Two-stage coprocessing of an Alberta subbituminous coal and Cold Lake vacuum bottoms

Abstract

Two-stage coprocessing of a Forestburg subbituminous coal and Cold Lake vacuum bottoms was performed. The effects of process variables such as recirculation gas velocity, temperatures in the first and second stages, operating pressures, and slurry space velocity were examined. The first stage temperature was varied from 375°C to 425°C, and for the second stage from 450°C to 455°C. The operating pressure was varied from 10.44 to 17.34 MPa, the superficial gas velocity in the second stage from 0.3 to 2 cm/s, and the slurry space velocity in the second stage from 0.8 to 1.2 kg/h/L. It was found that the recirculation gas velocity has important effects on the coal and pitch conversion. There is an optimum recirculation gas rate which results in the maximum pitch conversion at given operating temperature, pressure and slurry space velocity. There is a minimum recirculation gas rate to obtain the ultimate coal conversion at given operating temperature, pressure and space velocity. The pitch conversion increases almost linearly with decreasing slurry space velocity in the hydrocracking second stage. A pitch conversion of 82.2 wt % was reached without operational difficulties. However, when pitch conversion reached almost 85 wt % coke buildup occurred. The coal conversion reached 94.5 wt % but it is difficult to exceed this value. The use of the severity function allows the evaluation of coking runs in relation to successful non-coking runs. Above a severity value of 700 the coking becomes a serious operational problem. The effect of pressure between 10.45 MPa (1500 psig) and 17.34 MPa (2500 psig) on the pitch conversion or distillate yields is small. The quality of distillates between 65 and 82 wt % pitch conversion is very similar.