Drying of liquid digestate

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Summary
Drying of liquid digestate inside a drum dryer packed with cooling tower fills leads to deposition of tiny fibres on the fill material and clogging of air flow. Looking for new ideas to improve the contact surface area between the liquid digestate and hot air inside the drum dryer.
I am working on a project to dry liquid digestates (residue from the fermentation process of biogas plants).

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.

The liquid digestate at a temperature of 40°C is fed into a drum dryer. 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 gets deposited on the cooling tower fills (see image Fibre Deposition) 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.
 

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jrmichler

Science Advisor
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Your description sounds a lot like a description of paper stock. Paper stock is a slurry of water and cellulose fibers that is made into paper. The fibers in a sheet of paper stick together as a result of the drying process, not because of adhesives. The drying process as I recall (it's been a long time) starts with the wet fibers in contact. Then, as the water is evaporated, the surface tension of the remaining water pulls the fibers closer together, which eventually results in fibers bonding to each other. I think the process is Van der Walls bonding. If your fibers are similar to paper fibers, cooling tower clogging WILL happen.

The paper industry has a number of techniques for separating fiber from water, starting with fine screens. These screens are called wires in the paper industry, hence the term "twin wire former" for one type of process. These wires wear out and are replaced on a regular basis. You should be able to get a used wire from most any paper mill for the asking. Just call the mill and ask for the maintenance manager, the production manager, or the mill manager if it's a small mill.

I know this can be done because I once saw a demonstration of just that. They separated the fiber from liquid manure, then squeezed as much water out as they could. The result was a mass of fiber that did not even stink. It felt slightly damp to the touch. I do not remember what type of manure, nor do I remember any equipment names. And I do not know if all of this is directly applicable to biogas sludge.

Here's an introduction to the papermaking process: http://www.fibrelab.ubc.ca/files/2013/01/Topic-11-Papermaking-Introduction-text.pdf.
 
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Hi jrmichler,

Thanks a lot for your suggestion. I will look into it.

Apart from using cooling tower fills, is there any other better method to increase the heat transfer area inside the drum between the liquid digestate and air? I gave a thought about using dryer flights (fin like structures inside the drum), but I am not sure if the fibers would still get deposited on these flights and clog the flow again.

It would be nice to hear some suggestions about it as well.
 

jrmichler

Science Advisor
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It is many times more expensive to remove moisture by evaporation than by filtration. That is why paper machines use a three step process:
Step 1: Drain water through a wire, assisted by foils, in the former section.
Step 2: Squeeze out water in the press section.
Step 3: Evaporate the remaining water in the dryer section.
A very rough rule of thumb is that the cost of water removed increases by a factor of ten at each step. That's the cost per liter of water removed.

Paper machines dry the fiber mat by holding against successive hot surfaces in the dryer section. The hot surfaces are cast iron drums several feet diameter, heated by steam at typical pressures of 50 to 100 PSI (condensing temperature 298 to 338 deg F). The mat is held against the hot surface by dryer felts, a coarse fabric that looks almost like window screen. This type of process would work well if an acceptable end product was a dry mat.

If the desired end product is a loose dry pile of fiber, I would start by using the first two steps in the papermaking process. I would look at a rotary drum dryer for the final step.
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A rotary drum dryer (search the term) is a rotating drum with internal flights to tumble the product, and hot gases to dry the product. It uses the same principles as a clothes dryer, except larger and longer, and it is a continuous process. The flights break up lumps, resulting in good heat transfer to the hot gas.
 
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Thanks a lot again for your reply.

The aim of the project I'm working in 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 desired end product is definitely not a dry pile of fibers, but the same liquid digestate with a reduced volume.

Say if the dry matter content in the digestate is 4% before entering the dryer, it is supposed to be around 8% while exiting the dryer. So the digestate remains a liquid throughout, only the dry matter content increases.

The dryer has to run 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 dryer is in working condition, however the major issue is that the cooling tower fills placed inside the drum gets clogged quite often. Hence, there are only two ways moving forward.

1. Stay with the exisiting idea of cooling tower fills and try to get rid of the tiny fibers which get deposited on these fill surfaces. This is where, I think, the screening wires from the paper machines could be a possibility. But the problem is these wires have to be regularly replaced, which again is against the idea of a continuous process.

2. Or think of a new design inside the drum, such that this clogging can be avoided. I don't know, if mounting flights inside the drum would help in getting rid of clogging problem. The tiny fibers in the digestate would still deposit on the flights and clog the flow, isn't it?

Therefore, I am looking for new suggestions.

Thank you.
 

jrmichler

Science Advisor
889
793
Evaporation is about the worst way to concentrate a liquid to a slightly higher solids content. Since you only want to remove about half of the liquid, a screen type filter should do the job. There are many companies that make filtration systems. Talk to their application engineers, tell them what you want to filter, and clearly communicate that you do not want to fully filter the material, but only concentrate it by removing half of the water. They can supply a system where the liquid moves past the filter screen to keep it from clogging.

if the dry matter content in the digestate is 4% before entering the system / concentrator, it is supposed to be around 8% while exiting
You need to clearly communicate this up front. And say something like "system to partially remove liquid", rather than "dryer". If your boss is fixated on the idea of a dryer, you have a communication challenge.

Good luck, it sounds like an interesting project.
 
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The idea of evaporation was conceived with the sole aim of reducing the volume of the liquid digestate by evaporating the water into atmosphere, in order to cut the storage costs. So, the task is not just to concentrate the liquid to a slightly higher solids content, but also to reduce its volume.

If a filtration system is used, the liquid can be concentrated, agreed. But what to do with the filtered water? It can't be reused or pumped into the fields directly. It has to be stored somewhere. This is why, reducing the volume of the liquid is necessary, such that the storage costs are minimal.

In fact, there is already a separator in function, which separates the solid contents from the liquid digestate. Despite this separation process, the tiny fibers manage to pass through the filter medium and enter the dryer, causing a HUGE trouble.

And secondly, the heat from CHP unit has to be somehow used for this so-called "drying/evaporating" process. The farmers get some monetary benefits, if they reuse the heat from the CHP unit without letting it to atmosphere.

So, the crucial points to consider are the following.

1. Reduction of liquid digestate volume by removing water from it.
2. Achieving a performance of 1 liter evaporation for every 1 kW of heat supplied from the CHP unit.

Taking these points into account, the dryer is designed. However, it is not an optimal design, because of the clogging of flow. So, now the task is to find a workaround for fixing the clogging problem.
 

jrmichler

Science Advisor
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In that case, I see two possibilities:

1) Add a finer filter after your existing separator, then evaporate.

2) Periodically flood the dryer with a large amount of liquid to wash out the fiber buildup. Do this when the buildup starts, do not wait until the dryer is plugged.
 
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Ok thanks a lot for your reply.

You don't see anyother better design for the dryer, apart from using cooling tower fills? Mounting flights inside the drum won't solve the clogging problem completely, isn't it? The fibers may still get deposited on the surfaces of the flights and clog the flow.
 

jrmichler

Science Advisor
889
793
There are companies that specialize in cooling tower fills. I'm sure that one of their application engineers could find a fill that is more resistant to plugging, however the tradeoff would be poorer evaporation.
 

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