Safe Practices in Handling Molecular Sieves
General Use and in the Process Industry
General Precautions and Practice of Handling Molecular Sieves
Molecular sieve beads or powder are not flammable but if they are exposed to water the molecular sieve will give off heat. Molecular sieve dust can also irritate sensitive tissues and because of these reasons, safety precautions must be observed. Wear protective clothing (eye protection, dust masks, gloves and clothing to cover the body).
Molecular sieves are typically shipped in 55 gallon drums or in Supersacks. When removing the lid from the drum a vacuum condition may exist. Loosen the vent screw on the lid first to break the vacuum before removing the lid.
Preparation for Entering the Process Vessel
Purge the spent molecular sieve bed. This is done by first regenerating the bed by heating and cooling it with gas in a typical regeneration. If the bed was in hazardous or toxic service, use nitrogen to purge the system.
As a recommended last step to ensure safety, in hazardous service, pressure up the vessel to at least 100 psi then block it in and depressure it to flare. The rate of depressurization should be rapid in the downflow direction. 10 bed volumes is the typical for a very good purge to eliminate any residual hydrocarbons. Check the purge outlet until the outlet nitrogen level is below 50% the lower explosive limit and free of toxic materials. Then shut off the nitrogen purge and flare exhaust line.
Install blind flanges on all inlet and outlet piping to prevent accidental release of hazardous fluids into the vessel. Sample the environment inside the vessel to ensure it is breathable.
Purge the vessel of hazardous gases by using atmospheric air with an air mover.
Water flooding is an alternative to gas purging but will not allow for any reuse of molecular sieves.
To water flood, install blind flanges on all inlet and outlet lines but leave the flare line open and the water line at the bottom of the vessel. Open the water line and fill the vessel with water. The displaced gases will be forced into the flare line. Monitor the water filling rate to avoid flooding the flare line.
After the vessel is filled, shut the flare line off and drain the water to an appropriate disposal area.
Entering the Process Vessel
When entering the vessel, make sure the environment inside the vessel is breathable. Use an air mover to blow air continuously through the vessel. Never use plant air which might be contaminated or worse, piped to another gas other than air.
Make sure there is are two outside spotters holding the rope attached to a safety harness around the inside worker. The spotters can use the safety rope to prevent falls from the ladder or remove an unconscious or disabled worker.
Safety Procedures in Handling Molecular Sieves in Process Units
Clear the area of nonessential personnel. Make sure nobody walks under the load while the vessels are being loaded and rope off the area. Drums weigh over 300 lbs and full Supersacks weigh over 1200 lbs and are typically lifted to a height of over 30 feet.
If pneumatic trucks are used to load the vessel, keep the hose pointed away from personnel and ground the system electrically to prevent a static discharge. Flammable vapors in the area have the potential of being ignited.
Startup of a Molecular Sieve Unit with Straight Chain Hydrocarbons or Inert Gases
Remove all air in the molecular sieve vessel by purging with nitrogen. Use enough nitrogen to prevent channeling. To ensure a complete purge, pressure and depressure the vessel several times in the downflow direction. Continue purging until the mixture of nitrogen and contaminant is below 50% of the lower explosive limit.
Straight chain hydrocarbon or inert gases can be reintroduced for adsorption at this point.
Startup of a Molecular Sieve Unit with Reactive Fluids (Ethylene or Propylene)
Remove all air in the molecular sieve vessel by purging with nitrogen. Use enough nitrogen to prevent channeling. To ensure a complete purge, pressure and depressure the vessel several times in the downflow direction (about 10 bed volumes). Continue purging until the mixture of nitrogen and contaminant is below 50% of the lower explosive limit.
Regenerate with a non reactive gas like nitrogen.
Make sure that the reactive fluid and larger pore molecular sieves do not come into contact at temperatures above 100 degrees centigrade.
Be sure that the reactive fluid is not contacted with molecular sieves above 100 degrees centigrade. The beds must first be cooled with gas flow to ambient temperature prior to introducing the reactive fluid.
Molecular sieve beds must be preloaded with the reactive fluid (ethylene, propylene, etc.). This is done by using once through nitrogen and adding a precisely controlled amount of reactive fluid into the inlet purge gas. Start with a low flow rate of reactive fluid and gradually build to 10% of the nitrogen purge gas.
*Continuously monitor the outlet bed temperature such that the outlet gas does not exceed 100 degrees centigrade during the preload stage. This may require the preload to be done in two or more stages.
When the bed is purged and pressurized with the reactive fluid and the temperature has stabilized and does not exceed 100 degrees C anywhere in the bed it is ready for service. This is typically observed when the outlet temperature increases from the inlet temperature and then decreases to a level near the inlet temperature during preloading. When this condition is met, the preloading step has been completed. If the temperature decrease is not observed, the nitrogen/ethylene purge must be repeated again.
After preloading, the bed must be purged with one volume of the reactive fluid prior to pressurization. Always monitor the temperatures in the bed and the exit temperatures.
When the bed has been pressurized, the bed should never be left dead ended without flow. Flow must be established to flare or process to prevent a dangerous explosive hazard.
The adsorption vessel should be properly instrumented to monitor temperatures throughout the vessel.
Good flow distribution is essential at all times. This means good gas or liquid distributors and proper use of bed support material.
The vessel must be alarmed and monitored for temperature excursions at all times during operation.
If a heat excursion is observed and cannot be controlled below 140 centigrade, immediately shut down the unit and blow the vessel down to flare and follow with a nitrogen purge. Stop the reaction by closing the valves on the pipes leading in and out of the vessels to stop the flow of the fluid into the molecular sieve. Open the valve on the flare line to vent the reactants to flare. Purge with nitrogen to flare to remove the reactants and cool the bed down. This procedure is best if automated and provisions made to execute remotely.
When the molecular sieve bed is saturated with contaminants, the bed must be regenerated. This is accomplished by heating the bed to 300-650 Fahrenheit. If the heating outlet at any times exceeds the heating inlet temperature, stop the heating immediately. This may mean that a reaction is occurring. To stop the reaction, close the inlet and outlet valves to the vessel and depressure it to flare then purge with nitrogen.
Before putting a bed back into service, the bed must go through a cooling step before reintroducing the process feed. At high temperatures the feed can become highly reactive and equipment could be damaged causing a safety hazard. Temperatures in a molecular sieve bed can stay hot for weeks following a shutdown if the bed is not completely cooled. Always check to ensure that a hot bed is not switched to adsorption before the bed is cooled properly.