Patent Application
20070259973
The term “Cx”, where x is a number greater than zero, refers to a hydrocarbon compound having predominantly a carbon number of x. As used herein, the term Cx, may be modified by reference to a particular species of hydrocarbons, such as, for example, C5 olefins. In such instance, the term means an olefin stream comprised predominantly of pentenes but which may have impurity amounts, i.e. less than about 10%, of olefins having other carbon numbers such as hexene, heptene, propene, or butene. Similarly, the term “Cx+” refers to a stream wherein the hydrocarbons are predominantly those having a hydrocarbon number of x or greater but which may also contain impurity levels of hydrocarbons having a carbon number of less than x. For example, the term C15+ means hydrocarbons having a carbon number of 15 or greater but which may contain impurity levels of hydrocarbons having carbon numbers of less than 15.
The term “Cx—Cy”, where x and y are numbers greater than zero, refers to a mixture of hydrocarbon compounds wherein the predominant component hydrocarbons, collectively about 90% or greater by weight, have carbon numbers between x and y. For example, the term C5-C9 hydrocarbons means a mixture of hydrocarbon compounds which is predominantly comprised of hydrocarbons having carbon numbers between 5 and 9 but may also include impurity level quantities of hydrocarbons having other carbon numbers.
Unless otherwise specified, all quantities, percentages and ratios herein are by weight.
It has surprisingly been found that higher conversions can be sustained with no noticeable impact on the catalyst deactivation rate. The impact of the by product water is directly related to the partial pressure of the water. Embodiments of the invention utilize a synthesis gas that is diluted with a gas which is not reactive in the Fischer-Tropsch reaction. For example, synthesis gas generated with air or oxygen-enriched air has a substantial degree of nitrogen dilution, containing between about 20% to about 60% nitrogen. The dilution of the synthesis gas reduces the partial pressure of the water produced for a given conversion. Therefore it has now been shown that operation of one or more stages of Fischer-Tropsch reactions at CO conversions well above 60% per stage can be achieved and is desirable. According to embodiments of the present invention, the conversion of carbon monoxide in each Fischer-Tropsch reactor, i.e., each stage, exceeds 60%. In some embodiments of the invention, single stage conversion is up to 95%. As an example of the impact of higher single stage conversion rates on overall conversion, two FT reactors operating in series, each with 70% conversion results in overall conversion of carbon monoxide of 91%. Similarly, if each of two FT reactors has a 75% conversion rate, the overall CO conversion rate is 93.75%. Therefore, embodiments of the present invention permit the use of fewer stages resulting in substantial capital savings. As used herein the terms “% CO conversion” and “% conversion” refer to the percentage of CO in an initial feed stream which reacts by way of a Fischer-Tropsch reaction to form higher hydrocarbons on a once-through basis, i.e., achieved during a single pass through the Fischer-Tropsch reactor. The terms “overall CO conversion” and “overall conversion” mean the CO converted to higher hydrocarbons on a once-through basis after passing through all of the Fischer-Tropsch reactors in a given process. When referred to in connection with a specific Fischer-Tropsch reactor, the terms “CO conversion” and “conversion” mean the CO converted to higher hydrocarbons on a once-through basis upon passing solely through such specific Fischer-Tropsch reactor.
A Fischer-Tropsch liquid product is recovered from first vessel 12 by positioning a weir 34 in first vessel 12 below level 22 so that Fischer-Tropsch liquid product can collect in weir 34 and de-gas, thereby increasing the density of Fischer-Tropsch liquid product. The denser Fischer-Tropsch liquid product 36 is passed through a filter 38 from which a first Fischer-Tropsch liquid product 42 comprised primarily of C17+ hydrocarbon liquids is recovered. Following removal of the first Fischer-Tropsch liquid product 42, a remaining slurry 40 is returned via to a lower portion of first vessel 12. First Fischer-Tropsch liquid product 42 is removed at a rate sufficient to maintain the liquid level 22 in first vessel 12 at a desired level. Level 22 is maintained such that a sufficient disengaging zone is maintained above the slurry. Under preferred operating conditions, all liquid hydrocarbon recovery 42 from first vessel 12 is through filter 38.
The synthesis gas 24 charged to first vessel 12 in the embodiment shown in
The gaseous stream 28 comprises gaseous hydrocarbons, hydrogen, carbon monoxide, and nitrogen. The gaseous stream 28 is cooled to a temperature below about 150° F. by any acceptable processing means. For example, gaseous stream 28 may be passed to a heat exchanger 44. Preferably the temperature of gaseous stream 28 is reduced to a temperature of about less than about 100° F. Cooled gaseous stream 46 is then passed to a first separator 48 from which a second synthesis gas stream 52 is recovered. A recovered liquid stream 50 is recovered from first separator 48 and passed to a second separator 54 where the recovered liquid stream 50 is separated into a first recovered hydrocarbon stream 56 and an aqueous stream 58. First recovered hydrocarbon stream 56 is comprised primarily of C5-C17 hydrocarbons. Aqueous stream 58 is comprised primarily of water. The second synthesis gas stream 52 may be heated to a temperature suitable for charging to second stage reactor 70 using any acceptable processing means. For example, second synthesis gas stream 52 may be passed to a heat exchanger 62. For injection into second stage reactor 70, second synthesis gas stream 52 is typically heated to a temperature between about 300° and about 400° F. In some embodiments of the invention, second synthesis gas stream 52 may be passed from first separator 48 directly to second stage reactor 70 without additional heating.
Second stage reactor 70 comprises a vessel 72 including a plurality of heat exchange tubes 74 which are supplied with coolant 76 with recovered coolant 78 being recovered. In a preferred embodiment, the coolant 76 is water and the recovery stream 78 is steam. In some embodiments, the recovery stream 78 is re-cooled or otherwise appropriately disposed or recycled. A back pressure control valve 80 controls the pressure in heat exchange tubes 74 thereby regulating the temperature in second vessel 72. Second stage reactor 70 is a slurry bubble column reactor which contains a slurry comprised primarily of Fischer-Tropsch reaction products in which Fisher-Tropsch catalyst is suspended. A gas distributor 84 is positioned in the lower part of second vessel 72 to disperse second synthesis gas 52 into the slurry for movement upward through second vessel 72 as finely dispersed bubbles. A second gaseous stream 86 is recovered overhead.
A weir 92 is used to collect a portion of the slurry for withdrawal. The withdrawn slurry 94 is degassed before being passed through a second filter 96 from which a second Fischer-Tropsch liquid product 98 comprised primarily of C17+ hydrocarbons is removed. Following removal of the second Fischer-Tropsch liquid product 98 from withdrawn slurry 94, the remaining slurry 100 is reinjected into second vessel 72.
The second gaseous stream 86 which contains primarily gaseous hydrocarbons, hydrogen, carbon monoxide, and nitrogen, is recovered. Second gaseous stream 86 may be cooled to a temperature between about 100° F. and about 150° F. and preferably to a temperature of about 100° F. using any acceptable processing means. For example, as shown in
The third synthesis gas stream 110 may be passed for further reaction in an additional Fischer-Tropsch reactor. Alternatively, the third synthesis gas stream 110 may be further processed by adsorption, absorption, or low temperature processes to recover light hydrocarbons with any remaining gaseous components useful as low BTU fuel gas.
The present process is particularly adapted to the use of a synthesis gas containing nitrogen. Such synthesis gas streams are not well adapted to recycle, for increasing the conversion rate of other gases. Minimal recycling may be employed, if at all. For example, CO2 could be removed and recycled to synthesis gas generation or light olefins could be extracted and recycled to FT synthesis but recycle of nitrogen containing synthesis gas should be avoided.
The catalyst used in the present invention comprises cobalt, ruthenium, or cobalt and ruthenium supported on a support comprising an inorganic metal oxide selected from Group IIIA, IIIB, IVB, VB, VIB and VIIIB metal oxides, alumina, silica, silica-alumina and combinations thereof. The catalyst used in first stage reactor 10 and in second stage reactor 70 may be the same or different within the parameters set forth herein for the catalyst. Preferably the catalyst support comprises primarily alumina, titania, silica, silica-alumina, and combinations thereof with the preferred support comprising alumina.
Further, the catalyst may include a promoter. The promoter may be selected from those known to those skilled in the art for use with supported cobalt, ruthenium, or cobalt and ruthenium catalysts. Suitable promoters are selected from a group consisting of zirconium, titanium, thenium, cerium, hafnium, ruthenium, and uranium. In a preferred embodiment of the invention, the Fischer-Tropsch catalyst is cobalt supported on alumina promoted with ruthenium.
The liquid hydrocarbon products recovered from the process may be processed together or separately. First and second recovered hydrocarbon streams 56 and 114 are comprised primarily of C5-C17 hydrocarbons. First and second Fischer-Tropsch liquid products 42 and 98 are comprised primarily of C17+ hydrocarbons. Each of the streams 42, 98, 56, and 114 may be further processed for use as a variety of fuels, as chemical feedstocks and the like as is known to those skilled in the art.
Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments described are illustrative rather than limiting in nature and many variations and modifications are possible within the scope of the present invention. For example, while
This application claims priority to U.S. Provisional Application No. 60/746,302, filed May 3, 2006, which is incorporated herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60746302 | May 2006 | US |
