Pressure reversal

Fig. 6. High pressure reverse osmosis system for alcohol reduction.

A. Yarshuk, E. Staude. Charged membranes for low pressure reverse osmosis properties and applications. Desalination 55 115, 1992. 

J. E. MacNair, K. C. Lewis and J. W. Jorgenson, Ultraliigh-pressure reversed-phase liquid chromatography in packed capillaiy column . Anal. Chem. 69 983 (1997). 

MacNair, J.E., Patel, K.D., and Jorgenson, J.W., Ultrahigh-pressure reversed-phase capillary liquid chromatography isocratic and gradient elution using columns packed with 1.0 pm particles, Anal. Chem., 71, 700, 1999. 

Limit pressure reversal >3 MPa to fewer than 100 cycles per year... 

Figure 6. Low-pressure reverse osmosis performance of ammonia-modified cellulose diacetate membranes (IV) compared with formamide-modified reference membranes (I)...

Plate and frame systems offer a great deal of flexibility in obtaining smaller channel dimensions. Equations 4 and 5 show that the Increased hydrodynamic shear associated with relatively thin channels Improves the mass-transfer coefficient. Membrane replacement costs are low but the labor involved is high. For the most-part, plate and frame systems have been troublesome in high-pressure reverse osmosis applications due to the propensity to leak. The most successful plate and frame unit from a commercial standpoint is that manufactured by The Danish Sugar Corporation Ltd. (DDS) (Figure 15). 

A. The Pulsed Ionization, High-Pressure, Reverse Geometry,... 

Particulate management in the FCCU is critical because particulate emission rates can vary in response to upsets in the FCCU. For example, failure of the regenerator cyclones can lead to an order of magnitude increase in steady state particulate emission rates and pressure reversal upset incidents can result in massive, short-term particulate emission rates that must be accommodated by the SO2 scrubbing system. 

The Disc Tube system is a patented, ex situ process for the treatment of aqueous solutions ranging from seawater to leachate. The system uses high-pressure reverse osmosis through a semipermeable membrane to separate pure water from contaminated liquids. 

R. Gunther, B. Perschall, D. Reese and J. Hapke, Engineering for High Pressure Reverse Osmosis, 7. Membr. Sci. 121, 95 (1996). 

The goal of most of the early work on reverse osmosis was to produce desalination membranes with sodium chloride rejections greater than 98 %. More recently membranes with lower sodium chloride rejections but much higher water permeabilities have been produced. These membranes, which fall into a transition region between pure reverse osmosis membranes and pure ultrafiltration membranes, are called loose reverse osmosis, low-pressure reverse osmosis, or more commonly, nanofiltration membranes. Typically, nanofiltration membranes have sodium chloride rejections between 20 and 80 % and molecular weight cutoffs for dissolved organic solutes of 200-1000 dalton. These properties are intermediate between reverse osmosis membranes with a salt rejection of more than 90 % and molecular weight cut-off of less than 50 and ultrafiltration membranes with a salt rejection of less than 5 %. 

Many nanofiltration membranes follow these rules, but oftentimes the behavior is more complex. Nanofiltration membranes frequently combine both size and Donnan exclusion effects to minimize the rejection of all salts and solutes. These so-called low-pressure reverse osmosis membranes have very high rejections and high permeances of salt at low salt concentrations, but lose their selectivity at salt concentrations above 1000 or 2000 ppm salt in the feed water. The membranes are therefore used to remove low levels of salt from already relatively clean water. The membranes are usually operated at very low pressures of 50-200 psig. 

When the pilot senses that set pressure is reached, it vents the pressure above the piston sufficiently for it to be forced open by inlet pressure, reversing the unbalanced direction. During a relief cycle, when the reseat pressure is reached, the pilot shifts internally, admitting system pressure again into the volume (dome area) above the piston, and again closing the main valve. 

Figure 8.20. Pressure profiles in an underflow standpipe (from Knowlton, 1986) (a) Without pressure reversal (b) With pressure reversal.

H. Dach, J. Leparc, H. Suty, C. Diawara, A. Jadas-Hecart, A. Lhassani, M. Pontie, Innovative approach for characterization of nanofiltration (NF) and low pressure reverse osmosis (LPRO) membranes for brackish water desalination, Desalination, 2006 (submitted). 

Figure 2. High Pressure reverse-phase liquid chromatography trace of 2,3,7,8-TCDD standard and a typical human milk extract...

---