Olefin polymerization processes

In a process for producing an olefin polymer, at least one olefin monomer is polymerized in a polymerization reactor to produce a particulate polymer product containing hydrocarbon impurities including unreacted monomer and other C1 to C8 hydrocarbons. The polymer product is contacted with a gas-containing stream in a stripping vessel under conditions effective to strip hydrocarbon impurities from the polymer product and produce a stripped particulate polymer product and a gaseous first effluent stream containing inert gas and hydrocarbon impurities. The stripped particulate polymer product is recovered and the atmosphere adjacent the stripped particulate polymer product is sensed with a photoionization detector configured to ionize C4 to C8 hydrocarbons. The amount of the gas-containing stream supplied to the stripping vessel is then adjusted based upon such sensing.