Karl B. Schnelle, Jr.
Chemical Engineering Department, Vanderbilt University, Nashville, Tennessee, U.S.A.
Partha Dey
P. A. Consulting, Nashville, Tennessee, U.S.A.
INTRODUCTION
Absorption is a mass transfer operation in which a soluble gaseous component is removed from a gas stream by dissolving in a liquid. Absorption can be used to recover valuable gaseous components such as hydrocarbons or to remove unwanted gaseous components such as hydrogen sulfide from a stream.
A valuable solute can be separated from the absorbing liquid and recovered in a pure, concentrated form by distillation or stripping (desorption). The absorbing liquid is then used in a closed circuit and is continu-ously regenerated and recycled. Examples of regenera-tion alternatives to distillaregenera-tion or stripping are removal through precipitation and settling; chemical destruc-tion through neutralizadestruc-tion, oxidadestruc-tion, or reducdestruc-tion;
hydrolysis; solvent extraction; and liquid adsorption.
Absorption is one of the main methods of separation used in the chemical processing industry. Accompanied by chemical reaction between the absorbed component and a reagent in the absorbing fluid, absorption can become a very effective means of separation. Absorp-tion can also be used to remove an air pollutant like an acid gas from stream. Then, the system could be a simple absorption in which the absorbing liquid is used in a single pass and then disposed of while containing the absorbed pollutant.
Operations of Absorption Towers
In the past it was the custom to call absorbers operating as cleanup towers to remove undesirable gaseous efflu-ents by the name of scrubber. At that time most of the effluent gases being removed were acid gases being scrubbed with water. The designation of scrubber to scrub the discharge gas and clean it seemed rather natural. Today the same kind of operation is carried out, but with more stringent regulations imposed by the local air pollution control agency. The name scrubber is now applied to those operations in which particulate matter is removed but the scrubbing operation may also include the simultaneous removal of gaseous pollutants.
In this chapter the term absorber will refer to the removal of gaseous contaminants.
General Considerations
Filters, heat exchangers, dryers, bubble cap columns, cyclones, etc., are ordinarily designed and built by process equipment manufacturers. However, units of special design for one-of-a-kind operations such as packed or plate towers are quite often designed and built under the supervision of plant engineers. Thus, there is a large variety of this type of equipment, none of it essentially standard.
TYPES OF ABSORPTION EQUIPMENT
Absorption takes place in either staged or plate towers or continuous or packed contactor. However, in both cases the flow is continuous. In the ideal equilibrium stage model, two phases are contacted, well mixed, come to equilibrium, and then are separated with no carryover. Real processes are evaluated by expressing efficiency as a percentage of the change that would occur in the ideal stages. Any liquid carryover is removed by mechanical means.
In the continuous absorber the two immiscible phases are in continuous and tumultuous contact within a vessel that is usually a tall column. A large surface is made available by packing the column with ceramic or metal materials. The packing provides more surface area and a greater degree of turbulence to promote mass transfer. The penalty for using packing is in the increased pressure loss in moving the fluids through the column, which causes an increased demand for energy. In the usual countercurrent flow column, the lighter phase enters the bottom and passes upward. Transfer of material takes place by molecular and eddy diffusion processes across the interface between the immiscible phases. Contact may be also cocurrent or cross-flow. Columns for the removal of
Encyclopedia of Chemical ProcessingDOI: 10.1081/E-ECHP-120007642
Copyright # 2006 by Taylor & Francis. All rights reserved. 1
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air contaminants are usually designed for countercur-rent or cross-flow operation.
Absorption can take place in a countercurrent, cocurrent, or cross-flow device. Vertical countercurrent towers are either built with a metal, plastic, or ceramic packing or constructed as plate towers with various types of plates. This chapter will discuss the solvents used to carry out absorption and the various types of absorption equipment.
ABSORPTION SOLVENTS
Absorption systems can be divided into those that use water as the primary absorbing liquid and those that use a low-volatility organic liquid. The gas solubility should be high in the absorbing solvent. The gas leav-ing an absorber is usually saturated with the solvent;
therefore, the solvent should have a low vapor pres-sure. A lower viscosity solvent is advantageous to promote more rapid absorption rates and improve flooding characteristics. The solvent should not be cor-rosive to the materials of construction of the absorber.
It should be nontoxic and nonflammable. Depending on the region where the absorber is to be constructed, the solvent should have a low freezing point.
Nonaqueous Systems
At first glance, an organic liquid appears to be the preferred solvent for absorbing hydrocarbon and orga-nic vapors from a gas stream because of improved solubility and miscibility. The lower heat of vaporization of organic liquids results in energy conservation when solvent regeneration must occur by stripping. Many heavy oils such as No. 2 fuel oil or heavier and other solvents with low vapor pressure can do extremely well in reducing organic vapor concentrations to low levels.
Care must be exercised in picking a solvent that will have sufficiently low vapor pressure so that the solvent itself will not become a source of volatile organic pollu-tion. Obviously, the treated gas will be saturated with the absorbing solvent. An absorber–stripper system for recovery of benzene vapors has been described by Crocker.[1] Other aspects of organic solvent absorption requiring consideration are stability of the solvent in the gas solvent system, for example, its resistance to oxidation, and its possible fire and explosion hazard.
Although water is the most common liquid used for absorbing acidic gases, amines (monoethanol-, dietha-nol-, and triethanolamine; methyldiethanolamine; and dimethylaniline) have been used for absorbing SO2
and H2S from hydrocarbon gas streams. Such absor-bents are generally limited to solid particulate free systems because solids can produce difficult to handle
sludge as well as use up valuable organic absorbents.
Furthermore, because of absorbent cost, absorbent regeneration must be practiced in almost all cases.
Aqueous Systems
Absorption is one of the most frequently used methods for removal of water-soluble gases. Acidic gases such as HCl, HF, and SiF4 can be absorbed in water effi-ciently and readily, especially if the last contact is made with water that has been made alkaline. Less soluble acidic gases such as SO2, C12, and H2S can be absorbed more readily in a dilute caustic solution. The scrubbing liquid may be made alkaline with dissolved soda ash or sodium bicarbonate, or with sodium hydro-xide, usually with no higher a concentration in the scrubbing liquid than 5–10%. Lime is a cheaper and more plentiful alkali, but its use directly in the absorber may lead to plugging or coating problems if the calcium salts produced have only limited solubility. A technique often used is the two-step flue gas desulfurization pro-cess, where the absorbing solution containing NaOH is used inside the absorption tower, and then the tower effluent is treated with lime externally, precipitating the absorbed component as a slightly soluble calcium salt.
The precipitate may be removed by thickening and the regenerated sodium alkali solution is recycled to the absorber. Scrubbing with an ammonium salt solution can also be employed. In such cases, the gas is often first contacted with the more alkaline solution and then with the neutral or slightly acid contact to prevent stripping losses of NH3to the atmosphere.
When flue gases containing CO2are being scrubbed with an alkaline solution to remove other acidic com-ponents, the caustic consumption can be inordinately high if CO2 is absorbed. However, if the pH of the scrubbing liquid entering the absorber is kept below 9.0, the amount of CO2 absorbed can be kept low.
Conversely, alkaline gases, such as NH3, can be removed from the main gas stream with acidic water solutions such as dilute H2SO4, H3PO4, or HNO3. Single-pass scrubbing solutions so used can often be disposed of as fertilizer ingredients. Alternatives are to remove the absorbed component by concentration and crystallization. The absorbing gas must have ade-quate solubility in the scrubbing liquid at the resulting temperature of the gas–liquid system.
For pollutant gases with limited water solubility, such as SO2 or benzene vapors, the large quantities of water that would be required are generally imprac-tical on a single-pass basis, but may be used in unusual circumstances. An early example from the United Kingdom is the removal of SO2from flue gas at the Battersea and Bankside electric power stations, which is described by Rees.[2] Here, the normally alkaline
water from the Thames tidal estuary is used in a large quantity on a one-pass basis.
PACKED TOWERS
There are two major types of packing, random dumped pieces and structured modular forms. The structured packing is usually crimped or corrugated sheets. The packing provides a large interfacial area for mass transfer and should have a low-pressure drop. How-ever, it must permit passage of large volumes of fluid without flooding. The pressure drop should be the result of skin friction and not form drag. Thus, flow should be through the packing and not around the packing. The packing should have enough mechanical strength to carry the load and allow easy handling and installation. It should be able to resist thermal shock and possible extreme temperature changes, and it must be chemically resistant to the fluids being processed.
Random or Dumped Packing
Random packings are dumped into the tower during construction and are allowed to fall at random. The tower might be filled with water, first to allow a gentler settling and to prevent breakage, especially with ceramic-type packing. Random dumped tower packing comes in many different shapes. Two of the most popular are rings and saddles. Sizes range from 0.25 in. to 3.5 in., with 1 in. being a very common size. The choice of a packing is mostly dependent on the service in which the tower will be engaged. Packings are made of ceramic, metal, or plastic, depending on the service.
Ceramic materials will withstand corrosion and are therefore used where the solutions resulting are aqueous and corrosive. Metals are used where noncorrosive organic liquids are present. Plastic packing may be used in the case of corrosive aqueous solutions and for organic liquids that are not solvents for the plastic of which the packing is made. Metal packing is more expensive, but provides lower pressure drop and higher efficiency. When using plastic materials, care must be taken that the temperature is not too high and that oxidizing agents are not present. Ring-type packings are commonly made of metal or plastic, except for Raschig rings, which are generally ceramic. Ring-type packings lend themselves to distillation because of their good turndown properties and availability in metals of all types that can be press formed. Usually, ring-type packings are used in handling organic solu-tions when there are no corrosive problems. However, rings do not promote redistribution of liquids, and Raschig rings may even cause maldistribution. Saddles are commonly made from ceramic or plastic and
give good corrosion resistance. Saddles are best for redis-tribution of liquid and, thus, serve as a good packing for absorption towers.
Structured Packing
Early on after the production of random packings had been used extensively, stacked beds of the conventional random packings such as larger-sized Raschig rings were used as ordered packings. Owing to the high cost of installation of this type of packing, it was largely discontinued. At that time multiple layers of corru-gated metal lath formed into a honeycomb structure came into use. Later on, a woven wire mesh arranged in rows of vertically corrugated elements came into use. Subsequently, other wire-mesh structures have gained favor. Then, a sheet metal structured packing was developed to reduce the expense of the wire-mesh type. The use of this structured type of packing not only promotes mass transfer owing to increased surface area, but also has less pressure drop in many different services.
Tower Considerations Materials of construction
Random packing can be made from ceramic materials, plastic, or metal. Most structured packing and plates in staged towers are made from metal although there are simple woven types of plastic materials that can be considered as structured packing. The tower packing, plates, and tower materials must be compatible with the fluids flowing through the towers. Of particular sig-nificance would be acid gases that may have a deleter-ious effect on metal tower internal parts and organic solvents that may have a serious effect on plastic mate-rials. It is also necessary to consider the case where there may be a high heat of absorption emitted. The internal tower may have to be cooled to withstand the temperature that results from the heat of absorption.
Flow arrangements
In diffusional operations such as absorption where mass is to be transferred from one phase to another, it is necessary to bring the two phases into contact to permit the change toward equilibrium to take place.
The transfer may take place with both streams flowing in the same direction, in which case the operation is called concurrent or cocurrent flow. When the two streams flow in the opposite direction, the operation is termed countercurrent flow, an operation carried out with the gas entering at the bottom and flowing
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upward and the liquid entering at the top and flowing down. This process is illustrated in Fig. 1. A combined operation in which the contaminated gas is first cleaned in a countercurrent operation, as shown in Fig. 2, and then the gas is further treated to remove more of the contaminant as shown in the cocurrent operation that follows.
Countercurrent operation is the most widely used absorption equipment arrangement. As the gas flow increases at constant liquid flow, liquid holdup must increase. The maximum gas flow is limited by the pres-sure drop and the liquid holdup that will build up to flooding. Contact time is controlled by the bed depth and the gas velocity. In countercurrent flow mass transfer driving force is maximum at the gas entrance and liquid exit. Cocurrent operation can be carried out at high gas velocities because there is no flooding limit. In fact, liquid holdup decreases as velocity increases. However, the mass transfer driving force is smaller than in countercurrent operation.
Some processes for both absorption and the removal of particulates employ a cross-flow spray
chamber operation. Here, the water is sprayed down on a bed of packing material. The carrier gas contain-ing pollutant gas or the particulate flows horizontally through the packing, with the spray and packing caus-ing the absorbed gas or particles to be forced down to the bottom of the spray chamber where they can be removed. Fig. 3 illustrates a cross-flow absorber. The design of cross-flow absorption equipment is more dif-ficult than vertical towers because the area for mass transfer is different for the gas and liquid phases.
Continuous and steady-state operation is usually most economical. However, when smaller quantities of material are processed, it is often more advanta-geous to charge the entire batch at once. In fact, in many cases this is the only way the process can be done. This is called batch operation and is a transient operation from start-up to shut-down. A batch opera-tion presents a more difficult design problem.
Packed tower internals
In addition to the packing, absorption towers must include internal parts to make a successful piece of operating equipment. Fig. 4 illustrates the placement of the tower internals. These internals begin with a packing support plate at the bottom of the tower.
The packing support plate must physically support the weight of the packing. It must incorporate a high percentage of free area to permit relatively unrestricted flow of downcoming liquid. A flat plate has the disad-vantage in that both liquids and gases must pass coun-tercurrently through the same holes. Therefore, a substantial hydrostatic head may develop. Further-more, the bottom layer of packing partially blocks many of the openings reducing the free space. Both of these conditions lower tower capacity. A gas injec-tion plate provides separate passage for gas and liquid and prevents buildup of hydrostatic head.
Liquid distributors are used at all locations where an external liquid stream is introduced. Absorbers and strippers generally require only one distributor, while continuous distillation towers require at least two, at the feed and reflux inlets. The distributors should be 6–12 in. above packing to allow for gas disengagement from the bed. The distributor should provide uniform liquid distribution and a large free area for gas flow.
Liquid redistributors collect downcoming liquid and distribute it uniformly to the bed below. Initially, after entering the tower the liquid tends to flow out to the wall, the redistributor makes that portion of the liquid more available again to the gas flow. It also breaks up the coalescence of the downcoming liquid, and it will eliminate factors that cause a loss of efficiency in the tower and reestablish a uniform pattern of liquid Fig. 1 Countercurrent flow packed tower.
irrigation. A bed depth of up to 6 m (20 ft) should be alright before redistribution is needed.
Retaining and hold-down plates are used only with ceramic or carbon tower packing. They prevent the upper portion of the packed bed from becoming flui-dized and from breaking up during surges in pressure or at high-pressure drop. The plates rest directly on packing and restrict movement by virtue of the weight of the plate. Retainers or bed limiters prevent bed expansion or fluidization. When operating at high-pressure drops, retainers are fastened to the wall. They are designed to prevent individual packing pieces from passing through the plate openings.
PLATE TOWERS
Plate or tray towers are vertical cylinders in which the gas and liquid are contacted on horizontal plates in a stepwise fashion. By the nature of the operation plate towers are countercurrent flow devices. Fig. 5 shows a typical arrangement. In plate columns the gas is introduced at the bottom. Contact between gas and liquid is obtained by forcing the gas to pass upward through small orifices, bubbling through a liquid layer flowing across a plate. The liquid is introduced at the
Plate or tray towers are vertical cylinders in which the gas and liquid are contacted on horizontal plates in a stepwise fashion. By the nature of the operation plate towers are countercurrent flow devices. Fig. 5 shows a typical arrangement. In plate columns the gas is introduced at the bottom. Contact between gas and liquid is obtained by forcing the gas to pass upward through small orifices, bubbling through a liquid layer flowing across a plate. The liquid is introduced at the