The present invention relates to a method of producing liquid hydrocarbon using coal-based synthesis gas as raw materials, specifically a recycling synthesis method combining a cobalt-based synthesis reaction and an auto thermal reforming reaction.

Background

As the price of oil is rising in recent years, there is a growing interest in technologies of developing and producing alternatives to oil. It has become a hot research area to produce high-quality oil in an environment-friendly way by using coal, natural gas or other biomass as raw material, and applying the Fischer-Tropsch synthesis technology as the core technology. The energy resources of China are characterized by much more coal than oil. So preparing synthesis gas with coal and producing synthesis oil with synthesis gas have become one of the strategies of China to cope with oil shortage. Accordingly, it is urgent and pragmatic to develop the technologies for preparing oil with coal in China.

Synthesis gas prepared with coal mainly contains mixed gas of H ₂ and

CO. Under the action of a Fischer-Tropsch synthesis catalyst, the synthesis gas undergoes a Fischer-Tropsch synthesis reaction to generate a product of liquid hydrocarbon. The material flow coming from the Fischer-Tropsch synthesis reactor normally contains water vapor, C0 ₂ , unreacted synthesis gas (H ₂ and CO), CH ₄ , low carbon number hydrocarbons (C ₂ -C ₄ ), high carbon number hydrocarbons (C ₅₊ ), oxy-compounds, N ₂ and etc. After condensation and separation operation, fractions such as tail gas, water, oil, wax and etc. can be obtained. Components like oil and wax are processed in subsequent quality-improving processes to obtain refined oil.

To date, Fischer- Tropsch processes using iron-based catalysts and cobalt-based catalysts have been successfully developed. The activities of iron-based catalysts are relatively low. As a result, the growth rate of chain in the Fischer- Tropsch synthesis reaction is about 0.7, and the products contain much more light olefins and naphtha fraction than diesel oil fraction and wax. In contrast, the activities of cobalt based catalysts are relatively high. So the growth rate of chain can reach 0.9 or more, and the products mainly contain diesel oil fraction and wax.

A major difference between an iron-based Fischer- Tropsch catalytic reaction and a cobalt-based Fischer- Tropsch catalytic reaction is the quite low activity of the water-coal gas reaction during the Fischer- Tropsch catalytic reaction (Davis, BH, Catalysis Today, 84, 2003,83-98), which leads to a small amount of C0 ₂ generated during the reaction and high conversion rate of CO. However, such difference can lead to excessive water content in the reaction product of a cobalt-based Fischer-Tropsch catalytic reaction. Water has significant effects on a cobalt-based Fischer-Tropsch catalyst (Dalai, A.K., D., Davis, B.H., Applied Catalysis A, 348, 2008, 1-15). A research report (Rothaemel, M., Catalysis Today, 38, 1997, 79-84) indicates that a cobalt-based catalyst is prone to inactivation by oxidation in an environment having high water content. Van Berge (Catalysis Today, 58 (4), 2000, 321-334) further shows that the oxidation inactivation of cobalt is associated with the ratio of water partial pressure and H ₂ partial pressure (p(H ₂ ) /p(H ₂ 0)). When p (H ₂ )/p (H ₂ 0)<1, water may result in oxidation inactivation of a cobalt-based catalyst. Specifically, the present invention aims to solving the technical problem that the activity of a cobalt-based catalyst for the Fischer-Tropsch reaction is very low in a water-coal gas reaction expressed by Formula (1): CO + H ₂ 0 ^ C0 ₂ +H ₂ Formula ( 1 )

And the mechanism of a Fischer-Tropsch reaction is expressed by Formula (2):

CO + H ₂ → hydrocarbons + H ₂ 0 Formula (2)

Thus, a large amount of water tends to be gradually accumulated in a

Fischer-Tropsch reaction system using a cobalt-based catalyst. However, cobalt-based catalysts are sensitive to water and may lose activity due to negative effects of water. Therefore, it is essential for a successful industrial application of cobalt-based catalysts to avoid oxidation inactivation of the catalysts by effectively remove the water generated during reaction. The presently available Fischer-Tropsch processes normally involve removing water from the reaction product, but seldom involve the recycling gases and removing the water in reactor. It is therefore highly desired in the technical field to provide a cobalt-based Fischer-Tropsch process capable of overcoming the aforesaid defects of the prior art.

Summary of the Invention

Aiming at the problems that cobalt-based catalysts have low activity for water-coal gas reaction and are prone to be oxidized and inactivated under a high water partial pressure, the present invention provides a new method of conducting Fischer-Tropsch reaction comprising the following steps: i) introducing gas raw materials containing CO and H ₂ into a Fischer-Tropsch reactor, and incubating the gas raw materials with a cobalt-based catalyst of Fischer-Tropsch synthesis reaction to let the CO and H ₂ to react with each other under a controlled reaction condition and thereby generating hydrocarbon products containing two or more carbon atoms, and gaseous byproducts containing H ₂ 0, CH ₄ and C0 ₂ wherein the controlled reaction condition reduces the single-cycle conversion rate of CO; ii) taking at least a portion of the tail gas containing gaseous byproducts and unreacted gas raw materials from step (i) and mixing them with fresh gas raw materials containing CO and H ₂ , removing C0 ₂ and water from the mixed materials; iii) introducing the gas materials from step (ii) into the Fischer-Tropsch reactor and repeating steps (i) and (ii).

In another aspect of the present invention, there is provided a method for conducting Fischer-Tropsch synthesis reaction comprising the following steps: i) introducing gas raw materials containing CO and H ₂ into a Fischer-Tropsch reactor, and incubating the gas raw materials with a cobalt-based catalyst of Fischer-Tropsch reaction to let the CO and H ₂ to react with each other under a controlled reaction condition and thereby generating hydrocarbon products containing two or more carbon atoms, and gaseous byproducts containing H ₂ 0, CH ₄ and C0 ₂ wherein the controlled reaction condition reduces the single-cycle conversion rate of CO; ii) taking at least a portion of the tail gas containing gaseous byproducts and unreacted gas raw materials from step (i), and putting them under a condition of conversion to let CH ₄ to oxidize in the presence of water; iii) taking the resulting gaseous products from step (ii) and mixing them with fresh gas raw materials containing CO and H ₂ , removing C0 ₂ and water from the mixed materials; iv) introducing the gaseous materials from step (iii) into the Fischer-Tropsch reactor and repeating steps (i), (ii) and/or (iii). In an embodiment of the present invention, the single-cycle conversion rate of CO under the controlled condition is 20-70%, preferably 40-60%.

In another embodiment of the present invention, the controlled reaction condition comprises reaction pressure in the Fischer- Tropsch reactor being 10-30 Bar, preferably 15-25 Bar. In another embodiment of the present invention, the controlled reaction condition comprises the reaction temperature being 150-300 ^° C , preferably 180-250 ^° C . Preferably, the controlled reaction condition comprises reaction pressure in the Fischer- Tropsch reactor being 10-30 Bar, preferably 15-25 Bar and the reaction temperature being 150~300 ^° C , preferably 180~250 ^° C .

In an embodiment of the present invention, the total conversion rate of CO under the controlled reaction condition is at least 80%, preferably at least 90%.

In an embodiment of the present invention, in the CH ₄ oxidization reaction at least 95 mol%, preferably, 99 mol% of the CH ₄ , is consumed.

In an embodiment of the present invention, the mole percentage of water in the gaseous materials re-introduced into the Fischer- Tropsch reactor is less than 0.01%. In an embodiment of the present invention, the water is removed from the mixed materials by freeze drying. In an embodiment of the present invention, the water is removed from the mixed materials by cooling with liquid ammonia or washing with cold methanol, under the temperature of -20^0 ^° C , preferably -20 ^° C .

In an embodiment of the present invention, the volume percentage of C0 ₂ in the gaseous materials re-introduced into the Fischer- Tropsch reactor is 2% or less. In an embodiment of the present invention, the C0 ₂ is removed using ethanolamine aqueous solution or cold methanol, wherein the temperature of the cold methanol is -20^0 ^° C , preferably -20 ^° C . In an embodiment of the present invention, the C0 ₂ and water are removed by washing with cold methanol, wherein the temperature of the cold methanol is -20^0 ^° C , preferably -20 ^° C .

In an embodiment of the present invention, at least a portion of the tail gas from step (i) is directly expelled from the Fischer-Tropsch reactor as a purge gas. In an embodiment of the present invention, the Fischer-Tropsch reactor is selected from the group consisting of slurry bed reactor, fixed bed reactor, and fluidized bed reactor.

In an embodiment of the present invention, the ratio of the resulting gaseous products from step (ii) to the fresh gas raw materials is 1 6, preferably 1 3.

In an embodiment of the present invention, a portion of the gas raw materials and/or the C0 ₂ and/or water removed by step (iii) is supplied to the CH ₄ oxidization reaction in step (ii) for conducting the CH ₄ oxidization reaction.

In an embodiment of the present invention, the ratio of the volume of H ₂ to CO in the resulting gaseous products from step (ii) is 1 3, preferably 2.

In an embodiment of the present invention, the ratio of the volume of H ₂ 0, 0 ₂ and C0 ₂ to that of CH ₄ in the CH ₄ oxidization reaction is kept at 0-3 : 0-2: 0-5: 0-6. In an embodiment of the present invention, the amount of water present in the Fischer- Tropsch reactor is less than 5%.

Brief description of the drawings

Figure 1 is a process diagram of an embodiment according to the present invention.

Detailed Description of the Invention

The "range" disclosed herein is in the form of lower limit and upper limit. There can be one or more lower limits, and one or more upper limits, respectively. A given range is limited by selecting a lower limit and an upper limit. The selected lower limit and upper limit will determine the boundary of the specific range. The range limited by such way can be included or combined, i.e. any lower limit and any upper limit can be combined to form a range. For example, the range of "60- 120 and 80- 110" that is given by specific parameters can be understood to be 60-110, 60-80, 110- 120 and 80- 120. In addition, if the minimum value is 1 and 2, and a maximum value is 3, 4, and 5, the following range can thus be expected: 1-2, 1-3, 1-4, 1-5, 2-3, 2-4, 2-5, 3-4, and 4-5. Unless otherwise specified, all of the embodiments and preferred embodiments described herein can be combined to obtain new technical solution.

Unless otherwise specified, all of the technical features and preferred features described herein can be combined to obtain new technical solution.

Unless otherwise specified, all of the steps described herein can be performed in order or randomly, preferably in order. For example, said method comprises steps (a) and (b) means said method can comprise steps (a) and (b) in such order, or steps (b) and (a) in such order. For example, said method further comprising step (c) means step (c) can be added into said method in any order, for example, said method may comprise steps such as steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b).

Figure 1 shows a progress diagram of an embodiment of the present invention, and the present invention will be described in detail by reference to the accompany drawings. It should be noted that the following detailed description and specific embodiments are merely exemplary and for the purpose of better understandings of the present invention by the persons skilled in the art, and the scope of the present invention is not limited thereto.

As shown in Figure 1 , gas raw materials flow 6 is introduced by a compressor 104 into the Fischer- Tropsch reactor 105, and CO and H ₂ contact the catalyst in the Fischer- Tropsch reactor 105 under the reaction conditions so that the Fischer- Tropsch reaction occurs. The synthesis gas raw materials flow 6 comprises in part the fresh synthesis gas 1 and in part the materials flow 14 from the auto thermal reforming reaction, wherein the main components of the synthesis gas raw materials flow include H ₂ and CO, as described more specifically later. The fresh synthesis gas 1 used in the present invention may be synthesis gas from any known source, for example, coal gasification, synthesis gas as byproduct from the production of petroleum products and the like, preferably the synthesis gas made from coal. The Fischer- Tropsch reactor 105 may be any reactor known to those skilled in the art, for example, fixed bed reactors, fluidized bed reactors and slurry bed reactors. A person skilled in the art may set the reaction conditions of Fischer- Tropsch reaction according to information such as the type of reactor, the specific chain length of the desired product, specific ratios of components of the raw materials and etc. The cobalt-based catalyst used in the present invention may be any catalyst known in the art. When calculated based on the total weight of the catalyst, the catalyst contains 1— 35wt% of Co and one or more modifying agents selected from the group consisting of V, Cr, Pt, Pd, La, Re, Rh, Ru, Th, Mn, Cu, Mg, K, Na, Ca, Ba, Zn and Zr. The catalyst can be carried on one or more carriers selected from the group consisting of A1 ₂ 0 ₃ , Ti0 ₂ , Si0 ₂ and ZnO. In a preferred embodiment, the Fischer- Tropsch reactor is a slurry bed reactor. In another preferred embodiment, the pressure in the Fischer- Tropsch reactor is 10—30 Bar, preferably 15—25 Bar, and most preferably 20 Bar; and the reaction temperature in the Fischer- Tropsch reactor is 150— 300 ^° C , preferably 180— 250 ^° C , and most preferably 220 ^° C . Under such conditions, the single-cycle conversion rate of CO in the Fischer- Tropsch reaction is 20—70 % , preferably 40—60 % .

In the present invention, the terms "single-cycle conversion rate" and "total conversion rate" are used to indicate the effects of the reaction system on the reaction results. And the "total conversion rate" can be obtained by conducting the reaction process of the present invention in a continuous recycling way to mix at least a portion of the unreacted synthesis gas with fresh synthesis gas and then supply the mixed gas as synthesis gas raw materials flow to the Fischer- Tropsch reactor. In the embodiments described below, if the process of the present invention has auto thermo reforming operation, the fresh synthesis gas is mixed with the gas materials flow coming from the auto thermo reforming reactor, of which methane has been consumed. And for the embodiments not having auto thermo reforming operation, a portion of the tail gas is subject to a separation operation to remove C0 ₂ and H ₂ 0, and then directly mixed with the fresh synthesis gas. The ratio of the volume of the materials flow from the auto thermo reforming reactor to that of the fresh synthesis gas, i.e. flow rate of the recycling synthesis gas/flow rate of the fresh synthesis gas, is denoted as "recycling rate". For instance, if the "recycling rate" in a test is equal to 1 , it means that the materials flow from the auto thermo reforming reactor is first mixed with the fresh synthesis gas in a flow ratio of 1 : 1 before being decarburized, dewatered and etc. Under a steady-state condition, the molar flow rate (Nco _ta ii) of CO in the tail gas from the Fischer- Tropsch reactor is measured, while the molar flow rate of CO in the recycling synthesis gas (i.e. the materials flow from the auto thermo reforming reactor) is denoted as N _C o re« the molar flow rate of CO in the fresh synthesis gas is denoted as N _C o new ₅ and the amount of CO converted during the reaction is denoted as N _C o con- The total conversion rate is calculated according to the following formula:

Total Conversion Rate % = ^{Nc0 con} x 100% = ^N ∞" ^{ew + N} ∞ rec - Nco taii _{χ 100} o _/o

N ^iN CO new N ^iN CO new

And the "single-cycle conversion rate" refers to the ratio of the amount of CO converted N _C o con to the total molar flow rate of CO in the mixed synthesis gas entering into the reactor N _C o mi _X5 namely:

Single - cycle Conversion Rate % = ^{Nc0 con} x 100% = ^N ∞∞» ^{+ N} co rec- ^N co taii _{χ 100} o _/o

N ^iN CO mix N ^iN CO new + ^T N ^iN CO rec

It is apparent that the total conversion rate for a tail gas recycling system is higher than the single-cycle conversion rate.

Compared to a conventional Fischer-Tropsch reaction process, the Fischer-Tropsch reaction of the present invention has a relatively low single-cycle conversion rate. If the single-cycle conversion rate of the synthesis gas is too high, it may cause excessive high water vapor pressure which may lead to oxidation inactivation of a catalyst. And the large amount of reaction heat emitted by vigorous reaction may lead to too large a heat transfer area and reduce the spatial efficiency of the reactor. Furthermore, as the reaction proceeds, the partial pressures of CO and H ₂ drop, and the reaction rate is lowered, whereby the space-time yield of the reaction is reduced accordingly. Accordingly, by applying a relatively low single-cycle conversion rate for the synthesis gas, the present invention can effectively reduce the amount of water generated during a reaction and prevent a cobalt-based catalyst from being affected by the moisture and from losing activity, while obtaining a relatively higher spatial efficiency of a reactor and a relatively higher space-time yield rate.

The liquid products of a Fischer- Tropsch reaction mainly contain hydrocarbon oils and wax components. These components are first released as liquid product 8 from an opening at the bottom or middle of the reactor, and then conveyed downstream to the separation/condensation/refining process to obtain the desired products. The gas products, denoted herein as the tail gas 7, are discharged from the top of the reactor. The tail gas 7 mainly contains water vapor, C0 ₂ , unreacted synthesis gas (H ₂ and CO), CH ₄ , and possibly a small amount of low-carbon hydrocarbons (C ₂ -C ₄ ) and high-carbon hydrocarbons (C ₅₊ ), other oxygenate components, N ₂ and etc. It shall be noted that the description herein mainly discusses about water vapor, C0 ₂ , synthesis gas and CH ₄ in the tail gas for the purpose of simplification and clarification, and minor components such as hydrocarbons with two or more C atoms and N ₂ will inevitably exist in the subsequent reaction and processing steps. But these minor components will not have significant influences on the whole process of the present invention, because they are too little or too stable to affect the subsequent process significantly, or because they are removed in the subsequent separation operation. For example, the reaction gas is subject to a gas-liquid separation operation before the auto thermo reforming reaction so as to remove some water from the gas. In the separation operation, hydrocarbons having two or more C atoms are also removed from the tail gas. If necessary, persons skilled in the art may arrange an additional separation device to separate and retrieve one or more minor components.

The tail gas 7 then passes through a condensation unit 106 and a gas-liquid separation apparatus 107, whereby condensable components contained in the tail gas 7, such as hydrocarbons with two or more C atoms, water and the like, are separated. A portion of the gas phase is directly expelled as a purge gas 10 so as to maintain material and pressure balance in the whole reaction system, while the majority of the tail gas 12 is introduced into the auto thermal reforming reactor 108 for auto thermal reforming reaction.

In the present invention, it may consider not subjecting the tail gas 12 to the conversion reaction. Instead, the tail gas 12 can be mixed with fresh gas raw materials. Then C0 ₂ and water are removed from the mixed materials. As mentioned above, since a Fischer-Tropsch reaction of the present invention applies a relatively lower single-cycle conversion rate, recycling tail gas is necessary to improve the total conversion rate of the synthesis gas, i.e. a portion of the unreacted synthesis gas is recycled back to the synthesis reactor to improve oil yield rate. Because the synthesis gas made from coal contains about 10% of CH ₄ , direct recycling the tail gas from the Fischer-Tropsch reactor may cause accumulation of CH ₄ during recycling. The volume ratio of CH ₄ can reach 60% after several cycles, and excessive CH ₄ may reduce the space utilization rate of the Fischer-Tropsch reactor. Meanwhile, too much CH ₄ gas being recycled back to the inlet of the Fischer-Tropsch reactor may cause a large power consumption of the compressor. In order to solve the above problem, a majority of discharged gas materials flow is feed after condensation and gas-liquid separation to the auto thermal reforming reactor, in which the auto thermal reforming reaction is conducted to consume at least a portion of the CH ₄ inside the reactor. In particular, conversion reactions as shown in the following formulas (3)-(6) may- occur during the auto thermal reforming reaction to cause oxidation of methane.

CH ₄ + 20 ₂ = C0 ₂ + 2H ₂ 0 Formula (3)

CH ₄ +0 ₂ =C0 ₂ +2H ₂ Formula (4)

CH ₄ + H ₂ 0 = CO + 3H ₂ Formula (5)

CH ₄ + 2H ₂ 0 = C0 ₂ + 4H ₂ Formula (6)

The reaction shown in Formula (5) is most desired by the process of the present invention, because such a reaction can consume the byproducts CH ₄ and H ₂ 0 and generate the desired CO and H ₂ . The auto thermal reforming reaction preferably uses a supported catalyst, the active component of which may be a rare metal catalyst such as Pt, Pd, Ir, Rh and etc., or a common metal catalyst such as Ni, Co and etc. Besides to controlling the temperature and pressure inside the auto thermal reforming reactor, it is necessary to add 0 ₂ , H ₂ 0, C0 ₂ and other components to adjust the ratios of the reaction materials in the auto thermal reforming reactor so as to consume the majority of CH ₄ according to actual requirements.

In an embodiment of the present invention, the total gas pressure of the gas inside the auto thermal reforming reactor is 0.5^5MPa, preferably 1— 4MPa, more preferably 1.5 3MPa, and most preferably 2MPa; and the reaction temperature is 650^900 ^° C , preferably 750^ 880 ^° C , and more preferably 820^ 860 ^° C . In a preferred embodiment, when calculated using the molar amount of the CH ₄ in the auto thermal reforming reactor as a reference, the molar amount of 0 ₂ is 30^ 80 % of the molar amount of the CH ₄ , preferably 40^70 % , more preferably 40^70 % , and most preferably 60 % ; the molar amount of C0 ₂ is 5^25 % , preferably 8^ 15 % , more preferably 18 % ; and the molar amount of H ₂ 0 is 50^400 % of the molar amount of CH ₄ , preferably 100^300 % , more preferably 200 % . In a method of the present invention, the conversion rate of methane during the auto thermal reforming reaction is greater than 95 molar percentage, preferably greater than 98 molar percentage, more preferably equal to or greater than 99 molar percentage. In a preferred embodiment, the volume ratios of the H ₂ and CO contained in the materials flow from the auto thermal reforming reactor is set to 1 -3, preferably 2, by modifying reaction conditions of the auto thermal reforming reaction as described above.

The methane consumed during the auto thermal reforming reaction is from the methane remained after condensation and gas-liquid separation of the discharging gas material flow 7, and the water and 0 ₂ needed for the reaction come from the following sources: (1) the gas materials flow 13 containing 0 ₂ , H ₂ 0 and CO, which is externally supplemented; (2) the decarburizing device 101 , the dewatering device 102 and the gas-liquid separation device 103 for separation operations of the synthesis gas subsequently; and (3) the gas-liquid separation device 107. Internal recycling of said C0 ₂ and H ₂ 0 will be described in detail below.

After the auto thermal reforming reaction, the materials flow 14 from the auto thermal reforming reactor is mixed with the fresh synthesis gas 1 described before, and the mixed gas materials flow is subsequently subject to a C0 ₂ -removing separation operation in the decarburizing device 101 , a water-removing separation operation in the dewatering device 102, and a gas-liquid separation operation in the gas-liquid separation device 103. Gas from the top outlet of the gas-liquid separation device 103 is compressed by a compressor 104 and conveyed as the synthesis gas raw materials flow 6 to the Fischer- Tropsch reactor 105, and the next cycle of the process of the present invention is started.

In a preferred embodiment of the present invention, the materials flow 14 from the auto thermal reforming reactor is mixed with the fresh synthesis gas at a volume ratio of 1 6, preferably 1 3.

The decarburizing device 101 is used to remove at least a portion of C0 ₂ from the mixed gas flow and convey the C0 ₂ 15 separated to the auto thermal reforming reactor 108, so that the auto thermal reforming reacting system has the needed content of C0 ₂ . The decarburizing device 101 may apply any known method to separate C0 ₂ , for instance, by filling ethanolamine aqueous solution or cold methanol in the decarburizing device 101 and passing the gas materials flow containing C0 ₂ through the device. The cold methanol has a temperature of -20^0 ^° C , preferably -20 ^° C . Preferably, the gas material flow has a content of C0 ₂ in volume equal to or less than 2% after passing through the decarburizing device 101 , the dewatering device 102 and the separation device 103.

The dewatering device 102 may be any device known in the art that can remove moisture from the gas materials flow, so long the device will not affect other components of the gas stream, such as H ₂ and CO. Preferably, the dewatering device 102 removes moisture by drying agent or freeze drying. The freeze drying method includes drying utilizing liquid ammonia or washing with cold methanol at a temperature of -20 ^° C— 0 ^° C , preferably -20 ^° C .

In another preferred embodiment of the present invention, the decarburizing device 101 and the dewatering device 102 may be integrated in one device, in which the separation of C0 ₂ and H ₂ can be done simultaneously. For example, cold methanol can be introduced into the integrated device, and separation of C0 ₂ and water vapor can be realized in parallel by letting the gas materials flow pass through the cold methanol.

The gas-liquid separation device 103 is used to realize a further gas-liquid separation of the gas stream and remove moisture from the gas materials flow more thoroughly.

Although only one decarburizing device 101 , one dewatering device 102 and one separation device 103 are shown in the drawing, it is apparently possible to apply the same decarburizing devices 101 , the same dewatering devices 102 and the same separation devices 103 connected in series. After decarburization or dewaterization, C0 ₂ or liquid water in the solution is released by heating, pressure change or other means. C0 ₂ 15 and moisture 16 released by heating or pressure change or other means are conveyed to the auto thermal reforming reactor 108. In order to save energy, the heat needed for heating may come from the condensation unit 106 or the materials flow 14. Moreover, a portion of the water and C0 ₂ separated by the gas-liquid separation device 107 can be respectively conveyed as the raw materials to the auto thermal reforming reactor for consuming of CH ₄ .

In a preferred embodiment of the present invention, after the mixing, decarburizing and dewatering operations, the synthesis gas raw materials flow 6 fed into the Fischer- Tropsch reactor 105 contains 50—70 % of H ₂ , 20—40 % of CO, less than 2 % of CH ₄ , less than 2 % of C0 ₂ , and equal to or less than 0.01 % of moisture, wherein all the percentages refer to volume percentage calculated by applying the total volume of the materials flow 6 as reference.

Due to very low moisture content in the partially recycling synthesis gas raw materials 6, the water vapor pressure inside the reactor can be reduced when the gas materials flow enters into the Fischer- Tropsch reactor, whereas the moisture in the liquid phase and the catalyst enters into the gas phase. This improves the micro environment of the catalyst reaction and prevents the cobalt-based catalyst from inactivation due to excessive water vapor pressure.

During the reaction of the present invention, heat exchange operation can be arranged freely before, after or in each step so as to minimize power consumption. For example, the materials flow 14 from the auto thermal reforming reactor has a relatively high temperature, so it is preferred to recovery heat by guiding it through a heat exchanger first and subsequently mixing the gas material flow with the fresh synthesis gas 1 after cooling.

In conclusion, the process of the present invention makes the following progresses to improve the life and stability of cobalt-based catalysts and increase the space utilization rate and space-time yield of the Fischer- Tropsch reactor:

Single-cycle conversion rate of the synthesis gas is maintained at a low level to avoid excessive pressure of water vapor inside the Fischer- Tropsch reactor, while total conversion rate of the synthesis gas can be maintained by recycling the exhaust gas.

CH ₄ and other gases are removed from the recycling tail gas through the auto thermal reforming reaction to avoid excessive inert gases like CH ₄ and a lowered space utilization rate and a lowered space-time yield of the reactor.

Before the recycling synthesis gas returns to the Fischer- Tropsch reactor, moisture in the synthesis gas is removed by means of cryogenic refrigeration, drying or other processes so that the content of moisture inside the reactor can be reduced while the recycled synthesis gas has a low moisture content; A portion of the water and C0 ₂ separated in each decarburizing and dewatering operation is conveyed to the auto thermal reforming reactor to improve economical efficiency of the process of the present invention.

Compared with the presently known processes, the cobalt-based Fischer- Tropsch synthesis process of the present invention has the following advantageous effects:

The process of the present invention can convert the CH ₄ accumulated in the exhaust gas to a synthesis gas having a suitable H ₂ /CO ratio through the auto thermal reforming reaction, which can ensure a relatively high reaction rate and increase the space utilization rate and space-time yield rate of the reactor.

The process of the present invention has the auto thermal reforming device downstream of the Fischer- Tropsch reactor, which lowers the load of the auto thermal reforming reaction and the costs for production and operation.

The process of the present invention reduces the content of moisture inside the reactor effectively by combining the recycling operation with the low single-cycle conversion rate. At the same time, the dewatered recycling gas having low moisture content can further dilute the moisture inside the reactor so as to avoid oxidation inactivation of the cobalt-based catalyst caused by water and thereby improve the stability of catalyst.

The process of the present invention generating synthesis gas through the auto thermal reforming reaction can utilize byproducts of the cobalt-based Fischer- Tropsch reaction, such as CH ₄ , water and the like, and therefore improve economical efficiency of the whole process. In addition, the H ₂ /CO ratio of the synthesis gas entering into the reaction tower being adjustable within a certain range can lead to a higher adaptability of the process to catalysts. Examples

The following examples are used to illustrate the present invention specifically, and not to limit the scope of the present invention.

Example 1 :

In this example, a synthesis wax processing equipment having an annual production capacity of 100,000 tons is described. The process shown in figure 1 is performed in a reactor having a diameter of 5m and a height of 25m. A conventional cobalt-based catalyst for Fischer- Tropsch reaction is applied. There is 15% (mass percentage) of catalyst in the slurry. Detailed description about the slurry used by the present invention can be found in an article of Chang Jie et al (Chang Jie et al, Journal of Catalysis, 26(10), 859-868, 2005). The fresh synthesis gas applied in the example is the raw gas obtained by gasifying coal and

desulfurizing the gas, and the flow rate of the fresh synthesis gas is

940,000Nm /hr. Table 1 below lists in detail the components of this fresh synthesis gas, wherein the ratio of H ₂ /CO is about 1.6.

Table 1 : Components of the coal-made synthesis gas

Then, mix the raw gas with the recycled synthesis gas, which is the materials flow 14 from the auto thermal reforming reactor and contains the components listed below in Table 2.

Table 2: Components of the materials flow from the auto thermal reforming reactor

By adjusting the mixing ratio of the two gases (the recycling ratio is 1), the ratio of the volume of H ₂ and that of CO in the mixed gas is about 2. The mixed gas is washed in a washing tower filled with methanol of -20 ^° C so as to remove H ₂ 0 and C0 ₂ in the mixed gas. Purified synthesis gas is compressed by a compressor to 2MPa, and then supplied to the Fischer- Tropsch reactor. The gas entering into the tower has a flow rate of 137,000 Nm /hr and contains the components listed below in Table 3.

Table 3 : Components of the decarburized and dewatered gas entering into the reaction tower

The pressure in the Fischer- Tropsch reactor is 2MPa, the temperature therein is 220 ^° C , and the operation apparent flow rate of the synthesis raw materials flow is 0.2^0.4m/s. After reaction, a wax-based liquid phase hydrocarbon product is collected after filtering, and the tail gas of the reaction is discharged from the top of the reactor. When characterizing and analyzing the product with GC-14C Gas Chromatograph Analyzer manufactured by Shimadzu Corporation, the components listed in Table 4 are identified in the tail gas of the reaction. Thus, it can be calculated from the test results that the single-cycle conversion rate of CO is 60% and the total conversion rate is 93%.

Table 4: Components of the tail gas

The tail gas of the reaction passes through a condensation unit 106, and being cooled to room temperature by a heat exchange means. A portion of the resulting gas is expelled as a purge gas, while the remaining gas is conveyed to the auto thermal reforming reactor as the recycled gas. The present invention uses a fixed bed reactor having a diameter of 2.5m and a height of 18m as the auto thermal reforming reactor. The reactor uses a nickel catalyst carried on A1 ₂ 0 ₃ , wherein the catalyst may be prepared by the following steps: dissolving A1(N0 ₃ ) ₃ in water to prepare an aqueous solution with a concentration of 1M; adding an aqueous solution having 0.1M Na ₂ C0 ₃ into said A1(N0 ₃ ) ₃ aqueous solution to adjust the pH value of the solution to 9; stirring the solution for two hours at 70 ^° C ; filtering the solution to collect the solid precipitates; washing the filtered cake with deionized water to neutralize; roasting the filtered cake for one hour at 100 ^° C , and then for 4 hours at 800 ^° C ; preparing a A1 ₂ 0 ₃ powder of 20^40 mesh size by grinding the roasted filtered cake. The A1 ₂ 0 ₃ powder is immersed in 1M Ni(N0 ₃ ) aqueous solution at room temperature and stirred for one hour, and filtered by suction. Next, the solid remaining after the suction filtration is roasted for 3 hours at 650 ^° C to get a Ni/Al ₂ 0 ₃ catalyst having a capacity of about 10 wt%. This catalyst is added into the auto thermal reforming reactor to form a fixed bed.

By adjusting the amount of the supplement materials flow 13 from an external supply and that of the C0 ₂ and H ₂ 0 recycling internally, the molar ratio of 0 ₂ , C0 ₂ , H ₂ 0 and CH ₄ in the auto thermal reforming reactor is 0.6:0.18:2: 1. When examining the materials flow from the auto thermal reforming reactor with GC- 14C Gas Chromato graph Analyzer produced by Shimadzu Corporation, the results shown in Table 2 can be obtained, wherein the ratio of H ₂ /CO is about 2.6.

The heat from the materials flow 14 is collected by a heat exchanger, and the materials flow 14 is mixed, as the recycled synthesis gas, with the raw gas (the fresh synthesis gas 1), and the mixed gas enters into the slurry bed reactor after decarburization and dewaterization, whereby the above mentioned cycle can be repeated.

Reference Example 1

This reference example uses the same reaction process as Example 1 , but differs from it in the omission of the auto thermal reforming step. In other words, the recycled gas is directly mixed with the fresh synthesis gas, and conveyed to the Fischer-Tropsch reactor after decarburization and dewaterization. The gas entering into the Fischer-Tropsch reactor contains the components shown in Table 5. In this case, the total conversion rate is 88.51 % while the reaction CO single-cycle conversion is 60%. Components of the reaction gas entering into the tower (i.e. content of the synthesis gas raw material into the Fischer-Tropsch reactor) and the tail gas are listed in Table 5 and Table 6, respectively.

Table 5: Components of the synthesis gas entering into the reaction tower

Table 6: Components of the tail gas

From Table 5, it can be seen that the synthesis gas entering into the Fischer- Tropsch reactor includes about 30% of CH ₄ , while the total conversion rate of CO decreases 5%, whereby the space utilization rate and the space-time yield of the reactor are lowered significantly.

Example 2

The inventors of the present invention have conducted tests with different recycle ratio to determine the influence of recycled synthesis gas on the content of moisture in the Fischer- Tropsch reaction system. Specially, this example uses the same equipment and reaction conditions as Example 1 , but with different recycle ratio. After the reaction system enters into a balanced state, samples are taken from the liquid materials inside the Fischer- Tropsch reactor, and the water content in the liquid material is examined using gas chromatography. The results are listed in Table 7. In Table 7, a recycling ratio of zero means that the synthesis gas raw materials conveyed into the Fischer- Tropsch reactor 105 are completely fresh synthesis gas raw materials, while a recycling ratio of 5 means that the ratio of the volume of the recycling synthesis gas (i.e. material flow from the auto thermal reforming reactor) 14 to that of the fresh synthesis gas is 5: 1. It can be concluded based on Table 7 that recycling the completely dewaterized tail gas can significantly lower the content of moisture inside the reactor. The higher the recycling ratio is, the lower the liquid water content that the liquid phase in the reactor may have, whereby oxidation inactivation of cobalt-based catalyst due to existence of water can be effectively prevented. However, too large a recycling ratio may increase power consumption of the compressor and therefore increase the cost for operation, so it is necessary to choose a suitable recycling ratio in accordance with the actual conditions. In Example 1 of the present invention, the recycling ratio is set to 1.

Table 7: Influence of the recycling ratio on the moisture contents inside the reactor