Chemical pressure drop

Figure 3.10 is a plot of potential against distance from the wall for a liquid in a capillary of sufficient width for its middle A to be outside the range of forces from the wall. Since the capillary condensate is in equilibrium with the vapour, its chemical potential (=p represented by the horizontal line GF, will be lower than that of the free liquid the difference in chemical potential of the condensate at A, represented by the vertical distance AF, is brought about entirely by the pressure drop, Ap = 2y/r , across the meniscus (cf. Equation (3.6)) but at some point B. say, nearer the wall, the chemical potential receives a contribution represented by the line BC, from the adsorption potential. Consequently, the reduction Ap in pressure across the meniscus must be less at B than at A, so that again... 

The possible remedial and preventive actions are hot soaks and drains during cooldown to help remove soluble deposited material, chemical cleaning to remove corrosion products and reduce the pressure drop (see Metal surface treatments), and reduced corrosion product transport into OTSG using amines other than ammonia in feedwater (14). 

Rotameters The rotameter, an example of which is shown in Fig. 10-21, has become one of the most popular flowmeters in the chemical-process industries. It consists essentially of a plummet, or float, which is free to move up or down in a vertical, slightly tapered tube having its small end down. The fluid enters the lower end of the tube and causes the float to rise until the annular area between the float and the wall of the tube is such that the pressure drop across this constriction is just sufficient to support the float. Typically, the tapered tube is of glass and carries etched upon it a nearly linear scale on which the position of the float may be visually noted as an indication of the flow. 

Gate valves are used to minimize pressure drop in the open position and to stop the flow of fluid rather than to regulate it. The problem, when the valve is closed, of pressure buildup in the bonnet from cold liquids expanding or chemical action between fluid and bonnet should be solved oy a relief valve or by notching the upstream seat ring. 

FIG. 14-31 Pressure drop for a valve plate, measured versus model prediction ofBoUes [Chem. Eng. Progr. 72(9), 43 (1976)]. Reproduced with permission of the American Institute of Chemical Engineers. Copyright 1976 AlChE. All rights reserved. 

FIG. 14-32 Aeration factor for pressure drop calculation, sieve plates. [Bolles and Fair, Encyclopedia of Chemical Processing and Design, vols. 16, 86. J. M. McKetta (ed.), Marcel Dekker, New Yoik, J9S2.]... 

FIG. 14-50 Pressure drop correlation for random packings, as presented hy Robbins. [Cbem. Eng. Progr., 87(1), 19 (1990). Reproduced with peimission of the Ameiican Institute of Chemical Engineers. Copytight 1990 AlChE. All fights reseroed. ] To convert inches H20/ft to mm H20/m, multiply by 83.31. 

FIG. 14-73 Pressure-drop data for several sizes of paU rings test conditions same as described for Fig. 14-72. [Shaiiat and Kunesh, Ind. Eng. Cbem. Res., 34, 1273 (1.9.95).] Reproduced with permission. Copyright 1995 American Chemical Society. 

One of the problems in predicting efficiency and required pressure drop of a venturi is the chemical nature or wettability of the particulate, which on 0.5-pm-size particles can make up to a threefold difference in required pressure drop for its efficient coUection. Calvert... 

Determine the worst-case gas mixture combustion charac teris-tics, system pressure, and permissible pressure drop across the arrester, to help select the most appropriate element design. Not only does element design impac t pressure drop, but the rate of blockage due to particle impact, liquid condensation, and chemical reaction (such as monomer polymerization) can make some designs impractical even if in-service and out-of-seivdce arresters are provided in parallel. 

Energy needed for pumping can be a significant cost item for the inexpensive basic chemicals therefore, pressure drop must be known more accurately than calculation methods can provide. The needed accuracy can be achieved only by measuring pressure drop versus flow for every new catalyst. This measurement can now be done much better and more easily than before. Even so, for a basic understanding of correlation between pressure drop and flow, some published work must be consulted. (See Figure 1.4.1 on the next page.)... 

The catalyst volume is the same on both sides. It is assumed that no diffusional rate limitation exists even in the larger pellets. That is, the chemical reaction rate is controlling. Pressure drop must be the same for both sides, so the flow has to be less over the smaller pellets to maintain the AP (L/dp)(u /2g) = constant. 

Branan, Carl R. Estimating Pressure Drop," Chemical Engineering, August 28, 1978. 

Problem Three chemical plant recovery train towers were limited to half of design rates by bottoms pump cavitation and high tower pressure drop. 

Engineering factors include (a) contaminant characteristics such as physical and chemical properties - concentration, particulate shape, size distribution, chemical reactivity, corrosivity, abrasiveness, and toxicity (b) gas stream characteristics such as volume flow rate, dust loading, temperature, pressure, humidity, composition, viscosity, density, reactivity, combustibility, corrosivity, and toxicity and (c) design and performance characteristics of the control system such as pressure drop, reliability, dependability, compliance with utility and maintenance requirements, and temperature limitations, as well as size, weight, and fractional efficiency curves for particulates and mass transfer or contaminant destruction capability for gases or vapors. 

Palen, J. W. and Taborek, J., Solution of Shell-Side Flow Pressure Drop and Heat Transfer by Stream Analysis Method (Heat Transfer Research, Inc., Alhambra, CA), AICHE Chemical Engineering Progress Symposium Series No. 92, Vol. 65 (1969), pp. 53-63. 

---