Absorption towers composition

The screening of carriers, catalyst composition, particle sizes and shapes showed indeed, that a much more active catalyst could be made with Cs as a secondary promoter for the beds downstream the intermediate absorption tower. The best candidates were selected, and some m3 of each recipe were produced as 9-mm and 12-mm Daisy extrudates in a successful commercial-scale test production. The activities were as expected from the previous development work, and a 30 day activity test also confirmed it to be stable during this period. 

The evaluation of carriers and catalyst compositions showed that significantly higher SO2 oxidation activity could be achieved with Cs as a promoter under the operating conditions downstream the intermediate absorption tower as demonstrated by the results in Table 1, where the activity compared to the standard product is increased by more than a factor 2. This was clearly sufficient for the introduction of VK69 to the market as a new sulphuric acid catalyst. The activity results for different melt compositions were used to optimise the vanadium content and the molar ratios of K/V, Na/V. and Cs/V. However, the choice of Cs/V was not only a question of maximum activity, because of the significant influence of the Cs content on the raw material costs (the price of caesium is 50-100 times the price of potassium on a molar basis). Here, the economic benefits obtained by the sulphuric acid producer by the marginal activity improvement at high Cs content also had to be taken into account. 

FIG. 14-8 Graphical method for a three-theoretical-plate gas-absorption tower with inlet-liquor composition y and inlet-gas composition t/j. 

MATERIAL BALANCES. In a differential-contact plant such as the packed absorption tower illustrated in Fig. 22.9, there are no sudden discrete changes in composition as in a stage-contact plant. Instead the variations in composition are continuous from one end of the equipment to the other. Material balances for the portion of the column above an arbitrary section, as shown... 

Concentration analysers (e.g. for sulphuric acid at inlet of absorption towers) Gas composition analysers (stack gas analysers)... 

Complicated as all this seems to be, often a fairly simple relation can be derived in an effort to optimize part of the plant, or to partially optimize the whole plant. As an example, consider a simplified absorption tower where a gas stream F, containing z percent of a valuable material, is absorbed by a liquid stream L, which is to be manipulated to maintain minimum-cost operation. Assume that gas-exit composition y varies as follows ... 

Figure 24.1 Figure 23.1 s single contact absorption tower. Its quantities, temperatures, and gas compositions are used in Sections 24.1 and 24.2 calculations. The molar quantities are all per kg mol of Section 23.3 s first catalyst bed feed gas. The calculations assume that all input SO3 reacts to form H2S04(f). Note that output gas temperatureacid temperature. 

Figure 24.6 Equivalent composition and temperature of absorption tower product acid.

The bottom-packed bed is fed with 450 K, slightly diluted return acid from the steam-from-acid heat boiler. H20( ) in the descending acid reacts with the ascending gas SO3 to form hot H2S04( ) by exothermic reaction (1.2). Input acid composition and flow rate are controlled to give hot ( 500 K) absorption tower product acid. 

When acetylene is recovered, absorption—desorption towers are used. In the first tower, acetylene is absorbed in acetone, dimethylformarnide, or methylpyroUidinone (66,67). In the second tower, absorbed ethylene and ethane are rejected. In the third tower, acetylene is desorbed. Since acetylene decomposition can result at certain conditions of temperature, pressure, and composition, for safety reasons, the design of this unit is critical. The handling of pure acetylene streams requires specific design considerations such as the use of flame arrestors. 

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. 

Absorption of COj with carbonate solutions in a tower packed with 1-in. Raschig rings, (a) Correction factor /i for temperature and liquid rate (6) correction factors ft and /> for composition. [Sherwood and Pigford... 

The tray temperatures in our preflash tower, shown in Fig. 4.4, drop as the gas flows up the tower. Most of the reduced sensible-heat content of the flowing gas is converted to latent heat of evaporation of the downflowing reflux. This means that the liquid flow, or internal reflux rate, decreases as the liquid flows down the column. The greater the temperature drop per tray, the greater the evaporation of internal reflux. It is not unusual for 80 to 90 percent of the reflux to evaporate between the top and bottom trays in the absorption section of many towers. We say that the lower trays, in the absorption section of such a tower, are drying out. The separation efficiency of trays operating with extremely low liquid flows over their weirs will be very low. This problem is commonly encountered for towers with low reflux ratios, and a multicomponent overhead product composition. 

Algebraic Method for Concentrated Gases When the feed gas is concentrated, the absorption factor, which is defined in general as A = Lm/KGm and where K = t/°/x, can vary throughout the tower due to changes in temperature and composition. An approximate solution to this problem can be obtained by substituting the effective adsorption factors A, and A derived by Edmister [Ind. Eng. Chem. 35, 837 (1943)] into the equation... 

Effects of System Physical Properties on kG and kL When designing packed towers for nonreacting gas-absorption systems for which no experimental data are available, it is necessary to make corrections for differences in composition between the existing test data and the system in question. The ammonia-water test data (see Table 5-24-B) can be used to estimate HG, and the oxygen desorption data (see Table 5-24-A) can be used to estimate HL. The method for doing this is illustrated in Table 5-24-E. There is some conflict on whether the value of the exponent for the Schmidt number is 0.5 or 2/3 [Yadav and Sharma, Chem. Eng. Sci. 34, 1423 (1979)]. Despite this disagreement, this method is extremely useful, especially for absorption and stripping systems. 

Air at 25 °C is used to dry a plastic sheet containing acetone. At the drier exit, the air leaves containing 0.02 mole fraction acetone. The acetone is to be recovered by absorption with water in a packed tower. The gas composition is to be reduced to 5 X10 mole fraction at the colunm exit. The equilibrium relationship is y = 1.8x. The gas enters the bottom of the colunm (Figme 6.12) at a flux of 1000 Ibm/ft - hr, and the water enters the top at a flux of 1400 Ibm/ft hr. The tower is packed with... 

There is continuous transfer of material between phases, and the composition of each phase changes as it flows through the tower. At any given level, of course, equilibrium is not reached indeed, it is the departure from equilibrium that provides the driving force for material transfer. The rate of mass transfer is relatively low compared to distillation or absorption, and a tall column may be equivalent to only a few perfect stages. 

When molasses is fermented to produce a liquor containing ethanol, a C02-rich vapor containing a small amount of ethanol is evolved. The alcohol will be recovered by countercurrent absorption with water in a packed-bed tower. The gas will enter the tower at a rate of 180 kmol/h, at 303 K and 110 kPa. The molar composition of the gas is 98% C02 and 2% ethanol. The required recovery of the alcohol is 97%. Pure liquid water at 303 K will enter the tower at the rate of 151.5 kmol/h, which is 50% above the minimum rate required for the specified recovery (Seader and Henley, 1998). The tower will be packed with 50-mm metal Hiflow rings and will be designed for a maximum pressure drop of 300 Pa/m of packed height. 

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