Specifying Stream Properties

The specification of the feed pressure takes a little thought. We will discuss the selection of column pressure in more detail later in this chapter. We know that the distillate product is propane. We will want to use cooling water in the condenser because it is an inexpensive heat sink compared with refrigeration. Cooling water is typically available at about 305 K. A reasonable temperature difference for heat transfer in the condenser is 20 K. Therefore, reflux drum temperature will be about 325 K. The vapor pressure of propane at 325 K is about 14 atm (206psia). Therefore, the column will have a pressure at the feed tray of something a little higher than 14 atm. 

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Finally, we can solve the equations listed in Table 3.2.2 simultaneously using POLYMATH [19] or some other suitable mathematical software. The solution procedure used in POLYMATH is the bounded Newton-Raphson method described by Shacham and Shacham [20]. Table 3.2.5 lists the stream properties, which include the solution to the equations and specified temperatures and pressures at each line. The difference in the water flow rates into and out of the cooling tower is the water evaporated. Thus, to cool 164,700 Ibmol/h (74,700 kg mol/h) water requires evaporating 5,200 Ibmol/h (2,360 kg mol/h) of water. The evaporated water, along with water lost because of leaks, blowdown, and drift are a cost of operation. 

For a process system that involves a single condensable component, a vapor-liquid phase change, and specified or requested values of feed or product stream properties (temperature, pressure, dew point, relative saturation or humidity, degrees of superheat, etc.), draw and label the flowchart, carry out the degree-of-freedom analysis, and perform the required calculations. 

Now we get to specify one property of the bottom stream. We choose to specify the mole fraction of the bottoms product. Now the Operation specification is complete. 

The last part of this chapter was dealing with how to add and specify material streams for simulation. Variables specification is one of the important steps that users need to imderstand when dealing with HYSYS. When users wanted to specify streams especially materials, they need to specify at least four variables in order to have HYSYS to calculate the remaining properties. 

The Compressor operation is used to increase the pressure of an inlet gas stream. Depending on the information specified, the Compressor calculates ether a stream property (pressure or temperature) or compression efficiency. 

Figure 3.16 (a) Modifying reported stream properties, (b) Specifying moie fractions. 

The process flow sheet for a PFR in Aspen Plus is constructed in the same way as previous examples. In the data browser, specify the feed stream properties. Specify inlet reactions stoichiometry and parameters as shown in Figure 5.48 for the first reaction. For the thermodynamic data, Peng-Robinson is selected. The reactor is considered isothermal. The process flow sheet and stream property table are both shown in Figure 5.49. 

Separations are an important phase in almost all chemical engineering processes. Separations are needed because the chemical species from a single source stream must be sent to multiple destinations with specified concentrations. The sources usually are raw material inputs and reactor effluents the destinations are reactor inputs and product and waste streams. To achieve a desired species allocation you must determine the best types and sequence of separators to be used, evaluate the physical or chemical property differences to be exploited at each separator, fix the phases at each separator, and prescribe operating conditions for the entire process. Optimization is involved both in the design of the equipment and in the determination of the optimal operating conditions for the equipment. 

Most of the synthesis gas produced is captive. That is, it s consumed by the manufacturer. Synthesis gas plants are normally integrated into the adjacent application plant. When there is a two-party transaction involved, the properties of the synthesis gas stream are normally specified in a contract. There are no universally accepted standards that apply with this stream. 

Because the performance of a particular piece of equipment depends on its input, recycling of streams in a process introduces temporarily unknown, intermediate streams whose amounts, compositions, and properties must be found by calculation. For a plant with dozens or hundreds of streams the resulting mathematical problem is formidable and has led to the development of many computer algorithms for its solution, some of them making quite rough approximations, others more nearly exact. Usually the problem is solved more easily if the performance of the equipment is specified in advance and its size is found after the balances are completed. If the equipment is existing or must be limited in size, the balancing process will require simultaneous evaluation of its performance and consequently is a much more involved operation, but one which can be handled by computer when necessary. 

Next the two feedstreams must be specified. Clicking the + box in front of Streams in the Data Browser window produces a list of all streams. Opening the FB stream and clicking Input opens the window shown in Figure 2.41, on which all the properties of this feedstream are specified. This procedure is repeated for stream FE as shown in Figure 2.42. 

The composition of process streams can be of interest to guarantee product quality by keeping impurities below specified limits, because of safety and pollution concerns or to make sure that the heating values and other properties of intermediate streams are as they should be. Other reasons for installing process analyzers include reduction of by-products, decrease in analysis time, tightening of specifications, and monitoring of contaminants, toxicants, or pollutants. 

Pure component physical property data for the five species in our simulation of the HDA process were obtained from Chemical Engineering (1975) (liquid densities, heat capacities, vapor pressures, etc.). Vapor-liquid equilibrium behavior was assumed to be ideal. Much of the flowsheet and equipment design information was extracted from Douglas (1988). We have also determined certain design and control variables (e.g., column feed locations, temperature control trays, overhead receiver and column base liquid holdups.) that are not specified by Douglas. Tables 10.1 to 10.4 contain data for selected process streams. These data come from our TMODS dynamic simulation and not from a commercial steady-state simulation package. The corresponding stream numbers are shown in Fig. 10.1. In our simulation, the stabilizer column is modeled as a component splitter and tank. A heater is used to raise the temperature of the liquid feed stream to the product column. Table 10.5 presents equipment data and Table 10.6 compiles the heat transfer rates within process equipment. 

One complication is that often property-changing operators can only be applied to a stream when certain other properties of the stream are within specified values, which may not be true at the time. For example, a method to select only crystals greater than a given size can be applied only if a stream contains solids. Similarly, a separation method expected to exploit relative volatility differences can be applied only if enthalpy conditions permit simultaneous liquid and vapor phases. If the preconditions for the immediate application of an operator believed to be useful are not met, a new design subproblem may be formulated whose objective is to reduce property differences between the initial stream and the conditions necessary for the application of the operator. This recursive strategy is a common feature of the means-ends analysis paradigm. 

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