Double pass

Double pass (or two-pass) refers to further purification of permeate from one RO by running it through another RO. The first RO, as described in Chapter 5.1, would be the first pass. Permeate from the first pass is then sent to another RO known as the second-pass RO. 

The second-pass RO polishes the first-pass RO product to yield higher-quality water. 

Recovery of the second pass can be as high as 90% with only 2 stages. This high recovery can be achieved because of the relatively low-concentration of dissolved solids in the influent to the second pass. Overall system recovery will be 73% with 75% first pass and 90% second pass recoveries (recovery would be 67.5% without recycle). 

A tank is typically required between the first and second pass systems. This is so that flows can equalize between the passes. However, if the number of first-pass skids is equal to the number of second-pass skids, a tank may not be required. 

Inter-pass caustic injection is commonly used to drive out carbon dioxide from the first-pass RO permeate/second-pass feed. Carbon dioxide, a gas that is not rejected by the membranes, is converted to bicarbonate alkalinity, which is rejected (see Chapter 3.2). Removal of carbon dioxide is particularly important for applications that polish the second-pass RO permeate with ion exchange. The conversion and, therefore, elimination of carbon dioxide from the permeate will reduce the loading on the anion resin. 

Effluent quality from a double-pass RO system is generally high enough to allow for direct use in 600 to 900 psi boilers. Higher pressure boilers ( l,000psi) and higher pinity appHcations will still require some sort of post-treatment, typically a mixed-bed ion exchanger or electrodionization (see Chapter 16.4). 

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The frill width at half maximum of the autocorrelation signal, 21 fs, corresponds to a pulse width of 13.5 fs if a sech shape for the l(t) fiinction is assumed. The corresponding output spectrum shown in fignre B2.1.3(T)) exhibits a width at half maximum of approximately 700 cm The time-bandwidth product A i A v is close to 0.3. This result implies that the pulse was compressed nearly to the Heisenberg indetenninacy (or Fourier transfonn) limit [53] by the double-passed prism pair placed in the beam path prior to the autocorrelator. 

These modes of operation ate used in conjunction with the two most popular energy analyzers, the cylindrical mirror analyzer (CMA) and the concentric hemispherical analyzer (CHA). The most common form of the CMA used today is the double-pass version diagramed in Eigute 21. This device consists of two perfectly coaxial cylinders of radii r and r. The outer cylinder is held at a potential of (— ) and the inner cylinder is held at ground. The... 

Reverse flow Cross- flow Double pass Cascade double pass... 

Fig. 25-2. Double-beam, double-pass transmissometer for measuring smoke density in stacks. A[, chopper wheel A, beam gating wheel A3, aperture D, detector Fj, spectral filter F2, solenoid-activated neutral density filter L, lamp M, half-mirror/beam splitter Rj, solenoid-activated zero calibration reflector R2, retroreflector (alignment bullseye not shown). Design patented. Source Drawing courtesy of Lear Siegler, Inc.

Figure 8-73B. Type A Fiexitray with double pass. Used by permission, Koch Engineering Co., Inc.

Reverse Flow Cross Flow Double Pass Cascade Double-Pass... 

Double pass A split-flow tray with two liquid flowpaths on each tray. Each path handles half of the total liquid flow. 

Following the twin-bed with a third cation exchange bed or a mixed-bed (MB) polisher. Processes such as RO followed by twin-bed DI, plus twin MB polishers may yield treated water with silica leakage down to 0.5 ppb Si02. An alternative arrangement for the minimization of silica is the use of a double-pass RO followed by MB polishers. 

Very pure water To achieve this grade requires a further purification process, over and above that necessary to provide basic pure water. Second-stage processes include double-pass RO, MB polisher, or EDI. Typically, the equipment would be configured in a dual or triple train. 

XPS spectra were obtained using a Perkin-Elmer Physical Electronics (PHI) 555 electron spectrometer equipped with a double pass cylindrical mirror analyzer (CMA) and 04-500 dual anode x-ray source. The x-ray source used a combination magnesium-silicon anode, with collimation by a shotgun-type collimator (1.). AES/SAM spectra and photomicrographs were obtained with a Perkin-Elmer PHI 610 Scanning Auger Microprobe, which uses a single pass CMA with coaxial lanthanum hexaboride (LaBe) electron gun. 

HREELS experiments [66] were performed in a UHV chamber. The chamber was pre-evacuated by polyphenylether-oil diffusion pump the base pressure reached 2 x 10 Torr. The HREELS spectrometer consisted of a double-pass electrostatic cylindrical-deflector-type monochromator and the same type of analyzer. The energy resolution of the spectrometer is 4-6 meV (32-48 cm ). A sample was transferred from the ICP growth chamber to the HREELS chamber in the atmosphere. It was clipped by a small tantalum plate, which was suspended by tantalum wires. The sample was radia-tively heated in vacuum by a tungsten filament placed at the rear. The sample temperature was measured by an infrared (A = 2.0 yum) optical pyrometer. All HREELS measurements were taken at room temperature. The electron incident and detection angles were each 72° to the surface normal. The primary electron energy was 15 eV. 

Figure 11.21. Liquid flow patterns on cross-flow trays, (a) Single pass (b) Reverse flow (c) Double pass...

The relationship between weir length and downcomer area is given in Figure 11.31. For double-pass plates the width of the central downcomer is normally 200-250 mm (8-10 in.). 

At high liquid flowrates, the liquid gradient on the tray can become excessive and lead to poor vapour distribution across the plate. This problem may be overcome by the shortening of the liquid flow-path as in the case of the double-pass and cascade trays. The whole design process is discussed in Volume 6. 

RO systems today accomplish the conversion either with a single or a double pass. From the Information provided to me by permeator and system manufacturers, and from discussions with individuals active in the Industry, it appears that about 60% of seawater systems use single pass conversion. One must add here that there are also differences in the configuration of RO membranes within the permeators. While there are others, the spiral wound and the hollow fiber designs truly command the market place today. Again, estimating, it appears that the hollow fiber design also has about 60% of the market. 

Open-beam double-pass instruments using a retro-flector array to return the light beam back to the spectrograph are also in use (e.g., Mount, 1992 Mount and Harder, 1995 Harder et al., 1997a). Another variation is the use of a fast-scanning technique with pi-... 

Spray chambers commonly used for ICP-AES (a), double pass ... 

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