Polymer reactor systems, control

Minimum Variance Control Given that the objective of most continuous flow polymer reactor systems is to maintain the output... 

An automated pilot-scale 1-litre experimental polymer reactor system with facilities for on-line measurement of flow rate, temperature and density has been set up by Chien and Penlidis (1994a, b). These authors describe a set of open-loop process identification experiments and closed-loop control experiments performed on this system where monomer conversion is controlled in the presence of reactive impurities using the initiator flow rate as the manipulated variable. 

Achieving steady-state operation in a continuous tank reactor system can be difficult. Particle nucleation phenomena and the decrease in termination rate caused by high viscosity within the particles (gel effect) can contribute to significant reactor instabilities. Variation in the level of inhibitors in the feed streams can also cause reactor control problems. Conversion oscillations have been observed with many different monomers. These oscillations often result from a limit cycle behavior of the particle nucleation mechanism. Such oscillations are difficult to tolerate in commercial systems. They can cause uneven heat loads and significant transients in free emulsifier concentration thus potentially causing flocculation and the formation of wall polymer. This problem may be one of the most difficult to handle in the development of commercial continuous processes. 

Apparatus. Since all the polymer modification reactions presented in this paper involved gas consumption, an automated gas consumption measuring system was designed, fabricated and used to keep constant pressure and record continuously the consumption of gas in a batch type laboratory scale reactor. Process control, data acquisition, and analysis was carried out using a personal computer (IBM) and an interface device (Lab-master, Tecmar Inc.). 

This is the simplest process and is widely used for synthesis of condensation polymers. The system is homogeneous and consists of monomer/polymer. In this process the monomer and initiator are kept in a reactor and heated to suitable temperature. The chain transfer agent whenever used for controlling the Molecular weight is also dissolved in the monomer. 

Some polymerization reactions are highly exothermic, so the problems of temperature control, which are the major emphasis of this book, are important in these systems. However, beyond the issue of temperature control, polymer reactors must produce a product with the desired properties. The final polymer product properties, such as viscosity, molecular weight distribution, particle size, and composition, are important for consistent performance of the polymer. These properties depend on more than just temperature and few can be measured online.12... 

Any control procedure for polymer reactors must recognize and deal with these measurement problems. This rules out the use of conventional continuous control ideas taught to engineering students at most universities. These latter ideas are based on continuous measurements, in the presence of little or no measurement noise, and are applicable only to systems with rather short time delays in the feedback loop. 

Polymer reactor control then boils down to controlling all dominant variables to setpoint using manipulators with a fast response and then adjusting the setpoints of the controlled variables to achieve the desired economic objectives. The trick is to determine the dominant variables and manipulators in addition to their relationships. Some key manipulators are heat removal (for externally cooled systems) or conversion... 

Achievable values of Rp are also often limited by the heat removal capabilities of the reactor system, as the heat released by monomer addition is of the order 50-100 kj mol . While batch times are on the order of minutes or hours, individual polymer radicals live on average only a fraction of a second, as calculated by the expression X/(kp[M]). Thus, after the first few seconds of polymerization, the concentration of dead polymer chains is higher than that of polymeric radicals, and by the end of a typical polymerization the concentration of dead chains is orders of magnitude higher than [Ptot]- Final polymer MW and MWD (molecular weight distribution) are controlled by how the concentrations and kinetic coefficients in Equation 3.12 vary with polymer conversion. 

A simple microfluidic reactor system He et al. [124] for the effective synthesis of enzyme-functionalized nanoparticles offers many advantages over batch reactirais, including excelloit enzyme efficiencies. Better control of the process parameters in the microfluidic reactor system ovct batch-based methodologies enables the production of silica nanoparticles with the optimum size for efficient enzyme immobilization with long-term stability. The synthetic approach used glucose oxidase and two different nucleation catalysts of similar molecular mass the natural R5 peptide, and PEI polymer... 

In the solution reactor, the polymer is dissolved in a solvent/comonomer system. Typically, the polymer content in a solution reactor is controlled at between 10 and 30 wt-%. A hydrocarbon solvent in the range of C6 to C9 is typically used as the diluent in the solution process. 

The closed-loop stability of the batch motion can be established with the application of the standard singular perturbation [25] or small gain theorems [8, 10] available in the nonlinear dynamical systems literature, in eonjunction with the definition (7) of finite-time motion stability. In a chemical process context this closed-loop stability assessments can be seen in the cascade control of a continuous reactor [22], the cascade control of a continuous distillation [21, 24], and in the calorimetric estimation [15] of a batch polymer reactor. The closed-loop motion stability is ensured if the observer gain ( o) is tuned slower than the characteristic frequency ( j) of the fastest unmodeled dynamics, and the observer ( ), secondary ( ,.), and primary ( p) gains are sufficiently separated. This is. 

FIGURE 7.1 Schematic of a lab-scale 200 mm diameter iCVD reactor system. For a vinyl homopolymerization, a constant flow of monomer and initiator is metered into the pancake -style vacuum reaction chamber. An array of resistively heated wires, suspended a few centimeters above the substrate, heats the vapors. Laser interferometery provides real-time monitoring of the iC VD polymer thickness. The pressure of the chamber is controlled by a throttling value. Unreacted species and volatile reaction by-products are exhausted to a mechanical pump. For copolymerization, an additional monomer feed line would need to be added to the system (top image). Schematic cross-section of the iCVD reactor showing decomposition of the initiator by the heated filaments. Surface modification through polymerization of the monomer occurs on the actively cooled substrate (bottom image). 

Each type of support was evaluated in a 2.2-liter stainless steel autoclave fitted with a temperature control jacket and a marine stirrer. Each supported, finished catalyst was prepared in situ by sequentially adding to the polymerization reactor 10-100 mg of finished support, 2.0 ml of a toluene solution containing 0.5 wt% (n-butyCpj ZrCl, 0.6 liter of isobutane, 1.0 mmol triethylaluminum and a second addition of 0.6 liter of isobutane. The contents of the reactor were stirred at 400 rpm while the reactor was heated to 90°C, after which ethylene was added to the reactor system to maintain total pressure at 550 psig. After the polymerization experiment, the ethylene flow to the reactor was stopped, the reactor was cooled to ambient temperature and solvents were removed by evaporation, and the reactor was opened and the polymer was collected. Note that no reactor fouling was detected and the granular polyethylene had acceptable morphology important for a commercial process. 

The control system for a polymerization reactor must be sufficiently robust to handle unmeasured disturbances, which impact polymer reactor operation. These disturbances typically result either from trace amounts of polymerization inhibitors left over after monomer purification before the polymerization reaction or from trace amounts of other compounds which may be present in a typical polymerization recipe and which may be affecting the reaction. 

The composition of copol3mier produced in a steady-state CSTR will not, except for small stochastic variations, change with time. However, the copolymer produced in different reactors of a CSTR train will usually be different. The polymer formed in the first reactor will contain a higher fraction of the more reactive monomer than that formed in downstream reactors. Copol mier composition can be controlled by feeding monomer at various points along the reactor system. The relative flow rates of these intermediate feed streams can be controlled to achieve a variety of composition profiles within the reactor train. 

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