How to Figure Out the Efficiency of Your Molecular Sieve Beds

This article will explain how to figure out the working capacity in an ethanol plant.  Using the Measuring-Ethanol-Working-Capacity document I attached to my previous article Top 6 Signs It’s Time to Change out the Sieve in Your Ethanol Plant I will offer a more thorough explanation (or walk-through) on the attached document that describes how to figure out the working capacity of your ethanol plant. 

Ultimately the purpose of making these calculations is to let you know how efficient your sieve beds are running, a good working capacity for an ethanol plant is between 0.7% and 1.0%, the higher the percentage the better.  If your sieve bed is running at a lower working capacity than 0.7%, then it could be a sign that your sieve beds may need to be changed out. 

Looking at the example on the attached document here is how these numbers came to be.  (These numbers are used strictly as an example and are not representing an actual ethanol plant, however the mathematical process is still the same). 

In the example you start with following numbers:

Feed rate of 300 gallons a minute

Feed concentration of 188 proof, or 94% ethanol

Sieve bed load of 2000 pounds

Product concentration of 99.5% ethanol

I will explain this process in reverse order, which makes it easier to explain what/why we are making each of these calculations and then I will take you through the example in a straight forward order with the numbers given in the example. 

In order to find the working capacity you need to know how much water is getting adsorbed in the sieve bed and divide it by how much sieve is in the bed or…

Working Capacity= Water adsorbed in bed/lbs of sieve

Since we know how many pounds of sieve we are using, we need to figure out how much water is being adsorbed in the sieve beds.  That can be determined by subtracting the water that’s remains after running  ethanol/water based gas through the sieve beds (water out in product) from the water going in to the product (water in) or…

Water adsorbed in bed= Water In – Water Out in Product 

However, we need to figure out the water in and water out in product.  Before you can figure this out you need to know how much ethanol is being run through your sieve beds.  In order to figure out how much ethanol is going in you have to multiply the percentage of ethanol in your feed concentration by how many gallons of liquid you are putting through the sieve beds total, the equation looks like this. 

Ethanol In= Ethanol Feed Concentration % x Feed Rate

You can now figure out the water in which is figured by subtracting the number of gallons of ethanol from the feed rate or…

Water In=Feed Rate- Ethanol In 

Water out in product is figured by multiplying product concentration to ethanol in.  The resulting number from this equation is subtracted from ethanol in.  

Water Out in Product = Ethanol In- (Product Concentration x Ethanol In)

Now that we have all the parts you need to figure it out here is how it will look with the example.

1. Ethanol In= Ethanol Feed Concentration % x Feed Rate (0.94×300=282) Ethanol In=282 gallons/min.
2. Water In=Feed Rate- Ethanol In (300-282=18) Water In=18 gallons/min.
3. Water Out in Product = Ethanol In- (Product Concentration x Ethanol In) ((282-(0.995×282)) Water Out in Product= 1.41
4. Water Adsorbed in Bed= Water In – Water Out in Product (18-1.41=16.59) Water Adsorbed in Bed = 16.59 gallons/min.
5. Working Capacity= Water Adsorbed in Bed/lbs of sieve (16.59/2000=0.008) Working Capacity = 0.008lb of water/lb of sieve or 0.8% Working Capacity.

 We can conclude from this example that with a 0.8% working capacity the sieve beds are in good working order, and that a change out would not be necessary.