Unit Operations in Liquid Systems

The process patterns found in liquid systems are more diverse and frequently much more complex than those in vapor-phase applications. In part, this arises from the greater number of factors that can influence adsorption from solution. The various permutations in which these factors can be joined confer a flexibility that makes liquid-phase adsorption adaptable to many diverse situations.1 2 3 

Because this adaptability springs from the varied patterns in which the factors can join, it is often difficult to know the exact role of any single factor. Consequently, when things go wrong in an established operation we may be at a loss to know the exact cause. Fortunately such incidents usually are more of academic interest than of practical moment, because experience will generally enable the veteran operator to get everything quickly under control. 

Bone char, the forefather of activated carbon, has long been in the forefront of adsorbents for sugar refining. The granular char is placed in columns through which the hot syrups percolate until the char is exhausted, as evidenced by the appearance of color in the filtrate. The bone char is then regenerated by thermal means for re-use.3 

When activated carbon was first produced, efforts were made to enter markets using bone char. The decolorizing chars made initially, however,—and indeed for many years—were too soft and friable to be supplied in granular form and it became necessary 

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Separation of two liquid phases, immiscible or partially miscible liquids, is a common requirement in the process industries. For example, in the unit operation of liquid-liquid extraction the liquid contacting step must be followed by a separation stage (Chapter 11, Section 11.16). It is also frequently necessary to separate small quantities of entrained water from process streams. The simplest form of equipment used to separate liquid phases is the gravity settling tank, the decanter. Various proprietary equipment is also used to promote coalescence and improve separation in difficult systems, or where emulsions are likely to form. Centrifugal separators are also used. 

Examples of the application of the Grashof number in heat transfer problems can be found, for example, in [14]. For examples where the Archimedes number is applied in solid/liquid systems (suspensions) see [22] and for sedimentation as well as fluidization examples please refer to textbooks dealing with unit operations in chemical process engineering. 

Many effective control schemes have been established over the years for individual chemical units (Shinskey, 1988), For example, a tubular reactor usually requires control of inlet temperature. High-temperature endothermic reactions typically have a control system to adjust the fuel flowrate to a furnace supplying energy to the reactor. Crystallizers require manipulation of refrigeration load to control temperatui e. Oxygen concentration in the stack gas from a furnace is controlled to prevent excess fuel usage. Liquid solvent feed flow to an absorber is controlled as some ratio to the gas feed. We deal with the control of various unit operations in Chaps. 4 through 7. 

In Chap. 2 we illustrated the issues of plantwide control by using very simple unit operations. In each example, the reactor was a well-mixed, liquid-filled tank where we carried out isothermal, elementary reaction steps. A level control loop was sufficient to make the reactor fully functional. The point we tried to convey is that no matter how simple the individual unit operations (and their controls) may be, new control issues arise when the units become part of an integrated plant. Certainly these issues are still present when we introduce more complexity into the individual processing steps. In this chapter we study some industrially relevant reactor systems. 

Reactive distillation is a unit operation in which chemical reaction and distillation are carried out simultaneously within a fractional distillation apparatus. Reactive distillation may be advantageous for liquid-phase reaction systems when the reaction must be carried out with a large excess of one or more of the reactants, when a reaction... 

Classically, flat-sheet porous PTFE or polypropylene membranes are used as support for the membrane liquid and mounted in holders (cells, contactors) permitting one flow channel on each side of the membrane [1,3,6,8,25]. See Figure 12.1. Such membrane units are typically operated in flow systems and in principle apphcable to aU versions of membrane extraction for analytical sample preparation or sampling. Such a setup can be easily interfaced with different analytical instmments, such as HPLC and various spectrometric instmments, and thereby provides good possibdities for automated operation. Drawbacks of this type of devices are relatively large costs and limited availability, as well as some carryover and memory problems as the membrane units are utilized many times, necessitating cleaning between each extraction. 

Distillation is a widely used unit operation in pharmaceutical manufacture to remove components from a system to an acceptable level. The process engineer is able to assist in the selection of the optimum solvent system, providing vapour-liquid equilibrium information and predictions of the efficiency of the separation. A recent example highlighted the effectiveness of CAPE tools in the design and prediction of distillation performance. 

Transport of a species through a liquid membrane is superior to solvent extraction (SX) since extraction and stripping are performed in a single unit operation. Also, liquid membrane transport is a non-equilibrium, steady state process which depends upon kinetic factors in contrast to SX which is an equilibrium process. Furthermore, even solvents with low distribution coefficients for Ae desired species may be utilized in LM processes. Although LM systems generally have slower rates than ion-exchange (DC) processes, the latter are particularly sensitive to the presence of suspended solids and other foulants and also must be operated in cycles. 

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

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