Extended catalyst life two stage hydrocarbon synthesis process

Details Extended catalyst life two stage hydrocarbon synthesis process Extended catalyst life two stage hydrocarbon synthesis process

1999-09-17

2001-01-02

Richter, Johann (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Group viii metal containing catalyst utilized for the...

C518S700000, C518S706000

Utility Patent

active

06169120

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a two-stage Fischer-Tropsch synthesis process which provides for extended catalyst life. The carbon monoxide conversion in each of a first and second stage is controlled to a conversion of about 40 to about 60 percent of the carbon monoxide in each stage. Additional stages can be used if desired.
BACKGROUND OF THE INVENTION
Fischer-Tropsch hydrocarbon synthesis catalysts have been studied widely by a number of researchers in recent years. Preferred processes are currently slurry bubble column processes wherein the catalysts used typically comprise cobalt or ruthenium, cobalt and ruthenium or promoted cobalt catalyst. The catalysts are supported on a variety of supports but generally are supported on supports selected from metal oxides such as alumina, silica, titanium, silica-alumina and the like.
Promoters can be used to enhance the activity of or the stability of cobalt or ruthenium catalysts. For example ruthenium has been used to promote cobalt catalysts supported on either titania or alumina, see U.S. Pat. Nos. 4,568,663 and 4,801.573 respectively. Supported ruthenium catalysts are also quite useful for hydrocarbon synthesis (see U.S. Pat. Nos. 4,477,595; 4,171,320, and 4,042614). Also, ruthenium and zirconium have been used to promote cobalt supported on silica (see U.S. Pat. Nos. 4,088,671, 4,599,481, and 4,681,867). Two-stage hydrocarbon synthesis was disclosed in U.S. Pat. No. 4,443,561 relating to hydrogen:carbon monoxide ratios, but making no differentiation based on the pressure in each reaction stage. This process also requires that a hydrogen-rich gas be added between the stages.
Other two stage hydrocarbon synthesis processes have been reported in the literature. U.S. Pat. Nos. 4,547,609, 4,279,830, and 4.159,995 use an iron-based first stage catalyst for hydrocarbon synthesis and a second stage catalyst having activity for aromatization. Also U.S. Pat. No. 4,624,968 employs an iron-based first stage catalyst for producing olefins and a second stage catalyst for converting olefins to paraffins with additional CO and hydrogen. All of these systems are based on dual function catalyst systems, that is, where the first stage catalyst is active for a specific chemical reaction and the second stage catalyst is active for a different chemical reaction. However, none of these systems involve a two-stage process in which catalysts of essentially equivalent functionality are tailored to the specific operating conditions of each stage.
Hydrocarbon synthesis processes are known to be plagued with several problems. Of these problems, obtaining high conversion and dissipating heat are among the foremost. Since hydrocarbon synthesis is an exothermic reaction, heat must be removed from the reactor to avoid hot spots, catalyst deactivation, and loss of selectivity at higher temperatures. There is usually a preferred temperature range for operating the process which leads to the optimum selectivity to desired higher hydrocarbon products. Lack of efficient heat removal can lead to much higher temperatures in the reactor which, while increasing carbon monoxide conversion, severely debits the selectivity to preferred higher hydrocarbons. At the same time, increasing conversion generates more heat and thus, a greater burden on heat exchange facilities. Thus, the goals of high conversion and efficient heat transfer tend to oppose each other. To alleviate the problem, lower conversion in a first stage can be accommodated, thereby reducing the heat load in the first stage. However, this reduced conversion must be made up in the second stage.
It has been observed that when high conversions are achieved, the presence of the resulting large amounts of water are detrimental to catalyst life. It would be desirable to achieve the higher conversion levels without the corresponding decrease in catalyst life as a result of the increased water levels.
SUMMARY OF THE INVENTION
According to the present invention, an extended catalyst life two-stage hydrocarbon synthesis process is provided and comprises reacting a first synthesis gas stream comprising hydrogen, carbon monoxide and from about 30 to about 60 volume percent nitrogen in a first stage reactor in the presence of a catalyst comprising cobalt, ruthenium or cobalt and ruthenium supported on a support comprising at least one inorganic metal oxide selected from Group IIIA, IIIB. IVB, VB, VIB and VIIB metal oxides, alumina, silica, silica-alumina and combinations thereof at a temperature from about 380 to about 500° F. at pressure from about 15 to about 25 atmospheres at a carbon monoxide conversion from about 40 to about 60 percent to produce liquid hydrocarbon product; recovering at least a portion of the liquid hydrocarbon product stream from the first stage reactor; recovering a gaseous stream comprising gaseous hydrocarbons, hydrogen, carbon monoxide and nitrogen from the first stage reactor; cooling the gaseous stream to a temperature below about 150° F. to produce a cooled gaseous stream; separating water and hydrocarbons condensed from the cooled gaseous stream to produce a second synthesis gas stream comprising hydrogen, carbon monoxide and nitrogen; and reacting the second synthesis gas stream in a second stage reactor in the presence of a catalyst comprising cobalt, ruthenium or cobalt and ruthenium supported on an inorganic metal oxide selected from Group IIIA, IIIB, IVB, VB, VIB and VIIIB metal oxides, alumina, silica, silica-alumina and combinations thereof from about 380 to about 500° F. and at a pressure from about 15 to about 25 atmospheres at a carbon monoxide conversion from about 40 to about 60 percent to produce liquid hydrocarbon products.
Particularly preferred catalysts are catalysts comprising cobalt supported on alumina.
The gaseous stream leaving this second stage reactor may be treated for the removal of water and liquid hydrocarbons and passed as a synthesis gas stream to a third stage reaction zone.


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“Fischer-Tropsch Synthesis: Differential Reaction Rate Studies with Cobalt Catalyst”; R.B. Anderson, A. Krieg, R.A. Friedel and L.S. Mason,Industrial and Engineering Chemistry, Oct., 1949, pp. 2189-2197.

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