JPH0724840B2 - Methane fermentation control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a method for controlling methane fermentation.

B. Summary of the Invention In the methane fermentation control method of the present invention, while a pH sensor is provided in the methane fermentation tank, a solid-liquid separation tank is connected to the methane fermentation tank and digested sludge separated in the solid-liquid separation tank is PH supplied
A control tank is connected, and a gas containing methane gas and carbon dioxide generated in the methane fermentation tank is guided to a gas separation device using a polymer membrane via a compressor, and carbon dioxide is generated based on the difference in permeability of the polymer membrane. A pH controller installed in the pH control tank when the polymer membrane permeation gas with high concentration and the polymer membrane non-permeation gas with low carbon dioxide concentration are separated and a deviation occurs in the pH set value of the methane fermentation tank. Command, the gas of either the polymer membrane permeation gas or the polymer membrane non-permeation gas is selectively introduced into the pH control tank to bring the pH of the digested sludge
The pH of the methane fermentation tank can be surely controlled by a simple method by adjusting the temperature and recirculating the pH-adjusted digested sludge into the methane fermentation tank.

C. Conventional Technology and Problems The operation of methane fermentation tanks depends on the practical method of the operator, and sometimes the failure such as rancidity due to overload occurs. Therefore, it has been attempted to automatically control the operation of the methane fermentation tank, for example, a flow control method, a digested sludge recirculation control method, a base injection control method,
A gas cleaning recirculation control method has been proposed. On the other hand, pH, which can be easily monitored in the method of controlling the operation of the methane fermentation tank, is well known as an index.

Each control method will be described with reference to FIGS.

FIG. 2 is a block diagram of the flow rate control method, in which 1 is a methane fermentation tank, 7 is a pH sensor, 6 is a pH controller, 13 is a fermentation raw material storage tank, and 14 is a fermentation raw material injection cock. Further, 15 is a stirring blade inserted in the methane fermentation tank 1.
In this flow rate control method, the fermentation raw material once stored in the storage tank 13 flows into the methane fermentation tank 1, is separated into gas and digested sludge in the methane fermentation tank 1, and is discharged. At this time, the pH sensor 7 detects the pH in the methane fermentation tank 1, and the fermentation material injection cock 14 is opened and closed via the pH controller 6 to control the inflow of the fermentation material into the methane fermentation tank 1 to control the inside of the tank. It controls the pH of the liquid phase.

The above flow rate control method is the basic and indispensable system for the pH control of the methane fermentation tank, but if the concentration of the inflow substrate that is the fermentation raw material fluctuates greatly, simply control the inflow of the fermentation raw material into the methane fermentation tank. I can not handle it alone,
pH control becomes difficult.

Fig. 3 shows a block diagram of the digested sludge recirculation control method. According to the method shown in the figure, the sludge separated from the gas in the methane fermentation tank 1 is transferred to the solid-liquid separation tank 2. The liquid separated in the tank 2 is transferred to the pipe 16.

The pipe 16 has one end on the digested sludge discharge side and the other end on the digested sludge recirculation side, and the pump 12 is provided on the recirculation side.
Methane fermentation tank 1 with recycled digested sludge along with fermentation raw materials
It has come to return to. The pump 12 is a pH controller 6 connected to a pH sensor 7 that detects the pH in the methane fermentation tank 1.
The drive is controlled by. Therefore, the digestion gas generated in the methane fermentation tank 1 is a storage tank (not shown).
At the same time, the sludge liquid is transferred to the solid-liquid separation tank 11 to be separated into solid matter and liquid, the solid matter is discharged to the outside of the tank, and the liquid digested sludge is transferred to the pipe 16. At this time, the pH sensor 7 changes the pH in the methane fermentation tank 1
Is detected and the pump 12 is operated via the pH controller 6 in response to the change, the digested sludge discharged from the solid-liquid separation tank 2 is recirculated and reduced to the methane fermentation tank 1, and the tank liquid Control the pH of the phase.

The above-mentioned digested sludge recirculation control method has considerably good pH controllability and also has an inoculum seeding effect. However, it has a drawback that it cannot be applied when the solid-liquid separation tank 2 is required and solid-liquid separation of digested sludge is difficult.

FIG. 4 shows a block diagram of a base injection control method. In this method, a methane fermentation raw material is temporarily stored in a chemical tank 17, a base is injected therein, and then the methane fermentation raw material 1 is charged. Further, the pH sensor 6 detects the pH in the methane fermentation tank 1, and opens and closes the fermentation raw material cock 14 via the pH controller 7,
The input amount to the methane fermentation tank 1 is controlled according to the change in pH of the liquid phase in the tank.

Although the above-mentioned base injection control method has good pH controllability, it has a drawback in that it is costly in terms of chemical injection.

FIG. 5 is a block diagram of the gas cleaning recirculation control method.
This method guides the gas generated in the methane fermentation tank 1 to the scrubber 18 via the compressor 5, and in the scrubber 18, the digested gas is washed with water to dissolve the carbon dioxide in the digested gas and lower the carbon dioxide. After that, it is recirculated to the methane fermentation tank 1 through the gas stirring mechanism.

According to the control method described above, the soluble carbon dioxide in the liquid phase in the methane fermentation tank is volatilized to the gas phase, so
The pH rises. Although this method has a good pH controllability, it has a drawback that the mechanism is complicated and large in size because water for digestion gas cleaning must be supplied and a gas cleaning device is required.

As described above, each methane fermentation control method in the conventional methane fermentation tank has merits and demerits, and the fact is that it is rarely used other than the flow rate control method shown in FIG. The present invention proposes a new gas separation recirculation control method in order to remedy such drawbacks.

D. Means for Solving the Problems In the method for controlling methane fermentation according to the present invention, a gas containing methane gas and carbon dioxide generated in a methane fermentation tank is introduced into a gas separation device using a polymer membrane via a compressor. Then, based on the difference in permeability of the polymer membrane, it is separated into a polymer membrane permeable gas having a high carbon dioxide concentration and a polymer membrane non-permeable gas having a low carbon dioxide concentration. Then, when there is a deviation in the pH set value of the methane fermentation tank, either the polymer membrane permeation gas or the polymer membrane non-permeation gas is selected by the command of the pH controller installed in the pH adjustment tank. To pH
The pH of the digested sludge is adjusted by being guided to the adjustment tank.
The conditioned digested sludge is recycled into the methane fermentation tank.

E. Action According to the method for controlling methane fermentation, the gas containing carbon dioxide generated in the methane fermentation tank is guided to the gas separation device using a polymer membrane, and the gas is separated based on the difference in permeability of the polymer membrane. It is separated into a polymer membrane permeation gas having a high carbon concentration and a polymer membrane non-permeation gas having a low carbon dioxide concentration. When there is a deviation in the pH set value of the methane fermentation tank, either the polymer membrane permeation gas or the polymer membrane non-permeation gas is introduced to the pH control tank and digested by the command of the pH controller. The pH of the sludge is adjusted, and the digested sludge having the adjusted pH is recycled into the methane fermentation tank to adjust the pH in the methane fermentation tank.

F. Example An apparatus for carrying out the present invention will be described below with reference to FIG. It should be noted that the same parts as in the prior art will be described with the same reference numerals.

In the figure, 1 is a methane fermentation tank, 7 is a pH sensor provided in the methane fermentation tank 1, 6 is a pH controller connected to the pH sensor 7, and 2 is a solid-liquid separation tank connected to the methane fermentation tank 1. 3 is the pH connected to the solid-liquid separation tank 2
It is a control tank. Further, 4 is a gas separation device using a polymer membrane or the like, and the gas generated from the methane fermentation tank 1 is supplied to the gas separation device 4 via a compressor 5. Reference numeral 10 is a gas injection three-way cock, which is fed from two connection ports of the three-way cock 10. The permeated gas and the non-permeated gas separated by the gas separation device 4 are aerated to the pH adjusting tank 3 via the blower 11. 9 is pH adjustment tank 3
A pH sensor for detecting the pH of the inner liquid phase, which is connected to the pH controller 8. Further, the valve of the gas injection three-way cock 10 is switched by the pH controller 8.

The gas generated in the pH control tank 3 joins the gas supply path of the methane fermentation tank 1 and is sent to a storage tank (not shown), and the digested sludge is discharged to a predetermined place by a pipe 19. It A pipe 20 is branched from the middle of the pipe 19, and the pipe 20 is guided to the methane fermentation tank 1 with a pump 12 interposed in the middle thereof. The pump
12 is controlled by the pH controller 6. The gas separation device 4 has a gas separation membrane permeation gas 21 with a high carbon dioxide concentration, as will be described later in detail, based on the difference in permeability of the polymer membrane.
And a gas separation membrane non-permeable gas 22 having a low carbon dioxide concentration.

Next, the operation will be described.

The methane fermentation raw material enters the methane fermentation tank 1 and is gasified by the action of methane bacteria and other microorganisms in the methane fermentation tank. The generated gas is usually 60-70% methane and 40-30
Composed of% carbon dioxide. A part of the generated gas is pressurized by the compressor 5, and based on the difference in permeability of the gas separation device 4 using a polymer membrane or the like, the polymer membrane permeable gas 21 having a high carbon dioxide concentration and the high gas having a low carbon dioxide concentration are used. It is separated into a molecular film non-permeable gas 22 and this gas is separately guided to the connection port of the gas injection three-way cock 10.

On the other hand, the digested sludge in the methane fermentation tank 1 is sent to the solid-liquid separation tank 2 and separated into digested sludge and desorbed liquid, and the digested sludge enters the pH adjusting tank 3. When the gas separating membrane permeating gas or non-permeating gas is aerated by the blower 11 through the gas injection three-way cock 10 into the pH adjusting tank 3, the carbon dioxide partial pressure in the gas phase in the tank changes, and the liquid accompanying this changes. The concentration of carbon dioxide dissolved in the phase and the pH change. That is, the gas separation membrane non-permeable gas 22 having a low carbon dioxide concentration is fed to the pH adjusting tank 3
The pH of the digested sludge in the pH adjusting tank 3 rises when it is ventilated, and the pH of the digested sludge in the pH adjusting tank 3 decreases when the gas separation membrane permeating gas 21 having a high carbon dioxide concentration is vented to the pH adjusting tank 3. To do.

Therefore, the gas injection three-way cock 10 is switched by the pH controller 8 operated by the signal from the pH sensor 9 to control the pH of the liquid phase in the pH controller 3.

The digested sludge in the pH controller 3 thus adjusted in pH is discharged through the pipe 19. At this time, the pH sensor 7 detects the pH of the methane fermentation tank 1 and outputs the signal to the pH controller. When a deviation is generated by sending the data to the pump 6 and making a match with a fixed set value, an ON command signal is sent to the pump 12. As a result, the pH of the methane fermentation tank 1 rises when the pH-adjusted sludge whose pH has risen is recirculated to the methane fermentation tank 1 by passing a gas with a low carbon dioxide concentration through the pH regulation tank 3.
On the contrary, if a gas having a high carbon dioxide concentration is passed through the pH control tank 3 to recirculate the pH-adjusted sludge having a lowered pH, the pH of the methane fermentation tank 1 is lowered.

In this way, the pH of the methane fermentation tank 1 can be controlled by switching the gas injection three-way cock 10 and turning the pump 12 on and off.

G. Effects of the Invention As described above, according to the present invention, when a deviation occurs in the pH set value of the methane fermentation tank by using the recirculation control system, by the command of the pH controller provided in the pH adjusting tank. Either the polymer membrane permeation gas or the polymer membrane non-permeation gas is introduced to the pH control tank to adjust the pH of the digested sludge, and the pH-adjusted digested sludge is recycled into the methane fermentation tank. By doing so, there is an advantage that it is not necessary to constantly replenish a large amount of water for gas cleaning unlike the conventional gas cleaning recirculation control system, and the running cost is low. Further, although the gas cleaning recirculation system cannot lower the pH, the present invention also enables the pH to be lowered. Furthermore, since the digested sludge can be recycled by solid-liquid separation, the concentration of microorganisms required for methane fermentation can be increased and the fermentation rate of methane fermentation can be increased.

[Brief description of drawings]

FIG. 1 is a block diagram of an application example of a control method of methane fermentation according to the present invention, FIG. 2 and the following show a conventional example, FIG. 2 is a block diagram of a flow rate control method, and FIG. 3 is digestive sludge recirculation control. FIG. 4 is a block diagram of a method, FIG. 4 is a block diagram of a base injection control method, and FIG. 5 is a block diagram of a gas cleaning recirculation control method. 1 ... Methane fermentation tank, 2 ... Solid-liquid separation tank, 3 ...
… PH adjusting tank, 4 …… Gas separation device, 6 …… pH controller, 7 …… pH sensor, 8 …… pH controller, 9 …… pH sensor, 10 …… Gas injection three-way cock, 21 …… Gas Separation membrane permeation gas, 22 ... Gas separation membrane non-permeation gas.

Claims (1)

[Claims] 1. A pH sensor is provided in a methane fermentation tank, a solid-liquid separation tank is connected to the methane fermentation tank, and a pH adjusting tank to which digested sludge separated in the solid-liquid separation tank is supplied is connected. , A gas containing methane gas and carbon dioxide generated in the methane fermentation tank is guided to a gas separation device using a polymer membrane through a compressor, and a polymer having a high carbon dioxide concentration is produced based on the difference in permeability of the polymer membrane. When there is a deviation in the pH set value of the methane fermentation tank due to separation into the membrane permeation gas and the polymer membrane non-permeation gas with a low carbon dioxide concentration, the polymer is instructed by the pH controller installed in the pH control tank. Either the gas that permeates the membrane or the gas that does not permeate the polymer membrane is selectively guided to the pH control tank to adjust the pH of the digested sludge, and the pH-adjusted digested sludge is recirculated into the methane fermentation tank. A method for controlling methane fermentation, which is characterized in that