PROCESS AND DEVICE FOR THE PRODUCTION OF METHANE

Abstract

The present invention relates to a process for producing methane and to the device for producing methane, making it possible to increase the methane content of the outgoing gas and preferably simultaneously to increase the methane content of the outgoing gas and the productivity of the reactor.

Claims

1. A process for producing methane, comprising: a step (a), in a first bioreactor comprising methanogenic microorganisms in a liquid medium, of production of a gas mixture comprising methane, consisting in bringing said microorganisms into contact with incoming gases; a step (b), in a second bioreactor, of methane enrichment of the gas mixture obtained in step (a), consisting in at least partly transferring, from the first bioreactor to the second bioreactor, on the one hand the gas mixture obtained in step (a) and, on the other hand, the liquid medium contained in the first bioreactor, so as to increase the methane content in the gas mixture.

2. The process as claimed in claim 1, characterized in that the transfer of said liquid medium is carried out by sampling said liquid medium in the first bioreactor and then by injecting said liquid medium into the upper part of the second bioreactor so that said liquid medium circulates by gravity in the second bioreactor and is recovered in the first bioreactor.

3. The process as claimed in claim 1, characterized in that, in the first bioreactor, the liquid medium is a continuous liquid phase into which the incoming gases are injected, and in that the second bioreactor contains a continuous gas phase.

4. The process as claimed in claim 1, characterized in that the microorganisms are chosen from hydrogenotrophic methanogenic microorganisms, homoacetogenic microorganisms and acetoclastic methanogenic microorganisms or a mixture of these microorganisms.

5. The process as claimed in claim 1, characterized in that the incoming gases are CO.sub.2 and H.sub.2.

6. The process as claimed in claim 1, characterized in that H.sub.2 and CO.sub.2 can also be injected into the second bioreactor.

7. A device for producing methane, characterized in that it comprises: a first bioreactor comprising methanogenic microorganisms in a liquid medium, said liquid medium being a continuous liquid phase; a second bioreactor comprising a continuous gas phase and a system which makes it possible to increase gas exchanges; a device which makes it possible to inject the incoming gases into said continuous liquid phase contained in the first bioreactor; at least one means for supplying with liquid medium contained in the first bioreactor cooperating with the second bioreactor, said means being capable of supplying the second bioreactor with liquid medium, said supply means comprising pumping means ensuring the circulation of the liquid medium contained in the first bioreactor to the second bioreactor, said liquid medium circulating by gravity on said system which makes it possible to increase gas exchanges, said liquid medium being recovered in said first bioreactor; and at least one means for transferring the gas mixture contained in the first bioreactor to the second bioreactor.

8. The device as claimed in claim 7, characterized in that the second bioreactor contains a continuous gas phase.

9. The device as claimed in claim 7, characterized in that the device also contains a means for supplying incoming gases into the second bioreactor.

10. The device as claimed in claim 7, characterized in that the first bioreactor is chosen from a bubble column, a mechanically stirred column, an infinitely mixed reactor or an airlift reactor and in that the second bioreactor is chosen from a percolation reactor, a random packed column, a structured packed column, a spray column, a falling film column or a tray column.

11. The device as claimed in claim 7, characterized in that the device which makes it possible to inject the incoming gases into said continuous liquid phase is chosen from fine bubble diffusers, a porous column bottom diffuser, a pierced tube, a porous membrane made of polymers or ceramic material, a valve bubbler, or bubble-free membrane contactors such as hollow fiber membranes, or a hydroejector or a static mixer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0118] Other characteristics, details and advantages of the invention will emerge on reading the detailed description below, and on analyzing the appended drawings, on which:

[0119] FIG. 1 shows a device for producing methane according to the prior art;

[0120] FIG. 2 shows a device for producing methane according to one embodiment of the invention, in which the two bioreactors are combined (“two-stage system”);

[0121] FIG. 3 shows a device for producing methane according to another embodiment of the invention, in which the two bioreactors are connected by a means for transferring the gas mixture;

[0122] FIG. 4 shows a curve representing the productivity of NL CH.sub.4/L.sub.useful/h as a function of the methane content (% CH.sub.4) of a prior art process (“1 stage: bubble column”) and of the process according to the present invention (“2 stages: bubble column+percolation”) as represented in FIG. 2;

[0123] FIG. 5 shows a curve representing the CH.sub.4, CO.sub.2 and H.sub.2 composition of the outgoing gas as a function of time of the process according to the present invention (“two-stage system”). The composition is expressed on a dry gas basis.

DESCRIPTION OF EMBODIMENTS

[0124] The drawings and the description below contain essentially elements of a certain nature. They may therefore not only serve to better understand the present invention, but also contribute to the definition thereof, where appropriate.

[0125] Reference is now made to FIG. 1. The device 10 as represented in FIG. 1 is a methanation reactor of the prior art allowing the production of methane and comprising a single reactor 11 comprising a 22-liter bubble column (with a useful volume of 18 liters), which is gas-tight and thermally insulated. The temperature within the bubble column reactor is maintained at approximately 55° C. by the presence of a water circulation jacket 12.

[0126] A mixture of incoming gases 13, H.sub.2 and CO.sub.2, is injected into the bubble column 11 by a sintered fine bubble diffuser 14 in the lower part of the bubble column

[0127] The upper part of the reactor consists of a polyvinyl chloride (PVC) plate pierced with 7 orifices (not represented) allowing the passage of probes. The device 10 also comprises a gas outlet equipped with a condenser 15, a gas outlet 26 to a meter of, a gas loop 16 connected to analyzers 17 (analysis of the outgoing gases making it possible to quantify respectively the CO.sub.2, H.sub.2 and CH.sub.4 contents), a loop for gas recirculation 24 from the upper part of the reactor to the lower part, an orifice 21 for mixing the recirculated gases with the incoming gases, an inlet 18 for the supply of nutrient solution, and a purge of the liquid medium 27. A three-way valve 22 allows sampling of the gas in order to verify the composition of the gas by gas chromatography or for regenerating the gas and the anoxic medium in the gas headspace.

[0128] The dissolved carbon dioxide concentration, the redox potential and also the pH are measured by probes 23 immersed in the liquid medium (directly in the reactor). The probe allowing measurement of the pH also makes it possible to measure the temperature.

[0129] The bubble column comprises a liquid medium consisting of hydrogenotrophic methanogenic, acetoclastic methanogenic and homoacetogenic microorganisms, of nutrients and of trace elements.

[0130] According to this embodiment of the prior art, the gas supply is carried out via two synthetic-gas cylinders each comprising H.sub.2 and CO.sub.2. Mass flowmeters make it possible to finely adjust the entering flow rates of the incoming gases. Recirculation 24 of the gases from the top to the bottom of the column is carried out at constant speed, by means of a valve pump 25. The objective of this recirculation 24 is to increase the gas retention rate and the retention time of the gas in order to increase the dissolution of the H.sub.2 and the consumption by the microorganisms in order to reduce the residual H.sub.2 concentration in the outgoing gas mixture.

[0131] The flow rate of the outgoing gas mixture is measured using a Ritter gas meter. The gases leaving the reactor pass through a condenser 15 maintained at 4° C. Part of the condensation water is reintroduced into the reactor in order to maintain the volume of the liquid medium.

[0132] The reactor is continually supplied with incoming gas (H.sub.2 and CO.sub.2). On the other hand, the supply of nutrients and the purging of the liquid medium are carried out batchwise. The quantitative determination of the sulfur is carried out using a piston syringe system. The taking of samples of liquid for analysis of the compounds is carried out in the lower part of the reactor. Typically, the nutrients are injected using a concentrated solution of nutrients, in particular NH.sub.4Cl at 20 g/l, KH.sub.2PO.sub.4 at 10 g/l, MgCl.sub.2 at 2 g/l, CaCl.sub.2 at 1 g/l, Na.sub.2S at 26.7 g/l and NaHCO.sub.3 at 12.4 g/l.

[0133] The composition (proportion of H.sub.2, CO.sub.2, CH.sub.4) of the outgoing gas mixture is measured continually by sampling in the upper part of the column.

[0134] Reference is now made to FIG. 2 reproducing a device 30 for producing methane according to one embodiment of the invention. This device makes it possible to carry out an ex situ methanation process. The components represented in FIG. 2 and bearing the same references as those of FIG. 1 represent the same objects, which are not described again below.

[0135] The methanation device or reactor 30 is composed of a bubble column 31 and a percolation reactor 32, which are gas-tight and thermally isolated.

[0136] The 22-liter bubble column 31 (with a useful volume of 18 liters) is connected to the percolation reactor 32 by a part 33 made of PVC. The bubble column and the reactor thus connected are clamped by a collar. The percolation reactor 32 is packed with ⅝-inch Pall® rings having a diameter of 15 mm and a height of 15 mm (Techim France).

[0137] The bubble column 31 comprises a liquid medium comprising hydrogeno-trophic methanogenic, acetoclastic methanogenic and homoacetogenic micro-organisms, nutrients and trace elements. The liquid medium of the bubble column 31 is pumped into the lower part of the bubble column 31 and is conveyed via a peristaltic pump 40 into the upper part of the percolation reactor 32. The liquid medium is injected by spraying using a spray 38 present in the upper part of the percolation reactor. The liquid medium circulating by gravity on Pall® rings will percolate through the rings in order to increase the contact surface between the liquid and the gas, and will then fall back into the bubble column 31. The gas mixture generated in the bubble column 31 diffuses in the percolation reactor, through a stainless steel grid 34 retaining the Pall® rings of the reactor 32.

[0138] The methane productivity is high, in particular by virtue of a relatively high hydrogen partial pressure, and a high flow rate, which thus allow high microbial growth. The gas mixture thus generated will diffuse, according to a pressure differential applied between the inlet of the bubble column 31 and the outlet of the percolation reactor 32, in the percolation reactor 32 in which the hydrogen and the carbon dioxide will be converted into methane so as to achieve a high methane content in the outgoing gas mixture. According to this embodiment, more than 80% of the hydrogen contained in the incoming gases is converted.

[0139] The part 33 allows connection between the bubble column 31 and the percolation reactor 32 and comprises tappings 331, 332, allowing gas chromatography analysis and/or liquid sampling. The part 33 in which the tappings 331 and 332 are shown diagrammatically is an enlargement, on FIG. 2, of this part in order to illustrate the tappings.

[0140] The concentration of dissolved carbon dioxide, the pH and the redox potential are measured using probes 36, 37 immersed in the liquid, either directly in the reactor (not represented), or in a cell 35 connected to the reactor as represented in FIG. 2. The pH probe also makes it possible to measure the temperature.

[0141] In the same way as above, the gas supply is carried out via two synthetic gas cylinders each comprising H.sub.2 and CO.sub.2. Mass flowmeters make it possible to finely adjust the inlet flow rates of the incoming gases. The flow rate of the mixture of outgoing gases is measured by a Ritter gas meter. The gases leaving the reactor pass through a condenser maintained at 4° C. Part of the condensation water is reintroduced into the reactor in order to maintain the volume of the liquid medium.

[0142] The reactor is continually supplied with incoming gases (H.sub.2 and CO.sub.2). On the other hand, the supply of nutrients and the purging of the liquid medium of the reactor are carried out batchwise. The quantitative determination of the sulfur is carried out using a piston syringe system. The taking of liquid samples for the analysis of the compounds is carried out in the lower part of the reactor.

[0143] The composition (proportion of H.sub.2, CO.sub.2, CH.sub.4) of the outgoing gas mixture is measured continuously by sampling in the upper part of the column, by means of the same analyzers mounted in series as previously described.

[0144] Reference is now made to FIG. 3. The device represented also makes it possible to carry out an ex situ methanation process. The components represented in FIG. 3 and bearing the same references as those of FIG. 2 represent the same objects, which are not described again below.

[0145] In this embodiment, the bubble column 31 and the percolation reactor 32 are interconnected by pipelines and peristaltic pumps. This embodiment makes it possible in particular to use reactors of different diameters and to reduce the height of the reactor. The liquid medium contained in the bubble column 31 is pumped into the lower part of the bubble column 31 via a peristaltic pump 40 so as to be injected by spraying at the top of the percolation reactor 32 by means of a spray 38. The same liquid, in the same way as in the embodiment described in FIG. 2, will circulate by percolation on the Pall® rings and will be collected and reinjected via the tube 41 into the bubble column 31. The outgoing gas mixture generated in the bubble column 31 is transferred into the percolation reactor 32 by application of a pressure differential between the two bioreactors, if necessary by means of a compressor.

[0146] In the same way as previously, the gas supply is carried out via two synthetic gas cylinders each comprising H.sub.2 and CO.sub.2. Mass flowmeters make it possible to finely adjust the inlet flow rates of the incoming gases. The flow rate of the mixture of outgoing gases is measured by a Ritter gas meter. The gases leaving the reactor pass through a condenser maintained at 4° C. Part of the condensation water is reintroduced into the bubble column 31 in order to maintain the volume of the liquid medium.

[0147] The reactor is continually supplied with incoming gas (H.sub.2 and CO.sub.2). On the other hand, the supply of nutrients and the purging of the liquid medium of the reactor are carried out batchwise. The quantitative determination of the sulfur is carried out using a piston syringe system. The taking of liquid samples for analysis of the compounds is carried out in the lower part of the bubble column 31.

[0148] The composition (proportion of H.sub.2, CO.sub.2, CH.sub.4) of the outgoing gas mixture is measured continually by sampling in the upper part of the column, by means of the same analyzers mounted in series as previously described.

EXAMPLES

[0149] Other advantages, aims and particular characteristics of the present invention will emerge from the examples that follow, given purely by way of explanation and which are in no way limiting.

[0150] In the examples that follow, the various parameters were measured using the techniques detailed below.

[0151] Measurement of the Productivity with Respect to the Methane Produced

The productivity with respect to the methane produced is calculated by means of the following measurement:

[00001]$\begin{array}{cc}{P}_{C{H}_4}=\frac{\%{\mathrm{CH}}_4\mathrm{out}\times Q_{g,\mathrm{out}\left(\mathrm{dry}\right)}}{V_{useful}}& \left[\mathrm{Math}.3\right]\end{array}$

With: PCH.sub.5×10.sup.3=methane productivity in NL of CH.sub.4/L of useful volume/h [0152] % CH.sub.4out=percentage of methane in the outgoing gas expressed on a dry gas basis [0153] Q.sub.g,out=flow rate of outgoing gas in NmL/h expressed on a dry gas basis [0154] V.sub.useful=useful volume of the reactor in which the reaction takes place

[0155] Measurement of the Microbial Biomass Concentration (VSS)

[0156] The measurement of the biomass concentration is estimated once a week by measuring the volatile suspended solids (VSS) according to the Afnor NF T90-105-2 standard.

[0157] The principle consists in taking a sample of known volume (75 ml in this case). After centrifugation for 15 minutes at 13 200 rpm and at 4° C., the pellet is introduced into a previously dried and weighed aluminum cup. The cup is then placed in an oven at 105° C. for 24 hours. The water having thus evaporated, there remains in the cup only the suspended solids (SS). The cup is then weighed after cooling in a desiccator. The difference in mass between the empty cup and the cup after it has gone through the oven thus corresponds to the SS contained in the sample. Taking into consideration the initial liquid volume, the measurement is expressed in g.Math.L.sup.−1. The cup is then placed in a furnace at 550° C. for 2 hours. After cooling, the cup, which now contains only the mineral matter, is again weighed. The mass of VSS is obtained by the difference between the mass of VSS and the mass of the mineral matter.

[0158] Measurement of the Outgoing Gas Volume

[0159] The outgoing gas volume is measured by volumetry using a Ritter brand drum gas meter (TG 05 model 5). The volume is expressed on a dry gas basis.

[0160] Measurement of the H.sub.2, CO.sub.2 and CH.sub.4 contents

[0161] The H.sub.2, CO.sub.2 and CH.sub.4 composition of the outgoing gas is measured using various analyzers mounted in series:

[0162] H.sub.2 is measured by thermal conductivity by means of a Rosemount® Binos 100 2M analyzer.

[0163] CO.sub.2 and CH.sub.4 are measured with a non-dispersive infrared (NDIR) gas analyzer by using the Rosemount® X-stream analyzer.

[0164] Measurement of the pH and the Temperature

[0165] The pH and the temperature are measured using a probe and a transmitter from Mettler Toledo®.

[0166] Measurement of the Productivity/Methane Content Pairing

[0167] The productivity with respect to the methane produced is calculated by means of the formula detailed above and the CH.sub.4 content is measured using the appropriate analyzer. Once this information had been obtained for various operating points, graphs representing the CH.sub.4 content as a function of productivity were plotted and are as represented in FIGS. 4 and 5.

[0168] Operating Conditions [0169] Temperature: 52 to 57° C. [0170] Pressure: atmospheric pressure [0171] The incoming gas flow rate varied from 6.3 to 43.6 NL/h.

Example 1: Evaluation of the Enrichment in Biogas and of the Productivity According to a Prior Art Process

[0172] In the present example, the bioreactor used is the bioreactor as described in FIG. 1.

[0173] The bioreactor was inoculated using microorganisms (biomass) from organic waste methanizers. After biomass growth, the microorganism concentration was then adjusted around 3 g/l (VSS), by regular and appropriate purges of liquid and of biomass.

[0174] The pilot was continuously supplied with synthesis gases, H.sub.2 and CO.sub.2. The ratio between the flow rates of H.sub.2 and of CO.sub.2 was kept constant and the total flow rate gradually increased with the performance levels of the process.

[0175] The reactor operated for 300 days and the performance levels of the bioreactor were evaluated with regard to: [0176] the methane content in the gas headspace; [0177] the methane productivity in NL of CH.sub.4 per L of reactor per hour (PCH.sub.4 NL/Lreac/h).

[0178] The results are presented in the table below for various incoming gas flow rates:

TABLE-US-00001 TABLE 1 Q.sub.gin Q.sub.gout (dry) PCH.sub.4 × 10.sup.3 (NL/h) (NL/h) % CH.sub.4 (NL/L.sub.useful/h) 6.4 1.2 97.7 63.9 7.4 1.4 92.3 69.4 9.4 2.0 82.2 91.4 9.4 2.2 76.8 94.8

[0179] Q.sub.gin (NL/h) corresponding to the flow rate of incoming hydrogen and carbon dioxide in NL/h and Q.sub.gout (NL/h) corresponding to the flow rate of the outgoing gases in NL/h, expressed on a dry gas basis.

Example 2: Evaluation of the Enrichment in Biogas and of the Productivity According to the Process and Device of the Invention

[0180] In the present example, the bioreactor used is the bioreactor as described in FIG. 2.

[0181] The bioreactor was inoculated using microorganisms (biomass) from organic waste methanizers. After biomass growth, the microorganism concentration was then adjusted around 3 g/l (VSS), by regular and appropriate purges of liquid and of biomass.

[0182] The pilot was continuously supplied with synthesis gases, H.sub.2 and CO.sub.2. The ratio between the flow rates of H.sub.2 and of CO.sub.2 was kept constant (and the total flow rate gradually increased with the performance levels of the process).

[0183] The reactor operated for 50 days and the performance levels of the bioreactor were evaluated with regard to: [0184] the methane content in the gas headspace; [0185] the methane productivity in mL of CH.sub.4/L of reactor per hour (PCH.sub.4 mL/Lreac/h).

[0186] The results are presented in the table below for various incoming gas flow rates:

TABLE-US-00002 TABLE 2 Q.sub.gin Q.sub.gout (dry) PCH.sub.4 × 10.sup.3 (NL/h) (NL/h) % CH.sub.4 (NL/L.sub.useful/h) 14.5 2.9 94.5 76.7 14.5 3.0 93.9 77.9 29.2 6.1 90.8 153.6 43.6 9.7 83.3 225.0

[0187] Q.sub.gin (NL/h) corresponding to the incoming hydrogen and carbon dioxide flow rate in NL/h and Q.sub.gout (NL/h) corresponding to the outgoing gas flow rate in NL/h, expressed on a dry gas basis.

[0188] The results obtained show that, with only one bubble column (1-stage reactor), a significant compromise must be made between the methane content of the exiting gas and the productivity. The addition of a second percolation stage makes it possible to simultaneously increase the productivity of the process and the methane content of the gas produced. By way of illustration, the bioreactor described in FIG. 1 makes it possible to obtain a productivity of 91.4×10.sup.−4 NL.sub.CH4/L.sub.useful/h for a CH.sub.4 content of 82.2% at the reactor outlet, whereas the bioreactor described in FIG. 2 makes it possible to obtain a productivity of 153.6×10.sup.−4 NL.sub.CH4/L.sub.useful/h for a CH.sub.4 content of 90.8% at the reactor outlet, that is to say a productivity and a CH.sub.4 content of greater than 68.1% and 10.5% respectively.

[0189] Advantageously, the process and the devices according to the invention make it possible to simultaneously increase the productivity of the process and the methane content of the gas produced, with a simplified implementation, and in particular compared to the prior art process combining methanization and methanation steps.