Exergy analysis of methanol from the sewage sludge process

This paper concerns a new method of sewage sludge treatment that contributes more than traditional methods to the sustainable technology by achieving a higher rational efficiency of sludge processing. This is obtained by preserving the chemical exergy present in the sludge and transforming it into a chemical one-methanol. The proposed method combines a sludge gasification process and a modified methanol plant. The sludge gasification produces a synthetic gas mainly containing CO and H-2 which gas is next used as a reactant to make methanol. The plant, with a capacity of 50000 tons (dry) solids per year. is simulated by a computer model using the flow-sheeting program Aspen Plus (Aspen Technology). The exergy analysis is performed for various operating conditions and the optimal values of these conditions are found. The irreversibilities of different plant segments: thermal dryer, gasifier, gas cleaning, compressors, methanol reactor, distillation column and purge gas combustion are evaluated. It is shown that the rational efficiency of the overall process is much higher than that of traditional plants of thermal sludge treatment. (C) 2002 Elsevier Science Ltd. All rights reserved.