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How to decrease the shutdowns in a biogas plant

Background

 

The managing team of a waste water treatment plant (hereafter WWTP) contacted recently SACOME to solve the problem of blocking in an existing spiral heat exchanger, destined to the heating of sludge prior to its anaerobic digestion in a digester for the production of biogas.

The existing heat exchanger got plugged too often and this forced our client to do frequent and costly shutdowns to carry out cleaning works. After this first contact, our Staff did some visits on site to analyse closely the problem of our client and offer a state-of-the-art solution adapted to the requirements of our client.

 

Wastewater Treatment Plant

The solution

 

We can find different geometries of heat exchangers in the market: plate, spiral or tubular, to take the most representative ones.

The performance of the plate heat exchangers is not suitable for the sludge treatment, this is why traditionally the spiral and tubular tubular heat exchangers are widespread in the WWTP complexes.

Although it is true that the spiral heat exchangers are quite common in this field, the continuous search of a better performance of the anaerobic digester involves the processing of increasingly more complex sludges: thicker, more viscous and with higher solids and fibers contents. Under these conditions, a spiral heat exchanger can lead to problems of obstruction of the inner channels, which forces to do frequent cleaning works, causing production shutdowns.

Our Tubular Heat Exchanger I-TF20-I are the solution to this issue, as they have a larger process channel, avoiding the risk of plugging, and allowing a continuous operation, minimizing the number of required shutdowns for maintenance works.

SACOME tubular heat exchangers are the solution to this issue, as they have a larger process channel, avoiding the risk of plugging, and allowing a continuous operation, minimizing the number of required shutdowns for maintenance works.

Process

In this WWTP a large amount of organic waste is produced. With the aim to reduce the volume and limit the pollution, these residuals or sludges are sent to a special device known as anaerobic digester. In this equipment a biochemical degradation of the sewage sludge takes place, thanks to the bacteriological action in a controlled atmosphere of temperature and in the absence of oxygen, for further processing into biogas. The anaerobic digestion of these sewage sludge allows, not only the reduction of discharges, but its energy recovery, as biogas is a gas with high calorific value due to its rich methane composition, as we can see in the figure.

 

This biogas is stored in a gasometer where, once treated for the removal of polluting or corrosive components, it can be used as combustible in specially prepared engines, which, in turn, can drive an alternator and produce electrical power that can be discharged into the electrical lines.

  • METHANE 60%
  • CARBON DIOXIDE 30%
  • NITROGEN 5%
  • HYDROGEN 4%
  • OTHER GASES 1%

The heating of sludge previous to the digester: a critical process 

 

The sludge temperature at the inlet of the anaerobic digester is a key feature, as it influences meaningfully to the retention time necessary for the digestion process to be done by the bacteria. It has been proven that in 30 ºC atmospheres 10 days are required for the mesophilic digestion, while at 20 ºC 25 days are required. In view of the above, it is clearly relevant that the temperature must be controlled in a narrow margin in order to stabilize properly the sludges that go to the digester.

The heating of these sludge is done in a tubular heat exchanger, whose good performance is key to achieve an efficient digestion.

Product Specifications

 

Due to the different contents in solids of the sewage sludge from one WWTP to another, the thermal properties of these products, specifically the viscosity, can change meaningfully. Apart from the viscosity, the thermal properties as density, specific heat and thermal conductivity are features that may affect in a significant way to the thermal design of the heat exchanger. In this project, the thermal properties were agreed with the client taking into account our suggestions, according to our fluid database, where we keep values for sludges with different dry extract contents.

Tubular Heat Exchangers for Wastewater Treatment

 

Rheological tests. Although it was not needed in this case, for such variable fluids SACOME offers the possibility to do rheological tests on the basis of a product sample, in order to analyze these thermal properties. This is for sure a guaranteed success for the correct design of the heat exchanger.

Tubular Heat Exchangers Design

Our I-TF20-I tubular heat exchanger is the solution to this issue , as they this heat exchangers have a larger process channel, avoiding the risk of plugging, and allowing a continuous operation, minimizing the number of required shutdowns for maintenance works.
Tube in Tube Design
Our Tubular Heat Exchanger I-TF20-I are the solution to this issue, as they have a larger process channel, avoiding the risk of plugging, and allowing a continuous operation, minimizing the number of required shutdowns for maintenance works.

 

Monotubular Heat Exchanger for industrial applications

Modular Design
Easy removal for maintenance works. As it is a modular design, it is usually quite easy both to dismantle the units to proceed to its mechanical cleaning as, if required, to add more modules to enhance the duty.

Monotubular heat exchangers on a frame

Process Duty
Our tubular heat exchangers can process a wide range of capacities. We also offer the client the possibility to run the process by means of several lines in parallel, with the intent not only to increase the duty, but also to be able to work with a single line while operation and maintenance works are done in the other line, not stopping the production at any moment.
304 or 316L?
Due to the presence of phosphate and ammonium precipitated (struvite), we recommended our client to use 316L stainless steels, that responds in the best possible manner against the risk of uniform corrosion than the 304, because of its low carbon content and the content in molybdenum.
Corrugation

The inner tubes were HARD corrugated in order to increase the turbulence of the product, thus reducing the inherent risk of fouling and increasing the thermal efficiency, in such a manner way that we could propose the client a solution with an optimal exchange area.

 

Corrugation Technology for Tubular Heat Exchangers

Insulation
According to the client specifications, we included in our scope of supply the insulation of the heat exchangers in order to reduce the heat losses to the environment. The modules were individually insulated with 40 mm of mineral wool cladded with aluminium.
Installation and start-up
The equipment was installed on site by the clients operators, counting on the technical advice from our staff, and after the starting up it has been successfully running.

Key Figures

Process Flowrate (m³/h sewage sludge)

Dry extract content of sludge (%)

Sludge Pressure Drop  (bar)

Thermal Duty (Kcal/h)

Floor Space (mm)

Do you need heat exchangers for sludge heating?

Our Technical Staff shall be delighted to provide you with detailed information.

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