Process simulation—steady state recycle

Concerning its performance, steady-state recycling chromatography can be seen as an intermediate between simple batchwise or CLRC operation and continuous simulated moving bed (SMB) processes (Section 5.2). It combines lower complexity and equipment requirements than an SMB system with higher productivities and lower eluent consumption than obtained in batch chromatography. 

The stream conditions shown in Figure 14.1 are from the dynamic simulation of the process at steady-state conditions with the recycle of solvent loop closed. This loop did not converge in the steady-state Aspen Plus simulation. Other simulation issues are discussed in the next section. 

In the RSR approach the chemical reactor is the key unit, designed and simulated in terms of productivity, stability and flexibility. From the systemic viewpoint the key issue is the quality and dynamics of flows entering the reactor and less how they have been produced. Obviously, these flows include fresh reactants and recycle streams. The dynamics of flows must respect the overall material balance at steady state, as well as the process constraints. For this reason, the chemical-reactor analysis should be based on a kinetic model. 

Simultaneous, dynamic simulators require appreciably more computing power than steady-state simulators to solve the thousands of differential equations needed to describe a process, or even a single item of equipment. With the development of fast, powerful computers, this is no longer a restriction. By their nature, simultaneous programs do not experience the problems of recycle convergence inherent in sequential simulators however, as temperature, pressure, and flow rate are not fixed and the input of one unit is not determined by the calculated output from the... 

For steady-state simulation, be able to create a simulation flowsheet, involving the selection of models for the process units and the sequence in which process units associated with recycle loops are solved to obtain converged material and energy balances. 

The steady-state models in simulators do not solve time-dependent equations. They simulate the steady-state operation of process units (operation subroutines) and estimate the sizes and costs of process units (cost subroutines). Two other types of subroutines are used to converge recycle computations (convergence subroutines) and to alter the equipment parameters (control subroutines). These subroutines are discussed in this section. 

Recycle streams can present problems in process simulations. The process studied in this paper has two recycles and is very nonlinear, both of which complicate the convergence of the flowsheet simulation. Up to this point, the only successful solution of the problem discussed in the literature for this process used the approach of converting the simulation into a dynamic one and then closing the recycle loops with a plantwide control structure in place to drive the process to the desired steady state. 

The goal of plantwide control structure synthesis is to develop feasible control structures that address the objectives of the entire chemical plant and account for the interactions associated with complex recycle and heat integration schemes, and the expected multivariate nature of the plant. Many strategies have been proposed for accomplishing this task, and the majority of them have been demonstrated using dynamic process simulations. However, none have been accepted as the universal approach, in a manner similar to the steady-state process design synthesis hierarchy of Douglas [1]. 

As any user of flowsheet simulators knows, the convergence of steady-state simulators when recycle streams are present can be very difficult. Such is the case with both of these processes because they both involve two recycle streams. An alternative approach is to use a dynamic simulation to converge the process flowsheet to a steady state. The process conditions shown in Figures 8.23 and 8.24 are obtained in this way. This procedure is discussed in work by Luyben. ... 

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