WO2008111857A1

WO2008111857A1 - Treatment of organic material by digestion

Info

Publication number: WO2008111857A1
Application number: PCT/NZ2008/000048
Authority: WO
Inventors: Alan George Langdon; Xiaofen Li
Original assignee: Waikatolink Limited
Priority date: 2007-03-09
Filing date: 2008-03-07
Publication date: 2008-09-18
Also published as: NZ553786A

Abstract

The present invention relates to a method of treating organic material by digestion, the method including the steps of: a) creating conditions such that at least some of the volatile organic acids enter a gaseous state, and b) extracting at least some of the volatile organic acids in the gaseous state from the digester.

Description

TREATMENT OF ORGANIC MATERIAL BY DIGESTION

TECHNICAL FIELD

This invention relates to a treatment method for organic material.

More specifically this invention relates to a treatment method of organic biomass or waste material such as sewage, or algae.

BACKGROUND ART

Treatment of organic waste material is often required to ensure that waste is safe prior to release back in to the environment, or prior to further use. This is especially the case for sewage.

Sewage treatment usually includes at least three steps, being;

• A primary treatment. This is usually a physical separation, removing solid material (primary sludge) from water and other liquid components,

• A secondary aerobic treatment. In this step organic waste is oxidised and a secondary sludge of cellular biomass is formed.

• The sludges separated from both primary treatment and secondary are then typically treated by anaerobic digestion which incorporates a complex series of digestive and fermentative reactions are carried out by a host of different bacterial species.

The features of anaerobic treatment are discussed in detail below.

The net result of the anaerobic treatment of sewage sludges (or other organic waste material) is typically methane and carbon dioxide. Anaerobic decomposition includes three main steps, being:

1) A hydrolysis step. In this step large, complex organic molecules are converted to smaller, simpler, soluble organic molecules by the action of extracellular polysaccharidases, proteases, lipases and other enzymes.

2) A fermentation and acetogenesis step (digestion). In this step fermentation of the soluble organic materials to fatty acids, H2 and CO2 occur by acid- producing fermentative organisms.

This step also includes the conversion of fatty acids to acetate, CO2 and H2, by fermenting and acetogenic micro-organisms. This step results in the formation of simple organic acids,

3) A methogenesis step. In this step, acetate, H2 and CO2 are converted into methane and carbon dioxide by methanogenic micro-organisms.¹

Methane, the ultimate product of anaerobic decomposition can be used as a biogas fuel. This is generally burnt to produce electricity.

However, the production, and use of methane has a number of significant disadvantages. Methane is difficult to store, especially for long periods of time, and being a greenhouse gas, may lead to environmental damage or problems.

Anaerobic degradation of organic matter in a reactor is generally considered to be a two-stage process in which the acidogenic (fermenting and acetogenic) and methanogenic bacteria must be in a state of dynamic equilibrium, in which the volatile fatty acid (VFA) and other fermentation end-products of

1 Biology of Micro-organisms, sixth edition. Brock, T.D.; Madigan, MT. 1991 hydrolytic/fermentative bacteria are directly converted to CH4 and C.O2 by methanogenic species.

In a two-phase anaerobic digestion the acidogenic stage may be separated from the methanogenesis stage using the difference in growth rates of acidogens and methane-formers.

Two-phase acidogenic digestion has often been considered beneficial for the treatment of some wastewaters and other organic biomass. It has been reported in a number of papers that the optimal pH of the acidification process is about 6.0, while the optimal pH of a methanogenic reactor is about 7.0. Therefore, a two- phase system allows better optimization of both phases.

Anaerobic digestion can also be inhibited, or interrupted prior to the methanogenic stage. This allows the simple organic acids to be recovered from the acidogenic (fermenting and acetogenic) stages of the digestion. The organic acids can then undergo further chemical transformations into useful chemicals or fuels.

Processes have previously been developed to remove simple organic acids from anaerobic digestions.

The processes for removing organic acids from fermentation and acetogenic reactions in anaerobic digestion reactions include extraction of the organic acids in either a liquid or salt form.

Both of these methods have a number of significant disadvantages, and are discussed in turn below.

US 5,874,263 discloses a method of obtaining organic acids, such as acetic acid, and other low molecular weight C2-C6 organic acids from an anaerobic decomposition reaction in a liquid form. This requires a flow of water opposing the movement of organic biomass through the reactor, in which the acetic acid is collected.

In the preferred embodiment of US 5,874,263 the apparatus includes afleast two fermentation reactors. Fresh biomass is directed to the first reactor and conveying 5 means are provided to transfer the partially digested biomass from the first reactor to the second reactor for further digestion. Means to direct an aqueous product- extractant stream counter-current to the flow of biomass is provided to control product concentration and extraction in each of the reactors.

US 5,874,263 also discloses other possible product liquid extraction methods. For 10. example using a series of filters and mixers where the biomass is mixed and washed with water several times to wash the acetic (and other organic acids) out of the biomass, or washing and filtering the biomass between chambers of the reactor.

While the majority of the discussion has been in relation to anaerobic digestion, 15 many of the same principles and processes also apply to aerobic digestion but generally on a lesser scale. Therefore it should be appreciated that the present invention can apply to both types of digestion although anaerobic is preferred.

Using water, or other liquid to extract organic acids (such as acetic acid) from the liquid biomass slurry has a number of significant disadvantages, these include the 20 following:

• The liquid biomass slurry will also include a significant number of other soluble components. These may be extracted along with the desired organic acids, for example other simple organic molecules. The presence of additional extracted components will decrease the purity of acid 25 extracted, and increase the post-extraction processing required to produce the final product. • The liquid extraction requires complicated and expensive machinery and equipment. This includes multi-stage reactors, clarifiers, centrifuges, and filters. These all require money and time to maintain, and clean, thereby increasing the cost, time and maintenance required to keep the process 5 running.

These problems decrease the efficiency and cost effectiveness of producing low molecular weight organic acids from an anaerobic digestion via this method.

US 6.043.392 discloses a method of obtaining organic acids, such as acetic, ^' . propionic and butyric acid (C2-C4) from an anaerobic decomposition reaction in a 10_. volatile fatty acid (VFA) salt form.

The method includes the steps of precipitating metal salts of volatile fatty acids (VFA) from the fermentation liquor of an anaerobic fermentation, then recovering and drying precipitated metal salts of VFA's.

The dried precipitated metal salts can then be used in processed to produce 15 ketones, alcohols, or other desired products.

Column 3 line 65 to column 4 line 7 reads: "Biomass 205 is first passed through pretreatment stage 210, and then fed into fermentation stage 220 where the pretreated biomass is converted into VFA salts 215 and undigested residue 225. Fermentation liquor containing VFA salts 215 is transferred to amine dewatering 20 stage 230 where water is extracted, thus concentrating the VFA salts to approximately 20 % in concentrated stream. Undigested residue 225 from the fermentation stage is discarded or perhaps burned for process heat.

Again, this method has a number of significant disadvantages, these include the following:

25 • Additional chemicals are required to be added to the anaerobic decomposition reaction in order to form the desired organic acid salts. The requirement for additional chemicals increases the cost, and therefore the efficiency of the process.

• This process only provides salts as a product. If the organic acid is to be used as a feed stock for further processing, the salt may not be the desired form. Therefore, additional processing steps may be required. Again, these would increase the cost and efficiency of the process.

• The method disclosed in US 6,043,392 only removes the salt at the end of the process; therefore, the presence of high concentrations of salt may have a negative and possibly an inhibitory effect on the anaerobic decomposition reaction, taking place. This may therefore decrease the full potential amount of organic acid which may be produced.

All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.

It is acknowledged that the term 'comprise' may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term 'comprise' shall have an inclusive meaning - i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term 'comprised' or 'comprising' is used in relation to one or more steps in a method or process.

It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.

Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

DISCLOSURE OF INVENTION

According to one aspect of the present invention there is provided a method, of treating organic material by digestion,

the method including the step of:

a) digesting the organic material to produce volatile organic acids,

the method characterised by the steps of:

b) creating conditions such that at least some of the volatile organic acids enter a gaseous state, and

c) extracting at least some of the volatile organic acids in the gaseous state.

While the majority of the discussion has been in relation to anaerobic digestion, many of the same principles and processes also apply to aerobic digestion but generally on a lesser scale. Therefore it should be appreciated that the present invention can apply to both types of digestion although anaerobic is preferred.

In a preferred embodiment step a) is performed within a main digester vessel.

According to another aspect of the present invention there is provided a gaseous volatile organic acid produced by the method substantially as herein described.

In a preferred embodiment the term 'organic material' should be taken to include any biodegradable organic material. This may include any hand and mechanically sorted municipal solid waste, or sludge (sewage), any animal-based material, any plant-based material, or any marine or freshwater biomass, for example, algae..

It should be appreciated by one skilled in the art that the organic material includes complex organic molecules including but not limited to polysaccharides, fats and proteins.

Throughout this specification the phrase 'digesting the organic material to produce volatile organic acids' should be taken, as including steps of digestion (most likely anaerobic, but possibly aerobic) leading to, or resulting in the formation of volatile organic acids. This phase should include the following steps:

i) a hydrolysis step where complex organic molecules are converted into smaller and simpler organic molecules,

ii) a digestion process,

iii) an acetogenic step where the simple organic molecules are converted into volatile organic acids.

Both digestion and acetogenesis lead to the production of volatile organic acids, via the action of different micro-organisms, generally mixed acid bacteria and acetogens respectively.

It will be appreciated that the phrase 'digesting the biomass to produce volatile organic acids' occurs substantially prior to the conversion of simple organic acids to methane via methanogenesis.

Throughout this specification the term 'volatile organic acids' should be taken to include any volatile organic acids, but in particular low molecular weight organic acids with less than four carbon atoms. Such organic acids include acetic acid and propionic acid to name a couple.

Throughout this specification the organic acid shall herein be referred to as acetic acid. This is for ease of reference, and should not be seen as limiting.

In a preferred embodiment the digestion may be undertaken by any means known by one skilled in the art. It should be appreciated that the digestion may include additional components which ensure the desired solids content, pH, nutrient levels and substrate are provided.

It should be appreciated that digestion normally takes place in an organic material slurry.

As digestion proceeds, acetate and acetic acid are produced. These will initially be present in an aqueous phase according to the following equilibrium:

Acetate ion (aq) <-» Acetic acid (aq)

The acetate is produced by the digestion. The equilibrium is pH dependant, with lower pH levels driving the equilibrium towards the right (aqueous acetic acid).

The aqueous acetic acid is also in equilibrium with gaseous acetic acid, according to the following equilibrium:

Acetic acid (aq) <-> Acetic acid (g)

In one embodiment gaseous acetic acid is extracted from the digester during digestion.

The removal of gaseous acetic acid during digestion drives both of the above equilibriums to the right, converting acetate ions into the aqueous acetic acid, and aqueous acetic acid into gaseous acetic acid respectively.

Driving the equilibriums in this manner is beneficial as it prevents a build up of either acetate or aqueous acetic acid which may inhibit the digestion by lowering the pH too far. Driving the equilibriums in this manner also results in efficient extraction of acetic acid.

In a preferred embodiment the extraction of gaseous acetic acid may be continuous throughout the digestion, or part thereof.

However, this should not be seen as limiting, as the extraction of gaseous acetic acid may also be undertaken at discrete, regular or irregular intervals throughout the digestion.

The inventors envisage that the most efficient system is likely to be one pre- , requisite where the acetic acid is removed at substantially the rate it is being formed. If intermittent extraction was used the continuous stirring/mixing function (discussed later) would be lost. However there are some reports that claim that intermittent stirring may be an advantage. If intermittent extraction was used, this would need to occur prior to the pH falling to a level at which the rate of fermentation is inhibited.

In one embodiment the organic acid may be extracted from the digester by collecting the head-space gas, and separating an organic acid such as volatile gaseous acetic acid from the rest of the head-space gases.

In one preferred embodiment gaseous acetic acid may be extracted by pumping a gas through the digested organic material.. This will herein be referred to as 'purging' (removing the desired components) from the digester.

In one preferred embodiment the gas bubbled through the organic material may be a mixture of gaseous substances (and possibly vapour) produced by the digestion. This may be collected from the digester head-space and shall herein be referred to as 'head-space' gas. The head-space gas produced by digestion is likely to include carbon dioxide and gaseous acetic acid as its main components. However, it may also contain hydrogen, nitrogen and methane.

The use of the head-space gas mixture to 'bubble' through the organic material should not be seen as limiting. The present invention may alternatively utilise any other gas, or gas mixture, including: nitrogen, carbon dioxide, or inert gases. It should be appreciated that the gas should be such that it maintains the conditions required for digestion. For example, if the process is anaerobic digestion, then the gas should_. not contain oxygen.

In a preferred embodiment the head-space gas may be recycled. In this embodiment the head-space gas may undergo an extraction phase to remove, strip or condense out the acetic acid. The 'purge' gas (head-space gas with acetic acid or other volatile organic acid removed) is then recycled back into the digester. It is envisioned by the inventors' that other gaseous components from the digester will remain in the gas phase during this process and be recycled.

In one embodiment the head-space gas may be introduced at substantially the base of the digester. This means that the head-space gas will 'bubble' through the organic material slurry in the digester.

This is highly desirable, for several reasons, including the following:

• It brings the head-space gas into contact with a high percentage of the digester contents. This increases the efficiency with which gaseous acetic acid can be extracted from the digester,

• It acts to agitate the biomass undergoing digestion. This prevents the need to stir or physically agitate the reaction mixture. Stirring is commonly undertaken to ensure the reaction proceeds at the fastest rate possible. The bubbling of purge gas through the digester therefore decreases the complexity of the digester equipment required, thereby reducing equipment and maintenance costs.

• The movement of the purge gas through the digester quickly and easily extracts gaseous acetic acid from the digester. This acts to drive the acetic acid equilibriums in the direction of gaseous acetic acid, thereby increasing the efficiency of acetic acid production and extraction.

Actively purging the biomass slurry increases the efficiency and speed of gaseous acetic acid extraction.

In a preferred embodiment the gaseous acetic acid may be extracted at substantially the same rate at which it is produced by the digestion. This leads to the establishment of a steady state in the digester. This prevents any significant variations in pH of the organic material slurry which could adversely affect or inhibit digestion. The steady state allows maximum optimisation of digestion, and thereby gaseous acetic acid extraction.

Steady state operation implies continuous agitation, however it should be appreciated that there may be advantages in allowing the microorganisms some time to aggregate during periods without agitation

In a preferred embodiment the flow rate and volume of the head-space gas may be altered as required to optimise gaseous acetic acid extraction. For example, at higher rates of acetic acid production a higher volume, or a higher flow rate of purge gas may be utilised to ensure efficient extraction of the acetic acid.

Other things being equal it is expected that the production of acetic acid will depend on the level and type of substrate.

One aim of the present invention would be to maintain a constant maximum rate of production by maintaining all parameters at their optimum levels. The head-space gas is expected to become saturated with acetic acid when equilibrium is established. Generally the ideal situation would be to maintain the head-space gas under a condition close to saturation and increase the flow rate to the level that extracted acetic acid as fast as it was being produced.

A larger digester would be expected to produce more acetic acid than a small digester, and would thus require a faster purge rate.

The advantage of bubbling head-space gas through the organic material or slurry is to increase the gas/liquid interface through which the acetic acid equilibrium between the aqueous and the gaseous phases is achieved. The smaller the gas bubbles, the larger this area will be and the faster the equilibrium will be established. Therefore, excessively rapid 'bubbling' would not be expected to allow time for equilibration. This may ensure the acetic acid is extracted as fast as it was being formed but would consume more energy per mole of acetic acid extracted

This method of 'purgjng' the digester with head-space gas or other gas mixture is preferred. It is not limited by the re-establishment of the equilibrium between gaseous and aqueous acetic acid, in the bulk of the biomass slurry and the head space respectively. This would limit the speed of extraction when the head-space gas is simply collected from which the acetic acid is extracted, without being bubbled through the digester. The re-establishment of the equilibrium may be limited by the limited interface between the slurry and the head space, and the need for the aqueous acetic acid to move through the organic material slurry to the head-space/slurry interface and evaporate into the head-space.

It is anticipated that the presence of other gas constituents in the purge gas will not unduly affect the subsequent use of the gaseous acetic acid. However, in some embodiments the extracted gaseous acetic acid may be treated after separation from the purge gas to separate acetic acid other gas components, for example carbon dioxide. This may be via any known methods.

For example to separate acetic acid from carbon dioxide the head-space gas (including extracted acetic acid), or separated gaseous acetic acid may be bubbled through water. Acetic acid has greater solubility than carbon dioxide, and can therefore be collected as an aqueous phase while the carbon dioxide remains in the gaseous phase.

According to another aspect of the present invention there is provided acetic acid produced in accordance with the methods as previously discussed.

Ammonia could be removed by treatment with a mineral acid, such as sulphuric acid. In this example acetic acid would initially dissolve in the acid to a saturation level and would then not be affected. Neutralisation of the mineral acid with the ammonia would produce useful by-product salts eg ammonium sulphate.

In some embodiments it may be advantageous to remove the ammonia first as it the mixture of ammonia and acetic acid gases was passed through water, ammonium acetate would be formed.

However, very little ammonia will enter the head space under the acid conditions that favour evaporation of acetic acid

The inventors have recognised that a preferred embodiment may involve the extraction of the acetic acid using equipment separate from the main digester vessel.

With anaerobic digestion, it is preferred that the temperature remains within the main digester at around 35°C. This is a temperature at which mesophilic microorganisms digesting the organic material thrive. Any temperature significantly above this level can cause the micro-organisms to die, thus adversely affecting the digestion process. If thermophilic micro-organisms are used temperatures up to 55°C may be possible.

However, extraction efficiency of acetic acid is greatly improved with the use of even higher temperatures, say in the order of 95°C. If this temperature was applied to the main digester vessel, then certainly the micro-organisms would perish.

Another factor that the inventors are conscious of is that the equilibrium mole fraction of the acetic acid in vapour is less than that in liquid.

Taking these factors into account, a preferred embodiment of the present invention utilises the following equipment in addition to the main digester vessel,

a) a stripping cell, and

b) a recovery cell, and

c) fractional distillation apparatus.

It is envisioned that the function of the stripping cell would be to convert acetic acid in the liquid phase to the vapour phase - as per step b) of the present invention. In preferred embodiments the stripping cell exposes a small proportion of the reactor contents to water vapour saturated inert gas. Preferably the gas is. at a. sufficiently higher temperature so as to gain the greater efficiency of acetic acid extraction required.

The stripping can be achieved using a water saturated inert gas stream bubbled through the reactor fluid. Alternatively, a counter current flow of spray in a spray stripping column may be used. Yet alternatively a counter current flow in a packed column may also be used.

In preferred embodiments these technologies would be used in a stripping cell separate from the reactor utilising filtered or unfiltered reactor fluids. Although of course in some embodiments these may be included within the reactor.

It should be appreciated that in describing this additional technology, reference will be made to acetic acid. It should be appreciated that the principles in this technology could be applied to other volatile organic acids as well.

A suitable stripping gas can still be the head space gas from the reactor.

However, to ensure that the acetic acid can be recovered by the next stage of using recovery cell, it is important that the carbon dioxide has been removed from the head space gas.

Other embodiments of stripping apparatus are also envisioned.

Once the stripping cell has converted the acetic acid into a gaseous phase, then the acetic acid needs to be separated from the other gases (such as water vapour) as per step c) of the present invention. This is done in a preferred embodiment by a recovery cell.

The recovery cell operates by passing the water saturated gas stream containing the acetic acid through an appropriate solvent or reaction medium. The choice of solvent or reaction medium is very important. The solvent must have an affinity for the acid gas, but be a poor solvent for the water . The purpose of the solvent is to separate the acetic acid from water vapour and concentrate it in the solvent so that the acetic acid can then be distilled out from the resulting solution.

Not only does the solvent have to be immiscible with water and have an affinity for acetic acid, but it must also have a high boiling point so that the acetic acid can be readily separated therefrom in the fractional distillation stage.

The inventors have found therefore that suitable solvents include tertiary butyl amine or tertiary butyl phosphate dissolved in kerosene. At present the preferred solvent is tertiary butyl amine but it should be appreciated that solvents having similar properties to that described above could also be used.

The gas used in the stripping and recovery cells is maintained at a constant temperature to prevent water evaporation and/or condensation. This would be a source of energy loss and product dilution.

Volatile organic acids typically show negative deviations from ideality and typically have vapour pressures lower than that of water. The condensate from the head gas Would have an acetic acid concentration of less than that of the aqueous phase in the reactor.

The combination of stripping cell and recovery cell works particularly well together because the relatively low acid concentration in the vapour phase is effectively efficiently extracted by the reactive solvent with the water remaining in the gas phase.

The next stage is the recovery of acetic acid from the amine/acid liquid mixture.

In a preferred embodiment this is achieved through fractional distillation. Applying heat to the amine/acid mixture causes the acetic acid to evaporate first (as the solvent has a high boiling point). The evaporated acetic acid is drawn off from the distillation column as glacial acetic acid. The solvent can be reused in the recovery cell.

There are a number of optimisation issues to be considered when utilising the present invention. The inventors have shown that extraction efficiency is increased by gas flow rate. They also calculate that extraction efficiency will improve by maintaining the pH in the stripping cell at the pKa of the volatile organic acids or lower.

It may be an advantage to carry out the anaerobic digestion at a lower pH. Extraction efficiency is also increased by raising temperature. The inventors expect that recovery in the reactive solvent will decrease with increasing temperature. Therefore for a given volatile organic acid, there is likely to be a set of optimum conditions.

Energy efficiency can be optimised by ensuring that waste heat from the acid recovery process is used to maintain the elevated temperature required for the anaerobic digestion process. If the optimum temperature is different than that for digestion, the recovery process needs to be separated from the reactor to ensure that water evaporation/condensation does not occur.

In a preferred embodiment the extracted acetic acid may be used for one of the following purposes:

• As a feed stock.

Acetic acid may be used as a feed stock in any known reactions or processes. For example reacting the acetic acid with an alcohol to form an ester, this may be used as a liquid biofuel.

• The acetic acid may be dehydrated to acetic anhydride prior to further processing. This may be beneficial, as in some circumstances acetic anhydride is easier to handle and use than acetic acid.

• The acetic acid could serve as a liquid fuel carrier being catalytically converted to gaseous components immediately before combustion in a combustion engine or fuel cell.

• The extracted acetic acid can be used as a substrate for methanogenesis reactions, leading to increased efficiency of methane production.

• The acetic acid could be flavoured and sold as vinegar to general consumers or used in other food products.

• The acetic acid could be concentrated and sold as a commodity chemical (glacial acetic acid).

According to one embodiment of the present invention there is provided a methanogenesis method characterised by the step of

a) introducing acetic acid as a substrate into a digester for the second phase of anaerobic digestion

In this embodiment where the extracted acetic acid is used as a substrate for methanogenesis reactions, the method of the present invention may be undertaken in a first digester vessel (being the first-phase of anaerobic digestion). The acetic acid product can then be controllably feed into a second digester vessel under optimal conditions for methanogenesis (being the second-phase of anaerobic digestion) to produce methane.

This embodiment has a number of significant advantages over previous methods of producing methane where the organic material slurry is transferred from vessel to vessel. These include the following:

• Acetic acid is a purer substrate than organic material slurry.

• Using acetic acid as a substrate limits, or prevents the requirement for large volumes of biomass (organic material) slurry to be transferred between digester vessels when two-phase anaerobic digestion is undertaken.

This therefore simplifies the equipment required, as a gaseous substrate (and possibly smaller volumes of slurry) is the only components which need to be transferred between the first and second digester vessels. This process therefore allows methanogenesis to occur without the requirement for slurry, or to utilise a smaller volume of slurry which is optimised for methanogenesis.

This allows conditions for methanogenesis to be optimised, and maintained, thereby optimising methane production.

Therefore, one application of the present invention may be the retrofitting of existing anaerobic digesters, to incorporate the method of the present invention, and thereby increase the efficiency of methane production.

Retrofitting existing digesters would be highly desirable. It would significantly _, increase their efficiency, while being a cheap and easy modification.

According to yet another aspect of the present invention there is provided a digester configured to operate according to the methods previously discussed.

The present invention provides a number of significant advantages over liquid or salt extraction of volatile organic acids form anaerobic digestion reactions, these include the following:

• No additional chemicals are involved. The gas used to purge the digester contents of volatile acids is the reactor gas itself which will be recycled after the volatile acids have been stripped or condensed out.

Other gaseous reactor constituents will remain in the gas phase and be recycled to the reactor.

• Reactor conditions once optimised should reach a steady state -and not need chemical adjustment. This is because the volatile acid will be removed from the reactor as fast as it is being formed. In addition the gas purging will do away with the need to stir the reactor, thereby compensating for the energy requirement of purging.

• The invention can be retrofitted to existing two stage anaerobic rectors or digesters allowing the complete separation of the methanogenic stage from the hydrolysis and acetogenic stages of anaerobic digestion. This will allow better optimisation of both stages.

Retrofitting is important in those systems which have been designed for the production of methane, and for which it would be uneconomic to. add further . processing steps.

• Gas phase separation is inherently simpler and cleaner than solid, liquid or salt separation or extraction.

Gas phase extraction does not require the multistage reactors and associated clarifiers, centrifuges and filters etc. that are required for liquid phase extraction processes.

A further advantage is that the desired product will not be mixed with other reactor liquors as is the case for counter current liquid extraction.

• The present invention provides a new method of extracting gaseous acetic acid for use as feed stocks for further chemical processing. US Patent No. 6 043 392 describes some novel chemical reactions that yield useful products from the volatile fatty acid extracted. However other processes that are in the public domain could be used.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings in which:

Figure 1 Schematic of stripping apparatus contained within biological reactor

Figure 2 Schematic of extraction system separated from the biological reactor

BEST MODES FOR CARRYING OUT THE INVENTION

Experimentation has been undertaken on a laboratory scale, showing the process of the present invention.

Large scale experimentation is currently underway.

Details of experimentation to date is provided below.

Experiment 1 : Anaerobic digestion

In a preliminary experiment a bench scale anaerobic reactor was set up using a 5 L bottle, a stirring device, a constant temperature bath and appropriate tubing to allow collection of gaseous product and also allow gas circulation. Anaerobic digestion of a sucrose substrate was achieved using an active anaerobic sludge from a sewage treatment plant. Methane gas was produced at a temperature of 35⁰C. This is a well known phenomena as is the lowering of pH that will occur if the methanogenic stage producing methane is inhibited. The lowered pH produced by inhibition of the methanation step provides the required acid condition for the extraction of volatile organic acids.

Experiment 2: Stripping of acetic acid

The fermentation vessel maintained at 35⁰C was used to demonstrate the stripping of acetic acid from an aqueous solution typical of anaerobic digestion (3000 mg/L). Nitrogen gas was purged through a closed loop system using a diffuser. The purged acid was then determined by passing it through a standard sodium hydroxide solution and the rate of acid purging was determined by the rate of neutralisation of the sodium hydroxide. Table 1 summarises data for replicate runs at a purge gas flow rate of 880 mL min^"1

Table 1. HAc stripping rate at flow rate 880 mL mi rn_-f

Extraction Time NaOH Transferred HAc transfer rate

(day) (ml) HAc (mmol) mmol day^"

1.06 10.00 1.06 1.00

0.92 10.00 1.06 1.15

0.99 10.00 1.06 1.07

Average 1.07

Nitrogen gas was purged through a dilute solution of acetic acid and the extracted acid gas was reacted in a second vessel containing sodium hydroxide.

The data confirm that it is feasible to purge HAc out of dilute HAc solutions at a modest gas flow rate.

Experiment 3. Effect of purge rate on acid stripping

It was realised that optimum extraction conditions would involve purge gas flow rate. An experiment was performed in which the purge rate was increased over the range form 330 to 880 mL min^'1.

Data are summarised in Table 2.

Table 2. Effect of purge rate on acetic acid stripping rate

Time HAc

Purge, rate NaOH HAc stripping rate

(mL min^"1) (day) (mL) (mmol) (mmol day^"1)

330.00 3.03 14.00 1.48 0.10

680.00 1.00 10.00 1.06 0.21

880.00 0.99 50.00 5.30 1.07

The experimental arrangement was similar to that used to obtain the data of Table 1.

The extraction rate increased with gas flow rate. The non linear increase of extraction rate with purge rate could be due to a number of factors. It may be that smaller bubbles are produced at higher flow rates and these increase the gas/water surface area per unit volume of gas and hence the transfer rate. More vigorous flow may also cause a thinning of the boundary layer which would also enhance the transfer rate.

Effect of temperature on stripping rate

The stripping rate can be expected to increase with the partial pressure and hence concentration of the acetic acid in the gaseous phase. This is controlled by Henry's law. The Henry's law constant and its temperature variation is known and allow the effect of temperature on the partial pressure and hence concentration of the acetic acid in the gaseous phase to be calculated. The results of these calculations are shown in Table 3.

Table 3 Variation of Henry's Law constant (KH), acetic acid partial pressure

(PH_AC) and gas phase acetic acid concentration (CH_AC) with temperature¹

(⁰C) (K) ( mol L^"1 atnV¹) (Pa) (mol/M3)

25 298 4102 1.234 0.0004984

35 308 2075 2.441 0.0009533

45 318 1095 4.623 0.0017486

55 328 . 6010 8.420 0.0030880

65 338 342.2 14.80 0.0052680

75 348 201.0 25.19 0.0087079

85 358 121.7 41.62 0.0139843

95 368 75.71 66.91 0.0218704

99 372 63.06 80.33 0.0259734

The data show that the partial pressure and concentration of acetic acid in the vapour phase can be expected to rise steeply as temperature increases. Thus extraction efficiency can be expected to be more efficient at elevated temperatures.

1. Johnson, B.J., Betterton, EA, Craig, D. (1996), "Henry's law coefficients of formic and acetic acids", Journal of Atmospheric Chemistry, Springer Netherlands, 24 (2): 113-119.

Experiment 4: Effect of temperature on acid stripping rate

An experiment was performed where the stripping was carried out over a range of temperatures. Data are summarised in Table 4.

Table 4 Increase in stripping rate with temperature of stripping system

Time Time NaOH Transferred HAC transfer

Temperature ⁰C (min) (day) (ml) HAc mmol rate mmol/day

35 water bath &

UV lamp 1425.60 0.99 10.00 1.06 1.07

35 (Incubator) 300.00 0.21 10.00 1.00 4.81

66 (Incubator) 1020.00 0.71 45.00 4.51 6.37

There is a clear increase in extraction efficiency with temperature. The data also show the advantage of containing the whole apparatus in the constant temperature enclosure provided by an incubator.

Experiment 5 Recovery of the acetic acid by amine solvent

The ability of amine solvents to recover acid from gas phases is well known. We chose tributly amine because of its immiscibility with water and its high boiling point. The acid containing gas from a stripping cell was passed through tertiary butyl amine in a recovery cell and subsequently through standard sodium hydroxide. The time taken to neutralise the hydroxide when passed through the amine was compared with time taken when the amine was not included in the gas flow loop. The ratio of times provides a measure of the amine recovery efficiency. With 100% efficiency, an infinite time would be required.

When passed through amine no indicator change after 2980 minutes

Time taken when passed through hydroxide alone = 420

Maximum fraction of acid not extracted = 0.141

Minimum extraction efficiency = 86%

Experiment 6 Experiment to determine the purity of acetic acid produced after distillation

A conventional one stage distillation of amine solution containing the recovered acetic acid yielded glacial acetic acid with a purity, as determined by NMR of 98 - 99% as indicated by the data summarised in Figures 3.

Figure 3 Recovery of pure acetic acid by distillation of the solution of acetic acid reacted with tributyl amine

The data show that with single stage distillation efficiently separates the amine from the acid yielding a pure product. The small amount of amine carried over with the acetic acid can be removed during fractional distillation.

Claims

WHAT WE CLAIM IS:

1. A method of treating organic material by digestion, the method including the steps of:

a) digesting the organic material to produce volatile organic acids,

the method characterised by the steps of:

c) extracting at least some of the volatile organic acids from the gaseous state.

2. A method as claimed in claim 1 wherein step a) is performed within a main digester vessel.

3. A method as claimed in either claim 1 or claim 2 wherein step b) is performed in a stripping cell.

4. A method as claimed in any one of claims 1 to 3 wherein step c) is performed in a recovery cell.

5. A method as claimed in any one of claims 1 to 4 wherein the volatile organic acid is acetic acid.

6. A method as claimed in any one of claims 1 to 5 wherein the volatile organic acid is removed from the digester at substantially the same rate as the volatile organic acid is formed.

7. A method as claimed in any one of claims 1 to 6 wherein the organic acid is extracted from the digester by collecting head-space gas and separating the gaseous volatile organic acid from the rest of the head-space gases.

8. A method as claimed in any one of claims 1 to 7 which includes the step of pumping gas through at least some of the organic material.

9. A method as claimed in claim 8 wherein the gas is head-space gas.

10. A method as claimed in claim 9 wherein the head-space gas has had an volatile organic acid removed there from.

11. A method as claimed in any one of claims 8 to 10 wherein the gas. is introduced at substantially the base of the digester.

,

12. A method as claimed in any one of claims 8 to 11 wherein the gas is saturated with water vapour.

13. A method as claimed in any one of claims 1 to 12 wherein a solvent is used to remove the volatile organic acid from the gaseous state.

14. A method as claimed in claim 13 wherein the solvent has comparatively high boiling point with respect to the volatile organic acid.

15. A method as claimed in either claim 13 or claim 14 wherein the solvent is substantially water immiscible.

16.. A digester configured to operate according to the method as claimed in any one of claims 1 to 15.

17. An organic acid produced from the method as claimed in any one of claims 1 to 15.

18. An organic acid as claimed in claim 17 which is acetic acid.

19. A methanogenesis method characterised by the step of:

a) introducing acetic acid as a substrate into a digestive vessel for the second phase of anaerobic digestion.

20: A method as claimed in claim 19 wherein the acetic acid is derived from the method as claimed in any one of claims 1 to 15.

21. A method substantially as herein described with reference to and as illustrated by the accompanying examples.

22. A digester substantially as herein described with reference to and as illustrated by the accompanying examples.