Studies of Control During Runaway of Polymerisation Reaction to Improve Both Safety of Industrial Reactors and Product Quality
Tech Area / Field
- CHE-SYN/Basic and Synthetic Chemistry/Chemistry
- CHE-POL/Polymer Chemistry/Chemistry
- CHE-IND/Industrial Chemistry and Chemical Process Engineering/Chemistry
8 Project completed
Senior Project Manager
Institute of Problems of Chemical Physics, Russia, Moscow reg., Chernogolovka
- Dow Chemical Company, USA, MI, Midland
Project summaryIntroduction and overview. The objective of the Project is to perform a complex theoretical and experimental study of runaway of polymerization in batch reactors and to develop a scientific background of runaway forecasting and control to avoid industrial accidents and/or to improve product quality.
Synthesis of such widely used materials as polystyrene, polyvinylchloride, polyacrylate (plexiglass), polyvinylacetate and their co-polymers proceeds through radical exothermic polymerization. Since the reaction heat releases of such polymerization processes are rather high (12–24 kcal/mole) and heat removal methods (e.g. cooling jacket) insufficiently rapid, a sharp increase of the temperature of reaction mass can result in self-acceleration of the process leading to a sharp increase of the reactor pressure and possible reactor destruction, or production of low-quality products. From the data available (UK Health and Safety Executive, 1990) every two years five serious industrial accidents due to runaway polymerization reactions take place on average.
Chemical reactor safety systems usually include one or more of the following: a control system of the thermal mode of polymerization reactor; a system of rapid introduction of reaction inhibitors (“killers”) into the reactor; a system of emergency pressure relief of the reaction mass that is automatically activated following a sharp increase of the reactor pressure (due to runaway reaction).
Control of reactor thermal modes. The main requirements of the control system of the thermal mode is a well-timed detection of accident “origination”. Efficiency of this system depends on adequacy of mathematical model describing thermal state of the reactor, that is, whether the model completely enough considers the main physical, chemical and hydrodynamic phenomena at nonisothermal polymerization.
Emergency introduction of inhibitors (“killers”) into the bulk is not always efficient, because at high viscosity it is very problematic to distribute an inhibitor rapidly and uniformly over the bulk. It would be prospective to use substances capable to dissociate with a significant rate in a well-defined temperature range to form radical-chain inhibitors. Such substances, “sleeping” inhibitors, introduced into a monomer beforehand, could provide self-control of polymerization and prevent runaway. Preliminary experiments conducted at the Institute of Problems of Chemical Physics RAS showed that such a method of polymerization control is in principle possible.
Efficiency of the emergency pressure relief system depends on whether its design (the size of safety valve, pipeline layout, etc.) takes into account hydrodynamic properties of reacting two-phase fluid whose chemical composition is varying along the flowpath. This problem is, in particular, studied in the presently performed International project “INOVVATOR”.
The molecular-mass distribution (MMD) is an important characteristic of polymers governing their rheological, relaxation and, in the long run, exploitation properties. Control after polymerization conditions would allow not only to reduce wastage because of runaway to be excluded, but also MMD to be optimized when producing polymers with required properties.
The Project is devoted to theoretical and experimental studies of polymerization runaway regularities. Using the results obtained, a methodology is to be developed of assessment of safe operation of batch chemical reactors and improvement of product quality. Industrially important polymerization of styrene and vinyl acetate will be investigated in the Project.
The Project includes:
· Theoretical studies of nonstationary nonisothermal polymerization and development of a software to calculate characteristics of thermal modes in batch reactors.
· Experimental studies of regularities of polymerization runaway.
· Investigation of influence of hydrodynamic processes on regularities of polymerization runaway.
· Investigation of influence of temperature and hydrodynamic modes on evolution of MMD parameters.
· The search and synthesis of “sleeping” inhibitors reducing the rate of heat release at polymerization runaway; the experimental study of their efficiency and nonisothermal kinetic regularities of decomposition.
· Theoretical analysis and development of a mathematical model for formation of the two-phase (gas-liquid) system at emergency relief of the reactor pressure after polymerization runaway.
The Project will be performed by a team of different profile specialists (kinetics of radical-chain polymerization, physics of polymers, thermodynamics and thermochemistry, combustion and explosion, mathematical modeling) successfully cooperated earlier in the field of solid propellants and explosives. A significant part of experts took part in the studies of initiation and development of chemical (chain and thermal) explosions in reactors of nuclear and chemical industry (ISTC Project #124-94), as well as in the studies of structural singularities of polymers (ISTC Project #358-96).
Expected results and their application. The Project will result in:
· Development of physical and mathematical models for analysis of polymerization runaway in batch reactors taking into account the most essential hydrodynamic and physical-chemical phenomena.
· Software creation to calculate the temperature and degree of polymerization in batch reactor as functions of main technological parameters.
· Finding regularities of heat release and kinetic characteristics for nonisothermal polymerization of vinyl acetate and styrene.
· Development of physical and mathematical models for evolution of MMD parameters in the course of polymerization in batch reactors and on this basis formulating proposals for optimization of a technological regime to improve the quality of specific polymers due to diminishing scatter in MMD characteristics.
· Investigation of possibility of using of “sleeping” inhibitors, substances activated in a well-defined temperature range, to increase safety of polymerization processes.
The scientific significance of the Project is elucidation of regularities and critical modes of radical polymerization in batch reactors; regularities of molecular-mass distribution formation in nonisothermal conditions; studying possibility of creation of “sleeping” radical-chain inhibitors reducing the rate of heat release at polymerization runaway.
The practical significance of the Project relates to its ultimate objective, that is, creation of the fundamentals for assessment of conditions of runaway in industrial polymerization processes and control of this phenomenon to decrease the risk of industrial accidents; development on this basis of a methodology for both assessment of conditions of safe operation for chemical reactors and improving product quality. Results obtained in the course of Project performance can be of interest both for industry (Rhone Poulenc, ELF Acquitane and others), and modern methodology of design of safety systems of chemical reactors being developed at the Joint Research Center EC, US and European DIERS Users Groups.
Meeting ISTC goals and objectives. The Project entirely meets ISTC aims for it provides scientists and personnel formerly engaged in weapons development an opportunity to conduct research aimed at peaceful activity. Besides it promotes real integration of Russian scientists into the international scientific community and supports basic and applied research and technology development for peaceful purposes.
Scope of activities. The Project includes the following main stages and studies:
The study of regularities of polymerization runaway at different mechanisms of heat- and mass transfer including:
· Mathematical modeling of nonisothermal radical polymerization in reactors taking into account kinetic peculiarities and different mechanisms of heat- and mass transfer. Development of the software.
· The theoretical study of the effect of forced and natural convection on critical conditions and characteristics of polymerization runaway.
· Analysis of thermal modes, their characteristics and realization conditions.
· The experimental study of dynamics of runaway for polymerization of styrene and vinyl acetate in laboratory reactors. Checking up the results of theoretical calculations and adjustment of the models.
The study of regularities of heat release at nonisothermal polymerization taking into account variable viscosity including:
· Determination of kinetic characteristics and heat effects of styrene and vinyl acetate polymerization.
· The study of the effect of variable viscosity on characteristics of styrene and vinyl acetate polymerization.
The study of the influence of physical and hydrodynamic processes on evolution of characteristics of the molecular-mass distribution (MMD) including:
· Mathematical modeling of evolution of MMD characteristics at block polymerization in batch reactors taking into account space distribution of temperatures, polymerization degree and rate.
· The theoretical study of the effect of forced and natural convection on evolution of MMD characteristics.
· The experimental study of evolution of MMD characteristics at styrene and vinyl acetate polymerization in batch reactor.
Synthesis and experimental study of “sleeping” inhibitors including:
· The theoretical search for “sleeping” radical-chain inhibitors.
· Synthesis of “sleeping” inhibitors activated in a well-defined temperature range.
· The study of kinetics of “sleeping” inhibitors decomposition, determination of kinetic characteristics of inhibition.
· The theoretical study of the effect of “sleeping” inhibitors on regularities of polymerization runaway.
· The experimental study of the effect of “sleeping” inhibitors on regularities of runaway of styrene and vinyl acetate polymerization.
Mathematical modeling and analysis of regularities of formation of the two-phase system at emergency pressure relief and the study of the effect of volume portion of gas (vapor) on effective viscosity of the two-phase system.
Technical approach and methodology. In the course of project performance the experimental methods available at the IPCP will be used, in particular, thermogravimetry and calorimetry, as well as chemical, chromatographic and spectral analyses. In some cases new experimental techniques should be elaborated. When developing mathematical models and creating algorithms for their analysis, wide experience in mathematical modeling and optimization of various processes accumulated at the IPCP will be used. For experimental checking up of the developed models the existing reactors will be used or new ones will be manufactured.
Role of foreign collaborators. The proposed Project was discussed at the meeting with Mr. S. Morris, coordinator of the International project INOVVATOR (JRC EC, Ispra, Italy), in Chernogolovka on November 24-26, 1998. The agreement about collaboration has been also reached with Fraunhofer UMSICHT (Germany), Research Center Rossendorf (Germany) and Elf Aquitane (France), which implies constant exchange of information, discussion of models, investigation methods, technical reports, discussion and implementation of results, arrangement of joint Symposia and Workshops, etc.