WO2010046913A2

WO2010046913A2 - Selective filtration process for biogas production

Info

Publication number: WO2010046913A2
Application number: PCT/IN2009/000545
Authority: WO
Inventors: Nitant Vishnu Mate; Santosh Raghunath Gondhalekar; Arvind Purushottam Joshi
Original assignee: Kirloskar Integrated Technologies Ltd.; Gangotree Eco Technologies Pvt. Ltd.
Priority date: 2008-10-06
Filing date: 2009-10-05
Publication date: 2010-04-29
Also published as: WO2010046913A3

Abstract

The invention presented is a method for improving biogas generation by segregation of the two stages i.e. acidogenesis and methanogenesis by means of two separate reactors and semi- permeable membrane unit. The semi- permeable membrane unit permits the migration and concentration equilibration of low molecular size fatty acids but does not permit the migration of high molecular weight undigested, semi-digested feed as also of microbial cells. The transportation of volatile fatty acids is brought about by circulating the acid filtrate and the slurry containing the methanogenic microorganisms on two sides of a semi permeable membrane thus permitting concentration equilibrium of volatile fatty acids with the contents of the methanogenic reactor without exchanging the two microbial populations. The overall effect is faster conversion of feed into biogas without undue loss of the methanogens.

Description

FIELD OF INVENTION

The present invention relates to a method of biogas production, more particularly, it relates to a selective permeation process for biogas production.

BACKGROUND OF INVENTION

Generation, of biogas from biomass is one of the several alternatives that are being attempted to get over the problems arising from shortage of fossil fuels. Conversion of biomass to biogas proceeds through a number of steps. Initially, there is break down of the biomass into smaller units capable of penetrating the microbial cell wall. This is done through the mediation of exo-enzymes released by the microbial cell and results in hydrolysis or depolymerisation of complex biological matter. This is followed by acidogenesis that converts the smaller units into volatile fatty acids (VFA's). These fatty acids in turn are converted into methane and carbon dioxide by a process called methanogenesis that is brought about by another set of microbes.

The set of microbes that bring about acidogenesis are fast multipliers, can tolerate a downward shift of pH and an oxygenated environment. The set that brings about methanogenesis, however, are slow multipliers, perish at low pH values and need an anaerobic environment.

Prior Art and their Drawbacks:

1. United States Patent Application 4022665 discloses a "Two phase anaerobic digestion process".

An improved two phase anaerobic digestion process in which an initial phase continually receives an organic feed for short detention times of less than two days under conditions which efficiently liquefy and breakdown the feed to lower molecular weight acids and other intermediates for conversion to methane. A succeeding phase is operated to treat the lower molecular weight acids and intermediates for detention times of about two to about seven days under conditions which efficiently lead to production of methane. The feed is loaded in the first phase at rates from about 1 to about 10 pounds of total organics per cubic foot per day; and the products from the initial phase are loaded in the succeeding phase at rates of about 0.1 to about 0.5 pounds total organics per cubic foot per day.

Drawback

As there is no separation of Micro organisms, these get drained out through the output slurry. This limits the speed of the process.

2. United States Patent Application 4696746 discloses a "Two phase anaerobic digestion".

A two phase anaerobic digestion process for production of methane from organic carbonaceous material in which an active acid forming microbial population is maintained in a first acid forming digestion phase and an active methane forming microbial population is maintained in a second methane forming digestion phase, the liquid effluent from the acid forming digestion phase being passed to a first methane forming digester and gaseous product from the acid forming digestion phase being passed to a second separated methane forming digester for production of methane in the first and second methane forming digesters of the methane forming digestion phase. The two separated methane digestion phases provide increased overall methane production.

Drawback

Micro organisms are not retained in their respective digesters. Every time they get drained out with the output slurry. This limits the speed of the process. 3. Indian Patent Application. 210309, discloses "A triphasic Biomethanation process".

A three step biomethanation process, to convert starch or sugary agricultural feed stock into a methane rich gas mixture for facilitating generation of biogas to be used as kitchen fuels, electrical power or transportation from renewable biomass in a cost-effective manner. The process brings about conversion of starch-rich or sugar-rich biomass into methane through three stages, namely, hydrolysis, acidogenesis and methane formation. The present invention deploys enzymes/physical/microbial parameters to hasten the hydrolytic reaction in the first stage. It also deploys microbial consortia that have been enriched for a targeted feed in order to speed up the process of conversion. The formation of alcohol from starch is prevented, as a result of the consortia deployed, the conversion efficiency is substantially improved and the retention time is also reduced to cut the capital cost of the plant.

Drawback

Though the process is divided into three stages, it is observed that no attempt has been made to retain the microbial consortia in the respective digesters. This limits the process speed, as every time new bacteria needs to be generated.

4. United States Patent Application 5,529,692 discloses a "Method and apparatus for anaerobic biological hydrolysis and for subsequent biomethanization".

A method for the biological processing of organic substances and more particularly for anaerobic biological hydrolysis for subsequent biomethanation, with pH value control, in the case of which the dissolved and/or un dissolved organic substances supplied to a first reactor are at least subjected to slight acidification in such reactor, the major part of the un dissolved, at least partly acidified organic substances taken from the first reactor are supplied to a second reactor for the performance of at least one solids hydrolysis step and the main part of the dissolved, at least partly acidified organic substances from the first reactor and from the second reactor are supplied to a third reactor for the performance of at least one methanation step.

Drawback

As there is no separation of Micro organisms for acid digester as well as for methane digester, the consortia get lost, every time they get drained out through the output slurry. This limits the work force for conversion of biomass into methane and thus limits the speed of the process.

5. United States Patent Application 4,781,836 discloses a "Method for biomethanation".

An improved process for the biomethanation of an organic substrate includes treating the substrate in a first reactor to form organic acid anions, passing an aqueous preparation containing dissolved organic anions through an anion exchanger so that the organic acid anions are adsorbed and separated from the remainder of the aqueous preparation, desorbing the organic acid anions and passing the desorbed acids to a second reactor containing methanogenic bacteria which convert the acids to methane. In a preferred embodiment, a bicarbonate solution is produced in the second reactor and it is used to desorb the organic acid anions and regenerate the anion exchanger into the bicarbonate form.

Drawbacks

In this isolated case where introduction of an ion exchange column overcomes such a loss of microbial population, the feed streams have to be interrupted when the ion exchange resins get saturated with volatile fatty acids as also when the fatty acids are to be loaded.

Another drawback in this invention is the fact that non-ionic metabolites and toxins produced during microbial action keep accumulating in the methanogenic culture medium since there is no way these can be eliminated

Thus, after analyzing the limitations of these suggested Prior Art, the new inventive procedure is set up.

OBJECTS OF PRESENT INVENTION

Object of present invention is to provide a selective permeation process for biogas production for segregating the acidogenic and methanogenic stages and improving the efficiency of biogas generation.

Further object of present invention is to provide a selective permeation process for biogas production to bring about a much faster conversion of feed into biogas without undue loss of the methanogens.

SUMMARY OF INVENTION

The present invention provides a selective permeation process/method while segregating the acidogenic and methanogenic stages and improving the efficiency of biogas generation by eliminating the need to continuously replacing the contents of the methanogenic stage. As a continuous drain in the number of active cells is eliminated and the nutrient requirement of the methanogens is met, their number increases progressively leading to a faster completion of the conversion process. The selective transfer of volatile fatty acids from the acidogenic reactor to the methanogenic reactor without permitting either the larger molecules or microbial cells is brought about by circulating the fluids in the two reactors in a countercurrent manner along two sides of a semi permeable membrane.

The concentration of volatile fatty acids in the acidogenic stream is much higher on one side of the membrane compared to that in the stream carrying the methanogenic slurry. The volatile fatty acids migrate across the membrane to the methanogenic side in order to bring about concentration equilibrium. These volatile fatty acids are thus transported to the methanogenic reactor without increasing the volume of the reactor. The concentration of volatile fatty acids drops in the methanogenic reactor during their conversion to methane and the whole process keeps repeating.

It is important to ensure that there is no difference in the pressure existing in the two circulating streams that may result in physical migration of fluid from one stream to the other through the pores of the membrane.

The efficiency of the system can be enhanced by increasing the concentration of volatile fatty acids (e.g. by removal of water using the reverse osmosis process), by increasing the surface area of the interfacing membrane or by creating turbulence in the moving streams etc.

The net effect of the two recirculations thus, is to transport the volatile fatty acids to the methanogenic reactor without exchanging the microbial populations or physical movement of carrier fluid from the acidogenic reactor to the methanogenic reactor.

Other features, advantages, and objects of the present invention will become more apparent and be more readily understood from the following detailed description, which should be read in conjunction with the accompanying drawings. STATEMENT OF THE DSfVENTION

According to the present invention there is provided a selective permeation apparatus for producing biogas for various types of feeds, the selective permeation method comprising; an acidogenic reactor for mixing pulverized biomass with fresh water and an acidogenic slurry to, generate volatile fatty acids; a methanogenic reactor for the conversion of volatile fatty acids into methane and carbon dioxide; a filter unit capable of separating the volatile fatty acid solution from the acidogenic slurry received from the acidogenic reactor; an acid filtrate tank to store the acid solution received from the filter unit; a filter having plurality of semi-permeable membrane tubes enclosed in a chamber; a first peristaltic pump for pumping the acid solution in the acid filtrate tank through the semi-permeable membrane tubes of the filter and returning to the acid filtrate tank; and a second peristaltic pump for pumping the methanogenic slurry from the methanogenic reactor through the outer side of the semi-permeable membrane surface and collecting back to the methanogenic reactor, wherein the acidogenic slurry is allowed to pass through the plurality of semi-permeable membrane tubes and is collected back in the acid filtrate reactor, the methanogenic slurry is allowed to pass on the outer side of one or more of semi-permeable membrane tubes and collected back in the methanogenic reactor, thereby enabling passage of volatile fatty acids through the membrane and permitting the migration of volatile fatty acids from the acidogenic tank to methanogenic tank without exchanging microbial population through the mechanism of concentration equilibrium. Typically, wherein the membrane unit comprises; a chamber for enclosing; a plurality of semi-permeable tubing enclosed in the chamber; a first inlet and a first outlet openings for circulating the volatile fatty acid solution through the plurality of semi-permeable openings; and a second inlet and a second outlet openings for circulating the methanogenic slurry around the plurality of semi-permeable openings.

Typically, wherein the acidogenic reactor comprises a stirrer for stirring the pulverized feed and acidogenic microorganisms; and a heater for heating the acidogenic reactor for maintaining the temperature of the acidogenic reactor between 30 to 45 degree centigrade.

Further according to the present invention there is provided a selective permeation method/process for producing biogas from various types of feeds, the selective permeation method comprising steps of; feeding an acidogenic reactor with pulverized feed and fresh water; filtering the acidogenic slurry by filter unit for separating acid solution and slurry therefrom; collecting the acid solution in the acid filtrate tank for circulating therefrom; pumping the acidogenic solution by first peristaltic pump from the acid filtrate tank through a filter having plurality of semi-permeable membrane tubes and collecting back to the acid filtrate reactor; simultaneously, pumping the methanogenic slurry by second peristaltic pump around semi-permeable membrane tubes of the filter and collecting back to methanogenic reactor for circulation, wherein the acid solution is allowed to pass through one or more of semipermeable membrane tubes and is collected back in the acid filtrate reactor, the methanogenic slurry is allowed to pass around the plurality of semi-permeable membrane tubes and collected back in the methanogenic reactor, thereby enabling passage of acid through the membrane and permitting the migration of volatile fatty acids from the acid solution to methanogenic slurry without exchanging microbial populations through the mechanism of concentration equilibrium in the methanogenic reactor.

Typically, wherein organic matter used as feed is pre-treated using chemical, physical, or enzymatic means to convert it into small units to facilitate acidogenesis.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic view of a selective permeation apparatus for producing biogas according to the present invention.

Part list

1 Acidogenic Reactor

2 Filtration Unit

3 Acid Filtrate Tank

4 Peristaltic pump for Acid solution. 5 Filter having semi-permeable tubes

6 Peristaltic pump for Methanogenic Filtrate

7 Methanogenic Reactor

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The foregoing objects of the invention are accomplished and the problems and shortcomings associated with prior art techniques and approaches are overcome by the present invention as described below in the preferred embodiment.

The present invention is illustrated by way of example, and not by way of limitation, in the figure of the accompanying drawing and in which like reference numerals refer to similar elements in which.

Embodiments of the invention are discussed below with reference to the figure. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to the figure is for explanatory purposes and may be embodied in various forms as the invention extends beyond these limited embodiments. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure, or matter.

The present invention is best understood by reference to the detailed figure 1 and description set forth herein. The following description should be read in conjunction with accompanying figure 1.

Figure 1 shows a schematic view of a selective permeation apparatus for producing biogas according to the present invention. The acidogenic and methanogenic stages of the overall process are carried out in two separate reactors, such as acidogenic reactor (1) and methanogenic reactors (7).

At regular intervals when acidogenesis is complete, a portion of the slurry is filtered by means of a filtration unit (2) and transferred to an acid filtrate reactor (3) and acidogenic reactor (1) is topped up with fresh feed. The filtration unit (2) is capable of separating slurry and acid solution from acidogenic reactor. In an embodiment, the organic matter used as feed is pretreated using chemical, physical, or enzymatic means to convert it into suitable small units to facilitate acidogenesis.

A filter (5) having plurality of semi-permeable membrane tubes is provided for permeation of volatile fatty acids. The filters (5) such as, a dialyzer and a hollow fiber ultra filtration unit is placed between the above mentioned acid filtrate tank (3) and the methanogenic reactor (7).

The soluble contents of acidogenic reactor (1) and the methanogenic reactor (7) are then circulated through the filter (5) by means of peristaltic pumps and returned to the respective tanks. In an embodiment, the apparatus is shown to include two peristaltic pumps, such as first peristaltic pump (4) and second peristaltic pump (6). The first peristaltic pump (4) pumps the VFA solution from the acid filtrate tank (3) through the semi-permeable membrane of the filter (5) and back to the acid filtrate tank (3). The second peristaltic pump (6) pumps the methanogenic slurry from the methanogenic reactor (7) around the semipermeable membrane tubes of the filter (5) and back to the methanogenic reactor.

In an embodiment, the filter (5) includes a chamber, a plurality of semi-permeable tubes, first inlet and first outlet, and second inlet and second outlet. The plurality of semi-permeable tubes is enclosed in the chamber. The first inlet and first outlet, and second inlet and second outlet is configured on the Chamber. The acid solution from the acid filtrate tank (3) is passed thought the first inlet to enter into the semi-permeable membrane tubes of the filter (5). The first outlet is provided for egressing the used acid solution out of the semi-permeable membrane tubes of the filter (5).

Further, the methanogenic slurry from the methanogenic reactor (7) is passed thought the second inlet to enter into the chamber and flow around the semi- permeable membrane tubes of the filter (5). The second outlet is provided for egressing the used methanogenic slurry out of the chamber of the filter (5).

If convenient, the channels flowing inside and outside of the semi-permeable membrane tubes can be interchanged, thus the acidogenic stream may flow around of the semi-permeable membrane tubes, whereas the methanogenic stream may flow from within the semi-permeable membrane tubes.

The efficiency of the system can be enhanced by increasing the concentration of VFA' s (by removal of water using the reverse osmosis process in the acid digester stream), by increasing the surface area of the interfacing membrane or by creating turbulence in the moving streams etc. It can also be improved by manipulating stream pressures and creating a back and forth movement of the fluid across the membranes without any net migration of fluids.

Further, the present invention includes a selective permeation method/process (hereinafter referred as method) for producing biogas from various types of biomass. The method includes steps of feeding an acidogenic reactor (1) with biomass and fresh water. Then, filtering the acidogenic slurry by using filter unit (2) for separation of VFA solution from the acidogenic slurry. Further, collecting the acid solution in the acid filtrate reactor (3) for circulating it. Thereafter, pumping the acid solution by first peristaltic pump (4) from the acid filtrate tank (3) through a filter (5) having plurality of semi-permeable membrane tubes and collecting back to the acid filtrate tank (3).

Simultaneously, withdrawing the methanogenic slurry from the methanogenic reactor (7) for circulation, and pumping the methanogenic slurry by second peristaltic pump (6) from the methanogenic reactor (7) around semi-permeable membrane tubes of the filter (5) and collecting back to the methanogenic reactor (7) for circulation. Further, the acid solution is allowed to pass through one or more of semi-permeable membrane tubes and is collected back in the acid filtrate reactor, and the methanogenic slurry is allowed to pass around the plurality of semi-permeable membrane tubes and collected back in the methanogenic reactor, thereby enabling passage of acid out through the membrane and permitting the migration of volatile fatty acids from the acid solution to methanogenic slurry without exchanging microbial populations through a mechanism of concentration equilibrium in the methanogenic reactor.

When in operation, the semi-permeable membrane tubes of the filter (5) has two streams of fluids passing through it; the stream from the balancing tank (VFA stream), high in volatile fatty acid concentration and with a low pH passing through one (e.g. the inner channels of the hollow fibers) and the contents of the methanogenic reactor (the alkaline stream) (7) that is low in volatile fatty acids and with a high pH through the other (e.g. external to the hollow fibers in a hollow fiber unit).

The two streams are separated by the semi-permeable membrane (e.g. the hollow fiber membrane unit) (5) that permits the migration and concentration equilibration of the low molecule fatty acids and hydrogen ions as also accumulated metabolites but does not permit the migration of high molecular weight undigested/semi-digested feed as also of microbial cells.

The net effect of the two re-circulations thus, is to transport the volatile fatty acids to the methanogenic reactor (7) and acidity neutralization, without exchanging the microbial populations, through the process of concentration equilibrium.

The volatile fatty acids, on entering the methanogenic reactor (7) are converted into methane by the progressively increasing numbers of methanogens.

The overall effect is to bring about a much faster conversion of feed into biogas without undue loss of the methanogens. Suitably treated fresh feed may be mixed up with fresh water/ return stream of the acidogenic reactor 1 for further acidogenesis.

The return stream from the acidogenic reactor (1) may be drained to the extent fresh water is used to keep the volume of the recirculating stream constant and to allow for wash off of accumulating metabolites.

The embodiments of the invention as described above and the methods disclosed herein will suggest further modification and alterations to those skilled in the art. Such further modifications and alterations may be made without departing from the spirit and scope of the invention.

The invention is further described with the help of following illustration:

Illustration 1

In the following illustration corn starch is used as a feed for conversion into biogas.

222.22 grams of starch is slurried up in 866.67 ml of water and heated at 80 degrees centigrade to bring about its gelatinization. It is mixed with 0.22ml units of amylase and kept at 37 degrees centigrade for 4 hours with occasional stirring. The mixture is filtered to eliminate gross particles and subjected to the action of an acidogenic mixture in the form of slurried cow dung. In 24 hours the acidogenesis is found to be complete.

The resultant mixture is filtered through a coarse filter 2 and placed in tank 3 .The tank 7 is filled with an active methanogenic culture obtained from a biogas plant in routine operation.

The circulation of two streams is started using the two peristaltic pumps 4 and 6. Care is taken to ensure that the levels in the two reactors 3 and 7 remain unaltered by either manipulating the pump speeds or raising/lowering of the levels of the two reactors.

Generation of biogas starts after a lag of about one hour.

The experiment was repeated daily for 8 days and showed conversion of corn starch to methane in a reproducible manner.

ADVANTAGES OF PRESENT INVENTION

The method gets over the problems faced in the prior art of suboptimal conditions of a single reactor and the risk of digester failure.

As the micro organisms are retained in their respective reactors, the process speed is improved. This results in reduction of capital cost, over the conventional digesters, for getting the same output.

The process of biogas production is continuous i.e. without any break or requirement of stream reversals.

The invention is thus to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the following claims.

Claims

WE CLAIM:

1. A selective permeation apparatus for producing biogas for various types of slurry, the selective permeation method comprising; an acidogenic reactor for mixing pulverized biomass with fresh water and acidogenic microorganisms; a methanogenic reactor containing methanogens for conversion of volatile fatty acids to methane; a filter unit capable of separating acid solution and slurry from the mix received from acidogenic reactor; an acid filtrate tank capable of filtering the acid received form the filter unit; a filter having plurality of semi-permeable membrane tubes enclosed in a chamber, a first peristaltic pump for pumping the acid solution received from the acid filtrate tank through the semi-permeable membrane tubes of the filter and collecting back to the acid filtrate tank; and a second peristaltic pump for pumping the methanogenic slurry received from the methanogenic reactor around the semi-permeable membrane tubes of the filter and collecting back to the methanogenic reactor, wherein the acid solution is allowed to pass through the plurality of semipermeable membrane tubes and is collected back in the acid filtrate tank, the methanogenic slurry is allowed to pass around the plurality of semi-permeable membrane tubes and collected back in the methanogenic reactor, thereby enabling passage of volatile fatty acids out of the membrane and permitting the migration of volatile fatty acids from the acid solution to methanogenic reactor without exchanging microbial populations using the principle of concentration equilibrium.

2. The selective permeation apparatus as claimed in claim 1, wherein the filter comprises; a chamber for enclosing; a plurality of semi-permeable tubing enclosed in the chamber; a first inlet and a first outlet openings for circulating the acid solution through the plurality of semi-permeable openings; and a second inlet and a second outlet openings for circulating the methanogenic slurry around the plurality of semi-permeable openings.

3. The selective permeation apparatus as claimed in claim 1, wherein the acidogenic reactor comprises a stirrer for stirring the feed, water and culture mix; and a heater for heating the acidogenic reactor for maintaining the temperature of the acidogenic reactor between 30 to 45 degree centigrade.

4 4.. T Thhee sseelleeccttiivvee ppeerrmmeeaattiioonn mmeetthhoodd aass ccllaaiimmeedd i mn ccllaaiimm 11,, wwhheerreeiinn organic matter used as feed is pre-treated using chemical, physical, or enzymatic means to convert it into small units to facilitate acidogenesis.

5. A selective permeation method/process for producing biogas for various types of feeds, the selective permeation method comprising steps of; feeding an acidogenic reactor with a biomass, fresh water and acidogenic culture; filtering the acidogenic mix by filter unit for separating acid and slurry therefrom; collecting the acid solution in the acid filtrate tank for circulating therefrom; pumping the acid solution by first peristaltic pump from the acid filtrate tank through a filter having plurality of semi-permeable membrane tubes and collecting back to the acid filtrate tank; simultaneously, pumping the methanogenic slurry in the methanogenic reactor by second peristaltic pump from the methanogenic reactor around semi- permeable membrane tubes of the filter and collecting back to the methanogenic reactor for circulation, wherein the acid solution is allowed to pass through the plurality of semipermeable membrane tubes and is collected back in the acid filtrate tank, the methanogenic slurry is allowed to pass around the plurality of semi-permeable membrane tubes and collected back in the methanogenic reactor, thereby enabling passage of volatile fatty acids out of the membrane and permitting the migration of volatile fatty acids from the solution to methanogenic slurry without exchanging microbial populations using the principle of concentration equilibrium.

6. The selective permeation method as claimed in claim 4, wherein organic matter used as feed is pre-treated using chemical, physical, or enzymatic means to convert it into small units to facilitate acidogenesis.