Trays computer simulation

The preceding calculation assumes an infinite number of contacting stages in the slurry oil pumparound. Detailed tray-to-tray computer simulations show that for three theoretical stages in the slurry oil pumparound, the maximum tray loading is about 65% above that calculated using 700,000 Ib/hr vapor and 980"F. The computer simulation takes into account reduced temperature, liquid flows, and vapor density. 

This rather nebulous concept can best be appreciated by performing a tray-to-tray computer simulation of the column. In the field I first observed this phenomenon when the operators noticed that shutting down the intercooler shown in Figure 8-11 did not affect the propylene content of the absorber off-gas. 

In general, distillation columns should be operated at a low pressure. For example, Fig. 3.3 shows an isobutane-normal butane stripper. This fractionator is performing poorly. A computer simulation of the column has been built. The column has 50 actual trays. But in order to force the computer model to match existing operating parameters (reflux rate, product compositions), 10 theoretical separation stages (i.e., 10 trays, each 100 percent efficient) must be used in the model. This means that the trays are developing an actual tray efficiency of only 20 percent. 

The Erwin two-film (ETF) tray efficiency method is listed in the following four steps. This is a program subroutine copied from the fractionation tray computer program Chemcalc 13 [1], This tray efficiency method has been used successfully for over a decade and has hundreds of successful applications. It is presented here for you to apply as a supplemental to the other, more expensive computer simulation programs to make them more complete. Key variable nomenclatures follow. 

Ki and K/ are determined at the effective top and bottom section temperatures. If the temperature profile is available (e,g., from a computer simulation), the effective temperature is the arithmetic average of all tray temperatures in the column section. Alternatively, an arithmetic average of the feed-stage and end-stage temperatures is often used ... 

In plant or pilot tests, separation data are analyzed by computer simulation that reproduces test conditions. The number of theoretical stages in the simulation is varied until the simulated product compositions and temperature profile match those measured. Tray efficiency is determined from the number of stages that give a good match to test data. 

In a paper entitled A new simulation model for a real trays absorption column, Grottoli et al. (1991) present their version of a nonequilibrium model for computer simulation of absorption columns. Using the material presented in Chapter 14 as a basis, critically examine any novel aspects of their paper. 

The column is solved by computer simulation with vapor-liquid equilibrium calculations based on the van Laar liquid activity coefficient equation (Chapter 1). Initially the column is solved without a side draw to determine the composition profiles in the column. The column is solved at a reflux ratio of 1 and a bottoms rate of 800 kmol/h. The composition profiles with no side draw are shown in Figure 9.7. The trays are numbered from the top down, with the condenser as number one. A total of 20 trays are shown, which include the condenser and the reboiler. 

Chapter 10 Appendix. Tray and Downcomer Design with Computer Simulator... 

I was called to a refinery to help determine why kerosene production had declined 3,000 B/SD. The refinery manager explained that a process engineer had spent six months on the problem and had run several computer simulation analyses on 16 different crude oils, including complete tray-to-tray heat and material balances for three operating modes. 

Figure 49.4 shows the overhead system of the Coastal Refinery crude distillation tower in Aruba. The island of Aruba is a beautiful country in the Caribbean Sea off the coast of Venezuela. 1 often take my wife, Liz, on a romantic vacation to this tropical paradise. During one such exotic trip I was assigned by Coastal to find a plan to improve fractionation between naphtha and jet fuel. The problem was that the naphtha contained 20 percent jet fuel. A computer simulation of the crude tower indicated that the apparent tray efficiency of the upper five trays was... 

To discriminate between the fractionation efficiency of the trays and the hot drum, I obtained a sample of reflux from the discharge of the hot drum reflux pump. It contained 25 percent jet fuel and 75 percent naphtha. I compared the analysis of this sample to my computer simulation. The degree of fractionation observed between jet fuel and the reflux indicated a tray efficiency for trays 16 through 20 of 65 percent. This is a normal tray efficiency for this service. 

Small but powerful interactive computers with CRTs and printers, usually programmed in BASIC, are also available. The HP-9845B is a good example. It can be used as a smart terminal to access mainframe computers. In addition, we have terminals for minicomputers that permit large, tray-to-tray dynamic simulations coded in FORTRAN or Pascal. A smaller BASIC computer, the HP-85, is proving to be a very usefial portable machine. 

The hybrid Reynolds mass flux model and algebraic Reynolds mass flux model, which only need to solve simpler Eq. (1.8) instead of complicated Eq. (1.23), may be a proper choice for multiple tray computation if their simulated results are very close to the standard Reynolds mass flux model. For comparison, the simulated column trays in Sect. 4.1.1.1 for separating n-heptane and methylcyclohexane are used. Li [17] simulated concentration profiles of all trays at different levels above the tray floor, among which the tray number 8 and tray number 6 are shown in Fig. 4.17a and b. 

One might think that this problem can be very easily overcome by simply ratioing the flowrates of the two fiesh reactant feeds. This strategy works in computer simulations, but it does not work in a real plant environment. The reasons why ratio schemes are not effective are inaccuracies in flow measurements, which are always present, and/or changes in the compositions of the feedstreams. Either cause will result in an imbalance of the stoichiometry. Therefore, it is necessary to have some way to determine the amount of at least one of the reactants inside the column so that feedback control can be used to adjust a fresh feed flowrate. Sometimes temperatures or liquid levels can be used. Sometimes a direct composition measurement on a tray in the column is required. This issue is the heart of the reactive distillation control problem and will be quantitatively smdied in detail in subsequent chapters. 

For example, in distillation simulations the distillate and bottoms composition should be called XD(J) and XB(J) in the program. The tray compositions should be called X(NJ), where IV is the tray number starting from the bottom and J is the component number. Many computer scientists put all the compositions into one variable X(NJ) and index it so that the distillate is X(1J), the top tray is X(2J), etc. This gives a more compact program but makes it much more difficult to understand the code. 

Table 5,14 gives a digital computer FORTRAN program for this three-component batch distillation dynamic simulation. The specific example is a column with 20 trays and relative volatilities of 9, 3, and 1. The vapor flow rate is constant at 100 mol/h. 

A rigorous fractionation tower program has been developed with several features that broaden its applicability far beyond the capabilities of existing commercially available distillation tower simulation programs. In addition to the usual features of most modern multicomponent fractionation programs, the individual component material balances and the enthalpy balance written for each tray have been modified so that the resulting computer program is able to simulate ... 

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