Stripping operation design

Recovery of the solvent, sometimes by chemical means but more often by distillation, is almost always required, and the recoveiy system ordinarily is considered an integral part of the absorption-system process design. A more efficient solvent-stripping operation normally will result in a less costly absorber because of a smaller concentration of residual dissolved solute in the regenerated solvent however, this may increase the overall cost of solvent recoveiy. A more detailed discussion of these and other economic considerations is presented later in this section. 

The design of a plate tower for gas-absorption or gas-stripping operations involves many of the same principles employed in distillation calculations, such as the determination of the number of theoretical plates needed to achieve a specified composition change (see Sec. 13). Distillation differs from gas absorption in that it involves the separation of components based on the distribution of the various substances between a gas phase and a hquid phase when all the components are present in Doth phases. In distillation, the new phase is generated From the original feed mixture by vaporization or condensation of the volatile components, and the separation is achieved by introducing reflux to the top of the tower. 

Desorption with Chemical Reaction When chemical reactions are involved in a stripping operation, the design problem can become extremely complex. In fact, much less is known about this very important process than is known about absorption. A classic work on this subject is that of Shah and Sharma [Trans. In.st. Chem. Tng., 54, 1 (1976)], which is recommended to those in need of more details. 

Ammonia vapors from a stripping operation are to be condensed. Select the condenser pressure, which sets the top of stripper pressure, and design a condenser. Water at 90°F is to be used. 

A spherical preventer contains a packing element that seals the space around a drillpipe. This preventer is not designed to shut off the well when the drillpipe is out of the hole, although it allows stripping operations and some pipe rotation. Hydril Corporation, Shaffer and other manufacturers provide several models with different packing system designs for specific types of service. The ram-type preventer seals the annulus around the drillpipe however, each size of ram is designated for only one size of drillpipe. In other words, the preventer with 5-in. pipe rams can provide a seal only on 5-in. drillpipe. The preventer with blind rams is used to shut in the well if the pipe is not in the hole. If this type of preventer is activated with the pipe in the hole, the pipe can be cut. 

The engineering analysis and design of these operations addresses questions which are different than those addressed in connection with the shaping operations. This is illustrated in Fig. 1 which is a flow sheet, cited by Nichols and Kheradi (1982), for the continuous conversion of latex in the manufacture of acrylonitrile-butadiene-styrene (ABS). In this process three of the nonshaping operations are shown (1) a chemical reaction (coagulation) (2) a liquid-liquid extraction operation which involves a molten polymer and water and (3) a vapor-liquid stripping operation which involves the removal of a volatile component from the molten polymer. The analysis and design around the devolatilization section, for example, would deal with such questions as how the exit concentration of... 

The analysis and design of any stripping operation would be relatively straightforward provided that the velocity and concentration profiles that obtain in the extraction unit are known. Solutions to the momentum and diffusion equations provide this information, but, for most cases of interest in the chemical process industries, solutions to these equations are difficult to obtain since the flow geometry is often not well defined and flow may be both tortuous and turbulent. When these circumstances prevail, scientifically based, semiempirical relationships have often provided the basis for analysis and design procedures. 

A well-established concept in the design of stripping operations in the chemical processing industries is the use of equations in which the height of the unit required is expressed as the product of two terms the number of transfer units (NTU), a measure of the difficulty of the operation to be carried out, and the height of a transfer unit (HTU), a measure of the height required in order to carry out an operation of unit difficulty. The advantage of this procedure is that the effectiveness of a particular unit is directly reflected in the value of the HTU. On the other hand, the advan-... 

An air-stripping system designed to minimize energy requirements (8) was installed to remove volahle organic compounds at levels between 200 and 250 pg/L from drinking water supplied by a municipal well field. The system was capable of treahng 2.16 MGD (8,176 m /d) at effluent levels not exceeding 10 pg/L total VOCs. Total installed cost was approx 200,000, and operational costs were less than 20,000/yr. 

Although the ammonia-stripping operation is both reliable and flexible in its routine operation, it has some shortcomings which must be carefully considered by the design engineer. Some major problems are ... 

Absorption and stripping are technically mature separation operations. Design procedures are well developed for both packed columns and tray towers, and commercial processes are common. In most applications, the solutes are contained in gaseous effluents from chemical reactors. Passage of strict environmental standards with respect to air pollution by emission of noxious gases from industrial sources has greatly increased the use of gas absorbers (also known as scrubbers) in the past decade. 

Although design and construction extended over four years, all four machines have a similar configuration with the only significant difference being the orientation of the stacking conveyor. The latter difference is necessary to allow the stripping operation to proceed at the extreme ends of the cellhouse. 

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