Pressure-gradient controlled steady state

When an extruder is working under steady-state conditions several factors can influence the process. The primary parameters that in principle can be controlled externally are the screw speed, the throughput, the barrel temperature, and the die resistance. All other parameters are influenced by the changes in the primary variables, e.g., filled length, heat transfer, and pressure gradient, or are predetermined, e.g., screw geometry or material properties. 

Membrane-penetrant systems which show pressure (concentration) dependent permeation rates have been shown to develop highly non-linear concentration and activity profiles within the membranes [17]. Time is required to establish these profiles. In addition, water and other condensible vapors create large swelling gradients from the fully swollen wet (feed) side to unswollen dry (permeate) side of the membranes. The relaxation of the resulting stresses may control the time to attain steady-state flux [18]. 

---