This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-246913, filed Aug. 16, 2000, the entire contents of which are incorporated herein by reference.
This invention relates to a method of manufacturing a synthesis gas to be employed for the synthesis of gasoline, methanol or dimethyl ether by way of the GTL (Gas to Liquid) process.
A synthesis gas comprising hydrogen (H2) and carbon monoxide (CO) has been employed as a raw material for the synthesis of gasoline, etc. by way of the GTL (Gas to Liquid) process according to the Fisher-Tropsch reaction system.
This synthesis gas has been conventionally manufactured by a method wherein steam and carbon dioxide are added at first to a natural gas employed as a raw gas to prepare a mixed gas, which is then fed to the reaction tube of reformer which has been heated to a predetermined temperature so as to steam-reform the natural gas together with carbon dioxide, thereby manufacturing the synthesis gas containing hydrogen (H2) and carbon monoxide (CO).
By the way, depending on the place of origin, the natural gas occasionally contains hydrogen sulfide (H2S) and carbon dioxide (for example, CO2: 7.1% by volume, and H2S: 0.6% by volume). When such a natural gas is to be employed as a raw material, it is usually practiced to remove hydrogen sulfide prior to the step of adding steam and carbon dioxide to the natural gas. In this case, the removal of hydrogen sulfide from the natural gas is conventionally performed by means of amine absorption method.
However, if it is desired to remove hydrogen sulfide to a level of the order of ppm by means of the amine absorption method, carbon dioxide included in the original natural gas is also removed together with hydrogen sulfide. As a result, it becomes impossible to utilize the carbon dioxide that has been originally included in the natural gas, so that a large quantity of carbon dioxide is required to be supplied to the natural gas prior to the stage where the natural gas is to be transferred to the reaction tube of the reformer, thus increasing the manufacturing cost of the synthesis gas.
Therefore, an object of this invention is to provide a method for manufacturing a synthesis gas, which makes it possible to effectively utilize the carbon dioxide in a natural gas containing hydrogen sulfide and carbon dioxide by selectively removing only the hydrogen sulfide therefrom in the synthesis gas manufacturing method employing a reformer, thereby making it possible to reduce the quantity of carbon dioxide to be added to the natural gas prior to the stage where the natural gas is to be transferred to the reformer.
Namely, this invention provides a method of manufacturing a synthesis gas containing hydrogen and carbon monoxide, which comprises steps of;
removing only hydrogen sulfide from a natural gas containing hydrogen sulfide and carbon dioxide by permitting the natural gas to pass through a hydrogen sulfide-removing device filled with a hydrogen sulfide absorbent;
adding carbon dioxide and steam to the natural gas which the hydrogen sulfide has been removed to prepare a mixed gas; and
feeding the mixed gas into a reaction tube of a reformer, thereby permitting mainly a steam reforming reaction to take place in the mixed gas.
The method of manufacturing a synthesis gas according to this invention may be performed in such a way that before the natural gas is fed to the hydrogen sulfide-removing device, the natural gas is forced to pass through a convection portion communicated with a combustion radiation portion of the reformer, thereby heating the natural gas up to a temperature which is suited for the reaction between the hydrogen sulfide in the natural gas and the hydrogen sulfide adsorbent.
The method of manufacturing a synthesis gas according to this invention may be performed in such a way that the carbon dioxide recovered from a combustion exhaust gas generated at the combustion radiation portion of the reformer is utilized as a carbon dioxide source.
The method of manufacturing a synthesis gas according to this invention may be performed in such a way that the carbon dioxide recovered from the synthesis gas at a downstream side of the reformer is utilized as a carbon dioxide source.
It is preferable in the method of manufacturing a synthesis gas according to this invention that the hydrogen sulfide adsorbent is at least one oxide selected from triiron tetraoxide (Fe3O4) and zinc oxide (ZnO).
It is preferable in the method of manufacturing a synthesis gas according to this invention that the hydrogen sulfide-removing device is provided with at least one unit of first desulfurizing column filled with the hydrogen sulfide adsorbent comprising triiron tetraoxide and with a second desulfurizing column filled with the hydrogen sulfide adsorbent comprising zinc oxide, and that the natural gas containing hydrogen sulfide and carbon dioxide is permitted to successively pass through one column selected from these first desulfurizing columns and second desulfurizing column.
It is preferable that the hydrogen sulfide-removing device is provided with at least three units of first desulfurizing columns to be filled with triiron tetraoxide, wherein a first placed first desulfurizing column among the first desulfurizing columns is designed to execute an adsorption operation of hydrogen sulfide, a second placed first desulfurizing column among the first desulfurizing columns is designed to execute an operation of regenerating the adsorbent (iron sulfide) on which hydrogen sulfide is adsorbed, and a third placed first desulfurizing column among the first desulfurizing columns is designed to execute an operation of reducing the adsorbent that has been regenerated, these operations being sequentially executed.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.