Control Of Polymerization Reactor

Control of Polymerization Reactors

Presented By: - Muhammad Zafar Iqbal

- Amir Ali

M.Sc.Course Polymer Reactor Design

• General introduction to polymerization reactor control Problems

• Measurement and control of PR: Techniques introduction

• Control of Polymerization Reactors

05/03/23 Polymer & Process Engineering 2

Classical polymerization reaction control problems

1. Control of reaction rates and of reactor temperature2. Control of monomer conversion and polymer

production3. Control of molecular weight averages and MWDs4. Control of copolymer composition5. Control of particle size and PSDs6. Control of other reaction parameters


Control of reaction rates and of reactor temperature

• Polymerization reactions are usually very exothermic (heat of reaction about 100–200 kJ (g-mol)−1) and present very high apparent activation energies (around 10–30 kJ (g-mol)−1)

• A small increase of the reactor temperature may lead to a significant increase of the reaction rate (due to the high activation energies) and consequently to a large increase of the rate of heat release (due to the large heat of reaction).


CSTR Model


Control of monomer conversion and polymer production

• A second classical control problem is the control of the residual monomer content (monomer conversion) of the polymer productivity.

• Max. conversions are achieved through increase in residence time, batch time and is limited by viscous effects, diffusional effects.

• Min. residual monomer is required because max. conversion leads to max. productivity and also there are some constraints of EPA about VOCs in the products.

• This limiting concentration is usually much lower than the one achieved during polymerization, which means that additional post-polymerization treatments are required. As these are costly operations, it is interesting to minimize the amount of monomer at the end of polymerization (maximize monomer conversion).


Cont’dHow to minimize the residual monomer: (The control and optimization can be based on the above

statement)

• Reaction Times• Reactor Temperatures• Feed Rates• Using catalysts of different decay characteristics• Mixtures of initiators


Control of molecular weight averages and MWDs

• The reaction rates of all elementary reaction steps that constitute the complex network of the polymerization reaction mechanism also vary along the reaction time.

• As a consequence, the molecular properties of the produced polymer material usually change continuously along the time during transient operations.

• Similar drifts of the average molecular weights can be observed along the vessels that constitute the reactor trains of some emulsion and olefin polymerization processes.

• Control of the molecular weight averages and of the molecular weight distribution (MWD) is usually attained through manipulation of chain transfer agents (CTA).


Cont’d• Control of the molecular weight averages and of the molecular

weight distribution (MWD) is usually attained through manipulation of chain transfer agents (CTA).

• Manipulation of reactor temperatures for control of the MWD should be avoided because of the sluggishness of temperature responses and because of safety issues.

• Manipulation of initiator and monomer compositions should also be avoided because of the strong coupling with the production targets and with the energy balance variables.

• Mixing of CTAs (or other chemical species) during the reaction course can constitute a very complex task, due to the very small amounts of CTA that are required, to the high system viscosities and to the heterogeneous nature of the reaction medium.


Control of copolymer composition• Most monomers have different reactivity ratios, which lead

to production of copolymers that do not have the same composition of the monomer mixture.

• composition drift causes the production of heterogeneous polymer mixtures, which may be deleterious for the performance of the polymer material.

• This leads to the definition of the fourth classical control problem, which is the control of the copolymer composition along the reaction batch (or at the end of the batch).

• This objective is normally attained through manipulation of monomer feed flow rates.

• The strategy used to control the copolymer composition may exert a significant impact on the MWD of the final polymer material.


Control of particle size and PSDs• Many polymerizations are performed in heterogeneous

media, so that the final polymer product is obtained as a particulate material.

• The characteristics of the final particle size distribution (PSD) of the product may be of fundamental importance for many applications.

• To increase the polymer concentration of polymer latexes, multimodal and/or broad PSDs may be required.

• This leads to the definition of the fifth classical control problem, which is the control of some features of the PSD of the final polymer material.


The main controlling variables for PSD are:

• Modification of surfactant conc. (emulsion pzn)• Agitation Speed (mostly in suspension pzn)• Initial catalyst size distribution and reaction times (RTD

in continuous reactors especially coordination reactor)


Measurements and

control


Measurement & Control Hierarchy


MeasurementsMain types of measurements which are required in any

polymer process industry for reactors include:1. PTLF measurements2. Densitometry, dilatometry and gravimetry3. Viscosity measurement4. Measurement of composition5. Surface tension6. Molecular weight distribution (MWD)7. Particle size distribution (PSD)


Viscosity Measurement• Viscosities are of interest in polymer technology in order

to follow the course of a polymerization reaction or to monitor continuously the quality of a product.

• For polymer systems, solution or melt, the viscosity can be related to the molecular weight of the polymer.

• Viscosity is measured by capillary viscometers or rotating viscometers.

• Indirect method to obtain a measure of molecular size that is quick and inexpensive is the Melt Indexer.

• It is evident that the Melt Index varies inversely proportionally to the polymer molecular weight.


Measurement of composition• The composition of raw materials, finished products, and

samples of the various steps of a reaction is normally measured at the laboratory using the appropriate physical and chemical analytical methods

• In order to monitor compositions continuously, one needs automatically functioning analytical instruments that can continuously obtain the composition of a mixture.

• More recently the Fourier transform infrared technique (FTIR) has been used for faster data acquisition and handling than traditional IR spectrographic analysis. IR and FTIR can be applied to polymer solutions or solid films for composition analysis and are particularly useful for copolymer composition determination.


• The refractive index (RI) of a mixture is a function of the composition of the mixture and their respective refractive indices.

• Differential refractometer is commonly used as a concentration detector in the effluent of a gel permeation chromatography (GPC) column for molecular weight determination

• Raman scattering of light by molecules may be used to provide chemical composition and molecular structure and is currently being applied to polymers.

• Apart from optical methods, magnetic and electrical methods can also be used for composition measurement.


Measurement of Surface tension

• In emulsion polymerizations, particularly it may be of interest to measure the surface tension of the emulsion.

• The surface tension can give an indication of whether or not micelles are present, which is important in particle nucleation above the critical micelle concentration (CMC)

• The surface tension is measured through bubble pressure method

Ref: http://www.kruss.info/index.php?content=http%3A//www.kruss.info/techniques/bubble_pressure_e.html


Molecular weight distribution (MWD)• It is widely recognized that a reliable method of monitoring

molecular weight distribution, and the various molecular weight averages during the polymerization process is of importance to final polymer quality.

• Traditionally gel permeation chromatography (GPC) or size-exclusion chromatography (SEC) have been used to determine MWD.

• For each type of polymer an empirical correlation exists between molecular weights and elution volumes.

• More recently triple-detector instruments have been designed, which include a differential viscometer, a light-scattering instrument, and a differential refractometer that monitors the column effluent.

• Some vendors are commercializing more rapid GPC/SEC instruments for online control with some instruments already available.


Particle size distribution (PSD)• The particle size distribution can have a fundamental effect on the

physical properties of dispersions that are common polymer products.

• The presence of different size populations resulting in a multimodal distribution could have a strong influence on final properties and may need to be controlled.

• There are several particle size measurement techniques used such as optical imaging, electron imaging, optical diffraction and scattering, electrical resistance changes, sieving, sedimentation, and ultrasonic attenuation.

• Capillary hydrodynamic fractionation (CHDF) is a hydrodynamic method for measurement of nanometer-sized particles that is developed most recently.

• Very important and very complex invention is the acoustic attenuation technique for PSD.


Control Of Polymerization Reactor

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