Complex flowsheets

When dealing with more complex flowsheets than the one in Figure 13.13, the order in which the calculations... 

Filippov, E.A., Ruzin, L.I., Mamakin, I.V. Development of complex flowsheet of HLW partitioning by solvents on the base of crown ethers and polyorganophosphazenes in heavy diluent. In Khol kin, A.I. Ed., Khimiya i tekhnologiya ekstraktsii (Extraction Chemistry and Technology), Vol. 2. Moscow, 2001, pp. 50-63. (In Russian.)... 

Thus POLYBED PSA can be thought of as a process which alms for maximum recovery of the less-adsorbed component in very high purity at the expense of a complex flowsheet Its growing commercial acceptance suggests that it competes quite successfully with cryogenic separation in many cases ... 

Most real processes contain recycle streams. In this case the plantwide control problem becomes much more complex and its solution is not intuitively obvious. The presence of recycle streams profoundly alters the plant s dynamic and steady-state behavior. To gain an understanding of these effects, we look at some very simple recycle systems. The insight we obtain from these idealized, simplistic systems can be extended to the complex flowsheets of typical chemical processes. First we must lay the groundwork and have some feel for the complexities and phenomena that recycle streams produce in a plant. 

In the next two chapters, more complex flowsheets are considered. But many of the fundamental ideas developed in this chapter and in Chap. 2 are directly applicable to these complex, multiunit, multirecycle, multicomponent processes. 

It is generally a good practice to compile and test a user model in a simplified flowsheet or as a standalone program before adding it to a complex flowsheet with recycles. It is also a good practice to check the model carefully over a wide range of input values, or else constrain the inputs to ranges where the model is valid. 

Optimal pnofrt and oonfiguraljan presented In tables and ori the complex flowsheet... 

Following the convergence history is important at both units and flowsheet level. The simulation of a complex flowsheet is usually difficult and requires a deeper knowledge of flowsheeting techniques. 

Typical inconsistent specifications are those that violate the material balance. Figure 3.30 displays a very simple flowsheet that contains only mixer and splitter with recycle stream. This over-simplified scheme might be seen unrealistic but in fact can explain a basic source of non-convergence in more complex flowsheets, where the presence of large number of units and recycles could hide such simple situation. Several specifications of the splitter may be considered, as follows ... 

The extension of the above exercise at more complex flowsheets can be done by means of a topological analysis. The monograph of Mah (1990) is recommended for a detailed presentation of this topic. Nowadays most of the software performs an automatic identification of the computational sequence. However, the proposed sequence might be not always the most convenient for robust convergence. That is why it is still important to know how the topological analysis works. 

A more complex flowsheet, which contains three recycle loops, is shown in Figure 4.13a. Two calculation sequences are illustrated in Figure 4.13b and 4.13c. These involve the minimum number of tear streams, S5 and S8, and result in the following ouq>ut from ASPEN PLUS ... 

For our second nonideal system, we look at a process that has extremely nonideal VLB behavior and has a complex flowsheet. The components involved are ethanol, water, and benzene. Ethanol and water at atmospheric pressure form a minimum-boiling homogeneous azeotrope at 351K of composition 90mol% ethanol. Much more complexity is introduced by the benzene/water system, which forms two liquid phases with partial miscibility. The flowsheet contains two distillation columns and a decanter. There are two recycle streams, which create very difficult convergence problems as we will see. So this complex example is a challenging simulation case. 

A very full bag of distillation dynamic simulation techniques has been developed and demonstrated in this chapter. The example considered is a simple binary ideal vapor-liquid equilibrium (VLB) column. As the remaining chapters in this book demonstrate, these techniques can be readily extended to much more complex flowsheets and phase equilibrium. 

Now, we want to look at the dynamics of this complex flowsheet. There are three major issues that must be addressed in designing a control system for a heat-integrated column process that is operating under neat conditions. Auxiliary reboUers or auxiliary condensers are not used to balance the vapor boUup needed at the base of the low-pressure column with vapor condensation needed at the top of the high-pressure column. 

Solve equations—for unknown quantities. This can be difllcult, particularly if non-linear equations are involved. Overall balances usually give simpler equations. For complex flowsheets computer methods offer the only practical solution. 

Commercial flowsheet simulation software is now sufficiently user-friendly that undergraduates can produce steady-state and dynamic simulations of fairly complex processes. Figure 17 shows a typical flowsheet with all the controllers installed that our senior would develop and study. Computer speed has increased to the point that dynamic simulations of these complex flowsheets can be run in reasonable times. 

The final type of hybridization is the use of different models for different unit operations. Although this appears to be inconsistent at first, it is reality that thermodynamic models are not perfect and that some work much better for LLE than for VLE, some work better for low pressures and others for high pressures, and some work for hydrocarbons but not for aqueous phases. Furthermore, simulators perform calculations for individual units and then pass only component flowrates, temperature, and pressure to the next unit. Thus, consistency is not a problem. Therefore, one should always consider the possibility of using different models for different unit operations. All the simulators allow this, and it is essential for a complex flowsheet. An activity-coefficient model can be used for the liquid-liquid extractor and an equation of state for the flash unit. This hybridization can be extremely important when, for exanple, some units contain mainly complex organics and other units contain light hydrocarbons and nitrogen. 

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