Stripping with chemical reaction

Approximate design equations apphcable only to the case of pure physical desorption are developed later in this sec tion for both packed and plate stripping towers. A more rigorous approach using distiUation concepts may oe found in Sec. 13. A brief discussion of desorption with chemical reac tion is given in the subsec tion Absorption with Chemical Reaction. ... 

In situations in which one cannot assume that Hl and Hql I e constant, these terms must be incorporated inside the integrals in Eqs. (14-24) and (14-25). and the integrals must be evaluated graphically or numerically (by using Simpsons nile, for example). In the normal case involving stripping without chemical reactions, the hquid-phase resistance will dominate, making it preferable to use Eq. (14-25) in conjunction with the relation Hl — Hql. 

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. 

B Simple transport with chemical reaction in strip solution... 

The partial pressure of the transported species is maintained constant in the two gas phases separated by the membrane, by virtue of either the velocity of a sweep gas or the volume of closed gas phases. The membranes are generally in the form of a soaked filter oaoer, or of homogeneous solution supported by highly permeable polymer membranes. At one membrane face there is absorption with chemical reaction at the other there is stripping. Steady-state operation is ensured by zero net conversion within the membrane as a whole. 

A typical feed to a commercial process is a refinery stream or a steam cracker B—B stream (a stream from which butadiene has been removed by extraction and isobutylene by chemical reaction). The B—B stream is a mixture of 1-butene, 2-butene, butane, and isobutane. This feed is extracted with 75—85% sulfuric acid at 35—50°C to yield butyl hydrogen sulfate. This ester is diluted with water and stripped with steam to yield the alcohol. Both 1-butene and 2-butene give j -butyl alcohol. The sulfuric acid is generally concentrated and recycled (109) (see Butyl alcohols). 

Amine strippers use heat and steam to reverse the chemical reactions with CO2 and H2S. The steam acts as a stripping gas to remove the COo and HjS from the liquid solution and to cairy these gases to the overhead. To promote mixing of the solution and the steam, the stripper is a trayed or packed tower with packing normally used for small diameter columns. 

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... 

Under the reaction conditions described in the patents, methacrolein is always present in nonnegligible amounts, and therefore a commercial process necessitates an economical method for recycling methacrolein. The patents assigned to Asahi Chemical Industry claim the use of an organic solvent, a mixmre of decane, undecane, and dodecane, which can efficiently absorb isobutane and methacrolein from the off-gas, with 99.5% recovery efficiency. Isobutane and methacrolein are then stripped with air and recycled. 

The other type of chemical mechanism is more selective and is used when the solute is not soluble in the membrane phase, therefore requiring the addition of a selective reactant into the membrane to form a complex or an ion pair with the solute. The reaction product then diffuses across the membrane and at the second interface it reacts with a species added to phase 3 so that stripping also takes place by chemical reaction (Fig. 15.2b). This mechanism is called carrier-mediated membrane transfer. The reagent recovered from the reversed reaction then transfers back to the extraction interface. This is usually called the reagent shuttle mechanism. 

By scanning AFM tips along a surface, it is possible to induce chemical reactions along a line. One example is the application of a voltage to an AFM tip to oxidise strips of silicon with nanometre dimensions. The electric field from an AFM with a nanotube tip will strip hydrogen from the silicon surface, leaving a line of bare silicon which is readily oxidised in air so that the net result is a track of Si02 on the silicon surface. 

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