Meaning
Dimensionless groups representing the ratio of convective mass transfer to molecular diffusion characterize the transport behavior of volatile species in fluid systems. The Sherwood Number quantifies mass transfer efficiency at the boundary layer of polymer melts or solid pellets. A high value indicates that convective transport or agitation is dominant over slow molecular diffusion.
Transport Efficiency
Increasing the velocity of a carrier gas or the agitation speed of a melt reduces the resistance of the boundary layer. By raising the Sherwood Number, operators can dramatically speed up the solid-state polymerization of polyesters. This increase means that volatile byproducts are swept away from the pellet surface much faster than they can diffuse through the boundary layer.
This acceleration reduces the overall reaction time.
Reactor Design
Mathematical models of devolatilization equipment utilize transport correlations to predict resin purification rates. Designers calculate the Sherwood Number to optimize the size and gas flow rate of nitrogen stripping columns. This dimensionless value determines how much nitrogen is required to maintain efficient mass transfer without causing excessive pressure drop.
A properly calculated number prevents under-designing the reactor.
Polymer Processing
Extruder screws with intensive mixing zones enhance the removal of volatile monomers during polymer devolatilization. Within the melt phase, the Sherwood Number is highly dependent on screw speed and melt temperature. High shear forces increase the convective component of the transport process.
This ensures that the finished polymer has low residual monomer levels, which prevents taste or odor issues in food packaging. Optimizing this transport value is particularly important when processing recycled polymers that contain various volatile organic contaminants.
