Unit operation

This section deals with the most relevant operations that a robot can perform as part of a workstation. In some cases, a given operation is repeated along the operational sequence. 

Heat exchangers generally are based on cylindrical pipes that have a minimum surface area per unit volume through which heat is transferred (Kern, 1990). Plate heat exchangers are much more 

Incorporation of static mixer elements hy a Spain company in the shell-tuhe heat exchangers has led to a 55% increase in the heat transfer area. The volume of the heat exchanger was reduced to 16% in the shell side. In another case the area was reduced hy half and the volume hy 6.5% on the shell side. There are similar reductions, especially where viscous liquids are involved (Qi et ah, 2003). 

Distillation is predominantly about achieving high gas-liquid mass transfer rates, and the key parameters in achieving this goal that need attention are (McCahe and Smith, 1990 Treyhal, 1990) 

It has been observed that the overall pressure drop inside a packed distillation column comprises of three major components  

gas-liquid interaction at the interface along the flow channels, 

Most operations scaled up in the pharmaceutical industry use semibatch (and batch) processing in the general-purpose equipment. Such operations allow for fine control of slow unit operations, for example, reactions needing hours to complete, fermentation, and crystallization, and such fine control may be necessary to ensure high quality and productivity. 

A unit operation is any single step in an overall process that can be isolated and that also tends to appear frequently in other processes. For example, a car s carburetor is a single unit operation of the engine, just as the heart is a unit operation of the human body. The concept of a unit operation is based on the idea that general analysis will be the same for all systems because individual operations have common techniques and are based on the same scientific principles. In separations, a unit operation is any process that uses the same separation mechanism. For example, adsorption is a technique in which a solid sorbent material removes speciflc components, called solutes, from either gas- or liquid-feed streams because the solute has a higher affinity for the solid sorbent than it does for the fluid. The mathematical characterization of any adsorption column is the same regardless 

Because of the variety and complexity of modern processes, it is not practicable to cover the entire subject matter of chemical engineering under a single head. The field is divided into convenient, but arbitrary, sectors. This text covers that portion of chemical engineering known as the unit operations. 

An economical method of organizing much of the subject matter of chemical engineering is based on two facts (1) although the number of individual processes is great, each one can be broken down into a series of steps, called operations, each of which in turn appears in process after process (2) the individual operations have common techniques and are based on the same scientific principles. For example, in most processes solids and fluids must be moved heat or other forms of energy must be transferred from one substance to another and tasks like drying, size reduction, distillation, and evaporation must be performed. The unit-operation concept is this by studying systematically these operations themselves—operations that clearly cross industry and process lines—the treatment of all processes is unified and simplified. 

The unit operations are as applicable to many physical processes as to chemical ones. For example, the process used to manufacture common salt consists of the following sequence of the unit operations transportation of solids and liquids, transfer of heat, evaporation, crystallization, drying, and screening. No chemical reaction appears in these steps. On the other hand, the cracking of petroleum, with or without the aid of a catalyst, is a typical chemical reaction conducted on an enormous scale. Here the unit operations— transportation of fluids and solids, distillation, and various mechanical separations—are vital, and the cracking reaction could not be utilized without them. The chemical steps themselves are conducted by controlling the flow of material and energy to and from the reaction zone. 

Because the unit operations are a branch of engineering, they are based on both science and experience. Theory and practice must combine to yield designs for equipment that can be fabricated, assembled, operated, and maintained. A balanced discussion of each operation requires that theory and equipment be considered together. An objective of this book is to present such a balanced treatment. 

SCIENTIFIC FOUNDATIONS OF UNIT OPERATIONS. A number of scientific principles and techniques are basic to the treatment of the unit operations. Some are elementary physical and chemical laws such as the conservation of mass and 

Reaction in two-phase liquid-liquid systems. The Ruhrchemie process for the manufacture of butyraldehyde from propylene uses a water-soluble rhodium catalyst, while the product butyraldehyde forms an immiscible organic layer. Separation of the product from the catalyst is thus easily accomplished (see Section 5.2.5). 

The chemical anchoring of complexes to a solid, such as silica-supported chromium catalyst, has been successfully used by Union Carbide for ethylene polymerization (see Section 6.2). 

Confining the organometal in an organized structure like micelles, clay, or zeolite. 

The last two techniques are still being tested in the laboratory or pilot scale and have not so far been commercialized. 

Product separation and catalyst recovery at the end of the homogeneous catalyzed reactions, as explained, are in most cases carried out by crystallization, filtration, distillation liquid-liquid extraction, or gas-liquid absorption. These unit operations can be performed in batch or continuous mode. The salient features of these operations are described in the following. 

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Berkowitz, J. B., Funkhouser, J. T., and Stevens, J. I., Unit Operations for Treatment of Hazardous Industrial Wastes, Noyes Data Corporation, Park Ridge, N.J., 1978. 

Now, consider the case of spinless particles not subject to external electronic and magnetic fields. We may now choose the unitai7 operator U as the unit operator, that is, T = K. For the coordinate and momentum operators, one then obtains... 

Recovery nd Purifica.tion. The production of EH Lilly s human insulin requires 31 principal processing steps of which 27 are associated with product recovery and purification (13). The production process for human insulin, based on a fermentation which yields proinsulin, provides an instmctive case study on the range of unit operations which must be considered in the recovery and purification of a recombinant product from a bacterial fermentation. Whereas the exact sequence has not been pubUshed, the principle steps in the purification scheme are outlined in Figure la. 

The design of bioseparation unit operations is influenced by these governmental regulations. The constraints on process development grow as a recovery and purification scheme undergo licensing for commercial manufacture. 

Absorption, or gas absorption, is a unit operation used in the chemical industry to separate gases by washing or scmbbing a gas mixture with a suitable hquid. One or more of the constituents of the gas mixture dissolves or is absorbed in the Hquid and can thus be removed from the mixture. In some systems, this gaseous constituent forms a physical solution with the Hquid or the solvent, and in other cases, it reacts with the Hquid chemically. 

Gas-phase adsorption is widely employed for the large-scale purification or bulk separation of air, natural gas, chemicals, and petrochemicals (Table 1). In these uses it is often a preferred alternative to the older unit operations of distillation and absorption. 

Advances in fundamental knowledge of adsorption equihbrium and mass transfer will enable further optimization of the performance of existing adsorbent types. Continuing discoveries of new molecular sieve materials will also provide adsorbents with new combinations of useflil properties. New adsorbents and adsorption processes will be developed to provide needed improvements in pollution control, energy conservation, and the separation of high value chemicals. New process cycles and new hybrid processes linking adsorption with other unit operations will continue to be developed. 

C. J. Geankophs, Transport Process and Unit Operations, 2nd ed., AHyn Bacon, Newton, Mass., pp. 373. 

Trona Purification Processes. Two processes, named the monohydrate and sesquicarbonate according to the crystalline intermediates, are used to produce refined soda ash from trona. Both involve the same unit operations only in different sequences. Most ash is made using the monohydrate process. Eigure 2 shows simplified flow diagrams for each. 

Extraction, a unit operation, is a complex and rapidly developing subject area (1,2). The chemistry of extraction and extractants has been comprehensively described (3,4). The main advantage of solvent extraction as an industrial process Hes in its versatiHty because of the enormous potential choice of solvents and extractants. The industrial appHcation of solvent extraction, including equipment design and operation, is a subject in itself (5). The fundamentals and technology of metal extraction processes have been described (6,7), as has the role of solvent extraction in relation to the overall development and feasibiHty of processes (8). The control of extraction columns has also been discussed (9). 

C.. GeankopHs, Transport Processes and Unit Operations, AHyn and Bacon, Boston, 1978. 

Separation Efficiency. Similarly to other unit operations in chemical engineering, filtration is never complete. Some soflds may leave in the hquid stream, and some Hquid will be entrained with the separated soHds. As emphasis on the separation efficiency of soHds or Hquid varies with application, the two are usually measured separately. Separation of solids is measured by total or fractional recovery, ie, how much of the incoming solids is coUected by the filter. Separation of Hquid usually is measured in how much of it has been left in the filtration cake for a surface filter, ie, moisture content, or in the concentrated slurry for a filter-thickener, ie, solids concentration. 

In order to make a multipurpose plant even more versatile than module IV, equipment for unit operations such as soHd materials handling, high temperature/high pressure reaction, fractional distillation (qv), Hquid—Hquid extraction (see Extraction, liquid-liquid), soHd—Hquid separation, thin-film evaporation (qv), dryiag (qv), size reduction (qv) of soHds, and adsorption (qv) and absorption (qv), maybe iastalled. 

Plasma fractionation is unusual in pharmaceutical manufacturing because it involves the processing of proteins and the preparation of multiple products from a single feedstock. A wide range of unit operations are utilized to accompHsh these tasks. They are Hsted in Table 3 some are common to a number of products and all must be closely integrated. The overall manufacturing operation can be represented as a set of individual product streams, each based on the processing of an intermediate product derived from a mainstream fractionation process (Fig. 1). 

Principal Unit Operations. Figure 2 shows the principal unit operations involved in a typical fractionation operation. 

Process Rationale. The products of plasma fractionation must be both safe and efftcaceous, having an active component, protein composition, formulation, stabiUty, and dose form appropriate to the intended clinical appHcation. Processing must address a number of specific issues for each product. Different manufacturers may choose a different set or combination of unit operations for this purpose. 

Another type of combustion unit operates at about 1600°C to produce a molten slag which forms a granular frit on quenching rather than the usual ash. The higher operating temperature is obtained by preheating the combustion air or by burning auxiUary fuel. 

A survey of commercial thermal gasification in the United States shows that few gasifiers have been installed since 1984 (115). Most units in use are retrofitted to small boilers, dryers, and kilns. The majority of existing units operate at 0.14 to 1.0 t/h of wood wastes on updraft moving grates. The results of this survey are summarized in Table 36. Assuming all 35 of these units are operated continuously, extremely unlikely, the maximum amount of LHV gas that can be produced is about 0.003 to 0.006 EJ/yr (222—445 td /d). 

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