Fixed recycle flow rate

Step 6. Fix recycle flow rates and vapor and liquid inventories. The liquid inventories in the flash vessel and reactor are non-self-regulating, and therefore, need to be controlled (Guideline 1). Since the liquid product valve from the flash vessel has been assigned to control the product flow rate, the inventory control must be in the reverse direction to the process flow. Thus, the reactor effluent valve, V-4, controls the flash vessel liquid level, and the feed valve, V-1, controls the reactor liquid level. Both of these valves have rapid, direct effects on the liquid holdups (Guidelines 6, 7, and 8). The vapor product valve, V-5, which has been assigned to control the pressure in V-100, thereby controls the vapor inventory. 

Equations (4.37) to (4.42) involve eleven variables therefore five degrees of freedom must be specified. We assume constant purity of the fresh hydrogen, yH,2 = 0.95. The control structure fixes the fresh toluene flow rate Fj = 120 kmol/h and hydro-gen/toluene ratio at reactor inlet, yt,3/yH,3 =1/5. Specifying two additional variables, for example reactor volume Vand gas recycle flow rate FR, the mass-balance equations can be solved for six unknowns F2, FB, FT, FP, X and yHP. This is left as an exercise for the reader. 

The plantwide control deals, mainly, with the mass balance of the species involved in the process. The species inventory can be maintained based on two different principles, namely self-regulation and feedback control. Control structures based on self-regulation set the flow rates of fresh reactants at values determined by the production rate and stoichiometry. Control of inventory by feedback consists of fixing one flow rate in each recycle loop, evaluating the inventory by means of concentration or level measurements, and reducing the deviations from the setpoint by change of the feed rate of fresh reactants. 

The treatment of conflicting specifications leading to convergence problems has been developed elsewhere [7]. For example, this situation arrives when the distillation columns are specified by fixed product flow rates. These specifications, correct for standalone columns, lead to nonconvergence when the units are placed in recycles. The explanation is that during the iterative solution it is impossible to... 

In the second simulation (Figure 7.12b), the reactor-inlet flow was increased from 74000 kg/h to 81500kg/h. Initially, the amounts of VCM produced and of fresh EDC fed increase. However, these flows soon decrease to the initial values. This means that, when reaction conditions are fixed, production-rate changes can be achieved only at the expense of large variations of the recycle flow. Moreover, all the flow rates are very sensitive to disturbances if a control structure fixing the flow rate at the plant inlet is used. 

The chemical reactor will be designed in the context of a recycle system, as explained in Chapters 2 and 4. The strategy is fixing the flow rate and composition of the reactor-inlet mixture at values that are compatible with the operation requirements of the catalyst, as given in Table 10.3, for example 50% mol C2H4, 20% mol acetic acid, 6% mol oxygen and 24% mol C02. Figure 10.2 presents the simulation... 

It is worthy to note that we could be tempted to set these values as specifications when designing the plant control structure. However, some specifications might be in conflict. As it can be seen from the relation (7.5), it is not possible to control simultaneously gas recycle flow, ratio hydrogen/toluene and hydrogen concentration in purge. Normally, it is recommended to fix the recycle flow rate Rq at the maximum capacity of the compressor. If Mg is imposed since technological reasons, it comes out that yp must be let free. This is again an important plantwide control decision that can... 

Control structure CSl shown in Fig. 13.15 makes use of the feed in recycles of both reactants. Recycles flow rates are also fixed on flow control. Note that the make-up of A and B may be done directly in the reflux drum and in the reboiler sump, respectively. The reactor outlet is put on level control. Single-point composition control and fixed reflux are used for distillation columns. Composition controllers can be co-ordinated by the composition measurement for the end product. This structure works well, but has the disadvantage of an indirect setting of production. 

Fortunately, there is another possibility use an indirect measure by means of the separation system itself In fact, the separation system will recycle the amount of non-reacted toluene, which is a function of conversion. Thus, the solution is adding in the recycle an amount of fresh feed compatible with the achievable conversion. Hence, fixing the flow rate of a reactant in recycle proportional with the desired production is a feasible mean to change the production. 

Step 6. The recycle flow rate of toluene, after separations and make-up, can be fixed at a value compatible with the achievable production. Similarly, the gaseous recycle can be set at fixed flow rate. This solution is also advantageous for compressor operation. Therefore, make-up of hydrogen can be set on pressure control. 

Figure 13.35 Conversion vs. reactor volume, for fixed hydrogen / toluene ratio and different values of the gas recycle flow rate...

Steady-state component balances around the whole system and around each of the units are used to solve for the conditions throughout the plant for a given recycle flow rate D. The reactor holdup Vr and the reactor temperature Tr necessary to achieve a specified Qm JQ ratio are calculated as part of the design procedure. The other fixed design parameters are the kinetic constants (preexponential factors, activation energies, and heats of reaction for both reactions), the fresh feed flow rate and composition, the overall heat transfer coefficient in the reactor, the inlet coolant... 

The idea is to specify a control structure (fix the variables that are held constant in the control scheme) and specify a disturbance. Then solve the nonlinear algebraic equations to determine the values of all variables at the new steady-state condition. The process considered in the previous section is so simple that an analytical solution can be found for the dependence of the recycle flow rate on load disturbances. For realistically complex processes, analytical solution is out of the question and numerical methods must be used. Modem software tools (such as SPEEDUP, HYSYS, or GAMS) make these calculations relatively easy to perform. 

In Fig. 5.4, the distillate quality is plotted versus the Damkohler number at different recycling ratios for fixed thermodynamic parameters K = 2 and a = 1.5. As expected, by increasing the recycling flow rate the product quality is improved. At infinite Damkohler number, one obtains a maximum attainable composition level = (1 + q>)Xce. Obviously, there is a critical recycling ratio, that has to be attained in order to reach a specified product quality oP... 

Step 6. Fix a flow rate in every recycle loop and control vapor and liquid inventories (vessel pressures and levels). Process unit inventories, such as liquid holdups and vessel pressures (measures of vapor holdups), are relatively easy to control. While vessel holdups are usually non-self-regulating (Guideline 1), the dynamic performance of their controllers is less important. In fact, level controllers are usually detuned to allow the vessel accumulations to dampen disturbances in the same way that shock absorbers cushion... 

Fixing the flow rate of B in the permeate is a realistic basis since in many cases the aim in using a membrane system with recycle is just to obtain a specific flow rate of the product with a high conversion of the reactant. Obviously, the dependence on the recycle ratio R of the fraction of B to be removed in the membrane requires a suitable design of the membrane module which depends on the adopted value of R. 

In the above series, an important paper of Tyreus and Luyben [5] deals with second-order reactions in recycle systems. Two cases are considered complete one-pass conversion of a component (one recycle), and incomplete conversion of both reactants (two recycles). As general heuristic, they found that fixing the flow in the recycle might prevent snowballing. In the first case, the completely converted component could be fed on flow control, while the recycled component added somewhere in the recycle loop. In the second case, the situation is more complicated. Four reactant feed control alternatives are proposed, but only two workable. This is the case when both reactants are added on level control in recycles (CSl), or when the reactant is added on composition control combined with fixed reactor outlet (CS4). As disadvantage, the production rate can be manipulated only indirectly. Other control structures - with one reactant on flow control the other being on composition (CS2) or level control (CS3) - do not work. The last structure can be made workable if the recycle flow rates are used to infer reactant composition in the reactor. This study reinforces the rule that the flow rate of one stream in a liquid recycle must be fixed in order to prevent snowballing. 

Further, we will briefly comment other interesting works in this area. Pushpavanam Kienle [16] studied the reaction A- P in a non-isothermal CSTR / Separation / Recycle process. Assuming infinite activation energy and equal eoolant and reactor-inlet temperatures, they reported state multiplicity, isolated solution branches and instability, for both conventional and fixed-recycle control structures. In addition, the conventional structure showed regions of unfeasibility. The authors claimed the superiority of the fixed-recycle control structure over the fixed-fresh flow rate control. 

Hydrolytic Kinetic Resolution (HKR) of epichlorohydrin. The HKR reaction was performed by the standard procedure as reported by us earlier (17, 22). After the completion of the HKR reaction, all of the reaction products were removed by evacuation (epoxide was removed at room temperature ( 300 K) and diol was removed at a temperature of 323-329 K). The recovered catalyst was then recycled up to three times in the HKR reaction. For flow experiments, a mixture of racemic epichlorohydrin (600 mmol), water (0.7 eq., 7.56 ml) and chlorobenzene (7.2 ml) in isopropyl alcohol (600 mmol) as the co-solvent was pumped across a 12 cm long stainless steel fixed bed reactor containing SBA-15 Co-OAc salen catalyst (B) bed ( 297 mg) via syringe pump at a flow rate of 35 p,l/min. Approximately 10 cm of the reactor inlet was filled with glass beads and a 2 pm stainless steel frit was installed at the outlet of the reactor. Reaction products were analyzed by gas chromatography using ChiralDex GTA capillary column and an FID detector. 

There are different ways to connect the columns to build a SMB system. An important aspect is always the position of the recycling pump. The recycling pump ensures the internal flow of the mobile phase. Most often the recycling pump is placed between the last and the first column, i. e. columns 12 and 1 in Fig. 2. Once the recycling pump is fixed with respect to the columns, it moves with respect to the zones and is alternatively located in zones IV, III, II, and I. The flow rates required in the different zones are different and so the pump flow rates vary from... 

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