Reverse osmosis pressure vessel

The flow of water through a reverse osmosis membrane is primarily dependent on the applied pressure differential and the osmotic pressure differential across the membrane. The osmotic pressure is directly dependent on the salt concentration of the process stream. As a rule of thumb, each 100 mg/ of dissolved solids is roughly equivalent to one psi of osmotic pressure. Since the product stream usually has a very low salt content, the osmotic pressure of that stream is negligible. In addition, the product stream normally leaves the reverse osmosis pressure vessels at near atmospheric pressure so that the applied pressure differential is the feed pressure. Consequently, the term "net applied pressure" has come to mean the applied pressure minus the feed osmotic pressure. 

Spira.1- Wound Modules. Spiral-wound modules were used originally for artificial kidneys, but were fuUy developed for reverse osmosis systems. This work, carried out by UOP under sponsorship of the Office of Saline Water (later the Office of Water Research and Technology) resulted in a number of spiral-wound designs (63—65). The design shown in Figure 21 is the simplest and most common, and consists of a membrane envelope wound around a perforated central coUection tube. The wound module is placed inside a tubular pressure vessel, and feed gas is circulated axiaUy down the module across the membrane envelope. A portion of the feed permeates into the membrane envelope, where it spirals toward the center and exits through the coUection tube. 

Water produced by reverse osmosis (RO) is forced by an osmohc pressure through a semi-permeable membrane which acts as a molecular filter. The difiusion of solubles dissolved in the water is impeded, and those with a molecular weight in excess of 250 do not difftise at all. The process, which is the reverse of the natural process of osmosis, thus removes microorganisms and their pyrogens. Post-RO contaminahon m occur if the plant after the membrane, the storage vessel or the distribuhon system is not kept Ifee Ifom microorganisms. 

Figure 2. Schematic view of reverse osmosis test loop (I) hollow fiber membrane (2) pressure vessel (3) feed water (4) filter (5) pressure pump (6) relief valve.

Figure 3.44 Schematic of a spiral-wound module [115] installed in a multimodule pressure vessel. Typically four to six modules are installed in a single pressure vessel. Reprinted from Reverse Osmosis Technology, B.S. Parekh (ed.), Marcel Dekker, New York (1988), p. 81, by courtesy of Marcel Dekker, Inc.

Figure 5.24 Flow schematic of a typical brackish water reverse osmosis plant. The plant contains seven pressure vessels each containing six membrane modules. The pressure vessels are in a Christmas tree array to maintain a high feed velocity through the modules...

Reverse osmosis skids are typically contained within a frame of galvanized or urethane-coated steel. Skids should be designed for easy access for monitoring and maintenance. Access to controls, instruments, valves, the pump and motor, and membranes is essential. Access to the permeate from each pressure vessel is often overlooked. Without such access, profiling and probing used to troubleshoot poor performance is not possible (see Chapter 14.7). 

An industrial reverse osmosis plant usually will consist of three separate sections which are shown in Figure 4.2. The first section is the pretreatment section in which the feedwater is treated to meet the requirements of reverse osmosis element manufacturers and the dictates of good engineering practice. Following pretreatment, the feedwater is introduced into the reverse osmosis section where the feedwater is pressurized and routed to the reverse osmosis elements which are in pressure vessels. The feedwater flows across the membrane surface where product water permeates through the membrane and a predetermined amount remains behind as reject. The reject is discharged to waste while the product water is routed to the posttreatment section. The third or posttreatment section treats the product water to remove carbon dioxide and adds chemicals as required for industrial use of the product water. 

The tubular reverse osmosis device is shown in Figure 4.10. The tube serves as the pressure vessel and the membrane is installed inside the tube. Tubes with inside diameters of % and 1 inch have been used. Uniformly porous fiberglass reinforced plastic tubes have been used and nonporous but perforated copper, stainless steel and fiberglass tubes have also been successfully used. The membrane can be bonded to the tube in which case it is cast in situ or the membrane can be loose. The loose membrane is cast in sheet form and a cylindrical section is formed and placed in the tube. Packing densities for the Vi-inch diameter tube are about 100 square feet per cubic foot and about 50 square feet per cubic foot for the 1-inch diameter tube. 

The major advantages of the tubular reverse osmosis configuration are the ability to tolerate high suspended solids concentrations in the feed and the possibility of mechanical membrane cleaning. The disadvantages are the excessive number of tube end fittings in proportion to the active membrane area in each pressure vessel, the bulkiness of the reverse osmosis plant and the high cost. 

Figure 4.13 shows a flow diagram for a reverse osmosis unit with 75% recovery on a brackish feed. The pretreated feed is routed to the high pressure pump where the feed pressure is raised to between 250 and 400 psig as required for brackish water desalination. The pressurized feed is then pumped to the first pass pressure vessels where about 50% of the feed is recovered as product and 50% is reject. The reject from the first pass pressure vessels is then routed to the second pass pressure vessels where, again, about 50% of the first pass reject is recovered as product and 50% is reject which is sent to waste. Thus, the overall recovery of the unit is 75% as product. As can be seen, a normal array for a 75% recovery unit is two first pass pressure vessels feeding one second pass pressure vessel or a 2-1 array. If the system recovery were from 40 to 60%, all of the pressure vessels would be in parallel. However, if the system recovery were raised to between 85 and 90%, the pressure vessels would be arranged in a 4-2-1 array. 

Reverse osmosis unit, including membrane, pressure vessels, single pass system with either spiral wound or hollow fiber membranes. FOB cost 220000 at membrane area = 1600 m with n = 0.47 for the range 12-1600 and n= 0.85 for the range 1600-3 000000. L-i-M = 1.2-3.2 with large values for smaller size. 

The equipment for reverse osmosis is quite similar to that for gas permeation membrane processes described in Section 13.3C. In the plate-and-frame type unit, thin plastic support plates with thin grooves are covered on both sides with membranes as in a filter press. Pressurized feed solution flows between the closely spaced membranes (LI). Solvent permeates through the membrane and flows in the grooves to an outlet. In the tubular-type unit, membranes in the form of tubes are inserted inside porous-tube casings, which serve as a pressure vessel. These tubes are then arranged in bundles like a heat exchanger. 

The process of reverse osmosis is used to purify water because it removes many contaminants at relatively low cost. The osmosis process can be reversed by applying a pressure greater than the osmotic pressure to an aqueous solution. This causes the water, but not the dissolved solutes, to flow from the solution through the semipermeable membrane. The largest municipal desalination plant in the United States is in Sarasota, Florida. It treats salt water by reverse osmosis to produce more than 12 million gallons of fresh water per day. Other applications include the desalination of sea water on ocean-going vessels and wastewater purification. 

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