Testing a Processor for Mixing Ability

By Rick Da Tech


Testing a processor's mixing abilityDo you need to improve the mixing in your processor? Would it be a waste of time to add a mixing eductor or would adding one let you use less methanol? Over the years, the answer has always been subjective, leading to experimentation that may or may not have been fruitful. This simple test gives you a clear quantitative answer. 

To perform the test, take a sample of 100ml at the ouput of the pum just before you shut it down to settle out the glycerin. After both the sample and the batch settle, measure how much glycerin and how much biodiesel you have in the sample and in the full batch. If they have the same ratio of biodiesel to glycerin, you have good mixing. If your sample has a higher percentage of glycerin than the batch, your mixing can be improved. The greater the difference, the more your mixing can be improved.

Inside your processor are little balls of glycerin that want to fall to the bottom of the tank. Mixing keeps them suspended evenly throughout the biodiesel and relatively small in size. Over time these balls touch each other and stick together forming a larger ball. The larger the ball the faster it falls. These little balls of glycerin not only contain glycerin, but also contain methanol and catalyst. The methanol and catalyst can dissolve in both the glycerin and the biodiesel, but at different concentrations. The glycerin can hold much more than the biodiesel. As the reaction proceeds the methanol in the biodiesel is used up, throwing off the balance of methanol between the glycerin and biodiesel. This imbalance forces the methanol out of the glycerin, into the biodiesel. Since the methanol can't be in the reaction zone when it's needed, the process experiences methanol starvation This causes your reaction to be incomplete. How badly the reaction is starved for methanol is regulated by two major factors, the size of the balls and the distance the methanol needs to travel to get where it's needed for the reaction.

The smaller the ball, the larger the total surface area between the biodiesel and glycerin. Passing the beads through the pump or a static mixer will chop up the balls into smaller balls making it easier for the methanol to flow in and out of the glycerin.

The longer a ball of glycerin remains in the tank undistrubed by mixing, the larger it will grow and the faster it will fall. If most of the glycerin is in the bottom of the tank, the top of the will run out of methanol and stop reacting short of it’s full potential. We pump mix drawing the biodiesel and glycerin from the bottom of the tank. As more and more glycerin settles out, the ratio of glycerin to biodiesel going to our pump and our sample port will also change. This means that when we draw a sample, it is an indication of how much of the glycerin we see settling in our tank.If we see more glycerin in our sample than we see in the full batch, then we know we have glycerin settling in our processor.

There are some simple solutions to this, If you are using a high shear pump (centrifugal pumps), you can install a bigger pump using more energy or we can install a mixing eductor. If you are using a low shear pump (gear pumps), we can install a static mixer to reduce the size of the glycerin ball and slow settling that way.


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