Recovery and rejection

The RO system removes 90-95 % of the dissolved solids in the raw water, together with suspended matter (including colloidal and organic materials). The exact percent of product purity, product recovery and reject water depends on the amount of dissolved solids in the feedwater and the temperature at which the system operates. 

A single-pass experimental study used a 2.5 inch by 40 inch commercial membrane element. The recovery and reject results are calculated from ... 

Table 11.1 High-temperature membranes effect of temperature on recovery and rejection. 

Figure 9.12 Individual membrane module recovery and rejection as a function of position in a 2-stage RO system with 6 modules per pressure vessel.

The major advantage of RO for handing process effluents is its ability to concentrate dilute solutions for recovery of salts and chemicals with low-power requirements. No latent heat of vaporization or fusion is required for effecting separations the main energy requirement is for a high-pressure pump. It also requires relatively limited floor space for compact, high-capacity units, and it exhibits good recovery and rejection rates for a number of typical process solutions (15). 

The performance of RO and NF membrane processes is typicaUy determined by two key parameters, recovery and rejection, defined as foUows ... 

Most refrigeration systems are essentially the same as the heat pump cycle shown in Fig. 6.37. Heat is absorbed at low temperature, servicing the process, and rejected at higher temperature either directly to ambient (cooling water or air cooling) or to heat recovery in the process. Heat transfer takes place essentially over latent heat profiles. Such cycles can be much more complex if more than one refrigeration level is involved. 

The purified acid is recovered from the loaded organic stream by contacting with water in another countercurrent extraction step. In place of water, an aqueous alkafl can be used to recover a purified phosphate salt solution. A small portion of the purified acid is typically used in a backwashing operation to contact the loaded organic phase and to improve the purity of the extract phase prior to recovery of the purified acid. Depending on the miscibility of the solvent with the acid, the purified acid and the raffinate may be stripped of residual solvent which is recycled to the extraction loop. The purified acid can be treated for removal of residual organic impurities, stripped of fluoride to low (10 ppm) levels, and concentrated to the desired P2 s Many variations of this basic scheme have been developed to improve the extraction of phosphate and rejection of impurities to the raffinate stream, and numerous patents have been granted on solvent extraction processes. 

Fig. 4. The 341,000-m /d multistage flash (MSF) evaporation desalination plant A1 Taweelah B in Abu Dhabi, United Arab Emirates. Courtesy of Italimpianti SpA. It is a dual-purpose plant, composed of six identical power and desalination units. The desalination units at 56,800 m /d each are currently (1997) the largest ia the world. They have 17 recovery and 3 reject stages and a Performance Ratio of 8 1. The plant also produces 732 MWe of...

Consider now the consequences of placing simple distillation columns (i.e. one feed, two products, one reboiler and one condenser) in different locations relative to the heat recovery pinch. The separator takes heat Qreb into the reboiler at temperature Treb and rejects heat Qcond at a lower temperature Tcond There are two possible ways in which the column can be heat integrated with the rest of the process. The reboiler and condenser can be integrated either across, or not across, the heat recovery pinch. 

Mehra (2) [Named after the inventor] A gas separation process utilizing absorption in a solvent at moderate pressures. Developed by Advanced Extraction Technologies and applied to hydrogen recovery, nitrogen rejection, and recovery of natural gas liquids. 

The removal of PhCs by NF membranes occurs via a combination of three mechanisms adsorption, sieving and electrostatic repulsion. Removal efficiency can vary widely from compound to compound, as it is strictly correlated to (a) the physicochemical properties of the micro-pollutant in question, (b) the properties of the membrane itself (permeability, pore size, hydrophobicity and surface charge) and (c) the operating conditions, such as flux, transmembrane pressure, rejections/recovery and water feed quality. 

Purification of this crude stream, which contains a stoichiometric amount of 2-phenylacetamide, was achieved by crystallizahon of the amine 1 as its l -larirate salt with 90% recovery and upgrade in purity to 99.9% ee. The 2-phenylacetamide was completely rejected during this crystallizahon. 

Recovery levels for individual organic compounds were, as expected, less than the values predicted by using the membrane solute rejections. Differences in the actual recovery and the theoretical recovery were partially due to adsorption losses. Mass balance analysis still indicated a deficiency in some cases. Rectification of the inconsistencies in these data was complicated by the limited water solubility of compounds chosen for study, the necessity of using a cosolvent in spiking, and, in particular, the limitations of the analytical procedures at these extremely low concentrations. 

An RO system is rated based on product flow rate. An 800-gpm RO would yield 800 gpm of permeate. The influent and reject flows are typically not indicated except in the design details (they are usually calculated knowing the product flow rate and the percent recovery). 

The earlier plants operated at deficit, and needed an auxiliary boiler, which was integrated in the flue gas duct. Auxiliary burners in tunnels or flue gas duet were additionally used in some instances. This situation was partially caused by inadequate waste heat recovery and low efficiency in some energy consumers. Typically, the furnace flue gas was discharged in the stack at rather high temperature because there was no air preheating and too much of the reaction heat in the synthesis loop was rejected to the cooling media (water or air). In addition, efficiency of the mechanical drivers was low and the heat demand for regenerating the solvent from the C02 removal unit (at... 

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