Distillation complex operations

The choice of separation method to be appHed to a particular system depends largely on the phase relations that can be developed by using various separative agents. Adsorption is usually considered to be a more complex operation than is the use of selective solvents in Hquid—Hquid extraction (see Extraction, liquid-liquid), extractive distillation, or azeotropic distillation (see Distillation, azeotropic and extractive). Consequentiy, adsorption is employed when it achieves higher selectivities than those obtained with solvents. 

Other methods. A cycling procedure can also be used for the column operation. The unit operates at total reflux until a steady state is establisned. The distillate is then taken as total drawoff for a short time, after which the column is returned to total reflux operation. This cycle is repeated throughout the course of distillation. Another possibility is to optimize the reflux ratio to achieve the desired separation in a minimum time. More complex operations may involve withdrawal of sidestreams, provision for intercondensers, addition of feeds to trays, and periodic feed additions to the pot. 

Process Technology 3—Operations—combines process systems into operational processes with emphasis on operations under various conditions. Topics include typical duties of an operator. Instruction focuses on the principles of modem manufacturing technology and process equipment. Emphasizes scale-up from laboratory bench to pilot unit. Describe unit operation concepts solve elementary chemical mass/energy balance problems interpret analytical data and apply distillation and fluid flow principles. The purpose of this class is to provide adult learners with the opportunity to work in a self-directed work team, operate a complex operational system, collect and analyze data, start and stop process equipment, follow and write operational procedures. The course is advanced and requires the learner to apply classroom skills to real-life operational activities. Students are required to qualify and operate a process unit. 

In this section we turn our attention to more complex shapes represented by tower packings used in operations such as gas absorption, stripping, and distillation. The operation of such columns is addressed in more detail in Chapter 8. 

The readership at this level is broad. The topic of separation processes taught at all engineering schools is inextricably linked to mass transport, and students will benefit from an early introductory treatment of mass transfer combined with the basic concepts of separation theory. There is, in fact, an accelerating trend in this direction, which aims for students to address later the more complex operations, such as multicomponent and azeotropic distillation, chromatography, and the numerical procedures to simulate these and other processes. 

Introduce complex distillation configurations. Introduce prefractionation arrangements (with or without thermal coupling), side-rectifiers, and side-strippers to the extent that operability can be... 

The scope for integrating conventional distillation columns into an overall process is often limited. Practical constraints often prevent integration of columns with the rest of the process. If the column cannot be integrated with the rest of the process, or if the potential for integration is limited by the heat flows in the background process, then attention must be turned back to the distillation operation itself and complex arrangements considered. 

Distillation Columns. Distillation is by far the most common separation technique in the chemical process industries. Tray and packed columns are employed as strippers, absorbers, and their combinations in a wide range of diverse appHcations. Although the components to be separated and distillation equipment may be different, the mathematical model of the material and energy balances and of the vapor—Hquid equiUbria are similar and equally appHcable to all distillation operations. Computation of multicomponent systems are extremely complex. Computers, right from their eadiest avadabihties, have been used for making plate-to-plate calculations. 

Whereas there is extensive Hterature on design methods for azeotropic and extractive distillation, much less has been pubUshed on operabiUty and control. It is, however, widely recognized that azeotropic distillation columns are difficult to operate and control because these columns exhibit complex dynamic behavior and parametric sensitivity (2—11). In contrast, extractive distillations do not exhibit such complex behavior and even highly optimized columns are no more difficult to control than ordinary distillation columns producing high purity products (12). 

The simple and complex distillation operations just described all have two things in common (1) both rectifying and stripping sections are providea so that a separation can be achieved between two components that are adjacent in volatility and (2) the separation is effected only by the addition and removal of energy and not by the addition of any mass separating agent (MSA) such as in liquid-liquid extraction. 

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