Unit Operations - General

The pretreatment unit operations generally consist of equalization, neutralization, and oil separation. In addition, phenol recovery (to reduce the phenol concentration) and spill protection for spent acids and spent caustics may be required in some cases. 

Tanks are used in innumerable ways in the chemical process iadustry, not only to store every conceivable Hquid, vapor, or soHd, but also ia a number of processiag appHcations. For example, as weU as reactors, tanks have served as the vessels for various unit operations such as settling, mixing, crystallisation (qv), phase separation, and heat exchange. Hereia the main focus is on the use of tanks as Hquid storage vessels. The principles outlined, however, can generally be appHed to tanks ia other appHcations as weU as to other pressure-containing equipment. 

Lime-Sulfuric. Recovery of citric acid by calcium salt precipitation is shown in Figure 3. Although the chemistry is straightforward, the engineering principles, separation techniques, and unit operations employed result in a complex commercial process. The fermentation broth, which has been separated from the insoluble biomass, is treated with a calcium hydroxide (lime) slurry to precipitate calcium citrate. After sufficient reaction time, the calcium citrate slurry is filtered and the filter cake washed free of soluble impurities. The clean calcium citrate cake is reslurried and acidified with sulfuric acid, converting the calcium citrate to soluble citric acid and insoluble calcium sulfate. Both the calcium citrate and calcium sulfate reactions are generally performed in agitated reaction vessels made of 316 stainless steel and filtered on commercially available filtration equipment. 

A wide array of general-purpose distillation packages are available to the engineer. Some of the distillation software is stand-alone, whereas other packages are a part of a general-purpose flow sheet or process-simulation system. Because distillation is so universal, all process simulators have one or more distillation program modules for this unit operation. Often the nature of the distillation modules determines the suitabiUty of or the preference for the use of a specific simulator for an appHcation. 

The following subsections discuss the basic considerations involved in various unit operations of solid-solid separation and describe present industrial practice and equipment in general use. 

Both types of units have generally been operated in trace mode that is, background or elutaut electrolyte is fed to the unit along with the mixture to be separated. A desirable and possible means of... 

Performance of the power-recovery unit operating with a makeup driver (Fig. 29-56) is shown in Fig. 29-57 specific percentage values are shown, but the general characteristics and curwe shapes are typical. It should be noted that the flow scheme, the selection of... 

Because of these limitations, different models may appear to describe the unit operation equally well. Analysts must discriminate among various models with the associated parameter estimates that best meet the end-use criteria for the model development. There are three principal criteria forjudging the suitability of one model over another. In addition, there are ancillary criteria like computing time and ease of use that may also contribute to the decision but are not of general concern. 

Step 2.1 R ne the Initial Checklist. Table 1 is a checklist of unit operations. Apply this as an initial guide and checklist that can be used to identify each unit operation within the unit process that the audit will focus on. You may have to make additions to the list, because it is meant to be a general overview. Next to each unit operation that applies to the unit process of interest, provide a brief description of its function or purpose. This will prove useful in developing a process flow scheme in Step 3. Much of this sub-step can be accomplished by... 

The details of specifications for bag filter dust collectors are important to a proper and operable design selection. There are many variables which must be furnished by the manufacturer so that the user can understand how the unit operates mechanically and the unit s dust loading capabilities. The larger the air/cloth ratio for the unit, the smaller will be its physical dimensions and generally, cost however, the higher will be the frequency of cleaning. This can be quite troublesome, therefore low values of this ratio are preferable, consistent with the analysis of overall performance. 

Figure 5-5A. A flat blade turbine can handle the majority of all fluid mixing applications when correctly applied. Its high pumping capacity makes it preferable for general mixing operations. It Is well adapted to the application of protective coverings, such as lead, rubber and plastics. Courtesy of Lightnin (formerly Mixing Equipment Co.), a unit of General Signal.

Figure 5-52. A-410 Composite Impeller. Strong axial flow at very high flow efficiency. Operates through a wide range of viscosities. By permission, Lightnin, (Formerly Mixing Equipment Co.) a unit of General Signal.

In general, the number of boosters determines the operational flexibility of the unit with respect to the refrigeration load. A single booster unit operates continuously, regardless of load. A two booster unit can operate at 50% load by shutting off one unit at lower load levels it uses a pressure controller on the steam actuated by the condenser pressure. Because jets are not usually very flexible with respect to steam consumption and vacuum, load control may be in increments as compared to continuous variation. If a 100-ton unit is expected to operate an appreciable portion of the time at 25% of load, it may prove economical to install a four-booster unit and to operate only one for this period. Auxiliary ejectors remove uncondensed water vapor and air from the main condenser. 

While it is impossible to describe a system of storekeeping and distribution suitable for every site, there are certain essential principles which should be adhered to if cleanliness, order and economy are to be maintained. How these principles should be applied is for individual management to decide. The keynote, however, should be simplicity. A storekeeper familiar with both grades and needs should control distribution. While the lubrication schedule for any particular unit is generally the concern of the operator, the storekeeper must equally be aware of it and have a comprehensive list of the different grades, their applications, quantities, daily and other periodic needs. On such a basis he will be able to requisition and store the necessary lubricants in the most convenient and economic quantities and packages, and ensure that supplies are used on a first in, first out basis. 

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