Studies On Process Characteristics Of Energy Recovery Device For Reverse Osmosis Desalination System

The energy consumption for seawater reverse osmosis (SWRO) is one of the main factors that affect the cost of desalination product. Nowadays the actual specific energy requirements for SWRO are around four to five times as large as the thermodynamically calculated value, offering considerable area for improvement and cost reduction. The valve-controlled energy recovery device (ERD) designed in the experiment can reduce energy consumption efficiently by recovering the remaining energy in the brine steam. The ERD can be classified into two categories by their operating principle,the centrifugal type or positive displacement type. Most of the RO systems installed worldwide rely on centrifugal devices with low efficiency. Recently positive displacement devices are more used because of their high efficiency and have become a research focus of this field. The ERD designed in our research program is of the positive displacement principle, using the high pressure brine to pressurize the feed water directly. Two cylinders are used to recovery energy alternately to minish flow and pressure fluctuate of the brine and feed water. An experimental apparatus was set up and several valve controlling schemes were developed. Softwares of the programmable logic controller (PLC) were programmed. Dynamic characteristics of the unit under different controlling schemes were measured and analyzed. Time controlling system and flux auto control device were used to operate the solenoid valves regularly. The former scheme controls the corresponding valve by fixed pressurizing and depressurizing time and the latter by settled flux cumulation volume. Either of the two schemes could recovery the energy from the brine discontinuously and continuously, respectively. The investing and operating cost of time controlling means is low. But it requires the accuracy of time values. If the flow rate of brine or feed water increases, the piston may be got stuck on the cylinder port. Flux auto control device can be operated stably under different flow rate. But its investing cost is high comparing with the former control scheme. In the process of brine discontinuous feeding, the flow curves of the high pressure (HP) brine and the low pressure (LP) feed water present a downward variation periodically, while their pressure curves show an upward variation periodically. The pressure curves of the pressurized seawater present a horizontal liner variation. If the length or volume of the cylinder is enhanced, the fluctuating of flow and pressure is minished. In the process of brine continuous feeding, the flow and pressure curves of the HP brine are stable on the whole. The flow curves of the LP feed show a downward variation while its pressure curves present an upward trend periodically. A design of multifunctional valve for the ERD was put forward, using a specially designed valve to replace the four valves situated on the cylinder end. The unit was greatly simplified.