12.1. INTRODUCTION TO ADSORPTION PROCESSES
12.1A. Introduction
In adsorption processes one or more components of a gas or liquid stream are adsorbed on the surface of a solid adsorbent and a separation is accomplished. In commercial processes, the adsorbent is usually in the form of small particles in a fixed bed. The fluid is passed through the bed and the solid particles adsorb components from the fluid. When the bed is almost saturated, the flow in this bed is stopped and the bed is regenerated thermally or by other methods so that desorption occurs. The adsorbed material (adsorbate) is thereby recovered and the solid adsorbent is ready for another cycle of adsorption.
Applications of liquid-phase adsorption include removal of organic compounds from water or organic solutions, colored impurities from organics, and various fermentation products from fermentor effluents. Separations include paraffins from aromatics and fructose from glucose using zeolites.
Applications of gas-phase adsorption include removal of water from hydrocarbon gases, sulfur compounds from natural gas, solvents from air and other gases, and odors from air.
12.1B. Physical Properties of Adsorbents
Many adsorbents have been developed for a wide range of separations. Typically, the adsorbents are in the form of small pellets, beads, or granules ranging from about 0.1 mm to 12 mm in size, with the larger particles being used in packed beds. A particle of adsorbent has a very porous structure, with many fine pores and pore volumes up to 50% of total particle volume. The adsorption often occurs as a monolayer on the surface of the fine pores, although several layers sometimes occur. Physical adsorption, or van der Waals adsorption, usually occurs between the adsorbed molecules and the solid internal pore surface and is readily reversible.
The overall adsorption process consists of a series of steps in series. When the fluid is flowing past the particle in a fixed bed, the solute first diffuses from the bulk fluid to the gross exterior surface of the particle. Then the solute diffuses inside the pore to the surface of the pore. Finally, the solute is adsorbed on the surface. Hence, the overall adsorption process is a series of steps.
There are a number of commercial adsorbents and some of the main ones are described below. All are characterized by very large pore surface areas of 100 to over 2000 m2/g.
Activated carbon. This is a microcrystalline material made by thermal decomposition of wood, vegetable shells, coal, and so on, and has surface areas of 300 to 1200 m2/g with average pore diameters of 10 to 60 Å. Organics are generally adsorbed by activated carbon.
Silica gel. This adsorbent is made by acid treatment of sodium silicate solution and then drying. It has a surface area of 600 to 800 m2/g and average pore diameters of 20 to 50 Å. It is primarily used to dehydrate gases and liquids and to fractionate hydrocarbons.
Activated alumina. To prepare this material, hydrated aluminum oxide is activated by heating to drive off the water. It is used mainly to dry gases and liquids. Surface areas range from 200 to 500 m2/g, with average pore diameters of 20 to 140 Å.
Molecular sieve zeolites. These zeolites are porous crystalline aluminosilicates that form an open crystal lattice containing precisely uniform pores, which makes it different from other types of adsorbents, which have a range of pore sizes. Different zeolites have pore sizes from about 3 to 10 Å. Zeolites are used for drying, separation of hydrocarbons, mixtures, and many other applications.
Synthetic polymers or resins. These are made by polymerizing two major types of monomers. Those made from aromatics such as styrene and divinylbenzene are used to adsorb nonpolar organics from aqueous solutions. Those made from acrylic esters are usable with more-polar solutes in aqueous solutions.
12.1C. Equilibrium Relations for Adsorbents
The equilibrium between the concentration of a solute in the fluid phase and its concentration on the solid resembles somewhat the equilibrium solubility of a gas in a liquid. Data are plotted as adsorption isotherms as shown in Fig. 12.1-1. The concentration in the solid phase is expressed as q, kg adsorbate (solute)/kg adsorbent (solid), and in the fluid phase (gas or liquid) as c, kg adsorbate/m3 fluid.
Figure 12.1-1. Some common types of adsorption isotherms.
Data that follow a linear law can be expressed by an equation similar to Henry's law:
Equation 12.1-1
where K is a constant determined experimentally, m3/kg adsorbent. This linear isotherm is not common, but in the dilute region it can be used to approximate data for many systems.
The Freundlich isotherm equation, which is empirical, often approximates data for many physical adsorption systems and is particularly useful for liquids:
Equation 12.1-2
where K and n are constants and must be determined experimentally. If a log–log plot is made for q versus c, the slope is the dimensionless exponent n. The dimensions of K depend on the value of n. This equation is sometimes used to correlate data for hydrocarbon gases on activated carbon.
The Langmuir isotherm has a theoretical basis and is given by the following, where qo and K are empirical constants:
Equation 12.1-3
where qo is kg adsorbate/kg solid and K is kg/m3. The equation was derived assuming that there are only a fixed number of active sites available for adsorption, that only a monolayer is formed, and that the adsorption is reversible and reaches an equilibrium condition. By plotting 1/q versus 1/c, the slope is K/qo and the intercept is 1/qo.
Almost all adsorption systems show that as temperature is increased, the amount adsorbed by the adsorbent decreases strongly. This is useful since adsorption is normally at room temperatures and desorption can be attained by raising the temperature.
EXAMPLE 12.1-1. Adsorption Isotherm for Phenol in WastewaterBatch tests were performed in the laboratory using solutions of phenol in water and particles of granular activated carbon (R5). The equilibrium data at room temperature are shown in Table 12.1-1. Determine the isotherm that fits the data.
Solution: Plotting the data as 1/q versus 1/c, the results are not a straight line and do not follow the Langmuir equation (12.1-3). A plot of log q versus log c in Fig. 12.1-2 gives a straight line and, hence, follows the Freundlich isotherm, Eq. (12.1-2). The slope n is 0.229 and the constant K is 0.199, to give
Figure 12.1-2. Plot of data for Example 12.1-1.
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