13 Steps to Set Up Ethanol Plant Molecular Sieve Beds
3A molecular sieve is typically used in the final stage of ethanol dehydration in order to get ethanol to over 99% pure. Getting your molecular sieve beds to run optimally can be a challenging process, so this list of steps has been compiled in order to help with this challenge. Below are 13 steps that will help ethanol plant managers and operators set up their molecular sieve beds in their ethanol units.
1) Different plants have different bed configurations and the procedure will vary slightly depending on the number of beds and the layout. Some plants have two beds, some three beds some six and some nine. In general the following principles hold for the conditioning of all layouts but the sequencing of conditioning may vary. The important concept is to maintain control of the heat generated during conditioning, this can be as much as 1800 btu/lb of water adsorbed. The temperature should never be allowed to rise above 500 degrees Fahrenheit.
2) For maximum efficiency, the pressure in the sieve bed should be as high as the vessel construction allows with 10-15 degrees Celsius (50-60 degrees Fahrenheit) of super heat.
3) Set the 1st bed to accept the feed with the product valve open. Set the next bed in line to pull vacuum down through the first bed to be conditioned. This helps pull heat and water vapor down through the first bed to speed and it balances the conditioning while beginning the conditioning of the second bed from the bottom.
4) Feed to the beds should be set at 50-75% of normal operating flow rate. Usually a centrifugal pump is used to move the vapor to the beds. Sometimes there are limitations to the minimal amount of flow capacity from the pump. The initial feed should be set as low as possible, if the 50-75% rate is unachievable.
5) Temperature monitoring is critical. Ideally thermal transmitters are available to allow monitoring in the control room. Realistically, thermal readouts should be available for reading externally at the top 1/3 and the bottom 1/3 of the bed. Personnel should be stationed with communications equipment allowing information to be fed to the operator in the control room.
6) Ideally the feed should begin with 200 proof ethanol to wet the sieve bed and the surface of the beads. This will help reduce the evolution of heat from the water adsorption on the external surface of the bead while creating a heat sink within the bed. The heat capacity of the bed itself and the beads is low. The pre-wetting of the beads will allow some of the heat to be adsorbed by the ethanol. Ethanol when adsorbed onto the sieve media will generate about 700 btu/lb alcohol adsorbed, which is 29% less of the heat generated when exposed to water. If 200 proof is not available feed can begin with 190 proof.
7) Begin vapor feed at the recommended temperatures and pressures while pulling vacuum down through the bed into the second bed. Monitor the temperature at the top of the bed closely. When the temperature reaches 380 degrees Fahrenheit, cut feed to the first bed and begin feed to the second bed while pulling the vacuum on the first bed. Begin pulling vacuum on the third bed while pulling heat and vapor down through the second bed. If only two beds are available then open the first bed, while pulling the vacuum on the first bed, to the second bed. The vapor from the second bed will help cool the first bed. If this is a two bed system then you may get to a point where feed is cut while you are pulling vacuum on both beds waiting for them to cool.
8) The temperature in the first bed should begin to drop while the temperature in the second bed rises. When the temperature in the second bed reaches 380 degrees Fahrenheit, cut feed to the second bed and pull the vacuum to remove heat and adsorbed moisture. If this is a two bed system then you may get to a point where the feed is cut while you are pulling the vacuum on both beds waiting for them to cool.
9) Pull the vacuum on the beds until the temperature in the beds is below the vapor feed temperature. This will vary from plant to plant depending on the layout.
10) When the temperature in the bed is low enough feed may resume to that bed. In regards to temperature at the center of the bed, cut the feed to the bed when the center temperature reaches 380 degrees Fahrenheit. If no center bed temperature reading is available, then monitor the temperature at the bottom of the bed and cut the feed when the temperature at the bottom reaches 350 degrees Fahrenheit.
11) Resume feed to the next bed to be conditioned while pulling the vacuum to the bed being cooled. if the temperature of the next bed to be conditioned is not close to the feed temperature continue to cool down with vacuum and wait.
12) Repeat this sequence gradually increasing feed rate until the temperature throughout the bed has stabilized to the temperature of the feed. At this point the beds are commissioned and ready to resume normal operation.
13) The sieve beads can endure much higher temperatures than what is typically experienced in an ethanol plant, but this regimen is designed to protect not only the equipment, but the plant personnel from potentially dangerous temperature excursions.
Not every situation is the same – for assistance with your particular adsorbent/ethanol issue please contact Mark Binns – Hengye USA Technical Business Director. 502-232-5256 and/or email at firstname.lastname@example.org
Mark has a degree in Chemical Engineering from the University of Louisville, and has specialized in working with industrial adsorbents and the ethanol industry for over a decade.