Unit operation mechanisms

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. 

A change in size on scale-up is not the sole determinant of the seal-ability of a unit operation or process. Scalability depends on the unit operation mechanism(s) or system properties involved. Some mechanisms or system properties relevant to dispersions are listed in Table 2 (59). In a number of instances, size has little or no influence on processing or on system behavior. Thus, scale-up will not affect chemical kinetics or thermodynamics although the thermal effects of a reaction could perturb a system, e.g., by affecting convection (59). Heat or mass transfer within or between phases is indirectly affected by changes in size while convection is directly... 

Table 2 Influence of Size on System Behavior or Important Unit Operation Mechanisms...

System behavior or unit operation mechanisms Important variables Influence of size... 

Scaling based on wrong unit operation mechanism(s)... 

Fluid mixing is a unit operation carried out to homogenize fluids in terms of concentration of components, physical properties, and temperature, and create dispersions of mutually insoluble phases. It is frequently encountered in the process industry using various physical operations and mass-transfer/reaction systems (Table 1). These industries include petroleum (qv), chemical, food, pharmaceutical, paper (qv), and mining. The fundamental mechanism of this most common industrial operation involves physical movement of material between various parts of the whole mass (see Supplement). This is achieved by transmitting mechanical energy to force the fluid motion. 

Additional definition of the operative mechanisms can obviate the need for the larger unit. It maybe possible to assess limitations in a smaller unit that has only a few injection points on the distributor. The unit could be used to evaluate distributor designs that permit a wide range of acceptable operating conditions. However, if the acceptable range proves smaller than desired, the larger pilot unit would then be needed to estabUsh acceptable performance. 

Cleaners are most efficient on relatively large particles, 80—300 pm in diameter (see Fig. 1). Flat toner ink particles can fragment during processing, therefore it is probably best to locate mechanical cleaners early in the sequence of office paper deinking unit operations (40). 

Size reduction is an extremely important unit operation, whereby materials are subjected to stress in order to reduce the size of individual pieces. The stress is apphed by transmitting mechanical force to the soHd. 

It is likely that there will always be a distinction between the way CAD/CAM is used in mechanical design and the way it is used in the chemical process industry. Most of the computations requited in mechanical design involve systems of linear or lineatizable equations, usually describing forces and positions. The calculations requited to model molecular motion or to describe the sequence of unit operations in a process flow sheet are often highly nonlinear and involve systems of mixed forms of equations. Since the natures of the computational problems are quite different, it is most likely that graphic techniques will continue to be used more to display results than to create them. 

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). 

The simplest method is performing it manually. Once the total external resistance is known, the resistance unit can be built with a hand-operated mechanism to manually cut-off the external resistance. However, this method is suitable only for applications where the magnitude of torque during the pick-up period (torque at different... 

More shortcut design methods and rules of thumb have been developed for fractionation than probably any other unit operation. For example the paper reprinted in Appendix 5 on development of shortcut equipment design methods contains 18 references for fractionation shortcut methods out of 37 total. Both the process and mechanical aspects of fractionation design have useful rules of thumb. Many of the mechanical design rules of thumb become included in checklists of do s and don ts. 

Upon integration and reworking the relationship (details may be obtained from Fluid Mechanics and Unit Operations, D. S. Azbel and N. P. Cheremisinoff, Ann Arbor Science Publishers, Mich. (1983)), we obtain the following working formula ... 

In the analysis we assume Rf = 0, and then t = /2(Bq ), where B = /tr x /AP. The following relationships were developed and described by Azbel and Cheremisinoff (Fluid Mechanics and Unit Operations, Ann Arbor Science Publishers (1983)) ... 

Azbel, D. and N. P. Cheremisinoff, Fluid Mechanics and Unit Operations, Ann Arbor Seienee Pub, Mieh., 1983. 

There are a variety of ways of accomplishing a particular unit operation. Alternative types of process equipment have different inherently safer characteristics such as inventory, operating conditions, operating techniques, mechanical complexity, and forgiveness (i.e., the process/unit operation is inclined to move itself toward a safe region, rather than unsafe). For example, to complete a reaction step, the designer could select a continuous stirred tank reactor (CSTR), a small tubular reactor, or a distillation tower to process the reaction. 

The tests performed on E-cat samples provide refiners with valuable information on unit conditions. The data can be used to pinpoint potential operational, mechanical, and catalyst problems because the physical and chemical properties of the E-cat provide clues on the environment to which it has been exposed. 

In many cases, a refiner decides to revamp a cat cracker and employ a new technology without first identifying the unit s mechanical and process limitations. Sometimes money is spent to relieve a constraint and the unit hits another constraint almost immediately. Failure to perform a proper constraint analysis of the existing operation can result in focusing on the wrong issues for the revamp. In addition, the revamp goals must match the refinery s overall objectives. 

Advance Process Control (APC) is a mechanism which manipulates regulatory controls toward more optimum unit operation. 

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