GB2099846A

GB2099846A - Process for the preparation of synthesis gas

Info

Publication number: GB2099846A
Application number: GB8216177A
Authority: GB
Original assignee: Uhde GmbH
Current assignee: ThyssenKrupp Industrial Solutions AG
Priority date: 1981-06-04
Filing date: 1982-06-03
Publication date: 1982-12-15
Also published as: DE3122273A1; GB2099846B; JPS582203A; IT8221293A0; IT1198371B; FR2507169A1; NO821714L; FR2507169B1; DK251182A

Abstract

A two-stage process for the preparation of synthesis gas is provided wherein methane or a methane-rich gas mixture is first cracked at a pressure of 35 to 55 bar and a temperature of 650 to 800 DEG C to a CH4 content of 13 to 20% and then cracked at a temperature of 900 to 1200 DEG C with oxygen. Synthesis gas is produced which is especially suitable for use in methanol synthesis and which requires no modification of a methanol synthesis plant.

Description

SPECIFICATION Process for the preparation of synthesis gas The invention relates to a process for the preparation of synthesis gas and is particularly concerned with the preparation of synthesis gas which is suitable for use in methanol synthesis.

Synthesis gas is a gas which, as its name suggests, may be used for the synthesis of various compounds. In composition, it contains mainly carbon monoxide (CO), hydrogen (H2) and carbon dioxide (CO2). It is usually derived from carbonrich sources such as natural gas, naptha etc. For methanol synthesis, natural gas is often used and requires a high temperature and pressure for cracking to form the synthesis gas; the temperatures required may be in the range of from 700 to 12000C.

The substances required for the synthesis of methanol (CH3OH) are H2, CO, and CO2. Synthesis gas which is suitable for use in methanol synthesis has the correct stoichiometric composition if the following requirement is met:

Theoretically, the CO2 content could be zero which would result in the ratio being

While hydrocarbons such as naptha have a H/C ratio of 2, catalytic cracking of natural gas, especially gas having a high CH4 content, yields a gas composition with a H2-C02 CO+CO2 ratio of up to 3; in such cases the H2 content is too high for the gas to be useful for methanol synthesis without modification of the synthesis loop process in some way.

In previously described processes, too much hydrogen, i.e. ballast hydrogen, is produced during catalytic cracking. This ballast hydrogen requires the plant to be oversized to some extent which, for large-scale plants, imposes certain limits on the size of the plant equipment. The input of natural gas and energy per unit of methanol produced is, as a result, disproportionate.

One method previously proposed to reduce the unsuitable ratio comprises regularly bleeding and expanding a considerable part of the gas from the methanol synthesis loop. In this way a gradual H2 enrichment of the circulating loop gas may be prevented. After expansion, the gas bled from the methanol synthesis loop can be used as a fuel gas for catalytic cracking in the steam reformer as described in Canadian Patent Specification No.

786 393 or shown in Chemical Economy and Engineering Review of September 1 971, Vol. 3, No. 9 (No. 41), page 30, figure 2. This method of operating the methanol synthesis plant offers the advantage of simplicity in the gas generation section but has the disadvantage of excessive production of hydrogen, which has to be passed through the entire plant before subsequent reduction in pressure to almost atmospheric pressure to render it suitable for use as a fuel gas in the steam reformer.

Another process described in the Chemical Economy and Engineering Review of September 1971, Vol.3, No.9 (No.41), page 21, achieves an optimum adjustment of the H/C ratio for methanol synthesis using a CO converter arranged in parallel. In the converter, part of the H2.

rich and CO-bearing raw gas obtained from a single-stage catalytic cracking process is converted to CO2 and H2. Downstream the CO2 is absorbed and so is separated from the H2. The H2 can be put to another use; the CO2 is introduced into the main gas stream to aid adjustment to an optimum H/C ratio.

A further means to improve the unfavourable HCO ratio during catalytic cracking of the CH4 in the gas feed-stock consists in directly introducing CO2 into the synthesis gas stream. One example of such a process is disclosed in Petroleum 8 Petrochemical, August 1973, Vol. 13, No. 8, page 74, fig. 2. By that process, CO2 is removed from flue gas by scrubbing with chemicals, the CO2 is compressed and added to the synthesis gas. As the CO2 recovery from the flue gas takes place under atmospheric pressure, a large amount of energy is required for compression of the CO2; the equipment involved is also expensive.

The present invention comprises a process for the preparation of synthesis gas which comprises (i) cracking methane or a gas mixture containing a substantial amount of methane at a pressure in the range of from 35 to 55 bar and at a temperature in the range of from 650 to 8000C to a CH4 content in the range of from 13 to 20% by volume (ii) cracking the gas mixture obtained from step (1) at a temperature in the range of from 900 to 1 2000C with the addition of oxygen.

For a gas mixture containing a substantial amount of methane, any methane-rich gas mixture is suitable. Preferably, natural gas is used as the gas mixture. By the term "natural gas" both natural gas and synthetic natural gas is to be understood. Suitably, as a substantial amount of methane, the gas mixture contains more than 80% by volume of methane, preferably more than 90% by volume and especially more than 95% by volume.

The two steps of the process of the invention are both usually catalytic cracking steps, which are carried out at high temperatures and pressures, though it is possible for one or both of the steps to be carried out without catalysis. Step (ii) may be carried out at a high or super atmospheric pressure which is normal for such operations.

The technical advantage achieved by the process according to the present invention is that the optimum H/C ratio required for the methanol synthesis plant is adjusted in the catalytic cracking unit prior to entry to the methanol synthesis plant. The formation of the ballast hydrogen is avoided and, as a consequence, energy requirements for the compression of hydrogen as well as feedstock gas consumption are substantially reduced. Moreover, the entire plant equipment is, as regards volume, only designed for the gas throughput actually required. Thus the expensive modification of the methanol synthesis plant previously required to adjust the H2/CO ratio can be dispensed with.

The following Example illustrates the invention.

Example The flow rate, composition, temperature and pressure of the gas were recorded at various points in a catalytic cracking plant which has two cracking stages operating in accordance with the present invention. Percentages are given on a volume basis.

Feedstock Flow rate: 181,285 Nm3/h Composition: CH4 98% N2 2% Pressure: 51.8 bar Temperature: 920C Inlet of primary cracking unit Flow rate: 607, 666 Nm3/h Composition: CH4 29.24% N2 0.60% H2O 70.16% Pressure: 45 bar Temperature 5100C Outlet of primary cracking unit Flow rate: 730, 726 Nm3/h Composition: CO 2.78% CO2 6.519/0 H2 30.90% CH4 15.02% N2 0.50% H2O 44.29% Pressure: 42 bar Temperature 7200C Inlet of secondary cracking unit Flow rate: 730, 726 Nm3/h Oxygen flow rate: 67, 539 Nm3/h Purity of oxygen: 99.5% Pressure: 41.5 bar Temperature: 1 500C Outlet of secondary cracking unit Flow rate of synthesis gas produced: 927, 590 Nm3/h Composition: CO 12.06% CO2 6.10% H2 43.48% CH4 0.85% N2 0.42% H2O 37.09% Pressure: 40.5 bar Temperature: 10000C Composition of dry synthesis gas: CO 19.17% CO2 9.70% H2 69.11% CH4 1.35% N2 0.67% This resuits in a ratio of: H2-CO2 -2.06 (2d.p.) CO+CO2 Slight variations in the operating parameters forfeedstock, pressure and temperature permit a reduction of this calculated ratio to 2.0.

Claims

1. A process for the preparation of synthesis gas which comprises (i) catalytically cracking methane or a gas mixture containing a substantial amount of methane at a pressure in the range of from 35 to 55 bar and at a temperature in the range of from 650 to 8000C to a CH4 content in the range of from 13 to 20% by volume (ii) catalytically cracking the gas mixture from step (i) at a temperature in the range of from 900 to 1 2000C with the addition of oxygen.

2. A process as claimed in claim 1, wherein natural gas is used as the starting material.

3. A process as claimed in claim 1, which is substantially as described in the Example herein.

4. Synthesis gas whenever prepared by a process as claimed in any one of claims 1 to 3.

5. Synthesis gas whenever prepared by a process as claimed in any one of claims 1 to 3 for which the value of the ratio H2-CO2 H2+CO2 is 2.0.