WO2003083013A1 - Fischer-tropsch synthesis using industrial process off gas feedstreams - Google Patents

Abstract

A process is described in which industrial waste gases such as the waste gas from the production of acetylene are employed in the FT synthesis of liquid hydrocarbons. The process includes the steps of collecting the waste gas, compressing or expanding it to the proper pressure, adjusting the waste gas composition as needed to make it suitable for FT synthesis, passing the compressed and adjusted gas into a reactor containing a FT catalyst under the proper conditions of temperature, pressure, and space velocity, and collecting the liquid products thereby formed from the waste gas stream.

Description

FISCHER-TROPSCH SYNTHESIS USING INDUSTRIAL PROCESS OFF GAS FEEDSTREAMS

FIELD OF INVENTION The present invention relates to a process for synthesizing hydrocarbons from industrial waste gas streams such as Acetylene Off-Gases (AOG) .

BACKGROUND OF THE INVENTION Fischer-Tropsch (FT) synthesis of hydrocarbons has been used as a commercial process since the 1950' s. In a variety of guises, it is used to convert hydrogen (H₂) and carbon monoxide (CO) gas to hydrocarbons. Typically the products from FT synthesis are standard temperature and pressure (STP) liquids. Gases and waxes are also produced. The history of FT synthesis is elucidated in the following works:

"State of the Art in GTL Technology, Report issued by Joe Verghese, Vice President Technology Oil and Gas, ABB Lummus Global, 1998;

FT Technology, report issued by the investment firm Howard, Weil, Labouisse, Friedrichs of New Orleans, author Arthur W. Tower II, Dec. 18, 1998; FT Synthesis in the Liquid Phase, Kolbel et . al , Catal. Rev. Sci . Eng. 21(2), 225-274 (1980);

FT synthesis in the slurry phase, Schlesinger, M.D. efc al . , Industrial and Engineering Chemistry, 43(6) 1951 pp. 1474-79.

These works and the references contained therein are incorporated by reference in their entirety herein.

FT synthesis converts H₂ and CO into a wide boiling- point range of hydrocarbons. The H₂ and CO (synthesis gas) can be produced from a variety of carbon-bearing feedstocks and the resulting high-quality crude oil can be further processed to specific boiling-point fractions. Of special interest is the diesel fuel fraction because it requires little processing from the FT crude oil and it has desirable characteristics including very low sulfur and aromatic content, plus a high cetane index, and it burns exceptionally cleanly in a compression- ignition engine.

FT technology was invented in Germany in the 1920 's and supplied that country with its liquid fuels during World War II. Since that time, interest in the technology has come and gone, generally in phase with increases in the cost of crude oil or supply restrictions. Two of the first plants in the U.S. were the Carthage Hydrocol Plant in Brownsville, TX in the late 1940' s (See Keith "Gasoline from Natural Gas," Oil and Gas Journal, June 15, 1946, pp. 104-111) and the U.S. Bureau of Mines plant in Louisiana, TX in the early 1950's (See Linz, "Synthesis Test," Oil and Gas Journal, Aug. 31, 1950, pp. 42-43, and Kastens, et al . , "An American Fischer-Tropsh Plant," Ind. & Engr. Chem. , March 1952, pp. 450-466) .

A recent peak in FT interest appears to have been stimulated by several factors including environmental issues and the resulting interest in clean-burning liquid fuels, a desire for fuels derived from secure domestic feedstocks, interest in exploiting stranded or associated gas resources and heavy oil residues, among others. The 1998 investor research report by Howard, Weil, Labouisse, Friedrich of New Orleans covers the history and background of FT as well as current efforts in the field.

Generally, conventional FT synthesis employs a hydrocarbon feed stock having an undesirable characteristic to make synthesis gas containing CO and H₂ which is then passed over a FT catalyst which forms hydrocarbons with more desirable characteristics. Thus, for example, coal, which is unsuitable in its mined state for use as a motor fuel, can be converted into synthesis gas by oxidizing it under controlled conditions in the presence of water. This produces synthesis gas (primarily CO and H₂) , which is then used to produce hydrocarbons which are liquids under STP conditions and thus may readily be employed as motor fuel. In this manner the coal with its undesirable physical characteristics is converted into gasoline, kerosene, and diesel fuel, hydrocarbons having more desirable characteristics for engine fuels than the coal from which they were synthesized. But FT synthesis can normally only be carried out with feed streams that are carefully optimized for the process. Also, conversion of a hydrocarbon feedstock to synthesis gas and back to hydrocarbons does not have the thermal efficiency required to make it economically viable unless the FT catalysts have high carbon conversion ratios.

Additional inefficiencies are inherent in the energy waste of synthesis gas production.

The present invention notes that there are certain industrial processes which produce off gases that are particularly suited for conversion to liquid hydrocarbons, using processes specified in U.S. Patents Numbers 5,763,716, 5,645,613, 5,543,437, 5,506,272, and 5,324,335, all of which are incorporated by reference in their entirety herein. All these patents are assigned to Rentech, Inc., as is the present application. A major portion of a typical FT synthesis plant is dedicated to the necessary conversion of hydrocarbon feedstocks to "synthesis gas" upon which the FT catalyst may operate. In this first step, the feedstock is then converted to a mixture of H₂ and CO. This often requires extensive treatment to adjust the synthesis gas stream to a composition compatible with the requirements of the catalyst and operating conditions employed. This step may be eliminated by employing the waste gas streams from industrial processes which have compositions amenable to FT reactions. Such a scheme provides for improved efficiency in feedstock utilization since it eliminates the synthesis gas production step and adds value to a gas stream which would otherwise be discarded as a waste byproduct of the particular process being carried out .

Typically, the apparatus for synthesis gas preparation amounts to about two thirds of the cost of a FT plant. Utilization of waste gas streams often requires some pre-Fischer Tropsch reactor processing to render the waste gas usable in FT synthesis, but at a fraction of the cost of the synthesis gas preparation section of a typical FT plant.

The FT synthesis scheme of the present invention utilizes FT technology which can accept a wide range of variable conditions such as are disclosed in the above patents and U.S. Patents Numbers 5,621,155 and 5,620,670, which are totally incorporated by reference herein. This FT synthesis technology can be used in conjunction with waste gas streams produced in the production of acetylene by quenching a partially oxidized natural gas stream, such as that described in U.S. Patent No. 5,824,834, which along with the references contained therein is totally incorporated by reference herein. Applicants are presently unaware of any operating units subjecting AOG or related tail gases to FT reactions producing hydrocarbons.

SUMMARY OF THE INVENTION

One aspect of the present invention is the utilization of industrial waste gas streams containing H₂ and CO by converting what amounts to a crude synthesis gas to valuable liquid hydrocarbons.

A specific aspect of the present invention is a process for employing the waste gas of an acetylene plant to produce liquid hydrocarbon stocks which can be employed as motor fuels, thus adding value. Another aspect of the present invention is a process for optimizing carbon utilization of the carbon content of a acetylene production plant waste gas stream in the production of commercially useful hydrocarbons, including chemical feedstocks and waxes. Yet another aspect of the present invention is to efficiently and cost-effectively utilize the waste gas stream of an acetylene production plant in FT synthesis. In accordance with the present invention, a process for converting industrial waste gases to liquid hydrocarbons is provided, comprising steps of: a) collecting waste gases comprising H₂ and CO; b) optionally, compressing or expanding the collected waste gases to a pressure suitable for FT reactions; c) pretreating the waste gases to produce a FT reactor feed gas composition and proportions suitable for FT reactions; d) passing the compressed FT reactor feed gases through at least one FT synthesis reactor containing a FT catalyst under reaction conditions suitable for the production of hydrocarbons; and e) collecting liquid hydrocarbons from the products formed. In the processes of the invention, the pretreatment of the waste gases can be employed to adjust the molar hydrogen/CO ratio (H₂/CO ratio) in the feed gases provided to the FT reactor, and/or to remove at least a portion of at least one of the gases oxygen, acetylene, olefins and sulfurous gases. The FT reactor feed gas may comprise a maximum of 20 mole percent carbon dioxide, and the molar H₂/C0 ratio can range from about 0.5 to about 3.5.

A particularly suitable industrial waste gas is an acetylene off-gas, comprising from about 40 to about 80 mole percent H₂ and from about 15 to about 50 mole percent CO. The molar H₂/C0 ratio of the gas is preferably from about 0.7 to about 3. The waste gas can have any pressure, but the gas pressure should be adjusted for use as FT feed. The pressure of the FT reactor feed gas can be at least 200 psia, preferably from about 200 to about 450 or 500 psia, and most preferably from about 250 to about

400 psia. With iron-based catalysts, a preferred range is from about 300 to about 400 psia.

The catalyst used in the FT reactor can be based upon cobalt, ruthenium or iron, or combinations thereof, but is preferably an iron-based catalyst . When using an iron-based catalyst, the molar H₂/CO ratio of the FT reactor feed can be in the range of from about 0.6 to about 3.5, preferably from about 1.2 to about 1.8, while catalysts using cobalt or ruthenium permit a range of from about 1.8 to about 2.2.

When multiple FT reactors are used, they can be connected in series and/or parallel to suit the available feed gases and catalysts; with cobalt catalysts, series reactors may serve to increase carbon conversion.

As part of pretreatment , the compressed or expanded waste gases can be hydrogenated or hydrotreated before entering the FT reactor. The gases can also be subjected to conventional water-gas shift (or reverse shift) reactions or addition of H₂ from the system and/or external sources to adjust the molar H₂/C0 ratio. The FT reactor tail gases can be utilized in several ways to improve efficiency, including recycling at least a portion to the FT reactor, preferably via a compression step; and separating H₂ for export for sale or for feed in hydrogenation and product upgrading steps such as hydrotreating, hydrocracking, olefin saturation, pre- reforming, and steam methane reforming.

Other aspects, objects and advantages of the present invention will become apparent upon review of the following detailed description, the appended claims and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Before explaining the disclosed embodiments of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of the particular arrangements shown, since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. The present invention is defined and limited only by the appended claims .

FIGURE 1: A schematic diagram of an acetylene production plant coupled to a FT processing loop for utilization of the acetylene production waste gas stream. FIGURE 2: A schematic diagram elucidating the steps involved in the "Tail Gas Recycling" loop specified in the FT processing loop of Figure 1. FIGURE 3: Two Examples of the Production of Hydrocarbons From Typical Acetylene Off Gas Streams With and Without Tail Gas Recycle. FIGURE 4 : Two Further Calculated Examples of the

Production of Hydrocarbons From Acetylene Off Gas Streams Under a Variety of Conditions, Without Tail Gas Recycle.

FIGURE 5: One Additional Calculated Example of the

Production of Hydrocarbons From Acetylene Off Gas Streams, Without Tail Gas Recycle, and One Calculated Example of Production of Hydrocarbons from a Steel Mill Off-Gas Stream, Without Tail Gas Recycle, but with very high CO content, applying the water-gas shift reaction to adjust the H₂/CO ratio . FIGURE 6: Calculated examples of the Production of Hydrocarbons from Industrial Off-Gases, one example for a gas that does not require adjustment of the H₂/CO ratio, another example for a gas very rich in H₂, applying the water-gas back-shifting reaction by adding C0₂ to adjust the H₂/CO ratio. FIGURE 7: Calculated example of the Production of

Hydrocarbons from an Industrial Off-Gas with high H₂, applying a H₂-membrane to remove H and adjust the H₂/CO ratio. FIGURE 8: A schematic diagram of a steel mill coupled to an FT processing unit for utilization of the steel mill off gas stream, increasing a low non-optimum H_2/C0 ratio by applying a shift reactor and subsequently removing excess H₂0 and C0₂. FIGURE 9 : A schematic diagram of a generic industrial processing plant coupled to an FT processing unit for utilization of an industrial off gas stream, with optimum H₂/CO ratio as delivered. FIGURE 10: A schematic diagram of a generic industrial processing plant coupled to an FT processing unit for utilization of an industrial off gas stream, reducing a high non-optimum H₂/CO ratio by applying the back shift reaction and a separator to knock out the reaction water. FIGURE 11: A schematic diagram of a generic industrial processing plant coupled to an FT processing unit for utilization of an industrial off gas stream, reducing a high non-optimum H₂/CO ratio by removing H₂. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Although the present invention will be discussed in terms of converting off-gases from acetylene production plants to liquid hydrocarbons, it can be applied to any industrial waste gas containing reasonable amounts of H₂ and CO (e.g., at least twenty mole percent combined H₂ and CO) and a wide H/CO range (e.g., in principle a H₂/CO ratio in the range of from about essentially zero to nearly infinity) . Such gases may need to be compressed, expanded, heated, cooled or otherwise processed to optimize their properties for reaction in the FT reactor. Proportions of gaseous ingredients will normally be expressed in mole percentages. The term "and/or" may be used in the conventional sense, i.e. "A and/or B" meaning that either A or B may be present alone, or they may both be present.

With reference to Figure 1, the production of acetylene, as disclosed in U.S. Patent 5,824,834, which is incorporated herewith in its entirety, is shown as the upper process schematic, bracketed and labeled

"acetylene synthesis" (2) . As discussed in detail in that patent and well known in the art, such processes apply partial oxidation of hydrocarbons such as natural gas (4) with oxygen (6) in a reactor 10, after preheat (7) and mixing (8) , then quench the combustion products (12) and separate (14) an acetylene product 16.

The present invention takes the waste gas 18 directly from the production of acetylene, shown as an arrow labeled "AOG" emerging from the upper schematic box 14 labeled "Product Separation". Currently (20) this gas is typically diverted to the production of methanol (21) , or else it is stripped of H₂ and CO for other uses or burned as fuel . These uses can be characterized as being of relatively low value. In the present invention, this stream can be compressed (22) or otherwise treated to bring the pressure of the waste gas stream to at least about 250 psig, preferably from about 250 to about 400 psia, then introduced to an FT reaction scheme 26 such as is specified in any one of U.S.

Patents Numbers 5,324,335, 5,620,670, 5,763,716, and 6,068,760, which are all totally incorporated by reference herein.

A simple hydrogenation unit 24 may be required to remove trace compounds from the AOG, such as unsaturated hydrocarbons and residual oxygen. The FT liquid hydrocarbons 30 which can be produced are of much higher value than the methanol and other byproducts currently obtained. The FT reactor can be a conventional fixed bed reactor as described in many of the publications cited above, and in Dry, "The FT Synthesis," Chapt . 4 in Catalyst Science Technology, Anderson & Bowland, eds . , Vol. 1, Springer-Verlag, Berlin-Heidelberg-New York

(1985) . The reactor is preferably a slurry reactor as disclosed in Rentech's U.S. Patent No. 5,620,670, which is incorporated herein by reference. The catalyst can be a cobalt or ruthenium-based catalyst as described in the Rentech patents cited above, and disclosed in Exxon's U.S. Patent No. 5,348,982. Preferably, the catalyst is an iron-based catalyst as disclosed in the above Rentech patents, particularly U.S. Pat. No. 5,504,118, and is prepared as described in the latter patent, which is incorporated herein by reference.

These catalysts are preferably copper-potassium promoted iron catalysts of precipitated form. Iron-based catalysts in general, and the Rentech versions of U.S. Pat. No. 5,504,118 in particular, will accommodate broader ranges of molar H₂/C0 ratios in the feed gas entering the FT reactor, i.e., from about 0.6 to about

3.5. Cobalt-based catalysts typically require ratios in the range of from about 1.8 to about 2.2, and are limited by availability and price. Ruthenium-based catalysts may accommodate feed ratios in the range of from about 1.8 to about 2.2. The pressure of the feed gas entering the FT reactor should be in the range of from about 250 to about 400 psia, and the space velocity within the reactor can range from about 1.5 to about 4 Nl (H₂+CO) /hr/gram catalyst. The reactor temperature can range from about 220 to about 270 deg. C. The catalyst particles can range in diameter from about 1 to about 100 microns, and the catalyst concentration (in the slurry) can be in the range of from about five to about forty weight percent.

The AOG 18, which can include off-gases from a methanol synthesis step 21, can be compressed (22) and/or hydrogenated (24) as necessary to optimize the synthesis gas characteristics for FT reactions. Section 24 can be employed for additional pretreatments as necessary. For example, waste gases with low H₂/CO ratios can be adjusted to a H₂/C0 ratio compatible with the FT synthesis feed requirements by means of water-gas

shift pretreatment , using the reaction CO + H₂0 ^ C0₂ +

H₂. (See Example 6.)

Waste gases with high H₂/C0 ratios can be adjusted to a H₂/CO ratio compatible with the FT synthesis feed requirements by means of water-gas back-shift reaction pretreatment, as long as the waste gas contains sufficient C0_2/ or C0₂ can be added as needed. (See Example 8.) Alternatively H₂ may be removed selectively to lower the H₂/CO ratio by means of membrane or PSA (Pressure Swing Absorption) technology. (See Example 9.) The resulting processed gases 25 (the FT reactor feed) then enter the FT reactor 26, where products including liquid hydrocarbons 30 are synthesized. Exothermic heat of reaction from the FT unit generates steam (not shown) , which can be sent to a steam turbine if a combined cycle power plant is employed. FT overhead product is also cooled (not shown) .

The FT reactor is preferably a slurry reactor. In a slurry reactor, finely divided catalyst particles are suspended in a liquid hydrocarbon wax medium. Synthesis gas is introduced into the bottom of the reactor and bubbles up through the wax/catalyst slurry. Temperature control for the exothermic reaction is provided by cooling tubes suspended in the slurry and the resulting heat removed is usually used to generate steam for process or power requirements. The slurry reactor is attractive because of its low cost and excellent temperature control and catalyst inventory control (See e.g., Riekert, et al . ,

"Comparison of FT Reactors, " Chemical Engineering

Progress, April, 1982, pp. 86-90). See also Dry, supra . Hydrodynamics plays a critical role in a slurry reactor via bubble diameter, catalyst distribution, and mass transfer (See Inga et al . , "Hydrodynamic and Mass Transfer Characteristics in a Large-scale Slurry Bubble Column Reactor, " presented at 15^th Annual International Coal Conference, Pittsburgh, PA, Sept. 1998) .

Therefore, the design of the synthesis gas distributor, reactor internals, and the reactor operating parameters are critical to successful operation. Typically, empirical correlations have been used to estimate these design parameters although recently there have been successful attempts to predict gas holdup and backmixing by numerical methods, e.g. Sanyal et al . , "Numerical Simulation of Gas-Liquid Dynamics in Cylindrical Bubble Column Reactors", submitted to GLS '99: 4th International Conference on Gas Liquid and Gas-Liquid- Solid Engineering, Delft, The Netherlands, August 23-25, 1999.

Various FT catalysts containing at least one (i.e., combinations) of the metals iron, cobalt and ruthenium can be used in the reactor, and suitable catalysts from this field can be selected by those skilled in the art according to the composition and expected properties of the feedstock (s) and the desired products. Some of these catalysts are disclosed in the U. S. Patents cited above. Since the feedstock can contain industrial waste gases (such as AOG) and optionally related recycled tail gases, the composition and H₂/CO ratios can vary significantly. FT tail gas recycling steps and other measures can moderate some of these variations and control the H₂/CO ratio at the FT reactor inlet to a relatively narrow range to permit the use of a variety of existing FT catalysts. However, the iron-based FT catalysts described above are particularly suitable for a variety of industrial waste gas feedstocks, including AOG, because they will accommodate relatively wide ranges of H₂/C0 ratios while maintaining relatively high carbon conversion ratios.

Iron-based FT catalysts are preferred for a number of reasons including cost, availability, disposability/ toxicity, and the ability to work with a wide range of synthesis gas H₂ to CO ratios. Because of this last advantage of iron-based FT catalysts, a wide range of feedstocks can be considered for partial oxidation and subsequent FT synthesis, including natural gas or other

light hydrocarbon gases, liquids including Orimulsion™ (an aqueous bitumen emulsion), naphtha, heavy oil, asphalt, refinery bottoms, and solids including coal, bitumen, petroleum coke and biomass. Similarly, in the present invention, such catalysts will accommodate a variety of industrial off-gases which are suitable for FT conversion or can easily be made so. The FT products thus obtained are:

• Liquid hydrocarbons comprising compounds with about five or more carbon atoms, which may be fractionated into salable naphtha, diesel and wax fractions. These hydrocarbon products, STP liquids, can also contain lighter hydrocarbons, such as butane, in solution. These products can be used for low sulfur clean fuels (e.g., diesel), or for chemical feedstocks, formulation of lubricants, drilling fluids, etc. These are all considered high value products.

• Water fraction containing oxygenates, from which salable alcohols and organic acids may be recovered.

• Tail gas, which after proper processing (e.g., steam methane reforming, carbon dioxide removal and recycle) , may be recycled for additional FT product synthesis, and/or used for recovering the contained H₂ and as fuel .

Following the FT synthesis step in Figure 1, the products 27 are then separated (28) by fractional distillation or by any one of a number of processes such as are well known to those practiced in the art. The FT product stream is typically separated into five streams in the hydrocarbon recovery unit (28) : water/oxygenates 32, hydrocarbons 30, including naphtha (C₅ to C₉) , diesel (Cio to C₁₉) and wax (C₂₀+) fractions, and overhead tail gas 37. The FT tail gas 37 can be recycled to the FT reactor, as shown at 36 and 38. The hydrocarbon fractions are collected as the primary products. In some applications, FT waxes may have little or no value. Thermal cracking or hydrocracking can be used to convert the wax stream to liquids and thus increase the naphtha and diesel streams.

The preferred products are liquid hydrocarbons (30) comprising compounds having at least about five carbon atoms, preferably from about five to about 20 carbon atoms. These liquid hydrocarbons are valuable because of their suitability for motor fuel, including diesel fuel. Such FT fuel products are expected to be in high demand due to new environmental regulations for clean fuels. The waxy fraction (C₂₀+) can be converted to a variety of waxes and oils, and other components can produce a variety of useful chemicals and intermediates.

Water and oxygenates are separated at 32 for recovery or disposal .

After the FT products are separated, the "tail gas"

37 remaining may be reintroduced into the process (discussed below) or it may be processed (34) to recover H₂ and the remaining components burned as fuel in steam or electrical power generation. The tail gas 37 can be recycled to the FT reactor 26 via compressor 36 and further processing 38 including steam methane reforming. This will help to increase the conversion ratio of carbon in the synthesis gas which is converted to liquid hydrocarbons .

With reference to Figure 3, Examples 1 and 2 show results from typical Acetylene Off Gas (AOG) streams when subjected to a FT reaction scheme under the conditions such as are described in any one of the above cited patents, both with and without the optional step of tail gas recycling (described above) . These results show that it is economical to produce hydrocarbons with increased commercial value over methanol, H₂ and/or C0₂, or steam now being produced with the AOG off gas. The results show that a high percentage of the carbon content of the feedstream is incorporated into the products of the FT reactor. Additional efficiencies are realized if the tail gas recycle loop shown in Figure 1 is used. In the lower (FT) processing stream, the tail gas 37 from the

Product Separation section 28 can optionally be used as a "Tail Gas Recycle" loop. The tail gas recycle loop for tail gas 37 begins at the product separation process 28, passing the components of the "tail gas" through a compression stage 36 and returning to the feed stream for the FT synthesis stage 26 through a processing stage 38 labeled "Tail Gas Processing." Tail Gas Processing is further elucidated with reference to Figure 2 and U.S. Patents Numbers 5,621,155 and 5,620,670, which are incorporated herein by reference. This consists of a series of steps in which the unsaturated hydrocarbons contained in the FT tail gas 37 are first subjected to hydrogenation or adiabatic pre-reforming (at the processing stage 40 labeled "Olefin Saturation or APR"). Excess H₂ emerging from this process may then be stripped off (42) and reused in the hydrotreating (HDT) /44 of the FT liquid hydrocarbon products, exported for sale, or as fuel in the steam methane reformer 46 (SMR) . Hydrotreating, both in this process and in the alternative "hydrogenation" step 24 shown at the beginning of the FT processing loop of Figure 1 is well known to those skilled in the art. The catalysts employed for hydrogenation, both in the recycle loop and as a optional first step treatment of

AOG streams may be typical hydrogenation catalysts containing metals such as Cu, Ni or Pd, such as, for

example, Haldor Topsoe ' s ST- 101™ or Synetix's

Puraspec™, SudChemie ' s C31-7 and T-2047, or BASF's R0- 20/25 and R3-15, under conditions which are well known to those practiced in the art.

Following hydrogenation or adiabatic pre-reforming

(40) and removal of excess H₂ (42) , the gas stream 43 is then mixed with recycled C0₂ 45 and steam 47, and then the treated gases 49 subjected to steam methane reforming 46. Steam methane reforming is well known in the art, and is elucidated in the book "Catalytic Steam

Reforming" by J.R. Rostrup-Nielsen (Haldor Topsoe A/S) , Springer-Verlag : Berlin, Heidelberg, New York, Tokyo

(1984) . This work is incorporated by reference herein.

See U.S. Patent No. 3,531,263, which is incorporated herein by reference. The reformed hydrocarbon stream 51 from the reformer 46 is then cooled (52) and the excess C0₂ and water are removed (52, 54) under conditions which are well known to those practiced in the art and specified in the above referenced patents. The purified stream 53 of synthesis gas, thus prepared, is then directed back into the FT synthesis reactor as described above .

Reference to Figure 3 shows the additional carbon

(listed as all of the carbon containing species in the

"tail gas" below and including CH₄ as well as oxides of carbon) which may potentially be recovered using such a recycle loop. With particular reference to Figure 3, Example 2 in comparison to Example 1 shows the additional liquid hydrocarbons which may be realized from the processing scheme under identical conditions of processing and feed stream composition when tail gas recycle is employed, thus the production of materials incorporating five carbon atoms or more is increased more than two-fold when tail gas recycling is employed.

With further reference to Figure 1, in the processing steps of the present invention (lower schematic) , the Acetylene Off Gas (AOG) 18 is shown as passing through a hydrogenation step 24 prior to passing into the FT reactor 26 (shown as the "FT synthesis step") . This stage may be employed when the feed streams of industrial waste gas contain an excessive amount of unsaturated hydrocarbons and/or residual oxygen. This hydrogenation unit, as referenced above for the "tail gas recycle loop", may be any such method as is well known to those practiced in the art.

The use of the tail gas recycle loop and/or the hydrogenation step permits greater flexibility in feed stream composition that can be tolerated by the FT process. As such, utilizing these two optional

^•processing steps permits the use of FT catalysts based on metals other than iron, such as are well known to those practiced in the art. The ranges of waste gas feed stream compositions which work well with the present invention are listed as follows :

EXAMPLES The present invention will be further illustrated by the following calculated examples, using the various types of industrial offgases tabulated below. The calculated examples are presented in Figures 3, 4, 5, 6, and 7.

Samples of Industrial Offgases Suitable for FT Synthesis :

Possible Pre-treatment requirements before FT synthesis: Examples 3-5: Hydrogenation of 0₂ content may be required.

Example 6 : Requires adjustment of H₂/C0 ratio prior to FT synthesis; may also require reduction in C0₂ content, depending upon FT catalyst system. Example 7 : Depending on C0₂ level and FT catalyst system may require reduction in C0₂ content.

Examples 1-5 are AOG-related streams, Ex. 3 having the same composition as Ex. 1 of U.S. Patent No. 5,824,834 for acetylene production.

Example 6 employs an industrial off-gas from a steel mill, as illustrated in Figure 8.

Examples 7, 8 and 9 use industrial off-gas related streams . Calculated examples are provided in Figures 3 to 7 , and illustrated in Figures 1, 2 and 8-11. These examples (1, 2, 3, 4, 5, 6, 7, 8 and 9) illustrate the performance of the process over a wide range of molar H₂/CO ratios, and the versatility of the iron-based catalyst under such conditions. Example 6 illustrates the water-gas shifting adjustment (41) of an off-gas with very low H₂ content, as illustrated in Figure 8.

Example 7 is an example of an off-gas only requiring C0₂ removal (35), as illustrated in Figure 9. Examples 8 and 9 illustrate off gases with very high H₂ content requiring adjustment; Example 8 by water-gas back- shifting (Figure 10, 48) , and Example 9 by H₂ removal using a H₂ membrane as illustrated in Figure 11 (50) . Figure 8 illustrates the use of a steel mill off gas which is treated to provide a feed gas for a FT reactor. The off gas 17 is first compressed (22) and the compressed gas 19 heated (7) before entering water- gas shift reactor 41 for adjustment of the H₂/C0 ratio by the exothermic reaction CO + H₂0 = C0₂ + H₂. The shifted gas 29 is then cooled (39) and the cooled, shifted gas 31 is directed to the separator 33 where water is extracted. The dewatered gas is then passed through C0₂ removal section 35 where C0₂ is removed, and the resulting gas 43 is again heated (7) before entering the FT reactor 26. The remainder of the process is substantially as shown in Fig. 1. The calculated Example 6 in Fig. 5 is modelling the flow scheme in Fig. 8, where a steel mill off gas is supplied at low pressure (slightly above atmospheric) and with a very low H₂/CO ratio.

Figure 9 illustrates the use of off gases 17 from generic industrial processes as feed for a FT reactor, after preliminary processing. After compression (22), the gas 19 is passed through C0₂ removal section 35 for the removal of C0₂, then heated (7) and passed to FT reactor 26. The remainder of the process is substantially as shown in Fig. 1. The calculated Example 7 in Fig. 6 is modeling the flow scheme in Fig.

9, which does not adjust the H₂/CO ratio.

Figure 10 illustrates an alternate method of utilizing off gases 17 from a generic industrial process as feed for a FT reactor, wherein gas 17 is first compressed (22) and heated (7) before entering a water- gas back shift reactor 48 which employs heat and C0₂ to adjust the H₂/C0 ratio by the endothermic reaction

C0₂ + H₂ = CO + H₂0. The remainder of the process is

substantially as shown in Figs. 8 and 1. The calculated Example 8 in Fig. 6 is modelling the flow scheme in Fig.

10, which applies the back shift reaction to reduce the H₂/C0 ratio.

In Figure 11, a similar industrial off gas 17 is compressed (22) , passed through H₂ removal section 50 and C0₂ removal section 55 for removal of those gases, then heated (7) prior to entry into FT reactor 26. The remainder of the process is substantially as shown in Fig. 1. The calculated Example 9 in Fig. 7 is modelling the flow scheme in Fig. 11, which applies membrane technology for partial removal of H₂ to reduce the H₂/C0 ratio. Such membrane technology is commercially available from companies such as UOP (POLYSEP™) or Air Liquid (MEDAL™) . Clearly, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced in various ways other than as specifically described herein.

Claims

I CLAIM :

1. A process for converting industrial waste gases to FT liquid hydrocarbons comprising steps of: a) collecting waste gases comprising H and CO; b) optionally, compressing or expanding the collected waste gases to a pressure suitable for Fischer-Tropsch reactions; c) pretreating said waste gases to produce a FT reactor feed gas composition and proportions suitable for Fischer-Tropsch reactions; d) passing the compressed feed gases through at least one Fischer-Tropsch synthesis reactor containing a Fischer-Tropsch catalyst under reaction conditions suitable for the synthesis of hydrocarbons; and e) collecting FT liquid hydrocarbons from the products formed.

2. The process of claim 1 wherein said pretreatment of said waste gases is employed to adjust the ratio of H₂ to CO in gases fed to the Fischer-Tropsch reactor.

3. The process of claim 1 or claim 2 wherein said pretreatment of said waste gases is employed to remove at least a portion of at least one gas selected from the group consisting of oxygen, acetylene, olefins and sulfurous gases.

4. The process of any of the preceding claims wherein said FT reactor feed gas comprises a maximum of 20 percent carbon dioxide.

5. The process of any of the preceding claims wherein the molar ratio of hydrogen to CO in said FT reactor feed gas is in the range of from about 0.6 to about 3.5.

6. The process of any of the preceding claims wherein said industrial waste gas is an acetylene off- gas resulting from the production of acetylene.

7. The process of claim 6 wherein said acetylene off-gas comprises from about 40 to about 80 mole percent hydrogen and from about 15 to about 50 mole percent CO.

8. The process of claim 7 wherein the ratio of hydrogen to CO in said off-gas is in the range of from about 0.7 to about 3.

9. The process of claim 7 wherein the pressure of said FT reactor feed gas is in the range of from about

200 to about 450 psia.

10. The process of any of the preceding claims wherein said catalyst is an iron-based catalyst.

11. The process of claim 10 wherein said catalyst is a copper-potassium promoted, precipitated iron catalyst .

12. The process of claim 11 wherein said catalyst is prepared as described in U.S. Patent No. 5,504,118.

13. The process of claim 11 wherein the molar ratio of hydrogen to CO in the FT reactor feed gas is in the range of from about 0.6 to about 3.5.

14. The process of claim 10 wherein the molar ratio of hydrogen to CO in the FT reactor feed gas is in the range of from about 1.2 to about 1.8.

15. The process of claim 10 wherein the pressure of the FT reactor feed gas is in the range of from about 300 to about 400 psia.

16. The process of any of claims 1 to 9 wherein said catalyst is a cobalt-based catalyst.

17. The process of claim 16 wherein the molar ratio of hydrogen to CO in the FT reactor feed gas is in the range of from about 1.8 to about 2.2.

18. The process of any of claims 1 to 9 wherein said catalyst is a ruthenium-based catalyst .

19. The process of claim 18 wherein the molar ratio of hydrogen to CO in the FT reactor feed gas is in the range of from about 1.8 to about 2.2.

20. The process of any of the preceding claims wherein said hydrocarbons collected comprise molecules containing in the range of from about 5 to about 200 carbon atoms .

21. The process of claim 20 wherein said hydrocarbons collected include species classified as naphthas, middle distillates and waxes.

22. The process of any of the preceding claims wherein a plurality of said FT reactors are connected in series and/or parallel.

23. The process of any of the preceding claims wherein the compressed or expanded gases are subjected to a hydrogenation step prior to entering said Fischer- Tropsch reactor.

24. The process of claim 23 wherein hydrogen is separated from the tail gas of said Fischer-Tropsch reactor and employed in the FT feed gas hydrogenation step and/or product upgrading.

25. The process of any of the preceding claims wherein at least a portion of tail gas from said

Fischer-Tropsch reactor is recycled to said reactor after separation of hydrocarbons.

26. The process of any of the preceding claims wherein the catalyst of said Fischer-Tropsch reactor is contained in a slurry.

27. The process of any of the preceding claims wherein the H₂/CO ratio of the FT feed gas is adjusted by either shifting or back-shifting the gas prior to entering said Fischer-Tropsch reactor.

28. The process of any of the preceding claims wherein the H₂/CO ratio is adjusted by either adding or removing H₂ from the FT feed gas.

29. The process of claim 10 wherein at least a portion of tail gas from said Fischer-Tropsch reactor is recycled to said reactor after separation of hydrocarbons .

30. The process of claim 16 wherein at least a portion of tail gas from said Fischer-Tropsch reactor is recycled to said reactor after separation of hydrocarbons .

31. The process of claim 16 wherein a plurality of Fischer-Tropsch reactors are connected in series.