] The two synthesis gas streams 8 and 10, which, as a result of the production methods, have a different ratio of hydrogen to carbon monoxide, are then combined to give the third synthesis gas stream 5 having a hydrogen/carbon monoxide ratio in the range from 1.5 to 2.5.

[0028] The third synthesis gas stream 5, after being cooled and dried in the cooling unit K, is divided into a first synthesis gas substream 11 and a second synthesis gas substream 12, the volume flow rates of which have a ratio in the range from 0.01 to 0.05. While the first synthesis gas substream 11 is fed as a feed to a Fischer-Tropsch synthesis F, the second synthesis gas substream 12 is subjected to a water-gas shift reaction W in which carbon monoxide present is reacted with water to give hydrogen and carbon dioxide, so as to obtain a synthesis gas substream 13 with an elevated carbon monoxide content and reduced hydrogen content. The synthesis gas substream obtained in the water-gas shift is subsequently subjected to a known pressure swing adsorption D in order to obtain a hydrogen-rich stream 14 and a purge gas stream 15 laden with removed gas components, which subsequently serves as fuel and is used, for example, to supply heat to the steam reformer D.

[0029] In the Fischer-Tropsch synthesis F, the first synthesis gas substream 11 is converted to a crude product stream 16 comprising light hydrocarbons having four or fewer carbon atoms, heavy hydrocarbons having five or more carbon atoms, and unconverted synthesis gas. A tail gas 17 is separated from the crude product stream 16 in the cold trap S and consists in particular of unconverted synthesis gas and saturated and unsaturated light hydrocarbons. A first portion 18 of the tail gas 17 is recycled as feed into the Fischer-Tropsch synthesis, while a second portion 19 is sent to a removal unit T in which a stream 20 predominantly free of unsaturated hydrocarbons and a stream 21 largely consisting of unsaturated hydrocarbons are obtained. The stream 20 predominantly free of unsaturated hydrocarbons is then combined with the first feed substream 3, while the stream 21 largely consisting of unsaturated hydrocarbons can be released, for example, as a product.

[0030] The hydrogen-rich stream 14 produced in the pressure swing adsorption D is used in the hydrogenation reactor Z for hydrogenation of heavy or unsaturated hydrocarbons in the crude product stream 22 which remains after removal of the tail gas 17 and is converted in the process to the hydrocarbonaceous product stream 23.