WO2018185450A1

WO2018185450A1 - Butanol recovery method and apparatus

Info

Publication number: WO2018185450A1
Application number: PCT/GB2018/000057
Authority: WO
Inventors: John Dennis; Paul Hodgson
Original assignee: Cambridge Enterprise Limited
Priority date: 2017-04-04
Filing date: 2018-04-04
Publication date: 2018-10-11
Also published as: GB201705377D0

Abstract

The present invention relates to a method for producing a butanol-rich composition suitable for use as a fuel, the method comprising: (i) fermenting a feedstock in a fermenter to produce an aqueous fermentation broth comprising butanol; (ii) adding an organic solvent to the broth and performing a liquid/liquid extraction to recover a mixture of the butanol in said organic solvent; (iii) vaporising the organic solvent from the mixture to provide a butanol-rich composition; wherein the organic solvent comprises one or more alkanes and/or alkenes having a boiling point of less than 55°C at atmospheric pressure.

Description

Butanol Recovery Method and Apparatus

The present invention relates to a method of obtaining a butanol-rich composition from a fermenter. In particular, there is provided a more efficient method of obtaining butanol from an aqueous fermentation broth.

Biofuels have a long history ranging back to the beginning of the 20th century. However, petroleum-derived fuels displaced biofuels in the 1930s and 1940s due to increased supply, and efficiency at a lower cost. However, there is a desire to reduce dependence on non- renewable fossil fuels and it is now commonplace to supplement gasoline with amounts of ethanol. The use of butanol as a fuel supplement or as a fuel perse has also been considered and is desirable since it can be employed in conventional gasoline engines without requiring expensive modification. The acceptance of biofuels depends primarily on economic competitiveness of biofuels when compared to petroleum-derived fuels. Biofuels that cannot compete in cost with petroleum- derived fuels will be limited to specialty applications and niche markets.

Several factors influence the core operating costs of a carbohydrate based biofuel source. In addition to the cost of the carbon-containing, plant produced raw material, a key factor in product economic costs for ethanol or other potential alcohol based biofuels, such as butanol, is the recovery and purification of biofuels from aqueous streams.

Many technical approaches have been developed for the economic removal of alcohols from aqueous based fermentation media. The most widely used recovery techniques today use distillation and molecular sieve drying to produce ethanol. Butanol production via the

Clostridia-based acetone-butanol-ethanol fermentation also relies on distillation for recovery and purification of the products. Distillation from aqueous solutions is energy intensive. For ethanol, additional processing equipment to break the ethanol/water azeotrope is required. This equipment, molecular sieves, also uses significant quantities of energy.

Many unit operations have been studied for the recovery and purification of fermentation- produced alcohols, including filtration, liquid/liquid extraction, membrane separations (e.g., tangential flow filtration, pervaporation, and perstraction), gas and vacuum stripping, and "salting out" of solution, adsorption, and absorption. Each of the approaches has advantages and disadvantages depending on the circumstances of the product to be recovered and the product's physical and chemical properties and the matrix in which it resides. US8614077 describes a method to recover a C3-C6 alcohol from a fermentation broth containing microorganisms and the C3-C6 alcohol. The method includes the step of increasing the activity of the C3-C6 alcohol in a portion of the fermentation broth to at least that of saturation of the C3-C6 alcohol in the portion. The method also includes the step of forming a C3-C6 alcohol-rich liquid phase and a water-rich liquid phase from the portion of the fermentation broth. The method also includes the step of separating the C3-C6 alcohol- rich phase from the water-rich phase and discloses the use of higher boiling point solvents so that the C3-C6 alcohol is obtained as the distillate and the solvent as the bottom fluid.

WO2016/080531 relates to a method for concentrating and dehydrating butanol. The method involves the use, as extractants, of butane, isopentane and pentane, or mixtures thereof, in a liquid/liquid extraction. The liquid/liquid extraction is conducted at a temperature of from 100 to 250°C. This temperature range is selected to increase the distribution coefficient of butanol into the extractant in order to minimise the energy cost of the subsequent distillation step used to recover the butanol.

JP H0327336 relates to the dehydration of an aqueous solution of an alcohol while saving energy by subjecting the solution to primary, secondary and tertiary concentration steps using propylene, propane, n-butane or i-butane as a solvent under different temperature and pressure conditions.

Accordingly, it is desirable to provide an improved method with lower energy costs and/or tackle at least some of the problems associated with the prior art or, at least, to provide a commercially useful alternative thereto. According to a first aspect there is provided a method for producing a butanol-rich composition suitable for use as a fuel, the method comprising:

(i) fermenting a feedstock in a fermenter to produce an aqueous fermentation broth comprising butanol; (ii) adding an organic solvent to the broth and performing a liquid/liquid extraction to recover a mixture of the butanol in the organic solvent;

(iii) vaporising the organic solvent from the mixture to provide a butanol-rich composition;

wherein the organic solvent comprises one or more alkanes and/or alkenes having a boiling point of less than 55°C at atmospheric pressure.

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

Butanol has a boiling point of 118°C (pure n-butanol). The use of an organic solvent comprising one or more alkanes and/or alkenes having a boiling point of less than 55°C at atmospheric pressure allows for ready vaporisation of the organic solvent after liquid/liquid separation to provide butanol in suitable purity as a bottoms fluid from a distillation unit. The boiling point as referred to herein is the absolute boiling point of the pure organic solvent. Advantageously, the low boiling point alkanes and alkenes have a low toxicity for

fermentation organisms and can be made to boil off using the heat from the fermentation unit and, optionally, other low grade heat sources associated with the fermentation and separation process. The precise boiling point of the alkanes and/or alkenes can be adjusted in use by changing the process pressure without incurring a high energy cost. As a consequence the present invention provides a route to obtaining a butanol fuel or fuel supplement with low cost due to the utilisation of a low-grade heat-source. Indeed, for the use of pentane as the organic solvent it may be possible to achieve up to a 90% reduction in the energy consumption of the separation process compared to classical distillation separation systems.

The present invention relates to a method for producing a butanol-rich composition suitable for use as a fuel. A fuel grade is the lowest commercially viable grade of purity, whereas analytical grades, for example, have a higher purity. The butanol-rich composition obtained will be a liquid at room temperature.

The butanol-rich composition obtained as the final product in the present method is desirably substantially pure butanol, such as for example at least 95v/v% pure or at least 99v/v% pure. As discussed below, the process may involve the use of a single distillation step to produce the butanol-rich composition as the final product, or may involve a further distillation step to produce the butanol-rich composition as the final product. In the latter case, the product of the first distillation step (the butanol-rich composition) will be less pure than the final product (the refined butanol-rich composition) and may have a purity of, for example, of at least 50 v/v% butanol, more preferably at least 75 v/v% and most preferably at least 95 v/v%. Lower levels of butanol may also be obtained as the product of a single distillation step, such as at least 20v/v%. These lower levels may be useful for other specific processes, such as the method of enriching gasoline with butanol, as discussed below.

In another embodiment it is also possible that the organisms used in the process may additionally produce a secondary alcohol, such as ethanol and this may be present in an amount of up to 20 v/v% of the final product. Accordingly, the butanol-rich composition obtained as the final product in the present method in this embodiment is preferably at least 50 v/v% butanol, more preferably at least 75 v/v% and most preferably at least 95 v/v%. Another possible side-product may be acetone. The presence of ethanol and/or acetone depends on the fermentation system used.

The butanol produced in the butanol-rich composition may comprise one or more of the available butanol isomers, depending on which species are produced by the

microorganisms. Gas fermenter processes typically produce n-butanol, but other fermenters can also produce sec-, iso- and tert-butanol, or mixtures of two or more thereof, n-butanol and iso-butanol are most preferred. The preferred isomers produced can be tailored by selection of the appropriate microorganisms.

The method disclosed herein comprises a number of steps which may be conducted in order or simultaneously. Typically the process is a continuous process whereby all of the steps are being conducted simultaneously and concurrently. For a given portion of the process fluids the steps will be experienced sequentially. It is also possible that the fermentation process is conducted in a batchwise process.

The first step involves fermenting a feedstock in a fermenter to produce an aqueous fermentation broth comprising butanol. Fermenters and the selection of suitable feedstocks as fuels are well known in the art including biomass. A preferred fermenter is a gas fermenter which operates using a syngas as the feedstock fuel source. Syngas may, for example, be obtained by the gasification of waste or other bio-matter, and contains a mixture of hydrogen, carbon monoxide, and carbon dioxide. Other fermenters, such as

carbohydrate-based fermenters and second generation biochemical fermenters may use biomass materials directly as a feedstock, or may use derivative materials therefrom.

Suitable organisms for the fermentation are well known in the art. Preferred microorganisms are mostly of the class known as acetogens, including Clostridium ljungdahlii, Clostridium autoethanogenum, Eurobacterium limosum, Clostridium carboxidivorans P7,

Peptostreptococcus products, and Butyribacterium methylotrophicum. Most of these organisms use the Wood-Ljungdahl pathway and are particularly useful for gas

fermentation. Other fermentation approaches may use other biochemical conversion organisms which are known in the art. Under the fermentation conditions the organisms convert the feedstock into butanol and produce heat, so the fermentation unit operates at an elevated temperature. Depending on the heat-tolerance of the organisms the fermenter may run at up to 37°C or up to 55°C. It is preferred that the minimum operation temperature is at least 30°C. In most fermentation systems it is expected that the operation temperature will be between 30 and 37°C, preferably between 32 and 37°C, more preferably between 35 and 37°C.

The method involves adding an organic solvent to the broth and performing a liquid/liquid extraction to recover a mixture of the butanol in said organic solvent. Liquid/liquid extraction units are well known in the art. The level of butanol in the broth before extraction is typically up to 2 v/v%, but generally from 0.1 to 1.5 v/v% and more preferably about 1 v/v%. Higher concentrations are desired for process efficiency, but these can negatively impact on the fermentation microorganisms. Preferably the liquid/liquid separation is conducted in a separate treatment vessel from the fermenter, although it is also possible to conduct the liquid/liquid extraction within the fermenter itself. This is efficient in that it reduces the amount of equipment required.

The broth leaving the fermenter will be at the fermenter operating temperature and, preferably, the Iiquid/liquid extraction is performed at the same temperature. This is advantageous for a number of reasons. In particular, if the aqueous portion of the broth is returned to the fermenter in a recycle loop as discussed below, then it is provided at a suitable temperature for the fermenter. If the aqueous portion has cooled (or been cooled), then this can help to control the fermenter temperature. On the other hand, if the broth is heated before the Iiquid/liquid extraction then any organic components entrained in the broth, such as cells, may become damaged by the heat.

The organic solvent comprises one or more alkanes and/or alkenes having a boiling point of less than 55°C at atmospheric pressure. More preferably the organic solvent comprises one or more alkanes and/or alkenes having a boiling point of less than 50°C at atmospheric pressure, more preferably less than 37°C at atmospheric pressure. This avoids the need for unduly low pressures if the fermenter is operating at 37°C. Preferably the organic solvent comprises one or more alkanes and/or alkenes having a boiling point of at least -12°C at atmospheric pressure.

Preferably the organic solvent is predominantly one or more of these alkanes and/or alkenes, i.e. at least 50%, more preferably at least 75% and most preferably at least 95% or at least 99%. Preferable the organic solvent comprises or consists of just one alkane or alkene.

Preferably the organic solvent comprises a solvent selected from the group consisting of pentane, pentene, butane, butene, and mixtures of two or more thereof. The references to these compounds include all isomers thereof. Preferably the organic solvent consists of one or more of pentane, pentene, butane and butene. Butene may be especially preferred as it can readily be produced from the butanol product for use in the process. This makes it possible to replenish lost organic solvent in the process on site and with low cost. Isobutane has a boiling point at atmospheric pressure of -11 °C and therefore needs to be used under pressure to achieve liquid/liquid extraction. A pressure of at least 5 bar is sufficient to achieve this. Butene has a boiling point of -0.7°C at atmospheric pressure and about 37°C at 3.5 bar. Pentane is a preferred solvent as it has a boiling point of 36°C at atmospheric pressure and therefore can be made to boil with the fermenter heat at only a slightly reduced pressure (such as about 0.9 bar). This is preferable as the circulation of the organic solvent can be achieved with pumps and does not require the more energy-intensive use of compressors. The method further comprises a step of heating the mixture of organic solvent and butanol to vaporise, or "boil off', said organic solvent. This is step (iii). This leaves a recoverable butanol-rich composition.

Preferably the mixture from step (ii) is placed in thermal communication with the fermenter to provide heat in step (iii) to vaporise the organic solvent. The thermal communication is preferably via a heat exchanger, or more preferably the mixture is passed through one or more ducts within or in contact with the fermenter. The latter approach reduces any heat transfer losses and provides a simple plant design. In any event it is desirable to take heat from the fermenter because the fermenter needs to be cooled in order to prevent it overheating. This is typically achieved using a cooling water jacket which may be replaced or reduced in size with the use of ducts in the present method.

Advantageously placing the mixture from step (ii) in thermal communication with the fermenter helps to provide cooling to the fermenter. Fermenters are often cooled using actively chilled water, which is a significant energy cost in itself. Using a boiling organic solvent in a fermenter cooling tube/jacket (or even via a heat exchanger) is advantageous. Since the organic solvent is boiling, it has a much lower heat transfer resistance (than using specific heat of water) and so the temperature of the cooling jacket can be higher. In addition, using a boiling organic solvent in a fermenter cooling tube/jacket (or even via a heat exchanger) adds flexibility to the process. The temperature of the fermenter can be reduced further by reducing the pressure of the jacket fluid (the organic solvent and butanol mixture) further. If a very cold jacket fluid was wanted to simplify fermenter design, sub- atmospheric temperatures could be used, which would then require a compressor to compress the organic vapours to a pressure at which they could be condensed. This may also provide some further heat recovery as this would warm these gases as well. This miniature alkane heat pump set-up could replace the need for a chilled water set-up.

Preferably step (iii) is conducted under a pressure at which the boiling point of the organic solvent is below a temperature of the fermenter, whereby the temperature of the fermenter is sufficient to vaporise the organic solvent. That is, the pressure may be reduced to lower the boiling point of the solvent so that the fermenter heat (including any process heat losses) is sufficient to vaporise the organic solvent. Thus, for pentane, a pressure of 0.9 bar is enough to lower the boiling point such that it boils with the heat available from the fermenter at 37°C.

Preferably the liquid/liquid extraction is conducted under a pressure sufficient to ensure that the organic solvent is in liquid form. That is, if the organic solvent is especially volatile, such as butene, it may be necessary to operate at an increased pressure in the liquid/liquid extraction to ensure that the butene remains a liquid.

The inventors have found that the use of volatile organic solvents in this process can give a low-cost route to obtaining butanol. In particular, the process relies on a relatively inefficient but highly selective liquid/liquid extraction process, since it is possible to compensate for this inefficiency by utilising abundant sources of low grade heat energy. Since the fermentation process and associated processing steps yield so much low grade heat, the relatively low distribution efficiency of the liquid/liquid extraction does not mitigate the commercial utility of the process, since the energy requirements of step (iii) are easily met.

Preferably step (ii) further comprises recovering an aqueous solution and wherein the aqueous solution is recycled into the fermenter. The aqueous solution contains water, together with hydrophilic components of the fermentation broth such as butanoic acid and any dissolved gases, together with a small amount of the organic solvent, together with any broth nutrients. It is desirable to return this liquid to the fermenter for several reasons. It is cooler and provides a cooling effect on the fermenter. It contains butanoic acid which helps to encourage butanol production. It contains dissolved syngas to feed the organisms. The organic solvent, since it is largely immiscible with water and, hence, is present in a trace concentration in the recycled aqueous solution, does not have a significant effect on the organisms and is not toxic.

The inventors have also discovered that the temperature of the liquid/liquid extraction affects the distribution coefficient of butanol. Specifically, the distribution coefficient of butanol increases with an increase in temperature. This means that if the liquid/liquid extraction is conducted at a higher temperature then the amount of butanol carried into the solvent will increase. While this is desirable, this is counter-balanced by a lower selectivity, i.e. more water is then carried over to the solvent stream at higher temperatures. It is also counter- balanced by higher mutual solubility, i.e. more solvent is carried over to the aqueous stream at higher temperatures.

While it is most preferred that the liquid/liquid extraction is performed at the operating temperature of the fermenter (i.e. with no active heat changes to the broth), in another embodiment, the inventors have discovered advantages to using a higher temperature. Under all circumstances it is preferred that the temperature be less than 100°C and preferably less than 90°C.

In embodiments where the liquid/liquid extraction is performed above the operating temperature of the fermenter, the inventors have found that the optimum temperature is from 38 to 100°C, preferably from 40 to 90°C, and most preferably 50 to 70°C or about 60°C. Above 100°C and to an extent above 90°C, water carried over becomes more problematic.

The inventors have found that there is a further trade off, in that heating the broth up to achieve a higher extraction temperature is an additional heat demand. However, they have found that this can be recovered by cooling the output back to broth temperature.

That is, the heat used to raise the broth temperature can be recovered from the aqueous solution after the liquid/liquid extraction. This makes the increase in the temperature for the extraction process thermally efficient.

The inventors have found that there is no benefit in using even higher temperatures for the extraction, since this would require more than the abundantly available low-grade heat sources relied upon by the present invention. Moreover, significant heating is unnecessary at higher broth concentrations since there is sufficient low grade heat, such that the lower distribution coefficient does not matter.

Preferably step (iii) is conducted at a first pressure and the method further comprises:

(iv) heating the butanol-rich solution at a second pressure to vaporise residual organic solvent to provide a refined butanol-rich composition,

wherein the second pressure is greater than the first pressure. This additional purification step gives a more pure final product. That is, the further butanol-rich composition contains a higher percentage of butanol than the original (or first) butanol-rich solution.

In addition, heat needs to be put into the second heating step which can then be recovered from the butanol-rich solution and the vaporised residual organic solvent to help drive the initial vaporisation step. Preferably heat is recovered from condensing the residual organic solvent to provide a portion of the heating in step (iii). Other low grade heat sources associated with the process described herein may also be used to supplement the boiler heat in step (iii) which drives the distillation column. In particular, depending on the fermenter selection there may be abundant sources of low grade heat associated with the apparatus. For a biochemical fermenter there are lots of upstream processes at moderate temperatures which means that the separation may be performed in a single distillation step with a variety of sources for the boiler heat.

Examples of other low-grade heat sources which can be used to supplement the heat required for the heating steps disclosed herein include one or more of: waste heat from sterilisation processes; seed train fermenter heat (for producing organism innoculums); acid and enzymatic hydrolysis of biomass (saccharification); pH conditioning tanks and ion exchange columns used to remove any inhibitors (e.g. acetates, salts, organic acids);

wastewater treatment; and any vapor streams from upstream flashes to increase slurry concentration and remove volatiles. The final butanol-rich composition may contain small amounts of biological material from the fermenter, depending on whether there have been any filtration steps, because the butanol- rich composition is always recovered as the bottoms fluid from the distillation units. This is, however, unlikely as the selectivity of the discussed organic solvents in the liquid/liquid extraction step does not favour the incorporation of the biological material in the organic solvent. Similarly the composition may contain some of the organic solvent. However, neither of these contaminants adversely affects the utility of the butanol-rich composition as a fuel. If desired, biological solids may be filtered out of the composition using a conventional filtration process.

According to a further aspect there is provided a method for producing a fuel for use in an internal combustion engine of an automobile, the method comprising performing the method as described herein and blending the butanol-rich composition with gasoline to form the fuel, whereby the amount of butanol in the fuel is at least 5v/v%, preferably from 5 to 50v/v%. The use of butanol as a gasoline additive is desirable because its fuel properties much more closely match gasoline than bio-derived alternatives such as Ethanol. Moreover, it is not completely miscible with water, which may allow it to be piped with gasoline in existing networks rather than being blended at the station. Alternatively the butanol-rich composition can be used as a fuel per se.

According to a further aspect there is provided an apparatus for producing a butanol-rich composition suitable for use as a fuel, the apparatus comprising:

a fermenter for fermenting a feedstock having a first outlet for an aqueous

fermentation broth,

a liquid/liquid extraction unit in fluid communication with said first outlet and having a first inlet for an organic solvent and a second outlet for a liquid organic solution;

a first distillation unit in thermal communication with the fermenter, having a second inlet in fluid communication with said second outlet and having a third outlet for a butanol- rich composition and a fourth outlet for the organic solvent.

Biomass fermenters are well known in the art and any design could be suitably adapted for use in the present method. Preferably the biomass fermenter comprises microorganisms for digesting a feedstock to produce heat and butanol under aqueous conditions.

Similarly, liquid/liquid extraction units are well known in the art, as are distillation units.

Exemplary distillation units are discussed further below and typically comprise both a vaporisation chamber as well as a condenser and reboiler. Other types of distillation units include flash distillation columns. That is, a distillation unit as described herein comprises all of those components necessary to vaporise off the organic solvent and to obtain a liquid organic solvent and a liquid bottoms fluid. Preferably the thermal communication between the first distillation unit and the fermenter is via a heat exchanger, or more preferably at least a portion of the first distillation unit is located within the fermenter.

Preferably said fourth outlet is in fluid communication with said first inlet for recycling the organic solvent.

The apparatus may further comprise a second distillation unit having a third inlet in fluid communication with said third outlet and having a fifth outlet for a further butanol-rich composition and a sixth outlet for residual organic solvent. As discussed for the method above, this provides an improved final product purity and the energy supplied as heat in this apparatus can be recovered to drive the first distillation unit. Although it would be desirable from the perspective of apparatus simplicity not to have the second distillation unit, the presence of the second distillation unit can up to halve the energy costs of the process. Since the process is relatively mild, the aqueous fermentation broth may be recycled from the liquid/liquid extraction unit back into the fermenter.

Preferably the sixth outlet is in fluid communication with the first inlet for recycling the organic solvent, optionally via a condenser in thermal communication with the first distillation unit.

According to a further aspect there is provided the use of heat produced in a fermenter to recover a butanol-rich composition from an aqueous fermentation broth, wherein the heat is used to vaporise an organic solvent used to separate butanol from said broth after a liquid/liquid extraction process.

As mentioned above, in some embodiments it is also possible that the organisms used in the process may additionally produce a secondary alcohol, such as ethanol and this may be present in an amount of up to 20 v/v% of the final product. The organisms may also produce acetone. It is possible to simply ignore these components since they are more volatile than butanol and have much lower distribution coefficients with the solvents than butanol.

Accordingly, in the present method, the small amount of these components carried over into the solvent stream will be vaporised together with the organic solvent and ultimately reach equilibrium levels in the broth and recycled solvent. This may be desirable if the presence of these components in the broth can be used to encourage additional butanol production in the fermenter. According to a preferred embodiment, the ethanol and/or acetone may also be recovered as a separate product. In this embodiment the ethanol and/or acetone can be fractionated off when the solvent is vaporised. Alternatively, they may be distilled from the broth before it is returned to the fermenter. Since ethanol and acetone are low boilers they can readily be recycled in the solvent vapor recycle.

According to a further aspect, the process described herein which is for producing a butanol- rich composition suitable for use as a fuel, can be adapted for producing a butanol-enriched gasoline fuel. The method comprises performing the steps (i)-(iii) as described herein, and then:

(iv) adding water to the butanol-rich composition and performing a liquid/liquid extraction to recover a butanol-in-water mixture. In practice this has a relatively high concentration of butanol;

(v) adding gasoline to the butanol-in-water mixture and performing a liquid/liquid extraction to obtain a butanol-enriched gasoline fuel.

In this adapted process, the fermentation broth in step (i) will have a first concentration of butanol. This will be about 1v/v%, as discussed above. The butanol-in-water mixture will have a second concentration of butanol such as about 5v/v%. The second concentration of butanol will be higher than the first. The concentration of butanol in the final butanol-enriched mixture is determined by the concentration achieved in the vaporising step (iii).

Advantageously, this process is a lower energy route to supplementing gasoline with butanol. In particular, rather than having the second distillation step discussed herein, this aspect allows the introduction of the butanol directly into gasoline. This is achieved as a consequence of the high purity of butanol in water at a sufficient concentration. By circumventing the energy costs of the second distillation step, the method is a low energy route to gasoline enrichment.

The present disclosure will now be described further with reference to the figures, in which:

Figure 1 shows a schematic representation of the apparatus used in the method described herein.

Figure 2 shows a schematic diagram of a distillation unit.

Figure 1 shows an apparatus 1 comprising a gas fermenter unit 5. The gas fermenter 5 has an inlet 10 for introducing a feed gas. Typical feed gases are syn-gases, namely mixtures of hydrogen, carbon monoxide, and carbon dioxide. The gas fermenter contains

microorganisms which convert the syngas into butanol and release heat into the gas fermenter 5. The microorganisms are held in an aqueous environment and produce an aqueous broth comprising butanol. The temperature of the fermenter will typically be from 30 to 37°C.

The gas fermenter 5 is typically, but not necessarily, maintained at an elevated pressure such as about 5 bar in order to drive the gases in the syngas to dissolve in the aqueous fermentation broth. Fermenters of other designs may not need to be at such pressures. The gas fermenter 5 produces heat from the action of the organisms digesting and the heat maintained in the gas fermenter depends on the microorganisms used which determines their heat tolerance. Although a gas fermenter 5 is shown here, the fermenter could alternatively be a liquid fermenter. Gas and liquid fermenters are well known in the art.

The gas fermenter 5 has a liquid outlet 15 for passing a flow of the aqueous fermentation broth out of the gas fermenter 5. The temperature of the broth will reflect the temperature within the gas fermenter 5. The gas fermenter 5 has an off-gas outlet 20 to avoid a pressure build-up. The off-gas outlet 20 is provided with a condenser 25 from which liquids can be recovered into the flow of the aqueous fermentation broth passing out of the liquid outlet 15 from the gas fermenter 5. The liquid outlet 15 is connected to a liquid/liquid extraction unit 25. Between the liquid outlet 15 and a first inlet 30 of the liquid/liquid extraction unit 25, there may be a filtration unit 35. The filtration unit 35 is for recovering cells and other biological matter from the aqueous fermentation broth. The filtration unit 35 has an outlet 40 for the cells which may be recycled into the gas fermenter 5, and an outlet 45 for the filtered aqueous broth being passed to the inlet 30 of the liquid/liquid extraction unit 25.

The liquid/liquid extraction unit 25 is also provided with a second inlet 50 for an organic solvent such as liquid pentane, and has a first outlet 55 and a second outlet 60. In use the liquid/liquid extraction unit facilitates the contact of an organic solvent with the aqueous fermentation broth and provides two fractions. The first fraction is predominantly water and passes from the liquid/liquid extraction unit 25 through the first outlet 55 back to a recycle inlet 65 in the gas fermenter 5. In passing from the first outlet 55 the first fraction may pass through a pump 70 and may be supplemented with additional water at a water inlet 75 or a portion may be discarded at a purge outlet 80 to avoid excess water building up in the fermenter 5.

The liquid/liquid extraction unit 25 will typically operate at the same temperature as the broth obtained from the fermenter 5. That is, the liquid/liquid extraction unit 25 will contain the organic solvent and the broth at a temperature of about 30 to 37°C. A preferred temperature range would be 32 to 35°C. However, as discussed herein, the temperature may be higher, such as optimally about 60°C, depending on the set-up employed.

The second fraction is predominantly the organic solvent and contains butanol recovered from the aqueous fermentation broth. The second fraction is passed from the second outlet 60 to a first distillation unit 85, optionally via a pressure drop 90.

The first distillation unit 85 has a conventional form. A conventional distillation unit is discussed further in relation to Figure 2 below. The first distillation unit 85 has a first outlet 95 for the organic solvent which is boiled off from the mixture fed into the first distillation unit 85 via a first inlet 96, and a second outlet 100 for the bottoms liquid recovered from the first distillation unit 85 which is predominantly butanol. The distillation unit 85 is typically at a pressure of less than 1 bar, such as 0.9 bar and is driven by a heat source (or heat sources) providing a temperature of about 37°C. Suitable operation temperatures will again be from 30 to 37°C. The temperature is ideally within a few degrees of the fermenter temperature if this is the primary energy source, such as 1-5 or 2-3°C cooler than the fermenter.

The bottoms liquid recovered from the first distillation unit 85 via outlet 100 is optionally passed to a second distillation unit 105 via a pump 110. The pump 110 serves to increase the pressure in the second distillation unit, such as 1.2 bar.

The second distillation unit 105 also has a conventional form. The second distillation unit 105 has a first outlet 1 15 for residual organic solvent which is boiled off from the mixture fed into the second distillation unit 105 via a first inlet 116, and a second outlet 120 for the bottoms liquid recovered from the second distillation unit 105 which is more concentrated butanol. The second distillation unit 105 is at a pressure greater than the first distillation unit, such as 1.2 bar and is driven by a heat source. In view of the increased pressure, the heat source needs to provide a greater temperature in order to boil off the organic solvent, such as a temperature of about 120°C. Higher temperatures such as up to 200°C may be employed with higher pressures. This may be desirable since the heat can then be reused in other process steps and the purity obtained may be higher.

The residual organic solvent taken from the first outlet 115 may be passed to a heat exchanger 125 to recover heat from the solvent as it condenses.

The butanol-rich bottoms liquid taken from the second outlet 120 may be passed to a heat exchanger 130 to recover heat from the liquid as it cools. The cooled butanol-rich bottoms liquid is recovered as the output 135 from the process.

The organic solvent taken from the first outlet 95 and the residual organic solvent taken from the first outlet 115 may be recycled back to the second inlet 50 of the liquid/liquid extraction unit 25. The recycle loop may include a pump 140, an organic solvent inlet 145 for fresh solvent, and first and/or second purge outlets (150, 155) to avoid undue pressure build up.

The apparatus 1 has several points at which excess heat can preferably be provided in order to drive the first distillation unit 85. The primary source of heat in this example is the gas fermenter 5. Heat from the gas fermenter 5 may be passed to the first distillation unit 85 via a heat exchanger (not shown). Alternatively, a portion of the first distillation unit 85 may pass within the fermenter 5. That is, the fluid in the distillation unit may be passed within pipes located within the fermenter 5 in order to directly transfer heat from the fermenter 5 into the first distillation unit 85.

Another source of heat is the butanol-rich liquid obtained from the second outlet 120 of the second distillation unit 105. This provides heat at least partially obtained from the heat introduced into the second distillation unit 105.

Another source of heat is the top coil (not shown) of the second distillation unit 105. This provides heat at least partially obtained from the heat introduced into the second distillation unit 105. Another source of heat is the condenser 125. This provides heat originally obtained from the heat introduced into the second distillation unit 105.

Where a heat exchanger is used to recover heat from the process, a single combined heat exchanger may be used with multiple heat inlets and one heat outlet. This reduces cost and complexity.

Although this embodiment relies on a gas fermenter 5, other fermentation units would also be suitable for use in the method described herein. In use, the organisms in the gas fermenter 5 break down the syngas to produce butanoic acid. As the levels of butanoic acid build up, the organisms start to produce butanol instead. The maximum concentrations of butanol reach levels of up to 2 v/v%, but typically 1 v/v% in the aqueous fermentation broth. The fermentation broth is passed to the liquid/liquid extraction unit 25 where it is contacted with an organic solvent such as pentane. Pentane is largely immiscible with water so two layers are formed. The butanol in the fermentation broth passes largely into the pentane layer. Thus the liquid obtained from the second outlet 60 is substantially pentane with an amount of butanol therein.

The aqueous layer contains water, together with hydrophilic components of the fermentation broth such as butanoic acid and any dissolved gases, together with a small amount of pentane. It is desirable to return this liquid to the fermentation unit 5 for several reasons. It is cooler and provides a cooling effect on the fermentation chamber 5. It contains butanoic acid which helps to encourage butanol production. It contains dissolved syngas to feed the organisms. The pentane, since it is largely immiscible with water does not have a significant effect on the organisms and is not toxic.

The liquid obtained from the second outlet 60 which is substantially pentane with an amount of butanol therein is passed to the first distillation unit 85. Since the pentane has a substantially lower boiling point (36.1°C) than the butanol (118°C), the pentane can be removed at a high purity. The butanol which remains is generally of sufficient purity to be suitable for use as a fuel grade, particularly if passed through a second distillation unit 105. The presence of any pentane in the final butanol is not a concern for fuel applications.

Advantageously the first distillation unit may be operated at a reduced pressure. This lowers the boiling point of the pentane such that it can readily boil off using the heat produced by the fermenter 5. It is desirable to balance the boiling point of the organic solvent so that it is at least 1 °C, preferably from 5 to 15°C, more preferably 5 to 10°C below the temperature of the fermenter 5, since this gives room for any heat transfer resistance. It may be

advantageous to go to an even large delta T than 10°C since this gives additional overheads in the system design.

The first distillation unit 85 therefore produces a very pure pentane liquid and a relatively pure butanol-rich composition. This composition may then be passed to a second distillation unit 105. This is preferably the only part of the system where additional heat energy is directly added into the process fluids. The additional heat energy is sufficient to achieve a reboiler temperature of about 120°C. Since the second distillation unit 105 is at a higher pressure, the boiling points of the fluids are increased. This means that the butanol boiling point is still above the reboiler temperature. Heat can be recovered at the condenser which is operated with cooling water at the pentane outlet of the second distillation unit 105. The second distillation unit produces a purified butanol-rich composition and further pentane.

Pressures in the first and second distillation units 85, 105 can be controlled by conventional systems known in the art. In particular, restrictions can be used to cause pressure drops and pumps may be used to increase the pressure.

For the avoidance of doubt, the inlets and outlets discussed in the general description correspond as follows to those shown in Figure 1 : First inlet - First inlet 30 of the liquid/liquid separation unit 25

First outlet - First outlet 15 of the gas fermenter 5

Second inlet - First inlet 96 of the first distillation unit 85.

Second outlet - Second outlet 60 of the liquid/liquid separation unit 25

Third inlet - First inlet 116 of the second distillation unit 105.

Third outlet - Second outlet 100 of the first distillation unit 85

Fourth outlet - First outlet 95 of the first distillation unit 85.

Fifth outlet - Second outlet 120 of the second distillation unit 105.

Sixth outlet - First outlet 115 of the second distillation unit 105. Figure 2 shows an exemplary distillation unit 200 suitable for use as a distillation unit (85, 105) in the process described in Figure 1.

The distillation unit 200 has a process chamber 205 having an inlet 210 for a fluid to be distilled. Heat is provided by a reboiler 215. The reboiler 215 has an inlet 220 for fluid from the process chamber 205, a first outlet 225 for a fluid product 230 which is a butanol-rich composition in the method disclosed herein, and a second outlet 235 for vapour which is returned to the process chamber 205 via an inlet 240. Hot vapour rises in the process chamber 205 with some condensing and falling as a liquid. The hot vapour leaves the process chamber 205 via an outlet 245 to a condenser 250. The condensed fluid is passed to a reflux drum 255 from which condensed distillate is passed to a pump 260 and a first portion is recovered as an output 265 and a second portion is returned to the top of the process chamber 205 via an inlet 270 where it flows downward to provide cooling and condensation of the upflowing vapours.

As will be appreciated, the output from the distillation unit 200 is a liquid organic solvent and a liquid bottoms fluid.

Although a standard distillation column is shown, the distillation unit 200 could also be a flash column or other distillation column as known in the art, provided that the eventual output is a liquid organic solvent and a liquid bottoms fluid. Aspects of the present disclosure will now be demonstrated further with reference to the following non-limiting examples.

Example 1 - Boiling Point and Vapor pressure of pure C4 & C5 hydrocarbons The boiling point at atmospheric pressure and the vapor pressures at 25°C, 32°C and 37°C were measured for various C4 & C5 alkanes and alkenes. Results are shown in Table 1.

Table 1 - Atmospheric boiling points and vapor pressures at 25° C, 32° C and 37°C of C4 &

C5 hydrocarbons

These vapor pressures demonstrate that C4 & C5 alkanes and alkenes are suitable for use as liquid extractants at moderate pressures at typical fermenter broth temperatures (~37°C), and will boil at atmospheric pressures, or slightly reduced pressures, at the fermenter temperature.

Example 2 - Solubility of C4 & C5 hydrocarbons in water

The solubility of various C4 & C5 alkanes and alkenes was measured in water at 25°C, 32°C and 37°C. Results are shown in Table 2.

Table 2 - Solubility of C4 & C5 hydrocarbons in water

These solubilities are very low and demonstrate that minimal amounts of C4 & C5 extractants will be lost to any fermenter broth purge stream or recycled back to the fermenter. Example 3 - Solubility of water in C4 & C5 hydrocarbons

The solubility of water in various C4 & C5 alkanes and alkenes was measured at 25°C and 37°C. Results are shown in Table 3.

Table 3 - Solubility of water in C4 & C5 hydrocarbons

These water solubilities are very low, and demonstrate that minimal amounts of water will be carried over from the broth into the extractant hydrocarbon stream. This means that water will be excluded, making these hydrocarbon extractants very selective towards butanol over water. Example 4 - Activity and Selectivity of Butanol in water and n-pentane

The activity co-efficient of butanol in n-pentane was calculated by measuring the vapor-liquid equilibrium of butanol/pentane mixtures using a static still at 30°C and 60°C. The

temperature dependence of the activity co-efficient of butanol in pentane was calculated by measuring the excess enthalpy of mixing of butanol and pentane. The pressure dependence of the activity co-efficient of butanol in pentane was shown to be negligible by measuring the excess volume on mixing butanol and pentane. The activity co-efficient of butanol in water was calculated by measuring the vapor-liquid equilibrium of butanol/water mixtures using a static still at 25^°C, 30^°C and 50^°C. The limiting activity co-efficient of butanol in water at infinite dilution is well known in the literature. The temperature dependence of the activity co-efficient of butanol in water was calculated by measuring the excess enthalpy of mixing of butanol and water. The pressure dependence of the activity co-efficient of butanol in water was shown to be negligible by measuring the excess volume on mixing butanol and water.

The limiting activity co-efficient of butanol in water at 37^°C at infinite dilution was found to be 58. The limiting activity co-efficient of butanol in pentane at 37^°C at infinite dilution was found to be 26.

The molar and mass distribution coefficients are defined as:

Molar Distribution Coefficient Butanol mole fraction in pentane phase

Butanol mole fraction in aqueous phase

Mass Distribution Coefficient Butanol mass fraction in pentane phase

Butanol mass fraction in aqueous phase

The limiting molar distribution co-efficient at infinite dilution at 37^°C was found to be 2.23, which equates to a limiting mass distribution co-efficient at infinite dilution at 37^°C of 0.56. This means that at low concentrations, the mole fraction of the pentane phase in equilibrium with a broth of butanol mole fraction * (mass fraction is 2.23* (or 0.56"» in mass fraction). The mass distribution co-efficient for a 1 wt% butanol broth (0.24 mol%) at 37^°C was calculated to be 0.68 (equivalent to 2.2 molar distribution co-efficient), allowing a

concentration of up to 0.7 wt% (0.5 mol%) butanol in pentane to be produced in an extraction. The mass distribution co-efficient for a 2 wt% butanol broth (0.49 mol%) was calculated to be 0.83 (equivalent of 2.6 molar distribution co-efficient), allowing a

concentration of up to 1.7 wt% (1.3 mol%) butanol in pentane to be produced in an extraction. The selectivity of pentane was calculated as:

Selectivity = Mass Distribution Coefficient for Butanol

Mass Distribution Coefficient for water

= Butanol mass fraction in pentane phase . Water mass fraction in aqueous phase Butanol mass fraction in aqueous phase Water mass fraction in pentane phase

The selectivity for a 1 wt% butanol broth (0.24 mol%) at 37^°C was calculated to be 2,900. The selectivity for a 2 wt% butanol broth (0.49 mol%) at 37^°C was calculated to be 3,500. These selectivities are very high and show that butanol is extracted in a highly selective fashion from the broth, with minimal water carried over into the pentane stream. This allows simple separation of butanol from pentane downstream.

Example 5 - Activity and Selectivity of Butanol in water and n-butane

The activity co-efficient of butanol in n-butane was calculated by measuring the vapor-liquid equilibrium of butanol/pentane mixtures using a static still at 60^°C. The pressure dependence of the activity co-efficient of butanol in butane was shown to be negligible by measuring the excess volume on mixing butanol and butane.

The activity co-efficient of butanol in water was calculated by measuring the vapor-liquid equilibrium of butanol/water mixtures using a static still at 25^°C, 30^°C and 50^°C. The limiting activity co-efficient of butanol in water at infinite dilution is well known in the literature. The temperature dependence of the activity co-efficient of butanol in water was calculated by measuring the excess enthalpy of mixing of butanol and water. The pressure dependence of the activity co-efficient of butanol in water was shown to be negligible by measuring the excess volume on mixing butanol and water.

The limiting activity co-efficient of butanol in water at 60^°C at infinite dilution was found to be 64. The limiting activity co-efficient of butanol in butane at 60^°C at infinite dilution was found to be 38. The molar and mass distribution coefficients are defined as:

Molar Distribution Coefficient = Butanol mole fraction in butane phase

Butanol mole fraction in aqueous phase Mass Distribution Coefficient = Butanol mass fraction in butane phase

Butanol mass fraction in aqueous phase

The limiting molar distribution co-efficient at infinite dilution at 60^°C was found to be 1.7, which equates to a limiting mass distribution co-efficient at infinite dilution at 60 C of 0.53. This means that at low concentrations, the mole fraction of the butane phase in equilibrium with a broth of butanol mole fraction ^x (mass fraction ^m) is 1 J^x (or 0.53^m in mass fraction).

The mass distribution co-efficient for a 1 wt% butanol broth (0.24 mol%) was calculated to be 0.85 (equivalent to 2.7 molar distribution co-efficient), allowing a concentration of up to 0.8 wt% (0.7 mol%) butanol in butane to be produced in an extraction. The mass distribution coefficient for a 2 wt% butanol broth (0.49 mol%) was calculated to be 1.37 (equivalent of 4.4 molar distribution co-efficient), allowing a concentration of up to 2.7 wt% (2.2 mol%) butanol in butane to be produced in an extraction. The selectivity of butane was calculated as: Selectivity = Mass Distribution Coefficient for Butanol

Mass Distribution Coefficient for water

- Butanol mass fraction in butane phase Water mass fraction in aqueous phase

Butanol mass fraction in aqueous phase Water mass fraction in butane phase

The selectivity for a 1 wt% butanol broth (0.24 mol%) at 37°C was calculated to be 5,000. The selectivity for a 2 wt% butanol broth (0.49 mol%) at 37°C was calculated to be 8,000.

These selectivities are very high and show that butanol is extracted in a highly selective fashion from the broth, with minimal water carried over into the butane stream. This allows simple separation of butanol from butane downstream.

Example 6 This example demonstrates the non-toxicity of alkanes to Clostridium beyerinekii LMD 27.6

Droplets of various alkanes (hexane, heptane, octane, decane, dodecane) (analytical grade, Aldrich Europe) were added to a medium containing 10 kg/m³ yeast extract and 60 kg/m³ glucose, in vials in a jar with a GasPack system. A suspension of heat shocked spores or viable cells of Clostridium beyerinekii LMD 27.6 were used as an innoculum. Fermentation was conducted under an oxygen-free nitrogen atmosphere. All alkanes tested were found to be non-toxic to the organism.

Example 7 - Activity and Selectivity of Butanol in water and 1 -butene

The activity co-efficient of butanol in n-butane was calculated by measuring the vapor-liquid equilibrium of butanol/butane mixtures using a static still at 45 C and 91 °C. The pressure dependence of the activity co-efficient of butanol in 1-butene was shown to be negligible by measuring the excess volume on mixing butanol and 1-butene.

The activity co-efficient of butanol in water was calculated by measuring the vapor-liquid equilibrium of butanol/water mixtures using a static still at 25°C, 30°C and 50°C. The limiting activity co-efficient of butanol in water at infinite dilution is well known in the literature. The temperature dependence of the activity co-efficient of butanol in water was calculated by measuring the excess enthalpy of mixing of butanol and water. The pressure dependence of the activity co-efficient of butanol in water was shown to be negligible by measuring the excess volume on mixing butanol and water.

The limiting activity co-efficient of butanol in water at 45^°C at infinite dilution was found to be 61. The limiting activity co-efficient of butanol in water at 91 ^°C at infinite dilution was found to be 60. The limiting activity co-efficient of butanol in 1-butene at 45^°C at infinite dilution was found to be 15.9. The limiting activity co-efficient of butanol in 1-butene at 91 ^°C at infinite dilution was found to be 6.6.

The molar and mass distribution coefficients are defined as:

Molar Distribution Coefficient = Butanol mole fraction in 1-butene phase

Butanol mole fraction in aqueous phase

Mass Distribution Coefficient = Butanol mass fraction in 1-butene phase

Butanol mass fraction in aqueous phase The limiting molar distribution co-efficient at infinite dilution at 45^°C was found to be 3.8, which equates to a limiting mass distribution co-efficient at infinite dilution at 60^°C of 1.23. This means that at low concentrations, the mole fraction of the butane phase in equilibrium with a broth of butanol mole fraction * (mass fraction m) is 3.8* (or 1.23m in mass fraction). The limiting molar distribution co-efficient at infinite dilution at 91 ^°C was found to be 9.1 , which equates to a limiting mass distribution co-efficient at infinite dilution at 60 C of 2.9. This means that at low concentrations, the mole fraction of the butane phase in equilibrium with a broth of butanol mole fraction * (mass fraction is 9.1* (or 2.9^ in mass fraction). Example 8 - Secondary extraction directly to gasoline

A 1 wt% (0.24 mol%) aqueous butanol broth (mixture "A") was contacted with 1-butene at 37°C, 5 bara. Counter-current liquid-liquid extraction yeilded a mixture of butanol in 1-butene at 1.2 wt% (0.9 mol%) (mixture "B"). Utilizing heat at 37°C, some of the 1-butene was then boiled off at 3 bara in a distillation. The vapor 1-butene was the recovered by condensing at 25°C. Hence mixture B was concentrated to form a more concentrated mixture (mixture "C") of butanol in 1-butene at 26 wt% (21 mol%). Mixture "C" was contacted with clean water at 25°C, 5 bara. Using counter-current liquid- liquid extraction, this yeilded a mixture of butanol in water (mixture "D") at 4.3 wt% (1.1 mol%).

Finally, mixture D was contacted with gasoline at 25°C, 1 bara. Using counter-current liquid- liquid extraction, this yeilded a mixture of butanol in gasoline (mixture Έ") at 10.7 wt% (10 vol%), for use as a fuel.

Hence, only heat energy at 37°C was required in the process to produce a fuel mixture of 10 vol% butanol in gasoline.

Example 9 - Separation of butanol from aqueous broth using 1-butene at 37°C

Aqueous butanol broths at 0.25 wt% (0.061 mol%), 1 wt% (0.25 mol%) and 2 wt% (0.49 mol%) were contacted with 1-butene at 37°C. Counter-current liquid-liquid extraction yielded mixtures of butanol in 1-butene at 0.26 wt% (0.20 mol%), 1.2 wt% (0.91 mol%) and 2.9 wt% (2.2 mol%) respectively, and each containing 0.17 mol%, 0.17 mol%, 0.22 mol% water respectively.

Distillation was then conducted at 3 bara, with a reboiler temperature of 32°C and a condenser temperature of 25°C. 137, 29 and 11 MJ/kg recovery butanol of heat at 37°C were required for the reboiler in each case, which could be supplied from low grade heat sources such as the fermenter. 1-butene was recovered and condensed in the distillate. 56 wt%, 48 wt% and 48 wt% butanol in butene were recovered in the bottoms in each case respectively.

To recover butanol, a second distillation was performed at 5.5 bara. The condenser temperature was 37°C (hot enough to provide heat for the first distillation), and reboiler heat was supplied at around 170°C. Only around 0.8 MJ/kg recovered butanol of heat was required in this distillation at this elevated temperature. The remaining 1-butene was recovered in the distillate. Around 97% of butanol originally extracted into 1-butene was recovered in the bottoms, and was cooled to 25°C (providing further heat for the first distillation column).

Example 10 - Separation of butanol from aqueous broth using 1-butene at 60°C

Aqueous butanol broths at 0.25 wt% (0.061 mol%), 1 wt% (0.25 mol%) and 2 wt% (0.49 mol%) were contacted with 1-butene at 60°C. Counter-current liquid-liquid extraction yielded mixtures of butanol in 1-butene at 0.49 wt% (0.37 mol%), 2.2 wt% (1.7 mol%) and 5.0 wt% (3.8 mol%) respectively, and each containing 0.44 mol% water.

Distillation was then conducted at 3 bara, with a reboiler temperature of 32°C and a condenser temperature of 25°C. 88, 6 and 3 MJ/kg recovered butanol of heat at 37°C were required for the reboiler in each case, which could be supplied from low grade heat sources such as the fermenter. 1-butene was recovered and condensed in the distillate. 56 wt%, 48 wt% and 48 wt% butanol in butene were recovered in the bottoms in each case respectively. To recover butanol, a second distillation was performed at 10 bara. The condenser temperature was 70°C (hot enough to provide heat for heating the butene and aqueous broth to the extraction temperature of 60°C), and reboiler heat was supplied at around 190°C. Only around 0.9 MJ/kg recovered butanol of heat was required in this distillation at this elevated temperature. The remaining 1-butene was recovered in the distillate. Around 97% of butanol originally extracted into 1-butene was recovered in the bottoms, and was cooled to 25°C (providing further heat for the first distillation column). Following heat integration of the system, 88, 6 and 3 MJ/kg recovered butanol was required at 37°C respectively for the first distillation column reboiler; 22, 5 and 2 MJ/kg recovered butanol was required at 70°C respectively for heating the broth and butene to the extraction tempeature, and <0.9 MJ/kg recovered butanol was required at 190°C for the second distillation column reboiler. Hence, only 23, 6 and 3 MJ/kg butanol recovered of heat was required (as heat at 37°C can be supplied by low grade heat sources such as the fermenter).

Boiling Points More generally, the boiling points of the preferred organic solvents are listed below. These boiling points are affected by the pressure. In addition, the pressures required to achieve a desired boiling point of 32°C are listed below. The boiling points are for the pure solvent, and rises slightly due to the presence of butanol. This increases the difficulty of achieving a very pure product from the first distillation unit operating at low temperature alone, and favours the presence of the second distillation unit.

As will be appreciated, this list is not exhaustive. Rather, this list includes the preferred organic solvents. Thus, the list omits certain cyclo-com pounds (such as methylcyclobutane; 1 ,1-dimethylcyclopropane; 1 ,2-dimethylcyclopropane; ethylcyclopropane;

methylcyclopropane), certain dienes, alkynes and cycloalkenes.

The term atmospheric pressure as used herein takes it usual meaning and preferably means 1 bara (100 kPa).

Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the invention or of the appended claims.

Claims

Claims:

1. A method for producing a butanol-rich composition suitable for use as a fuel, the method comprising:

(!) fermenting a feedstock in a fermenter to produce an aqueous fermentation broth comprising butanol;

(ii) adding an organic solvent to the broth and performing a liquid/liquid extraction to recover a mixture of the butanol in said organic solvent;

2. The method according to claim 1 , wherein the butanol-rich composition comprises at least 95 v/v% butanol.

3. The method according to claim 1 or claim 2, wherein the organic solvent comprises a solvent selected from the group consisting of pentane, pentene, butane, butene, and mixtures of two or more thereof.

4. The method according to any of the preceding claims, wherein the mixture from step (ii) is placed in thermal communication with the fermenter to provide heat in step (iii) to vaporise the organic solvent.

5. The method according to claim 4, wherein the thermal communication is via a heat exchanger, or wherein the mixture is passed through one or more ducts within or in contact with the fermenter.

6. The method according to any of the preceding claims, wherein step (iii) is conducted under a pressure at which the boiling point of the organic solvent is below a temperature of the fermenter, whereby the temperature of the fermenter is sufficient to vaporise the organic solvent.

7. The method according to any of the preceding claims, wherein the liquid/liquid extraction is conducted under a pressure sufficient to ensure that the organic solvent is in liquid form.

8. The method according to any of the preceding claims, wherein step (ii) further comprises recovering an aqueous solution and wherein the aqueous solution is recycled into the fermenter.

9. The method according to any of the preceding claims, wherein step (iii) is conducted at a first pressure and wherein the method further comprises:

wherein the second pressure is greater than the first pressure.

10. The method according to claim 9, further comprising recovering heat from

condensing the residual organic solvent to provide a portion of the heating in step (iii).

1 1. A method for producing a fuel for use in an internal combustion engine of an automobile, the method comprising performing the method according to any of the preceding claims and blending the butanol-rich composition with gasoline to form the fuel, whereby the amount of butanol in the fuel is at least 5v/v%.

12. Apparatus for producing a butanol-rich composition suitable for use as a fuel, the apparatus comprising:

a fermenter for fermenting a feedstock having a first outlet for an aqueous fermentation broth,

13. Apparatus according to claim 12, wherein the thermal communication between the first distillation unit and the fermenter is via a heat exchanger, or wherein at least a portion of the first distillation unit is located within or in direct contact with the fermenter.

14. Apparatus according to claim 12 or claim 13, wherein said fourth outlet is in fluid communication with said first inlet for recycling the organic solvent.

15. Apparatus according to any of claims 12 to 14, wherein the apparatus further comprising a second distillation unit having a third inlet in fluid communication with said third outlet and having a fifth outlet for a further butanol-rich composition and a sixth outlet for residual organic solvent.

16. Apparatus according to any of claims 12 to 15, wherein the sixth outlet is in fluid communication with the first inlet for recycling the organic solvent, optionally via a condenser in thermal communication with the first distillation unit.

17. Use of heat produced in a fermenter to recover a butanol-rich composition from an aqueous fermentation broth, wherein the heat is used to vaporise an organic solvent used to separate butanol from said broth after a liquid/liquid extraction process.