Flowsheet the Process

Based on work by G.E.H Joosten and J. Schilder (formerly from Shell) 

You have played in the lab and now have the product as you want it. You have a recipe a list of ingredients, the lab-equipment required, and a description of the procedures to make the product. However, this is all on a scale of perhaps 100 g in a laboratory setting. We still have a lot to do before the product is in the market. Here we start thinking about making the product on a larger scale, say a thousand or a million times more than in the recipe. We hrst do this on paper. 

This is the last point in the method used here where you can go back to the lab and improve the process without needing to redo expensive large scale experiments. 

Start with the recipe with this in hand, constmct a process diagram showing the steps in the process, and how they are connected. At this point do not worry about equipment - that comes later. Then consider the ingredients and where they are added to the process. It is often instructive to look also at the amounts of energy that go in and out of the process. And to look at the waste produced. 

All too often you will hnd that the lab recipe is no good on a large scale. The ingredients cannot be obtained with the same quality, they are too expensive, or the source is too 

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Although precipitation constitutes the concluding part of the hydrometallurgical process flowsheet, the process is also put to use as a purification step in order to separate impurities. The different methods of precipitation are shown in Figure 5.25. The crystallization process is probably the simplest method to remove metals from solutions as compounds. No reagents are added, and the process simply involves the complete dissolution of the metal salt at a suitable temperature and pressure to obtain as high a metal content as possible. As the temperature is raised or lowered, depending on the particular system, the solubility of the metal salt in solution is exceeded and the salt precipitates or crystallizes... 

Formulate the Product I Flowsheet the Process Estimate the Cost. -" I Equip the Process Scale Up... 

Flowsheet the Process. Draw a flow diagram of the process showing the materials (and energy) entering and leaving the process. Consider process changes and recycles. 

Figure 16.1 shows two versions of the flowsheet. The process on the left uses both heat exchangers and tubular reactors in the external loops. Preheating the feed to the reactor with the hotter reactor effluent reduces the size of the reactor. The process on the right uses only tubular reactors. The reactors must be significantly larger because of the lower inlet temperature. However, reactor vessels are relatively inexpensive compared to heat exchangers. Thus, we will limit the study to the reactor-only process. 

Once the flowsheet structure has been defined, a simulation of the process can be carried out. A simulation is a mathematical model of the process which attempts to predict how the process would behave if it was constructed (see Fig. 1.1b). Having created a model of the process, we assume the flow rates, compositions, temperatures, and pressures of the feeds. The simulation model then predicts the flow rates, compositions, temperatures, and pressures of the products. It also allows the individual items of equipment in the process to be sized and predicts how much raw material is being used, how much energy is being consumed, etc. The performance of the design can then be evaluated. 

The decisions made in the reactor design are often the most important in the whole flowsheet. The design of the reactor usually interacts strongly with the rest of the flowsheet. Hence a return to the decisions made for the reactor must be made when the process design has progressed further and we have fully understood the consequences of those decisions. For the detailed sizing of the reactor, the reader is referred to the many excellent texts on reactor design. 

However, factors such as this should not he allowed to dictate design options at the early stages of flowsheet design because preheating the cold feed hy heat integration with the rest of the process might be possible. 

Where possible, introducing extraneous materials into the process should be avoided, and a material already present in the process should be used. Figure 4.6h illustrates use of the product as the heat carrier. This simplifies the recycle structure of the flowsheet and removes the need for one of the separators (see Fig. 4.66). Use of the product as a heat carrier is obviously restricted to situations where the product does not undergo secondary reactions to unwanted byproducts. Note that the unconverted feed which is recycled also acts as a heat carrier itself. Thus, rather than relying on recycled product to limit the temperature rise (or fall), simply opt for a low conversion, a high recycle of feed, and a resulting small temperature change. 

Clearly, the time chart shown in Fig. 4.14 indicates that individual items of equipment have a poor utilization i.e., they are in use for only a small fraction of the batch cycle time. To improve the equipment utilization, overlap batches as shown in the time-event chart in Fig. 4.15. Here, more than one batch, at difierent processing stages, resides in the process at any given time. Clearly, it is not possible to recycle directly from the separators to the reactor, since the reactor is fed at a time different from that at which the separation is carried out. A storage tank is needed to hold the recycle material. This material is then used to provide part of the feed for the next batch. The final flowsheet for batch operation is shown in Fig. 4.16. Equipment utilization might be improved further by various methods which are considered in Chap. 8 when economic tradeoffs are discussed. 

Consider again the simple process shown in Fig. 4.4d in which FEED is reacted to PRODUCT. If the process usbs a distillation column as separator, there is a tradeofi" between refiux ratio and the number of plates if the feed and products to the distillation column are fixed, as discussed in Chap. 3 (Fig. 3.7). This, of course, assumes that the reboiler and/or condenser are not heat integrated. If the reboiler and/or condenser are heat integrated, the, tradeoff is quite different from that shown in Fig. 3.7, but we shall return to this point later in Chap. 14. The important thing to note for now is that if the reboiler and condenser are using external utilities, then the tradeoff between reflux ratio and the number of plates does not affect other operations in the flowsheet. It is a local tradeoff. 

Once the process route has been chosen, it may be possible to synthesize flowsheets that do not require large inventories of materials in the process. The design of the reaction and separation system is particularly important in this respect, but heat transfer, storage, and pressure relief systems are also important. 

The design of the reactor usually interacts strongly with the rest of the flowsheet. Hence a return must be made to the reactor when the process design has progressed further. 

Classification Process simulation refers to the activity in which mathematical models of chemical processes and refineries are modeled with equations, usually on the computer. The usual distinction must be made between steady-state models and transient models, following the ideas presented in the introduction to this sec tion. In a chemical process, of course, the process is nearly always in a transient mode, at some level of precision, but when the time-dependent fluctuations are below some value, a steady-state model can be formulated. This subsection presents briefly the ideas behind steady-state process simulation (also called flowsheeting), which are embodied in commercial codes. The transient simulations are important for designing startup of plants and are especially useful for the operating of chemical plants. 

Preliminary Process Flowsheet. This will show major equipment and lines, preliminary equipment details (vessel diameter, number of trays, pump flow and driver horsepower, etc.), major instrumentation, and, it is hoped, have a material balance at the bottom of each drawing with flows keyed to a numbering system on the diagram. The process flowsheets should cover both the process and utility sides of the plant. 

Figure 3.24 shows the process flowsheet for an ethylene/ethylbenzene plant, Gas oil is cracked with steam in a pyrolysis furnace to form ethylene, low BTU gases, hexane, heptane, and heavier hydrocarbons. The ethylene is then reacted with benzene to form ethylbenzene (Stanley and El-Halwagi, 1995). Two wastewater streams are formed R ... 

As can be seen from the process flowsheet, the following terminal wastewater streams are generated ... 

Kraft pulping is a common process in the paper industry. Figure 8.4 shows a simplifled flowsheet of the process. In this process, wood chips are reacted (cooked) with white liquor in a digester. White liquor (which contains primarily NaOH, NaiS, Na2C03 and water) is employed to dissolve lignin from the wood chips. The cooked pulp and liquor are passed to a blow tank where the pulp is separated from the spent liquor weak black liquor which is fed to a recovery system for... 

The initial aim of the procedure is to generate a reasonable base case design that can be used for preliminary economic evaluation of the process. This can subsequently be optimized and/or compared with any process alternatives that are identified. The complete process is always considered at each decision level, but additional fine detail is added to the structure of the flowsheet at any stage. Established heuristics and equipment selection procedures are used together with new process synthesis insights to guide each flowsheet decision. 

Listed across the top of the matrix are the various shutdown valves in the facility. A mark in each box indicates the function performed by each device to assure that it protects the process component. By comparing the functions performed by each device to the mechanical flowsheet, it is possible for an auditor to quickly ensure that the process component is indeed isolated. 

Designs and/or specifies items of equipment required to define the process flowsheet or flow system specifies corrosion resistant materials of construction. 

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