Flowsheets with Recycle

Recycles of solvents, catalysts, unconverted feed materials, and byproducts are found in many processes. Most processes contain at least one material recycle, and some may have six or more. Furthermore, when energy is recovered by process-to-process heat transfer, then energy recycles are created, as discussed in Section 4.5.4. 

For a sequential-modular simulation program to be able to solve a flowsheet with a recycle, the design engineer needs to provide an initial estimate of a stream somewhere in the recycle loop. This is known as a tear stream, as the loop is torn at that point. The program can then solve and update the tear stream values with a new estimate. The procedure is repeated until the difference between values at each iteration becomes less than a specified tolerance, at which point the flowsheet is said to be converged to a solution. 

Connections ] Parameters Formulas Spieadsheet Cabtiadon Order User Variables ] Notes [ Delete 

The methods used to converge recycle loops in the commercial process simulation programs are similar to the methods described in Section 1.9. Most of the commercial simulation programs include the methods described below. 

In this method, an initial estimate, x, is used to calculate a new value of the parameter, f(x ). The estimate is then updated using the calculated value  

---

Figure 5.19 Flowsheet with recycles, separation and heat integration.

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. 

Advantages and disadvantages of each approach have been discussed in Chapter 2. For steady state flowsheeting most of the simulators have adopted the SM architecture. The situation is more complex in dynamic flowsheeting, mainly because of the way in which the time-derivatives are handled. Thus, the time may be seen locally, at unit level, or globally, at flowsheet level. In the case of flowsheets without recycles, the time is seen at the same horizon from every unit. The information is transmitted sequentially in a synchronised manner. In the case of flowsheet with recycles the count of time is less obvious. The integration should take place simultaneously in all units, with a time-... 

Example 60.1 A Drying Flowsheet with Recycled Exhaust Gas Stream... 

FIGURE 60.8 Flowsheet with recycled exhaust gas stream. 

FIGURE 60.10 Flowsheet with recycled material stream. 

Figure 14.6 Absorber/stripper flowsheet with recycle closed. Block ABSORBER vapor composition profiles...

In some other cases, these methods have to be used. For example, if a binary mixture contains an azeotrope whose composition is highly dependent on pressure, it is possible to separate this mixture into two columns, operating under different pressures, according to the flowsheet with recycle (Fig. 2.12). 

Figure 4,10 illustrates the revised flowsheet with segregation, interception, recycle and sink/generator manipulation. As can be seen from the figure, the flowrate of the terminal wastewater stream has been reduced to 4.8 kg H2O/S. This is exactly the same target predicted in Fig. 4,4. In order to refine the...

From a computational viewpoint, the presence of recycle streams is one of the impediments in the sequential solution of a flowsheeting problem. Without recycle streams, the flow of information would proceed in a forward direction, and the cal-culational sequence for the modules could easily be determined from the precedence order analysis outlined earlier. With recycle streams present, large groups of modules have to be solved simultaneously, defeating the concept of a sequential solution module by module. For example, in Figure 15.8, you cannot make a material balance on the reactor without knowing the information in stream S6, but you have to carry out the computations for the cooler module first to evaluate S6, which in turn depends on the separator module, which in turn depends on the reactor module. Partitioning identifies those collections of modules that have to be solved simultaneously (termed maximal cyclical subsystems, loops, or irreducible nets). 

The older modular simulation mode, on the other hand, is more common in commerical applications. Here process equations are organized within their particular unit operation. Solution methods that apply to a particular unit operation solve the unit model and pass the resulting stream information to the next unit. Thus, the unit operation represents a procedure or module in the overall flowsheet calculation. These calculations continue from unit to unit, with recycle streams in the process updated and converged with new unit information. Consequently, the flow of information in the simulation systems is often analogous to the flow of material in the actual process. Unlike equation-oriented simulators, modular simulators solve smaller sets of equations, and the solution procedure can be tailored for the particular unit operation. However, because the equations are embedded within procedures, it becomes difficult to provide problem specifications where the information flow does not parallel that of the flowsheet. The earliest modular simulators (the sequential modular type) accommodated these specifications, as well as complex recycle loops, through inefficient iterative procedures. The more recent simultaneous modular simulators now have efficient convergence capabilities for handling multiple recycles and nonconventional problem specifications in a coordinated manner. 

Materia] Balance of a Chlorination Process with Recycle A plant for the chlorination has the flowsheet shown. From pilot plant work, with a chlorine/benzene charge weight ratio of 0.82, the composition of the reactor effluent is... 

Figures 7.13 and 7.14 give results using the FS2 flowsheet with the furnace for this hot-reaction case. Figure 7.13 shows that a 10% decrease in recycle flowrate can be handled, but a 20% decrease produces a reactor mnaway. This occurs despite the fact that the reactor inlet temperature increases only slightly ( 0.5 K) during the transient. Figure 7.14 gives results for changes in the setpoint of the reactor inlet temperature controller. Rather surprisingly, inlet temperature can be increased by 2 K without a runaway. This is unexpected since the isolated reactor (Fig. 7.12) showed a runaway with a +2 K change in Tm. The difference may be due to the effect of pressure. In the isolated reactor simulation, pressure is held constant at 50 bar. In the simulation of the whole process, pressure drops as reactor temperature increases due to the increased consumption of reactants. Since the reaction rate depends on the square of the total pressure (P2), the decrease in pressure lowers the reaction rates. However, a 3 K increase cannot be handled.

The reactor and separation systems described above can be assembled in a flowsheet In a preliminary simulation, the goal is closing the material balance with recycles. In a first approach shortcut models may be used to simulate the distilla-... 

Additional details of the economic and sizing calculations can be found in Luyben (1993). Notice that the flowsheet with the smallest annual cost has four CSTRs. Now let s compare this system with a process that has one CSTR and a column whose overhead product is recycled back to the reactor. Economic studies of this system have shown that a simple stripping column is cheaper than a full column. Table 2.3 gives size and cost data over a range of reactor sizes. 

In the simple binary process considered above, the 2500 ft3 reactor with a 17-tray stripper gives the process with the smallest total annual cost 936,000/yr versus 1,550,000,Nr for the best of the CSTR-in-series flowsheets. Thus this process with recycle is more economical, from the viewpoint of steady state, than the alternative process consisting of reactors in series. This is the point we made in Sec. 2.2 about the economic advantage for recycle. 

Because the reactor/stripper process is much more attractive economically, it may be the flowsheet of choice despite its larger short-term variability in product quality. This illustrates how plants with recycle are generally more difficult to control than units in series. 

Figure 2.12 Ternary process flowsheet with incomplete conversion of both reactants and one recycle stream.

Figure 2.13 Ternary process flowsheet with incomplete conversion and two recycle streams (.heavv-out-first sequence . iai Control structure CS4 reactor composition and level control (workable. (61 control structure CS1 reactant makeup control based on component inventories t workable).

The final example to illustrate our plantwide control design procedure comes from Luyben and Tyreus (1998), who present design details of an industrial process for the vapor-phase manufacture of vinyl acetate monomer. This process is uniquely suited for researchers pursuing process simulation, design, and control studies. It has common real chemical components in a realistically large process flowsheet with standard chemical unit operations, gas and liquid recycle streams, and energy integration. 

Description The flowsheet shown is only one of several possible schemes. The raw materials to a free-standing glycol plant are refined ethylene oxide and pure water. These are mixed with recycle waters and pumped from a feed tank (1) to the hydration reactor after being preheated with hot recycle water and steam. When the glycol unit is part of a combined oxide/glycol plant, it is economically desirable to feed it bleed streams from the ethylene oxide... 

We cannot go into all of the details of flowsheeting program construction and application here. Refer to Chapter 5 and the to supplementary references at the end of this chapter for further information. However, let us look at an example of a very simple process with recycle to compare the simultaneous equation and modular solution techniques just to illustrate the concepts. 

Figure E5.8 is a hypothetical process used for demonstration by Diamond Shamrock Co. of their flowsheeting code PROVES. Makeup gas is compressed, combined with recycle gas, and fed, together with liquid raw material, into a three-phase, suspended bed catalytic reactor. The reactor is cooled by recirculating liquid through a heat reclamation steam generator. Reaction products are condensed and the pressure of the exit stream reduced in two stages. The gas from the first-stage separator is recirculated, whereas the liquid from the second-stage separator is fed into a distillation column. Pure product is withdrawn from the bottom of the column. The distillate is a by-product that is pumped to another plant.

Figure 11-11 shows the flowsheet of the two-filtration process with recycle of second filtration mother liquor. For the recycle process, at the beginning of the batch, the starting materials Iso and 7-chloro were charged to a reactor. The Iso charged included the recycled Iso from the previous batch and the fresh Iso. The batch was heated and aged at the desired reaction temperature. After the reaction was complete, the batch was transferred to a crystaUizer and cooled over a period of time. Both... 

---