Gas-Solid Separations

20

GAS-SOLID SEPARATIONS his chapter consists of two subsections: a section about gas-solid separations and a section in which a variety of other topics of interest in chemical processing are discussed. The subjects in this latter section do not readily fit into categories in other chapters but are nevertheless valuable in applications in the

T

chemical process industries. The objective here is to describe the principles involved, to point out the main applications, and to refer to sources of more information. Equipment manufacturers mentioned in this chapter can be identified in the Thomas Register and in the Chemical Engineering Buyers’ Guide.

20.1. GAS-SOLID SEPARATIONS

force on the smaller ones because the latter particles have less mass and therefore spiral downward into the dust hopper. When the gas velocity is not sufficient to suspend the particles, gravity causes the particles to fall into the dust collection chamber. Further, a smalldiameter cyclone generates a greater centrifugal force than a large diameter unit. To obtain the same performance, a design engineer has the choice of either a smaller diameter and a short chamber or a larger diameter with a longer barrel chamber. Because the tapered section of the cone is smaller than the main chamber, higher velocities are encountered, which may reentrain the finer dust particles and these then may be swept to the outlet of the cyclone. Figure 20.1(a) shows the vortex pattern in a cyclone separator. Vatavuk (1990) pointed out that a key dimension in the sizing of a cyclone is the inlet area. Properly designed cyclones can remove nearly every particle in the 20–30 micron range. Typically, cyclone separators have efficiencies in the range of 70–90%. Because of the relatively low efficiency of these units, they are often used as a first stage of dust collection, and are referred to as primary collectors. Typical cyclone dimension ratios are indicated in Figure 20.1(b). Inlet velocities should be in the range of 100–150 ft/sec, but may be limited by the occurrence of reentrainment of dust particles or by an unacceptable pressure drop. The pressure drop is estimated in terms of velocity heads, a value of 4 being commonly used. Equations (18.24) and (18.25) (shown here again) are expressions for the pressure drop.

The removal of solids from a gas or air stream is of great industrial importance. This is especially true in the last two or three decades with the increased requirements for effective solids removal from solid-laden streams mandated by law. In addition to environmental control requirements, there are health considerations not only in the workplace but also in the environment surrounding the plant site. Companies have an interest in being good corporate citizens by controlling various emissions from the plant. Dust collection is the process of removal and collection of solids in a gas phase. Its purpose is to 1. Control air pollution from various industrial plants 2. Eliminate safety and health hazards from the workplace in which grinding, milling, and packaging operations take place 3. Recover valuable products from dryers, conveyors, bagging equipment, and so on, for recycling back into a process 4. Reduce equipment maintenance on rotating equipment caused by dusts Newer, more effective control equipment has led to more efficient designs and simultaneously to lower operating expenses. This section describes various dust collection equipment, ranging from the simple to the more sophisticated. Design equations for this equipment are proprietary; when contacting equipment manufacturers, they will require certain information so that they can design or recommend the proper equipment applications. There are four broad groups of gas-solid separation equipment:

ΔP = 4ρV2 =2g = 4:313 ρðft=sec=100Þ2 psi And for atmospheric air ΔP = 0:323ðft=sec=100Þ2 psi

1. Cyclone and inertial separators for removal of large solid particles 2. Baghouse collectors for removal of intermediate-sized particles 3. Wet scrubbers employing liquid sprays to entrap solid particles 4. Electrostatic precipitators to collect fine particles

(18.25)

The size of the inlet is selected at a specific inlet velocity and required volumetric rate; the other dimensions then are fixed. Capacity and efficiency of the cyclone depend on the inlet velocity and dimensions of the vessel. Correlated studies have been made with a rectangular inlet whose width is D/4 and whose height is 2–3 times the width. A key concept is the critical particle diameter which is one that is removed to the extent of 50%. As shown in Chapter 18, the critical particle diameter is given by Equation (18.26).

There are subgroups under each of these four categories and they will be considered where appropriate. CYCLONE AND INERTIAL SEPARATORS

ðDp Þcrit = ½9μD=4πNt Vðρ − ρg Þ0:5

Cyclone Separators. The most commonly used equipment for the separation of dust particles from an air/gas stream is the cyclone separator. The literature on design and operation of cyclones has been extensively reviewed by Rietemer and Vetver (1961), Maas (1979), Zenz (1982), and Pell and Dunson (1999). A sketch of a cyclone separator and typical dimensional ratios is found in Figure 20.1(b). The dust-laden gas stream enters near the top of the collection chamber tangentially. The force on the larger particles is greater than the

(18.26)

where D = diameter of the vessel, ft V = inlet linear velocity, ft/sec Nt = number of turns made by the gas in the vessel Zenz (1982) presented a graphical correlation that can be represented by Equation (18.27).

709 Copyright © 2012 Elsevier Inc. All rights reserved. DOI: 10.1016/B978-0-12-396959-0.00020-3

(18.24)

710 GAS-SOLID SEPARATIONS AIR OUTLET

DUST LADEN AIR INLET

EDDY AIR OUTLET INLET DUST LAGEN AIR

VORTEX CORE MAIN VORTEX

(a)

(d)

DUST OUTLET

DUST OUTLET

Figure 20.1a. Cyclone separators. (a) Vortex pattern in a cyclone separator. (d) Multiclone separator. (Courtesy of Scientific Dust Collectors, 2002). Nt = ½0:1079 − 0:00077 V + 1:924ð10−6 ÞV2 V

DB W

D0

with V in ft/sec. With a height opening equal to 2.5 times the width, the volumetric rate is (18.28)

Q = AV = 2:5 D2 V=16 Skirt

A

(18.27)

S

These relations are used in Example 20.1 to determine the size of a separator corresponding to a specified critical particle diameter. Figure 20.1(c) is a plot of the percent removal of particles in a cyclone as a function of their diameters relative to the critical particle diameter given by Equations (18.26) and (18.27).

HC H

Multiclones. A multiclone separator consists of a number of small cyclones arranged in parallel in a chamber to handle large volumes of dust-laden air. They are capable of having very high particle removal efficiencies. Vatavuk (1990) reported that multiclones might have efficiencies up to 80% on 5-micron particles. Figure 20.1(d) is a sketch of a multicone separator. Cyclone dimensional proportions for Figure 20.1(b) are:

B

Figure 20.1b. Typical dimension ratios of a cyclone separator.

W = DB =4

S = 2DB + ðDB =8Þ

Do = DB =2

H = 2DB + 2DB = 4DB

A = DB =2

B = usually DB =4

HC = 2DB

20.1. GAS-SOLID SEPARATIONS

is diverted, changing direction, therefore slowing the air velocity so that the air stream cannot support the particles. The dust impinges on the louvers, falls into the lower chamber, and is discharged at the bottom of the collector. The “clean” gas leaves at the top of the unit. These collectors are used in applications where the inlet dust loads are small, usually less than 0.5 grain per cubic foot. Heavier loads quickly plug the equipment. As one might expect, this class of gas-solid separator is not as efficient and will not perform as well as baghouses, wet scrubbers, and the like. There are other types of inertial separators. One such design consists of a fan-type unit that has specially designed blades and housing for handling what might be abrasive dusts. Due to the design, a fan accelerates the dust entering the unit and throws the particles against the housing of the inertial separator. This type unit is often installed on vents from grinding operations (Scientific Dust Collectors, 2002).

10 8 6 4

2 Dp /(Dp)critical

711

1 0.8 0.6 0.4

BAGHOUSE COLLECTORS 0.2

2 5

10

20

40

60

80

90 95 98

99.0 99.5 99.8

0.1

Percent removed

Figure 20.1c. Percent removal of particles in a cyclone as a function of their diameters relative to the critical diameter given by Equations (18.26) and (18.27) (Zenz, 1982). Inertial Collectors. Inertial separator designs consist of a louver or baffle device mounted in a plenum chamber, as shown in Figure 20.2. Dust-laden air enters beneath the louvers and the flow

EXAMPLE 20.1 Size and Capacity of Cyclone Separators Air at 1000 cuft/sec and density of 0.075 lb/cuft contains particles with density 75 lb/cuft. 50% of the 10 µm diameter particles are to be recovered. Find the sizes and numbers of cyclones needed with inlet velocities in the range of 50–150 ft/sec. The inlet is rectangular with width D/4 and height 2.5D/4, where D is the diameter of the vessel. Equation (18.26) becomes  2 4πðρ − ρg ÞD2p 4πð75 − 0:075Þ D 10 = = = 0:00876, 9μ Nt V 9ð1:285Þð10−5 Þ 304, 800 where Nt is given by Eq. (18.16). The number of vessels in parallel is n=

Q′ 100 6400 = 2 : = ð2:5=16ÞD2 V DV AV

Industrial processes for which noncleanable filters are not applicable may emit large quantities of dust. The capture of such dusts requires a cleanable filter and this is an application for baghouses. A typical baghouse design is shown in Figure 20.3(a). The baghouse consists of a number of filter bags attached at the top of the bag to a shaker arm enclosed in a rectangular chamber. Most collectors of this type have a device to hold the bags in tension. Dust-laden air enters near the bottom of the chamber and flows inside the bag. The dust is trapped on the inside of the bag and the clean air flows through the bag and exits at the top of the chamber. Periodically, when the pressure drop rises to a predetermined value, a shaker device is activated, loosening the dust that falls into the dust hopper at the bottom of the chamber. The shakers are operated automatically and the frequency of operation is important. The more frequent the shaking operation, the more wear and tear on the bags.

From Figure 18.11, the percentage recoveries of other-sized particles are: Dp /(Dp)crit

% Recovered

0.3 0.5 0.6 1 2 6 9

9 22 30 50 70 90 98.5

When the smallest of these cyclones, 1.62 ft dia, is operated at 150 cuft/sec, Nt = 5:35  0:5 9ð1:285Þð10−5 Þð1:62Þ ðDp Þcrit = 4πð5:35Þð150Þð75 − 0:075Þ

The results at several velocities are summarized. V (cfs)

Nt

D (ft)

n

50 100 144

3.71 5.01 5.32

1.62 4.39 6.71

48.8 3.32 1.0

= 1:574ð10−5 Þft, 4:80 μm:

712 GAS-SOLID SEPARATIONS Cleaned-gas outlet

Dust-laden gas inlet

Dust outlet

Figure 20.2. Louver dust collector. (Courtesy of Scientific Dust Collectors, 2002). Another type bag collector is designed so that the dust collects on the outside of an envelope bag. The bags are attached to a shaker device mounted near the bottom of the unit. In this design, the dustladen air enters near the bottom of the unit; the air passes through the bag and exits at the top of the collector. Figure 20.3(b) is a diagram of the envelope collector. A typical wire retainer device for holding the bags vertically and preventing them from collapsing under high air pressure is shown in Figure 20.3(c). When selecting fabric materials for baghouses, the following criteria must be considered: Temperature of the process air stream Electrostatic characteristics of the dust Abrasiveness of the dust Moisture in the air and collector Hygroscopic nature of the dust Acid or alkali chemical resistance Ease of disengaging the dust from the filter material Size of the dust particles Permeability of the fabric so that only air will pass through the filter bag Cost of the material

Fan-Pulsed Dust Collector. The next advance in the development of dust collection equipment was to be able to clean the filter bags continuously. The fabric filter tubes are arranged in a radial fashion in a cylindrical housing, as shown in Figure 20.4. A rotating arm has a traveling manifold through which air is supplied by a fan mounted outside of the shell of the chamber. Reverse air is admitted through the arm as it travels over the filter element openings, blocking the airflow to adjacent elements in the cleaning step. Pulsed Jet Baghouse Collector. Another type of continuous cleaning collector of the pulsed jet type is also known as the blow ring collector. The dust-laden air enters the unit in a manifold at the top of the collector and the air flows from inside the cloth tube through the media to the clean air outlet at the bottom of the collector. The dust collects on the inside of the cloth tubes and a blow ring travels up and down on the outside of the bag. Jets of air are emitted by the blow ring and pass through the fabric, dislodging the dust inside the bag that falls to the dust hopper below. Figure 20.5 is a diagram of the pulsed jet baghouse collector with a blow ring. Although pulsed jet baghouse collectors operate at low pressure and can accommodate a wide range of dust loadings, they are not suitable for high temperatures and in corrosive environments. The main disadvantages are the abrasion of the bags by the traveling jet ring and the high maintenance of the blow rings. All bag collectors discussed in this section require regular inspection. The bags or filter elements should be inspected for Coating of dust that cannot be removed by cleaning Deposit of moist material on one side of the filter element Hardness of the coating for evidence of condensation

SHAKER ARM SHAKER MOTOR

AIR OUTLET

FILTER BAGS, DUST CAKE ON INSIDE OF BAG

The bags are made of the following materials (Scientific Dust Collectors, 2002):

DUST LADEN AIR INLET

Polyester—the standard and most commonly used material Polypropylene—for superior chemical resistance Fiberglass—for high temperatures and in acid and alkaline conditions Aramid—for high temperature applications Polytetrafluoroethylene—used to capture fine particles where an artificial dust cake is required DUST HOPPER

Shaker filters have the drawback that they must be taken off stream to clean the bags. Since continuous on stream operation is required, several baghouse chambers are installed in parallel with dampers, permitting one section to be in the cleaning cycle while the rest of the baghouse is filtering the dust-laden gas stream. Baghouses are high-maintenance items due to internal movement in a dust-laden stream. They operate at low air-to-cloth ratios and the collectors are large and more costly than some other devices described in this section.

(a)

Figure 20.3. Baghouse collectors. (a) Tubular shaker collector. (b) Baghouse collector with envelope filter bags. (Courtesy of Scientific Dust Collectors, 2002). (c) Bag wire retainers.

20.1. GAS-SOLID SEPARATIONS

713

Collars OUTLET

Induced flow

FAN MOTOR

Wire retainers

ENVELOPE FILTERS SHAKER MOTOR

Filter bags

AUTOMATIC CONTROLS SHAKER ARM DUST LADEN AIR

DUST HOPPER Collector housing

(b)

(c)

Figure 20.3.—(continued )

REVERSE AIR PRESSURE BLOWER DRIVE MOTOR

OUTER ROW REVERSE AIR INNER ROW REVERSE AIR

Thickness of the coating along the length of the filter element Color of the coating compared to the color of the dust Condition of the filter element, openings of the weave, tears, and wear due to flexing of the material Further, baghouse interiors should be inspected for buildup of powder on the walls of the housing and in the dust hopper.

AIR AIR

OUTLET

OUTLET

FABRIC FILTER TUBES

DUST-LADEN AIR

PARTICLE DEFLECTOR

INLET BLOWER

HOPPER

DUST LADEN AIR INLET

Figure 20.4. Fan-pulsed dust collector. (Courtesy of Scientific Dust Collectors, 2002).

BLOW RING MAKES CONTACT WITH CLOTH TUBE

JET DUST HOPPER

Figure 20.5. Pulsed jet baghouse showing blow ring. (Courtesy of Scientific Dust Collectors, 2002).

714 GAS-SOLID SEPARATIONS A disadvantage of cyclones and inertial collectors is that dust particles are frequently swept back into the exiting air stream. Wet scrubbers were designed to overcome this disadvantage. WET SCRUBBERS Wet Cyclone Scrubbers. This air washer is a variation on the dry cyclone separator. The wet collector is equipped with spray nozzles that atomize water. Dust-laden air enters tangentially near the bottom of the unit spirally upward into the water spray. Figure 20.6(a) is a sketch of this equipment showing a manifold equipped with spray nozzles. Plugging of the nozzles is a high-maintenance item. In one design, the spray nozzles are mounted in the wall of the collector, spraying water inward into the dust-laden air, making the nozzles more accessible for maintenance. A baffle or impingement device is often installed in the center of the chamber to break up the swirling air-water-dust stream. Spray Scrubbers. Spray scrubbers consist of an empty cylindrical chamber in which dust-laden air is contacted with water from spray nozzles, as shown in Figure 20.6(b). The dust-laden air enters the tower near the bottom and passes upward through the water spray. This type of equipment is similar to the spray towers used in mass transfer operations. Proper water distribution can be a problem, so multiple banks of spray nozzles mounted on a manifold produce better air-water contact. The spray knocks down the dust that leaves the bottom of the unit as a dust-water mixture. An entrainment separator is mounted in the upper part of the chamber to reduce the potential spray carryover into the exiting gas stream.

CLEAN

Venturi Scrubbers. Venturi scrubbers are used to separate air streams from solids that are noxious, hazardous, or explosive. The exiting liquid stream, usually a solid-water suspension or a slurry, may be returned to the process for recovery. This type scrubber operates typically at high air velocities between 15,000 and 20,000 ft/min, causing high shear stresses forming very fine water droplets (Bonn, 1963). Water is added in the range of 5 gallons/1,000 cfm in the venturi throat (Scientific Dust Collectors, 2002). The water droplets cause the collection of fine dust particles that may be recovered as a suspension or slurry. Near the exit of the scrubber, a mist eliminator of the inertial or cyclone type is essential to separate the mist from the exiting air stream by changing direction of the airflow. Figure 20.6(c) is a sketch of a venturi scrubber. Any surface that was not wet would form a mud, causing frequent cleaning of the collector interior. In order to have a scrubber operating efficiently, the velocity in the scrubber has to be such as to drive the dust particles into the water. Venturi scrubbers have efficiencies in the range of 90–95% compared to dry cyclones in the range of 70–90%. Orifice Scrubbers. This type scrubber is essentially an inertial trap in which air impinges against a water-wet surface. In the unit, large water droplets are formed using large quantities of water. The collision of the air with the water causes wetting of the dust and the droplets are separated from the air by changing the flow direction, sometimes two or more direction changes, before the air leaves the unit, as in Figure 20.6(d). This design has considerable CLEAN

AIR OUTLET

AIR OUTLET

MIST ELIMINATOR SPRAY HEADER WATER IN BAFFLE

SPRAY MANIFOLD

DUST

DUST

LADEN AIR

LADEN AIR

WATER IN

WATER AND SOLIDS OUT

(a)

WATER AND SOLIDS OUT

(b)

Figure 20.6. Wet scrubbers. (a) Wet cyclone scrubber. (b) Spray scrubber. (c) Venturi scrubber. (d) Orifice scrubber.

20.1. GAS-SOLID SEPARATIONS

CLEAN

AIR OUTLET

MIST ELIMINATOR

WATER

IN

(c)

DUST LADEN AIR IN

VENTURI

WATER AND SOLIDS OUT CLEAN AIR OUTLET

AIR OUT

(d)

DUST LADEN AIR INLET

WATER

Figure 20.6. —(continued )

715

716 GAS-SOLID SEPARATIONS appeal since there is an absence of ledges, moving parts, and restricted passages, making the unit easier to clean. Wet Dynamic Scrubbers. These scrubbers are also known as mechanical scrubbers, as seen in Figure 20.7. They have a powerdriven rotor to produce a spray that is centered in the inlet of the unit such that the blades of the rotor are coated with water. As the dust-laden stream enters, it contacts the water surfaces

AIR OUTLET

WATER IN THROUGH SPRAY NOZZLE

and the dust-water mixture is thrown outward against the walls. An entrainment separator is attached to the scrubber near the exit to prevent spray carryover. This design is limited in the dust loading because the wear on the rotor blades is high due to the solids. Other Types of Wet Scrubbers. Plate towers, like sieve, valve, and bubble cap towers, and packed beds have been used in the past for dust collection but these are all subject to plugging. Comments about Wet Scrubbers. Despite numerous claims “that wetting of dust particles by the scrubbing liquid plays a major role in the collection process, there is no unequivocal evidence that this is the case” (Pell and Dunson, 1999). There have been suggestions that wetting agents in the scrubbing liquid may be beneficial but this is controversial. ELECTROSTATIC PRECIPITATORS

DUST LADEN AIR IN

WATER AND SOLIDS OUTLET

Figure 20.7. Dynamic wet precipitator. (Adapted from Scientific Dust Collectors, 2002).

An electrostatic precipitator is a rectangular chamber enclosing a number of grounded vertical plates that are equally spaced to allow dust-laden air to flow between them, as shown in Figure 20.8. Electrodes at high voltage, between 40,000 and 60,000 volts, are suspended between collector plates. This high voltage causes the gas to ionize and thus dust becomes negatively charged. Some dust particles have a high charge and the forces to attract the particles to the grounded collecting plates will be high. The forces depend on the dielectric characteristics of the dust. The precipitators operate on dust streams of low concentration. In Figure 20.8, several chambers are included in the rectangular chamber, each consisting of electrodes and collection plates. Generally, electrostatic precipitators are high-efficiency units but CABLE FROM RECTIFIER

RAPPERS

SUPPORT FRAME CORONA WIRES

AIR OUTLET DUST−LADEN AIR INLET

HOPPER

SHELL

COLLECTING PLATES

DUST OUTLET

Figure 20.8. Electrostatic precipitator. (Courtesy of Scientific Dust Collectors, 2002).

20.2. FOAM SEPARATION AND FROTH FLOTATION

20.2. FOAM SEPARATION AND FROTH FLOTATION

the efficiency depends on the velocity of the gas stream. The lower the gas velocity, the higher the efficiency, which may be 99%, but at high velocity this figure may drop to 50%. The pressure drop through an electrostatic precipitator is low, on the order of 0.5 in water or less, but in order to maintain good collection efficiency, it is necessary to have a uniform velocity distribution through the unit. There are several electrode designs, such as stretched wires and framed electrodes with points jutting out to rods or flat plates. There is also a tubular design that consists of pipes with electrodes in the center of the pipes. There are two-stage precipitators that have a high voltage zone followed by a zone of lower intermediate voltages. The dust passes through both zones and in order to maintain high efficiency, the air distribution must be even. To remove the dust from the collector, plates are wrapped with an air-powered device during which the electric power to the precipitator is shut off and the dust falls into the hopper. The advantages of an electrostatic precipitator are: 1. 2. 3. 4. 5. 6. 7.

Foams are dispersions of gas in a relatively small amount of liquid. When they are still on the surface from which they were formed, they also are called froths. Bubbles range in size from about 50 µm to several mm. The data of Table 20.1 show densities of water/air foams to range from 0.8 to 24 g/L. Some dissolved or finely divided substances may concentrate on the bubble surfaces. Beer froth, for instance, has been found to contain 73% protein and 10% water. Surface active substances attach themselves to dissolved materials and accumulate in the bubbles whose formation they facilitate and stabilize. Foam separation is most effective for removal of small contents of dissolved impurities. In the treatment of waste waters for instance, impurities may be reduced from a content measured in parts per million to one measured in parts per billion. High contents of suspended solids or liquids are removed selectively from a suspension by a process of froth flotation.

Efficiency is very high, often exceeding 99%. The particle size must be very small. Standard precipitators operate up to temperatures of 700°F. Large flow rates are possible. Collectors can tolerate extremely corrosive conditions. The collected dust is dry. Electric power requirements are low.

FOAM FRACTIONATION Some dissolved substances are attracted to surfactants and thus are concentrated and removed with a foam. Such operations are performed in batch or continuous stirred tanks, or in continuous towers as in the flowsketch of Figure 20.9. Compressed air may be supplied through a sparger or ambient air may be drawn into a high speed rotating gas disperser. Improved separation is achieved by staged operation, so that a packed tower is desirable. Moreover, packing assists in the formation of a stable foam since that is difficult to do in an empty tower of several feet in diameter. Larger contents of surfactant usually are needed in large towers than in laboratory units. In pilot plant work associated with the laboratory data of Table 20.1, a tower 2 ft square by 8 ft high was able to treat 120 gal/hr of feed. The laboratory unit was 1 in. dia, so that the gas rate of 154 cm3/min of Table 20.1 corresponds to a superficial gas velocity of 1.1 ft/min. Most of the work on foam fractionation reported in the literature is exploratory and on a laboratory scale. A selected list of about 150 topics has been prepared with literature references by Okamoto and Chou (1979). They are grouped into separation of metallic ions, anions, colloids, dyes and organic acids, proteins, and others. Stable foams that leave the fractionator are condensed for further processing or for refluxing. Condensation may be effected by a blast of steam, by contact with a hot surface, by chemical

The disadvantages are: 1. The initial capital investment is high. 2. Due to very high or low resistivity, particles may be difficult to collect. 3. Variable air flow can significantly affect the efficiency adversely. 4. Space requirements are greater than baghouses. 5. A cyclone may be needed upstream from a precipitator to reduce the dust load on the unit. ARRANGEMENT OF COLLECTION EQUIPMENT In many cases, more than one collection device may be necessary to control dust problems. For example, a cyclone collector may be followed by a baghouse, and then perhaps by an electrostatic precipitator, or the cyclone may be upstream from a wet scrubber, since a single unit may not do a thorough cleaning job.

TABLE 20.1. Data of Foam Separation Experiments Made in a 1 in. Dia Column on a Waste Water Containing Radioactive Components and Utilizing Several Different Surfactants Flow rates (cm3/min)

Surfactant Aerosol AY Alipal CO-436 Alipal LO-529 Deriphat 170C Igepon CN-42 Tergitol 7 Ultrawet SK

Surf. conc. (gm/liter) 6.5 0.375 0.4 0.5 0.12 2.0 0.08

717

Gas, V

foam, Q

154 154 154 154 154 154 154

176 186 174 60 72 202 173

(Davis and Haas, 1972, pp. 279–297, Walas, 1988).

Foam Foam cond., F 0.197 0.950 0.415 4.92 1.6 0.763 0.137

Average bubble Foam diameter, density, ρt (gm/liter) D (cm) 1.12 5.10 2.40 74 24 3.77 0.79

0.06 0.05 0.06 0.025 0.038 0.05 0.10

718 GAS-SOLID SEPARATIONS antifoaming agents, sonically or ultrasonically, or by contact with a high speed rotating disk as appears in the flowsketch, Figure 20.9. FOAM

FOAM BREAKER

FROTH FLOTATION Finely divided solids or immiscible liquids can be made to adhere to gas bubbles and then can be removed from the main liquid. Affinity of a solid for an air bubble can be enhanced with surfactants which adhere to the surface of the solid and make it nonwetting. The main application of froth flotation is the separation of valuable minerals from gangue. Ores of Cu, Zn, Mo, Pb, and Ni are among those commercially preconcentrated in this way. Reagent requirements of each ore are unique and are established by test. A large amount of experience exists, however, and information is supplied freely by reagent manufacturers. Some recipes are given with descriptions of flotation processes in books on mineral dressing, for example, that of Wills (1985). Promoters or collectors give the mineral the water-repellent coating that will adhere to an air bubble. Frothers enhance the formation and stability of the air bubbles. Other additives are used to control the pH, to prevent unwanted substances from floating, or to control formation of slimes that may interfere with selectivity. Air is most commonly dispersed with mechanical agitation. Figure 20.10 illustrates a popular kind of flotation cell in which the gas is dispersed and the pulp is circulated with impellers. Such vessels have capacities of 300–400 cuft. Usually several are connected in series as in Figure 20.10(b). The froth is removed from each cell as it is formed, but the pulp goes through the battery in series. The froth is not highly stable and condenses readily without

CONDENSED FOAM FEED REFLUX

RECOVERED MATERIAL

SURFACTANT AIR RAFFINATE

Figure 20.9. Sketch of a foam fractionating column. Surfactants or other foaming agents may be introduced with the feed or separately at a lower feed point. Packing may be employed to minimize axial mixing. (Walas, 1988).

UPPER ROTOR DISPERSER

LOWER ROTOR FALSE BOTTOM (a)

Pulp flow

level

Pulp Pulp

Pulp

level

level

(b)

Figure 20.10. Several flotation cells connected in series. The interaction of air and pulp in a froth flotation cell and a series arrangement of such cells: (a) Sectional schematic of flotation cell. Upper portion of rotor draws air down the standpipe for thorough mixing with pulp. Lower portion of rotor draws pulp upward through rotor. Disperser breaks air into minute bubbles. Larger flotation units include false bottom to aid pulp flow. (WEMCO Division, Envirotech Corp.). (b) A bank of three flotation cells. The floating concentrate is withdrawn continuously from each stage but the remaining pulp flows in series through the cells. (Walas, 1988).

20.3. SUBLIMATION AND FREEZE DRYING

special provisions as it overflows. Since some entrainment of gangue occurs, usually it is desirable to reprocess the first froth. The flowsketch of Figure 20.11 illustrates such reprocessing. The solids to the first stage are ground here to −65 mesh, which normally is fine enough to release the mineral, and to −200 mesh in the final stage. Total residence time in a bank of cells may range from 4 to 14 min. A table of approximate capacities of several makes of flotation cells for a pulp with 33% solids of specific gravity = 3 is given in the Chemical Engineers’ Handbook (1984, p. 21.49); on an average, an 8-cell bank with 4-min holdup has a capacity of about 1.5 tons solid/(hr) (cuft of cell) and a power requirement of about 0.6 HP/ (cuft of cell). The chief nonmineral application of froth flotation is to the removal or oil or grease or fibrous materials from waste waters of refineries or food processing plants. Oil droplets, for instance, attach themselves to air bubbles which rise to the surface and are skimmed off. Coagulant aids and frothers often are desirable. In one kind of system, the water is saturated with air under pressure and then is pumped into a chamber maintained under a partial vacuum. Bubbles form uniformly throughout the mass and carry out the impurities. The unit illustrated in Figure 20.12 operates at 9 in. mercury vacuum and removes both skimmed and settled

350 TPD Solids 29% Solids −65 Mesh

719

Drive unit Slimmer arm

Effluent welr Effluent chennel Scurn trough Scure bottle Scum pipe

Sludge pipe

VLL.

Influent cone

Shaft seal

Skimmer blede Sludge scraper arm

Vaccum control chember Duffer to vacuum

Nozzle control levers

Effluent chamber

Influent pipe

Figure 20.12. Vacuator of the “constant-level” type. The cylindrical tank with a dome-shaped cover is under a constant vacuum of about 9 in. of mercury. Sewage enters a central draft tube from which it is distributed by means of a flared-top section. Floating solids, buoyed up by fine air bubbles, are skimmed from the liquid surface and carried to a trough. Settled solids are removed from the bottom with a scraper mechanism. (Courtesy of Engineering News-Record). (Walas, 1988).

sludges. Because of the flocculation effect it is able to process waste water at an enhanced rate of about 5000 gal/(sqft)(day) instead of the usual rate of 800–1000. In another application, particles of plastics in waste stream are chopped to diameters of 5 mm or less, passed through flotation cells containing proprietary surfactants, and removed as an air froth.

3-43" Denver Cells

8-43" Denver Cells 75 TPD, 40% Solids

Tails 299, TPD Solids 17% Solids

7' x 7' Ball Mill

Water 36" Akins Classifier 75 TPD 10% Solids −200 Mesh

Water

30' Thickener 75 TPD 50% Solids

8-24" Denver Cells RECYCLE 24 TPD 10% Solids

51 TPD 40% Solids

Leaf Filter CONCENTRATE 51 TPD Solids

Figure 20.11. Flotation section of a flowsheet for concentration of 350 tons/day of a copper ore. (data of Pima Mining Co., Tucson, AZ). (Walas, 1988).

20.3. SUBLIMATION AND FREEZE DRYING Sublimation is the transformation of a solid directly into vapor and desublimation is the reverse process of condensing the vapor as a solid. The term pseudosublimation is applied to the recovery of solid condensate from the vaporization of a liquid. The goal of a commercial sublimation is the separation of a valuable material from nonvolatile ones at temperatures low enough to avoid thermal degradation. The preservation of cell structure (and taste) is a deciding factor in the choice of freeze drying, a special instance of sublimation, foods, pharmaceuticals, and medical products. Only a few solids have vapor pressures near atmospheric at safe temperatures, among them CO2, UF6, ZrCl4, and about 30 organics. Ammonium chloride sublimes at 1 atm and 350°C with decomposition into NH3 and HCl, but these recombine into pure NH4Cl upon cooling. Iodine has a triple point 113.5°C and 90.5 Torr; it can be sublimed out of aqueous salt solutions at atmospheric pressure because of the entraining effect of vaporized water. Sublimation pressures down to 0.001 bar are considered feasible. At lower pressures and in some instances at higher ones, entrainer gas is used, usually air or nitrogen or steam. By such means, for instance, salicyclic acid is purified by sublimation at 150°C with an entrainer of air with sufficient CO2 to prevent decarboxylation of the acid. At the operating temperature, the vapor pressure is only 0.0144 bar. Operating conditions corresponding to equilibrium in a salicylic acid sublimer appear in Figure 20.13. Equilibrium may be approached in equipment where contact between phases is intimate, as in fluidized beds, but in tray types percent saturation may be as low as 10%.

720 GAS-SOLID SEPARATIONS Heater

Sublimer

Condenser

Cold trap 40 C

Makeup

Air (5−10% CO2) 2000 kg/hr

150 C

Salicyclic acid 137 kg/hr

Figure 20.13. Sublimation of salicyclic acid at 1 bar.Vapor pressures are 14.4 mbar at 150°C and 0.023 mbar at 40°C. The air rate shown corresponds to equilibrium in the sublimer, but in some kinds of vessels percent saturation may be as low as 10%. The conditions are those of Mullin [Crystallisation, Butterworths, London, 288 (1972)].

TABLE 20.2. Materials That May Be Purified by Sublimation or Are Being Freeze-Dried (a) Substances Amenable to Purification by Sublimationa Aluminium chloride Anthracene Anthranilic acid Anthraquinone Benzanthrone Benzoic acid Calcium Camphor Chromium chloride Ferric chloride Iodine Magnesium a

Among substances that are sublimed under vacuum are anthranilic acid, hydroxyanthraquinone, naphthalene, and β-naphthol. Pyrogallol and d-camphor distill from the liquid state but condense as solids. Several metals are purified by sublimation, for instance, magnesium at 600°C and 0.01–0.15 Torr. The common carrier gases are air or nitrogen or steam. Condensate from a carrier usually is finely divided, snowlike in character, which is sometimes undesirable. Substances which are sublimed in the presence of a carrier gas include anthracene, anthraquinone, benzoic acid, phthalic anhydride, and the formerly mentioned salicylic acid. A partial list of substances amenable to sublimation is in Table 20.2.

Naphthalene β-Naphthol Phthalic anhydride α-Phthalimide Pyrogallol Salicylic acid Sulphur Terephthalic acid Titanium tetrachloride Thymol Uranium hexafluoride Zirconium tetrachloride

Some others are mentioned in the text.

(b) Products That Are Being Freeze-Dried Commercially Foodstuffs

Pharmaceuticals

Coffee extract Fish and seafood Fruits Fruit juices Meat Milk Tea extract Vegetables

Antibiotics Bacterial cultures Serums Virus solutions

Animal Tissues and Extracts Arteries Blood Bones Hormones Skin Tumors

(Walas, 1988).

EQUIPMENT The process of sublimation is analogous to the drying of solids so much the same kind of equipment is usable, including tray dryers (Fig. 9.6), rotary tray dryers (Fig. 9.8), drum dryers (Fig. 9.11), pneumatic conveying dryers (Fig. 9.12), and fluidized beds (Fig. 9.13). The last of these requires the subliming material to be deposited on an inert carrier which is the fluidized material proper. Condensers usually are large air-cooled chambers whose walls are kept clear with brushes or scrapers or even swinging weights. Scraped or brushed surface crystallizers such as Figure 16.11(a) should have some application as condensers. When a large rate of entrainer gas is employed, a subsequent collecting chamber will be needed. One of the hazards of entrainer sublimation with air is the possibility of explosions even of substances that are considered safe in their normal states. FREEZE DRYING Certain highly heat-sensitive materials such as biological products, pharmaceuticals, high flavor-content foods, etc. listed in Table 20.2(b) may be freeze dried but the cost of the process is at least one order of magnitude greater than that of spray drying. The moisture removal from such materials is by sublimation. The process is preceded by quick freezing which forms small crystals and thus minimum damage to cell walls, and is likely to destroy bacteria. Some of the materials that are being freeze dried commercially are listed in Table 20.2(b). Most industrial freeze driers are batch type like simple tray driers of low capacity or vacuum tunnel driers. Liapis and Bruttini (1995) have published a detailed analysis of the freeze drying operation including costs and processing details of freeze-dried products. The most advanced technique of quick freezing is by pouring the material onto a freezing belt. Before drying, the material is granulated or sliced

to improve heat and diffusional mass transfer. These operations are conducted in cold rooms at about −46°C. Sublimation temperatures are in the range of −10 to −40°C and corresponding vapor pressures of water are 2.6–0.13 mbar. Tray dryers are the most commonly used type. The trays are lifted out of contact with hot surfaces so the heat transfer is entirely by radiation. Loading of 2.5 lb/sqft is usual for foodstuffs. Drying capacity of shelf-type freeze dryers is 0.1–1.0 kg/(hr)(m2 exposed surface). Another estimate is 0.5–1.6 lb/(hr)(sqft). The ice surface has been found to recede at the rate of 1 mm/hr. Freeze drying also is carried out to a limited extent in vacuum pans, vibrating conveyors, and fluidized beds. Condensers operate as low as 70°C. Typical lengths of cycles for food stuffs are 5–10 hr, for bacterial pellets 2–20 hr, and for biological fluids 20–50 hr. A production unit with capacity of 500 L may have 75 kW for refrigeration and 50 kW for heating. Conditions for the preparation of freeze dried coffee are preparation of an extract with 20–25% solids, freezing at 25–43°C, sublimation at approx. 200 Torr to a final moisture content of 1–3%, and total batch processing time of 6–8 hrs.

20.4. SEPARATIONS BY THERMAL DIFFUSION Separation of mixtures based on differences in thermal diffusivity at present are feasible only for analytical purposes or for production on a very small scale of substances not otherwise recovered easily. Nevertheless, the topic is of some interest to the process engineer as a technique of last resort. In a vessel with a temperature gradient between a hot and cold surface, a corresponding concentration gradient of a fluid likewise can develop. The substance with the smaller molecular volume usually concentrates in the high temperature region, but other factors

20.4. SEPARATIONS BY THERMAL DIFFUSION

including that of molecular shape also affect the relative migrations of components of mixtures. Thus, the sequence of separation of hydrocarbons from hot to cold regions generally is: light normal paraffins, heavy normal paraffins, naphthenes and mono cyclic aromatics, and bicyclic aromatics. Isotopes with small differences in molecular weights were the first substances separated by thermal diffusion, but isomers which have identical molecular weights also are being separated. The basic construction of a horizontal thermal diffusion cell is sketched in Figure 20.14(a). When gases are to be separated, the distance between the plates can be several mm; for liquids it is a fraction of a mm. The separation effects of thermal diffusion and convection currents are superimposed in the equipment of Figure 20.14(b), which

721

is called a thermogravitational or Clusius-Dickel column after the inventors in 1938. A commercially available column used for analytical purposes is in Figure 20.14(c). Several such columns in series are needed for a high degree of separation. Clusius and Dickel used a column 36 m long to make 99+% pure isotopes of chlorine in HCl. The cascade of Figure 20.15 has a total length of 14 m; most of the annular diameter is 25.4 mm, and the annular widths range from 0.18 to 0.3 mm. The cascade is used to recover the heavy isotope of sulfur in carbon disulfide; a production rate of a 90% concentrate of the heavy isotope of 0.3 g/day was achieved. Separation of the hydrocarbon isomers of Table 20.3(a) was accomplished in 48 hr in the column of Figure 20.14(c) with 50°C

PACKING NUT GASKET INNER TUBE

Cold wall Hot wall Light product

ANNULAR SPACE "SLIT"=0.0115 in.

6 in. TAKEOFF CLOSURE

OUTER TUBE 60 in. Feed HOR WALL

Light stream

SLIT WIDTH

NICHROME WIRE HEATNG COIL

ASBESTOS TAPE DISULATION

Heavy stream Heavy product

COLD WALL

(a)

(b)

(c)

1.425 TRANS-1,2DIMETHYL CYCLOHEXANE CETANE TOP PROCUCT PRODUCT COMPOSITION

REFRACTIVE INDEX AT 25⬚C

1.0

1.430

1.435 CIS-1,2-DIMETHYL CYCLOHEXANE

1.438 0

1

2

3

4 5 6 FRACTIONS

(d)

7

8

9

10

0.8

0.6

0.4

0.2

BOTTOM PRODUCT

CUMENE 0.2 0.4 0.6 0.8 1.0 CHARGE COMPOSITION )VOL. FRACTION CENTANE)

(e)

Figure 20.14. Construction and performance of thermal diffusion columns. (a) Basic construction of a thermal diffusion cell. (b) Action in a thermogravitational column. (c) A commercial column with 10 takeoff points at 6 in. intervals; the mean dia of the annulus is 16 mm, width 0.3 mm, volume 22.5 mL (Jones and Brown, 1960). (d) Concentration gradients in the separation of cis and trans isomers of 1,2-dimethylcyclohexane (Jones and Brown, 1960). (e) Terminal compositions as a function of charge composition of mixtures of cetane and cumene; time 48 hr, 50°C hot wall, 29°C cold wall (Jones and Brown, 1960). (Walas, 1988).

722 GAS-SOLID SEPARATIONS 35 G/DAY

occur. Thus mixtures of benzene and cyclohexane are not separated, nor can mixtures of benzene and octadecane when the latter is in excess. Examples of separations of isotopes are in Table 20.3(b). The concentration of U-235 listed there was accomplished in a cascade of 2100 columns, each with an effective height of 14.6 m, inner tube 5 cm dia, gap 0.25 mm, hot surface 87–143°C, and cold surface 63°C, just above the condensation temperature at the operating pressure of 6.7 MPa. Although the process was a technical success, it was abandoned in favor of separation by gaseous diffusion which had only 0.7% of the energy consumption. For separation of hydrocarbons, thermal requirements are estimated to range from 70,000 to 350,000 Btu/lb, compared with heats of vaporization of 150 Btu/lb. Although thermal diffusion equipment is simple in construction and operation, the thermal requirements are so high that this method of separation is useful only for laboratory investigations or for recovery of isotopes on a small scale, which is being done currently.

2.8% C34S34S 0.01% C34S34S

32 34 8.0%, C S S 0.02% C34S34S

CASCADE I

6 G/DAY

CASCADE II

61.0% C32S34S 34 34

2.4% C S S REACTOR 32 34 4.6% C S S 84.2% C34S34S 32 36 1.5% C S S 8.0% C34S36S

32 34 45.4% C S S 34 34 9.4% C S S

Figure 20.15. Sketch of liquid thermal diffusion system. The liquid thermal diffusion system for the recovery of heavy sulfur isotope in carbon disulfide. The conditions prevailing at the time after 90% 34S is reached. Each rectangle in the cascades represents a column, each height being proportional to the length of the column. The two cascades have a combined height of 14 m, annular dia 25.4 mm, and annular width 0.18–0.3 mm. Production rate of 90% concentrate of 34 S was 0.3 g/day. (Rutherford, 1978). (Walas, 1988). hot wall and 20°C cold wall. The concentration gradient that develops in such a column is shown in Figure 20.14(d). The equilibrium terminal compositions depend on the overall composition, as indicated in Figure 20.14(e). Other kinds of behaviors also

20.5. ELECTROCHEMICAL SYNTHESES Electrolysis plays a role in the manufacture of some key inorganic chemicals on an industrial scale, but rather a minor one in the manufacture of organic chemicals. Chlorine, alkalis, metals, hydrogen, oxygen, and strong oxidizing agents such as KMnO4, F2, and Cu2O are made this way. Electroorganic processes of commercial or potentially commercial scale are listed in Table 20.4, which implies that much research is being done in pilot plants and may pay off in the near future. In the United States, the three large tonnage applications are to the manufacture of adiponitrile, the Miles process for dialdehyde starch, which is on standby until the demand picks up, and the 3M electrofluorination process for a variety of products. Pros and cons of electrochemical processes are not always clear cut. In a few cases, they have lower energy requirements than conventional chemical methods but not usually according to the survey of Table 20.5. The process for manufacturing adiponitrile by electrochemical reduction of acetonitrile is an outstanding example; moreover, comparison of the performances of the original and improved cells [sketched on Figures 20.16(e) and (f)] suggests the

TABLE 20.3. Examples of Separations by Thermal Diffusion (a) Hydrocarbon Isomers Final Composition, Vol. % Components n-Heptane Triptane Isoöctane n-Octane 2-Methylnaphthalene 1-Methylnaphthalene trans-1,2-Dimethylcyclohexane cis-1,2-Dimethylcyclohexane p-Xylene o-Xylenea m-Xylene o-Xylenea p-Xylene m-Xylene

Vol. % 50 50 50 50 50 50 40 60 50 50 50 50 50 50

(Jones and Brown, 1960). (Walas, 1988). o-Xylene contains paraffinic impurity.

a

Mol. wt. 100 100 114 114 142 142 112 112 106 106 106 106 106 106

Density 0.6837 0.6900 0.6919 0.7029 0.9905 1.0163 0.7756 0.7963 0.8609 0.8799 0.8639 0.8799 0.8609 0.8639

Top 95 5 58 42 55.5 44.5 100 0 92 8 100 0 50 50

Bottom 

10 90  40 60  42:5 57:5  0 100  0 100  19 81  50 50

Separation, % 75.4 11.4 13.1 100 92 80 0

20.5. ELECTROCHEMICAL SYNTHESES

723

TABLE 20.3.—(continued )

(b) Isotopes Working fluid

Isotope separated 35

HCl

Cl Cl 34 Kr 86 Kr 17 O 18 O 235 U 15 N 134 Xe 136 Xe 3 He 36 A 38 A 20 Ne 22 Ne 78 Kr 86 Kr 3 He 21 Ne 13 C 124 Xe 37

Kr O2 UF6 N2 Xe He A Ne Kr He§ Ne CH4 Xe

mo1 % product 99.6 99.4 98.2 99.5 0.5 99.5 0.86 99.8 1 99 10 99.8 23.2 99.99 99.99 10 96.1 99 33.9 90 4.4

Phase

Single Column (S) or Cascade (C)

Investigator

Year

Gas

S

Clusius and Dickel

1939

Gas

S

Clusius and Dickel

1941

Gas

C

Clusius and Dickel

1944

Liquid Gas Gas

C S C

Manhattan Dist. Clusius & Dickel Clusius et al.

1945 1950 1956

Gas Gas

C C

Bowring and Davies ORNL†

1958 1961

Gas

C

ORNL

1961

Gas

C

ORNL

1961

Gas Gas Gas Gas

C C C C

Mound Lab.‡ ORNL Mound Lab. ORNL

1962 1963 1963 1964

†

Oak Ridge National Laboratory, U.S. AEC, Oak Ridge, Tennessee. Mound Laboratory, U.S. AEC, Miamisburg, Ohio. § Feed not of normal abundance, contained 1 percent 3He from nuclear reaction. (Benedict et al., 1981). (Walas, 1988). ‡

often great leeway in cell design. Small scale electrode processes frequently are handicapped because of the expense of developing efficient cell components of cells such as electrodes, diaphragms, membranes, and electrolytes which usually can be justified only for large scale operation. In comparison with chemical oxidations and reductions, however, electrode reactions are nonpolluting and nonhazardous because of low pressure and usually low temperature. Although electricity usually is more expensive than thermal energy, it is clean and easy to use. Electrolytic processes may become more attractive when less expensive sources of electricity are developed. ELECTROCHEMICAL REACTIONS

in the electrolyte and other elements of a cell and particularly the overvoltages at the electrodes. The latter are due to adsorption or buildup of electrolysis products such as hydrogen at the electrode surfaces. Figure 20.17(a) shows magnitudes of hydrogen overvoltages for several metals and several currents. The several contributions to voltage drops in a cell are identified in Figures 20.17(b) and (c), whereas Figure 20.17(d) indicates schematically the potential gradient in a cell comprised of five pairs of electrodes in series. Electrochemical cells are used to supply electrical energy to chemical reactions, or for the reverse process of generating electrical energy from chemical reactions. The first of these applications is of current economic importance, and the other has significant promise for the future.

An equilibrium electrical potential is associated with a Gibbs energy of formation by the equation:

FUEL CELLS

E 0 = −ΔG 0 =23:06n, where n is the number of gram equivalents involved in the stoichiometric equation of the reaction, ΔG0 is in kcal/g mol, and E0 is the potential developed by the reaction in volts. Thus, for the reaction H2 O ⇄ H2 + 12 O2 at 25°C, E 0 = 54:63=ð2Þð23:06Þ = 1:18 V and for HCl ⇌ 12 H2 + 12 Cl2 at 25°C, E 0 = 22:78=23:06 = 0:99 V: Practically, reactions are not conducted at equilibrium so that amounts greater than equilibrium potentials are needed to drive a reaction. Major contributions to inefficiency are friction

A few chemical reactions can be conducted and controlled readily in cells for the production of significant amounts of electrical energy at high efficiency, notably the oxidations of hydrogen or carbon monoxide. Some data of such processes are in Figure 20.18. The basic processes that occur in hydrogen/air cells are in Figure 20.18(a). Equilibrium voltage of such a cell is in excess of 1.0V at moderate temperatures, but under practical conditions this drops off rapidly and efficiency may become less than 40%, as Figure 20.18(b) shows. Theoretical cell potentials for several reactions of fuel cell interest are in Figure 20.18(c), in theory at least, the oxidations of hydrogen and carbon monoxide are competitive. High temperatures may be adopted to speed up the electrode processes, but they have adverse effects on the equilibria of these particular reactions. Figure 20.18(d) shows the characteristics of major electrochemical fuel systems that have been emphasized thus far. Most of the development effort has been for use in artificial satellites where cost has not been a primary consideration, but

724 GAS-SOLID SEPARATIONS TABLE 20.4. Electroorganic Synthesis Processes Now Applied Commercially or Past the Pilot Plant Stage Producta

Raw Materiala

Company (country)

Scale

Type of Process

Commercialized Adiponitrile

108 kg/yr 108 kg/yr 2 × 107 kg/yr Not available Not available Not available Not available Not available Not available Not available 3 × 105 kg/yr Not available Not available 1:2 × 05 kg/yr 6 × 104 kg/yr Not available Not available Past pilot-plant Past pilot-plant

Oxidation of functional group Reduction of functional group Reduction Reduction Reduction Crum Brown-Walkerc Anodic Paired synthesis

Past pilot-plant Past pilot- plant Past pilot-plant Past pilot-plant Commercial? Past pilot-plant Commercial? Past pilot-plant

Paired synthesis Reduction of functional group Crum Brown-Walkerc

Benzaldehyde Dihydrophthalic Acid Hydroquinone or Quinone

Monsanto (US) Monsanto (UK) Asahi (Japan) Nitrobenzene (Japan) Holliday (UK) Anthracene Holliday (UK) Furan (Japan) BASF (West Germany) Hydrocarbons, aliphatic Dia Nippon (Japan) carboxylic acids, sulfonic 3M (US)b (India) acids, amines, etc. Glucose Oxalic acid (Japan) Tetrahydrocarbazole BASF (West Germany) Pyridine Robinson Bros. (UK) Maleic acid (India) Monomethylazelate Soda Aromatic Co. (Japan) Ethylmagnesium halide Nalco (US) Propylene BASF (West Germany) others in UK and West Germany Pyridinium salts (Japan) Salicylic acid (India) Adipic acid half esters BASF (West Germany) (Japan) (USSR) Toluene (India) Phthalic acid BASF (West Germany) Benzene Several

Maltol Pinacol

Furfuryl alcohol Acetone

Past pilot-plant Past pilot-plant

p-Aminophenol Anthraquinone 2,5-Dimethoxydihydrofuran Fluorinated Organics

Gluconic Acid Glyoxylic Acid Hexahydrocarbazole Piperidine Succinic Acid Hexadecanedioic Acid Tetraethyl Lead Propylene Oxide

4,4’-bis-Pyridinium Salts Salicylaldehyde Sebacid Acid Diesters

Acrylonitrile

Otsuka (Japan) (Japan) BASF (West Germany)

Reductive coupling

Reductive rearrangement Indirect oxidation Oxidative addition Anodic substitution

Indirect oxidation [Mn(III)] Reduction Paired synthesis or anodic oxidation + chemical reduction Oxidation Reductive coupling

a

Formulas are given in Appendix A. Added by author. Oxidative coupling. (Baizer, 1980). (Walas, 1988). b c

TABLE 20.5. Comparative Energy Requirements of Electrochemical and Chemical Processes kcal/kg Chemical

Electrochemical

a

Chemical

Adiponitrile Aniline Nitrobenzene route Phenol route Sorbitol Terephthalic Acid Phenol Methyl Ethyl Ketone

b

43,177 (10,520)

65,808

b b

36,172 – 9,649 17,382 35,592 6,187

Melamine Hydroquinone Dichloroethane HCl route Cl2 route

b b

30,159 52,739

13,919 16,736 958 700 12,251 6,690 3,233 15,472 30,814

b b

17,773 –

6,131 14,819

a

b

c

c

c c

c

Electrochemical energy adjusted for generating plant efficiency. Improved Monsanto process. c Energy charged is for hydrocarbon raw materials (different compounds); other compounds begin with the same raw materials. d Chemical route energy given by Rudd et al.; others estimated by Beck et al. (Beck et al., 1979). (Walas, 1988). b

20.5. ELECTROCHEMICAL SYNTHESES

725

Figure 20.16. Basic designs of electrolytic cells. (a) Basic type of two-compartment cell used when mixing of anolyte and catholyte is to be minimized; the partition may be a porous diaphragm or an ion exchange membrane that allows only selected ions to pass. (b) Mercury cell for brine electrolysis. The released Na dissolves in the Hg and is withdrawn to another zone where it forms salt-free NaOH with water. (c) Monopolar electrical connections; each cell is connected separately to the power supply so they are in parallel at low voltage. (d) Bipolar electrical connections; 50 or more cells may be series and may require supply at several hundred volts. (e) Bipolar-connected cells for the Monsanto adiponitrile process. Spacings between electrodes and membrane are 0.8–3.2 mm. (f) New type of cell for the Monsanto adiponitrile process, without partitions; the stack consists of 50–200 steel plates with 0.0–0.2 mm coating of Cd. Electrolyte velocity of 1–2 m/sec sweeps out generated O2. (Walas, 1988). spinoff to industrial applications has some potential for the near future. CELLS FOR SYNTHESIS OF CHEMICALS Cells in which desired chemical reactions can be conducted and controlled are assemblages of pairs of anodes and cathodes between which the necessary potential difference is impressed. The regions

near the electrodes may be separated by porous diaphragms to minimize convective mixing of the products formed at the individual electrodes. In recent years, semipermeable or ion exchange membranes have been employed as diaphragms. In Figure 20.16(a), the membrane allows only Na+ ions to pass so that the caustic that is made in the cell is essentially free of NaCl. In the mercury cell of Figure 20.16(b), no partition is necessary because the released Na dissolves in the mercury; the amalgam is reacted with water in an

726 GAS-SOLID SEPARATIONS

Figure 20.17. Overvoltage and distribution of voltage drops in cells (Hine, 1985). (a) Overvoltage of hydrogen on some metals. (b) Voltage distribution in two kinds of cells for electrolysis of brine. (c) Variation of voltage distribution with current density in the electrolysis of HCl. (d) Schematic of voltage profile in a bipolar cell with five pairs of electrodes. (Walas, 1988).

electrically neutral zone of the cell to make salt-free caustic. Because of pollution by escaped mercury, such cells have been largely phased out for production of salt-free caustic. The same process sometimes can be performed efficiently in cells either with or without diaphragms. Figures 20.16(e) and (f) are for making adiponitrile by reduction of acetonitrile. In the newer design, Figure 20.16(f), the flow rate of the electrolyte is high enough to sweep out the generated oxygen quickly enough to prevent reverse oxidation of the product. Either parallel, called monopolar, or series, called bipolar, electrical connections can be made to the pairs of electrodes in a complete cell. The monopolar types have individual connections to each electrode and thus require only individual pair potential to be applied to the cell assembly. The bipolar mode has electrical connections only to the terminal electrodes. One design such as Figure 20.16(f) has 48

pairs of electrodes in series and requires 600 V. The equipment of Figure 20.19(a) also has bipolar connections. The voltage profile in such equipment is indicated schematically in Figure 20.16(c) and Figure 20.17(d). Bipolar equipment is favored because of its compactness and, of course, the simplicity of the electrical connections. No adverse comments appear to be made about the high voltages needed. Although the basic cell design shown schematically in Figures 20.16(a) and 20.19(d) is effective for many applications when dimensions and materials of construction are properly chosen, many special designs have been developed and used, of which only a few can be described here. For the cracking of heavy hydrocarbons to olefins and acetylenes, for instance, the main electrodes may be immersed in a slurry of finely divided coke; the current discharges from particle to particle generate the unsaturates. Only 100–200 V appears to be sufficient.

20.5. ELECTROCHEMICAL SYNTHESES

727

Figure 20.18. Data of electrochemical fuel cells. (a) Processes in a fuel cell based on the reaction between hydrogen and oxygen. (b) Voltagecurrent characteristic of a hydrogen-air fuel cell operating at 125°C with phosphoric acid electrolyte [Adlhart, in Energy Technology Handbook (Considine, Ed.), 1977, p. 4.61. (c) Theoretical voltages of fuel cell reactions over a range of temperatures. (d) Major electrochemical systems for fuel cells (Adlhart, in Considine, loc. cit., 1977, p. 4.62). (Walas, 1988). The most widely used brine electrolytic cells are the Hooker and Diamond Shamrock which are both monopolar, but bipolar designs like that of Figure 20.19(a) also are popular. That figure does not indicate the presence of a diaphragm but one must be used. Rotating electrodes characterize the BASF cell of Figure 20.19(b), which is used for making adiponitrile. The cell described in the

literature has 100 pairs of electrodes 40 cm dia spaced 0.2 µm apart. The rapid flow rate eliminates the need for diaphragms by sweeping out the oxygen as it is formed. Lead alkyls are made by the action of Grignard reagents on lead anodes in the equipment of Figure 20.19(c). Lead pellets serve as the anode and are replenished as they are consumed. Several tubes 5 cm dia are housed in a single shell for temperature control

728 GAS-SOLID SEPARATIONS

Figure 20.19. Some special designs of electrolytic cells. (a) Glanor bipolar diaphragm-type cell assembly for chlor-alkali production (PPG Industries). (b) BASF capillary gap cell has 100 pairs of graphite plates with gaps of 0.2 mm used for adiponitrile synthesis; anodes are electroplated with lead dioxide [Beck and Guthke, Chem. Ing. Tech. 41, 943(1969)]. (c) Principle of the shell-and-tube reactor for electrolytic oxidation of Grignard reagents to lead alkyls. Lead shot serves as consumable anode which is replenished continuously. Individual tubes are 5 cm dia by 75 cm long [Danly, Encycl. Chem. Technol. 8, 702 (1979)]. (d) Simple cells of the type used for electrolysis of HCl and water; voltage breakdown is shown in Figure 20.16(c). (Walas, 1988).

REFERENCES

and as required for capacity. Lead chemicals have been slowly phased out due to environmental and health problems. The simplest kind of cell construction, shown in Figure 20.19 (d), suffices for the production of hydrogen by electrolysis of water and for the recovery of chlorine from waste HCl. The term filterpress cell is applied to this kind of equipment because of the layered construction. These two electrolyses are economically feasible under some conditions. Some details are given by Hine (1985). It has been mentioned already that only a few inorganic and organic electrochemical processes have made it to commercial scale, but the potential may be there and should not be ignored. Surveys of the field and of the literature have been made by Hine (1985), Fletcher (1982), and Roberts et al. (1982). REFERENCES Gas-Solids Separation Anon. ——, A Scientific Review of Dust Collection, Scientific Dust Collectors, Alsip, IL, 2002. D.E. Bonn, Wet type dust collectors, Chem. Eng. Prog., 59(10), 69–74 (October 1963). J.M. Maas, Cyclone separators, in P.A. Schweitzer (Ed.), Handbook of separation methods for Chemical Engineers, McGraw-Hill, New York, 1979, pp. 6.10–6.17. M. Pell and J.B. Dunson, Gas-solid separations and equipment, in D. Green (Ed.), Perry’s Chemical Engineers’ Handbook, 7th ed., Section 17, McGraw-Hill, New York, 1999. K. Rietemer and C.G. Verver, Cyclones in Industry, Elsevier, New York, 1961. W.M. Vatavuk, Estimating Costs of Air Pollution Control, Lewis Publishers, Chelsea, MI, 1990. F.A. Zenz, Cyclones, Encyclopedia of Chemical Processing and Design, Dekku, New York, Vol. 14, 1982, pp. 82–97. Foam Separation and Flotation D. Green (Ed.), Perry’s Chemical Engineers’ Handbook, 6th ed., McGrawHill, New York, 1984. J. Davis and D. Hass, in R. Lemlich (Ed.), Adsorptive Bubble Separation Techniques, Academic, New York, 1972. R. Lemlich, Adsorptive Bubble Separation Techniques, Academic, New York, 1972. Y. Okamoto and E.J. Chou, Foam separation processes, in P.A. Schweitzer (Ed.), Handbook of Separation Techniques for Chemical Engineers, McGraw-Hill, New York, 1979, pp. 2.183–2.197. P. Somasundaran, Foam separation methods: A review, in Perry and van Oss (Eds.), Separation and Purification Methods, Vol. 1, 1972, pp. 117–199. T. Sorensen, Flotation, in D. Green (Ed.), Perry’s Chemical Engineers’ Handbook, 6th ed., McGraw-Hill, New York, 1984, pp. 21.46–21.52. B.A. Wills, Mineral Processing Technology, Pergamon, New York, 1985.

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Sublimation and Freeze Drying L.F. Albright, Albright’s Chemical Engineering Handbook, CRC Press, Boca Raton, FL, 2009. W. Corder, Sublimation, in D. Green (Ed.), Perry’s Chemical Engineers’ Handbook, 6th ed., McGraw-Hill, New York, 1984, pp. 17.12–17.14. N. Ganiaris, Freeze drying, in D.M. Considine (Ed.), Chemical and Process Technology Encyclopedia, McGraw-Hill, New York, 1974, pp. 523–527. C.A. Holden and H.S. Bryant, Sublimation, Separation Sci., 4(1), 1 (1969). L. Liapis and R. Bruttini, Freeze Drying, in A.S. Majundar (Ed.), Hand book of Industrial Drying, 2nd ed., Dekker, New York, 1995, pp. 309–343. C.J. Major, Freeze drying, in D. Green (Ed.), Perry’s Chemical Engineers’ Handbook, 5th ed., McGraw-Hill, New York, 1973, pp. 17.26–17.28. G. Matz, Sublimation, in F. Uhlmann’s (Ed.), Encyclopedia of Chemical Technology, Vol. 2. Verlag Chemie, Weinheim, 1972, pp. 664–671. J.W. Mullin, Sublimation, in Crystallization, Butterworths, London, 1972, pp. 284–290. L. Roy and J.C. May (Eds.), Freeze drying-Lyophilization of Pharmaceutical and Biological Products, Dekker, New York, 1999. Thermal Diffusion M. Benedict, T.H. Pigford, and H.W. Levi, Nuclear Chemical Engineering, McGraw-Hill, New York, 1981. A.L. Jones and G.B. Brown, Liquid thermal diffusion, in McKetta and Kobe (Eds.), Advances in Petroleum Chemistry and Refining, Vol. III, Wiley, New York, 1960, pp. 43–76. W.M. Rutherford, Separation of highly enriched 34S by liquid phase thermal diffusion, Ind. Eng. Chem. Proc. Des. Dev., 17, 17–81 (1978). G. Vasaru, et al., The Thermal Diffusion Column, VEBN Deutscher Verlag der Wissenschaften, Berlin, 1969. Electrochemical Syntheses J. Adlhart, in D.M. Couriclices (Ed.), Energy Rheumology Handbook, McGraw-Hill, New York, 1977, p. 4.61. M.M. Baizer, Isotope effects in electrochemical production, J. Appl. Electrochem., 18, 285 (1980). T. Beck and M. Guthke, Organic electrochemistry, in Organische Verbundigen, Wiesbaden, Germany, Chem. Ing. Tech., 41, 943 (1969). T. Beck, et al., A Survey of Electrolytic Processes, ANL/OEPM, 79–5, Electrochemical Technology Corporation, Wiesbaden, Germany, 1979 D.E. Danly, Separation of dibasic acids, in R.E. Kirk and D.F. Othmer (Eds.), Encyclopedia of Chemical Technology, J. Wiley and Sons, New York, 1979. D. Fletcher, Industrial Electrochemistry, Chapman and Hall, London, 1982. F. Hine, Electrode Processes and Electrochemical Engineering, Plenum, New York, 1985. R. Roberts, R.P. Ouellete, and P.N. Cheremisinoff, Industrial Applications of Electroorganic Synthesis, Ann Arbor Science, Ann Arbor, MI, 1982.