How can water be evaporated from the liquid digestate? 1

[Chentock]

I am working on a project to dry liquid digestates (residue from the fermentation process of biogas plants). The aim of the project is to reduce the volume of the liquid digestate, in order to cut the transport and storage costs for the farmers. This volume reduction is achieved by evaporation of water from the digestate.

The liquid digestate has a dry matter (DM) content of about 3%. The digestate is primarily made up of animal manure and maize silage. It is a non-newtonian fluid having a viscosity of 4 - 15 mPa-s. It exhibits a shear thinning behaviour, where the viscosity decreases with shear rate.

A drum dryer is used, which runs continuously (24x7) with an automatic setting. The dryer makes use of the heat from the combined heat and power (CHP) unit. The task is to remove/evaporate 1 liter of water for every 1 kW of heat supplied from the CHP unit.

The liquid digestate after the separation process is fed into the drum dryer at a temperature of 40°C. Hot air with a temperature of 70°C is blown into the drum to dry the digestate. The drum is packed with cooling tower fills (see image Packed fills) in order to facilitate faster drying by increasing the heat transfer surface area between digestate and air.

The problem is that the tiny fibres from the liquid digestate, inspite of pre-separation process, gets deposited on the cooling tower fills and the clog the air flow. Thereby the whole functioning of the dryer is affected. These tiny fibres are suspended and float in the liquid digestate, as it is difficult to filter them out.

Hence, I would like to know, if heating the liquid digestate helps? What happens to these tiny floating fibres in the liquid, when the temperature is raised from 40°C to 60°C? Definitely the viscosity of the liquid would decrease, but does it affect these tiny fibres?

How can this fibre deposition and clogging of flow be avioded? Are there better methods to increase the heat transfer area, other than cooling tower fills? Any new suggestions are welcome.

Thank you.

 

[chicopee]

Sublimation of ice into a vacuum chamber is a way to restore documents that were wet from fire extinguishing efforts, so perhaps a similar procedure can be exercised in your case. I always try thing outside the box in unusual situations. In unusual situations, sometimes you succeed and most of the time you fail.

 

[IRstuff]

Just for reference evaporating 1 liter of water conventionally requires 0.628 kWh of energy, not counting the energy required to get to 100C


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[MartinLe]

It's been a while since I worked in the field som maybe I missed a few new developements. But evaporating liquid digestate is a huge hassle.

3% TS is low for digestate, is there a separator or centrifuge upstream?

IMO the fills will alway block. For comparison, look at this drum dryer

http://www.mineralit.com/resources/files/mineralit Universal-Trommetrockner TypII Infoblatt.pdf

with far more internal space.
When you remove the fills, you will have far less air-digestate contact area in your dryer. But removing small openings that can and will be blocked could be worth the attempt.

I don't have a good idea on how to remove the fine solids from the digestate stream.

Pr-heating the digestate would help, but would no solve your blocking problem. One issue with heating is that you get struvite (and maybe organic) deposits on your HX, expect to clean often.

You can also look at alternative systems, Dorset has a belt dryer that recircualtes some dried digestate and mixes it with fresh input though it needs an input TS of 8% (

https://www.dorset.nu/wp-content/up...m-for-Biomass-and-Poultry-Litter_OKT-2016.pdf

)

You also need to consider that the vapor will contain ammonia (how muc hdepends on pH, temperature and a ton of other factors), check if you can legally vent this in your locale. Ammomnia from can be removed with an acid scrubber, ammoniumsulfate has also some value as a fertilizer.

All of that said, digestate management needs to be considered when planning a biogas plant, while the digestate is a good fertilizer it's also very diluted and costs lots of fuel to truck around, finding fields near the plant is key. There's also limits how much fertilizer you can responsilby dump on a field.