KR101770286B1 - Production method of high purity methane gas - Google Patents

Abstract

The present invention relates to a method for producing high purity methane gas, comprising the following steps: (a) filling a high pressure reactor with a reaction solution consisting of anaerobic microbes and organic waste containing food waste; and (b) pressurizing the high pressure reactor and then fermenting the same so as to produce biogas containing methane.

Description

Production method of high purity methane gas [0002]

The present invention relates to a method for producing high purity methane gas, and more particularly, to a method for increasing the content of methane gas in a biogas generated in the process of fermenting organic wastes into anaerobic microorganisms.

The anaerobic fermentation process can be defined as a process in which biodegradable organic matter undergoes a multistage biochemical reaction under the condition of absolutely no oxygen and is finally decomposed into biogas such as methane (CH ₄ ) and carbon dioxide (CO ₂ ).

The biogas generated by the anaerobic fermentation process is predominantly composed of about 50 to 75 vol.% Methane, about 25 to 50 vol.% Carbon dioxide, impurities less than about 0.1 vol.% Air, about 7,000 to 8,000 ppm hydrogen sulfide, And about 40 ppm of siloxane.

Such biogas can be utilized as natural gas, vehicle fuel, etc. In order to achieve this, it is essential to improve the quality of methane gas through a gas purification process.

Carbon dioxide, a typical impurity in the purification of biogas, can generally be removed by methods such as pressure swing adsorption and water scrubbing.

The high-pressure adsorption method removes carbon dioxide using the difference in the adsorbed amount of the adsorbent. The high pressure adsorption method removes carbon dioxide by using activated carbon, zeolite, silica gel, carbon molecular sieve, zeolite molecular sieve, It can be used as an adsorbent. The high pressure adsorption method has advantages such as low maintenance cost, easy operation and low required energy as compared with other processes, but has a disadvantage that the recovery rate of methane is lower than other methods (Korean Patent Publication No. 2010-0037249).

In the aqueous solution absorption method, carbon dioxide is removed by using an absorbent that absorbs carbon dioxide, and absorbents such as amine-based absorbents, ammonia, potassium carbonate, and amino acids are used. The aqueous solution absorption method is the most commonly used method, but it has a disadvantage that the energy used for regenerating the absorbent is excessive compared to other processes, and under a high concentration of carbon dioxide, the absorbent reacts with carbon dioxide to form a particulate matter Open Patent Publication No. 2010-500168).

Therefore, it is necessary to develop a method for producing methane gas of high purity at a high recovery rate, which is easy to operate without excess energy required.

Korean Patent Publication No. 2010-0037249 (October 28, 2011) Japanese Laid-Open Patent Publication No. 2010-500168 (2010.01.07.)

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing high purity methane gas which is easy to operate without requiring excess energy and can produce high purity methane gas at a high recovery rate .

According to an aspect of the present invention, there is provided a method for preparing a biodegradable bioreactor, comprising: a) filling a high pressure reactor with a reaction solution containing organic waste and anaerobic microorganisms including food waste; And b) pressurizing and fermenting the high-pressure reactor to produce a biogas comprising methane.

As the amount of biogas generated by the anaerobic fermentation gradually increases, the internal pressure of the high-pressure reactor increases and the amount of carbon dioxide dissolved in the reaction solution increases, The purity of the remaining methane gas can be improved. Accordingly, the efficiency of the biogas purification in the reactor itself is excellent, and the cost of the rear end refining facility of the biogas can be reduced.

It is also advantageous in that it is possible to provide a method for producing high purity methane gas which is easy to operate without requiring excess energy and can produce high purity methane gas with high recovery rate.

FIG. 1 is a view showing the concept of the present invention. FIG. 1 (a) shows the solubility of methane and carbon dioxide according to the initial internal pressure of a high-pressure reactor, and FIG. 1 (b) shows an anaerobic fermentation process And the solubility of methane and carbon dioxide as the internal pressure of the high-pressure reactor increases as the biogas is generated.
2 is a schematic view showing a high-pressure reactor according to an embodiment of the present invention.
3 is a flowchart illustrating a process of purifying biogas according to an embodiment of the present invention.
FIG. 4 (a) is a graph showing a change in pressure according to anaerobic fermentation period according to Example 1, and FIG. 4 (b) is a graph showing changes in methane content according to pressure according to Example 1 and Example 2 Graph.
5 is a graph showing changes in the content of methane in the high-pressure environment (Example 3) and the atmospheric-pressure environment (Comparative Example 1) according to the fermentation period.

Hereinafter, a method for producing high purity methane gas of the present invention will be described in detail with reference to the accompanying drawings. The following drawings are provided by way of example so that those skilled in the art can fully understand the spirit of the present invention. Therefore, the present invention is not limited to the following drawings, but may be embodied in other forms, and the following drawings may be exaggerated in order to clarify the spirit of the present invention. Also, throughout the specification, like reference numerals designate like elements.

Hereinafter, the technical and scientific terms used herein will be understood by those skilled in the art without departing from the scope of the present invention. Descriptions of known functions and configurations that may be unnecessarily blurred are omitted.

The present invention relates to a biogas produced from an anaerobic fermentation process, wherein the solubility constant difference between gases constituting the biogas and the Henry's law are used to increase the amount of carbon dioxide dissolved in the reaction solution, thereby increasing the purity of methane present in the gas phase And more particularly, to a method for producing a high purity methane gas.

Specifically, a method for producing high purity methane gas according to an embodiment of the present invention includes the steps of: a) filling a high-pressure reactor with a reaction solution containing organic wastes including food wastes and anaerobic microorganisms; And b) pressurizing and fermenting the high-pressure reactor to produce biogas comprising methane.

As shown in FIG. 1, as the amount of biogas existing in the gaseous phase in the high-pressure reactor gradually increases due to anaerobic fermentation, the internal pressure of the high-pressure reactor also increases. As a result, The purity of the methane gas which is dissolved in the solution and remains in the vapor phase can be improved. Accordingly, the efficiency of the biogas purification in the reactor itself is excellent, and the cost of the rear end refining facility of the biogas can be reduced.

Hereinafter, a method for producing high purity methane gas according to the present invention will be described in detail.

First, a) a step of filling a high-pressure reactor with a reaction solution containing organic wastes including food waste and anaerobic microorganisms may be performed.

In an embodiment of the present invention, the high-pressure reactor is a reactor capable of withstanding high pressures, and can be used without particular limitation as long as it can withstand the target pressure. In one embodiment, . At this time, the limit pressure to which the high-pressure reactor can withstand is not particularly limited, and the maximum pressure of the high-pressure reactor to which the present technology can be reached is considered as the limit pressure. As a specific example, a high-pressure reactor may be used which can stand up to 300 atm.

When the high-pressure reactor is prepared, the reaction solution containing the organic waste and the anaerobic microorganism used for the fermentation reaction is put in the reactor and completely sealed. At this time, it is preferable that the reaction solution is not completely filled so that the interior of the high-pressure reactor is divided into a liquid phase and a gaseous phase. When the reaction solution is filled in the high-pressure reactor, the amount of biogas produced even when the internal pressure reaches the limit pressure may be small. On the contrary, when the amount of the reaction solution to be filled is too small, the amount of generated biogas is insignificant or the internal pressure of the high-pressure reactor is not increased so much that the amount of carbon dioxide dissolved in the reaction solution is lowered, . Preferably, the reaction solution can be filled in from 50 to 95% by volume of the total volume of the autoclave, more preferably from 75 to 95% by volume. In such a range, a large amount of biogas can be produced up to the limit pressure, and the internal pressure of the high pressure reactor can be easily increased later, so that a larger amount of carbon dioxide can be dissolved in the reaction solution.

In one embodiment of the present invention, the reaction solution may include organic wastes and anaerobic microorganisms.

The organic waste may include food waste, and the food waste may collectively refer to food waste discharged from a home and / or a restaurant. Such food wastes can be effectively fermented by anaerobic microorganisms to produce biogas with high methane content.

In addition, the organic waste according to an exemplary embodiment of the present invention may further include sewage sludge, wherein the sewage sludge may be sludge generated in each step of sewage treatment.

That is, the organic waste according to an example of the present invention may include food waste and sewage sludge, and when the anaerobic fermentation process is performed by mixing the two at an appropriate ratio, the food waste and the sewage sludge are each fermented The production rate of methane gas and the content of methane gas can be increased. It is expected that iron, aluminum, calcium and phosphorus, which are contained in sewage sludge at high concentration, affect the anaerobic fermentation process.

More preferably, the mixing ratio between the food waste and the sewage sludge is suitably adjusted. In one embodiment, the organic waste may be mixed with food waste: sewage sludge in a volume ratio of 1: 0.01 to 0.5 , More preferably food waste: sewage sludge may be mixed in a volume ratio of 1: 0.1 to 0.2. In such a range, it is more preferable to mix food waste and sewage sludge in order to effectively improve the production rate and content of methane gas.

The mixing ratio between the organic waste and the anaerobic microorganism in the reaction solution is also important. For example, the reaction solution according to the present invention may satisfy the following relational expression (1).

[Relation 1]

1? I / S? 5

(Where, I is the concentration of anaerobic microorganism (g VS / L) and S is the concentration of organic waste (g COD / L).

By mixing the organic waste and the anaerobic microorganism at an appropriate concentration, the anaerobic fermentation process can be smoothly performed to improve the production rate of methane gas and increase the content of methane gas. Preferably, the I / S ratio may be from 2 to 4, and the effect of increasing the methane gas production rate and the effect of increasing the content may be greatly increased.

In addition, the concentrations of the organic waste and the anaerobic microorganism in the reaction solution may be adjusted to the levels commonly used in the art. For example, the concentration (S) of the organic waste may be 0.5 to 10 g COD / L, Preferably 1 to 5 g COD / L. The concentration (I) of the anaerobic microorganism may be 0.5 to 50 g VS / L, and more preferably 2 to 20 g VS / L. The anaerobic fermentation process can be effectively carried out in such a concentration range.

Meanwhile, in one embodiment of the present invention, the anaerobic microorganism is not particularly limited as long as it is commonly used in the anaerobic fermentation reaction. In one embodiment, the anaerobic fermentation sludge, the methanogenic microorganism, or a mixture thereof may be used. More specifically, the anaerobic fermentation sludge may be a fermentation sludge containing anaerobic microorganisms as sewage sludge operated in a sewage treatment facility. Methane-producing bacteria means an organism producing methane, and may be any one or two or more selected from Methanobacterium, Methanococcus, Methanosarcina and Methanospirillum, but is not limited thereto.

In addition, the reaction solution according to an example of the present invention may further include sodium hydroxide. Since sodium hydroxide has a property of absorbing water vapor and carbon dioxide in the air well, carbon dioxide present in the gas phase can be dissolved more in the reaction solution, and thus the purity of methane in the biogas can be further improved.

Although the amount of sodium hydroxide to be added is not particularly limited, it is preferable to appropriately adjust the amount of sodium hydroxide in a line which does not disturb the fermentation reaction of the reaction solution. In one embodiment, the initial pH of the reaction solution to which sodium hydroxide is added is 7.5 to 11, more preferably pH 8.5 to 10. Within this range, the carbon dioxide can be more readily dissolved in the reaction solution without interfering with the anaerobic fermentation process. The term " initial " means a period immediately after sodium hydroxide is completely dissolved in the reaction solution.

Next, b) pressurizing and fermenting the high-pressure reactor to produce a biogas containing methane. At this time, the pressurization may be self-pressurized as the biogas is generated, or may be pressurized through the pressure regulator as described below.

In detail, when the reaction solution is put into a high-pressure reactor and fermented, biogas including methane and carbon dioxide can be produced. As the fermentation time becomes longer, the amount of biogas present in the gas phase in the high-pressure reactor is increased, The internal pressure can be increased. As the internal pressure increases, the amount of carbon dioxide dissolved in the reaction solution can be greatly increased. That is, the method of producing high purity methane gas according to the present invention may include a step in which, in step b), a part of the carbon dioxide contained in the biogas is dissolved in the pressurized reaction solution.

This is by Henry's law, Henry's law is as follows.

[Henry's Law]

C = K x P

(MolL ^-1 atmospheric pressure- ¹ ), P is the partial pressure of the gas present in the gas phase (mol / L), K is the Henry's constant of the gas Atmospheric pressure).

That is, as the biogas is generated, the amount of carbon dioxide present in the gas phase in the high-pressure reactor is increased. As a result, the partial pressure of carbon dioxide is increased and the amount of carbon dioxide dissolved in the liquid phase in the high- It will be done.

However, as the amount of methane present in the gas phase also increases, the partial pressure of methane also increases, thereby increasing the amount of methane dissolved in the reaction solution. However, the Henry's constant of methane is significantly lower than that of carbon dioxide The amount of methane dissolved in the solution may be significantly less than that of carbon dioxide. As a result, the content of methane in the biogas existing in the gas phase gradually increases, so that methane gas of high purity can be produced. At this time, the Henry's constant of methane is about 0.0016 molL ^-1 atmospheric pressure- ¹ , the Henry's constant of carbon dioxide is about 0.031 molL ^-1 atmospheric pressure- ¹ , and the Henry's constant in the present invention can be used in the same meaning as the solubility constant.

Meanwhile, in an example of the present invention, the internal pressure of the high-pressure reactor may gradually increase as the biogas is generated, and may be increased to the limit pressure of the high-pressure reactor, and more effectively dissolves carbon dioxide into the reaction solution, In order to produce methane gas, the internal pressure of the step b) is preferably increased to 5 atm or higher, more preferably 8 atm or higher. At this time, the upper limit of the internal pressure may vary depending on the high-pressure reactor, and may be equal to the limit pressure of the high-pressure reactor, but it may preferably be 30 atm or less.

Also, as shown in FIG. 2, the high-pressure reactor 100 according to an exemplary embodiment of the present invention may further include a pressure regulator 300 on the high-pressure reactor. The pressure regulator 300 regulates the internal pressure of the high-pressure reactor 100. By increasing the internal pressure of the high-pressure reactor 100 from the beginning of the fermentation process, the carbon dioxide is dissolved in the reaction solution 200 from the beginning of the fermentation process . As a specific example, the initial internal pressure of the high-pressure reactor 100 controlled by the pressure regulator 300 may be 3 atm or higher, more preferably 5 atm or higher. The upper limit of the initial internal pressure by the pressure regulator 300 is not particularly limited, but may be less than half of the limit pressure of the high-pressure reactor 100. That is, when the critical pressure of the high-pressure reactor 100 is 100 atm, the initial internal pressure may be 50 atm or less. When the initial internal pressure is close to the limit pressure of the high-pressure reactor 100, the internal pressure of the high-pressure reactor 100 can easily reach the limit pressure, and the amount of methane gas produced may be small. Here, 'initial' means a period immediately after the internal pressure is controlled and the high-pressure reactor 100 is completely sealed.

In addition, by separately providing the pressure regulating unit 300, even if the amount of the biogas is gradually increased, the position of the pressure regulating unit 300 is gradually adjusted to increase the volume of the gas phase part in the high pressure reactor 100, The advantage is that the amount of gas can be increased. At this time, when the volume of the gas phase part is suddenly increased too much through the pressure regulator 300, the carbon dioxide dissolved in the reaction solution 200 may be eluted again due to a sudden decrease in the internal pressure. In one embodiment, in the case of a high-pressure reactor 100 having a limiting pressure of 10 atmospheres, it may be desirable to maintain the internal pressure at a level of 8 to 10 atmospheres, but the present invention is not limited thereto .

In addition, the method of producing high purity methane gas according to an embodiment of the present invention may further include a step of passing the biogas produced in step b) through a gas separation membrane and purifying it. By refining the refined biogas through the high-pressure reactor, methane gas with higher purity can be produced.

At this time, it is preferable that the gas separation membrane is a porous membrane in which only one gas of carbon dioxide or methane is passed. In order to reduce the loss of methane gas, methane can not pass through or the passing speed is remarkably low, It is preferable to use a gas separation membrane which can easily pass impurities and the like. In one embodiment, the selectivity of carbon dioxide / methane in the gas separation membrane is preferably 35 or more, and more preferably 50 or more. More specifically, it is preferable to use a gas separation membrane having a carbon dioxide permeability of 100 GPU to 1,000 GPU.

The type of the gas separation membrane is not particularly limited as long as it is commonly used in the art, and specific examples thereof include cellulose acetate, polysulfonic acid, polyimide, polycarbonate, polystyrene, polyethylene terephthalate, polyphenylene oxide, polysiloxane, polyethylene oxide , Polypropylene oxide, or a mixture thereof, or may be made of a zeolite-based inorganic material, a carbon molecular sieve material, or the like.

In more detail, the gas separation membrane includes a first separation membrane and a second separation membrane. The first separation membrane firstly discharges the carbon dioxide remaining in the first purified biogas to secondly purify the biogas, The second step of purifying the biogas by secondly discharging the carbon dioxide remaining in the second purified biogas can produce the methane gas of higher purity and higher purity through the multi-step purification process. At this time, the first separation membrane and the second separation membrane may be different from each other or may be the same.

Meanwhile, the biogas can be pressurized to discharge carbon dioxide through the first separation membrane or the second separation membrane, and the biogas can be used without any particular limitation as long as it is commonly used in the art. For example, The pressure of the biogas can be increased by reducing the volume of the reservoir being operated. As the pressure of the biogas increases, the carbon dioxide can be discharged more effectively through the first separator or the second separator, and the purity of the methane gas to be concentrated can be remarkably improved. Specifically, the purity of the methane gas according to an exemplary embodiment of the present invention may be at least 90 vol%, more preferably at least 95 vol%, even more preferably at least 99 vol%, particularly preferably at least 99.9 vol%.

In addition, the method of producing high purity methane gas according to an embodiment of the present invention may further include a pretreatment step such as dehumidification, desulfurization, deammonia and desiloxane treatment ordinarily practiced in the art, It is of course possible to improve the efficiency of production.

Hereinafter, a method for producing high purity methane gas according to the present invention will be described in more detail with reference to examples. It should be understood, however, that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention. Unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Also, the singular forms as used in the specification and the appended claims are intended to include the plural forms as well, unless the context clearly indicates otherwise. In addition, the unit of the additives not specifically described in the specification may be% by weight.

[Example 1]

The reaction solution was placed in a high-pressure reactor having a critical pressure of 10 atmospheres so that the volume ratio of the reaction solution to the gaseous phase was 10: 1. The high-pressure reactor was completely sealed and the anaerobic fermentation process was performed for 2 weeks.

At this time, the reaction solution was prepared by mixing 2.0 g COD / L of food waste and 5.0 g VS / L anaerobic fermentation sludge.

As shown in FIG. 4A, the internal pressure of the high-pressure reactor increased with an increase in the anaerobic fermentation process period, and it was confirmed that the pressure reached 10 atm, which is the limit pressure of the high-pressure reactor after about 7 days.

4B, which shows the change in the content (volume%) of methane, the methane content gradually increases as the internal pressure of the high-pressure reactor increases. After about 9 days, the content of methane Was about 80 vol%.

[Example 2]

All processes except that food waste and sewage sludge were mixed and used as organic waste were carried out in the same manner as in Example 1. Organic waste was mixed with food waste and sewage sludge at a volume ratio of 9: 1 and used at a concentration of 2.0 g COD / L.

4B showing the change in the content (volume%) of methane accordingly shows that the purity of methane was improved faster than that in Example 1 as the methane content reached about 80% by volume after about 8 days And it can be confirmed that methane of higher purity is produced at the same pressure.

[Example 3]

As the anaerobic fermentation process progresses, the volume of the vapor phase is gradually increased to maintain the internal pressure of the high-pressure reactor at 10 atm. Proceed in the same manner as in Example 1.

As a result, as shown in Fig. 5, the methane content reached about 80 vol% after about 5 days.

[Comparative Example 1]

As in Example 3, the internal pressure of the autoclave was maintained constant, but not at 10 atmospheres.

As a result, unlike Example 3 where the methane content reached about 80% by volume after about 2 days, the methane content of Comparative Example 1 did not exceed 60% by volume even after 10 days.

From this, it was confirmed that the content of carbon dioxide dissolved in the reaction solution was remarkably increased by increasing the internal pressure of the high-pressure reactor.

[Examples 4 to 7]

The procedure of Example 1 was repeated except that NaOH was added to the reaction solution to adjust the pH of the initial reaction solution as shown in Table 1 below.

pH Methane content (vol%) Two days later After 5 days Example 4 7 38 78 Example 5 8 64 86 Example 6 9 76 89 Example 7 10 81 93

As shown in Table 1, when NaOH is further added to the reaction solution, NaOH absorbs gaseous carbon dioxide and moisture, and the purification rate and purity of methane are further improved.

[Example 8]

The anaerobic fermentation and primarily purified methane gas for 30 days from Example 1 were transferred to a methane storage tank through a condensate line. At this time, as shown in FIG. 3, the first purified methane gas from the high-pressure reactor was moved to a concentration pipe, and then an opening / closing valve connected to the first separator was opened to discharge a small amount of residual carbon dioxide. Next, the opening / closing valve connected to the first separating membrane is closed, the volume of the condensing pipe is adjusted to raise the internal pressure of the condensing pipe to 4 atmospheres, and then the opening / closing valve connected to the second separating membrane is opened to discharge the remaining carbon dioxide Purity methane gas having a purity of 99% or more was obtained. The first separation membrane was made from cellulose acetate, and the second separation membrane was made from polysulfonic acid.

While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims. Therefore, the above description does not limit the scope of the present invention, which is defined by the limitations of the following claims.

100: High-pressure reactor
200: reaction solution
300: Pressure regulator
400: opening / closing valve

Claims (12)

a) filling a high-pressure reactor with a reaction solution containing organic waste including food waste and anaerobic microorganisms; And
b) pressurizing the high pressure reactor to an initial pressure of 3 atm or higher and then fermenting to produce a biogas containing methane,
Wherein during the fermentation in step b), the volume of the high-pressure reactor is increased as the biogas is generated while maintaining the initial pressure or higher.
The method according to claim 1,
Wherein the internal pressure of step b) is 5 atm or higher.
The method according to claim 1,
Wherein a portion of the carbon dioxide contained in the biogas is dissolved in the pressurized reaction solution in the step b).
The method according to claim 1,
Wherein the organic waste further comprises sewage sludge in step a).
5. The method of claim 4,
Wherein the organic waste is mixed with food waste: sewage sludge in a volume ratio of 1: 0.01 to 0.5.
The method according to claim 1,
Wherein the reaction solution satisfies the following relational expression (1).
[Relation 1]
1? I / S? 5
(Where, I is the concentration of anaerobic microorganism (g VS / L) and S is the concentration of organic waste (g COD / L).
The method according to claim 1,
Wherein the reaction solution further comprises sodium hydroxide.
8. The method of claim 7,
Wherein the initial pH of the reaction solution is from 7.5 to 11.
The method according to claim 1,
Wherein the high-pressure reactor further comprises a pressure regulator in the upper portion of the high-pressure reactor.
delete The method according to claim 1,
further comprising the step of purifying the biogas produced in step (b) by passing the biogas through a gas separation membrane.
12. The method of claim 11,
Wherein the gas separation membrane includes a first separation membrane and a second separation membrane, the first separation membrane separating the purified biogas from the biogas by first discharging the remaining carbon dioxide, and the second separation membrane separating the second purified biogas A method for producing high-purity methane gas by purifying the biogas three times by releasing the remaining carbon dioxide in the gas.

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