Digester Mixing Theory and Landia GasMix Review on their 30th Anniversary!

Read on for our discussion of CSTR Digester Mixing Theory and our Landia GasMix Review, plus congratulations to Landia UK on celebrating 30-years in business!

Introduction to Digester Mixing Theory

Manufacturers that offer anaerobic digestion plant mixing systems will tell you that operators that seek better biogas plant efficiency and have digesters that lack mixing, can generally improve their biogas yield by adding a stirring and mixing system.

Even those digesters which have mixing systems installed will often benefit from an upgraded mixer. And the cost of the installation plus pump energy costs are soon paid back by the additional output of biogas.

Not only that, they cite a wide range of other benefits, which all add up to easier plant operation and reduced maintenance costs.

(This assumes that the plant operator has already paid attention to balancing the carbon to nitrogen ratio (C:N ratio), and feeding the plant regularly and consistently in accordance with good operating practice.)

Achieving Better Biogas Plant Efficiency Through Improved Mixing

Biogas plant owners are right to question their mixing efficiency as one of the first and simplest ways to raise their biogas yield.

So, when we were asked by sponsor Landia to review their GasMix biogas reactor mixing system I decided to go back to the first principles, to check the value of mixing to all chemical and biochemical reactions.

Mixing Theory

Without a mixing system a reactor will still mix, but only slowly. There will be some flow within the tank induced by the effects of inlet and outlet flow, bubbles of gas rising through the liquor.

There will also be some relatively weak convection “eddy” currents induced by temperature differences between the body of liquid (liquor) and the ambient temperature outside. In addition, any heating provided by the plant operator will also produce gentle upward currents above the heating pipes.

However, this mixing will virtually all be by what water engineers call “laminar flow” in localised “eddies”. That is not the best type of flow for efficient chemical and biochemical reactions to take place.

When considering this recently I turned back to one of my textbooks from the 1970s for guidance as to the mechanics common to all chemical and biochemical processes.

It was not long before I found what I was looking for, to confirm the basic theory applied by process design engineers when deciding on whether to mix or not mix, but seldom aired in the biogas industry, as follows:

“If the motion of the fluid is turbulent, the transfer of fluid by eddy motion is superimposed on the molecular transfer process. In this case, the rate of transfer to the surface [so that a reaction can occur] will be a function of the degree of turbulence. When the fluid is highly turbulent, the rate of transfer by molecular motion will be negligible compared with that by eddy motion. “

Ref: Page 325, MOMENTUM, HEAT AND MASS TRANSFER; Chemical Engineering, Volume One, Revised second edition, By J. M. COULSON and J. F. RICHARDSON, Pergamon Press.

The higher the rate of transfer by molecular motion (or mass transfer) the more rapidly the biogas producing organisms (methanogens) will be able to grow and reproduce.

So, it follows that the higher the yield and quality of biogas will be until some other factor becomes rate-limiting. In other words, in order for the reaction to take place the “food” must come into contact with the “bug” that wants to eat it. This happens at a molecular level, and the more molecular motion through physical mixing the better!

The above book continues, by the use of mathematics in the form of advanced fluid dynamics, to provide equations. These are the equations used by chemical process designers to calculate the mass and heat transfer rates needed to optimize their chemical and biochemical reactions.

However, for our purposes, the excerpt above is enough. It says to me that introducing turbulent flow by using a mixer system, and not simply relying on laminar convection in “eddy currents”, will be beneficial in raising the biogas output rate for any feed.

So, the main point of this article appears to be backed up by the chemical process engineering theory.

But, the question which remains for biogas plants with or without any engineered mixing system, is:

“Will it really often be financially viable to invest in new, or retro-fitted, mixing equipment, and if so does Landia's GasMix system come up to scratch”.

Landia GasMix Review

After recently studying the Landia GasMix system, we decided that it did meet our expectations of such a system.

Not only that, the manner in which it works is innovative and ingenious, and deserves to be understood by as wide an audience within the biogas industry as possible.

Rather than continuing here with a written review, we thought that the best way to explain the way the GasMix system works was by producing a video to explain it in detail, as below:

<< Coming Soon >>

Returning now to the question we posed above, which was: “Will it really often be financially viable to invest in new, or retro-fitted, mixing equipment, and if so does Landia's GasMix system come up to scratch”, our conclusion to this review is yes it is and, yes it does – in all technical aspects. This is a system well worth considering.

More information is available from Landia at www.landia.co.uk or for the US: www.landiainc.com

Press Release

30-year Milestone For Chopper Pump Manufacturer Landia UK

Landia UK is marking its 30^th anniversary of business. Set up in 1994 as a subsidiary of Landia A/S in Denmark (established 1933), the UK company began as a humble, two-person operation with Liz Robinson, and the late Hugh Vaughan.

Today, Liz Robinson, together with Paul Davies, lead the company that prides itself on helping companies find solutions to their wastewater/slurry pumping needs, serving the water, food, farming and biogas industries, plus many more. Landia UK’s longevity mirrors the proven long-lasting operation of its low maintenance equipment, which they say offers customers unrivalled total cost of ownership.

“From a standing start as a complete unknown 30 years ago, we’ve firmly established our reputation as problem solvers, with great products and tailored solutions,” said Liz Robinson.

“I want to thank our hard-working team for their dedication and belief in what we provide, which to some at first, doesn’t appear to be cheapest, but will, in the long-term almost certainly prove to be so.”

Recent successes for Landia UK include its digester mixing system for the biogas sector, with landmark installations for Yorkshire Water, Scottish Water, Eco Verde Energy and Cannington Bio Energy.

In addition, Landia UK has won significant praise for its work in helping provide pumping/aeration solutions to the Timothy Taylor’s Brewery, Deeside Cereals, plus numerous farms all over the United Kingdom and Ireland.

Landia UK’s Paul Davies, added: “We’re very proud of marking our 30^th anniversary, but year in, year out, demand continues to grow for our pumps, especially where hard-to-handle solids in wastewater and slurries create big problems for equipment that isn’t robust enough.

We also pride ourselves on the back-up we always give to our customers. After a sale, unlike some, we don’t just walk away.

“We’re also into our 18^th consecutive year of offering training and career opportunities to prospective local young engineers, which is now paying dividends. Members of our team who learnt their trade as apprentices in the workshop are now out in the field, servicing and selling Landia equipment. These talented young people will play a key part in our future.”

PR ends