PRINCIPLES OF OPERATION

Transcription

1 ., o PRINCIPLES OF OPERATION Reverse Osmosis is a process of the removal of dissolved ions from water in which pressure is used to force the water through a semipermeable membrane element which will pass the water but reject most other dissolved materials. To understand reverse osmosis, the naturally occurring phenomenon of osmosis must be understood. Osmosis can be defined as the spontaneous passage of a liquid solvent from a dilute solution across an ideal semipermeablemembraneelement. The transfer ofthe solvent water- butnot the solutes (dissolved solids) - will continue until the concentrations of the solution on either side of a membrane element are equal. The flow rate of the water is directly proportional to the concentrations of the two solutions. This driving force, called the osmotic pressure, can be measured, and resulting flow can be halted by applying a pressure equal to osmotic pressure on the more concentrated solution side. If this external pressure is increased further, the flow of water will be reversed from its natural flowing direction and towards the more dilute solution. The reversing of the flow is the process of reverse osmosis. For example, if a variable pressure were applied on the more concentrated solution side of a semipermeable membrane element, the following conditions could be realized: (1) P equals the osmotic pressure of the solution: The solvent flows at the same rate in both directions; i.e., there is no net change in water volumes. This condition, as shown Fig. RO-l, represents the phenomenon of osmosis. (2) P is greater than the osmotic pressure of the solution: Solvent flows from the more concentrated solution to the "pure" solvent side of the membrane. This condition, as shown in Fig. RO-2, represents the phenomenon of reverse osmosis. There are many membrane elements which have good qualities of rejection (salt separation). The main problem with the early membrane elements was that the water flow rate (flow per unit area) was very low at any reasonable pressure drop across the membrane element. The breakthrough that made RO feasible was the creation of an asymmetric cellulose acetate membrane element; one surface of the element having a dense layer of skin (0.2 micron thick), and the remainder being a relatively spongy porous mass.!fthis element was magnified many times its actual size, it would be seen that the initial pore size in the dense skin is very small. 6010!OOA Paqe 3

2 Pure Water... Semipermeable Membrane Figure RO-! Osmosis Normal Flow From Low Concentration Solution to High Concentration Solution Pressure Pure Water... Semipermeable Membrane Figure RO-2 Reversed by Application of Pressure to High Concentration Solution A Pace 4

3 Figure RO-3 5~ral Wound Aaaem~y II" Preuure v... Suppcn PiaII J?igure RO-4 The Spiral-Wound Cartridge FEE ~~ Anli TolescoPing DevlC1l D-- ~~~-==---"_1I11- BRINE FIb6rgl~ OUlefWr~ ~_... PflOOUC T FEED~~:J\S~ L I Food ChannII SpaoeI' 1000( Producl W.le( eou.cuon Chatv* Product Flow

4 Outsode Dlameler 4.25 in. \ ~ Concenlrale FlOW Feed :~- -~--8: Flow I 1'211"1 ).~ I-: ~_ ~ i \ t., I _l.i.~t.j ' '12 in. Extension SPECIFICATIONS Materials of Construction: , 316 SS, Polished 316 S5 Maximum Operating Pressure: 304 & 316 SS: 1000 psig (6890 kpa) Polished 316 S5:1000 psig (6890 kpa) D.D.: 4.25 in. (108 mm); J.D.: 4.0 in. (102 mm) manufactured by Osmonics to ASTM specification A269 for industrial and polished sanitary. End Cap Assembly: 304 & 316 5S: Victaulic-type Style 77 or HP70 to 1000 psig (6890 kpa);victaulic-type Style 75 to 500 psig (3445 kpa); Polished 316 SS: high pressure dairy clamp to 1000 psig (6890 kpa). Number of Elements: Length (apj,j"qj.){jncludjn~ CQIIDlin~sand ead cads) 4 in. Industrial: in. (em) 4 in. Sanitary: lil. (em) Weight (...ithout clemelllsl: Ibs. (kgs) 1 49 (124) 52 (132) 20 (9.1) 2 89 (226) 92 (234) 30 (13.6) 3 l29 (328) 132 (335) 40 (18.l)

5 SYSTEM DESCRIPTION The RO system is composed of two major parts; the high-pressure pump, and the pressure tubes containing membrane elements. The high-pressure pump is water lubricated and must not be run in the dry condition. The correct direction of rotation is shown on the pump housing. When it is connected electrically, the direction of rotation should be checked. The pump should not be run for more than 60 seconds with reverse rotation as this may damage the internal parts. IONICS ULTRAPURE WATER CORPORATION uses membrane elements which are placed in pressure tubes rated at working pressures in excess of 400 psi. RO systems used by IONICS ULTRAPURE WATER CORPORATION (IUWC) generally operate below 300 psi. The supply water, which is pressurized by the pump, flows over these membrane elements. The system is carefully designed to make certain that minimum flow rates over the elements are maintained. This factor is critical to the efficient operation ofro membrane elements. The reason for this is that as pure water passes through the element under pressure, it leaves behind, at the element surface, a very high percentage of the dissolved substances originally present in the supply water. For example, if the water in contact with the element is 500 ppm, then the product water going through the element at that point will be about 10 ppm (2%). A little further downstream, the water in contact with the element may be concentrated to 1,000 ppm, (2%) and so on. By maintaining the water flow velocity across the element smface above a critical valve, this concentrated boundary layer is kept at a minimum, and product water quality is possible. The other benefit ofproper flow rates is that suspended matter tends to be carried out of the system more effectively. For these reasons, the design flow rates should not be changed except in the "safe" direction. The "safe" direction in general is increasing the concentrate flow from the system. It should be noted that the less concentrated the supply water is in the RO system, the better the quality of the product water. In other words, the lower the water recovery rate, the better the product water quality. For some applications, the economic benefits of better quality product water far out weigh the extra cost of rejected water. An example of this is where RO water is to be subsequently deionized A Paqe 10

6 OPERATING SPECIFICATIONS It appears that there are longer tenn benefits to be gained by operating at lower percent water recovery. particularly in reducing maintenance to the system, and minimizing precipitation problems which may arise do to chemical additive failures. Itis important to realize that the product water from an RO system is delivered essentially at atmospheric pressure. In general, it cannot be operated by opening and closing a valve in the product water line. The reason for this is that the high pressure in the system drives the water across the membrane elements and no flow of water would be possible if the pressure on both sides of the elements were the same. If, for some reason, the product water line was closed while the system was operating, the pressure would build up. Ofcourse, ifthe product water side ofthe system were strong enough, which it is not, pressure would end up the same as the pump pressure but at that point no water would flow across the membrane. The membrane elements used by "IUWC" are capable oftaking very high "forward" pressure; i.e., from the direction of the supply water side to the product water side. However, they cannot tolerate much "back" pressure; i.e., in the direction from the product water side to the supply water side. The maximum back pressure should be 40 psi. The quality ofthe product waterproduced by ROis aconstant percentage ofthe feedwater. Forexample, if the feedwater is running 50 ppm, the product water may run about 2 to 5 ppm or less (90% or more rejection of dissolved minerals). When the feed water is running 500 ppm, the product water would run about 50 ppm or less OOA.l Pnnt:> 11