Liquid-phase limited

Data courtesy of the Norton Company.

Operating conditions (see text): 38-mm ceramic Intalox saddles; solute gases, 0.5-1.0 percent mole; reagent conversions = 33 percent; pressure, 101 kPa (1 atm); temperature, 16-24°C; gas rate = 1.3 kg/(s-m2) = 1.1 m/s; liquid rates = 3.4 to 6.8 kg/(s-m2); packed height, 3.05 m; tower diameter, 0.76 m. Multiply table values by 0.0624 to convert to (lb-mol)/(h'ft3).

Data courtesy of the Norton Company.

Operating conditions (see text): 38-mm ceramic Intalox saddles; solute gases, 0.5-1.0 percent mole; reagent conversions = 33 percent; pressure, 101 kPa (1 atm); temperature, 16-24°C; gas rate = 1.3 kg/(s-m2) = 1.1 m/s; liquid rates = 3.4 to 6.8 kg/(s-m2); packed height, 3.05 m; tower diameter, 0.76 m. Multiply table values by 0.0624 to convert to (lb-mol)/(h'ft3).

may be neglected). However, in 1975 Dankwerts and Alper [Trans. Inst. Chem. Eng., 53, T42 (1975)] showed that by placing a stirrer in the gas space of the stirred-cell laboratory absorber, the gas-phase mass-transfer coefficient kG in the laboratory unit could be made identical to that in a packed-tower absorber. When this was done, laboratory data for chemically reacting systems having a significant gas-side resistance could successfully be scaled up to predict the performance of a commercial packed-tower absorber.

If it is assumed that the values for kG, kL, and a have been measured for the commercial tower packing to be employed, the procedure for using the laboratory stirred-cell reactor is as follows:

1. The gas-phase and liquid-phase stirring rates are adjusted so as to produce the same values of kG and kL as will exist in the commercial tower.

2. For the reaction system under consideration, experiments are made at a series of bulk-liquid and bulk-gas compositions representing the compositions to be expected at different levels in the commercial absorber (on the basis of material balance).

3. The ratios of rA(cbB°) are measured at each pair of gas and liquid compositions.

For the dilute-gas systems, one form of the equation to be solved in conjunction with these experiments is hT =

Gm ryl dy a y2 rA

where hT = height of commercial tower packing, GM = molar gas-phase mass velocity, a = effective mass-transfer area per unit volume in the commercial tower, y = mole fraction solute in the gas phase, and rA = experimentally determined rate of absorption per unit of exposed interfacial area.

By using the series of experimentally measured rates of absorption, Eq. (14-73) can be integrated numerically to determine the height of packing required in the commercial tower.

A number of different types of experimental laboratory units could be used to develop design data for chemically reacting systems. Charpentier [ACS Symp. Ser., 72, 223-261 (1978)] has summarized the state of the art with respect to methods of scaling up laboratory data and has tabulated typical values of the mass-transfer coefficients, interfacial areas, and contact times to be found in various commercial gas absorbers, as well as in currently available laboratory units.

The laboratory units that have been employed to date for these experiments were designed to operate at a total system pressure of about 101 kPa (1 atm) and at near-ambient temperatures. In practical situations, it may become necessary to design a laboratory absorption unit that can be operated either under vacuum or at elevated pressure and over a range of temperatures in order to apply the Dankwerts method.

It would be desirable to reinterpret existing data for commercial tower packings to extract the individual values of the interfacial area a and the mass-transfer coefficients kG and k0L to facilitate a more general usage of methods for scaling up from laboratory experiments. Some progress has already been made, as described later in this section. In the absence of such data, it is necessary to operate a pilot plant or a commercial absorber to obtain kG, kL, and a as described by Ouwerkerk (op. cit.).

Modern techniques use rigorous modeling computer-based methods to extract fundamental parameters from laboratory-scale measurements and then apply them to the design of commercial absorption towers. These techniques are covered next.

Rigorous Computer-Based Absorber Design While the techniques described earlier in this section are very useful to gain an understanding of the key effects in commercial absorbers, current design methods used in industrial practice for chemically reactive systems are increasingly often based upon computerized rigorous methods, which are commercially available from software vendors. The advantages of these rigorous methods are as follows: (1) Approximations do not have to be made for special cases (e.g., fast chemical reactions or mass-transfer resistance dominated by the gas or liquid phase), and all effects can be simultaneously modeled. (2) Fundamental quantities such as kinetic parameters and mass-transfer coefficients can be extracted from laboratory equipment and applied to commercial absorber towers. (3) Integrated models can be developed for an entire absorption process flowsheet (e.g., the absorber-stripper system with heat integration presented in Fig. 14-3), and consequently the entire system may be optimized.

Computer programs for chemically reacting systems are available from several vendors, notably the following:




AMSIM Schlumberger Limited

ProTreat Sulphur Experts

TSWEET Bryan Research and Engineering RateSep Aspen Technology

Zhang and Ng, Proc. Ann. Conv.—Gas Proc. Assoc., Denver, Colo.; 1996, p. 22. Weiland and Dingman, Proc. Ann. Conv., Gass Proc. Assoc., Houston, Tex., 2001, p. 80. Polasek, Donnelly, and Bullin, Proc. 71st GPA Annual Conv., 1992, p. 58. Chen et al., AIChE Annual Meeting, San Francisco, Nov. 12-17, 2006.

The specific approaches used to model the chemically reacting absorption system are slightly different among the different vendors. The general approach used and the benefits obtained are highlighted by considering a specific example: removal of CO2 from flue gases discharged by a power plant using aqueous monoethanolamine (MEA), as presented by Freguia and Rochelle [AIChE J., 49, 1676 (2003)].

The development and application of a rigorous model for a chemically reactive system typically involves four steps: (1) development of a thermodynamic model to describe the physical and chemical equilibrium; (2) adoption and use of a modeling framework to describe the mass transfer and chemical reactions; (3) parameterization of the mass-transfer and kinetic models based upon laboratory, pilot-plant, or commercial-plant data; and (4) use of the integrated model to optimize the process and perform equipment design.

Development of Thermodynamic Model for Physical and ChemicalEquilibrium The first and perhaps most important step in the development of the thermodynamic model is the speciation, or representation of the set of chemical reactions. For CO2 absorption in aqueous MEA solutions, the set of reactions is

In addition, a model is needed that can describe the nonideality of a system containing molecular and ionic species. Freguia and Rochelle adopted the model developed by Chen et al. [AIChE J., 25, 820 (1979)] and later modified by Mock et al. [AIChE J., 32, 1655 (1986)] for mixed-electrolyte systems. The combination of the specia-tion set of reactions [Eqs. (14-74a) to (14-74«)] and the nonideality model is capable of representing the solubility data, such as presented in Figs. 14-1 and 14-2, to good accuracy. In addition, the model accurately and correctly represents the actual species present in the aqueous phase, which is important for faithful description of the chemical kinetics and species mass transfer across the interface. Finally, the thermodynamic model facilitates accurate modeling of the heat effects, such as those discussed in Example 6.

Rafal et al. (Chapter 7, "Models for Electrolyte Solutions," in Models for Thermodynamic and Phase Equilibria Calculations, S. I. Sandler, ed., Marcel Dekker, New York, 1994, p. 686) have provided a comprehensive discussion of speciation and thermodynamic models.

Adoption and Use of Modeling Framework The rate of diffusion and species generation by chemical reaction can be described by film theory, penetration theory, or a combination of the two. The most popular description is in terms of a two-film theory, which is

Gas Interface Liquid


Was this article helpful?

0 0

Post a comment