01-12-2012, 04:29 PM
A “How To” Guide for Adsorber Design
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Introduction
Adsorption is considered complicated, compared with distillation, absorption, and extraction.
Just because the subject is perplexing, however, is no reason to avoid it. Hence, this article is an
overview of taking a separation application from basic data and conditions to a preliminary design.
The aim is to give simple, step-by-step procedures for designing ordinary adsorbers, as well as basic
pressure swing adsorbers and temperature swing adsorbers. As background information, an
introduction to adsorbents, their characteristics, and adsorption technology, in general, appeared in
the other article, “Adsorbent Selection.”
Some of the methods mentioned here are semiempirical, while others rely solely on first
principles (solutions of differential mass balance equations) with no adjustable parameters. All the
models featured here require only a calculator to solve. The biggest constraint, however, is that the
basic properties required for simulation or design are not widely available, e.g., in handbooks or in
property databases. Unfortunately, the more features a model takes into account, the more adjustable
parameters are required, and the more experimental data are necessary to evaluate them, as explained
below. The good news is that, with a relatively simple model and minimal data (described in the
next section), one can get to an approximate design in the span of a few minutes.
Necessary Data
Certain general properties of adsorbents are involved in all adsorber design calculations. They
can virtually never be predicted, but must be measured. In fact, vendor-supplied charts and tables
are sometimes available, but are seldom guaranteed to be valid for design purposes. In those cases,
measurements may be justified, too. The necessary properties are: densities and void fractions,
isotherms (or other equilibrium data), kinetics, and fixed bed dynamics. These factors are intimately
involved in adsorber models, which are covered in the next sections. In addition, though not strictly
a property, cost is involved in every design decision.
Frequently, rough values of density or a range of nominal values are available from the vendor.
Likewise, cost data are available as a function of quantity and required pretreatment, if any. Most
vendors provide additional generic properties. If the potential sale is significant, they may even
produce data. Otherwise, data for density, isotherms, and kinetics, might be found in books or
monographs (e.g., Valenzuela and Myers, “Adsorption Equilibrium Data Handbook,” Prentice Hall,
1989; Dobbs and Cohen, “Carbon Isotherms for Toxic Organics,” EPA-600/8-80-023, 1980), journal
articles (see, e.g., Adsorption, AIChE Journal, Chemical Engng. Science, Industrial and Engng.
Chem. Fundam., Journal of Colloid and Interface Science, Langmuir), graduate research theses from
university libraries, or the world-wide web. Despite the spectrum of sources, it is rare to find the
correct combination of adsorbent-adsorbate-temperature, range of data, lot number, pretreatment
conditions, etc. As a result, those resources may be considered risky. Alternatively, you might
arrange to conduct the measurements, either yourself or by someone else in your firm. Finally, since
the tests are sometimes tedious and require special apparatus, you might arrange to have tests
conducted by an independent firm. They frequently offer unbiased evaluations of adsorbents from
various vendors, skill in conducting and analyzing the tests, and since they perform such
measurements routinely, they are likely to be cost effective.
Isotherms
Adsorption equilibrium data are commonly gathered at a fixed temperature and plotted or
tabulated as capacity or loading versus the fluid-phase concentration (or partial pressure for gases
and vapors). In that format the data comprise an isotherm. As mentioned earlier, adsorption
capacity governs the capital cost because it dictates the amount of adsorbent required, which also
fixes the volume of the adsorber vessels. Information about the general nature of isotherms and
about the multitude of equations that are used to fit data can be found in the previous article
(Chemical Engng., Nov. 1995). Some will be repeated here. For example, Figure 1 shows
classifications suggested by Brunauer, Deming, Deming, and Teller, i.e., Types I-VI. Types I , II,
and IV represent “favorable” equilibrium (concave downwards), while Types III and V represent
“unfavorable” equilibrium (concave upwards). Type VI has two regions that are favorable and two
that are unfavorable. Furthermore, Types IV and V exhibit hysteresis, which occurs when desorption
occurs along a different path than adsorption, e.g., as a result of liquid-filled pores, and implies that
uptake and release may be slow. For all that, only Type I adsorbents, over the range of relevant
conditions, are generally suited to cyclic applications.