| In the present study we investigate design alternatives of recycle systems with a single adiabatic fixed-bed reactor in autothermal operation with a feed-effluent heat exchanger with cold bypass. Ternary systems with second order kinetics and two fresh-feed makeup streams are considered. Three possible reaction systems are covered: irreversible with moderate activation energy, irreversible with high activation energy, and reversible. When relative volatilities are assumed, the product component is the heaviest of all three.; It was found that adding a furnace to the preheating section on the reactor side improves the operability of the reactor/preheater when the reactor temperature rise is small. The resulting economic penalty is not large.; It is typical of the irreversible reaction cases that the optimum design presents a very temperature sensitive reactor. One way to overcome this situation is to lower the reactor per-pass conversion by increasing the recycle flowrate at constant exit temperature. A more effective approach for the high activation energy case consists in unbalancing the reactant concentrations at the reactor inlet, selecting as the limiting reactant preferably the one with the smallest molar heat capacity.; The flowsheets that better compromise cost and controllability are, for the reversible reaction case, a liquid recycle flowsheet and, for the irreversible reaction cases, a flowsheet with both gas and liquid recycles. In addition to this, the final design of the high activation energy irreversible reaction case must have imbalanced reactant concentrations, taking as the limiting reactant the one with the highest relative volatility.; Control structures are given for each particular design. In the dual recycle design for the high activation energy reaction case, a self-regulatory scheme is used, in which the fresh feed of the limiting reactant remains fixed and the corresponding composition in the recycles is floating. In the dual recycle design for the irreversible, moderate activation energy case and also in the liquid recycle design for the reversible reaction case, none of the fresh feeds is fixed, so throughput is controlled indirectly by changes in the reactor inlet temperature or the recycle flowrate. |
