Adsorption processes bulk-separation

In contrast to trace impurity removal, the use of adsorption for bulk separation in the liquid phase on a commercial scale is a relatively recent development. The first commercial operation occurred in 1964 with the advent of the UOP Molex process for recovery of high purity / -paraffins (6—8). Since that time, bulk adsorptive separation of liquids has been used to solve a broad range of problems, including individual isomer separations and class separations. The commercial availability of synthetic molecular sieves and ion-exchange resins and the development of novel process concepts have been the two significant factors in the success of these processes. This article is devoted mainly to the theory and operation of these Hquid-phase bulk adsorptive separation processes. 

In contrast to trace impurity removal, the use of adsorption for bulk separation in the liquid phase on a commercial scale is a relatively recent development. This article is devoted mainly lo the theory and operation of these liquid-phase bulk adsorptive separation processes. 

The traditional application of adsorption in the process industries has been as a means of removing trace impurities from gas or liquid streams. Examples include the removal of H2S from hydrocarbon streams before processing, the drying and removal of C02 from natural gas, and the removal of organic compounds from waste water. In these examples the adsorbed component has little value and is generally not recovered. Such processes are generally referred to as purification processes, as distinct from bulk separations, in which a mixture is separated into two (or more) streams, each enriched in a valuable component, which is recovered. The application of adsorption to bulk separations is a more recent development that was stimulated to a significant extent by the rapid... 

BulkSepa.ra.tlon, The adsorptive separation of process streams into two or more main components is termed bulk separation (see Fig. 12). The development of processes and products is complex. Consequently, these processes are proprietary and are purchased as a complete package under licensing agreements. High purities and yields can be achieved. 

Adsorption as a gas phase separation process fills a space in the spectrum of separations processes that encompasses both purification and bulk separations. The market for gas phase adsorptive separations is of the order of several billion US dollars armuaUy when aU sorbent, equipment and related products are included. 

Commercial gas-adsorption processes (see Table I) can be divided into bulk separations, in which about 10 weight percent or more of a stream must be adsorbed, and purifications, in which usually considerably less than 10 weight percent of a stream must be adsorbed. Such a differentiation is desirable to make because in general different process cycles are used for the different categories, as will be discussed later. 

Gas bulk-separation processes consist mostly of pressureswing adsorption (PSA) variations and displacement-purge and Inert-purge processes Recently, however, a chromatographic cycle has been commercialized Below, these cycles will be discussed ... 

Large-scale Sorbex processes have been developed for a variety of different bulk separations a brief summary is given in Table IV. In recent years, the same principle has been applied also to a wide range of chiral separations and other difficult separations that are important in the pharmaceutical industry. Several novel system configurations have been developed. In one system, a carousel of 12 small columns rotates between two stationary circular headers, which act as the switch valve, thus effectively incorporating the adsorption and the flow switching functions within a single unit. 

In the case of gel permeation or size-exclusion HPLC (HP-SEC), selectivity arises from differential migration of the biomolecules as they permeate by diffusion from the bulk mobile phase to within the pore chambers of the stationary phase. Ideally, the stationary phase in HP-SEC has been so prepared that the surface itself has no chemical interaction with the biosolutes, with the extent of retardation simply mediated by the physical nature of the pores, their connectivity, and their tortuosity. In this regard, HP-SEC contrasts with the other modes of HPLC, where the surfaces of the stationary phase have been deliberately modified by chemical procedures by (usually) low molecular weight compounds to enable selective retardation of the biosolutes by adsorptive processes. Ideally, the surface of an interactive HPLC sorbent enables separation to occur by only one retention process, i.e., the stationary phase functions as a monomodal sorbent. In practice with porous materials, this is rarely achieved with the consequence that most adsorption HPLC sorbents exhibit multimodal characteristics. The retention behavior and selectivity of the chromatographic system will thus depend on the nature and magnitude of the complex interplay of intermolecular forces... 

Farooq, S D. M. Ruthven. Numerical simulation of a kinetically controlled pressure swing adsorption bulk separation process based on a diffusion model. Chem. Eng. Sci. 1991,46,2213. 

The experimental results and dynamics in the bed were analyzed by using a non-isothermal dynamic model incorporating mass, energy, and momentum balances. Although the temperature increase inside a bed is undesirable, in case of bulk separation, the temperature variation during the adsorption process is inevitable. Therefore, the... 

The adsorption dynamics of binary and ternary hydrogen mixture in activated carbon and zeolite SA bed was studied by experimentally and theoretically through breakthrough and desorption experiments. Energy balance is an essential element for accurate adsorption process modeling in case of bulk separation. Especially in ternary system, sinusoidal... 

Commercial processes have been arbitrarily divided into two groups purifications and bulk separations. In purification processes, relatively dilute streams of adsorbate (e.g., 10 weight percent or less) are absorbed, whereas in bulk separations, 10 weight percent or more is adsorbed. Different adsorption cycles are used for each group. 

In the PSA cycle, the regeneration step takes place by lowering the pressure on the bed and purging the adsorbent. The lower pressure shifts the adsorption equilibrium and affects the regeneration of the adsorbent. In the PSA cycles, the time required to depressure, regenerate, and increase the pressure on the bed is of the order of minutes. Because of the short time cycle, the PSA process is attractive for bulk separations (see Figure 15.17). 

The formalism of density functional theory (DFT) has received considerable attention as a way to describe the adsorption process at the fluid-solid interfece. The older approach was to treat the adsorbate as a separate, two-dimensional phase existing in equilibrium with the bulk gas phase. This model works well... 

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