Back-pressure

Kipp s apparatus Equipment for the production of a gas by interaction of a liquid and a solid. It consists of three receptacles, the top is reservoir for the liquid and is connected to the bottom. The middle contains the solid and a tap for the gas. When gas is released the liquid rises and reacts with the solid, when the tap is closed the back pressure returns the liquid lo the lower reservoir and reaction cease . Once widely used for the production of H.S (HCl and FeS) and C O (HCl and CaC(3,<). 

The experimental conditions used to determine the CFPP do not exactly reflect those observed in vehicles the differences are due to the spaces in the filter mesh which are much larger in the laboratory filter, the back-pressure and the cooling rate. Also, research is continuing on procedures that are more representative of the actual behavior of diesel fuel in a vehicle and which correlate better with the temperature said to be operability , the threshold value for the Incident. In 1993, the CEN looked at two new methods, one called SFPP proposed by Exxon Chemicals (David et al., 1993), the other called AGELFI and recommended by Agip, Elf and Fina (Hamon et al., 1993). 

In addition to obviating the need of velocity selection, the increased backing pressure over that attainable with an eflfiisive source leads to significantly higher downstream beam densities. 

Fig. 8. Fluid-bed MTG demonstration plant schematic diagram. BPR = Back pressure regulator TC = temperature controller.

Cleaning is frequently aided mechanically. Foam balls scour the center of tubes, and hoUow-filter systems can be back-flushed. HoUow fibers and membranes attached to rigid supports can be back-pressured, thereby eliminating the pressure drop that holds redispersed films on the membrane surface. 

The simplest osmotic dosage form, ALZA Corporation s OROS elementary osmotic pump (Fig. 7), combines the dmg and sometimes an osmotic agent in a monolithic core and deflvers the dmg in solution (102). The mass dehvery rate with time dm df) of the dmg solution is described by equation 4, where is the hydrauHc permeabiUty of the membrane, a is the membrane reflection coefficient, Atz is the osmotic pressure gradient, APis the hydrostatic back pressure, A is the area of the membrane, C is the dissolved concentration of the dmg, and b is the membrane thickness. 

The dehvery port can be sized sufficiently large that the back pressure is negligibly small. Equation 4 then reduces to equation 5, where k represents the osmotic water permeabihty of the membrane and S represents the solubihty of the dmg at saturated concentration. 

A Back-Pressure Efficiency Factor. Because a gaseous diffusion stage operates with a low-side pressure p which is not negligible with respect to there is also some tendency for the lighter component to effuse preferentiahy back through the barrier. To a first approximation the back-pressure efficiency factor is equal to (1 — r), where ris the pressure ratiopjpj. 

These efficiency factors are discussed in more detail in References 26—28. Actually, the barrier and back-pressure efficiencies are interrelated and caimot be formulated independently, except only as an approximation. A better formation that has been found to fit the experimental results is ... 

Because of increased emphasis on maximizing cogenerated power, newer plants are trying to utilize back-pressure turbines only in applications where efficiencies above 70% can be attained. This typically means limiting the applications to the large (>1000 kW) drives, and usiag small machines only where they are necessary for the safe shutdown of the unit. Multistage turbiaes are used even on the smaller loads. 

It should be noted that the highest possible absorption rates will occur under conditions in which the hquid-phase resistance is negligible and the equilibrium back pressure of the gas over the solvent is zero. Such situations would exist, for instance, for NH3 absorption into an acid solution, for SO9 absorption into an alkali solution, for vaporization of water into air, and for H9S absorption from a dilute-gas stream into a strong alkali solution, provided there is a large excess of reagent in solution to consume all the dissolved gas. This is known as the gas-phase mass-transfer limited condition, wrien both the hquid-phase resistance and the back pressure of the gas equal zero. Even when the reaction is sufficiently reversible to allow a small back pres-... 

The liquid-phase rate coefficient is strongly affected by fast chemical reactions and generally increases with increasing reac tion rate. Indeed, the condition for zero hquid-phase resistance m/k-d) imphes that either the equilibrium back pressure is negligible, or that... 

Tlie safety valve is similar to the relief valve except it is designed to open fiillv, or pop, with onlv a small amount of pressure over the rated limit. Conventional safety valves are sensitive to dovvmstream pressure and niav have iinsatisfactorv operating characteristics in variable back pressure applications. The balanced safety relief valve is available and minimizes the effect of dovvmstream pressure on performance. 

For most chemical plants, process steam is used at pressures of 1.825 MN/m" (250 psig), saturated or lower. When combined heat and power generation is economically justified, the steam may be generated at about 5.96 MN/m" (850 psig) appropriately superheated and used to drive back-pressure steam turbines passing out process steam at the required pressure level. 

Use of Operating Curve Frequently, it is not possible to assume that = 0 as in Example 2, owing to diffusional resistance in the liquid phase or to the accumulation of solute in the hquid stream. When the back pressure cannot be neglected, it is necessary to supplement the equations with a material balance representing the operating line or curve. In view of the countercurrent flows into and from the differential section of packing shown in Fig. 14-3, a steady-state material balance leads to the fohowing equivalent relations ... 

Variable-pre.s.sure, vaiiable-rate filtration. The use of a centrifugal pump results in this pattern the discharge rate decreases with increasing back pressure. 

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