Sizing of equipment

The chemical engineering undergraduate spends most of his time sizing equipment. Usually in the problems assigned the type of equipment to be used is specified. For a distillation column the student would be told whether it is a bubble cap, a sieve plate, a valve tray, a packed column, or something else, and then asked to size it for a desired separation. In other cases he would be given the size of the specific equipment and asked to determine what the output would be for a given input. 

If an answer is still not forthcoming, an equipment manufacturer may be asked to provide the information. The vendor should be told that this is a preliminary budget estimate so he will not provide a detailed quotation. That may take a couple of man-weeks (a man-week is the time spent by a man working one week), while the budget estimate may only take a few man-days. The added expense for the detailed quote will have to be absorbed by the vendor and will eventually result in increased prices for the buyer. 

Sometimes not enough is known about the processing rates for even a vendor to size the equipment. In this case it may be possible for the engineer to have the equipment manufacturer obtain the necessary data. 

For instance, the Swenson Evaporator Company maintains a pilot plant for sizing crystallization equipment. They will rent it to prospective customers, who can then 

One problem that frequently arises when processes for new products are being designed is that the physical properties of these compounds are unknown. Often these unknown properties may need to be estimated before the equipment can be sized. A list of references on estimating physical properties is given at the end of this chapter. 

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Increasing the size of equipment in the steps which limit the batch cycle time to reduce the dead time for those steps which are not limiting. 

Apart from determinating the optimum size of equipment, the degree of flexibiHty is another key plant design parameter. FlexibiHty means cost, thus only as much flexibiHty as required by the processes should be buHt. Excessive flexibiHty is counterproductive (2). 

Nusse/t Number. Empidcal correlations can be obtained for a particular size of tube diameter and particular flow conditions. To generalize such results and to apply the correlations to different sizes of equipment and different flow conditions, the heat-transfer coefficient, Z, is traditionally nondimensionalized by the use of the Nusselt number, Nu named after Wilhelm Nusselt,... 

Table 23-14 gives data for common types of L/L contactors. Since the given range of k a is more than 100/1, this information is not of direct value for sizing of equipment. The efficiencies of various kinds of small liquid/liqiiid contactors are summarized in Fig. 23-38. Larger units may have efficiencies of less than half these values. 

Once the candidate corrective measure alternatives have been identified, a more detailed evaluation of each alternative needs to be undertaken. From an engineering perspective, the first step in the evaluation process would include the development of a conceptual design for each alternative. The conceptual design would consist of a process description, a process flow diagram and a layout drawing. Preliminary sizing of equipment and utility and land requirements would be developed. In addition, chemical requirements and residuals produced can be estimated. From the conceptual design, permitability and residuals disposal issues can be identified and addressed. 

Design strategies which result in an inherently safer design may also tend to improve process economics. For example, minimizing the size of equipment or simplifying a process by eliminating equipment will usually reduce capital investment and reduce operating costs. However, overall process economics are very complex and are impacted by many factors, and it may not always be true that an inherently safer process is also economically more attractive. 

When designing a plant, every piece of process equipment should be specified as large enough to do its job, and no larger. We should minimize the size of all raw material and in-process intermediate storage tanks, and question the need for all in-process inventories, particularly of hazardous materials. Minimizing the size of equipment not only enhances inherent process safety, but it can often save money. 

Figure 1-17 [2] can be used as a guide in establishing relative sizes of equipment as represented on a flowsheet. This chart is based on approximate relative proportions pictured by the mind s eye [2]. For example, the 10-foot diameter x 33-foot high tank would scale to 1.5 inches high. By using the height-developed scale factor, the diameter would be (1.5"/33 ) (10 ) = 0.45" or say 0.5" diameter on the flowsheet. 

Equipment schedules developed for separate types and sizes of equipment and showing the number of units required for time period. 

The size of equipment can vary from a 200 W unit for a commercial dry-cleaning machine to systems of megawatt size for synthetic fibre processes. 

Example 7.4-2. Sizing of equipment units for a single-product plant... 

Addition of parallel equipment units reduces the size of equipment for faster stages (see section 7.4.1.1). This is done at the cost of a greater number of units for processing slower stages whereby the total volume of these units remains constant. An economic trade-off between the larger number of equipment items and their smaller size will decide whether installing parallel units will improve the process. 

The values given in Table 12.1 and Figure 12.1 can be used for the preliminary sizing of equipment for process evaluation, and as trial values for starting a detailed thermal design. 

As noted before, standard sizes of equipment are cheaper and are thus preferentially specified. Because the equipment must assuredly be large enough to produce the amount of product desired, undersized items are rarely specified. From this the reader may infer that the predicted utility requirements may be low, since for a preliminary design the standard items are usually not specified. Also, numerous small items of equipment have not been included, and possibly the designer may have mistakenly omitted at least one major item. Therefore, it is wise to increase the power requirements by 5%- 10%. 

An exact calculation of inventory is difficult in the conceptual design phase, since the size of equipment is not usually known. The mass flows in the process are however known from the design capasity of the process. Therefore it is practical to base the estimation of inventory on mass flows and an estimated residence time. Consequently the inventory has been included to the ISI as a mass flow in the ISBL equipment including recycles with one hour nominal residence time for each process vessel (e.g. reactor, distillation column etc). For large storage tanks the size should be estimated. The total inventory is the sum of inventories of all process vessels. 

A plant is divided into inside and offsite battery limit areas. The configurations of ISBL and OSBL areas differ considerably. Generally the size of equipment, the amount of chemicals and also the spacings are larger in OSBL area. The safety of the process structure is also affected by these factors. Therefore this aspect is included also into the database. 

The part sizes are limited by the tool sizes such as moulds, dies, autoclaves or winding machines and by the power and the size of equipment such as presses, bags, pultrusion machines, etc. 

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