Chemistry: fischer-tropsch processes; or purification or recover – Plural zones each having a fischer-tropsch reaction
Reexamination Certificate
2000-02-14
2002-11-12
Parsa, Jafar (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
Plural zones each having a fischer-tropsch reaction
C518S700000, C518S707000, C518S728000
Reexamination Certificate
active
06479557
ABSTRACT:
The present invention relates to a process for the preparation of hydrocarbons from a mixture of carbon monoxide and hydrogen. The present invention relates in particular to a process for the preparation of hydrocarbons from a mixture of carbon monoxide and hydrogen having a given H
2
/CO molar ratio.
Mixtures of hydrogen and carbon monoxide are often referred to as synthesis gas. Such synthesis gas can be used in numerous well-known processes to produce a large variety of organic compounds, containing carbon, hydrogen and, optionally, oxygen moieties.
A process for the preparation of paraffinic hydrocarbons from a mixture of carbon monoxide and hydrogen (syngas) is commonly known as Fischer-Tropsch synthesis. This synthesis process involves contacting syngas at elevated temperature and pressure with a catalyst, comprising as catalytically active component a Group VIII metal, in particular Fe, Ni, Ru or Co, to yield paraffinic hydrocarbons. A detailed description of an example of such a Fischer-Tropsch synthesis process can be found in EP-A-0428223 and EP-A-0174696.
The Fischer-Tropsch synthesis process referred to hereinabove, in particular a process using a catalyst comprising Ni, Ru or Co, and having no Co-shift activity, typically consumes syngas at a H
2
/CO molar usage ratio of 2.0 to 3.0, in particular 2.0 to 2.3, mainly depending on the length of the paraffinic hydrocarbons formed. It will be understood that when a given Fischer-Tropsch synthesis process consumes syngas at a H
2
/CO molar usage ratio of for example 2.1, the H
2
/CO molar feed ratio should preferably be 2.1 as well in order to avoid a surplus of either H
2
or CO.
A problem underlying the present invention is that a syngas feed having a H
2
/CO molar ratio of 2.1 is not readily available and can only be prepared at unacceptably high costs.
In the art a number of processes are known to prepare syngas. Depending on the process for preparing syngas and the type of organic feed to such processes, the H
2
/CO molar ratio of the syngas may vary widely. If coal is used as organic feed in such a process, the H
2
/CO molar ratio of the resulting syngas is generally lower than if natural gas is used as organic feed. A steam methane reforming process, using natural gas as organic feed, typically produces syngas having a H
2
/CO molar ratio of at least 3. A process involving partial oxidation, either autothermal or catalytic, of natural gas typically produces syngas having a H
2
/CO molar ratio of 1.7. A partial oxidation process using coal or residual oil as organic feed typically produces syngas having a H
2
/CO molar ratio of 0.5.
Accordingly, in order to obtain a syngas having a H
2
/CO molar ratio of for example 2.1, one has to prepare syngas using at least two different processes, such as a combination of steam methane reforming and partial oxidation of natural gas, and mix the two H
2
/CO mixtures thus obtained. For example EP-A-0168892 and EP-A-0178007 disclose energy efficient methods to prepare syngas from two different processes. Alternatively, one could prepare syngas by one syngas preparation process and mix the syngas thus prepared with a separate hydrogen or carbon monoxide stream to arrive at the desired hydrogen to carbon monoxide ratio.
It will be understood that it will be most desirable to be able to use a syngas feed in a synthesis process, which syngas feed is obtained from one syngas preparation process only, without the need to mix the syngas feed with another syngas having a different hydrogen to carbon monoxide ratio or separate hydrogen or carbon monoxide streams.
GB-A-2243616 discloses a process for the production of paraffinic and aromatic hydrocarbons from a carbonaceous feedstock. This process involves converting the carbonaceous feedstock into synthesis gas having a H
2
/CO molar ratio of less than 2.1, converting the synthesis gas partially into paraffinic hydrocarbons and converting the remaining synthesis gas into aromatic hydrocarbons. The process for the preparation of paraffinic hydrocarbons consumes synthesis gas at a H
2
/CO usage ratio higher than the H
2
/CO feed ratio, whereas the process for the preparation of aromatic hydrocarbons consumes synthesis gas at a H
2
/CO usage ratio lower than the H
2
/CO feed ratio. As outlined in this document the most efficient conversion is obtained when the overall H
2
/CO usage ratio is the same as the H
2
/CO feed ratio.
Examples of other well-known processes for the preparation of organic products from synthesis gas are processes for the preparation of oxygen-containing hydrocarbons like methanol, higher alcohols or dimethylether and are well known in the art.
GB-A-2092172 discloses a process for the preparation in a first step, of oxygen-containing organic compounds from synthesis gas having a H
2
/CO molar ratio of at least 0.5, and, in a second step, paraffinic hydrocarbons from unconverted synthesis gas from the first step. The oxygen-containing compounds obtained in the first step can be used as intermediates in the production of other organic compounds like olefins.
The preparation of olefins directly from synthesis gas is well known to those skilled in the art. U.S. Pat. No. 4,518,707 and EP-A-0446035 disclose examples of such preparation process.
U.S. Pat. No. 4,624,968 discloses a multi-stage Fischer-Tropsch process for the production of paraffinic waxes by converting in a first step synthesis gas into olefins, and converting in a second step the olefins and any additional synthesis gas into paraffins. The two steps may be combined into one step by using a mixture of the required different catalysts in one catalyst bed. This two-stage arrangement for the preparation of paraffinic hydrocarbons is said to offer the advantage that it results in increased amounts of heavy hydrocarbons and lower selectivities for methane and ethane as compared with the production of paraffins in one step.
One of the problems with processes using synthesis gas as feed is the production of by-products. EP-A-0153781 discloses a two-step process for the preparation of paraffinic hydrocarbons. In the first step a catalyst is used which is capable of yielding a product containing only limited amounts of by-products. Nevertheless, by-products are present in the product of this process. The formation of by-products becomes an even more important problem if it is desired to prepare organic products for use in the chemical industry. For example, if it is desired to produce olefinic hydrocarbons, the formation of paraffinic hydrocarbons represents a problem.
By-products can be separated from the desired product by means known in the art, such as by molecular sieves, adsorption, distillation or washing, but this is relatively expensive and is often not economically viable in view of the limited volumes of by-products produced (that is, no economy-of-scale).
It would be desirable if by-products of a synthesis process, using a synthesis gas feed, could be separated from the main product and used in an economically viable way.
As with the separation of by-products, the production of organic products from synthesis gas is also governed by the economy-of-scale. Thus, preferably, the production of organic products from synthesis gas is carried out on a large scale. However, if it is desired to prepare organic products for use in the chemical industry, the market could be relatively small as compared with the scale required to make preparation of those organic products economically viable, thus prohibiting the economic production of those products.
Despite the maturity of research in this field and despite the wealth of publications that are published each year in this field, it is believed that no one has considered to prepare in a first step a first organic product and a first by-product, and prepare in a second step a second organic product and a second by-product, and in a third step, e.g. separate the first by-product from the first organic product and mix the first by-product with the product from the second step.
The present invention therefore provides a pr
Lange Jean-Paul
Maxwell Ian Ernest
Scheffer Bob
Parsa Jafar
Shell Oil Company
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