Read on for our discussion of CSTR Digester Mixing Theory and our Landia GasMix Review.
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.
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:
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.
[Article first published in August 2014: Latest update July 2021]