CA2105613A1

CA2105613A1 - Process and an arrangement for recovering heat in the chemical degradation of sewage sludge or wastewater

Info

Publication number: CA2105613A1
Application number: CA 2105613
Authority: CA
Inventors: Thomas Melin; Otto Horak; Klaus Holzer
Original assignee: Individual
Current assignee: Bayer AG
Priority date: 1992-09-10
Filing date: 1993-09-07
Publication date: 1994-03-11
Also published as: DE4230266A1; EP0588138A1; JPH06190382A

Abstract

A PROCESS AND AN ARRANGEMENT FOR RECOVERING HEAT IN THE
CHEMICAL DEGRADATION OF SEWAGE SLUDGE OR WASTEWATER

A b s t r a c t In a process for the wet oxidation, pressure hydrolysis or chemolysis of sewage sludge or wastewater at elevated temperature and pressure, heat exchange between the cold influent and the hot effluent is achieved by the fact that pressure is released in the hot effluent, the vapors formed are separated from the aqueous phase, returned to the cold influent and brought into direct contact therewith, so that at least part of the vapors condense out and heat the influent. In the arrangement for carrying out this process, the reaction stage 8 is followed by a pressure release element 10 and a phase separator 11. A vapor return pipe 12 leads from the phase separator 11 to a gas/liquid contactor 3 pre-ceding the reaction stage 8.

Fig. 1

Description

21056~L3 A PROCESS AND AN ARRANGEMENT FOR RECOVERING HEAT IN THE
CH~MICAL DEGRADATION OF 8EWAGE SLUDGE OR WASTEWATER

This invention relates to a process for heat exchange between cold influent and hot effluent in an installation for the wet oxidation, pressure hydrolysis or chemolysis of sewage sludge or wastewater at elevated temperature and pressure. The invention also relates to an arrangement for carrying out the process.
The chemical degradation of sewage sludges or wastewaters by wet oxidation or pressure hydrolysis is normally carried out at elevated temperatures, i.e. at temperatures well above lOO~C. A high heat demand and hence high energy costs can only be avoided in these reactions when a heat exchange is carried out between the hot effluent stream fromithe installation and the cold influent stream, i.e. when steps are taken to recover heat. Hitherto, indirect heat exchangers opera-ted in countercurrent have been used for this purpose.
This technique is described, for example, in Chemie-Ingenieur-Technik 62 (1990), No. 7, pages 555 to 557.
However, it has been found that coatings are often formed on the heat exchanger surfaces, adversely affecting heat exchange and - more seriously - leading to serious limitations in the availability of the installation. This disadvantage arises in particular on the side of the stream to be heated if, for example, insoluble substances are formed during the heating process. Iron phosphates are one example of such substances. In addition, however, many other substances are also precipitated in the event of an increase in temperature which leads to coatings, incrustations and, hence, to a deterioration in heat transfer and, in many cases, to complete blockage. These effects are known in ~, ~
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210~ 3 the hydrolysis of activated sludges. However these effects can also occur in other suspensions and also in wastewaters.
Another disadvantage of conventional heat ex-changers lies in the increased probability of gas cushions being formed on the effluent side. Where the material typically used to withstand the aggressive chemical ef~ects occurring in heat exchangers, namely titanium, is used for wet oxidations, spontaneous reactions can occur between oxygen and the material ("titanium burn") and can lead to serious dangers and even to destruction of the installation.
The problem addressed by the present invention was to increase the availability and operational reli-ability of the installation in a process for recovering heat in the chemical degradation of sewage sludges or wastewaters.
According to the invention, the solution to this problem is characterized in that pressure is released in the hot effluent of the reaction stage (wet oxidation or pressure hydrolysis), the vapors formed are separated from the aqueous phase, returned to the cold influent and then brought into direct contact therewith. On mixing with the influent, at least part of the vapors condense out and heat the influent. The condensed-out component of the steam present in the vapors preferably makes up at least 70%. Accordingly, a direct rather than indirect heat exchange takes place in the process according to the invention.
If the heat transferable to the influent where pressure release and condensation are carried out in a single stage is not sufficient to cover the energy demand, the process is preferably carried out, above all in large installations, in such a way that pressure release in the effluent takes place in several pressure Le A 29 332 2 21~5~.3 stages and the vapors formed are fed into the influent in countercurrent in as many stages.
The heat exchange between vapors and influent takes place in one or more gas/liquid contactors. This may take place in countercurrent, co-current or cross-flow, preferably in countercurrent where the vapors contain relatively high percentages of non-condensable gases.
According to the invention, the installation used to carry out the described process is characterized in that the reaction stage is followed by a pressure release element and by a phase separator and in that a vapor return pipe leads from the phase separator to a gas/liquid contactor preceding the reaction stage.
In preferred embodiments, pressure-regulated release valves are used as the pressure release elements while cyclone separators are used as the phase separa-tors. The gas/liquid contactor advantageously consists of an injection-type condenser.
In one embodiment preferred for wet oxidation, the reaction stage is formed by one or more bubble columns arranged in tandem.
The following advantages are afforded by the invention:
- The malfunction and danger source attributed to coatings, incrustations and blockages of the heat exchangers in the hitherto known process can be completely eliminated.
- - There is no longer any need for the expensive heat exchanger generally made of special materi-als.

- It has also been found that, during the flash L~ A 29 332 3 ~

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, 21~ L3 evaporation in the effluent, the effluent is almost completely degassed or stripped. Volatile organic constituents and CO2 are thus removed from the liquid phase of the effluent.
Embodiments of the invention are described in detail in the following with reference to the accompany-ing drawings, wherein:
Figure 1 is a process flow chart for the wet oxidation of industrial sewage sludge.
Figure 2 is a process flow chart for the pressure hydrolysis of wastewater with pressure release in two stages.
The sewage sludge concentrated to a dry matter content of 40 to 45 g DM per hour accommodated in a tank 1 is injected by means of a pump 2 into an injection-type condenser 3 (nozzle 4). Sulfuric acid is added to the bottom of the injection condenser 3 - in the form of a stirred tank 5 - for preliminary hydrolysis and to improve the subsequent oxidation process. The pre-hydrolyzed sludge is then introduced into the reaction stage for the wet oxidation by means of a pump 6 and a two-component injector 7. The reaction stage consists of three bubble columns 8 in tandem which are flooded, operated and traversed in co-current by the solid sus-pension and gas. The oxygen required for the oxidation process is also introduced into the reaction stage through the two-component injector 7 and dispersed in the sludge. The operating temperature in the reaction stage is dependent upon the,composition of the sludge and the-materials used. In the present example,-the starting temperature is 160C - temperature at which relatively inexpensive materials (steel-enameled) can be used. To obtain the final temperature required for the oxidation process, heat has to be supplied to the ~e A 29 332 4 2l~5l6l3 installation during starting up. To this end, super-heated steam may be introduced through the inlet 9.
After reaching the final temperature, the exothermic oxidation reaction begins. Under the described condi-tions, there is then no longer any need for ~eat to besupplied.
The fully reacted mixture of gas (mainly CO2, steam and excess oxygen) and oxidized sludge is then relieved of pressure by a pressure-regulated valve 10 made of wear-resistant material. At the temperatures and pressures prevailing in the present embodiment, the optimum pressure after pressure release is approximately 1 bar, i.e. atmospheric pressure, so that there is no need for pressure control after the pressure release valve. The pressure does not necessarily have to be re-leased to atmospheric pressure and may even be released to another pressure level, for example in the vacuum range. Pressure release is accompanied by partial evaporation of the hot effluent mixture under pressure.
The vapors formed during pressure release are then separated from the liquid cooled by the flash evapora-tion in the cyclone separator 11 and are brought into direct contact with the influent in the injection condenser 3 by way of the return pipe 12. The vapors contain the entire enthalpy difference of the effluent stream between the exit temperature and the boiling temperature under the selected pressure. This enthalpy is returned to the influent. The non-condensed residual vapors rich in inert gas are substantially freed from steam in a condenser 13 arranged at the head of the injection condenser 3 and are subsequently subjected to thermal waste gas purification. In cases where the vapors have hardly any inert gas component lmajority of pressure hydrolyses), phase flow between vapors and influent is not an important factor. In wet oxidations Le A 29 332 5 - , - : ~ :, , , .

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21~613 where air or oxygen and also carbon dioxide are present in the influent, heat utilization can be improved by countercurrent flow. Through the condensation of the vapors in the injection condenser 3, the heat of evapo-ration of the vapors is largely transferred to theinfluent. In this process, however, the clear sludge sprayed in can only be maximally heated to the boiling temperature of the water under the selected release pressure. Accordingly, if the selected release pressure is too low, the enthalpy of the vapors is only partly utilized so that condensation is not complete. If, by contrast, the release pressure is too high, the heat of condensation of the vapor stream can be fully utilized, but in this case the mass flow of vapors is smaller so that optimal heat utilization is again not achieved.
Where the release pressure is optimally selected, only about half the heat required to heat the influent to the effluent temperature can be recovered in the single-stage arrangement shown in Fig. 1. However, providing sufficiently large quantities of heat (heat of combus-tion) are released during the wet oxidation, this recovery of heat is suffient to guarantee heat-autarchic operation of the installation as a whole under the de-scribed conditions in the case of a sewage sludge con-centrated to a dry matter content of at least 4%. Theheat losses are made up by the heat of oxidation in the reaction stage.
In principle, any of the gas/liquid contactors known from heat and mass transfer technology may be used to achieve the direct transfer of heat from the vapors - to the infl~uent. More particularly trickle-film contac-tors, two-phase flow packings, columns, bubble columns and also spray and jet washers may be used. Injection-type condensers in the form of jet washers are parti-cularly suitable because they do not have any built-in Le A 29 332 6 .

21~S613 parts, show a low pressure loss on the gas side and represent compact, robust and inexpensive apparatus.

Example Concentration of the sewage sludge in the tank 1: 40 to 45 g DM per 1 Temperature of the sewage sludge on entry into the injection condenser 3: 30C
Temperature of the prehydrolyzed sewage sludge àfter leaving the stirred tank 5: 90C, pH value = 1 to 2 Compression to 22 bar by pump 6 Temperature in the reaction stage: 150 to 160C

Pressure in the reaction stage: 20 bar (head pressure) Pressure after release: approx. 1 bar Temperature of vapors and waste gas in the return pipe 12: 96C.

A pressure hydrolysis with pressure release in two stages is described as a second example of applica-tion. The associated process flow chart is shown in Fig~ 2. The raw wastewater to be treated or the waste-water suspension is delivered by the pump 14 into the injection condenser 15 of which the bottom is in the form of a stirred tank 16. Inert gases released are removed at the head of the injection condenser 15 under pressure control (17) by means of a vacuum pump 18. The wastewater to be treated is delivered by a second pump 19 from the first stirred tank 16 into a second injec-L~ A 29 332 7 - ~
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' 2105~13 tion condenser 20 of which the bottom forms the second stirred tank 21. From the second stirred tank 21, the prehydrolyzed wastewater is pumped by the pump 22 into the hydrolysis reactor 23 (reaction stage). The hydro-lyzed wastewater is then relieved of pressure in twostages. The first pressure release stage consists of the pressure-regulated release valve 24 and the follow-ing cyclone separator 25 while the second pressure release stage consists of the level-controlled release valve 26 connected to the exit of the cyclone separator 25 and the following (second) cyclone separator 27. A
return pipe 28 leads from the cyclone separator 25 of the first pressure release stage to the second stirred tank 21. Through this pipe the vapors released in the first pressure release stage are returned to the stirred tank 21 where the vapors are mixed with the wastewater introduced, condensed and give off their heat to the wastewater. As in the case of the first stirred tank 16, inert gases can escape overhead through a pressure-regulated valve 29. There is no need for a vaccum pumpbecause the second injection condenser is operated under excess pressure.
The vapors released in the second pressure release stage are separated from the liquid phase in the cyclone separator 27 and are returned through another return pipe 30 to the first stirred tank 16 where they also condense in the wastewater stream introduced and give off their heat in the process. In this two-stage release of pressure and recycling, therefore, the vapors of the second pressure release stage which are at a lower pressure and temperature level are used to heat - the wastewater in the first injection condenser 15 while the vapors returned through the pipe 28, which are at a higher pressure and temperature level, are used for further heating the wastewater in the second injection Le A 29 332 8 ~-2105~13 condenser 20. Pressure release and recycling of the vapors may also take place in more than two stages. In this case, as in the two-stage embodiment described, the vapors from the first pressure release stage of the effluent stream, which have the highest condensation temperature, are fed into the last condensation stage of the influent stream while the vapors from the last pressure release stage of the effluent stream are used for heat recovery in the first condensation stage of the influent stream. More than half the available heat can be utilized by countercurrent operation in this way.
The number of stages to be sensibly selected is of course limited by the expense involved which increases with any increase in the number of stages. The energy required to cover the heat balance is supplied by the introduction of steam 31 into the influent stream for pressure hydrolysis.
The above-described example of pressure hydroly- `
sis with pressure release in two stages was carried out with the following operational parameters:

Wastewater temperature on entry into the first injection condenser 15: 25C -Wastewater temperature on leaving the rirst stirred tank 16: 87C

Wastewater temperature on leaving the second stirred tank 21: 137C
Pressure in the reaction stage 23: 10 bar (head pres-sure) Temperature in the reaction stage 23: 180C

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~, 21~13 Temperature of the effluent in the cyclone separator 25:

Temperature of the effluent in the cyclone separator 27:
sOC

Pressure and temperature in the return pipe 28: 3.6 bar/

Pressure and temperature in the return pipe 30: 0.7 bar/
90C.

All the pressures are absolute pressures. ;~
The quantity of steam required to cover the heat balance given effective insulation of the plant is just under 0.12 t steam per t influent. If no heat were to be recovered, 0.32 t steam per t influent would have to be used, reducing the energy demand to almost one third (steam introduced: 30 bar saturated steam)..

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Claims

1. A process for heat exchange between cold influent and hot effluent in an installation for the wet oxida-tion, pressure hydrolysis or chemolysis of sewage sludge or wastewater at elevated temperature and pressure, characterized in that pressure is released in the hot effluent, the vapors formed are separated from the aqueous phase, returned to the cold influent and brought into direct contact therewith, so that at least part of the vapors condense out and heat the influent.

2. A process as claimed in claim 1, characterized in that pressure release takes place in several pressure stages and the vapors formed are fed in countercurrent into the influent in as many stages.

3. A process as claimed in claims 1 and 2, charac-terized in that the influent is contacted with the recycled vapors in countercurrent, co-current or cross-flow in a gas/liquid contactor.

4. An arrangement for carrying out the process claimed in claims 1 to 3 consisting of a pressure reaction stage (8,23) for wet oxidation, pressure hydrolysis or chemolysis, characterized in that the reaction stage (8, 23) is followed by a pressure release element (10, 24, 26) and by a phase separator (11, 25, 27) and in that a vapor return pipe (12, 28, 30) leads from the phase separator (11, 25, 27) to a gas/liquid contactor (3, 16, 20) preceding the pressure reaction stage (8, 23).

5. An arrangement as claimed in claim 4, charac-terized in that the gas/liquid contactor consists of an injection-type condenser.

6. An arrangement as claimed in claims 4 and 5, characterized in that, for wet oxidation, the reaction stage consists of one or more bubble column(s) (8) arranged in tandem.