Vacuum manifolds, also known as Schlenk Lines, are commonly used for the final drying of samples removed from a rotary evaporator. Organic solvents and/or acids left behind during the distillation process are removed over several hours or more depending on the sample size. A foreline cold trap upstream of the pump is always recommended to prevent the ingestion of liquids. An acid neutralization trap is also recommended between the cold trap and the pump when strong acids are present in the sample.
When the drying is finished for the day, it is very important to either turn the pump off and remove and clean the trap, or isolate the trap from the pump using a valve. The reason for this is to prevent sublimation of condensed solids or vaporized liquids from the cold trap from recondensing in the pump.
The use of large capacity pumps with flow rates above 40 l/min (2.4 m³/h) on vacuum manifolds will actually shorten the oil change interval. This occurs because the larger pump will accelerate the sublimation process. The vapor flows through the trap too quickly to condense.
When a large capacity pump is used, it is common to see at the end of a drying run that no condensables are in the trap because the chemicals have been drawn into the pump. There is a common misconception that a pump with a large pumping capacity will shorten the drying time. Due to tubing restrictions in the manifold and stopcock, this is not the case.
Drying time differences between a large and a small pump occur only when the manifold system is leaky! Leaky vacuum systems should be repaired. Our application experts will help you to choose the right vacuum pump for your laboratory and application.
Welch offers various lab vacuum pumps for manifolds / Schlenk line applications. We have a range of compact gear vacuum pumps and very durable ones. Our CRVpro series of direct-drive rotary vane vacuum pumps are built for reliability to provide stable operations and long product lifespan due to their cool running and enhanced airflow. This leads to reduced chemical activity within the pump and slows down rates of oil consumption. As mentioned, a foreline cold trap is always recommended with vacuum manifold systems to remove the risk of any vapors condensing in the pump.
Furthermore, Welch offers a variety of vacuum accessories for its lab vacuum pumps to improve your Schlenk line applications – since PVC and silicone tubing in vacuum manifold setups are not recommended due to their poor chemical resistance to many common organic solvents present in samples undergoing drying, we offer tubings based on gum rubber, flexible stainless steel and PTFE to increase resistance against organic solvents from samples.