Absorbers and Strippers

7 Consider three different configurations of absorbers for removing an organic pollutant, P, from air. For this particular system, the equilibrium line is straight. 

8 Your lungs can be modeled as an absorber that transfers O2 from the air to your blood. 

The map below is a graphical analysis of lungs for a normal breathing rate and a normal pulse rate. Because you will use this map to graph your answers to parts (A)-(D) of this exercise, you are advised to photocopy and enlarge the map. 

We are indebted to Professor Richard Seagrave for suggesting the concept for this exercise. Similar interesting applications to biological systems can be found in Sea-grave s text. Biological Applications of Heat and Mass Transfer, Iowa State University Press, 1971. 

9 Consider an absorber with the flow rates and compositions given in the diagram below. 

RCMs also have limitations in that they only give information at infinite reflux (see Section 2.7). 

In this section, we will look at both absorption and stripping towers as stand-alone separation units. However, because of the configuration of these units being single CSs, they do also offer important information and ideas about complex columns, which consist of internal CSs that in essence behave similarly to either a stripper or absorber. 

In order to solve the problem by means of the DPE, the two critical parameters, namely and have to be specified. By merely specifying the flowrate of vapor and liquid streams it is simple to determine the reflux ratio. It is important to emphasize again that constant molar overflow (CMO) is assumed, meaning that the vapor and liquid flows remain constant throughout the column (see Section 3.3.2). This is a standard assumption in absorber and stripper design, and is generally a very good one, but it can be relaxed if the situation calls for it [5,6]. Recall that the reflux ratio is defined as 

The remaining parameter, X, requires by definition that the flowrates and compositions of streams either at the top or the bottom of the column be specified. Since the vapor and liquid flowrates are assumed to remain constant throughout the column, the streams entering and leaving the column can be determined by specifying R. We need only specify the compositions of the streams at the top or the bottom of the column to determine X. Again, the definition of Xa is given as 

For the absorption problem it is convenient to use the top of the column for specifying Xa, since one usually knows the composition of the clean absorbent (usually a pure liquid) and also the desired impurity composition that the exiting vapor stream at the top of the column has to achieve. Thus, through these compositions and flowrates all the parameters for integrating the DPE have now been specified 

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The overhead temperatures of both the absorber and stripper are kept as low as possible to minimise solvent carryover. A temperature of about 311 K is typically used ia the high pressure absorber. The overhead temperature ia the stripper is set by the boiling poiat of the saturated complex solution and by the operating pressure of the stripper. At a stripping pressure of 0.166 MPa (1.7 atm), a temperature of 378 Kis used. The solvent-rich gas from the stripper is cooled to recover as much solvent as possible by condensation prior to the final aromatics-recovery section. Fiaal solvent recovery is accomphshed by adsorption on activated carbon (95). 

Generahzed prediction methods for fci and Hi do not apply when chemical reaction occurs in the liqmd phase, and therefore one must use ac tual operating data for the particular system in question. A discussion of the various factors to consider in designing gas absorbers and strippers when chemical reac tious are involved is presented by Astarita, Savage, and Bisio, Gas Treating with Chemical Solvents, Wuey (1983) and by Kohl and Ricseufeld, Gas Purification, 4th ed., Gulf (1985). 

The SB method is not presented here, but is presented in detail in the sixth edition of Peny s Chemical Engineers Handbook. Extensions of the SB method to nonideal mixtures and complex configurations are developed by Eckert and Hlavacek [Chem. Eng. ScL, 33, 77 (1978)] and Eckert [Chem. Eng. Sci., 37, 425 (1982)] respectively but are not discussed here. However, the approximate and very useful method of Kremser [Nat. Pet. News, 22(21), 43 (May 21, 1930)] for application to absorbers and strippers is discussed at the end of this subsec tion. 

Tray Efficiencies in Plate Absorbers and Strippers Compn-tations of the nnmber of theoretical plates N assnme that the hqnia on each plate is completely mixed and that the vapor leaving the plate is in eqnihbrinm with the liqnid. In actnal practice a condition of complete eqnihbrinm cannot exist since interphase mass transfer reqnires a finite driving-force difference. This leads to the definition of an overall plate efficiency... 

Some performance data of plants with DEA are shown in Table 23-11. Both the absorbers and strippers have trays or packing. Vessel diameters and allowable gas and liquid flow rates are estabhshed by the same correlations as for physical absorptions. The calciilation of tower heights utilizes data of equilibria and enhanced mass-transfer coeffi-... 

Maxwell s correlation was generated from hydrocarbon data only. Ludwig states that the Drickamer and Bradford correlation is good for hydrocarbons, chlorinated hydrocarbons, glycols, glycerine and related compounds, and some rich hydrocarbon absorbers and strippers. 

Absorbers and strippers frequently operate with a liquid having essentially the same physical characteristics regardless of the pressure. An example of this is a gas absorber. The same lean oil is used if the tower is operating at 100 or l,000psi. This type of system is excluded from the CAFq limiting value. 

Rousseau, R. W. and Staton, S. J., Analyzing Chemical Absorbers and Strippers, Chemical Engineering, July 18, 1988. 

This method [18] is well suited to handling the details of a complicated problem, yet utilizing the concept of average absorption and stripping factors. It also allows for the presence of solute components in the solvent and the loss of lean oil into the off gas. Reference 18 presents more details than are included here. Reference 18 is Edmister s original publication of the basic method for absorbers and strippers. Reference 18 also generates the... 

Select design pressure drop for operations. Suggested values of below 1.0 in. water/ft. Low-pressure, atmospheric, and pressure columns usually require 0.5 to 0.7 in. water/ft, with absorbers and strippers around 0.2-0.6 in. water/ft. For vacuum distillation low values of 0.05-0.6 in. water/ft are often necessary, usually depending on the required boiling point of the bottoms. 

For independent absorbers and strippers, the Kremser-Brown formulas apply. The fraction absorbed is... 

Figure 13.18. Algorithm for the SR (sum rates) method for absorbers and strippers [Birningkam and Otto, Hydrocarbon Processing 46(10), 163-170 (1967) Henley and Seader, 1981].

The jmethod of O Connell is popular because of its simplicity and the fact that predicted values are conservative (low). It expresses the efficiency in terms of the product of viscosity and relative volatility, pa, for fractionators and the equivalent term HP In for absorbers and strippers. The data on which it is based are shown in Figure 13.43. For convenience of use with computer programs, for instance, for the Underwood-Fenske-Gilliland method which is all in terms in equations not graphs, the data have been replotted and fitted with equations by Ncgahban (University of Kansas, 1985). For fractionators,... 

Figure 13.43. Efficiencies of fractionators and absorber-strippers. The original curves of O Connell (Trans. AIChE 42, 741 1946)] have been replotted and fitted with equations, as shown on the figures, by S. Negahban (University of Kansas, 1985). (a) Fractionators (the viscosity ju is in cP). (b) Absorbers and strippers H = Henry s law constant in lb mol/(cuft)(atm), P is in atm, and fi is in cP.

Operating conditions for the absorption and stripping towers are important design parameters for the process. Due to vapor pressure and entrainment, propane will be present in the effluent gas streams from both the absorber and stripper. Usually this quantity of propane is not recovered and is considered an economic loss. The amount of propane in the gas phase is mainly dependent on the operating temperature and pressure of the towers. 

Figure 1 shows the vapor pressure of some of the relevant compounds as a function of temperature. At -230.8°F and -184°F, the vapor pressure of propane is about 0.1 mm Hg and 1 mm Hg respectively. Absorption/stripping process is a conventionally practical process so it is decided to evaluate the separation of methane from hydrogen and carbon monoxide by this process. Since the operating pressures of the absorber and stripper are about 500 psia and 487 psia respectively, the mole fractions of propane in the outlet gas streams of the absorber and stripper are about 6.75 x 10 6 and 1 x 10 4 at -230.8°F and -184°F respectively. 

When the absorber operates at higher temperature as shown in Table II, propane losses increase. When the absorber and stripper operate at -230.8°F and -184°F respectively, the propane loss is equivalent to about 0.0850% and 3.28% respectively of the cost of methane assuming the cost of methane and propane is the same. 

Table III. Summary of Dimensions and Characteristics of Absorbers and Strippers...

From Eq. (A-l), we can calculate the loss per set of pipes (10 pipes) connecting each absorber and stripper is... 

Figure 6 shows a simplified C02 capture process with two major process units absorber and stripper Zhang et al., 2009. A lean amine solvent (low C02 loading) is fed into the top of the absorber and is in counter-current contact with the gas containing C02. The C02 is chemically absorbed by the amine solvent and the treated gas exits the top of the absorber. The rich (high C02 loading) amine leaves the bottom of the absorber and is preheated by a cross heat exchanger before... 

Figure 6 Typical C02 capture unit with absorber and stripper.

Step 1. The first step in the procedure is to determine the key component equilibrium curve slope M. This is for one component only. You must choose a light key and a heavy key component for this tray efficiency calculation. Select components that are keys in the fractionator split. A single K = Y/X equilibrium value is to be applied to absorbers and strippers as this M value. The change of Y per the change of X is sought out for the light key component. 

In these systems, the interface between two phases is located at the high-throughput membrane porous matrix level. Physicochemical, structural and geometrical properties of porous meso- and microporous membranes are exploited to facilitate mass transfer between two contacting immiscible phases, e.g., gas-liquid, vapor-liquid, liquid-liquid, liquid-supercritical fluid, etc., without dispersing one phase in the other (except for membrane emulsification, where two phases are contacted and then dispersed drop by drop one into another under precise controlled conditions). Separation depends primarily on phase equilibrium. Membrane-based absorbers and strippers, extractors and back extractors, supported gas membrane-based processes and osmotic distillation are examples of such processes that have already been in some cases commercialized. Membrane distillation, membrane... 

The SR method is suitable for modeling absorbers and strippers. For some extremely wide boiling systems, especially those with noncondensables, it is the best method. It has been found to work very well for the side strippers of a refinery fractionator. Absorbers typically have a rich gas bottom stage feed and a lean oil top stage feed. The equations of the SR method do not allow its direct use for reboiled absorbers, absorbers with condensers, or distillation columns. For these columns, other methods like that of Tomich (Sec. 4.2.8) or Russell Sec. 4.2.10) can be used. 

Chapters 7 and 9 present that stage efficiency prediction and scaleup can be difficult and unreliable. Section 4.1.2 points out that the computational form in which stage efficiencies are often applied, as multipliers to the equilibrium If-values, may inadequately reflect actual equilibrium or column operation. For highly nonideal, polar, and reactive systems, such as amine absorbers and strippers, prediction and... 

Sum rates Strata f35> McNeese >36) Burningham and Otto (34) Absorbers and strippers, especially the wjdest-bojling systems Steam strippers All Bide products Jlows and duties must be specified... 

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