Distillation with Other Unit Operations

Combination of Distillation with Other Unit Operations 

Unit operations are based on different phase equihbria (e.g. distillation is based on the vapor-liquid equilibrium, crystallization is based on the soHd-Hquid equili- 

Combinations of distillation and pervaporation (membrane separation with a vapor permeate stream) have been applied to many separation problems. One example is the separation of ethanol-water mixtures. Ethanol and water form an azeotrope and therefore conventional distillation is not feasible. Using a combination including pervaporation, pure ethanol and water can be obtained as can be seen from Fig. 3.2-10. Water can be obtained from the bottom of the distillation column. The distillate stream consists of the azeotropic mixture which is fed into the pervaporation unit The highly selective polyvinyl alcohol membrane is capable of separating water from ethanol. Pure ethanol can be obtained from the retentate, and the permeate stream, which consists of ethanol and water, is recycled into the distillation column. 

Short-cut methods as well as rigorous techniques have been used for the development and ophmization of hybrid separation processes. Research continues on the development of generally applicable rules for the design and optimizahon of hybrid separations. A review of the current status has been published by Franke et al. (2004). 

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Multifunctional Reactors Reaction may be coupled with other unit operations to reduce capital and/or operating costs, increase selectivity, and improve safety. Examples are reaction and distillation and reaction with heat transfer. Concepts that combine reaction with membrane separation, extraction, and crystallization are also being explored. In each case, while possibly reducing cost, the need to accommodate both reaction and the additional operation constrains process flexibility by reducing the operating envelope. 

In this chapter the focus is on reactors. First we introduce the general factors that affect the selection of the reactor. In Section 6.2 are given general guidelines. Section 6.3 considers details for different types of reactions that affect the size of the reactor. The rest of the chapter discusses some specifics about the different types of reactor. Section 6.4 considers burners. Plug flow tubular reactors, PFTR, are considered in Sections 6.5 to 6.26. Stirred tanks reactors, STR, are considered in Section 6.27 to 6.33. Finally Sections 6.34 to 6.37 explore combining reactors with other unit operations such as distillation, extmsion, membranes and vacuum pumps. 

The transfer of energy in the form of heat occurs in many chemical and other types of processes. Heat transfer often occurs in combination with other unit operations, such as drying of lumber or foods, alcohol distillation, burning of fuel, and evaporation. The heat transfer occurs because of a temperature difference driving force and heat flows from the high- to the low-temperature region. 

Evaporator systems will be combined with other unit operations such as distillation or extraction. The Carver-Greenfield process, which uses a carrier oil to permit a product to be evaporated to nearly complete moisture removal, will find wider application. Ion-exchange will be integrated with evaporation. 

Reactors combined with other unit operations Reactive distillation Overcomes Not easy to find Methyl acetate... 

Silicon microreactors have been used in number of flow systems, such as gas-liquid [10], liquid-liquid [11], and gas-liquid-soHd [12] reactions, which are often used in conjunction with other unit operations, such as extraction [13] and distillation [14]. Furthermore, silicon microreactors have been shown to be able to operate at high temperatures and pressures [15]. In addition, the small volumes of microreactors allow potentially dangerous chemistry to be conducted more safely. For example, fluorination and chlorination of aromatics, nitration to form highly energetic compounds, and reactions carried out in the explosive regime have all been conducted safely in microreactors [2]. 

We first review in Part 1 the basics of plantwide control. We illustrate its importance by highlighting the unique characteristics that arise when operating and controlling complex integrated processes. The steps of our design procedure are described. In Part 2, we examine how the control of individual unit operations fits within the context of a plantwide perspective. Reactors, heat exchangers, distillation columns, and other unit operations are discussed. Then, the application of the procedure is illustrated in Part 3 with four industrial process examples the Eastman plantwide control process, the butane isomerization process, the HDA process, and the vinyl acetate monomer process. 

The goal of this book is to help chemical engineering students and practicing engineers develop effective control structures for chemical and petroleum plants. Our focus is on the entire plant, not just the individual unit operations. An apparently appropriate control scheme for a single reactor or distillation column may actually lead to an inoperable plant when that reactor or column is connected to other unit operations in a process with recycle streams and energy integration. 

When compared to the development of models and methods for other unit operations, it is obvious that crystallization has not been generalized to the degree that has been accomplished for distillation, extraction, adsorption, etc. This situation is changing rapidly, however, with increasing research now being carried out at academic and industrial centers on CrystaUization fundamentals to model and predict nucleation and/or growth rates as well as other key properties, including polymorph formation. 

The emergence of chemical engineering as a professional field of specialized knowledge was catalyzed to a major extent by the systematic classification of apparatus in terms of the Unit Operations. With further progress, the design methods evolved for particular apparatus types have proved equally applicable to other unit operations similar in physical arrangement, material and energy balances, rate behavior, and phase equilibrium. Thus there has been a very extensive development of parallel calculation methods for the separation operations conducted under countercurrent flow conditions—the fluid-fluid operations of distillation, absorption, and extraction. 

Membrane processes, lika other unit operations, should be approached with a systematic design procedure supported by a solid data hese. Because of their neveity, Ibe del a hese for membrane processes is still mnch smaller lhan thei for corresponding older unit operations such as distillation. The current lack of information often necessitates the estimation and use of constant values for component permeabilities although thase coalficients are kaown to be functions of pressure and gas composition in many cases.28,29... 

CGCC can indicate if a reduction in reflux is possible (Fig. 11.12). At minimum reflux the Pinch touches the vertical axis. Obviously, both reboiler and condenser duties are reduced in the same proportion. Note that the reduction in reflux might be justified from a stand-alone operation point of view, but not necessarily when the integration with other units is considered, such as in the case of the heat integrated distillation columns (see later in this chapter). 

This chapter discusses several general pitfalls inherent in distillation startup and shutdown operations. Hazards and pitfalls specifically related to the nature of the chemicals processed (e.g., toxicity, flammability), the simultaneous startup/shutdown of the column with other units (e.g., reactors, furnaces), or to the overall startup/shutdown policy (e.g., operator training procedures) are outside the scope of this distillation text. The safe handling procedures of the relevant chemicals, as well as the overall startup/shutdovm policy, must be followed to cater for the latter hazards and pitfalls. 

Distillation is one of the most important unit operations in chemical engineering. It forms the basis of many processes and is an essential part of many others. It presents a more difficult control problem then with many other unit operations, as at least five variables need to be controlled simultaneously and there are at least five variables available for manipulation. Thus, a distillation column provides an example of a multiple-input-multiple-output control problem. It is critical that variable pairing is done appropriately between controlled and manipulated variables. The overall control problem can usually be reduced to a 2 x 2 conposition control problem since the inventory and pressure loops frequently do not interact with the composition loops. This workshop will highlight some fundamental mles of distillation control and show how a basic distillation control scheme can be selected. 

The SR method can be applied to distillation columns, but the equations of the algorithm do not allow the solution of the condenser and the reboiler with the other stages in the column. Because only the energy balances are used as independent functions, reboiler and condenser duties, reflux ratio, and the boilup ratio have to be specified. This overspecifies the column and the solution cannot be found. The condenser and the reboiler can be solved as separate unit operations in a flowsheet as demonstrated by Fonyo et al. (39). The SR method is used in the ABSBR step of FLOWTRAN of Monsanto, St. Louis, Missouri, and also in both the public release version of ASPEN and in ASPENPlus of AspenTech, Cambridge, Massachusetts. 

The reactor effluent usually contains a mixture of reactants and products. It is fed into a separation section where the products are separated by some means from the reactants. Because of their economic value, reactants are recycled back to upstream units toward the reactor. The products are transported directly to customers, are fed into storage tanks, or are sent to other units for further processing. The separation section uses one or more of the fundamental unit operations distillation, evaporation, filtration, crystallization, liquid-liquid extraction, adsorption, absorption, pressure-swing adsorption, etc. In this book we typically use distillation as the separation method because of its widespread use and our considerable experience with it. Everyone is a victim of his or her experience. Our backgrounds are in petroleum processing... 

In this chapter we have presented some fundamental concepts of distillation control. Distillation columns are without question the most widely used unit operation for separation in the chemical industry. Most final products are produced from one end or the other of a distillation column, so tight control of product quality requires an effective control system for the column. However, the column is usually an integral part of an entire plant, so its control scheme must also be consistent with the plantwide control structure. 

The product gases are cooled and compressed (4) to facilitate separation of products and by-products. The suction-side of the compressor ensures that upstream units operate at a low pressure. The product gases are first dried (5) and the cooled product passed to a cryogenic separator (6) which removes hydrogen from the system. Some is recycled with the other portion passed-on for other uses. A selective hydrogenation unit (7) removes dienes and acetylenes. A final distillation train removes light hydrocarbon (C2-), propylene product, propane, which is recycled, and a C4 by-product. 

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