Digital control of a coupled fluidized-bed thermal processing system for tar sands

A digital control system, consisting of an HP-1000 host computer, an HP-2250 interface, and several actuators and sensors, was used to control a laboratory unit for the thermal processing of tar sand. The unit included two coupled fluidized beds for pyrolyzing the bitumen content of the tar sand and combusting the resulting coke left behind on the sand. The condensed pyrolyzate is suitable for upgrading to a synthetic crude oil. The control system for solids retention times in both fluidized beds included four pressure gauges, two dp cells, two pressure transmitters and two solids-flow control valves. A digital proportional-integral-derivative (PID) algorithm plus valve linearization to eliminate hysteresis of control valves were found to be the best control method. The preferred control constants, ( K c = 80, τ i = 40, τ d = 0 for the pyrolysis reactor, and K c = 50, τ i = 5, τ d = 0 for the combustion reactor), were found experimentally. The temperature in the pyrolysis reactor (450–550°C) could be controlled by the amount of energy transferred into the combustion reactor from the surroundings. An SCR power controller was used to regulate power into a heating wire wrapped around the combustion reactor wall. The best PID algorithm controller constants, ( K c = 1000, τ i = 10, τ d = 0) for controlling the thermal cracking of bitumen in the pyrolysis reactor were found experimentally. From the controlled variable responses, it was concluded that the digital control system used here was much superior to the analog controllers used previously.