DE8336265U1 - Column reactor for continuous anaerobic degradation processes - Google Patents

Description

Jülich Nuclear Research Facility Limited liability company

Official reference number: G 83 36 265.7 Description

Column reactor for continuous anaerobic degradation processes

The innovation relates to a column reactor for the continuous implementation of anaerobic Abbe* processes with a fixed bed column with solid carrier bodies covered with biomass with adjustable lower inlet and upper outlet, a pH control and level regulator, which is to be operated in particular at 30 - 40 ⁰ C and pH 6 - 7 using microorganisms that selectively degrade the substrates in a continuous process with biogas production.

Anaerobic microbial processes generally proceed very slowly and the bacterial concentrations are low compared to aerobic ones due to the poor use of the carbon source by the anaerobic growing cell (ATP formation). As a consequence, long residence times and low space-time yields must be accepted during continuous operation in a stirred tank reactor. If the residence time is shortened below the critical value, which corresponds to the reciprocal of the maximum growth rate MY/, the microorganisms are washed out and the process tends to become unstable if the feed is not throttled immediately.

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An increase in the space-time yield is possible if ' did biomass with a reduction in the hydraulic residence time

either retained in the reactor or outside the |

reactor and recycled, resulting in | Decoupling of the solid and liquid residence time takes place. | Processes based on sedimentation of microorganisms

However, they have low efficiency and are strongly dependent on the separability of the cells, which also

is disturbed by biogas formation and flotation. | ■

Filtration is possible in principle, but it requires |

However, special filter units and is too expensive for |

the treatment of larger quantities of liquid. §

An effective way of biomass retention is to grow the microorganisms on an inert carrier (see European patent application 0 028 846).

In this technique, the liquid to be treated is continuously fed into a reactor filled with a bacterial suspension, in which a residence time is maintained that is below the washout point. Under these stress conditions, a selection takes place in which only those microorganisms can multiply that develop sufficiently strong adhesion forces to the carrier and can feed on the carbon source provided. Additional selection is achieved by applying different levels of shear force depending on the

speed depends on the flow velocity of the liquid.

The above-mentioned implementation in a fluidized bed reactor does have the advantage that a high flow velocity can be selected and the biogas separation is unproblematic and that approximately the same conditions prevail within the reactor, but a relatively high level of abrasion due to more intensive friction and loss of bacteria must be accepted and a relatively large amount of energy must be used to reach the fluidization point. (Furthermore, solids that are lighter than the carrier can hardly be kept in the fluidized bed, although this is generally advantageous).

In contrast, a fixed bed reactor with continuous flow would have the advantage of low abrasion and low energy requirements, but only relatively low flow velocities are possible and dead volumes form in the reactor due to biogas cushions. There is also the risk of blockages due to suspended solids and a concentration and pH gradient forms within the reactor from the inlet to the outlet.

Both variants have disadvantages.

The following aspects are particularly important for acetic acid degradation, which is currently of particular interest:

- The high chemical oxygen demand of the acetic acid-containing wastewater to be treated;

- its low pH value of 2 to 4;

- the very long doubling time of the microorganisms in question;

- their low tendency to form larger aggregates or flakes, so that only weak interactions with the carrier can be assumed;

- the pH optimum for their growth is 6.4 to 7.

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For these reasons, very long residence times of the liquids to be treated in the reactor are necessary and the achievable space/time yields are therefore only low. Furthermore, the pH value of the liquid to be treated must be monitored, especially before it enters the reactor, brought to the optimum value by adding buffer or alkali. The economic efficiency of the process is therefore unsatisfactory.

The innovation is therefore based on the task of improving the economic efficiency of anaerobic degradation, in particular acetic acid methanation, whereby specifically a fixed bed reactor should be used.

The innovative column reactor developed for this purpose is characterized by pH sensors both at the lower and upper ends of the column and a recirculation of reactor effluent with a dosing device subject to pH measurements.

In operation, the liquid to be treated, such as an aqueous liquid containing lower carboxylic acid and/or furfural and/or methanol, with a pH value possibly truncated to 2 to 4, is fed to the fixed bed reactor with microorganisms grown on a solid support as feed mixed with such an amount of recycled reactor effluent that the pH value of the reactor liquid measured at the lower and upper ends of the fixed bed is kept within a narrow range not exceeding 0.3 pH units within 6 to 7.2.

The advantages of the fixed bed reactor mentioned above are used, while at the same time

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but its disadvantages are avoided by ensuring that the reactor flows quickly through the appropriate mixing of recycled reactor effluent, so that practically uniform conditions prevail in the reactor and the decomposition takes place under optimal conditions over the entire length of the reactor. At the same time, the perfect discharge above the reactor ensures that blockages cannot occur.

If

In this reactor, the pH value of the reactor liquid can be kept in the optimal range for bacterial growth through recycling without external influence, so that the addition of chemicals that would otherwise be necessary to establish compatible conditions can generally be omitted.

The economic efficiency of such a fixed bed reactor operation with microorganisms grown on a carrier by autoselection and with recycling of the reactor effluent in such a way that the pH gradient in the reactor practically disappears can now surprisingly be considerably improved by choosing a "very careful run ^- in phase": In the start-up phase, the cultivation is sufficiently successful.

adhering microorganisms only with an average residence time that is not less than 5o% of the generation time of the microorganisms involved. After such cultivation has been successful, the space/time yield of the $ reactor can be increased by reducing the average residence time, although this must be done in relatively small steps while maintaining equilibrium, while a reduction in the residence time by about 5o% or more results in a completely negative result, ie the microorganisms are completely washed out of the reactor, so that, viewed globally, an increase in yield initially does not seem possible.

Preferably dater

After the first cultivation with a residence time that is not less than 50 % of the generation time of the microorganisms involved, an iterative shortening of the residence time is carried out until the space/time yield can no longer be increased.

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In the tests carried out, anthracite has proven to be particularly suitable as a carrier material, which should be used in a grain size of around 2 to 8 mm, especially 4 to 6 mm. This carrier material is very hard and shows low friction losses and has an irregular surface.

To test the innovation,

In the laboratory, a reactor is used as shown schematically in the attached Figure 1.

Figure 2 shows a series of measurements for the dynamic behavior of a fixed bed circulation reactor with iterative shortening of the residence time.

The reactor illustrated in Figure 1 was essentially formed by a Plexiglas column with a height of 65 cm and a diameter of 5 cm with a filling height to diameter ratio of 12:1 (generally ratios of 8 to 16:1 are appropriate). The inner wall of the reactor was provided at regular intervals with ring-shaped inserts (as "flow breakers") to prevent the formation of channels along the walls of the fixed bed. The temperature was kept at 37 to 38°C by thermostatting.

Filter anthracite N from PREUSSAG AG dbr Ibbenbüren deposit was used as the carrier material. This anthracite had the following characteristic properties:

Density: 1.4o - 1.45 g/cm ³

Bulk density j 74o kg/m ³

Carbon content: Over 9o %

Grain size: Type 1 Type 2 Type 3

o,8 -1,6 mm 1,4-2,5 mm 2,5-4,? mm

This anthracite is a natural product that is mined, crushed and sieved . It is a
non-porous material with a defined grain size.

Methanosarcina barkeri was fixed as a microorganism on this carrier.

By varying the filling height, the free contact volume was changed, to which the respective residence time can be related.

For the degradation tests, test water containing acetic acid was used, which was provided with vitamins, minerals and trace elements and had a chemical oxygen demand (CCSB value) of about 20 kg/m and a pH value of 4. The feed and recycling streams were dosed separately and combined at the bottom of the reactor shortly before entering the reactor.

In the upper part of the reactor there was a level sensor to limit the maximum filling level of the reactor, from which the liquid was withdrawn by means of a liquid pump controlled by the level controller. The pH values of the liquid at the top and bottom of the reactor were monitored daily and the ratio of circulation to feed was then
adjusted so that a practically uniform pH value is achieved in the
reactor (the circulation is usually about 15 to 20 times the feed).

In case of irregular behaviour of the reactor, where a desired

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If the pH value cannot be equalized by (manually) controlling the circulation, the inflow is temporarily throttled to allow the system to recover. For safety reasons, constant pH control is provided in the middle area of the system, which intervenes in an emergency by adding corrective agents to the system.

To start up the reactor, the feed was set to &ogr; ml /hour (residence time 3/1 ^) and its pH value was adjusted to 5.o blunted to prevent over-acidification of the reactor as a result of the initially only low sales.

After about two weeks, a sufficient bacterial population had formed on the carrier (recognizable by the acetic acid degradation rate and biogas formation in the reactor), so that the feed could be gradually passed through the reactor more quickly without prior pH adjustment, thereby shortening the residence time accordingly.

Figure 2 shows the acetate concentration in the reactor effluent as a function of time at different residence times. As can be seen, the residence time from step 1 to step 2 was reduced by about 30% and the changed conditions were maintained for about 12 days each.

As can be seen, after each shortening of the residence time, the acetic acid concentration increases, because the bacterial population that grew under the stationary conditions of the previous stage cannot initially cope with the excess supply of ctg* C source. (Particularly strong increases are due to

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a "wash out" of microorganisms that sometimes initially occurs due to the increase in flow).

A little later, however, as one can see, more biomass is formed and the acetic acid is again largely broken down. After stationary conditions have been established, this procedure (shortening the residence time with equilibrium adjustment) is repeated until a clear deterioration in the acetic acid conversion becomes apparent or a further increase in the space/time yield is no longer possible. In the present case, this applies to a residence time of around 7 hours.

The total time from start-up to achieving such short residence times is approximately 10 to 12 weeks.

Larger jumps in the reduction of the residence time or even a sudden change in the residence time from the initial state to economically optimal conditions (from about 3 days to about &bgr; hours) are not feasible, since the system would collapse immediately.

As already mentioned, the circulation speed depends on the pH gradient between the lower and upper parts of the reactor and is set so that this gradient just disappears (the pH value in the reactor increases on the one hand due to the reduction in the acetic acid concentration caused by degradation and on the other hand due to the buffering effect of the carbonic acid formed, so that under stationary conditions a pH value of 6.7 to 7.0 is established in the reactor).

Example__1

The reactor shown in Fig. 1 was used with

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a height of 63 cm and a diameter of 5 cm. It was filled with 3oo g of Filter-Anthrazit N from PREUSSAG with a grain size of 1.4 to 2.5 mm. The free reaction volume was 0.76 l.

The inflow was increased in the manner described to 83 ml/h (hydraulic flow rate: 9.2h), corresponding to a volume load of 56 kg COD per m and day. Under stationary conditions, a COD reduction of 95% was achieved; the volume-specific biogas formation was 41 m per m ³ and day.

Example_2

In this case, only 1oo g of filter anthracite It of the same grain size as in Example 1 were filled into the reactor. The free contact volume was 1.1 l.

Due to the smaller surface area available, less biomass was fixed overall, so that the operating limit was reached at a residence time of about 17 hours. Under these conditions, the volume load was 32 kg COD per m ³ and day, given the insufficient amount of carrier, and the volume-specific biogas formation was 19m per m and day.

In a further experiment, 32o g of filter anthracite with a grain size of 2.5 to 4.0 mm were filled into the reactor. The free contact volume was 0.75 l. The feed was gradually changed to 1oo ml/h, corresponding to a residence time of 7.5 h and a volume loading of 7o kg COD per m and day . With a COD reduction of approx. 8o%, the volume-specific biogas formation was 42 m ³ per m and day.

- 11 -Example 4

In another experiment, 300 g of filter anthracite N was used to process vapour condensate from the pulp industry with a chemical oxygen requirement of 14 kg per m. In addition to acetic acid, this also contains methanol, furfural/formic acid and sulphite. In this case, instead of the acetic acid-degrading microorganism Methanosarcina barker!, a mixed culture containing microorganisms that also degrade furfural, methanol and oxidised sulphur compounds was used.

Within a period of 4 weeks, the residence time was reduced to 15 hours. The volume load was 23 kg per m ³ and day and the volume-specific biogas formation 14 m per m and T. of more than 90%.

14 m per m and day with a COD reduction

The above-mentioned acetic acid degradation

The inventive process explained in more detail can be applied analogously to other lower carboxylic acid and/or methanol and/or furfural degrading anaerobic processes become.

Claims (1)

Julien Nuclear Research Facility Limited Liability Company Protection claims Column reactor for the continuous implementation of anaerobic degradation processes with a fixed bed column with solid carrier bodies covered with biomass with adjustable lower inlet and upper outlet, a pH control and level regulator, characterized by pH sensors at both the lower and upper ends of the column and a return of reactor outlet with a dosing device subject to pH measurements. Column reactor according to claim 1, characterized by a column with a height:diameter ratio of 8 to 16:1, in particular about 12:1 Column reactor according to claim 1 or 2, characterized by grained carrier bodies with a grain size of 2 to 8 mm. Column reactor according to claim 3, characterized by carrier bodies made of anthracite, in particular filter anthracite N from the Ibbenbüren deposit of PREUSSAG, with a grain size of 2 to 8 mm, in particular 4 to 6 mm. PT 1.656a nö/hö