Reduction of energy consumption in process plants using nanofiltration and reverse osmosis

Pressure driven membrane processes are characterized by their low specific energy consumption. Since they constitute a mechanical separation without phase or temperature change, they are least energy intensive. This study focused on further energy reduction. Process industries (chemical, food, mechanical) use numerous membrane processes where the concentrate stream is throttled to reduce the membrane pressure. Significant energy losses are incurred there. Examples of such processes are; de-salting and concentration of fine chemicals, de-mineralization and concentration of cheese whey, purification of sugar and wastewater treatment. Highest losses occur in typical batch processes, whereas semi-batch and continuous plants are energetically more efficient. The use of energy recovery systems is well known in the seawater RO industry. This study looked at all the existing technologies available today. They include pressure exchangers of various kinds, Francis type turbines, Pelton turbines and hydraulic pressure boosters (turbochargers). Several existing plants including simple batch, simple batch with feed and bleed and continuous plants were analyzed to evaluate the use of such devices. Returns on investment were also estimated. It was found that only hydraulic pressure boosters (turbochargers) are suitable for process applications. They are compact, maintenance free and are available in sizes typically encountered in the process plants. Integration into new plants as well as retrofit of existing plants is possible. Depending on the process, an investment in the energy saving system can be recovered within a period from 6-24 months at the current European energy prices.