Process design equipment details

Hole or perforated area = 4 to 14 percent of bubble area. By bubble area. I m referring to the tray area excluding the area under the downcomer. 

To select the hole area. I ll first calculate the weight of liquid on the tray. This consists of two parts the weir height and the crest height. 

For perforated tray decks (sieve or grid trays), I calculate the pressure drop of the vapor flowing through the holes (inches). The pressure drop I want is the weight of liquid on the trays that I calculated above in step c. The idea is to keep the tray from leaking through the tray deck perforations. 

K for sieve trays is 0.3 K for grid trays is 0.6. Only an idiot would use movable valve trays. The valve caps stick to the deck and do not greatly retard tray deck leakage. 

To summarize. I ll select a hole area for the tray, so that the velocity of vapor flowing through the holes will be big enough to keep the tray from leaking. Of course, if the tray decks are badly out of level, the above calculations are meaningless. So don t forget to inspect your tray installation for tray deck levelness (see my book. Process Equipment Malfunctions, McGraw-Hill, 2011). 

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After the estimates are made, process design has taken shape, and the funds to build the plant are authorized, it s time to buy the equipment. To the uninitiated, the task can appear quite formidable. Even to the veteran, the chore is great with a multitude of details to remember. Note, the title of the chapter is not how to, because that is left to each engineer and his client or company to decide. What will be presented here can be more nearly described as a checklist and outline to help the purchaser recall all the details and avoid as many surprises as possible. 

As the process moves from the process development stage to the design and construction stage the chemistry, unit operations, and type of equipment have been set. The design and construction stage needs to focus primarily on equipment specifications, piping and instrumentation design, installation details, and layout for an inherently safer installation. 

The first step in minimizing accidents in a chemical phuit is to evaluate the facility for potential fires, explosions, and vulnerability to other liazards, particularly those of a chemical miture. This calls for a detailed study of plant site and layout, materials, processes, operations, equipment, and training, plus an effective loss prevention program. The technical nature of industry requires detailed data and a broad range of experience. Tliis complex task, today becoming the most important in plant design, is facilitated by the safety codes, standiu ds, and practice information available. The technical approach to evaluating die consequences of hazards is discussed later in tliis cliapter and in Part V (Chapters 20 and 21). 

The process designer must be aware of costs as reflected in the (1) selection of a basic process route (2) the equipment to be used in the process and (3) the details incorporated into the equipment. The designer must not arbitrarily select equipment, specify details or set pressure levels for design wdthout recognizing the relative effect on the specific cost of an item as well as associated equipment such as relieving devices, instruments, etc. 

The process designer or mechanical engineer in a process plant is not expected to, nor should he, actually design a mechanical vacuum pump or steam jet, biit rather he should be knowledgeable enough to establish the process requirements for capacity, pressure drops, etc., and understand the operation and details of equipment available. 

Emphasizes how to apply techniques of process design and interpret results into mechanical equipment details... 

The purpose of these 3 volumes is to present techniques of process design and to interpret the results into mechanical equipment details. There is no attempt to present theoretical developments of the design equations. The equations recommended have practically all been used in actual plant equipment design, and are considered to be the most reasonable available to the author, and still capable of being handled by both the inexperienced as well as the experienced engineer. A conscious effort has been made to offer guidelines to judgment, decisions and selections, and some of this will be found in the illustrative problems. 

Because melts have different properties and there are many ways to control processes, detailed factual predictions of final output are difficult to arrive. Research and hands-on operation have been directed mainly at explaining the behavior of melts or plastics like with other materials (steel, glass, and so on). Modem equipment and controls are overcoming some of this unpredictability. Ideally, processes and equipment should be designed to take advantage of the novel properties of plastics rather than to overcome them. 

In this book the discussion of optimisation methods will, of necessity, be limited to a brief review of the main techniques used in process and equipment design. The extensive literature on the subject should be consulted for full details of the methods available, and their application and limitations see Beightler and Wilde (1967), Beveridge and Schechter (1970), Stoecker (1989), Rudd and Watson (1968), Edgar and Himmelblau (2001). The books by Rudd and Watson (1968) and Edgar and Himmelblau (2001) are particularly recommended to students. 

Detailed (Quotation) estimates, accuracy 5-10 per cent, which are used for project cost control and estimates for fixed price contracts. These are based on the completed (or near complete) process design, firm quotations for equipment, and a detailed breakdown and estimation of the construction cost. 

Capital cost estimates for chemical process plants are often based on an estimate of the purchase cost of the major equipment items required for the process, the other costs being estimated as factors of the equipment cost. The accuracy of this type of estimate will depend on what stage the design has reached at the time the estimate is made, and on the reliability of the data available on equipment costs. In the later stages of the project design, when detailed equipment specifications are available and firm quotations have been obtained, an accurate estimation of the capital cost of the project can be made. 

The chemical process industries are competitive, and the information that is published on commercial processes is restricted. The articles on particular processes published in the technical literature and in textbooks invariably give only a superficial account of the chemistry and unit operations used. They lack the detailed information needed on reaction kinetics, process conditions, equipment parameters, and physical properties needed for process design. The information that can be found in the general literature is, however, useful in the early stages of a project, when searching for possible process routes. It is often sufficient for a flow-sheet of the process to be drawn up and a rough estimate of the capital and production costs made. 

Whenever possible, experimentally determined values of physical properties should be used. If reliable values cannot be found in the literature and if time, or facilities, are not available for their determination, then in order to proceed with the design the designer must resort to estimation. Techniques are available for the prediction of most physical properties with sufficient accuracy for use in process and equipment design. A detailed review of... 

A brief outline of the technique is given in this section to illustrate its use in process design. It can be used to make a preliminary examination of the design at the flow-sheet stage and for a detailed study at a later stage, when a full process description, final flow-sheets, P and I diagrams, and equipment details are available. 

Before the details of a particular reactor are specified, the biochemical engineer must develop a process strategy that suits the biokinetic requirements of the particular organisms in use and that integrates the bioreactor into the entire process. Reactor costs, raw material costs, downstream processing requirements, and the need for auxiliary equipment will all influence the final process design. A complete discussion of this topic is beyond the scope of this chapter, but a few comments on reactor choice for particular bioprocesses is appropriate. 

Also indices such as the Dow Fire and Explosion Hazard Index and the Mond Index have been suggested to measure the degree of inherent SHE of a process. Rushton et al. (1994) pointed out that these indices can be used for the assessment of existing plants or at the detailed design stages. They require detailed plant specifications such as the plot plan, equipment sizes, material inventories and flows. Checklists, interaction matrices, Hazop and other hazard identification tools are also usable for the evaluation, because all hazards must be identified and their potential consequences must be understood. E.g. Hazop can be used in different stages of process design but in restricted mode. A complete Hazop-study requires final process plans with flow sheets and PIDs. 

The sieve analysis only gives an approximation of the particle distribution. The geometric shape of the particles is a factor in its moving to the proper-size sieve. For many process operations, more detail about the shape and surface area of the particles is important for the proper design and operation of equipment. 

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