Continuous stirred tank reactors (CSTRs) are of high interest for industrial biogas plants e.g. being part of a waste water treatment plant (WWTP).
CSTRs digester for production waste
This reference shows two continuous stirred tank reactors made of stainless steel. They are integrated to a company-owned waste and sewage treatment plant of food producer in Germany. With the new digesters they can make the most of their energetic production residues. On the one hand, they can cut energy costs and on the other hand they can improve their ecological footprint. In this context, two CSTRs became necessary and the decision was made to stainless steel digesters by Stallkamp.
The production residues undergo a long process: first of all the sludge is sieved before it comes into the digester. The continuous stirred tank reactors offer the advantage of optimal and steady homogenising of the biomass. The biomass is decomposed and converted into energy. The gas yield is among others dependent on the mixing and homogenisation of the biomass. After digestion the digestate is separated into solids and liquids. The solids are dried, pelletised and further processed as fertiliser, whereas, the liquids undergo a further clarification process until it is dischargeable water.
How to build continuous stirred tank reactors?
It is a segment-like desgin that is used to construct the tank of this size. Each of them has a diameter of 15m and a height of almost 20m. This leads to a digestion volume of 3,300m³. Single shell plates (3 to 6mm) are rolled to the desired diameter which are then screwed and sealed to form a large ring on site. After finishing one ring, the tank is lifted up with electrical lifting supports. For this project new supports were developed because the final weight of more than 40 tons had to be liftedsynchronously and as smooth as possible. Then the next tank ring can be constructed. After thirteen repetitions the final height was reached. "We knew from multiple tests that the concept would work. Nevertheless, the calm and even lifting of the structure was a special moment," reports Erich Stallkamp (managing partner), who followed the construction progress daily.
Stainless steel CSTRs
It was decided for stainless steel due to its corrosion resistant characteristic which is valuable in the anaerobic digestion process. And the plant operator decided for stainless steel in its highest quality: the tank wall as well as the stainless steel floor (4mm) are made of duplex stainless steel sheets. The material 1.4462 has a particularly high yield strength as well as a very high corrosion resistanc. Therefore it is very suitable for use in anaerobic processes. The roofs of the digesters are also made of stainless steel. Stable rafters attached to an inner and an outer ring build the basis. From the inside and outside thin stainless steel sheets are attached to the basis to build the roof. The opening for the central mixer is in the middle of the roof. The mixers constantly homogenises the biomass and thus ensures optimum process biology.
Technical Data for both CSTR digesters
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