DE3751934T2 - Process for reducing methane formation and increasing the yield of liquid products in a Fischer-Tropsch process - Google Patents

Description

The present invention relates to a process for reducing methane production and increasing liquid yields in Fischer-Tropsch reactions.

US-A-4 547 525 describes and claims a method for reducing methane formation in a Fischer-Tropsch process for synthesizing hydrocarbons, which comprises contacting, at an elevated temperature of 100°C to 500°C and a pressure of about 100 to 10,000 kPa, a feed mixture of R₂/CO with a heterogeneous catalyst comprising one or more Group VIII metals supported on an inorganic heat-resistant oxide for a time sufficient to produce hydrocarbons including methane, wherein one or more olefins are added to the feed mixture of H₂/CO in an amount sufficient to reduce methane formation to a level below that which would occur without the addition of the olefin to the feed. Preferably, one or more olefins are added to the feedstock in an amount, based on a molar ratio of olefin to CO, in the range of about 1:100 to 5:1 and preferably the olefin comprises one or more α-olefins.

Our European patent EP-B-0 321 616, from which the present patent application is divided, describes and claims a process for reducing methane formation and increasing liquid yield (C5+) in a Fischer-Tropsch process for synthesizing hydrocarbons from CO and H2, in which at least one olefin is added to a catalyst-containing reactor bed, characterized in that a gas mixture containing CO and H2 is introduced at the feed region of the reactor bed and the olefin is separately introduced directly into the bed at a location more than 10% of the length of the bed from the feed region of the bed and more than 10% of the length of the bed from the exit region of the bed.

The present invention provides a process for reducing methane formation and increasing liquid yields (C₅₋) in a Fischer-Tropsch process for synthesizing hydrocarbons by introducing CO and H₂ and at least one olefin into the feed region of a reactor bed containing catalyst, characterized in that steam is used as a co-feed material in the process.

It has been found according to the invention that liquids (C5+) yields and methane production in Fischer-Tropsch hydrocarbon synthesis reactions are increased or decreased, respectively, by adding one or more olefins and steam in the feed section of the reactor bed. The olefin may be obtained by separation from the product stream and recycled to the reactor bed or from an independent source, such as from processing the paraffinic product of the Fischer-Tropsch reaction through a dehydrogenation reactor to convert the paraffins to olefins. According to a preferred embodiment, the olefin or olefins may comprise one or more C2 to C20 α-olefins, the olefin to CO molar ratio in the reactor being in the range of 1:100 to 5:1, e.g. B. 1:20 to 5:1, especially 1:10 to 2:1 and most preferably about 1:5 to 1:1. The molar ratio of water to olefin may be in the range of 0.1 to 50, especially 0.2 to 10 and most preferably 0.5 to 10. The catalyst may comprise at least one Group VIII metal on an inorganic heat-resistant oxide support. According to a particularly preferred embodiment, the catalyst comprises ruthenium on a titania support. A cobalt or iron catalyst may also be used.

The molar ratio of water to CO can range from 0.1:1 to 5.0:1.

Preferred olefins useful in the process of the invention include α-olefins of the type R-CH=CH₂, where R is hydrogen or an alkyl group having 1 to 17 carbon atoms. More preferably, the alkyl group has 1 to 11 carbon atoms and most preferably, the alkyl group has 1 to 8 carbon atoms. More preferably, they are C₂ to C₁₀ α-olefins. The The amount of alpha-olefin recycled to the reactor bed is sufficient to maintain an olefin to CO molar ratio of 1/100 to 5/1, more preferably 1/20 to 5/1, and most preferably 1/10 to 2/1. Internal olefins in which the unsaturation is located away from the terminal carbon atom may also be usefully added to the CO/H2 feed.

Although the process of the invention can be carried out in the presence of any suitable Fischer-Tropsch catalyst, in a preferred embodiment it is carried out in the presence of a catalyst comprising one or more Group VIII metals on an inorganic heat-resistant oxide support, preferably ruthenium or cobalt on such a support. Thus, suitable supports include oxides of titanium, niobium, vanadium, tantalum, silicon, aluminum, manganese and mixtures thereof. Preferably, the catalyst support is selected from the group consisting of titania, zirconium titanate, mixtures of titania and alumina, mixtures of titania and silica, alkaline earth titanates, alkaline titanates, rare earth titanates and mixtures of any of the foregoing with supports selected from the group consisting of vanadium oxide, niobium oxide, tantalum oxide, alumina, silica and mixtures thereof. Thus, according to a particularly preferred embodiment of the invention, the process is carried out in the presence of a catalyst comprising ruthenium on a titania support. Alternatively, the catalyst may comprise cobalt or iron on any of the foregoing inorganic heat-resistant oxides.

In general, the amount of catalytic cobalt metal present is 1 to 50 wt.% of the total catalyst composition, in particular 10.0 to 25 wt.%.

Iron catalysts containing 10 to 60 wt.%, especially 20 to 60 wt.% and most preferably 30 to 50 wt.% iron are unsupported but have promoters of heat-resistant metal oxide (SiO₂, Al₂O₃, etc.), alkali (K, Na, Rb) and Group IB metal (Cu, Ag). These catalysts are usually calcined, but usually not reduced, instead they are brought to reaction temperature directly in the CO/H₂ feedstock.

Generally, the amount of ruthenium present on the catalyst as a catalytic metal is in the range of 0.01 to 50 wt.% of the total catalyst composition, more preferably 0.1 to 5.0 wt.%, and most preferably about 0.5 to about 5 wt.%.

Generally, in the process of the invention, the temperature in the Fischer-Tropsch reactor is in the range of about 100°C to 500°C, especially 150°C to 300°C and most preferably 200°C to 270°C at a pressure of 100 to 10,000 kPa, especially 300 to 5,000 kPa and most preferably 500 to 3,000 kPa. The space velocity (V,) of the feed gas (H₂+CO) is in the range of 10 to 10,000, especially 100 to 4,000 and most preferably 200 to 2,000 standard cm³/h (cm³ catalyst). The space velocity (V2) of the α-olefin is in the range of 0.1 to about 20,000, especially 3 to 4,000, and most preferably 10 to 1,000 standard cm3/hr.(cm3 catalyst). Thus, the volume ratio of V1/V2 is 0.005 to 10,000, especially 0.02 to 1,000, and most preferably 0.5 to 200. The molar ratio of hydrogen to carbon monoxide in the feed gas, H2/CO, is in the range of 0.5 to 10, and preferably 1 to 3.

The reactor bed may comprise a single reactor bed of a catalyst system or it may comprise two catalyst beds containing different Fischer-Tropsch synthesis catalysts separated by an inert interstage separation zone.

The invention is explained in more detail with reference to the following description and the following examples.

Fischer-Tropsch hydrocarbon synthesis Experimental procedure

An amount of about 5 to 40 g of catalyst was placed in a stainless steel reactor with an inner diameter of 0.78 cm with a temperature sensor placed in the catalyst bed to measure the temperature. The reactor was purged with helium at a flow rate of 500 cm3/min and then pressurized with He to 30 atm to ensure the integrity of the equipment. Then the Re in the reactor was vented to 1 atm and replaced with H2 at a space velocity between 200 and 600 V/V/h at room temperature. The temperature was raised to a final reduction temperature between 400ºC and 500ºC in approximately 2 to 3 hours and maintained at this temperature overnight.

After reduction, each catalyst was operated in one of two ways, which were found to be essentially equivalent. According to one method, the reduced catalyst was treated with 2 H₂/CO at 200°C for 15 to 50 hours at 1 atm (the space velocity of the H₂/CO used was varied from 200 to 1,000 cm3 H₂+CO/h (cm3 catalyst)). The conditions in the reactor were then adjusted to the desired temperature, pressure and space velocity. According to the other method, no treatment in H₂/CO atmosphere was carried out and the catalysts were started up at the desired experimental pressure of H₂/CO but at an initial low temperature of 150 to 170°C, which was then subsequently increased (in 1 to 5 h) to the desired temperature of the experiment.

Examples

In all examples, the determination of Fischer-Tropsch synthesis activity to the various products was measured using an on-line gas chromatograph using an isothermal stainless steel fixed bed reactor (0.78 cm internal diameter) into which about 5 to 40 g of catalyst was placed. A temperature probe was placed in the catalyst bed to measure the reaction temperature.

Preparation of the Ru catalyst

Titanium dioxide supports were prepared by pressing Degussa P-25 powder (20 to 70% rutile) into wafers using a hydraulic press at a pressure of about 20,000 lb/in2 (137.9 mPa). The wafers were then crushed in a mortar to a coarse powder and sieved to obtain the 80 to 140 mesh (US standard) fraction for all TiO2 supported catalysts. The titanium dioxide support samples thus formed were calcined in air flow (about 1 liter per minute) overnight at a temperature of 550°C to 600°C. The calcined samples were then reduced in hydrogen at a flow rate of 1 L/min by heating from room temperature to 450°C for four hours, after which the sample was cooled in flowing H2, purged with He and removed. These reduced samples had the characteristic blue-gray color of surface reduced TiO2.

Catalysts were prepared by impregnating the support material with a ruthenium acetate/acetone mixture. The nitrate was obtained from Engelhardt as 10 wt% ruthenium metal in nitric acid.

The nitrate-impregnated TiO2 was reduced in flowing hydrogen (1 L/min) as follows: (a) heated from room temperature to 100°C and held for 0.5 h; (b) heated from 100°C to 450°C at 3°C/min and held for 4 h; (c) cooled in H2 to room temperature overnight and (d) purged in the reduction cell with He or Ar (1 L/min) for 1 h. Each sample of reduced catalyst was then passivated by introducing 1% O2 into the He stream and then increasing the O2 content to 100% over a period of about 2 to 3 h.

Preparation of the cobalt catalyst

Degussa P-25 TiO₂ was treated with acetone, dried at 90°C and mixed with 9.6 wt.% Sterotex (vegetable stearin). This material was tabletted to 3/8 inch (9.525 mm) using a pelleting pressure that produced a material with a pore volume of 0.35 to 0.40 ml/g. The 3/8 inch (0.525 mm) pellets were crushed and screened to 42 to 80 mesh and calcined in flowing air at 500ºC for 4 hours to burn off the Sterotex. Portions were then recalcined in flowing air at 650ºC for 16 hours to obtain a high rutile content and were screened to 80 to 150 mesh. The support composite used to prepare this catalyst was 97% rutile, 13.6 m²g, 0.173 ml/g.

Impregnation process

108.8 g of cobalt(II) nitrate (Co(NO3)2 6h2O) and 16 mL of aqueous perrhenic acid solution (52 mg Re/mL) were then dissolved in about 300 mL of acetone in a 1 L round bottom flask. 160 g of TiO2 (YAX-0487) were added and the mixture was hung on a rotary evaporator until dry. The catalyst was dried in a vacuum oven at 140°C for 18 h and calcined in air flow at 250°C for 3 h. The calcined catalyst was sieved again to remove the small amount of particulate matter generated during preparation. Preparation of the cobalt catalyst Analytical investigations of the catalyst are given below:

In addition to the quantitative indication of the rutile content, the X-ray diffraction spectrum also showed the presence of Co₃O₄.

Comparison example 1

The general procedure described for Fischer-Tropsch hydrocarbon synthesis was modified in Examples 1 and 2 as follows.

The catalyst was started in H₂/CO at a space velocity required to achieve 25 to 45% CO conversion and at 1700 to 2500 kPa. An ethylene/argon mixture was mixed with H₂/CO feed and then introduced into the reactor at the inlet in an amount of ethylene to give 2.5% and 6.2% ethylene in the feed with Co and Ru catalysts, respectively. The total pressure of the reactor was then increased to the level necessary to maintain the partial pressure of H₂ and CO in the reactor at the level they were at prior to ethylene addition. The reactor operated at these conditions for 4 to 24 hours. The ethylene/argon feed was then directed to enter the reactor undiluted by H₂+CO at a point 1/3 x L (L is the reactor length) downstream from the inlet while the space velocity and partial pressure of H₂+CO were left at their previous values. The reactor was operated at these conditions for 4 to 24 hours. The ethylene/argon feed was stopped and the reactor conditions returned to the initial conditions to exclude irreversible deleterious effects of the ethylene addition on the synthesis activity and selectivity.

* % converted ethylene that appeared as this product

** % added ethylene that appeared as this product

*** Selectivity is defined as the percentage of converted CO that appears as the given product (is almost identical to wt.%). However, when ethylene is added, some of the C atoms come from it and the sum of the selectivities is more than 100%.

11.7% Co/0.5 Re/TiO₂, 200ºC, 2050 kPa, H₂+ CO

H2/CO = 2.1/1

25 to 27% CO conversion

The addition of ethylene (C2-) at the reactor inlet decreases the CH4 selectivity and increases the C5+ selectivity. Most of the ethylene (at a concentration of 2.5%) is converted, but only 13.5% of the converted ethylene appears as desirable C3+ products, the rest (86.5%) is hydrogenated to ethane, which is unreactive to further chain growth to C3+ under Fischer-Tropsch synthesis conditions and thus cannot be recycled without converting it to ethylene in a dehydrogenation reactor.

Comparison example 2 Effect of ethylene feed ruthenium catalysts

* % converted ethylene that appeared as this product

** % added ethylene that appeared as this product

*** Selectivity is defined as the percentage of converted CO that appears as the given product (is almost identical to wt.%). However, when ethylene is added, some of the C atoms come from it and the sum of the selectivities is more than 100%.

1.2% Ru/TiO₂, 200ºC, 1 700 kPa, H₂/CO = 2.1/1, 35 to 45% CO conversion

Addition of ethylene to H2/CO feed at the reactor inlet (at a level of 6.2%) decreases the CH4 selectivity from 5.5 to 4.3, while increasing the C5+ selectivity from 87.5 to 105.6%. Most of the added ethylene (97%) is converted, with only 18% appearing as the desired C3+ products, the remainder (82%) is hydrogenated to ethane, which is unreactive to further chain growth to C3+ at Fischer-Tropsch synthesis conditions and thus cannot be recycled without converting it to ethylene in a dehydrogenation reactor.

Comparison example 3 Addition from above versus addition at the inlet Ethylene-Ru

There is a clear advantage to bypassing the reactor inlet by introducing the C2 below the inlet at the same concentration. The CH4 selectivity continues to decrease (from 4.3% to 3.9%), the C5+ selectivity increases (from 105.6% to 117.0%), and the activity of the catalyst is unaffected by the presence of ethylene either at the inlet or below the inlet of the reactor. The apparent reason for the improved product selectivity is an increase in the yield of C3+ from ethylene (the percentage of the ethylene feed that has been converted to C3+) from 17% to 26% despite the lower ethylene conversion when added below the inlet. The addition below the inlet dramatically reduces the selectivity to hydrogenation (from 82% to 44%) while increasing the C3+ selectivity (from 18% to 56%). Therefore, it dramatically increases the efficiency of ethylene utilization while avoiding the formation of species such as ethane, which cannot be further polymerized under Fischer-Tropsch conditions.

* % converted ethylene that appeared as this product

** % added ethylene that appeared as this product

*** Selectivity is defined as the percentage of converted CO that appears as the given product (is almost identical to wt.%). However, when ethylene is added, some of the C atoms come from it and the sum of the selectivities is more than 100%.

1.2% Ru/TiO₂, 200ºC, 1 700 kPa H₂/CO = 2.1/1, 35 to 45% CO conversion

Comparison example 4 Feed from above versus feed below the inlet Ethylene-Co

There is a clear advantage in bypassing the reactor inlet region by introducing the C2 below the inlet at the same concentration. The CH4 selectivity continues to decrease (from 8.3% to 7.2%), the C5+ selectivity increases (from 92.9% to 99.3%) and the activity of the catalyst is not affected by the presence of ethylene either at the inlet or below the inlet of the reactor. The apparent reason is an increase in the yield of C3+ from ethylene (the percentage of the ethylene feed that has been converted to C3+) from 13% to 27.3% despite the lower ethylene conversion. when added below the inlet. The addition below the inlet dramatically reduces the selectivity to hydrogenation (from 86.5% to 69.5%) while increasing the C3+ selectivity (from 13.5% to 30.5%). Therefore, it dramatically increases the efficiency of ethylene utilization while avoiding the formation of species such as ethane which cannot be further polymerized under Fischer-Tropsch conditions.

* % ethylene converted that appeared as this product

** % added ethylene that appeared as this product

*** Selectivity is defined as the percentage of converted CO that appears as the given product (is almost identical to wt.%). However, when ethylene is added, some of the C atoms come from it and the sum of the selectivities is more than 100%.

11.7% Co/0.5 Re/TiO₂, 200ºC, 2 050 kPa H₂ +CO, H₂/CO = 2.1/1, 25 to 27% CO conversion

Comparison example 5 Experimental procedure for ethylene addition

The catalyst was started in H₂/CO at a space velocity required to achieve a conversion of 15 to 30% CO at 2000 to 2100 kPa. The conditions were maintained for at least 48 h. Then an ethylene/argon mixture was mixed with the CO/H₂ feed and introduced at the inlet with an ethylene concentration of 2 to 7 vol.%. The total pressure in the reactor was then increased so as to maintain the partial pressure of H₂ + CO at the values it had been before the simultaneous introduction of ethylene. The reactor operated under these conditions for at least 24 h.

1.2% Ru/TiO₂, 191 to 192ºC, 2 100 kPa H₂ + CO, H₂/CO = 2.1/1, 18 to 22% CO conversion

* % converted ethylene that appeared as this product

** % added ethylene that appeared as this product

*** Selectivity is defined as the percentage of converted CO that appears as the given product (is almost identical to wt.%). However, when ethylene is added, some of the C atoms come from it and the value of these "selectivities" can exceed 100%.

The addition of ethylene (C₂=) to the H₂/CO feed at the reactor inlet reduces the CH₄ selectivity and increases the C₅+ selectivity with the Ru/TiO₂ catalyst. A large proportion of the added ethylene (concentration 7% based on the feed) is converted (73.4%), but only 49.5% of the converted C2= appears as the desired C3+ products, the remainder (50.5%) is hydrogenated to ethane, which is unreactive towards further chain growth under Fischer-Tropsch synthesis conditions and thus cannot be recycled without converting it to ethylene in a dehydrogenation reactor.

Example Experimental procedure for the addition of ethylene and water together with the feed (as co-feed)

The catalyst was started in H₂/CO at a space velocity required to achieve 15-30% CO conversion at 2000-2100 kPa. Conditions were maintained for at least 48 hours. An ethylene/argon mixture was then mixed with the CO/H₂ feed and introduced at the inlet at an ethylene concentration of 2-7 vol.%. The total pressure in the reactor was then increased so as to maintain the partial pressure of H₂ + CO at the levels prior to the simultaneous introduction of ethylene. The reactor operated at these conditions for at least 24 hours. Water vapor was then introduced at a concentration of 10 to 20 vol% by passing the H2+CO feed through a saturator maintained at 120 to 145°C while the ethylene/argon flow was kept constant. The total pressure of the reactor was increased so as to maintain the partial pressures of H2+CO and ethylene at their levels prior to the simultaneous addition of water. The H2/CO and ethylene/argon flow rates were also increased so as to maintain the CO conversion at the level prior to the addition of water. The reactor operated at these conditions for at least 24 hours. Effect of addition of ethylene and water - ruthenium catalyst

1.2% Ru/TiO₂, 191 to 192ºC, 2 100 kPa H₂+ CO, H₂/CO = 2.1/1, 18 to 22% CO conversion

* % ethylene converted that appeared as this product

** % added ethylene that appeared as this product

*** Selectivity is derived as the percentage of converted CO that appears as the given product (almost identical to wt.%). However, when ethylene is added, some of the C atoms come from it and the value of these "selectivities" can exceed 100%.

The table shows that the simultaneous supply of steam with the ethylene and with the H₂/CO feedstock offers a clear advantage. While the ethylene conversion drops from 73.5% to 38.2% with the addition of 18% H₂O, the yield decreases only slightly (from 36.3% to 33.8%). But most importantly:

ο The selectivity to the desired C₃&sbplus; products increases from 49.5% to 88.5%.

ο The rate of ethylene conversion to C₃₋ products and CO conversion increases almost threefold, in other words, the activity of the catalyst for both reactions, C₂H₄ incorporation and Fischer-Tropsch synthesis, increases threefold.

ο The rate of conversion of ethylene to ethane (i.e. hydrogenation rate) actually decreases three-fold upon addition of 18% steam to the H₂/CO/C₂= feed.

Associated beneficial effects are a decrease in CH4 selectivity (from 3.0% to 0.9%) and an increase in effective C5+ selectivity from 138.5% to 199.5%.

These results suggest that the effect of water is to increase the re-incorporation and chain growth of the added ethylene, while dramatically inhibiting its hydrogenation to undesirable ethane.

Remarks:

ο Mass expressed in pounds (1b) is converted to kg by multiplying by 0.4536.

ο Length expressed in inches is converted to cm by multiplying by 2.54.

Claims (10)

1. Process for reducing methane formation and increasing liquid yields (C5+) in a Fischer-Tropsch process for synthesizing hydrocarbons by introducing CO and H2 and at least one olefin into the feed region of a catalyst-containing reactor bed, characterized in that steam is used as a co-feed material in the process. 2. The process of claim 1, wherein the amount of olefin fed to the reactor bed is based on a molar ratio of olefin to CO in the range of 1:100 to 5:1. 3. A process according to claim 1 or claim 2, wherein the olefin(s) is/are α-olefin of the type R-CH=CH₂, where R is hydrogen or an alkyl group having 1 to 17 carbon atoms. 4. A process according to any one of claims 1 to 31, wherein the olefin comprises a C₂ to C₂₀ α-olefin. 5. A process according to any one of claims 1 to 4, wherein the catalyst comprises one or more catalytic Group VIII metals. 6. The process of claim 5, wherein the catalytic metal is selected from iron, cobalt and ruthenium. 7. A process according to any one of claims 1 to 6, which is carried out at a temperature of at least 100°C. 8. Process according to one of claims 1 to 71, in which the molar ratio of water to CO is in the range of 0.1:1 to 5.0:1. 9. A process according to any one of claims 1 to 8, wherein the molar ratio of water to olefin is in the range of 0.1:1 to 50:1. 10. The process of claim 9 wherein the molar ratio of water to olefin is in the range of 0.2:1 to 10:1.