Absorption system

In the following cases absorption processes with and without chemical reaction will be demonstrated. 

The flow system shown below comprises of three reactors. In the first two reactors absorption of a single component takes place, whereas the third reactor is assumed as total collector for the absorbed gas. If this is not the case, Eq.(5.2-1(c)) is applicable and in the following matrix P33 = 1- )i3 At. The fluid in which the species are absorbed, enters the first reactor and leaves the third one at a rate 

A simpler case which can be treated by applying the above model is the absorption of one component, f = 1, into a single reactor, i.e. reactor 1 where reactor 

3 is the total collector . Under these conditions, ai2 = 0 and P13 = 1 in Fig.5.2-2(1). In the numerical solution it was assumed that the reactors are of an identical volume, i.e. a constant p,. Other parameters kept unchanged were C j = 310-6 and 

The flow system is shown in Fig.5.2-l(l). It comprises of two reactors and only in the first one the chemical reaction given below takes place, i.e.  

As the equilibrium partial pressure of hydrogen chloride over a 35.2 per cent hydrochloric acid is only pxHci = 135 mm Hg the gas under partial pressure Phci = 228 mm Hg will be readily absorbed in the acid (the difference in pressures pHci — Pan — AB determines the absorption rate). 

The line CD represents the absorption of gas containing 50 per cent HG1. From its position we can see that with this gas the difference pHC( — in 

Curve 2 represents the partial pressure of hydrogen chloride over the hydrochloric acid at 80 °C. It can be seen that this curve does not intersect the ordinate of the 35.2% hydrochloric acid. This means that from a gas with partial pressures of hydrogen chloride comprised in the diagram an acid of the required concentration cannot be produced. 

When hydrogen chloride is absorbed by water a great amount of heat is released which causes a rise in temperature in the absorbers. Hence, if an acid with a high concentration is to be produced, the gases must not only be cooled before entering the absorption equipment, but also care must be taken to cool the absorbing liquid during the operation. 

When hydrogen chloride produced in sulphate furnaces is used for the manufacture of hydrochloric acid, the gases after being eooled to about 60 °C in quartz pipes outwardly sprayed with water are first led to stoneware towers packed with ceramic fillings or coke to remove the entrained mist of the sul- 

Benfield LoHeat System (UOP) [677], The high thermal efficiency of this two-stage adsorption process using lean and semi-lean solvent is achieved by recompression of the flash steam with an injector or a mechanical vapor compressor. UOP has also developed a number of other process configurations [404], [669], [678] -[680], The diethanolamine promoter enhances the mass transfer by carbamate 

Recently UOP has introduced a new activator, called ACT-1 [673] which is claimed to reduce C02 levels in the absorber exit gas in existing plants and should reduce the solvent circulation rate as well. For new installations the enhanced mass transfer achieved with the new activator translates into smaller towers and therefore to investment capital savings. The activator is an amine compound, but speculations on its nature are still going on. 

LRS 10 of British Gas [675], [676], [681]. This new activator combination for hot potash systems was commercially introduced in 1988 and since then used in 30 plants which include ammonia, LNG, hydrogen and vinyl acetate. A corrosion inhibitor is still used in the absorption solution. 

Exxon s Flexsorb process [669], [670] uses a sterically hindered amine as an effective promoter of a hot potash system. A lower energy requirement and less solution makeup than in other processes are claimed. So far it has been successfully used in several large plants. 

Other hot potash processes offered are the Carbosolvan process [671] and the SPIC process [681]. 

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If the values of in the combining states are very different the dissociation limit of a progression may be observed directly as an onset of diffuseness. However, the onset is not always particularly sharp this is the case in the B Uq+ absorption system of iodine... 

Similarly, the acetylenic group, —C=C—, shows an intense absorption system at about... 

Discussion of the concepts and procedures involved in designing packed gas absorption systems shall first be confined to simple gas absorption processes without compHcations isothermal absorption of a solute from a mixture containing an inert gas into a nonvolatile solvent without chemical reaction. Gas and Hquid are assumed to move through the packing in a plug-flow fashion. Deviations such as nonisotherma1 operation, multicomponent mass transfer effects, and departure from plug flow are treated in later sections. 

Fig. 6. Operating lines for an absorption system line A, high -L /ratio soHd line, medium -L /G ratio line B, -L /G ratio at theoretical minimum...

Absorption Systems. Absorption refrigeration cycles employ a secondary fluid, the absorbent, to absorb the primary fluid, refrigerant vapor, which has been vaporized in the evaporator. The two materials that serve as the refrigerant—absorbent pair must meet a number of requirements however, only two have found extensive commercial use ammonia—water and water—Hthium bromide. 

Recovery and Purification. AH processes for the recovery and refining of maleic anhydride must deal with the efficient separation of maleic anhydride from the large amount of water produced in the reaction process. Recovery systems can be separated into two general categories aqueous- and nonaqueous-based absorption systems. Solvent-based systems have a higher recovery of maleic anhydride and are more energy efficient than water-based systems. 

Phjsica/ absorption systems A/-methyl-2-pyrrolidinone methanol... 

The Rectisol process is more readily appHcable for acid gas removal from synthesis gas made by partial oxidation of heavy feedstocks. The solvents used in Purisol, Fluor Solvent, and Selexol processes have low vapor pressures and hence solution losses are minimal. Absorption systems are generally corrosion-free. 

In a typical batch operation, carbon disulfide is added to four molar equivalents of 25—30 wt % aqueous ammonia in a stirred vessel, which is kept closed for the first one to two hours. The reaction is moderately exothermic and requires cooling. After two to three hours, when substantially all of the disulfide has reacted, the reaction mixture is heated to decompose dithiocarbamate and trithiocarbonate and vented to an absorption system to collect ammonia, hydrogen sulfide, and any unreacted carbon disulfide. 

Effects of System Physical Properties on Ug and Ui When designing packed towers for nonreacting gas-absorption systems for... 

Absorption Refrigeration Systems Two main absorption systems are used in industrial application lithium bromide-water and ammonia-water. Lithium bromide-water systems are hmited to evaporation temperatures above freezing because water is used as the refrigerant, while the refrigerant in an ammonia-water system is ammonia and consequently it can be applied for the lower-temperature requirements. 

The ammonia-water absorption system was extensively used until the fifties when the LiBr-water combination became popular. Figure 11-103 shows a simplified ammonia-water absorption cycle. The refrigerant is ammonia, and the absorbent is dilute aqueous solution of ammonia. Ammonia-water systems differ from water-lithium bromide equipment to accommodate major differences Water (here absorbent) is also volatile, so the regeneration of weak water solution to strong water solution is a fractional distillation. Different refrigerant (ammonia) causes different, much higher pressures about 1100-2100 kPa absolute in condenser. 

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. 

An appropriate procedure for executing the design of an absorber-stripper system is to set up a carefully selected series of design cases and then evaluate the investment costs, the operating costs, and the operability of each case. Some of the economic factors that need to be considered in selec ting the optimum absorber-stripper design are discussed later in the subsec tion Economic Design of Absorption Systems. ... 

For gas-absorption systems in which the inlet gas is concentrated, the correct equation is... 

Introduction Many present-day commercial gas absorption processes involve systems in which chemical reactions take place in the liquid phase. These reactions generally enhance the rate of absorption and increase the capacity of the liquid solution to dissolve the solute, when compared with physical absorption systems. 

Recommended Overall Design Strategy When considering the design of a gas-absorption system involving chemical reactions, the following procedure is recommended ... 

Traditional Design Method The traditionally employed conventional procedure for designing packed-tower gas-absorption systems involving chemical reactions makes use of overall volumetric mass-transfer coefficients as defined by the equation... 

The reader may refer to the data in the preceding edition. For the current work, emphasis will be given to one absorption system, carbon dioxide-air-caustic, and to several distillation systems. 

TABLE 25-18 Advantages and Disadvantages of Absorption Systems (Packed and Plate Columns)... 

The hydrogen chloride absorption system must be of such design that there is no possibility for water to suck back into the reaction flask after a sudden surge of escaping hydrogen chloride. 

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