Abstract:
There is disclosed, in one aspect, a continuous process for producing n-butanol. This process comprises (a) continuously contacting at least one carbohydrate-containing substrate, such as blackstrap molasses, with an n-butanol producing culture, such as Clostridium acetobutylicum (Weizmann), in water to effect the fermentation of the substrate and form a product mixture comprising n-butanol, (b) continuously extracting the product mixture from the substrate, culture, and water by forming a solution of the product mixture with an extraction solvent, such as monofluorotrichloromethane, (c) continuously separating the extraction solvent from the product mixture by vaporizing substantially all of the solvent without substantial vaporization of the product mixture, and (d) continuously condensing the vaporized solvent for reuse as an extraction solvent in step b. In another aspect, there is disclosed an apparatus for conducting such a process.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of copending U.S. patent application Ser. No. 076,250, filed Sept. 17, 1979, now abandoned entitled &#34;Method For Continuous Anaerobic Fermentation Of Carbohydrates, Sugars, And The Like To Produce Solvent Materials Such As N-Butanol And Apparatus For The Continuous Anaerobic Fermentation Process&#34;, the disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Normal (n)-butanol and acetone may be produced by anaerobic fermentation processes as disclosed by Weizmann. Such processes were first commercialized during the period 1914 to 1918. The cultures which were developed and discovered by Weizmann are the anaerobic bacteria Clostridium Acetobutylicum Weizmann. Using feed stock materials such as corn and horse chestnuts, these bacteria produce n-butanol and acetone in commercial yields. During World War I, the process was used primarily for obtaining acetone for the manufacture of explosives. 
     Over the years, improvements were made in the process. There were developed different Clostridium cultures which produced both better yields and different mixtures of solvents including ethanol and propanol. Different feedstock materials such as corn cobs, blackstrap molasses, beet molasses, and others were also used. Furthermore, there were employed other additives as well as nutrients which speeded up the fermentation reaction. The process was operated on a commercial scale until the middle 1950&#39;s when cheap petroleum feed stocks as well as processes for making butanol and the other solvents from petroleum became available and petroleum became the primary source for producing butanol. 
     These fermentation processes have become of greater interest of late due to the recent sharp increase and apparent long term rise in the price of petroleum, coupled with the stable price of farm product feeds for the fermentation. Fermentation products are of current commercial interest as a potential internal combustion fuel to replace the petroleum based fuels. A problem exists, however, with the Weizmann fermentation process because the maximum concentration of butanol is of the order of 2.5% before the bacteria are inactivated. The distillation recovery methods used to separate these solvents requires a tremendous amount of energy. 
     The search has continued for improved processes and apparati for producing n-butanol on a continuous, economical basis. This invention was made as a result of that search. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Accordingly, is is a general object of the present invention to avoid or substantially alleviate the above-noted problems. 
     A more specific object of the present invention is to provide a continuous process for the production of butanol and other solvents by the anaerobic fermentation of substrates such as sugar and starch-containing agricultural products. 
     Another object of this invention to provide an apparatus for continuously passing a fermentation liquor through a stationary culture bed. 
     It is a further object of this invention to provide a reactor structure which will allow the removal of fermentation products from the fermentation liquor during continuous processing so that the culture medium will not be inactivated. 
     Still another object of the invention is to provide means for extracting a desired product from a fermentation liquor during the fermentation process without expending substantial energy. 
     A further object of the invention is to provide a reactor system which will provide for the continuous recycle of a solvent system and fermentation liquor without requiring more than small make-up amounts of water and solvent. 
     Yet another object of the invention is to provide means for making n-butanol at low cost for use as a fuel for internal combustion engines. 
     Other objects and advantages of the invention will become apparent from the following summary of the invention and description of its preferred embodiments. 
     The present invention provides, in one aspect, a continuous process for producing n-butanol from the starting materials of an anaerobic fermentation process. This process comprises (a) continuously contacting at least one carbohydrate-containing substrate with an n-butanol producing culture in water to effect the fermentation of the substrate and form a product mixture comprising n-butanol, (b) continuously extracting the product mixture from the substrate, culture, and water by forming a solution of the product mixture with an extraction solvent while substantially avoiding the formation of a solution of the solvent with the substrate, culture, and water, (c) continuously separating the extraction solvent from the product mixture by vaporizing substantially all of the solvent without substantial vaporization of the product mixture, and (d) continuously condensing the vaporized solvent for reuse as an extraction solvent in step b. 
     In another aspect, the present invention provides an apparatus for conducting a continuous anaerobic fermentation of at least one carbohydrate-containing substrate with a culture to form a reaction product mixture. The apparatus comprises (a) means for holding the substrate and water, (b) reactor means for holding the culture, and facilitating contact between the substrate and culture, (c) means for feeding the substrate from the holding means to the reactor means for reaction with the culture to produce the reaction product mixture, (d) extraction means for contacting the reaction product mixture with an extraction solvent to form a solution of the solvent and product mixture, while substantially avoiding the formation of a solution of the solvent with the substrate, culture, and water, (e) means for vaporizing the extraction solvent while substantially avoiding the vaporization of the product mixture, and (f) means for condensing the vaporized extraction solvent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention is described further in accordance with the devices, materials, constructions, combinations, and arrangements of parts shown by way of example and illustrated in the accompanying drawings of a preferred embodiment in which: 
     FIG. 1 is an elevation view, partly in section, of a reactor for anaerobic fermentation of carbohydrate and sugar materials. 
     FIG. 2 is a section through the view of FIG. 1 which shows the general arrangement of the flow structure in the main body of the fermentation reactor. 
     FIG. 3 is an enlargement in section of the central pipe in the main body of the fermentation reactor. 
     FIG. 4 is a sectional view of the membrane wall of the main fermentation reactor body. 
     FIG. 5 is a view into the section of FIG. 4 showing the construction of a membrane support. 
     FIG. 6 is an alternative construction of the membrane wall of the fermentation reactor which is designed to reduce the resistance to flow of the fermentation liquor. 
     FIG. 7 is a plan view of FIG. 6. 
     FIG. 8 is an alternative arrangement of the extractor unit using low density extraction solvents. 
     FIG. 9 is a partial section of the main reactor body with additional membranes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the drawings, like parts are designated by like reference numerals throughout. 
     Referring first to FIG. 1, the reference numeral 10 refers to a fermentation system made in accordance with the present invention. The fermentation system comprises four main units: fermentation reactor 11, extraction unit 12, feed tank 13, and extraction separation unit 14. Reactor 11 is connected to feed tank 13 by means of pipelines 15 and 16 which are connected by pump 17. Pipe line 16 is connected into central distribution pipe 18 of reactor 11. Central distribution pipe 18 has a porous or perforated wall 19 shown in FIG. 3. This perforated or porous wall extends from liquid level 20 (FIG. 1) to reactor bottom 27 at the point designated as 21. 
     FIG. 2 illustrates that reactor 11 comprises several concentric walls. Central distribution pipe 18 is sealed to reactor bottom 27 at the point designated as 21. Membrane wall 22 comprises filter medium 23 as shown in detail in FIG. 4. Filter medium 23 is supported by structural mesh 24 as shown in detail in FIG. 5. Outer wall 26 of reactor 11 is a solid-liquid retaining wall as is reactor bottom 27. 
     Reactor 11 is connected by means of pipe line 28 to extractor unit 12. The point of entry of pipe line 28 into extractor unit 12 is on the lower portion of extractor body 30, designated as 29. Extraction unit 12 is connected by means of pipe line 31 to the extraction separation unit 14 which has a set of injector units 32 attached to standing pipe 33 which runs down unit 12. Standing pipe 33 is connected by pipe line 34 to compressor 35. Compressor 35 is connected by pipe line 36 to extraction separation unit 14. Pipe line 34 has a valved make-up port 37 for introducing additional extraction solvent. 
     Extraction unit 12 is connected to feed tank 13 by means of pipe line 38. Feed tank 13 is equipped with a valved entry 39 for new fermentation materials. Pipe line 31 which connects extraction unit 12 to extraction separation unit 14 is terminated inside separation unit 14 via nozzle section 40. Pipe line 34 which connects compressor 35 to extraction unit 12 is equipped with cooling condenser 41. 
     When the unit is in operation, feed tank 13 is filled with a solution of fermentable material 42. The fermentable materials are well known to those skilled in this art. Such materials are carbohydrate-containing and include blackstrap molasses, invert sugar, sucrose, fructose, glucose, wood sugar, xylose, beet molasses, citrus molasses, hydrol, sulfite liquors, saccharified starches, potatoes, corn, wheat, oats, and other fermentable materials well known to those skilled in this art. 
     This material is pumped by pump 17 into central distribution pipe 18 of reactor 11. Surrounding central distribution pipe 18 and within membrane wall 22 is contained fermentation liquids 43 which include the cultures that do the fermentation. 
     These cultures are also well known to those skilled in this art and include Clostridium acetobutylicum (Weizmann), Clostridium butylicum, Clostridium beijerinckii, B. butylaceticum, B. butylicus B.F., B. granulobacter pectinovorum, and others well known to those skilled in this art. Other cultures are set forth in Chapter 13 of Industrial Microbiology by Dunn (McGraw Hill, 1959), the disclosure of which is hereby incorporated by reference. 
     The filter action of filter medium 23 prevents the bacteria in the culture from passing through and retains them in the space between pipe 18 and membrane wall 22. Liquid 44 which passes through membrane wall 22 collects in the space between membrane wall 22 and outer wall 26 and then flows through pipe line 28 into extraction unit 12 at the point designated as 29. 
     Pump 17 is operated at an appropriate rate to pass the fermentation liquor 43 through reactor 11 at a rate that will cause the fermentation to proceed to a point where somewhat less than the toxification level of butanol is reached in the liquor. This level would be up to about 4%, typically from about 1.5% to about 2% by weight. The exact level would depend upon the specific culture used. 
     The liquor which enters extraction unit 12 at the point designated as 29 passes up the extraction unit. Countercurrent to the liquor flow, extraction solvent 47 passes in the form of fine droplets generated by exiting from injector units 32. As shown in FIG. 1, it is assumed that the extraction operation is conducted with a solvent which is more dense than the mixture of water, butanol, excess sugars, and other solvents. The solvents used in this invention are set forth in detail in U.S. Pat. No. 4,260,836 to Levy, entitled &#34;Solvent Extraction Of Alcohols From Water Solutions With Fluorocarbon Solvents&#34;. The disclosure of this patent is hereby incorporated by reference. A preferred solvent is fluorocarbon 11 (F-11) (monofluorotrichloromethane) which has a high solvency for butanol and a very low solvency for water. In addition, the density is high (1.464), the boiling point is approximately room temperature at one atmosphere pressure (23.8° C.), and it has a low heat of vaporization (43.5 calories per gram). These features, combined with its low flammability, make F-11 an ideal extraction solvent. 
     Extraction unit 12 may also be operated with solvents such as isopentane, which have a comparatively low density. Isopentane is a preferred solvent in this regard in that it has a specific gravity of 0.621, a boiling point of 28° C., a heat of vaporization of 88.7 calories per gram, and good solvency for n-butanol and the other products produced by the fermentation. The main difference vis-a-vis solvent density in the extraction unit operation is that, for a low density solvent such as isopentane, the solvent would rise to the top of the column so that the entire unit would be essentially inverted. This is shown in FIG. 8 where the corresponding parts of the inverted extraction unit 12a are numbered 14a, 29a, 30a, 31a, 32a, 33a, 34a, 35a, 36a, 37a, 38a, 40a, 41a, 47a, 48a, 49a, and 50a. 
     Two differences result from the use of isopentane rather than F-11. First, the heat of vaporization of isopentane is somewhat greater than that of the F-11. Second, isopentane is highly flammable which makes the unit more difficult to operate safely. The relative cost of isopentane is presently much lower than that of F-11 so that the use of isopentane may be advantageous with larger units which require large amounts of solvent material. 
     Monofluorotrichloromethane and isopentane are merely illustrative of the solvents that may be used in this invention. Other solvents that have the appropriate solvency action and that use relatively small amounts of heat for vaporization may also be used. Furthermore, mixtures of two or more solvents may also be used. Mixtures of solvents may be devised with particular properties such as solvency action. 
     Referring again to FIG. 1, compressor 35 pumps extraction solvent 47 into extractor injection units 32 and pulls a reduced pressure in the extraction separation unit 14 which vaporizes extraction solvent 47 at nozzle 40 to deposit mixture 48, which contains n-butanol in the bottom of unit 14 where it is periodically removed via pipe line 50 and valve 49. Condenser 41 cools the extraction solvent vapors so that solvent 47 is delivered to standing pipe 33 as a liquid. 
     The extracted liquor passes out of the top of extractor unit 12 into pipe line 38 to feed tank 13 where additional feed material is added to the liquor via valve 39. Feed tank 13 is equipped with mixer 53 which has agitators 54 to conveniently mix recycled liquor 42 with the new feed material. Carbon dioxide and other gases are generated by the fermentation. These gases are controllably exhausted from reactor 11 by means of pipe line 51 and control pressure valve 52. Reactor 11 is equipped with drain 55 in order to selectively or completely remove the contents thereof. 
     Compressor 35 is operated at a rate sufficiently high to provide enough extraction solvent 47 to completely remove all of the butanol from the liquor. This rate is dependent upon the pass rate of liquor through the reactor system. 
     The cylindrical design of reactor 11 is advantageous in the continuous fermentation process. One advantage results from the fact that the velocity of recycled liquor 42 is highest when it enters region 43 where the culture is present. At that point in reactor 11 there is little fermentation product to inhibit the fermentation and reduce the fermentation rate and the concentration of the substrate is highest vis-a-vis any other point in reactor 11. Thus, reaction proceeds at a high rate. As the liquor moves outward, the velocity decreases, the cross section of available culture increases, and the reaction is continued at a high rate because of the longer residence time and the larger number of bacilli operating on the liquor which compensates for the inhibiting action of the fermentation products produced. The result is an efficient fermentation conversion. 
     There is another advantage to the preferred cylindrical shape of reactor 11. At the point where the liquor passes through membrane filter 22, the velocity is very low compared with the entrance velocity at central distribution pipe 18. This avoids the difficulty of clogging filter medium 23 by having the fluid pass through at high rates. 
     Clogging may also be reduced by the use of a scraper unit against the inner wall of membrane wall 22 and also by the introduction of a pulsating pressure at sonic to ultra sonic frequencies into space 46 by the use of conventional transducers. The problem associated with clogging of filter medium 23, i.e., decreased flow through filter medium 23, may also be overcome by increasing the area of the membrane wall. One means of accomplishing this is illustrated in FIG. 7 where membrane wall 22 is pleated at points 56 and 58. This pleated structure substantially increases the area of the membrane wall and reduces the pressure drop caused by the flow. This construction may be used with parallel plate constructions of the reactor that would be suitable for some applications. 
     Certain culture variations of the bacteria act at different stages in the fermentation process for converting the carbohydrate-containing substrate to butanol. In order to improve the rate and efficiency of operation of the fermentation unit, it may be desirable to segregate these different types of cultures in reactor 11. This may be accomplished by employing membranes similar to membrane 22, but having different pore sizes to suit the particular circumstances. This arrangement is shown in FIG. 9 by membranes 57. 
     The apparatus used in the fermentation system described herein may be made with a variety of materials. These materials should not be affected by the fermentation process or by the fermentation products. Reactor 11 and other equipment may be made, for example, from steel, stainless steel, aluminum, polyvinyl chloride plastic, polyethylene, and other resistant polymers. The porous pipe may be made from sintered stainless steel, bronze, aluminum, or other materials known to those skilled in this art. The membrane filter material may be made, for example, from cellulose fiber and may be essentially a filter paper. It may also be made from a synthetic fiber paper using polyester or polypropylene fibers. Furthermore, certain filter membranes are made of polyvinyl chloride and of polypropylene. These membranes are made with pores in the range of from about 0.1 to about 2 microns, which is the normal size range for the bacteria spores. Other filter materials, well known to those skilled in this art, may also be used as the filter material. The particular pore size of the filter material depends upon the culture employed and may be readily determined by those skilled in this art. 
     The materials of construction for the other portions of the apparatus of the fermentation system described herein are metals which are conventionally used in chemical processing equipment. 
     Other additives may be used in reactor 11 in order to, for example, adjust the pH of the system to enhance fermentation. Typical additives to control pH include calcium hydroxide, calcium carbonate, and dilute solutions of sulfuric acid. The usual amount of calcium carbonate is from about 5 to about 7 percent by weight of the substrate. 
     Nutrients, such as amino acid-containing substances, may also be added to reactor 11. Ethyl grain, butyl grain, and molasses stillages may be used to buffer the fermentation mass and serve as a source of nutrients. The use of stillage results in better yields. Other additives to increase yield and control the fermentation rate are set forth in Chapter 13 of Industrial Microbiology (McGraw Hill 1959). 
     The fermentation reaction requires certain conditions. The optimum fermentation temperature is 31° C. Although the pH may vary from about 5 to about 7, the initial pH is usually from about 5.5 to about 6.5 while the final pH is typically from about 5.2 to about 6.2. Since the fermentation is anaerobic, it may be advantageous to maintain sterile carbon dioxide at a pressure until the bacteria have had an opportunity to build up a pressure of their own. 
     The cultures in reactor 11 are preferably contained in a fixed bed. The tendency of n-butanol in sufficiently high concentrations to inhibit the fermentation reaction is reduced by the presence of the filter member which effectively removes the n-butanol from the culture shortly after production. 
     The present invention is further illustrated by the following example. All parts and percentages in the example as well as in the specification and claims are by weight unless otherwise specified. 
     EXAMPLE 
     Maize or other similar carbohydrate material is ground to a coarse meal which is then mixed with sufficient water to give approximately an 8% by weight concentration of maize. If desired, the corn germ and a portion of the bran may be removed prior to mashing. The resulting mixture of maize meal and water is introduced into pressure cookers provided with suitable agitators, where it is heated with live steam at approximately thirty pounds pressure for two hours. This operation serves the double purpose of thoroughly sterilizing the mash and at the same time converting the starch of the maize into a form more easily acted upon by the bacteria. 
     Two &#34;cooks&#34; of mash consisting of about 5,600 gallons each, prepared in the manner described above, are cooled to approximately 37° C. and then introduced into reactor 11 which has previously been thoroughly sterilized. The mash is next inoculated with a culture of butylacetonic bacillus, preferably Clostridium acetobutylicum (Weizmann). The amount of inoculum may vary, but it is generally preferred to use about 2% by volume. At subsequent intervals of about four hours, additional mash is added in lots of either one or two &#34;cooks&#34; (consisting of about 5,600 gallons each) until reactor 11 contains a total of seven &#34;cooks.&#34; It is thus seen that twelve hours or more time must elapse before reactor 11 reaches its maximum fermenting capacity. A normal fermentation is usually completed in approximately 52 hours from the time of inoculation. During the course of the fermentation, abundant quantities of hydrogen and carbon dioxide gases are liberated and solvents in approximately the ratio of six parts of n-butanol, three parts of acetone and one part of ethyl alcohol are formed. At the conclusion of the fermentation, the product mixture is extracted as discussed in detail hereinabove. 
     Further details of the fermentation process, including substrates, cultures, etc. are set forth in U.S. Pat. No. 1,875,536, the disclosure of which is hereby incorporated by reference. 
     The principles, preferred embodiments, and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in this art without departing from the spirit of the invention.