WO2025101191A1 - System and method for sequestration of bioinhibitors from a fermentation broth - Google Patents

Abstract

A system and method are provided for using semi-permeable membrane-assisted solvent extraction for sequestering a bioinhibitor from a fermentation broth containing the same, by means of a plurality of tubular membrane modules arranged in a cascading sequence wherein the tube-side material preferably flowing counter-currently with the corresponding material flowing on the shell-side of the module cascades from one module in sequence to the next module in sequence using gravity as the motive force, and the shell-side material is pumped along the exteriors of the tubular membranes contained in each of the modules. A tube-in-tube mixing vessel is associated with each module to enable a recycle loop of a portion of the tube-side material, with another portion being cascaded forward to the next module in series.

Description

SYSTEM AND METHOD FOR SEQUESTRATION OF BIOINHIBITORS FROM A FERMENTATION BROTH

FIELD OF THE INVENTION

The present invention is related broadly to systems and methods for producing organic molecules, such as ethanol, by fermentation, and in particular to systems and methods of dealing with the presence of bioinhibitors in a fermentation broth.

BACKGROUND OF THE INVENTION

On a volume basis, alcoholic fermentation represents one of the largest fields of industrial biotechnology, being used, for example, for production of traditional alcoholic beverages (wine, beer, strong alcoholic beverages, etc.) as well as industrial and fuel ethanol. The feedstock for fermentation is typically a sugar source, such as glucose derived from corn starch or sucrose from sugar beets and sugar cane, which are renewable agricultural crops.

Other alcohols are also produced or susceptible of being produced commercially (with improved process economics) by fermentation. N-butanol for example occurs naturally as a minor product of the ethanol fermentation of sugars and other saccharides and is present in many foods and drinks, being permitted as an artificial flavorant in the United States in butter, cream, fruit, rum, whiskey, ice cream and ices, candy, baked goods, and cordials.

The largest use of n-butanol currently is as an industrial intermediate in the production of varnishes, though n-butanol has also been proposed as a large-scale substitute for diesel fuel and gasoline, in that at 85 percent strength n-butanol can be used in cars designed for gasoline without any change to the engine (unlike 85% ethanol) and provides almost as much energy for a given volume as gasoline. Most n-butanol today is produced via a couple of known methods from the petrochemical propylene, though in recent years increasing efforts have been undertaken toward the development of economically viable methods of producing n-butanol by sugars fermentation from biomass.

Isobutanol is yet another alcohol that can be produced from biomass by fermentation and is attracting a great deal of commercial interest as a potential feedstock for the production of sustainable aviation fuel (SAF).

However, there are significant challenges to be addressed in respect of the production by fermentation of these alcohols.

In respect of ethanol, for example, one of the most costly components of production is the energy required for distillation of the ethanol from the aqueous fermentation broth in which it is made. Typically, the energy used for distillation is obtained from conventional fossil fuels, so that any savings that can be realized from lowering the energy costs incident to distillation will simultaneously lower the cost of ethanol production and the carbon dioxide emissions incident to its production.

One of the reasons distillation costs for ethanol production are high is that the water forming the greatest proportion of the fermentation broth has one of the highest heat capacities of the ethanol miscible liquids. Another reason is that ethanol and water form an azeotrope, making it difficult to separate the species efficiently by distillation, especially because their boiling points are only separated by 22°C.

In respect of both n-butanol and isobutanol, a different complication adversely impacting distillation costs is introduced, in that operational titers for both are limited because both are to the organisms that produce them at greater percentages. As well, in common with ethanol, it is difficult to separate n-butanol and isobutanol from the respective aqueous fermentation broths in which they are produced by distillation because of their low boiling points.

Commonly-owned United States Patent No. 10,752,875 represents one approach to solving the separation difficulties associated with recovering ethanol from a clarified fermentation broth, and provides a system and method for improving the distillation economics of producing ethanol, wherein a clarified fermentation broth from which suspended solids (such as biomass and other nonsolubilized materials such as grain particles) have been removed is supplied to a membrane module configured to receive the flow of clarified broth and employing a selectively permeable membrane to permit preferential passage of ethanol from the aqueous fermentation broth. The membrane module is described as including: a broth inlet port to receive the flow of clarified broth from the solids separation means in a first flow direction tangential to the membrane; a broth exit port to conduct a flow of solvent- extracted broth comprising residual extraction solvent from the membrane module; a solvent inlet port to receive a flow of an extraction solvent in a second flow direction tangential to the membrane; an outlet port configured to conduct the flow of extraction solvent containing the organic compound from the membrane module; and, a pressurizing means to maintain a positive pressure on the flow of clarified broth relative to the flow of extraction solvent in the membrane module. The pressurizing means may further include a vent for releasing carbon dioxide contained in the clarified broth from the membrane module. A decanter is described for receiving the flow of solvent-extracted broth from the membrane module, separating an extraction solvent phase comprising a portion of said residual extraction solvent from an aqueous phase of the solvent-extracted broth and returning the aqueous phase minus the extraction solvent phase to the fermentation vessel. Extraction solvent containing ethanol sequestered from the aqueous fermentation broth by means of the selectively permeable membrane is meanwhile distilled to recover the ethanol therein, with recycling the extraction solvent to be reused in the membrane module.

BRIEF SUMMARY OF THE INVENTION

The present invention provides in one aspect an improved system and method for using semi-permeable membrane-assisted solvent extraction (or, “MSE” for “membrane-assisted or mediated solvent extraction”) in the context of continuous processing of fermentation broths.

Direct liquid-liquid extraction has the disadvantage of being susceptible to or at least admitting of the possibility of the formation of emulsions, so MSE was proposed in commonly assigned US 10,752,875 for enabling the continuous processing of a clarified ethanol-containing fermentation broth without the formation of such emulsions.

The system and method of the present invention differ in a number of ways from the system and method disclosed in US 10,752,875, however, in that rather than employing stacked flat sheet membranes with relatively larger pore sizes operating preferably in cross-flow mode, in the present system and method a plurality of tubular membrane modules are provided in which each module contains a number of tubular semi-permeable membranes having much smaller pore sizes and arranged in a shell-and-tube configuration.

The tubular membrane modules are arranged in a cascading sequence, wherein the tube-side material, whether a preferably clarified aqueous fermentation broth containing a bioinhibitor needing to be removed therefrom (which can be, as noted previously, a desired product such as n-butanol or isobutanol that can presently only be produced at titers in the fermentation broth entailing much higher, including economically prohibitively high, distillation energy expense for recovery) or a solvent for extracting the bioinhibitor through the tubular membrane from the aqueous fermentation broth - preferably flowing counter-currently with the corresponding material flowing on the shell-side of the module (i.e., the broth if the solvent is tube-side or the solvent if the broth is tube-side) - cascades from one module in sequence to the next module in sequence using gravity as the motive force, and the shell-side material is pumped along the exteriors of the tubular membranes contained in each of the modules. A tube-in-tube mixing vessel is associated with each tubular membrane module, whereby the tube-side material from a given module - whether solvent enriched in the targeted bioinhibitor or preferably first clarified fermentation broth depleted in the targeted bioinhibitor - is well-mixed with the tube-side material from the preceding module or from a source of the tube-side material entering the cascading series of tubular membrane modules, with respect to the tube-in-tube mixing vessel associated with the first tubular membrane module to be encountered by the tube-side material, and a portion is recycled to the tube-side of the associated respective tubular membrane module while the remainder is forwarded to the tube- in-tube mixing vessel associated with the next tubular membrane module in the cascading sequence of modules.

The capabilities and advantages of an apparatus or system as thus summarized will become more evident to those of skill in the art from a consideration of the following detailed description and the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one embodiment of a cascading sequence of tubular membrane modules as employed in a system and method according to the present invention.

FIG. 2 depicts the compositions of various streams in the apparatus of FIG. 1 as a function of time, in Example 1 evaluating processing an aqueous n-butanol solution at a certain high broth proxy (n-butanol solution):solvent volumetric flow ratio.

FIG. 3 depicts the effect in Example 2 of substantially reducing the broth proxy: solvent volumetric flow ratio for the same solution, in the same system producing FIG. 2.

FIG. 4 depicts the effect seen in Example 3 of reducing the broth proxy:solvent volumetric flow ratio still further from the experiment giving rise to FIG. 3.

FIG. 5 depicts the effect seen in Example 4 of using higher volumetric flow rates for both the broth proxy and solvent compared to the experiment giving rise to FIG. 4, but the same relative volumetric flow ratio as used for FIG. 4.

FIG. 6 provides a tabular summary of the steady state flows and compositions of the experiments shown graphically in FIGS. 2 through 5.

DETAILED DESCRIPTION

Described herein, broadly, is a system and method for efficiently sequestering a bioinhibitor present within a fermentation broth, in certain embodiments preferably to such a degree whereby at least a portion of the processed fermentation broth having a reduced to a much reduced to preferably an inconsequential concentration of the bioinhibitor, but retaining virtually all other, beneficial constituent species (various bionutrients), is suitable for being recycled back with minimal further processing to the fermentation step for reuse.

In certain other embodiments, where the bioinhibitor is an objective of the fermentation in the first place, for example, n-butanol or isobutanol from sugars fermentation (which as mentioned above can be produced presently only in limited titers because of their toxicity to the microorganisms known to be useful for their production through fermentation), the system and method, independently of whether the bioinhibitor-depleted fermentation broth is recycled at least in part, provide a means for producing the bioinhibitor continuously by fermentation, extracting the desired bioinhibitor into a solvent in higher concentrations than the same can be produced in the fermentation broth because of its toxicity and indeed at concentrations whereby distillation becomes economically viable for separating the desired bioinhibitor from the extracting solvent.

The system and method in still other embodiments allow for both the extraction of a sufficiently high concentration of a desired bioinhibitive product such as n-butanol or isobutanol from the fermentation broth that the desired product may be economically recovered from the extracting solvent and the recycling at least in part of the biomass, of the processed remaining, bioinhibitive-product-depleted aqueous fermentation broth and of the extracting solvent following its separation from the desired bioinhibitve product, preferably all in a continuous mode of operation.

Those of skill in the art will appreciate, in respect of these several scenarios, that how much bioinhibitor is ideally removed and how much remains will depend for any particular context of use on a number of considerations, including but not being limited to the identity of the bioinhibitor, and particularly whether the bioinhibitor in question is desirably recovered for other applications or uses and what the technoeconomic requirements of those other applications or uses may be, the amount of bioinhibitor that is initially present in the clarified fermentation broth and the effect on the generating fermentation of having various amounts of bioinhibitor present, the capital and operating costs involved in removing enough bioinhibitor from the broth to realize certain degrees of removal, the comparative costs of not recycling a certain amount of processed clarified fermentation broth depleted in the bioinhibitor by means of the present invention and, in the circumstance where the bioinhibitor is desirably separated from the extracting solvent for other applications and uses, the capital cost and operating expense associated with making this separation.

With these observations made, however, we have considered that with respect to the removal of n-butanol from a clarified fermentation broth (as exemplified below), since we had known that concentrations of n-butanol above about 1.0 weight percent tended to slow the fermentation we were using to produce that material and a concentration of 2.0 weight percent in the broth would result in a complete loss of production, an appropriate, non-optimized target for the concentration of n-butanol in the processed broth should be about 0.3 weight percent or less and the corresponding concentration of n-butanol in the 2,6- dimethy 1-4 -heptanol solvent we contemplated and evaluated for carrying out MSE on the broth should be at least about 4.6 weight percent, to enable distillation to be economically attractive for separating the n-butanol from the solvent - and we are satisfied that we have demonstrated these objectives are readily achievable by means of the system and method we have developed.

With these introductory comments having been made to establish the ways in which the system and method of the present invention can be beneficially employed, the present invention can be more readily understood in the context of an integrated continuous fermentation process wherein a fermentation vessel is provided for containing a fermentation broth in fluid communication with a solids separation means, which receives at least a portion of the fermentation broth. The solids separation means is configured to continuously separate suspended solids (i.e., biomass and/or unsolubilized material such a grain particles) from the fermentation broth to produce a clarified broth and conduct a flow of the clarified broth out of the separation means to a cascading series of tubular membrane modules as will be described in greater detail hereafter.

The fermentation broth containing a bioinhibitor to be sequestered by means of that cascading series of tubular membrane modules is in any event preferably drawn from the fermentation vessel during the optimum production rate of the associated principally desired fermentation product (which as previously mentioned may be one and the same as the bioinhibitor targeted for sequestration according to the system and method of the present invention).

The solids separation means is configured to separate the suspended solids from the fermentation broth to form a clarified fermentation broth. Removal of suspended solids from the fermentation broth is important to prevent fouling of the selectively permeable tubular membranes in the cascading series of tubular membrane modules and enable improved extraction of the bioinhibitor/desired bioinhibitive fermentation product across the tubular membranes in the series of tubular membrane modules. These suspended solids include cell biomass, and in a dry grind process can include grain particles and other solid elements present in the fermentation broth as a result of the fermentation process.

In certain embodiments, the solids separating means are preferably configured to return all, or more preferably, a portion of the suspended solids to the fermentation vessel. By separating the suspended solids, including the fermentation organism, from the broth prior to the processing of the clarified broth in the cascading series of tubular membrane modules, and returning at least a portion of the material separated out by the solids separating means to the fermentation vessel, an optimally productive level of the fermentation organism and solid material can be maintained in the system. In this regard, the suspended solids removed from the fermentation broth typically contain some residual fermentation broth. The residual fermentation broth can be separated from the suspended solids by the solids separating means and returned to the fermentation vessel, while the separated portion of the suspended solids may be used for other purposes - such as being dried to form an animal feed ingredient.

In certain embodiments, the fermentation organism is a flocculating strain of yeast. In some other embodiments, a flocculent, preferably a divalent cation that induces yeast flocculation, such as Zn, Mg or Mn, is used in the fermentation broth. Flocculation of the fermentation organism can assist in separation of the yeast cells from the fermentation broth.

The suspended solids may be removed from the fermentation broth by any suitable means known in the art for solid/liquid separation, including, but not limited toa means for one or more of sedimentation, centrifugation, or other solids concentrating steps, such as filtration. The solids separation means preferably comprises at least one solid/liquid separation device and preferably comprises a series of solid /liquid separation devices.

In certain embodiments the solids separation means comprises a centrifuge, preferably a continuous flow centrifuge configured with a means to return at least a portion of the suspended solids containing the fermentation organism to the associated fermentation vessel. Typically, such means are a conduit which may preferably be put into fluid communication with a separate pump to return the suspended solids or portion thereof to the fermentation vessel.

In certain embodiments, the solids separation means comprises one or more filtration apparatus to separate the suspended solids from the fermentation broth and preferably return at least a portion of the suspended solids to the fermentation vessel. At least one filtration apparatus preferably comprises an ultrafiltcr, such as an ultrafilter bag. Other filtration apparatus known in the art or hereafter developed may be employed.

In certain embodiments the solids separation means comprises at least one centrifuge in fluid communication with at least one filtration apparatus. The filtration apparatus may be placed between the fermentation vessel and the solids separation means and/or between the solids separation means and the cascading series of tubular membrane modules. In some embodiments, one, two, three or more of filtration apparatus are employed in addition to a centrifuge.

The solids separation means is in any case configured to conduct a continuous flow of clarified broth still containing a bioinhibitor to be removed out to a cascading series of tubular membrane modules, an illustrative embodiment (TO) of which is schematically depicted in Fig. 1 in a mode of operation wherein the clarified broth is on the shell side of the succession of membrane modules each containing a plurality (and typically numerous) of tubular, semi-permeable membranes in a shell-and-tube configuration.

Turning now to Fig. 1, a source 12 of a suitable extraction solvent for extracting the bioinhibitor from within the aqueous clarified broth by a concentration gradient across the membranes, for example, for n-butanol or isobutanol being 2,6-dimethyl-4-heptanol or dodecane, provides a flow or stream 14 of solvent that is controlled by a flow control means 16 (e.g., a metering valve) into a central, inner tube 18 of a tube-in-tube mixing vessel 20 comprised of the inner tube 18 and an outer tube 22. The inner tube 18 extends for substantially the entire length of the mixing vessel 20, and empties into the mixing vessel 20 proximate its downstream end wherein the solvent from within the inner tube 18 is turbulcntly mixed with a solvent stream 24 received from the collective interior lumens (not shown) of the first tubular membrane module 26 in the cascading series of three tubular membrane modules (26, 28 and 30) via a back pressure valve 32 controlling the gravity-driven flow of solvent through and from the tubular membranes in the first module 26.

A recycle portion 34 of the mixed solvent from the source 12 and from that solvent that has been received from the tubular membranes in module 26 is recycled with the assistance of a pump 36 to the upstream end 38 of the module 26, setting up a recycle loop whereby a given flow of solvent is maintained in contact with the exteriors of the tubular membranes in module 26 and enabled to extract additional bioinhibitor from within the tubular membranes and attain a desired steady state concentration of the bioinhibitor within the solvent that is greater than that which would be achieved in a once-through or single pass configuration.

Meanwhile, a cascading portion 40 of the combined solvent from source 12 and the preceding module 26 is supplied to the central or inner tube 42 of the tube- in-tube mixing vessel 44 associated with the next tubular membrane module 28. The mixing vessel 44 is configured and operates in a precisely analogous fashion as the mixing vessel 20, again receiving a solvent flow 46 mediated via a corresponding back pressure valve 48 from the collective interior lumens of the tubular membranes within the module 28 and mixing that solvent flow 46 with the cascading portion 40 received from the preceding mixing vessel 20 (and thus in part from the preceding tubular membrane module 26), then forwarding its own cascading portion 50 of mixed, increasingly bioinhibitor-enriched solvent and recycling its own recycle portion 52 for being recirculated through the interior lumens of the tubular membranes in the module 28.

Cascading portion 50 from the second in series of tube-in-tube mixing vessels (44) is then supplied to the central, inner tube 54 of the tube-in-tube mixing vessel 56 associated with the third module 30 of the sequence of three cascading tubular membrane modules (26, 28 and 30). The cascading portion 58 of mixed solvent comprising solvent from the inner tube 54 and solvent 60 received from the collective interior lumens of the tubular membranes in the third module 30 is then collected in a vessel for further processing in or is preferably conveyed directly via a conduit to a solvent separation means 62 (such as a distillation column or train) for separating the accumulated bioinhibitor in the cascading portion 58 from the solvent used in the system 10 to extract the bioinhibitor from an aqueous, clarified fermentation broth containing the bioinhibitor and received from the solids separation means described at length above. Preferably, at least a portion of the solvent returned by the solvent separation means 62 is suited for recycle and reuse via source 12.

Turning now to more briefly consider the receipt of clarified broth from the solids separation means and the processing of the same on the shell side of the apparatus 10, incoming broth from a source 64 thereof - typically a continuous flow received from the associated solids separation means as described above - is conveyed as stream 66 by means of a pump 68 to the shell side of tubular membrane module 30 and flows counter currently therein along the exteriors of the tubular membranes in the module, with respect to the solvent flowing therethrough on the tube side. The broth flowing from the shell side of module 30 is communicated to the shell side of module 28, with the assistance of a further pump if needed, and then the broth from the shell side of module 28 is in similar fashion communicated to the shell side and exteriors of the tubular membranes in module 26 before finally being collected in a vessel or conveyed directly for further processing as appropriate for being recycled at least in its useful parts to the fermentation process wherein the fermentation broth was originally produced - returning useful nutrients, unfermented sugars etc. to the extent practicable.

The utility of the present invention is further illustrated by the following examples, which make use of the three-module arrangement depicted in Fig. 1:

EXAMPLES

For the counter-current flow experiments reported herein, samples were taken at four sites within the solvent stream and four sites in the broth stream (Figure 1 - S1-S4/B5-B8) of a lab-scale three module apparatus constructed as depicted in FIG. 1, at regular intervals. Each of the three, 2.5-inch O.D. by 8 inches long, Liqui- cel Cartridge™ fiber membrane modules (3M Company, St. Paul, MN) employed 9,950 Celgard polypropylene hollow fiber membranes with an average pore size of 0.04 microns and providing a cumulative exterior or shell-side contact area of 1.4 square meters and a cumulative interior or tube-side contact area internally of 1.13 square meters.

FIGS. 2 through 5 show the n-butanol concentration vs time response during startup of the experiments, where gas chromatography was used for the analysis of all collected samples (broth and solvent). Because majority water aqueous solutions tend to show some data scatter when analyzed by GC (a well-known phenomenon associated with the uneven volatilization of water as a sample is injected onto the hot platen at the entrance to a GC column), the broth concentration curves show a degree of variability over time that analysis by HPLC should successfully avoid, but we considered analysis by GC sufficient for our purposes and indeed, the broth and solvent concentration versus time response curves demonstrate that the system and method of the present invention were effective in removing n-butanol from the simple broth proxy solution used throughout (containing 2 wt percent of n-butanol and 98 wt. percent of water) and into an extracting solvent (in all experiments, 2,6- dimethyl-4-heptanol). FIG. 6 contains a tabulated summary of the n-butanol concentrations and extraction rates for all experiments.

Example 1

The first countercurrent experiment was conducted with a broth proxy feed to the tube-side of 330 ml/min and solvent feed of 25 ml/min on the shell-side (13 to 1 broth: solvent flow ratio) and with internal solvent re-circulation rates of approximately 350 ml/min in each stage. Solvent was recirculated to the shell side of each cartridge. Flow rates (ml/min) of the solvent ternary and broth exiting the system were measured by graduated cylinder and stopwatch. The n-butanol concentrations were measured by gas chromatography as previously mentioned, using an FID detector for Examples 1-4 and a TCD detector for Examples 5-7.

For this Example 1, Figure 2 shows the n-butanol concentrations measured at the four sampling sites for the broth proxy and the solvent, as the process ramped up to steady state. At steady state, the exit concentration of n-butanol in the solvent ternary (measured at port S4) was 10.1 wt.%. The n-butanol recovery rate at steady state was 108.5 g/hour. The exit concentration in the broth phase was 1.4 wt.% (measured at port B8, compared to an inlet feed concentration of 2 wt.% measured at port B5.

Example 2

Flow rates for this next experiment were adjusted to get a flow rate ratio of 3.5 to 1 (broth to solvent), to get more complete removal of n-butanol from the broth. Broth flow was 80 ml/min and solvent flow was 25 ml/min. After 4 hours, as shown by Figure 3, n-butanol concentration in the solvent reached 5.77 wt%, and the extraction rate was 69.7 g/hr. In the broth, the exit concentration of n-butanol was 0.4 wt%.

Examples 3 and 4

In the third and fourth experiments, again both with solvent to the shell sides of the cartridges, we kept the same ratio of flow rates (3 to 1) but varied the overall flows to determine that effect on transfer:

For Example 3, the broth flow rate was set at 75 ml/min, with the solvent flow set at 25 ml/min, while for Example 4, the broth flow rate was 150 ml/min and the solvent flow rate was 50 ml/min.

These examples were run back-to-back on the same day. In Example 3, as shown by Figure 4, the final butanol concentration in the solvent was 5.4 wt% and the extraction rate was 63.1 g/hr. Butanol concentration in the broth was taken down to 0.24 wt%. In Example 4, as shown by Figure 5, the final butanol concentration in the solvent was 4.6 wt% but the butanol extraction rate increased to 109.4 g/hr. Final butanol concentration in the broth was 0.31 wt%. The decrease in concentration in the solvent occurred within an hour after making the flow increases in the broth proxy and solvent. The reduction in concentration is probably an indicator that we are approaching a transfer rate limit at those flow rates.

Example 5

In this experiment, the solvent was switched tube-side and the broth proxy was flowed on the shell side of the cartridges. With flowing the solvent through the interior lumens of the tubular membranes, it was anticipated that there could be a substantial pressure drop generated at higher solvent flow rates, e.g., on the order of 500 ml/min. For a multi-stage process like this, with solvent recirculation at rates as high as 300 ml/min, there was thus some question whether there would enough pressure difference between the broth and solvent phases in all three modules, whereby the solvent pressure inside the lumens would exceed the broth pressure on the exterior of the tubular membranes. For series broth flow, we found that if the exit pressure on the module 26 was kept at 6 psig, the exit pressure on the module 28 will be about 8 psig, and the exit pressure on module 30 will be about 10 psig, without any intermediate valving.

We were able to circulate solvent to the lumen sides of the modules at approximately 300 ml/min and still maintain solvent pressures below about 4 psig on the inlet sides of the modules, so that we had at least a 2-psi margin in all three modules. A small back pressure was kept on the solvent phase in each module, such as 1 to 2 psig, to ensure that each module remained filled.

Broth flow for this Example 5 was set to 265 ml/min and the solvent flow within the lumens was 65 ml/min (for a broth to solvent ratio by volume of 4.1 to 1). The n-butanol exit concentration in the solvent was 6.19 wt%, and n-butanol was extracted at a rate of 196 g/hr, the highest extraction rate we observed. The n- butanol concentration in the broth was 1.36 wt.%.

From the foregoing it will be seen that this invention is one well adapted to attain all ends and objectives herein-above set forth, together with the other advantages which are obvious and which are inherent to the invention.

Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matters herein set forthor shown in the accompanying drawings are to be interpreted as illustrative, and not ina limiting sense. For example, those of skill in the art will recognize that similar associated mixing vessels and recycle loops may be implemented for the shell-side material proceeding through the succession of tubular membrane modules 30, 28 and 26 (in the illustrative embodiment, the clarified broth from which bioinhibitor is being progressively sequestered in an extracting solvent). As another example, while the extracting solvent has been described as the tube-side material in membrane modules 26, 28 and 30 and fermentation broth as the shell-side material, those skilled in the art will appreciate that the broth may be use as the tubeside material and the extracting solvent as the shell-side material, though generally to avoid the risk of fouling the interior lumens of the tubular membranes we consider it will be preferable to employ the extracting solvent as the tube-side material.

Further and relatedly, while specific embodiments have been shown and discussed, various modifications may of course be made, and the invention is not limited to the specific forms or arrangement of parts and steps described herein, except insofar as such limitations are included in the following claims. Further, it will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

Claims

1. A system for continuously sequestering a bioinhibitor from a clarified aqueous fermentation broth in which the bioinhibitor may be found, comprising: a cascading sequence of a plurality of tubular membrane modules, each comprising a plurality of tubular semi-permeable membranes in a shell and tube configuration; a source of clarified aqueous fermentation broth containing a bioinhibitor to be removed by means of the cascading sequence of tubular membrane modules, in fluid communication with the interior lumens or exteriors of the plurality of tubular semi- permeable membranes in a first, upstream or last, downstream tubular membrane module in the cascading sequence; a source of a solvent into which the bioinhibitor preferentially partitions, in fluid communication with the other of the interior lumens or exteriors of the plurality of tubular semi-permeable membranes in the last, downstream tubular membrane module in the cascading sequence where the clarified aqueous fermentation broth is in fluid communication with the first, upstream module, or in the first, upstream module where the clarified aqueous fermentation broth is in fluid communication with the last, downstream tubular- membrane module in the cascading sequence so that in continuous operation the clarified broth and solvent are flowing counter-currently with respect to one another and one of the two flows within the interior lumens of the tubular semi- permeable membranes in series and the other flows counter-currently in contact with the exteriors of the tubular semi-permeable membranes; a tube-in-tube mixing vessel associated with each tubular membrane module in the cascading sequence, in which a central tube for the vessel associated with a given tubular membrane module is adapted to receive from a source thereof, whether the supply of the material for the first tubular membrane module in the cascading sequence or the immediately preceding tubular membrane module in the cascading sequence, that material, whether clarified broth or solvent, that is to flow within the interior lumens of the tubular semi-permeable membranes in series and which central tube extends for substantially the length of the surrounding outer tube and concludes with one or more openings through which the broth or solvent will be placed in fluid communication with and be mixed with corresponding material, whether clarified broth or solvent, received from the interior lumens of the tubular membranes after the same has traversed the length of the associated tubular membrane module, with a recycle loop drawing a recycle portion of mixed material from the end of the mixing vessel proximate where the material from the central tube and that received from the interior lumens of the tubular' membranes from the associated membrane module are combined and returning that recycle portion to the associated membrane module, and with a means provided at the opposing end of the mixing vessel for supplying a cascading portion to the central tube of the next tubular membrane module in the cascading sequence or to a line or vessel for receiving the clarified broth or solvent from the last in the cascading sequence of tubular membrane modules; flow control means associated with the provision of material received from the interior lumens of a tubular membrane module to its associated tube-in-tube mixing vessel for being combined with material from the central tube of said mixing vessel; pump means for causing that material, whether clarified broth or solvent, that is to pass counter-currently along the exteriors of the tubular membranes in the sequence of tubular membrane modules to flow from a source of that material, whether the supply of the material with respect to the last of the sequence of cascading tubular membrane modules or material from the shell side of a preceding tubular membrane module, to an associated tubular membrane module; and conduits for communicating the shell-side material from a source thereof to the last of the sequence of cascading tubular membrane modules, then to each of the remaining tubular- membrane modules in succession and finally to a line or vessel for receiving the clarified broth or solvent from the shell-side of the final tubular membrane module to receive the shell-side material.

2. The system of claim 1, wherein the solvent is the material supplied to the interior lumens of the tubular membranes in the cascading tubular membrane modules, and the clarified broth moves counter-currently through the cascading tubular membrane modules on their shell side, along the exteriors of the tubular membranes within.

3. A method for continuously producing a fermentation product from the fermentation of sugars whose titers conventionally must be limited because of the product’s toxicity to the organism used to produce it, comprising: continuously fermenting a sugar substrate in the presence of a biocatalyst to produce a fermentation broth comprising cell mass and other undissolved solids and the fermentation product; removing cell mass and other undissolved solids from the fermentation broth to continuously produce a clarified fermentation broth; providing a system according to either of claim 1 or claim 2; continuously supplying the clarified fermentation broth and a solvent into which the fermentation product can be extracted to that system; continuously collecting solvent into which the fermentation product has been extracted, from the system; continuously collecting clarified fermentation broth from which the fermentation product has been removed at least in part by extraction into the solvent, from the system; and continuously separating fermentation product from the collected solvent.

4. The method of claim 3, further comprising recycling at least a portion of the solvent after the fermentation product removal step to the system.

5. The method of claim 3, wherein enough of the bioinhibiting fermentation product is removed into the solvent, such that at least a portion of the collected processed clarified fermentation broth is recycled back for use in the fermentation step.