A Tubeless Evaporator

Abstract

The first author of this paper has invented a process, which incorporates direct contact evaporation, for saline water conversion, and waste concentration, which can include by-products recovery. The process has several potential advantages: Simplicity - major components are four vessels without internals, pumps, and a fan (unneeded in one version of the process); wide application - seawater, waste water, and membrane plant effluents; produces distilled water, co-produces brine, or concentrated brine, or moist solid, or by-products; no pretreatment in the ordinary case (on some applications, trash removal and chlorination is required); no scaling problem in the evaporator; minimum corrosion (no acid addition) ; can be powered by electricity of heat, including waste heat; low energy consumption in the single stage version of the process, extremely low energy consumption in the multistage version of the process. The process uses a working substance (WS), such as a high melting organic compound, at its freezing point, to effect the direct contact evaporation. An acceptable WS must be insoluble in water, inert, and easily separated by specific gravity difference from brine, product water, and any solids produced. It is also desirable that they be non-toxic, non-flammable, cheap, and have a high heat of fusion. Several WSs were identified that had the potential to be used in a commercial plant. At this point, the Office of Water Research and Technology (OWRT) sponsored a research project to investigate the operating characteristics of various WSs in the laboratory and a bench scale plant (BSP). A WS screening program tested melting range, solubility, tendency to entrain or emulsify, and stability, in the laboratory program. The BSP program simulated the operation of major pieces of equipment with WSs that passed the screening program, and seawater and other saline waters. A total of eight WS candidates were subjected to the WS screening program. Of these, two proved to be unsatisfactory. It was believed that these substances could have been made to work, but with other satisfactory materials available, there was little incentive to pursue this effort. The other six were found to be satisfactory for use in the BSP. Three of these were selected for testing in the BSP. Prior to the above effort, there was no data with which to design a pilot plant. Any such attempt would have had to be based on engineering judgement. Now one set of data (feed rates, operating conditions, etcetera) has been taken. These data could be far from optimum. Additional WSs, particularly those with melting points above 100°C are needed for multistage operation. Finally, the direct contact concept should be extended to the preheaters as well as the evaporator in order to make the process completely tubeless. Such a program, sponsored by OWRT, has just commenced. This paper describes the process and its potential advantages in more detail, reports on the results of the laboratory and bench scale programs referred to above, and outlines the program for the future development of the process.