Impurities plantwide control

The goal of this case study is to illustrate some generic issues raised by the conceptual design of a large-scale process involving several reaction and separation sections interconnected in a complex plant with recycles. The study emphasizes the intricate effects of handling the removal of numerous impurities generated in different reactors by a common separation system, with implications on process dynamics and plantwide control. 

Some processes make use of the first method. The second one has been highlighted by studies in the field of process dynamics and plantwide control [7, 8]. More generally, the chemical conversion of impurities is a powerful method for diminishing positive feedback effects through recycles with negative influence on operation and control. 

The inventory of impurities is a plantwide control problem acknowledged in industry for a long time. This problem can be handled systematically by means of... 

Figure 7.13 presents a simplified flowsheet, which concentrates the essential features the balanced VCM technology, as conceptually developed in the previous sections, but this time with the three plants and recycles in place chlorination of ethylene (Rl), thermal cracking of EDC (R2) and oxyclorinahon of ethylene (R3). As mentioned in Section 7.3, from plantwide control three impurities are of particular interest (I]) chloroprene (nbp 332.5 K), (12) trichloroethylene (nbp 359.9K), and (13) tetrachloromethane (nbp 349.8). I, and 12 are bad , since the first can polymerize and plug the equipment, while the second favors the coke formation by EDC pyrolysis. On the contrary, I3 has a catalytic effect on the VCM formation, in some patents being introduced deliberately. 

The source of 13 is the pyrolysis reactor, 12 appears mainly by the oxychlorinahon, while I3 is produced in both chlorination and oxychlorinahon steps. These impurities must be removed selectively from EDC. I, and 12 should be kept at low ppm levels, while 13 is close to an optimal value. This is the incentive of the plantwide control problem. 

The three quality specifications regarding the impurities in EDC, available by direct concentration measurements, such as by IR spectroscopy or online chromatography, are the outputs of the plantwide control problem. The degrees of freedom indicate as first choice manipulated variables belonging to the large column S2 D2-distillate flow rate, SS2-side-stream flow rate, and Q2-reboiler duty. We may also consider manipulated variables belonging to the small column... 

Summing up, if the inventory of the main components can be handled by local control loops, the inventory of impurities has essentially a plantwide character. The rates of generation, mainly in chemical reactors, and of depletion (exit streams and chemical conversion), as well as the accumulation (liquid-phase reactors, distillation columns and reservoirs) can be balanced by the effect of recycles in order to achieve an acceptable equilibrium state. Interactions through recycles can be exploited to create plantwide control structures that are not possible from a standalone unit viewpoint. 

Examine plantwide control problems, as the input of reactants, the manipulation of the production rate, and the control of waste and impurities. Develop a plantwide control structure (see Chapter 13). 

A similar controllability analysis can be applied to solve various plantwide control problems, as the handling of impurities in a complex plant (see Case Study 3 in the Chapter 17). The next example will illustrate the main aspects of a controllability analysis by means of a distillation column. 

The first topic developed in this chapter is the interrelation between steady state design and controllability. Particular attention will receive the generic stmcture Reactor-Separator-Recycle. We consider that material balance has priority in establishing the plantwide control strategy. That is why we will examine in the first place the effects induced by the recycle of mass. Particular attention will receive the feed policy of the reactants, as well as the handling of impurities, both fundamental plantwide control issues. Then we will examine the effect of recycling energy, with emphasis on heat... 

The management of impurities is an issue of greatest significance in Process Design. Impurities affect the product quality, but also can generate troubles in operation and maintenance, and cause environmental damages. The inventory of impurities is a plantwide control problem, because it involves reactors and separators connected through recycles. Ideally, each component should be followed in each unit and each. stream. Downs (1992) pointed out that the material balance must be preserved not only from an overall viewpoint, but also for each component. He presented a qualitative... 

A quantitative approach of the plantwide control of Impurities has been proposed by Dimian et al. (2000). They demonstrate that the inventory of impurities in a large plant is a plantwide control problem that can be solved efficiently by managing positive and negative feedback effects, as well as by taking profit from interactions through recycles. 

Chemical conversion is an effective way to counteract the accumulation of impurities due to positive feedback. Also, changing the connectivity of units may be used to modify the effect of interactions, for example by preventing an excessive increase in recycles due to snowball effects. Effective plantwide control structures may imply controlled and manipulated variables belonging to different but dynamically neighbouring units. The methodology to evaluate the dynamic inventory of impurities consists of a combination of steady state and dynamic flowsheeting with controllability analysis. This is used to assess the best flowsheet alternative and propose subsequent design modifications of units. Case Study 3 in Chapter 17 will present this problem in more detail. 

In the ease of eomplex flowsheets with interaction between reeycles, the handling of impurities can be formulated as a plantwide control problem and solved by means of controllability analysis. 

The control of waste and impurities is a key issue in developing a plantwide control strategy. The accumulation of impurities must be avoided by providing exit points in purges after transformation in benign materials. Obviously, the amount of waste should be minimised by design. The control system can help to solve the waste minimisation problem by its capacity to measure and control the dynamic inventory of components. Controllability tools can be used to solve this plantwide problem. 

The implementation of a plantwide control structure on a given design can follow a step-wise procedure (Luyben Tyreus, 1999). The actions with plantwide character regard energy management, production rate, product quality, safety and environmental protection, and control of impurities. 

Bildea, C. S., S. Cruz, Dimian, A. C., ledema, P., 2002, Design of tubular reactors in recycle systems. Proceedings ESCAPE-12, Elsevier, 439-444 Dimian, A. C., A. J. Groenendijk, P. ledema, 2001, Recycle interaction effects on the ontrol of impurities in a complex plant, 2001, Ind. Eng. Chem. Res., 40,5784-5794 Downs, J., 1992, Distillation control in a plantwide control environment, in W. Luyben (ed), Practical Distillation Control, van Nostrand Reinhold, New York Fisher, W. R., M. F. Doherty, J. M. Douglas, 1988, The Interface between design and control, Ind. Eng. Chem. Res., 27, 597-611... 

The inventory of impurities is a plantwide control problem, because it involves both the reaction and separation subsystems through recycles. Ideally, the inventory of each component should be traced from the source to its final destination. Recent systematic studies on the dynamics and control of the recycle systems have been started, as described in the Chapter 13. Luyben and Tyreus (1998) proposed a ten steps plantwide control design procedure (section 13.7). The step 7 consists of Checking component balances, identify how chemical components enter, leave, and are generated or consumed in the process. At this stage it is necessary to find the specific mechanism or control loop to guarantee that there will be no uncontrollable build-up of any chemical component within the process . 

Dynamic flowsheeting. The detail of modelling depends on the dynamics of units involved in the plantwide control problem. However, the availability of suitable dynamic models for the wide variety of unit operations Involved in practice is questionable. The simplification of the steady-state Plant Simulation Model to a tractable dynamic model, but still able to represent the relevant dynamics of the actual problem, is a practical alternative. In this case detailed models are necessary for the key units, where impurities are generated and eliminated, as kinetic models for reactors and dynamic models for some distillation columns. For other units, steady-state models are sufficient. 

The quality of the intermediate DCE must fulfil strict purity specifications. Low impurity levels imply high energetic consumption, but higher impurity amounts are not desired for operation. The intermediate DCE is conditioned mainly in the distillation column S2. In the bottom product the concentration of the two bad impurities E and E must not exceed an upper limit, of 100 and 600 ppm, respectively, while the concentration of the good impurity E must be kept around optimal value of 2000 ppm. Because these impurities are implied in all three reaction systems through recycles that cross in the separation system, their inventory is a plantwide control problem. The problem is constraint by technological and environmental constraints, as mentioned. 

The dynamic simulation model has been adapted to meet the constraints of a large scale problem and of the equation solving mode of Aspen Dynamics. The final model contained more than 6000 equations. Since the change in material balance (inventory) takes place at long time scales, some substantial simplifications of the local control of units can be considered. Finally, the plantwide control problem is reduced to analyse a 3x7 system, where three outputs (concentration of impurities li, I2, and I3) should be controlled with three among five inputs (D2, SS2, Q2, D4, and Q4), in the presence of two disturbances (Fdce, X ). Because of decentralised control, at most three SISO controllers should be physically implemented. 

Interactions through recycles can be exploited to create plantwide control structures that are not possible when a stand-alone approach is adopted. In this case, acceptable control of three key impurities can be achieved with only two control loops. 

Dimian and Bildea in Chapter C3 explore the issues related to the plantwide control of the material balance. The control of the reactants and impurity inventories in complex reactive systems with recycle are interrelated to the design of the reactor and the separation units. Nonlinear analysis of the reactor model and the recycle structure reveal the conditions for good dynamic performance and guides through the selection of the most appropriate plantwide control strategy. The interactions induced by the recycle streams in the plant can be favourably exploited to build effective control structures that are impossible with stand-alone units. 

Plantwide control problems arise in the context of intricate recycles of mass and energy that characterise modem process plants. Positive feedback effects complicate the dynamics and control because of interactions and non-linear phenomena. Managing the production rate and the formation of waste are important plantwide control problems that originate primarily from the control of component inventory. In this paper we will make the distinction between the inventory of main components and of impurities. Main components designate reactants, intermediates and products that ensure the targeted production rate and the economic efficiency. Although much less as material amount, the inventory of impurities is equally important since they may be harmful for the environment, affect the product quality and lower the price, plug the equipment and lead to troubles in operation, etc. 

Crude DCE from Rl and R3 are sent to washing/drying operation simulated by the blackbox unit SO. Dissolved gases and very light impurities are removed in the unit SI, and further in the distillation column S4, which is the exit point of the light impurities. We will see that S4 plays a very important role in solving the plantwide control of impurities. 

Because the key impurities are implied in all-three reaction systems through recycles that cross in the separation system, their inventory is a plantwide control problem. This is completed by technological and environmental constraints, as mentioned. 

Closed-loop simulations with only the controller Q2-Ii show that the effect of the disturbance Fdce on I2 is reduced by 80%. I2 is below its maximum so that further reduction is not needed. However, a controller on I2 would be required only if this impurity must be kept on setpoint, but the operation would create more problems that it solves. Hence, leaving I2 free, but having the guarantee of a bounded variation, is a rational compromise that preserves the robustness of the control system. Hence, the plantwide control objective could be achieved with only two control loops, Q2-Ii and D4-I3. Over a practical range of fiequency should be almost de-coupled. 

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