Hydrocarbons Synthesis

Abstract

A process for synthesising hydrocarbons includes feeding a gaseous feedstock comprising hydrogen, carbon monoxide and carbon dioxide, into a dimethyl ether (DME) synthesis stage, and in the DME synthesis stage, converting a portion of the gaseous feedstock into a DME product and gaseous products. The DME product is separated from unreacted gaseous reactants and the gaseous products to obtain a tail gas comprising hydrogen and carbon monoxide. The tail gas is fed into a Fischer-Tropsch hydrocarbon synthesis stage, and the hydrogen, carbon monoxide and carbon dioxide are allowed at least partially to react catalytically in the Fischer-Tropsch hydrocarbon synthesis stage to form hydrocarbons.

Description

EXAMPLE 1

A stand-alone DME process was modelled using a computerised simulation to set a base case for comparison with the improvement derived from the present invention.

The simulated DME process consists of a cooled methanol reactor followed by an adiabatic combined methanol synthesis and dehydration reactor that contains a bed of dual function catalyst (i.e. combined methanol formation and methanol dehydration) and a bed of methanol dehydration catalyst. The process operates at a pressure of 100 bar. The molar composition of the fresh synthesis gas is 66.2% hydrogen, 24.7% carbon monoxide, 5.2% carbon dioxide and 0.2% water. This corresponds to a syngas number of 2.05.

Recycled synthesis gas is mixed with fresh synthesis gas and preheated to 225° C. 15% of the preheated stream is split from the preheated stream (forming a bypass stream) prior to feeding the remaining 85% to the methanol reactor. The outlet temperature from the methanol reactor is controlled to 274° C. The effluent from the methanol reactor is mixed with the bypass stream and fed to the combined synthesis and dehydration reactor. The effluent from the combined synthesis and dehydration reactor is cooled to condense approximately 99% of the water and methanol and 20% of the DME. The uncondensed gas is split into a recycle stream (93%) and a purge stream (7%). The recycle stream is admixed with the fresh synthesis gas. The purge stream is subjected to an additional cooling step to remove all of the DME.

With a recycle ratio of 2.9 and a per pass H₂ and CO conversion of 27.5%, an overall H₂ and CO conversion of 84.4% and an overall CO and CO₂ conversion of 87.7% is achieved. The mass ratio of methanol product to DME product achieved is 1:1.56. The actual yield over maximum possible yield is 84%.

EXAMPLE 2

In a comparative example to illustrate the benefits of the present invention, a process in which a natural gas-based feed is partially converted to DME and the tail gas converted to hydrocarbons in a two-phase high temperature Fischer-Tropsch reaction stage, was modelled using a computerised simulation.

A typical synthesis gas composition ex an autothermal reformer was used as fresh synthesis gas, i.e. a molar composition of 64.3% hydrogen, 28.6% carbon monoxide, 3.3% carbon dioxide, 2.3% methane and 1.5% inerts. A hydrogen rich gas with a molar composition of 55.3% hydrogen, 2.1% carbon monoxide, 29.9% methane, 12.4% inerts and 0.3% heavier hydrocarbons is separated from a Fischer-Tropsch synthesis stage tail gas (see below). This hydrogen-rich gas is mixed with the fresh feed gas to yield a feedstock to the DME reaction stage with a syngas number of 2.03. The operation of the DME synthesis stage is similar to that described in example 1, except that a lower overall conversion of reactants is targeted. The DME synthesis stage is operated with a recycle ratio of 1.1 and a per pass H₂ and CO conversion of 28%. In this manner an overall conversion over the DME synthesis stage of 50.2% and 50.7% is achieved for H₂ and CO, and CO and CO₂ respectively.

The tail gas from the DME synthesis stage now serves as feedstock for the Fischer-Tropsch synthesis stage, without the need for any composition adjustment. The DME that may still be present in the tail gas from the DME synthesis stage is passed through to the Fischer-Tropsch synthesis stage. The Fischer-Tropsch synthesis stage includes a Fischer-Tropsch reactor which operates at a pressure of 25 bar and a temperature of 350° C. Tail gas from the Fischer-Tropsch reactor is treated to recover hydrocarbons and water. The Fischer-Tropsch tail gas is subjected to a first condensation stage at 30 to 70° C., whereafter a potion of the tail gas is recycled to the inlet of the Fischer-Tropsch reactor, while the remainder is subjected to CO₂ removal followed by further cooling and separation in a cold separation unit to recover light C₂+ hydrocarbons. The DME present in the effluent from the Fischer-Tropsch reactor is recovered together with the products from the Fischer-Tropsch synthesis stage. A hydrogen-rich gas is separated in a cold separation unit and used to adjust the syngas number of the fresh synthesis gas to 2.03.

The Fischer-Tropsch synthesis stage is operated with per pass H₂ and CO conversion of 45.6% and a recycle ratio of 2. This results in an overall conversion of 85.7% and 84.7% for H₂ and CO, and CO and CO₂ respectively over the Fischer-Tropsch synthesis stage.

For the process as a whole, the overall H₂ and CO conversion is 96.7%, while the CO and CO₂ conversion is 92.5%.

The mass ratio of products for methanol:DME:hydrocarbons is 1:2.14:0.63.

The actual yield of the process to the maximum theoretical yield is 91%.

Claims

1. A process for synthesising hydrocarbons, which process includes feeding a gaseous feedstock comprising hydrogen, carbon monoxide and carbon dioxide, into a dimethyl ether (DME) synthesis stage;in the DME synthesis stage, converting a portion of the gaseous feedstock into a DME product and gaseous products;separating the DME product from unreacted gaseous reactants and the gaseous products to obtain a tail gas comprising hydrogen and carbon monoxide;feeding the tail gas into a Fischer-Tropsch hydrocarbon synthesis stage; andallowing the hydrogen, carbon monoxide and carbon dioxide at least partially to react catalytically in the Fischer-Tropsch hydrocarbon synthesis stage to form hydrocarbons.

2. The process as claimed in claim 1, in which the Fischer-Tropsch hydrocarbon synthesis stage is a two-phase high temperature catalytic Fischer-Tropsch hydrocarbon synthesis stage, the hydrocarbons formed in the Fischer-Tropsch hydrocarbon synthesis stage thus being gaseous hydrocarbons at the operating pressure and temperature of the Fischer-Tropsch hydrocarbon synthesis stage.

3. The process as claimed in claim 1 or claim 2, which includes adjusting the composition of the gaseous feedstock so that the gaseous feedstock has a syngas number (SN) between 1.8 and 2.2, where

4. The process as claimed in any one of the preceding claims, in which converting a portion of the gaseous feedstock into a DME product and gaseous products includes contacting the gaseous feedstock with a catalyst or catalysts that enhance methanol synthesis and methanol dehydration reactions.

5. The process as claimed in any one of the preceding claims, in which the DME product includes a mixture of DME and methanol and which includes converting the DME product into light olefins in a light olefins production stage without increasing the DME concentration in the DME product.

6. The process as claimed in any one of the preceding claims, which includes recycling a portion of the tail gas from the DME synthesis stage to the DME synthesis stage, a ratio of tail gas recycle to gaseous feedstock being between about 0:1 and about 2:1.

7. The process as claimed in any one of the preceding claims, in which the DME synthesis stage is operated at conditions suitable to ensure that overall CO+CO2 conversion in the DME synthesis stage is between about 20% and about 80%.

8. The process as claimed in any one of the preceding claims, which includes recycling some of the Fischer-Tropsch hydrocarbon synthesis stage tail gas to the Fischer-Tropsch hydrocarbon synthesis stage, to obtain high overall CO+CO2 conversions in the Fischer-Tropsch hydrocarbon synthesis stage of at least 80%.

9. The process as claimed in any one of the preceding claims, which includes recycling some of the Fischer-Tropsch hydrocarbon synthesis stage tail gas to the Fischer-Tropsch hydrocarbon synthesis stage, a ratio of Fischer-Tropsch tail gas recycle to the tail gas from the DME synthesis stage fed to the Fischer-Tropsch hydrocarbon synthesis stage being between 2.5:1 and 1:1.5.

10. The process as claimed in claim 5, which includes, in a separation stage, separating light hydrocarbons from the Fischer-Tropsch hydrocarbon synthesis stage tail gas and converting these light hydrocarbons, together with the DME product, into light olefins with a carbon number from 2 to 4 in the light olefins production stage.

11. The process as claimed in claim 5 or claim 10, in which gaseous hydrocarbons and any unreacted hydrogen, unreacted carbon monoxide, and CO2 are withdrawn from the Fischer-Tropsch hydrocarbon synthesis stage, and separated into one or more condensed liquid hydrocarbon streams, a reaction water stream and a Fischer-Tropsch hydrocarbon synthesis stage tail gas, the process further including treating the condensed liquid hydrocarbons from the Fischer-Tropsch hydrocarbon synthesis stage, to provide a light hydrocarbon fraction, including naphtha, which is converted, together with the DME product, in the light olefin production stage to light olefins, and to provide a diesel fraction.

12. A process as claimed in claim 5 or claim 10 or claim 11, which includes using separation equipment to recover C2-C4 light olefins from the Fischer-Tropsch hydrocarbon synthesis stage and in which C2-C4 light olefins from the light olefins production stage are recovered using the same separation equipment that is used to recover the C2-C4 light olefins produced by Fischer-Tropsch synthesis.