{HEAD ~ The packed bed} of fixed packing elements acts as {TAIL ~ a gas distributor}, as well as causing good gas/liquid contact, to provide an even flow of gas across essentially the entire cross sectional area of the packed bed of mobile packing elements 
acts as
{ HEAD ~ The packed bed } of fixed packing elements acts as { TAIL ~ a gas distributor } , as well as causing good gas / liquid contact , to provide an even flow of gas across essentially the entire cross sectional area of the packed bed of mobile packing elements
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH