| Abstract: | Models of parallel-flow (cocurrent and countercurrent) dryers have been developed within a steady-state process flowsheeting package (ASPENplus?) and applied to a case study involving a countercurrent timber veneer dryer to evaluate the ease of control for arrangements which include the use of recycle or a heat exchanger at the dryer exit to preheat the incoming air. The technique involves interfacing 
models, which allow both dynamic and steady-state analyses to be performed, with the ASPENplus? package. These
models may also be called from the SPEEDUP? package, but in this case only the steady-state behaviour has been studied in ASPENplus? by running the unsteady-state analyses to steady state using a false time-stepping technique. For the case study of the countercurrent timber veneer dryer, the use of a heat exchanger is predicted to require 13% less fuel gas than no recycle and 6% less than the use of 30% outlet gas recycle at the optimum operating condition (minimum fuel gas use) for each system. The system is also predicted to be easier to control with a heat exchanger than with recycle, according to the Relative Gain Arrays for the systems studied. For the countercurrent veneer dryer, the use of 30% outlet gas recycle increases the outlet solids temperature compared with cases both with no recycle (corresponding to the dryer on its own) and with a heat exchanger between the outgoing and incoming gas, since recycle moves the operating region up the vapour pressure/temperature curve so that the sensitivities of both the outlet solids temperature and the outlet solids moisture content to the gas flowrate increase dramatically. This means that 30% recycle changes the preferred control pairings in this case from (solids outlet temperature, inlet air flowrate), (solids outlet moisture content, fuel gas flowrate) to (solids outlet temperature, fuel gas flowrate), (solids outlet moisture content, inlet air flowrate). The indicated pairings of controlled and manipulated variables differ with the amount of recycle, suggesting that dynamic analysis needs to be performed to assess the optimum control method for this system. |
