Unless otherwise specified, all quantities, percentages and ratios herein are by weight.
Three basic techniques may be employed for producing a synthesis gas, or syngas, which is used as the starting material of a Fischer-Tropsch (“FT”) reaction. These include oxidation, reforming and autothermal reforming. As an example, a Fischer-Tropsch conversion system for converting hydrocarbon gases to liquid or solid hydrocarbon products using autothermal reforming includes a synthesis gas unit, which includes a synthesis gas reactor in the form of an autothermal reforming reactor (“ATR”) containing one or more reforming catalysts, such as a nickel-containing catalyst. A stream of light hydrocarbons to be converted, which may include natural gas, is introduced into an ATR along with an oxygen-containing gas which may be compressed air, other compressed oxygen-containing gas, or pure oxygen. The ATR reaction may be adiabatic, with no heat being added or removed from the reactor other than from the feeds and the heat of reaction. The reaction is carried out under sub-stoichiometric conditions whereby the oxygen/steam/gas mixture is converted to syngas.
Known autothermal processes for the production of synthesis gas are disclosed in, for example, U.S. Pat. Nos. 6,085,512; 6,155,039; and 4,833,170, the disclosures of each of which are incorporated herein by reference.
The Fischer-Tropsch reaction for converting syngas, which is composed primarily of carbon monoxide (CO) and hydrogen gas (H2), may be characterized by the following general reaction:
2nH2+nCO→(—CH2—)n+nH2O (1)
Non-reactive components, such as nitrogen, may also be included or mixed with the syngas. This may occur in those instances where air, enriched air, or some other non-pure oxygen source is used during the syngas formation.
Referring to
Examples of Fischer-Tropsch systems are described in U.S. Pat. Nos. 4,973,453; 5,733,941; 5,861,441; 6,130,259, 6,169,120 and 6,172,124, the disclosures of which are herein incorporated by reference.
When the FTR is operated below about 260° C., the liquid products (5) from the Fischer-Tropsch reaction include hydrocarbons ranging from methane (CH4) to high molecular weight paraffinic waxes containing more than 100 carbon atoms.
The reactor exit gas (3) may comprise nitrogen, carbon dioxide, carbon monoxide, hydrogen, water and light hydrocarbons typically having a molar composition range of about 15 to 90% N2, 5 to 10% CO2, 0.5 to 15% CO, 1 to 30% H2, 0.1 to 10% H2O and the remainder hydrocarbons. Thus, the reactor exit gas contains inert non-combustible components in a range of about 20 to 94 mole % with the remainder being water and combustible components. As such, the reactor exit gas has a heating value in a range of about 2,500 to 15,800 kJ/m3. Inert non-combustible components are defined herein as components which will not react exothermically with oxygen. Such components include nitrogen, argon, carbon dioxide and water. Combustible components are defined herein as components which may react exothermically with oxygen at elevated temperatures. Such components include carbon monoxide, hydrogen, alcohols, methane and heavier hydrocarbons.
The reactor exit gas (3) is cooled using a cooler. In some embodiments, the cooler is an air cooler (6), water coolers (7) and feed-product exchanger (25), or a combination thereof. Upon exiting the cooler (25), the tail gas is at a temperature of about from 25° C. to 40° C. The cooled tail gas is fed to a three phase separator (8). The three phase separator (8) is configured to permit the separation of a gas phase and two liquid phases within a bottom portion of the separator (8). An FT produced water stream (4) exits the bottom, a light hydrocarbon stream (10) is withdrawn from a side port and a tail gas (9) exits the top. The tail gas (9) is fed to a dehydrator (11) to remove entrained water and then further cooled by a cooler (12) and a refrigeration system (13). In a preferred embodiment, the dehydrator (11) contains alumina fill to extract any remaining water. The refrigeration system (13) is preferably a propane refrigeration system which cools the tail gas to a temperature of about −33° C., condensing the majority of the remaining hydrocarbons in the gas. After exiting the refrigeration system (13), the tail gas is fed to a final separator (15) which operates to separate a gas phase and a liquid phase. A lean tail gas (16) exits the top of the final separator (15) and exchanges heat with the dehydrated tail gas in cooler (12) and the reactor exit gas in cooler (25) and the lean tail gas (23) is heated to a temperature of about 30° C. to 45° C. The warm lean tail gas (23) may then be used as a fuel source to generate power or may then be further processed. The typical molar (mol %) composition of the cooled lean tail gas at about 19 atms and 38° C. is about 84.3% N2, 4.5% CO2, 2.0% CO, 4.3% H2, 3.1% CH4 and 0.8% C2+.
A light hydrocarbon stream (18) exits the bottom of the separator (15) and is combined with the light hydrocarbon stream (10) from the three-phase separator (8). The combined light hydrocarbon stream (19) may be sent for further processing such as stabilization, hydrogenation, etc. Recovery using this approach can recover approximately between 85% to 88% of the hydrocarbons from the reactor exit gas (3) in the light hydrocarbon stream (19). The light hydrocarbon stream (18) includes hydrocarbons as light as propane. The typical temperature is −20° C. The typical pressure is 370 psig. The coolant (20) of water cooler (7) may be cooling water or any process stream
Referring to
The following table lists the equipment for the processes shown in
Referring to
While the invention has been described with respect to a limited number of embodiments, the specific features of one embodiment should not be attributed to other embodiments of the invention. No single embodiment is representative of all aspects of the inventions. Moreover, variations and modifications therefrom exist. For example, other stripping mediums may be used to increase hydrocarbon recovery in the scrubber. Additionally, heat exchangers and preheaters may be designed for maximum heat efficiency. The appended claims intend to cover all such variations and modifications as falling within the scope of the invention.
This Application claims priority over U.S. Provisional Application No. 60/820,028, filed Jul. 21, 2006, which is incorporated herein in its entirety.
Number | Date | Country | |
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60820028 | Jul 2006 | US |