Temperature, feed water rejection

Simplified Cycle. A simplified fossil steam cycle appears in Figure 19. The water accumulates in the bottom of the condenser, called the hotweU. It goes through a feed pump to pressurize it. The pressurized water passes through one or more feedwater heaters, which raise the temperature. The water then enters the boiler where heat from the fuel converts it to steam. The steam expands through the engine, usually a turbine, which extracts work. In the middle of the turbine some of the steam is extracted to supply heat to the feedwater heater. The remainder expands through the turbine and is condensed. The rejected heat is carried away by the condenser coolant, which is usually water, but sometimes air. The condensed steam then returns to the... 

The solubihty of sihca is a function of pH and temperature. Lime softening and lime plus soda ash softening are most effective in removing sihca. Other options are process related (a) run the RO system at reduced recovery, (b) increase the feed water temperature - sihca solubihty increases with temperature, and (c) use silica inhibitors. Colloidal silica is difficult to remove by IX because it is not ionised, and can foul the resins when the levels are high. Colloidal silica flocculates easily in boundary layers resulting in severe fouhng. It can be removed by UF membranes with a MW CO of up to 100,000 Da. Since the solubihty of the sihca increases below a pH of about 6.0 and above a pH of about 9.0, the actual solubihty of sihca in the concentrate stream is further affected by the pH of reject water. 

The difficulty with controUing membrane feed-pressure and reject-flow rate simultaneously is that one affects the other. This gets more compficated with a two-pass system. If the membrane feed pressure is reduced, the reject-flow rate also wiU decrease if aU other parameters (feed water temperature, quafity and membrane condition) in the system remain constant. The danger, then, is that the whole system may osciUate if the PID controUers are not properly tuned. Tuning one controUer for fast operation and the other for relatively slow operation is often the solution. Fast operation generaUy is best applied to the membrane feed-pressure controUer because during start-up this parameter must be adjusted first. Manual control of these valves is, therefore, often preferred. 

The steam turbine of AHWR is designed for a steam quality of 99.75%. During normal operation or in a bypass mode of operation the steam from the turbine exhaust is condensed in a condenser, which rejects the heat to seawater. The condensate is heated in heat exchangers by the moderator system. The feed water temperature is finally raised to 403 K through LP (low pressure) heaters and de-aerators using the steam bled from the turbine. The feed water pumps then pump the feed water into the steam drum where it mixes with the water separated from steam-water mixture. 

The curves between sihca rejection efficiency and SWE in the beginning show a high sUica rejection plateau as the SWE decreased, which evenmaUy gives way to deterioration in product sUica at lower SWE values (Fig. 13.13). The rudimentary requirement is to operate the EDI modules at high SWE (low current efficiency) to attain the desired product water quahty in terms of resistivity and sUica rejection performance for a given feed water conductivity and temperature. 

Table V displays data recorded at the test facility in Roswell, New Mexico, maintained by the Office of Water Research and Technology, U. S. Department of Interior. This facility delivers a feed of brackish water pretreated to control bacterial growth and to deliver a feed free of chlorine. Modules 148 and 152 were nominally identical samples. They are constructed of fiber bundles approximately 2 inches in diameter, 10 inches in length and containing approximately 25 square feet of membrane surface area. The increase in productivity over time can be explained by an increase in feed temperature over the course of the test. The decline in rejection of module 148 is not fully understood. However, it is probable that the decline is similarly the responsibility of a temperature increase. Recent data indicates a stabilization at a rejection level of 94%.

The results show that, at temperatures below 60 °C and an air feed stoichiometry below three, the cathode exhaust is fully saturated (nearly fully saturated at 60 °C) with water vapor and the exhaust remains saturated after passing through a condenser at a lower temperature. In order to maintain water balance, all of the liquid water and part of the water vapor in the cathode exhaust have to be recovered and returned to the anode side before the cathode exhaust is released to the atmosphere. Because of the low efficiency of a condenser operated with a small temperature gradient between the stack and the environment, a DMFC stack for portable power applications is preferably operated at a low air feed stoichiometry in order to maximize the efficiency of the balance of plant and thus the energy conversion efficiency for the complete DMFC power system. Thermal balance under given operating conditions was calculated here based on the demonstrated stack performance, mass balance and the amount of waste heat to be rejected. 

The poly(ether/amide) thin film composite membrane (PA-100) was developed by Riley et al., and is similar to the NS-101 membranes in structure and fabrication method 101 102). The membrane was prepared by depositing a thin layer of an aqueous solution of the adduct of polyepichlorohydrin with ethylenediamine, in place of an aqueous polyethyleneimine solution on the finely porous surface of a polysulfone support membrane and subsequently contacting the poly(ether/amide) layer with a water immiscible solution of isophthaloyl chloride. Water fluxes of 1400 16001/m2 xday and salt rejection greater than 98% have been attained with a 0.5% sodium chloride feed at an applied pressure of 28 kg/cm2. Limitations of this membrane include its poor chemical stability, temperature limitations, and associated flux decline due to compaction. 

Dynamically formed membranes were pursued for many years for reverse osmosis because of their high water fluxes and relatively good salt rejection, especially with brackish water feeds. However, the membranes proved to be unstable and difficult to reproduce reliably and consistently. For these reasons, and because high-performance interfacial composite membranes were developed in the meantime, dynamically formed reverse osmosis membranes fell out of favor. A small application niche in high-temperature nanofiltration and ultrafiltration remains, and Rhone Poulenc continues their production. The principal application is poly(vinyl alcohol) recovery from hot wash water produced in textile dyeing operations. 

Mass transfer in the feed and strip solutions is limited by the extent of concentration polarization. On the feed side of the membrane, concentration polarization refers to an increase in the concentration of solutes at and near the feed-membrane interface because of evaporation of water into the membrane pores (Fig. 1). The resulting solute concentration gradient between the membrane-feed interface, where the concentration is greatest, and the bulk solution induces diffusive transport of rejected solutes back through the concentration polarization boundary layer into the bulk stream. Bulk solution is simultaneously transported to the membrane wall by convection. When equilibrium has been established under a given set of operating conditions (stream flow rate, temperature, fluid dynamics imposed by membrane module design), the rate of back diffusion is equal to the rate at which the solutes are carried to the membrane surface by convective flow. ... 

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