GB1566698A

GB1566698A - Treatment of gases containing hydrogen and carbon monoxide

Info

Publication number: GB1566698A
Application number: GB36710/75A
Authority: GB
Original assignee: Amec Foster Wheeler Holdings Ltd
Current assignee: Amec Foster Wheeler Holdings Ltd
Priority date: 1975-09-05
Filing date: 1975-09-05
Publication date: 1980-05-08
Also published as: CA1093822A; JPS5241607A

Description

(54) THE TREATMENT OF GASES CONTAINING HYDROGEN AND CARBON MONOXIDE (71) We, FOSTER WHEELER LIMITED, a British Company, of Foster Wheeler House, Station Road, Reading, Berkshire RG1 lLX, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to the treatment of gases containing hydrogen and carbon monoxide, e.g. coke oven gases and in particular to reducing the propensity to carbon formation of such gases.

Coke oven gas has been used in the United Kingdom and in Europe as a town gas. The gas is generally produced by heating rough coal to a temperature in the range 1000 to 12000C to yield the coke oven gas together with coke. The gas is subjected to several stages of cooling and purification to remove impurities such as tar, naphthalene, ammonia and sulphur and the resulting gas is stable and easily transportable. Coke oven gases normally have a composition within the following ranges: H2 51 to 56%, CO 5 to 10%, CO2 1.8 to 5%, CH4 25 to 30%, C2H4 2 to 3%, N2 5 to 10%, 02 0.5 to 2.0%.

The extension of the use of natural gas has led to the gradual elimination of coke oven gas from domestic use. Large volumes of coke oven gas are still produced, however, as a by-product in the preparation of metallurgical coke. This gas has found few convenient outlets.

A recent development of a direct reduction process for the production of substitute scrap for electric arc furnaces has provided a possible opening for coke oven gas as a source of reducing gas. In this process a reducing gas is reacted with ferric oxide in a shaft furnace at a temperature in the range 500 to 9000C to produce iron. The iron in the form of pellets may then be used in electric arc furnaces.

In the preparation of a suitable reducing gas, coke oven gas is preheated to a temperature in the range 400 to 5300C and introduced into a steam reformer where it reacts with steam in the presence of a catalyst such as nickel on a refractory support. The hydrocarbons in the coke oven gas are converted to hydrogen and carbon dioxide and the carbon monoxide to carbon dioxide. The disadvantage with this process however is that the preheating performed by conventional heaters is liable to cause carbon formation is accordance with the Boudouard reaction: 2COxC+CO2 since the equilibrium favours carbon formation at this temperature range. This carbon formation is highly undesirable.

It is an object of the present invention to treat gases and in particular coke oven gas to reduce this formation of carbon.

Therefore according to the invention there is provided a method of preparing a reducing gas suitable for use in the direct reduction of ferric oxide from a source gas stream which comprises: 5 to 120/, by volume of carbon monoxide, 1.8 to 7% by volume of carbon dioxide, and 50 to 65 /ó by volume of hydrogen, in which the gas is treated to reduce the content of carbon monoxide therein by, (i) passing the gas at an initial temperature of 250 to 3750C and at elevated pressure over a catalyst to convert carbon oxides to methane, or (ii) mixing the gas with steam and passing the mixture an an initial temperature of 200 to 2700C over a low temperature shift catalyst to convert the carbon monoxide to carbon dioxide, the resulting gas being subjected to steam reforming in the presence of a catalyst to give the desired reducing gas.

The gas which has been treated to reduce the carbon monoxide content has a low carbon formation propensity when subjected to the steam reforming step and therefore may be reformed in a conventional steam reformer to produce a reducing gas without undue carbon formation.

In one embodiment of the invention the carbon oxides in the coke oven gas are reacted with the hydrogen in the gas to form methane and water in the presence of a catalyst such as nickel on a refractory support. Suitable catalysts are well known in the art and are commercially avaiiable, for example, from Imperial Chemical Industries Limited or from C.C.E. The methanation reaction is favoured by a low temperature in the range 250 to 375"C and elevated pressure, although the pressure effect is not very pronounced. The reaction however, is exothermic and the temperature rise must be moderated, for example, by inclusion of a relatively inert component. Generally, it has been found that the total temperature rise for a single reactor should be limited to about 200"C.

A reducing gas prepared from coke oven gas should have a reducing ratio, defined as (H2+CO)/(H2O+CO2) equal to about 9.0 if the gas is to be used for direct reduction of iron ore. This means that in a steam reformer for converting coke oven gas that the steam to hydrocarbon molar ratio should be in the range of 1.2 to 1.5. It is therefore convenient to use steam as an inert component to modify the temperature of the methanation reaction in the process of the invention since the resulting gaseous product may then be passed directly for steam reforming.

In a preferred embodiment of the invention coke oven gas and steam are passed through a reactor having a plurality of reaction zones containing the catalyst in such a way that a portion of the coke oven gas to be reformed is mixed with at least a major portion of the steam required for steam reforming and this mixture is passed through the first reaction zone of the reactor. The mixture leaving the first reaction zone is mixed with a cold second portion of coke oven gas thereby lowering the temperature of the mixture and the gases passes through a second reaction zone. This quenching operation can be repeated several times. The resulting steam/treated coke oven gas mixture may be passed on to a steam reformer for subsequent conversion to a reducing as with negligible carbon formation.

Other methods of controlling the temperature rise during the methanation reaction include recycling some of the product gas to increase the inert component content in the mixture and utilising the temperature rise to convert water to steam.

In a second method of reducing carbon formation from coke oven gas in accordance with the invention, coke oven gas is mixed with steam and reacted at a temperature in the range 200 to 2700C in the presence of a low temperature shift catalyst to convert carbon monoxide to carbon dioxide. This reaction is also exothermic and similar arrangements to those illustrated above in the conversion of carbon oxides to methane may be used to control the temperature rise. The resulting gaseous product may be used directly in a steam reforming reactor.

Suitable catalysts are similar to the methanation catalysts and are commercially available from Imperial Chemical Industries Limited and C.C.E.

The process of the invention is not limited to the treatment of coke oven gas and similar gas streams containing hydrogen and carbon oxides in the above defined ranges may also be treated.

The process of the invention is however ideal for coke oven gas treatment and may be incorporated as a stage in the purification and preparation of coke oven gas for use as a reducing gas. Thus suitable purification and processing stages would include in the following order de-tarring, ammonia removal, benzol removal, H2S removal, preheating organic sulphur removal, reduction of carbon monoxide content, preheating and direct reduction by steam reforming.

The invention will now be illustrated by the following Examples with reference to the accompanying drawings in which Figures 1, 2 and 3 represent flow diagrams of the reactions in Examples 1, 2 and 3 respectively.

Example 1 100 Moles of coke oven gas to be steam reformed with 50.99 moles of steam, which is equivalent to a steam to hydrocarbon mole ratio of 1.4 were treated by the methanation process in accordance with the invention.

Figure 1 of the accompany drawings represents a flow diagram of the system used. It includes a methanation reaction 2 having a number of stages 2a to 2d.

The coke oven gas stream was divided such that one fifth was fed to the methanator reactor 2 at the inlet 4 with the major portion of process steam. The amount of steam fed was 37.6 moles the remaining 13.39 moles required was produced in the methanator reactor and fed in as an additional moderator.

The feed to the first stage 2a was: moles mole ^ dry mole on wet CO 1.02 5.1 1.77 H2 12.3 61.5 21.39 CH4 5.14 25.7 8.94 C2H6 0.52 2.6 0.90 CO2 0.36 1.8 0.63 02 0.10 0.5 0.17 N2 0.56 2.8 0.97 H2O 37.6 65.23 The inlet temperature was 250"C.

The oulet gas composition from the first stage 2a was: moles H2 7.6 CH4 6.52 C2H6 0.52 N2 0.56 H2O 39.44 The outlet temperature was 4160C.

The gas leaving the first stage 2a of methanation was quenched with a further one fifth of the incoming coke oven gas added through an inlet 6 such that the mixture temperature was 3190C and the gas composition was: moles CO 1.02 H2 19.90 CH4 11.66 C2H6 1.04 CO2 0.36 O2 0.50 N2 1.12 H2O 39.44 The gas was fed into a second stage 2b of methanation and the outlet gas from that stage had the following compositions: moles H2 15.20 CH4 13.04 C2H8 1.04 N2 1.12 H2O 41.38 The temperature of the gas at the outlet from the second stage 2b was 4410C.

The gas leaving the second stage 2a was again quenched with a further one fifth of the feed coke oven gas introduced through inlet 8 and the temperature of the mixture was 362"C and its composition was: moles CO 1.02 H2 27.50 CH4 18.18 C2H6 1.56 CO2 0.36 O2 0.10 N2 1.68 H2O 41.38 The gas mixture was fed into the third stage 2c of the methanation reactor and the resulting gas left it at 4580C and the gas composition was as follows: moles H2 22.80 CH4 19.56 C2116 1.56 N2 1.68 H2O 43.32 The gas was then again mixed with a further one fifth of coke oven gas introduced through an inlet 10 and the remaining process steam of 5.73 moles to give a mixture temperature of 3710C.The gas was then fed into the fourth stage 2d of the methanation reactor to give a product gas of the following composition of 452"C: moles H2 30.40 CH4 26.08 C2H8 2.08 N2 2.24 H2O 50.99 This gas was then mixed with the remaining portion of coke oven gas feed passing along a line 11 and the resulting as had the following composition: moles mole % wet mole % dry Co 1.02 0.77 1.26 H2 42.70 32.44 52.84 CH4 31.22 23.69 38.64 C2H6 2.60 1.97 3.22 CO2 0.36 0.27 0.45 O2 0.10 0.08 0.12 N2 2.80 2.12 3.47 H2O 50.99 38.70 The resulting gas had a low carbon monoxide content and did not produce large carbon deposits when subjected to steam reforming to give a reducing gas.

Example 2 Figure 2 represents a flow diagram of the reaction system used in this Example. This Example describes the use of an isothermal reactor.

Coke oven feed gas of composition shown below and at a temperature in the range 250 to 375"cm was fed into a multitube reactor 12 in which the tubes are packed with catalyst and the tubes were surrounded by water at its boiling point (2200 C) at a pressure of about 24 kg/cm2 absolute.

The coke oven gas composition was: moles CO 10.20 H2 56.20 CH4 26.20 C2H6 2.50 CO2 3.20 02 0.50 N2 1.10 H2O 44.84 A portion of steam required for the subsequent steam reforming reaction was also fed into the reactor.

The heat of reaction was sufficient to produce 736 kg of steam at 24 kg/cm2 absolute.

The gas leaving the reactor had the following composition: moles mole % wet mole Ó dry H2 11.80 10.05 12.00 CH4 39.60 33.72 72.00 C2H6 2.50 2.13 4.55 N2 1.10 0.94 2.00 H2O 62.44 53.16 and were suitable for being subjected to steam reforming to give a reducing gas without large carbon deposition.

Example 3 This Example illustrates how gas treated by the present invention may be used as an inert component to control the reaction temperature.

Figure 3 of the accompanying drawings represents a flow diagram of the system used.

Coke oven gas of composition as in Example 2 was mixed with product gas resulting from the methanation of coke oven gas and passed into a reactor 20. The product gas left the reactor at a temperature of 392"C and was passed through at heat exchanger 22 in which it gives up some heat to recycled product gas. After the heat exchanger the product gas was cooled in a cooler 24 and the water content removed in a water separator 26. A portion of the resulting product gas was then recycled via a compressor 28, heat exchanger 22 and heater 30 to provide an inert component to help to moderate the methanation of fresh incoming coke oven gas.

The inlet gas mixture to the reactor 20 was as follows: moles CO 10.20 H2 127.00 CH4 263.80 C2H6 85.00 CO2 3.20 O2 0.50 N2 7.70 H2O 4.92 The gas was heated to 250"C by heat exchanger with the product gases leaving the reactor 20. The product gas left the reactor at 392"C and had the following composition: moles H2 82.60 CH4 277.20 C2H6 85.00 N2 7.70 H2O 22.52 The resulting gas was suitable for steam reforming without significant carbon deposition.

Example 4 This Example illustrates carbon monoxide conversion to carbon dioxide and hydrogen to give a gas suitable for steam reforming to a reducing gas without significant carbon deposit.

A portion of gas of composition as in Example 1 was preheated to 2270C together with 48.06 moles of steam and was passed through the first stage of the low temperature catalytic shift reactor.

The inlet composition was: moles CO 2.55 H2 30.75 CH4 12.85 C2116 1.30 CO2 0.90 O2 0.25 N2 1.40 H2O 48.06 The gas at the outlet from the reactor had the following composition at a temperature of 287"C.

moles CO 0.15 H2 32.65 CH4 12.85 C2H6 1.30 CO2 3.30 N2 1.40 H2O 46.16 The gas was quenched with the remaining portion of the coke oven feed gas and the temperature of the mixture was 207"C and its composition was: moles CO 2.70 H2 63.40 CH4 25.70 C2116 2.60 CO2 4.20 02 0.25 N2 2.80 H2O 46.16 The mixture was passed through the second stage of the 'flow temperature shift' reactor and composition of the resulting gas at a temperature of 237 C was:: moles CO 0.30 H2 65.30 CH4 25.70 C2H6 2.60 CO2 6.60 N2 2.80 H2O 44.26 The steam to hydrocarbon carbon ratio was 1.43 and the ratio of oxidising components to hydrocarbon carbon i.e. H2O+0.5 CO2 to carbon, was 1.54. The gas was suitable for steam reforming without significant carbon deposition.

Example 5 Gas of composition as in Example 2 was preheated together with steam to 226"C and passed through a tubular reactor, in which the tubes were packed with the 'low temperature shift' catalyst and were surrounded by boiling water at approximately 15 kg/cm2 absolute.

The gas was of the following composition: moles CO 10.20 H2 56.20 CH4 26.20 C2116 2.50 CO2 3.20 02 0.50 N2 1.10 H2O 46.00 The resulting gas left the reactor at 2260C and the gas composition was.

moles CO 0.30 H2 65.10 CH4 26.20 C2H6 2.50 CO2 13.10 N2 1.10 H2O 37.10 The heat of reaction evaporated 148 kg of water at 15 kg/cm2 absolute.

The ratio of oxidising components to hydrocarbon carbon the resulting gas was 1.4 the gas was suitable for steam reforming without formation of carbon.

WHAT WE CLAIM IS: 1. A method of preparing a reducing gas suitable for use in the direct reduction of ferric oxide from a source gas stream which comprises: 5 to 12% by volume of carbon monoxide, 1.8 to 7% by volume of carbon dioxide, and 50 to 65% by volume of hydrogen, in which the gas is treated to reduce the content of carbon monoxide therein by (i) passing the gas at an initial temperature of 250 to 3750C and at elevated pressure over a catalyst to convert carbon oxides to methane, or (ii) mixing the gas with steam and passing the mixture at an initial temperature of 200 to 2700C over a low temperature shift catalyst to convert the carbon monoxide to carbon dioxide, the resulting gas being subjected to steam reforming in the presence of a catalyst to give the desired reducing gas.

2. A method as claimed in Claim 1 in which the source gas stream is coke oven gas.

3. A method as claimed in Claim 1 or Claim 2 in which the catalyst employed to convert carbon oxides to methane or carbon monoxide to carbon dioxide is nickel on a refractory support.

4. A method as claimed in any preceding claim in which an inert gas inert to the reaction to convert carbon oxides to methane or carbon monoxide to carbon dioxide is added to moderate the temperature rise during the conversion of the carbon oxides to methane or carbon monoxide to carbon dioxide, respectively.

5. A method as claimed in Claim 4 in which steam is the inert gas added.

6. A method as claimed in Claim 5 in which sufficient steam is added to give a steam to hydrocarbon carbon molar ratio of from 1.2 to 1.5.

7. A method as claimed in any of claims 4 to 6 in which the gas stream is passed through a succession of a plurality of reaction zones, fresh portions of the gas steam and/or portions of steam being added to the treated gas stream after each reaction zone so as to moderate the temperature rise.

8. A method as claimed in Claim 4 in which some of the said resulting gas is recycled as the inert gas.

9. A method as claimed in any of claims 1 to 3 in which to moderate the temperature rise during the conversion of the carbon oxides to methane or carbon monoxides to carbon dioxides the gas stream is in indirect heat exchange with boiling water.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The gas was of the following composition: moles CO 10.20 H2 56.20 CH4 26.20 C2116 2.50 CO2 3.20 02 0.50 N2 1.10 H2O 46.00 The resulting gas left the reactor at 2260C and the gas composition was. moles CO 0.30 H2 65.10 CH4 26.20 C2H6 2.50 CO2 13.10 N2 1.10 H2O 37.10 The heat of reaction evaporated 148 kg of water at 15 kg/cm2 absolute. The ratio of oxidising components to hydrocarbon carbon the resulting gas was 1.4 the gas was suitable for steam reforming without formation of carbon. WHAT WE CLAIM IS: 1. A method of preparing a reducing gas suitable for use in the direct reduction of ferric oxide from a source gas stream which comprises: 5 to 12% by volume of carbon monoxide,

1.8 to 7% by volume of carbon dioxide, and 50 to 65% by volume of hydrogen, in which the gas is treated to reduce the content of carbon monoxide therein by (i) passing the gas at an initial temperature of 250 to 3750C and at elevated pressure over a catalyst to convert carbon oxides to methane, or (ii) mixing the gas with steam and passing the mixture at an initial temperature of 200 to 2700C over a low temperature shift catalyst to convert the carbon monoxide to carbon dioxide, the resulting gas being subjected to steam reforming in the presence of a catalyst to give the desired reducing gas.

5. A method as claimed in Claim 4 in which steam is the inert gas added.

10. A method of preparing a reducing gas as claimed in claim 1 substantially as

herein described in any Examples.

11. A reducing gas when made by a method as claimed in any preceding claim.

12. A method of reducing ferric oxide in which ferric oxide is contacted with a reducing gas as claimed in Claim 11 at a temperature of from 500 to 9000 C.