Choosing The Best Evaporation Technique For Digestate And Sludge

Due to their thick, viscous nature digestates and sludges are among the most challenging materials to evaporate efficiently and effectively. However, the benefits in terms of handling, disposal and treatment can be significant. While there is considerable interest in using mechanical vapour recompression (MVR) evaporation techniques, the complex nature of digestate means that MVR may not always be the best choice.

The electrical energy employed in MVR is normally considerably cheaper than the thermal energy needed for traditional evaporation. However, there are a number of key considerations when using MVR – particularly for very thick and viscous products– which can add to the capital cost (and complexity) of an MVR-based evaporation solution. It is therefore very important that all these elements are considered from the outset.

MVR VERSUS TRADITIONAL EVAPORATION

Traditional evaporation techniques use a high temperature service fluid (such as pressurised steam) to raise the temperature of the product above its boiling point so that water (and other volatile compounds) is driven off, leaving a more concentrated solution. The principal source of energy for this process is therefore the fuel used to heat the water (steam) in the boiler, such as gas or oil.

In MVR, the steam which comes off the product in the evaporator is channelled into a compressor which increases the pressure (and therefore the temperature). This steam, which is now above the boiling point of the product, is then used as the service fluid for the evaporator. As the compressor uses an electric motor, the process is driven by electricity rather than thermal energy. Because the compressor reuses/recycles evaporated steam, a lot of latent heat is recovered, making MVR one of the cheapest methods of evaporating water in terms of operational costs.

 

LIMITATIONS OF MVR

However, because of the way MVR works, there is a relatively small temperature difference between the service fluid and the boiling point of the product, meaning heat transfer between the two is limited and you need a large surface area to achieve it.

In contrast, boilers can deliver maximum steam pressure of up to 8 or 10 bar, meaning an effective temperature of 160°C or 180°C. The greater temperature difference means less transfer surface area is required, and you can utilise a much smaller heat exchanger.

Because of the thick nature and high fouling potential of many digestates and sludges, in most cases you need a relatively large surface area to achieve sufficient heat transfer. Because of the limitations off compressors in terms of service fluid temperature, the required heat exchangers and pumps can be exceptionally large indeed, increasing capital cost and the amount of energy needed.

 

SPECIFIC ISSUES WITH DIGESTATE AND SLUDGES

Digestate and sludges often require pre-treatment to remove suspended solids and/or ammonia before evaporation – adding additional costs. As no two sludges are the same, at HRS we always test the material that any client will be working with in order to determine not only the best heat exchanger solution for the evaporation process, but also what pre-treatment may be necessary.

 

CONCLUSIONS

Evaporating digestate and sludges using MVR is certainly possible and HRS Heat Exchangers have supplied such systems where they are appropriate. However, our experience has shown that careful consideration, including the system modifications which may be required to ensure efficient and effective evaporation of the project, is essential in order to determine the most suitable and most economical technology for each project.