VACUUM TECHNOLOGY FOR CHEMICAL AND PHARMACEUTICAL APPLICATIONS14.1 Introduction

At present in chemical industry, there is increasing demand for separation and enrichment processes at low temperatures. This is possible through vacuum distillation and drying/degassing techniques. Vacuum distillation is routinely used in petroleum refineries to separate crude oil into fuels and petrochemical feed stocks. Vacuum processing is used in the chemical industries to refine a wide range of products and recover raw materials for reuse. The purification of food products such as vegetable oil also requires the use of vacuum distillation. Molecular distillation under high vacuum conditions is used on an industrial scale to purify and separate high-boiling materials such as vitamins, oils, waxes, fatty acids, glycerides, and plasticizers. Vacuum concentration is extremely useful in pharmaceutical and Biomedical industries involving heat sensitive chemicals. This article briefly discusses the technical aspects of vacuum distillation, drying, and concentration processes. This is followed by a detailed discussion of different vacuum pumps, fluid flow in pipes and design of vacuum systems applicable to chemical process applications.

Principles of Distillation:

When a binary liquid mixture boils, the vapor produced will differ in composition from the liquid; the vapor will contain more of the lower boiling component. Conversely, when a portion of a binary vapor mixture is condensed, the liquid produced will be richer in the higher-boiling component. These principles of vapor-liquid equilibrium form the basis for distillation. When a series of successive vaporizations and condensations is carried out, a mixture of two or more volatile components can be separated until products of a specified purity are obtained. Distillation is distinguished from evaporation, as in the concentration of sodium hydroxide-water solutions, some components are volatile; at least one is not.

Phase Equilibria:

The volatility of a compound is a measure of the ease with which it is vaporized. Volatility K is equal to the ratio of a component concentration in the vapor phase to the component concentration in the liquid phase.

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14.1

Where yi xi = vapor and liquid component mole fractions. Differences in component volatilities allow the use of distillation to separate a mixture of several compounds. Of particular interest in distillation is the ratio of the volatility of one component to another. This ratio, the relative volatility, is a measure of the ease of separation of the two components

Where = relative volatility between component 1 and a less volatile component2Large ratios indicate easier separations. The volatility of a pure compound depends on the manner in which individual molecules interact. The volatility of the same compound may differ in a mixture, depending on the effect of molecular interaction between the different compounds. A mixture is termed ideal if the volatility of each component is unaffected by the presence of the other components. Raoults’s law can be used to describe the behavior of ideal solutions

.. 14.3Where pi = component partial pressure in vapor phase

Xi = liquid component mole fraction = Pure component vapor pressure at equilibrium temperature

Dalton’s law can be combined with raoult’s law to give

14.4Where yi = vapor component mole fractionP = total pressure

Rearrangement of Eq. 14.1 and substitution into Eq. 14.2 yields

14.5

Thus the relative volatility of two components in an ideal liquid solution is equal to the ratio of pure component vapor pressures.Many compounds don not from ideal solutions when mixed. In these mixtures, the volatility of each component is altered by the presence of the others and Raoult’s law does not apply. The effect of this “activity” between different molecules cannot be neglected. A more rigorous form of Eq. 5-3 must be used. For liquid phase nonideality, eq. 14.3, 14.4, and 14.5 become

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14.2

Where is the liquid phase activity coefficient. For a solution of two or more compounds that is not ideal, the activity coefficients vary with both composition and temperature. Activity coefficients are most useful for distillation calculations when determined from constant-pressure data in which the vapor and liquid phase compositions have been measured at different equilibrium temperatures.Example:If a binary mixture of 37 mole percent hexane and 63 mole percent n- propanol has a bubble point of 700C, calculate the relative volatility assuming ideal liquid phase behavior. The liquid phase activity coefficients are actually 2.0 and 1.2 for hexane and propanol at these conditions. Recalculate the relative volatility based on these activity coefficients and compare the result with the first answer. The vapor pressure of hexane at 700C is 240 torr.

Solution: 1. Ideal solution: from Eq. 5.5,

2.Non-ideal solution: from Eq. 5-8,

3. Percent error =

Raoult’s law, which assumes = 1.0 for all components, serves only as a rough approximation for most mixtures. Mixtures that obey Raoult’s law at all concentrations usually contain compounds, which are similar in chemical structure. Two such systems are methanol –ethanol and benzene-toluene. Liquid phase behavior can be approximated as ideal, with certain concentration ranges, for first estimates of equilibria in many other systems. For mixtures of light hydrocarbons, the component volatilities or “K values” (Eq. 14.1) are relatively independent of composition. Nomographs of such K values are presented De

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Priester [1]. If possible, one should always examine the Vapor liquid equilibria data for each system before making any simplifying assumptions. One assumption that is almost always justified in vacuum distillation is ideal vapor phase behavior. Because of the low gas densities at low pressures, interaction between molecules in the vapor phase is usually negligible.

Equilibrium Stages:

The purpose of distillation equipment is to provide a means by which vapor and liquid streams can be repeatedly contacted so that they reach equilibrium, causing the vapor phase to be enriched in the more volatile components. The liquid stream is routed so that it flows countercurrent to the vapour, creating series of equilibrium stages, the number of equilibrium stages required to enrich the vapor to a specified purity depends on the relative volatility between the light and heavy components.

Figure (a) below illustrates a chain of several equilibrium stages. These stages are typically arranged in a vertical column or tower so that the liquid flows downward by gravity and the vapor is forced upward by pressure differences. The liquid and vapor streams shown in the figure are numbered, by convention according to the number of the stage in which the stream originated. In other words, the streams are named for the stage in which they last reached equilibrium with the other phase.

The streams entering a contact stage are not in equilibrium with each other; the lack of phase equilibrium provides the driving force for mass transfer between the two phases. The liquid stream from above is at it’s bubble point, the vapor from below at it’s dew point. As the vapor contacts the cooler liquid, heat is transferred from the vapor to the liquid. The cooled vapor is partially condensed and the heated liquid partially vaporized. The vapor condensed will contain a larger fraction of heavy components than the remaining vapor, while the liquid vaporized will contain more light components than the remaining liquid. The transfer of heat and mass between phases continues until they are in equilibrium. At equilibrium, both streams will the same temperature and the compositions of the two phases will be related by Eq. 14.7. The liquid will have been partially stripped of volatile components and the vapor will have been enriched in volatiles. The two phases are separated and are routed on to additional equilibrium stages.

Distillation requires simultaneous fluid flow, heat transfer, and mass transfer. Since the vapor stream is enriched in lower boiling components as it rises in the column, the equilibrium temperature in each succeeding stage will be lower than that of the stage below. Likewise, a pressure profile will exit across the column from the lowest pressure at the top of the highest at the bottom. A pressure drop across each stage results from the countercurrent contact between vapor and liquid

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streams and obstructions with in the contacting stage. The top stage in a distillation column will be at the lowest temperature and pressure of any stage and will be richest in light components.

Referring again to Fig. (a) below, the steam leaving the top and bottom stages, V l

and VN are the two product streams. The top product stream is usually condensed and recovered as a liquid. The top and bottom product streams are called the distillate and bottoms, respectively. The mixture to be separated can be fed to the cascade at either the top, the bottom, or any stage in between. Fig. (a), (b) & (c) illustrate these three feed locations.

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229

Bottoms (B)

(c)

Feed

L1 V2

Stage N

Stage F

(D)

Distillate Lo

(Reflux)

Vent

L1 V2

V1

LN Vo

Condenser

Base heater

Stage 1

Stage F - 1

Stripping Section

Enriching Section

Distillate Lo

(Reflux)

(d)

Bottoms(B)

(b)

Vent

L1 V2

V1

LN Vo

Feed

Condenser

Base heater

Enriching SectionStage 1

Stage N

Distillate

Feed L0

(a)

L1 V2

(D)

Bottoms(B)

V1

LN Vo

Vent

Condenser

Base heater

Stripping Section

Stage 1

Stage N

V3

VN-1

LN V0

LN-1 VN

L1 V2

Lo V1

L2

LN-2

Stage 1

Stage N

Stage N - 1

Stage 2

14.2Vacuum Distillation

The principle of vapor-liquid equilibrium forms the basis for distillation. When a series of successive vaporizations and condensations is carried out, a mixture of two or more volatile components can be separated until products of a specified purity are obtained. The basic principles of distillation are unchanged by vacuum operation. In vacuum distillation, however, the column condenser is not vented to the atmosphere but to a vacuum pump, which maintains the system, pressure below atmospheric.

High distillations temperatures must be avoided in some process to prevent undesirable reactions, thermal degradation, or the polymerization of process materials. Vacuum distillation is often the only technically feasible means of purifying heat sensitive materials. Most food products and pharmaceutical compounds, for example, are processed under vacuum. The material being vaporized in the column base heater may be high boiling liquid that can not be heated to its bubble point except under reduced pressure. In other applications, vacuum distillation is used so that less expensive, low-pressure steam can be employed as the heating medium. Low-temperature vacuum operation frequently has the additional advantage of increasing mixture separability. As a rule, the ratio of vapor pressures for two compounds will increase with decreasing temperature. If the two compounds from an ideal solution, reducing the temperature will increasing relative volatility. For compounds that do not from ideal liquid solutions, the activity coefficient ratio some times decreases at low temperatures. An increase in the vapor pressure ratio, how ever, often overrides this effect, and the relative volatility increases. The equilibrium diagram shown in Fig. 1 illustrates the effect of vacuum on the phase equilibria of many binary mixtures. The equilibrium curve is a plot of y1 as a

function of x1. Since K = , the more the curve is bowed away from the diagonal

(y = x), the higher the light component volatility (K) and the easier the separation. When vacuum operation facilities the separation of a mixture by distillation, operating or capital costs may be reduced by lowering the required reflux ratio or reducing the number of equilibrium stages required at higher pressure.

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Fig. 14.1b. Elimination of an azeotrope by vacuum

operation

X1

Y1

Vacuum

Atmospheric Pressure

Fig. 14.1a. Typical Equilibrium Diagram

The most frequent drawback of vacuum distillation is probably the need for larger column diameters. Larger diameter columns are required because vapors are less dense at low pressures. An important point that is often overlooked, however, is the effect of improving the separation by reducing the column temperatures. Vacuum distillation can often be done at substantially lower reflux ratios so that the vapor and liquid flows within the column are much smaller. In such cases, the effect of decreased vapor densities can be partially or totally overcome.

Some highly non-ideal binary mixtures have equilibrium curves similar to those shown in Fig.14.1b. The equilibrium curve at atmospheric pressure crosses the diagonal, indicating an azeotrope. At this point of intersection, the equilibrium compositions of both liquid and vapor are identical. Further enrichment of the vapor phase beyond this azeotrope composition is impossible by conventional distillation techniques. Vacuum distillation, how ever, allows further enrichment since no azeotrope is formed. Vacuum is used to “break the azeotrope”. Vacuum distillation can be used to advantage in a wide range of other applications. When toxic materials are distilled, vacuum operation may be required for safety reasons.

Example:

A mixture of 90 mole percent acetone and 10 mole percent water has a double point at 760 torr of 57.60C. the liquid phase activity coefficients at this composition and temperature are 0.99 for acetone and 3.7 for water. The same mixture at 200 torr has a bubble point of 22.80C and activity coefficients of 1.0 and 5.3. Would operation at 200 torr aid in the separation of this mixture?

Data

Solution 1. 760 torr. From eqn. 5.8

2. 200 torr

Note: The separation is improved at this concentration even through lowering the temperature lowered the ratio of liquid phase activity coefficients. If additional equilibrium stages are required, the effect of vacuum operation should be checked

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for all compositions concerned. Operation of a distillation column at a top temperature of 230C or less would require a refrigerated condenser in many localities. The optimum column pressure probably lies some where between 200 and 760 torr.

Some highly nonideal binary mixtures have equilibrium curves similar to those shown in Fig. 14.1b. the equilibrium curve at atmospheric pressure crosses the diagonal, indicating an azeotrope. At this point of intersection, the equilibrium compositions of both liquid and vapor are identical. Further enrichment of the vapor phase beyond this azeotrope composition is impossible by conventional distillation techniques. Vacuum distillation, how ever, allows further enrichment since no azeotrope is formed. Vacuum is used to “break the azeotrope”. Vacuum distillation can be used to advantage in a wide range of other applications. When toxic materials are distilled, vacuum operation may be required for safety reasons.

Vacuum Distillation Equipment:

Fig. 14.2 illustrates a typical layout for a vacuum distillation system. This wide assortment of equipment must be designed to work together in order to carry put an efficient separation of the feed mixture. A presentation of the design procedures and equations for distillation equipment is not with in the scope of this text. We will, however, undertake a short overview of equipment types.

232(a) Tray column (b) Packed column14.2 Equipment layout for vacuum distillation

The distillation Column:

The distillation column has been previously described as a vertical arrangement of several successive equilibrium stages. The manner in which vapor and liquid streams are contacted to from an equilibrium stages varies according to the particular design. The contact stages can be either discrete or continuous. A column made up of discrete stages is called a plate or tray column, while continuous contact is accomplished in a packed column. Figure 14.2(b) illustrates the differences in vapor and liquid paths for each design. Vapor and liquid flows generally follow two separate paths in the tray column. Both vapor and liquid flow countercurrently through the same passages in a packed column. The discrete trays with in a tray column cause the vapor liquid streams to be combined and then separated in multiple contact stages. The trays have openings that allow the vapor bellow to pass through the trays. While the liquid typically flows in a layer across each tray and is conveyed to the tray below through downcomers. The vapor bubbling up through the liquid layer on each tray forms a forth, creating surface area for mass transfer between the two phases. The pressure drop for a vacuum column is typically 3 to 5 torr per tray. Higher pressure drop produces more turbulence and thus higher mass transfer rates in the forth or contact zone.

The liquid and vapor streams do not usually reach equilibrium on a single tray. Partial entrainment, bypassing, and insufficient contacting will limit ray efficiencies to less than100 percent.

The vacuum pump:

Vacuum pumps are sized according to both suction pressure and capacity requirements. Thermal degradation concerns will frequently dictate the maximum permissible operating pressure. In the absence of this limitation, the suction pressure of vacuum pumps serving distillation columns is usually set by an economic optimization. The benefits of lower distillation temperatures are weighed against increasing capital costs until an optimum pressure level is selected. The required pump capacity is usually the sum of steady-state air leakage and vapor flow rates, but evacuation loads must be considered in batch distillations. Air leakage can occur at any point under vacuum, in e column, reboiler, condensers and connecting lines.

14.3Molecular Distillation

Some heat sensitive commercial products have such a high molecular weight that conventional rough-vacuum distillation techniques cannot bee used as a means of separation. Because of their low volatility, these products begin to decompose before they can be heated to the temperatures required for rough vacuum

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distillation. Some compounds may exert a vapor pressure of only a fraction of a torr at temperatures as high as 3000C. These products must be distilled at comparatively high vacuums to maintain moderate temperatures, and exposure to heating must be minimized. The device that meets this need is the molecular still. Molecular stills are used on an industrial scale to purify and separate high-boiling materials such as vitamins, oils, waxes, fatty acids, glycerides, and plasticizers. Industrial molecular distillation is generally regarded as a major vacuum processing operation. It is, but molecular distillation is obviously only a small segment of commercial vacuum distillation when compared with the massive rough-vacuum operations that dominate the processing industries.

As methods for producing better vacuum were developed, early laboratory work sought to apply these high vacuum to conventional lab stills in an effort to further reduce distillation temperatures. Initial efforts were unsuccessful. Pressures below about 5 torr seemed to provide little change in the boiling point of mixtures distilled in the ordinary flask-condenser-receiver arrangement. In reality, the high vacuum measured in the receiver were not present in the distilling flask because of flow restrictions that were present. The normal mode of viscous gas flow was changing to molecular flow. The conductance of the glassware and tubing that had been ample before was now limiting the obtainable distillation pressure. Molecular distillation requires operating pressures between 0.1 and 10 microns (10-4 and 10-2

torr) and a short, unobstructed path between the evaporator and condenser surfaces. The total separation between these two surfaces is on the order of the mean free path of the distilling molecules at these pressures.

Molecular distillation is actually a special case of evaporative distillation. In contrast to conventional distillation, evaporative distillation is carried out below the boiling point of the evaporating liquid. Evaporation is usually slow because most liquid molecules that obtain sufficient energy to leave the liquid surface collide with the surrounding gas molecules and return. In a closed vessel, the net evaporation is zero once the partial pressure of the liquid component equals its vapor pressure. In an open vessel, gas movements carry away some escaping molecules and evaporation continues until the liquid is consumed. Evaporation can proceed more quickly in a molecular still because the residua gas pressure is so low as to make the escape of molecules from the liquid much easier. If the condensing surface is placed within the mean free path of the evaporating molecules, a molecule is more likely to collide with the condenser than with another molecule.

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Industrial Equipment: Typical operating pressures for industrial molecular stills range between 3 and 30 microns. The gap between the evaporator and condenser surfaces is often widened on commercial stills to reduce radiation losses and thus improve thermal efficiency. The combination of these changes produces a gap that may be 5 to 10 times the mean free path of evaporating molecules. Technically, such conditions are not true molecular distillation but short- path or unobstructed-path distillation. Short-path distillation is often termed molecular distillation because it has the same characteristics of high evaporation coefficients and low thermal hazard to the distilled products.

Falling-film molecular stills such as the one shown in Fig.14.3 were common in the 1930s. The material to be distilled is distributed over a central column and allowed to flow down the sides. The column is heated from the inside so that the distilland is partially vaporized, the residue being collected at the bottom and removed. The evaporated molecules travel across a small annular space and reach the cool vessel wall, which serves as the condenser. The distillate flows down the outer wall and is removed. The wall can be cooled by the surrounding air or wrapped with external cooling coils. Gravity spreads the distilland in a thin film, giving the surface renewal needed for efficient molecular distillation.

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Fig.14.3 Falling film molecular still

The capacity of a failing-film still is limited by the surface area of the heated column. This area cannot be increased beyond certain limits because tall columns give long exposure times and large diameters result in maldistribution of the distilland. The most significant improvement in still capacity was made possible by the addition of moving wipers to distribute and accelerate the failing film.

The centrifugal molecular still (Fig. 14.4) was first made around 1935, and by the end of World War II it dominated industrial applications. The distilland is fed to the center of a, rapidly spinning evaporator so that the feed is spread across the evaporator cone and the residue flows over the sides to a collection gutter. This produces an extremely thin, well-distributed liquid film. The centrifugal action quickly moves the liquid across the evaporator surface, minimizing thermal exposure times. Smaller stills use the vessel dome to condense the distillate; larger stills employ stationary water-cooled condensers placed inside the rotating evaporator cone.

A 5-ft centrifugal molecular still with a capacity of 2000 lb of feed per hour marketed by D.P.l. (Distillation Products, Inc., a subsidiary of Eastman Kodak) was practically the only choice for large-scale molecular distillation for some time. Centrifugal stills, as well as other molecular still designs, are now available from a number of companies.

Because of the relatively poor separation efficiency of a single molecular still, some attempts have been made at providing fractionation or multiple stages within a still. In most industrial applications, however, product purity is improved either by multiple passes through the same still or by feeding to the center of a series of counter currently cascaded stills.

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Fig.14.4 Centrifugal molecular still

Degassing:

Degassing is similar to distillation--components of differing volatilities are separated in both operations. In degassing operations, however, the volatile components are dissolved gases and the high-boiling component is the liquid solvent. Small quantities of volatiles are removed overhead, and the desired product is removed underneath. The product need not be heated to its bubble point. Dissolved gases are removed by changing their solubility in the product. This primarily accomplished in vacuum degassers by reducing the pressure. Vacuum degassing is widely used to prevent corrosion in equipment sensitive to the presence of gases like oxygen and carbon dioxide. The feed water to many industrial boilers undergoes vacuum deaeration to reduce the levels of oxygen and carbon dioxide and prevent corrosion with in the boiler. Dissolved oxygen is usually the controlling factor in the corrosion of iron in water.

Vacuum degassers are also used in conjunction with molecular stills and other operations in order to remove dissolved gasses under a rough vacuum before subjecting the feed to a higher vacuum. Because the pimping of dissolved gasses is less expensive at higher pressures, the feed to many high vacuum operations is degassed in several stages, each stage at a lower pressure than the one before.

Pervaporation:

Pervaporation is a membrane process which differs from all other membrane process because of the permeate phase change from liquid to vapour. The fig. 14.5 shows the principle of the process. The feed mixture is a liquid. The driving force in the membrane is achieved by lowering the activity of the permeating components at the permeate side. Components in the mixture permeate through the membrane and evaporate as a result of the partial pressure on the permeate side being lower than saturation vapour pressure. As shown in the fig. 14.5, the driving force is generally controlled by applying vacuum.

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Pervaporation is expensive, owing to the phase change, which is inextricably linked with the material transport across the membrane. Because of its high selectivity however, pervaporation is of interest in cases where conventional separation processes either fail or result in a high specific energy consumption and/or high investment costs.

The most important category of separation problems for which pervaporation is promising are mixtures with an azeotrope and/or small differences in boiling characteristics.

14.4 Vacuum Freeze Drying

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Fig. 14.5 Principle of pervaporation

Freeze drying is used extensively in processing certain food stuffs, pharmaceuticals and biological materials, which get damaged if dried by heating to even moderate temperatures. Typical examples are vegetables, seafood, coffee, concentrated beverages, and fruit juices. The product of freeze-drying is a stable solid that can be reconstituted by simple addition of water. The substance to be dried is usually frozen by exposure to very cold air. In freeze drying the water is removed as a vapor by sublimation from the frozen material in a vacuum chamber. After the moisture sublimates to vapor, it is removed by mechanical vacuum pumps or steam jet ejectors.

Process steps in freeze-drying: The objective of freeze-drying is the removal of water from a solid. The process involves three distinct steps: freezing, sublimation and desorption. Product materials are introduced into the vacuum dryer as frozen solids. If freezing is the initial step of the operation, it is normally accomplished by placing the product in trays on refrigerated shelves inside a controlled environmental chamber. Vacuum chilling is usually not a viable alternative as it results in the collapse of the cellular structure and additional damage that can inhibit reconstitution.

Sublimation, the second step in the process, is subject to both mass-transfer and heat-transfer limitations, and both chamber pressure and shelf temperatures are important process variables. Lower pressures increase drying rates at the expense of increased costs for refrigeration and the vacuum pumping system. Economics will normally dictate pressures in the range 0.02 to 0.1 Torr for the production of pharmaceuticals and biologicals used in medical research. Freeze dryers used for food preservation normally operate in the range 0.25 to 0.75 Torr. The production of freeze-dried coffee and freeze-dried orange juice requires pressures in the range 0.1 to 0.2 Torr, the melting point of ice at the dryer operating pressures determines permissible operating temperatures. Dryer temperatures span the range 200K to 374K. In most commercial applications, sublimation is controlled by a combination of conduction and radiation.

The final step of the process, desorption, follows sublimation of the ice crystals. The moisture remaining in the product following sublimation is known as “bound moisture”. The bulk of the moisture remaining in the solid is chemically or physically adsorbed on solid surfaces. Bound moisture is removed by gradually increasing the temperatures from below freezing (typically 222-245K) to 310-374 K. bound moisture normally constitutes less than 10% of the total moisture removed from the solid, but desorption is much slower than sublimation, and it will normally require 20 to 30 % of total drying time.

Vacuum Pumping Systems for Freeze Drying:

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Depending on the drying requirement, various types of freeze drying systems are used commercially. They are batch, multi-cabinet and tunnel freeze dryers. All these freeze dryers in general operate in the range 0.02 to 0.75 Torr. This requirement dictates the design of the vacuum pumping system. Four and five stage steam jets have been used for freeze drying applications, particularly those that require high capacity at low operating pressures. But rotary-piston oil-sealed pumps and rotary blowers dominate process applications. Vacuum pumps are sized on the basis of shelf area. For a rough estimate, 2.5 CF/min per one square foot of shelf area can be assumed considering that a rotary-piston oil-sealed pump will be used for application in the range 0.1 to 0.75 Torr and a “rotary-blower-rotary-piston” oil-sealed combination system will be used for applications in the range 0.02 to 0.1 Torr. Filtration:

Vacuum filters are used extensively in the process industries in liquid-solids separation operations involving chemicals, pharmaceuticals, food, beverages building materials, and other products. A solid can be separated from a liquid by pulling a vacuum across a filter medium that passes the liquid but retains the solid. Cake filtration involves the measurable accumulation of solids on the surface of the filter medium. A requirement for cake filtration is the formation of a porous cake, that is, a cake that will allow passage of the liquid as filtration progresses. The process objective of cake filtration is usually either solids recovery or liquid clarification, but cake filters are some times used to recover both the filter cake and the filtrate. Vacuum filters dominate cake filtration operations because they can be designed for continuous operation and because capital costs for a vacuum filter are normally considerably less than capital costs for a centrifuge. Vacuum filters are limited to a maximum pressure differential of 1 atm, and performance in handling high-viscosity liquids in markedly inferior to the performance of a centrifuge or a pressure filter.

Vacuum Pumps for Process Applications:

Vacuum pumps are sized according to both suction pressure and capacity requirements. Thermal degradation concerns will frequently dictate the maximum permissible operating pressure. In the absence of this limitation, the suction pressure of vacuum pumps serving distillation columns is usually set by an economic optimization. The benefits of lower distillation temperatures are weighed against increasing capital costs until an optimum pressure level is selected. The required pump capacity is usually the sum of steady-state air leakage and vapor flow rates, but evacuation loads must be considered in batch distillations. Air leakage can occur at any point under vacuum, in e column, reboiler, condensers and connecting lines.

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Because of the low pressures required for molecular distillation, vacuum is provided by a combination of several vacuum pumps working in series. The required ultimate pressure and capacity of the vacuum pumps can vary considerably with each application. Air leakage is minimized by the tight design of molecular stills. The primary load to the vacuum pump in most molecular stills is made up of air and volatile vapors that degas from the distilland and are not condensed. In nearly all processes, the feed is degassed in a separate chamber before entering the molecular still. The required suction pressure in commercial operations can vary between 1 and 50 microns depending on the need for true molecular distillation. The high-vacuum pumps used with most molecular stills are the oil diffusion pump and the oil booster pump.

Vacuum Gauge Selection: The two most important considerations in vacuum gauge selection are:

1. The range of pressure to be measured 2. The nature of the gases present in the system

Naturally, one should choose a gauge that can accurately measure across the range of expected system pressures. The range of vacuum gauges commonly used in process vacuum systems are shown in fig. 23. The second consideration refers to the way in which the gauge measures pressure. For instance, the McLeod gauge, which measures only the pressure exerted by noncondensable gases, would not be well suited for use in a process system where large amounts of condensable vapors are released. Likewise, a gauge that gives a pressure reading that is composition-dependent should not be used in a general-purpose central vacuum system.

241Fig 14.6 : Pressure ranges of common process vacuum gauges

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