Selecting a distillation scheme for purifying ditolylmethane for a nuclear heat source

Abstract

Ditolylmethane can be used as a coolant in nuclear heat plants [i], ~Ithough it needs ongoing purification from radiolytic products, including low-boillng and high-boiling components2* Periodic distillation to remove radiolytic products represents world practice [2, 3].Our task was to develop a distillation system less laborious to service, simpler to automate, and costing less to operate.A continuous-flow plant can have those advantages [4] if it has the parameters given in the last column in Table i. If those parameters can be attained, one could use the heat from the first loop in the plant (200-240~ to evaporate the ditolylmethane and the low-boiling components instead of the more expensive electrical power, while the vapor could be condensed by cheaper air cooling to +30~ instead of water cooling, while one could reduce the loss of ditolylmethane with the wastes and almost eliminate the loss of low-boiling components in the vacuum pump.The laboratory tests were based on outgassed ditolylmethane coolant from the first loop in the ARBUS nuclear heat plant, which had the following mass-fraction composition:ditolylmethane 82 • I, low-boiling components 2.3 • 0.2, and high-boiling ones 16 • 1%. The lowboiling components included benzene, toluene, xylene, and four unidentified compounds.The high-boiling ones included compounds containing three or more benzene rings and having a mean molecular weight 450.Continuous purification from these components and water was examined in laboratory glass systems simulating vacuum distillation in two ways. In a one-stage system, the two types of organic component and the water are removed under a common vacuum by sequential partial condensation from the common vapor flow as the temperature is reduced.The two-stage scheme differed in that the low-boiling components and the water were removed at a lower vacuum (first stage) and the high-boillng ones at a higher vacuum (second stage).The model for single-stage distillation contained all the major parts of the commercial analog [3]. In models for the two-stage distillation, the distillation units in each stage were based on two types of equipment: a rotor-film evaporator and a packed evaporator. The tests were made with two forms of laboratory model for a stage (Fig. i).In each case, we examinedfirst the initial stage and then the second one. The differences lay only in the work{ng conditions and raw-material composition: the first stage was provided with the initial coolant, while the second was provided with the distillation residue arising from separating the low-boiling components. The heated raw material entered the distiller, where it was separated into vapor and liquid.The vapor condensed in a condenser, and the condensate ran into a collector.There were traps cooled to -70~ before the vacuum pumps in all the models, which were used to estimate the vapor loss to the vacuum pump to be expected in the industrial plant.A basic component in the rotor-film evaporator is the column heated by the coolant (internal diameter 20 mm)...