Pressure Drop Introduction

Definitions Gas absorption is a unit operation in which soluble components of a gas mixture are dissolved in a liquid. The inverse operation, called stripping or desorption, is employed when it is desired to transfer volatile components from a liquid mixture into a gas. Both absorption and stripping, in common with distillation (Sec. 13), make use of special equipment for bringing gas and liquid phases into intimate contact. This section is concerned with the design of gasliquid contacting...

Transition Between Flow Regimes

Froth-Spray Froth-spray transition has been investigated for sieve trays using a variety of techniques. The gradual nature of this transition bred a multitude of criteria for defining it, and made its correlation difficult. Excellent overviews were given by Lockett (Distillation Tray Fundamentals, Cambridge University Press, Cambridge, England, 1986) and Prado, Johnson, and Fair Chem. Eng. Progr. 83(3), p. 32 (1987) . Porter and Jenkins I. Chem. E. Symp. Ser. 56, Summary Paper (1979) presented...

Other Packing Considerations

Liquid Holdup Liquid holdup is the liquid present in the void spaces of the packing. Reasonable liquid holdup is necessary for good mass transfer and efficient tower operation, but beyond that, it should be kept low. High holdup increases tower pressure drop, the weight of the packing, the support load at the bottom of the packing and tower, and the tower drainage time. Most important, when distilling thermally unstable materials, excessive holdup raises product degradation and fouling, and...

Francis Weir Equation

P 0.0825 ln - 0.269 ln Fh + 1.679 (14-107) Fh F-factor for flow through holes, Fh UhpGs, m s (kg m3)05 hds hw + how + 0.5hhg (14-108) how height of crest over weir, mm clear liquid hhg hydraulic gradient across tray, mm clear liquid The value of weir crest how may be calculated from the Francis weir equation and its modifications for various weir types. For a segmental weir and for height in millimeters of clear liquid, where Q liquid flow, m3 s Lw weir length, m

B

FIG. 14-108 (a) Liquid entrainment from the bottom of a vessel by centrifugal flow. (Rietema and Verver, Cyclones in Industry, Elsevier, Amsterdam, 1961. ) (b) Gas-outlet skirt for liquid cyclones. (Stern et al., Cyclone Dust Collectors, American Petroleum Institute, New York, 1955. ) Reentrainment is generally reduced by lower inlet gas velocities. Calvert (R-12) reviewed the literature on predicting the onset of entrainment and found that of Chien and Ibele (ASME Pap. 62-WA170) to be the most...

Q65

Fits experimental data reasonably well. Surface tension and liquid viscosity tend to increase the bubble size at a low Reynolds number. The effect of surface tension is greater for large orifice diameters. The magnitude of the diameter increase due to high liquid viscosity can be obtained from Eq. 14-208 . For conditions approaching constant pressure at the orifice entrance, which probably simulates most industrial applications, there is no independently verified predictive method. For air at...

Info

Overall Mass-Transfer Coefficient In systems with relatively sparing soluble gases, where the gas-phase resistance is negligible, the mass-transfer rate can be determined by using the concept of an overall volumetric mass-transfer coefficient kLa as follows where Ms solute molar mass-transfer rate, kgmol s kLa overall mass-transfer coefficient, 1 s C solute concentration in equilibrium with the liquid phase, kg-mol s and C,b solute concentration in bulk of liquid. Bakker et al. op. cit. have...

Liquidphase Continuous Systems

Practical separation techniques for gases dispersed in liquids are discussed. Processes and methods for dispersing gas in liquid have been discussed earlier in this section, together with information for predicting the bubble size produced. Gas-in-liquid dispersions are also produced in chemical reactions and electrochemical cells in which a gas is liberated. Such dispersions are likely to be much finer than those produced by the dispersion of a gas. Dispersions may also be unintentionally...

P JTf p

Where dW W is the mass of particles in a given narrow size distribution and Pt is the average penetration for that size range. When the particles to be collected are log-normally distributed and the collection device efficiency can be expressed_by Eq. 14-224 , the required overall integrated collection efficiency Pt can be related to the ratio of the device aerodynamic cut size Dpc to the mass median aerodynamic particle size Dpa50. This required ratio for a given distribution and collection is...

Other Hydraulic Limits

Weeping Weeping is liquid descending through the tray perforations, short-circuiting the contact zone, which lowers tray efficiency. At the tray floor, liquid static head that acts to push liquid down the perforations is counteracted by the gas pressure drop that acts to hold the liquid on the tray. When the static head overcomes the gas pressure drop, weeping occurs. Some weeping usually takes place under all conditions due to sloshing and oscillation of the tray liquid. Generally, this...

L2 j e hb

Values of pp and dp are droplet density, g cm3, and droplet diameter, cm is the gas viscosity, P. All other terms were defined previously. Table 14-27 gives values of j calculated from experimental data of Jackson and Calvert. Values of j for most manufactured packing appear to fall in the range from 0.16 to 0.19. The low value of 0.03 for coke may be due to the porosity of the coke itself. Calvert R-12 has tested the correlation in cross-flow packed beds, which tend to give better drainage...

System Limit The Ultimate Capacity Of Fractionators

Liquid drops of various sizes form in the gas-liquid contact zones of tray or packed towers. Small drops are easily entrained upward, but their volume is usually too small to initiate excessive liquid accumulation flooding . When the gas velocity is high enough to initiate a massive carryover of the larger drops to the tray above, or upward in a packed bed, liquid accumulation entrainment flooding takes place. This flood can be alleviated by increasing the tray spacing or using more hole areas...

Maldistribution And Its Effects On Packing Efficiency

Modeling and Prediction Maldistribution may drastically reduce packing efficiency. HETP may increase by a factor as high as 2 or 3 due to maldistribution. Shariat and Kunesh Ind. Eng. Chem. Res., 34 4 , 1273 1995 provide a good demonstration. Early models Mullins, Ind. Chem. Mfr., 33, 408 1957 Manning and Cannon, Ind. Eng. Chem. 49 3 , 347 1957 expressed the effect of liquid maldistribution on packing efficiency in terms of a simple channeling model. A portion of the liquid bypasses the bed,...

Comparing Trays And Packings

Most separations can be performed either with trays or with packings. The factors below represent economic pros and cons that favor each and may be overridden. For instance, column complexity is a factor favoring trays, but gas plant demethanizers that often use one or more interreboilers are traditionally packed. Vacuum systems. Packing pressure drop is much lower than that of trays because the packing open area approaches the tower cross-sectional area, while the tray's open area is only 8 to...

Distributors

Fractionating Reflux Still

Liquid Distributors A liquid distributor or redistributor should be used in any location in a packed column where an external liquid stream is introduced. Liquid redistributors are also used between packed beds to avoid excessive bed lengths that may impair packing efficiency or mechanical strength. It is best to have the packing supplier also supply the distributor, with the user critically reviewing the design. The user must provide the supplier with concise information about the plugging,...

Schoepentoeter Pressure Drop

Where hhg is the hydraulic gradient head, mm, and vH is the horizontal velocity in the troughs, m s. Flashing Feed and Vapor Distributors When the feed or reflux is a flashing feed, the vapor must be separated out of the liquid before the liquid enters a liquid distributor. At low velocities only , this can be achieved by a bare nozzle Fig. 14-70a . A V baffle Fig. 14-70b is sometimes installed as a primitive flashing feed or vapor distributor. For better vapor-liquid separation and...

Swept Back Weir

Vapor Vane Distributor

FIG. 14-71 High-velocity flashing feed and vapor distributors. a Vapor horn. b Radial vane distributor. c Schoepentoeter. Parts a, b, courtesy oof Koch-Glitsch LP part c, courtesy oof Sulzer Chemtech. FIG. 14-71 High-velocity flashing feed and vapor distributors. a Vapor horn. b Radial vane distributor. c Schoepentoeter. Parts a, b, courtesy oof Koch-Glitsch LP part c, courtesy oof Sulzer Chemtech.

What Is Chimney Tray In Distillation Column

Distillation Column Chimney Tray

FIG. 14-72 Chimney tray vapor distributor. Reprinted courtesy of KochGlitsch LP. FIG. 14-72 Chimney tray vapor distributor. Reprinted courtesy of KochGlitsch LP. correlation solely by the readily available specific surface area Tables 14-13 and 14-14 . Minimum Wetting Rate The minimum wetting rate MWR is the lower stability limit of packings. Below this liquid load the liquid film on the packing surfaces breaks up and dewetting occurs. The area available for mass transfer diminishes, and...

Equipment For Distillation And Gas Absorption Packed Columns

Packings are generally divided into three classes 1. Random or dumped packings Figs. 14-48 and 14-49 are discrete pieces of packing, of a specific geometric shape, that are dumped or randomly packed into the column shell. 2. Structured or systematically arranged packings Fig. 14-50 are crimped layers of corrugated sheets usually or wire mesh. Sections of these packings are stacked in the column. 3. Grids. These are also systematically arranged packings, but instead of wire mesh or corrugated...

Oldershaw Column

Kirschbaum, Distillier-Rektijiziertechnik, 4th ed., Springer-Verlag, Berlin and Heidelberg, 1969. 2. Kastanek and Standart, Sep. Sci. 2, 439 1967 . 3. Billet and Raichle, Chem. Ing. Tech., 38, 825 1966 40, 377 1968 . 4. AIChE Research Committee, Tray Efficiency in Distillation Columns, final report, University of Delaware, Newark, 1958. 5. Billet R., IChemE., Symp. Ser. 32, p. 4 42 1969 . 6. Mayfield et al., Ind. Eng. Chem., 44, 2238 1952 . 7. Fractionation Research, Inc. Report of Tests of...

Fair S Entrainment Correlation

Sieve Tray Flooding Capacity

Low values of FLG indicate vacuum operation, high values indicate operation at higher pressures or at high liquid vapor loadings. The liquid gas ratio L G is based on mass flow rates. For multipass trays, the ratio needs to be divided by the number of passes. The strength of the correlation is at the lower flow parameters. At higher flow parameters high L G ratios, high pressures, emulsion flow , Fig. 14-31 gives excessively conservative predictions, with the low values of Csbf to the right...

Ln [y y

In which subscripts 1 and 2 refer to the bottom and top of the absorption tower respectively, y mole-fraction solute in the gas phase, and y gas-phase solute mole fraction in equilibrium with bulk-liquid-phase solute concentration x. When the equilibrium line is straight, y mx. The traditional design method normally makes use of overall KGa values even when resistance to transfer lies predominantly in the liquid phase. For example, the CO2-NaOH system which is most commonly used for comparing...

Packing Efficiency

Gives the following transfer unit relationship where HOG height of an overall transfer unit, gas concentration basis, m Hg height of a gas-phase transfer unit, m HL height of a liquid-phase transfer unit, m X m LM GM slope of equilibrium line slope of operating line In design practice, a less rigorous parameter, HETP, is used to express packing efficiency. The HETP is the height of packed bed requiredto achieve a theoretical stage. The terms HOG and HETP may be related under certain conditions...

Hetp Pall Rings

Hetp For Pall Rings

FIG. 14-57 Superimposing experimental pressure-drop data for a given packing generates a GPDC interpolation chart for this packing. a A random packing chart is based on Eckert's GPDC, Fig. 14-55. b A structured packing chart is based on Kister and Gill's GPDC SP , Fig. 14-56. From Kister, H. Z., Distillation Design, copyright McGraw-Hill, 1992 used with permission. Kister and Larson in Schweitzer, Handbook of Separation Techniques for Chemical Engineers, 3d ed., McGraw-Hill, 1997 extended Eq....

Obtaining Tray Efficiency

Efficiency prediction methods are listed here in decreasing order of reliability. Rigorous Testing Rigorous testing of a plant column is generally the most reliable method of obtaining tray efficiency. Test procedures can be found elsewhere AIChE Equipment Testing Procedures Committee, AIChE Equipment Testing Procedure Tray Distillation Columns, 2d ed., 1987 Kister, Distillation Operation, McGraw-Hill, New York, 1990 . Scale-up from an Existing Commercial Column As long as data are for the same...

Flow Regimes On Trays

Three main flow regimes exist on industrial distillation trays. These regimes may all occur on the same tray under different liquid and gas flow rates Fig. 14-19 . Excellent discussion of the fundamentals and modeling of these flow regimes was presented by Lockett Distillation Tray Fundamentals, Cambridge University Press, Cambridge, 1986 . An excellent overview of these as well as of less common flow regimes was given by Prince PACE, June 1975, p. 31 July 1975, p. 18 . Froth regime or mixed...

Tray Efficiency

Murphree Efficiency

Overall Column Efficiency This is the ratio of the number of theoretical stages to the number of actual stages Since tray efficiencies vary from one section to another, it is best to apply Eq. 14-132 separately for the rectifying and stripping sections. In practice, efficiency data and prediction methods are often too crude to give a good breakdown between the efficiencies of different sections, and so Eq. 14-132 is applied over the entire column. Point Efficiency This is defined by Eq. 14-133...

Packedcolumn Flood And Pressure Drop

Structured Packed Column Image

Pressure drop of a gas flowing upward through a packing countercur-rently to liquid flow is characterized graphically in Fig. 14-53. At very low liquid rates, the effective open cross section of the packing is not appreciably different from that of dry packing, and pressure drop is due to flow through a series of variable openings in the bed. Thus, pressure drop is proportional approximately to the square of the gas velocity, as indicated in the region AB. At higher liquid rates, the effective...

O Connell Correlation

Connell Efficiency Correlation

FIG. 14-45 Overall column efficiency of 25-mm Oldershaw column compared with point efficiency of 1.22-m-diameter-sieve sieve-plate column of Fractionation Research, Inc. System cyclohexane-n-heptane. Fair, Null, and Bolles, Ind. Eng. Chem. Process Des. Dev., 22, 53 1982 . given system, higher Oldershaw column efficiencies were measured under cellular foam conditions than under froth conditions. For this reason, Gerster Chem. Eng. Progr. 59 3 , 35 1963 warned that when cellular foam can form,...

When Blood Pressure Drop

Tray Efficiency Versus Vapour Rate

FIG. 14-41 Sequence of steps for theoretical prediction of tray efficiency. From H. Z. Kister, Distillation Design, copyright 1992 by McGraw-Hill reprinted by permission. FIG. 14-41 Sequence of steps for theoretical prediction of tray efficiency. From H. Z. Kister, Distillation Design, copyright 1992 by McGraw-Hill reprinted by permission. Factors Affecting Tray Efficiency Below is a summary based on the industry's experience. A detailed discussion of the fundamentals is found in Lockett's book...

Flooding

Flooding is by far the most common upper capacity limit of a distillation tray. Column diameter is set to ensure the column can achieve the required throughput without flooding. Towers are usually designed to operate at 80 to 90 percent of the flood limit. Flooding is an excessive accumulation of liquid inside a column. Flood symptoms include a rapid rise in pressure drop the accumulating liquid increases the liquid head on the trays , liquid carryover from the column top, reduction in bottom...

Swept-back Weir

Weir Liquid Crest Distillation Column

FIG. 14-21 Flow passes on trays. a Single-pass. b Two-pass. c Three-pass. d Four-pass. FIG. 14-21 Flow passes on trays. a Single-pass. b Two-pass. c Three-pass. d Four-pass. The straight, segmental vertical downcomer Fig. 14-23a is the most common downcomer geometry. It is simple and inexpensive and gives good utilization of tower area for downflow. Circular downcom-ers downpipes Fig. 14-23b , are cheaper, but poorly utilize tower area and are only suitable for very low liquid loads. Sloped...

Traytower Design

The design of a tray tower for gas absorption and gas-stripping operations involves many of the same principles employed in distillation calculations, such as the determination of the number of theoretical trays needed to achieve a specified composition change see Sec. 13 . Distillation differs from absorption because it involves the separation of components based upon the distribution of the various substances between a vapor phase and a liquid phase when all components are present in both...

Gas Absorption Tray Columns

Moving valve trays Fig. 14-18c have the perforations covered by movable disks valves . Each valve rises as the gas velocity increases. The upper limit of the rise is controlled by restricting legs on the bottom of the valve Fig. 14-18c or by a cage structure around the valve. As the gas velocity falls, some valves close com-letely, preventing weeping. This gives the valve tray good turn-own. Table 14-5 is a general comparison of the three main tray types, assuming proper design, installation,...

Heat Effects In Gas Absorption

Overview One of the most important considerations involved in designing gas absorption towers is to determine whether temperatures will vary along the height of the tower due to heat effects note that the solute solubility usually depends strongly on temperature. The simplified design procedures described earlier in this section become more complicated when heat effects cannot be neglected. The role of this section is to enable understanding and design of gas absorption towers where heat...

Pressure Drop

Pressure Drop Correlation

In vacuum distillation, excessive pressure drop causes excessive bottom temperatures which, in turn, increase degradation, polymerization, coking, and fouling, and also loads up the column, vacuum system, and reboiler. In the suction of a compressor, excessive pressure drop increases the compressor size and energy usage. Such services attempt to minimize tray pressure drop. Methods for estimating pressure drops are similar for most conventional trays. The total pressure drop across a tray is...

Distillery Pressure

The terms in Eqs. 14-123 to 14-126 are in English units and are explained in the Nomenclature. For sieve trays, m 1.94 and Cw 0.79. Note that the constants are a slight revision of those presented in the original paper C. L. Hsieh, private communication, 1991 . Clear liquid height is calculated from Colwell's correlation Eqs. 14-115 to 14-122 . The Hsieh and McNulty correlation applies to trays with 9 percent and larger fractional hole area. For trays with smaller hole area, Hsieh and McNulty...

Other Tray Types

Distillation Trays Fouling

Bubble-Cap Trays Fig. 14-27a These are flat perforated plates with risers chimneylike pipes around the holes, and caps in the form of inverted cups over the risers. The caps are usually but not always equipped with slots through which some of the gas comes out, and may be round or rectangular. Liquid and froth are trapped on the tray to a depth at least equal to the riser or weir height, giving the bubble-cap tray a unique ability to operate at very low gas and liquid rates. The bubble-cap tray...

Primary Tray Considerations

Downcomer Backup

Number of Passes Tray liquid may be split into two or more flow passes to reduce tray liquid load QL Fig. 14-21 . Each pass carries 1 Np fraction of the total liquid load e.g., j in four-pass trays . Liquid in each pass reverses direction on alternate trays. Two-pass trays have perfect symmetry with full remixing in the center down-comers. Four-pass trays are symmetric along the centerline, but the side and central passes are nonsymmetric. Also, the center and offcenter downcomers only...

Glitsch Downcomer Backup Equation

Distillation Theory and Practice an Intensive Course, University of New South Wales University of Sydney, August 9-11, 1977. f Glitsch, Inc., Ballast Tray Design Manual, 6th ed., 1993 available from Koch-Glitsch LP, Wichita, Kans. R. H. Perry and D. W. Green eds. , Perry's Chemical Engineers' Handbook, 7th ed., McGraw-Hill, 1997. The author believes that low-molecular-weight hydrocarbons refers to light hydrocarbons at near atmospheric pressure or under vacuum. The foam stability of...

Lockett Distillation Tray Fundamentals

Glitsch, Inc., Ballast Tray Design Manual, Bulletin 4900, 6th ed., 1993. Available from Koch-Glitsch, Wichita, Kans. 2. Koch Engineering Co., Inc., Design Manual Flexitray, Bulletin 960-1, Wichita, Kans., 1982. 3. Nutter Engineering, Float Valve Design Manual, 1976. Available from Sulzer ChemTech, Tulsa, Okla. 4. M. J. Lockett, Distillation Tray Fundamentals, Cambridge University Press, Cambridge, England, 1986. To convert lb ft3 to kg m3, multiply by 16.0. source H. Z. Kister, Distillation...

Tray Capacity Enhancement

Multichordal Downcomer

High-capacity trays evolved from conventional trays by including one or more capacity enhancement features such as those discussed below. These features enhance not only the capacity but usually also the complexity and cost. These features have varying impact on the efficiency, turndown, plugging resistance, pressure drop, and reliability of the trays. Truncated Downcomers Forward Push Trays Truncated downcomers forward push trays include the Nye Tray, Maxfrac Fig. 14-26a , Triton , and MVGT ....

Design Of Gas Absorption Systems

Chemical And Physical Solvents Co2

General Design Procedure The design engineer usually is required to determine 1 the best solvent 2 the best gas velocity through the absorber, or, equivalently, the vessel diameter 3 the height of the vessel and its internal members, which is the height and type of packing or the number of contacting trays 4 the optimum solvent circulation rate through the absorber and stripper 5 temperatures of streams entering and leaving the absorber and stripper, and the quantity of heat to be removed to...

Absorption With Chemical Reaction

Van Krevelen Hatta

Introduction Many present-day commercial gas absorption processes involve systems in which chemical reactions take place in the liquid phase an example of the absorption of CO2 by MEA has been presented earlier in this section. These reactions greatly increase the capacity of the solvent and enhance the rate of absorption when compared to physical absorption systems. In addition, the selectivity of reacting solutes is greatly increased over that of nonreacting solutes. For example, MEA has a...

Paul Mathias Fluor

This is a copyrighted work and The McGraw-Hill Companies, Inc. McGraw-Hill and its licensors reserve all rights in and to the work. Use of this work is subject to these terms. Except as permitted under the Copyright Act of 1976 and the right to store and retrieve one copy of the work, you may not decompile, disassemble, reverse engineer, reproduce, modify, create derivative works based upon, transmit, distribute, disseminate, sell, publish or sublicense the work or any part of it without...

Handbook

Equipment for Distillation, Gas Absorption.Phase Dispersion, and Phase Separation HENRY Z. KISTER, PAUL MATHIAS D. E. STEINMEYER, W. R. PENNEY B. B. CROCKER, JAMES R. FAIR Copyright 2008, 1997, 1984, 1973, 1963, 1950, 1941, 1934 by The McGraw-Hill Companies, Inc. All rights reserved. Manufactured in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored...