Tower, absorption

The BASF process uses /V-methy1pyrro1idinone as the solvent to purify acetylene in the cracked gas effluent. Alow pressure prescmbbing is used to remove naphthalenes and higher acetylenes. The cracked gas is then compressed to 1 MPa (10 atm) and fed to the main absorption tower for acetylene removal. Light gases are removed from the top of this tower. 

Acetylene Absorption. The gaseous feedstock containing the hydrocarbons is introduced into the acetylene absorption tower at a pressure... 

First Alternative. Figure 1 illustrates the first of the two alternative production processes. Here the mother Hquor from the sodium nitrate crystallization plant, normally containing about 1.5 g/L iodine as iodate, is decanted for clarification and concentration homogenization. From there the solution is spHt into two fractions. The larger fraction is fed into an absorption tower where it is contacted with SO2 obtained by sulfur combustion. In the absorption tower iodate is reduced to iodide according to the following reaction ... 

After leaving the absorption tower, the resulting iodide solution, together with the iodide solution coming from the kerosene extraction plant as described below, is contacted with the smaller iodate fraction in the stoichometric proportion shown, producing iodine ... 

This secondary reaction starts at about 180°C, but the mass must be heated to 350—400°C to bring the reaction to completion and produce a nitrate-free product. The off-gases are extremely corrosive and poisonous, and considerable attention and expense is required for equipment maintenance and caustic-wash absorption towers. Treatment of the alkaline wash Hquor for removal of mercury is required both for economic reasons and to comply with governmental regulations pertaining to mercury ia plant effluents. 

Gas contact is typically carried out in absorption towers over which the alkaline solutions are recirculated. Strict control over the conditions of absorption are required to efficiendy capture the NO and convert it predominantly to sodium nitrite according to the following reaction, thereby minimizing the formation of by-product sodium nitrate. Excessive amounts of nitrate can impede the separation of pure sodium nitrite from the process. 

Ma.nufa.cture. In a typical process, a solution of sodium carbonate is allowed to percolate downward through a series of absorption towers through which sulfur dioxide is passed countercurrently. The solution leaving the towers is chiefly sodium bisulfite of typically 27 wt % combined sulfur dioxide content. The solution is then mn into a stirred vessel where aqueous sodium carbonate or sodium hydroxide is added to the point where the bisulfite is fully converted to sulfite. The solution may be filtered if necessary to attain the required product grade. A pure grade of anhydrous sodium sulfite can then be crystallized above 40°C because the solubiUty decreases with increasing temperature. 

Plants producing oleum or Hquid SO typically have one or two additional packed towers irrigated with oleum ahead of the normal SO absorption towers. Partial absorption of SO occurs in these towers, and sulfuric acid is added to maintain desired oleum concentrations. Normally, oleum up to about 35 wt % free SO content can be made in a single tower two towers are used for 40 wt % SO. Liquid SO is produced by heating oleum in a boder to generate SO gas, which is then condensed. Oleums containing SO >40 wt % are usually produced by mixing SO with low concentration oleum. 

Gas leaving the converter is normally cooled to 180—250°C using boiler feedwater in an "economizer." This increases overall plant energy recovery and improves SO absorption by lowering the process gas temperature entering the absorption tower. The process gas is not cooled to a lower temperature to avoid the possibiUty of corrosion from condensing sulfuric acid originating from trace water in the gas stream. In some cases, a gas cooler is used instead of an economizer. 

Gas leaving the economizer flows to a packed tower where SO is absorbed. Most plants do not produce oleum and need only one tower. Concentrated sulfuric acid circulates in the tower and cools the gas to about the acid inlet temperature. The typical acid inlet temperature for 98.5% sulfuric acid absorption towers is 70—80°C. The 98.5% sulfuric acid exits the absorption tower at 100—125°C, depending on acid circulation rate. Acid temperature rise within the tower comes from the heat of hydration of sulfur trioxide and sensible heat of the process gas. The hot product acid leaving the tower is cooled in heat exchangers before being recirculated or pumped into storage tanks. 

Oleum Ma.nufa.cture, To produce fuming sulfuric acid (oleum), SO is absorbed in one or more special absorption towers irrigated by recirculated oleum. Because of oleum vapor pressure limitations the amount of SO absorbed from the process gas is typically limited to less than 70%. Because absorption of SO is incomplete, gas leaving the oleum tower must be processed in a nonfuming absorption tower. 

Lime-Kiln Operation. Gases containing up to 40% carbon dioxide from the lime kiln pass through a cyclone separator, which removes the bulk of entrained dust. The gas is then blown through the two scmbbers, which remove the finer dust, cooled, and passes iato an absorption tower. Here carbon dioxide may be recovered by the sodium carbonate or Girbotol process. 

The effects of various catalysts (47,48), contaminants (49,50), acid concentration (51), temperature (52), and pressure (53—57) on the rate of absorption have been studied. The patent Hterature indicates that absorption can be improved by making the contact between the gaseous ethylene and hquid sulfuric acid more efficient (58—61), by suitable design of the absorption tower (62), and by various combinations of absorption and hydrolysis (63-68). 

FIG. 5-27 Identification of concentrations at a point in a countercurrent absorption tower,... 

Use of Mass-Transfer-Rate Expression Figure 14-3 shows a section of a packed absorption tower together with the nomenclature that will be used in developing the equations which follow. In a differential section dh, we can equate the rate at which solute is lost from the gas phase to the rate at which it is transferred through the gas phase to the interface as follows ... 

FIG. 14-5 Nnmher of overall gas-phase mass-transfer units in a packed absorption tower for constant mGf /LM solution of Eq. (14-23). (From Sherwood and Pigford, Absorption and Extraction, McGraw-Hill, New York, 1952. )... 

Overview One of the most important considerations involved in designing gas-absorption towers is to determine whether or not temperatures will vaiy along the length of the tower because of heat effects, since the sohibility of the solute gas normally depends strongly upon the temperature. When heat effects can be neglected, computation of the tower dimensions and required flows is relatively straight-... 

If the solute-rich gas entering the bottom of an absorption tower is cold, the liquid phase may be cooled somewhat by transfer of sensible heat to the gas. A much stronger cooling effect can occur when the solvent is volatile and the entering rich gas is not saturated with respeci to the solvent. It is possible to experience a condition in which... 

Temperature and Humidity of Rich Gas Cooling and consequent dehumidification of the feed gas to an absorption tower can be very beneficial. A high humidity (or relative saturation with solvent) limits the capacity of the gas phase to take up latent heat and therefore is unfavorable to absorption. Thus, dehumidification of the inlet gas prior to introducing it into the tower is worth considering in the design of gas absorbers with large heat effects. 

For dilute systems in countercurrent absorption towers in which the equilibrium curve is a straight line (i.e., yj = mXi) the differential relation of Eq. (14-60) is formulated as... 

Substitution of Eq. (14-61) into Eq. (14-62) and integration lead to the following relation for an extremely slow first-order reaction in an absorption tower ... 

Table 14-2 illustrates the observed variations in values for different packing types and sizes for the COg-NaOH system at a 25 percent reactant-conversion level for two different liquid flow rates. The lower rate of 2.7 kg/(s-m ) or 2000 lb/(h-ft ) is equivalent to 4 (U.S. gal/min)/ft and is typical of the liquid rates employed in fume scrubbers. The higher rate of 13.6 kg/(s-m ) or 10,000 lb/(h-fU) is equivalent to 20 (U.S. gal/min)/ft and is more typical of absorption towers such as are used in CO9 removal systems. For example. We note also that two different gas velocities are represented in the table, corresponding to superficial velocities of 0.59 and 1.05 m/s (1.94 and 3.44 ft/s). 

FIG. 14-91 Particle -size distribution and mist loading from absorption tower in a contact HC1SO4 plant. [Gillespie and Johnstone, Cbem. Eng. Prog., 51(2), 74 (1955).]... 

Of the three categories, the packed column is by far the most commonly used for the absorption of gaseous pollutants. Miscellaneous gas-absorption equipment could include acid gas scrubbers that are commonly classified as either wet or diy. In wet scrubber systems, the absorption tower uses a hme-based sorbent liquor that reacts with the acid gases to form a wet/solid by-product. Diy scrubbers can be grouped into three catagories (1) spray diyers (2) circulating spray diyers and (3) dry injection. Each of these systems yields a diy product that can be captured with a fabric filter baghouse downstream and... 

Zenz, F. A., Designing Gas-Absorption Towers, Chemical Engineering, November 13, 1972. 

Other Considerations For organic vapor HAP control applications, low outlet concentrations will typically be required, leading to impractically tall absorption towers, long contact times, and high liquid-gas ratios that may not be cost-effective. Wet scrubbers will generally be effective for HAP control when they are used in combination with other control devices such as incinerators or carbon adsorbers. 

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