Fluidized Bed For Catalytic Polymerization

Fluidized Bed Catalytic Polymerization

FLUIDIZED BED FOR CATALYTIC POLYMERIZATION The present work discusses fluidized bed polymerization. It includes the catalytic processes by using fluidized bed, fixed bed, monolith reactor, by using different parameters like type of catalyst, flowrate, temperature etc. and their effect on conversion A source code written in C to get the concentration profile and the rate of reaction as well as conversion at different temperatures is calculated by numerical methods.

INTRODUCTION We know that a polymer is made up of many small molecules which are combined to form a single long or large molecules. The individual small molecules from which the polymer is formed is called monomer and process by which the monomer molecules are linked to form a big polymer molecule is called ‘Polymerization’. The polymerization process in which catalyst is used is called as catalytic polymerization. eg. In many process such as UNIPOL process for making poly ethylene, Ziegler Natta Catalyst is used. It is fluidized bed catalytic polymerization so we come across

FLUIDIZATION Fluidization is the process by which solid particles are transformed into fluid like state through suspension in gas or liquid. The fluidized bed is mainly used for heterogeneous catalytic reactions and the reaction of high effect. C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

In Heterogeneous catalytic reaction catalyst often looses its activity with operating time. If this decrease is rapid and severe, it is desirable to generate catalyst continuously without shutting down the reactor. The Fluidized bed provide an effective way to achieve this objective.

THE VARIOUS STAGES DURING FLUIDIZATION PROCESS 1)

FIXED BED

2)

MINIMUM FLUIDIZATION

3)

SMOOTH FLUIDIZATION.

4)

BUBBLING FLUIDIZATION

5)

SLUGGING.

6)

TURBULENT FLUIDIZATION.

7)

LEAN PHASE FLUIDIZATION.

FIXED BED If a fluid is passed upward through bed of fine particles as shown in fig (a) at a low flow rate, the fluid merely percolates through voids spaces between stationary particles. Such a bed is called as fixed Bed.

MINIMUM FLUIDIZATION At still higher velocity, a point is reached where all the particles are just suspended by the upward flowing gas or fluid. At this point the frictional force between particle and fluid just counterbalances the weight of the particles, the verticle component of the compressive force between adjacent C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

particles disappears, and pressure drop through any section of the bed about equals the weight of fluid and particles in that section. The bed is considered to be just fluidized and is referred as an incipiently fluidized bed or a bed at minimum fluidization fig.(b). SMOOTH FLUIDIZATION In Liquid-solid systems, an increase in flowrate above min. fluidization usually results in a smooth, progressive expansion of the bed. Cross flow instabilities are damped and remain small and heterogeneity, or large scale voids of liquid, are not observed under normal conditions. Such a bed is called as articulately fluidized bed, or homogeneously fluidized bed or smoothly fluidized bed. Fig.( c ). BUBBLING FLUIDIZATION Generally gas-solid systems behave quite differently. With an increase in flow rate beyond min. Fluidization, large instabilities with bubbling and channeling of gas are observed. At higher flow rates, agitation becomes more violent and movement of solid becomes more vigorous. In addition the bed does not expand much beyond it’s volume at min. fluidization. Such a bed is called as aggregative fluidized bed, heterogeneously fluidized bed or Bubbling Fluidized bed. Fig. (d). SLUGGING In gas solid systems, gas bubbles coalesce and grow as they rise, and deep enough bed of small diameter. They may eventually become large enough to spread across the vessel. In the case of fine particles, they flow smoothly

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

down by wall around the rising voids of gas. This is termed as slugging with axial sluges, fig (e). For coarser particles, the portion of the bed above the bubble is pushed upward, as by a piston. Particles rain down from the slug, which finally disintegrates. At about this time another slug forms, and this unstable oscillatory motion is repeated. This is called a flat slug fig.(f). TURBULENT FLUIDIZATION When particles are fluidized at a sufficiently high gas flow rate, the terminal velocity of the solids is exceeded, the upper surface of the bed disappears, entrainment becomes appreciable and instead of bubbles, one observes a turbulent motion of solid clusters and voids of gas of various sizes and shapes. This is the turbulent fluidized bed. Fig.(g). LEAN PHASE FLUIDIZATION WITH PNEUMATIC TRANSPORT With further increase in velocity above turbulent fluidization, solids are carried out of the bed with the gas. In this we have a disperse, dilute, or lean phase fluidized bed with pneumatic transport of solids. In both turbulent and lean phase fluidization, large amounts of particles are entrained, precluding steady state operations.

For steady state

operation in these contacting modes, entrained particles have to be collected by cyclones and returned to the beds. In turbulent fluidized beds, inner cyclones can deal with the moderate rate of entrainment as shown in fig and this system is called a fluid bed. On the other hand the rate of entrainment is far larger in lean phase fluidized beds, which usually necessitates the use of big cyclone collector outside the bed as shown in fig. This system is called as fast fluidized bed. C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

In fluid beds and fast fluidized beds, smooth and steady recirculation of solids through the dipleg or other solid trapping device is crucial to good operations. These beds are called circulating fluidized beds. In the spout bed, a high velocity spout of gas punches through the bed of solids, thereby transporting particles to the top of the bed. The rest of the solids move downward slowly around the spout and through gently upward percolating gas, behaves somewhere between bubbling and spouting is also seen and this may be called spouted fluidized bed behaviour.

MECHANISM OF CATALYTIC POLYMERIZATION – Typical catalyst for catalytic polymerizations include, in order of importance, a protonic acids ( Lewis acids & Friedel-crafts halides ), protonic acids & stable carbonium – ion salts. All these are strong electron accepters. Many of them particularly the lewis acids, require a cocatalyst, usually a Lewis base or other proton donor, to initiate polymerization. A polymerization of isobutylene by AlCl3 or BF3 takes place within a few seconds at –1000C. producing polymer of molecular weight upto several millions. MECHANISM :In the polymerization of isobutylene with boron trifluoride catalyst, the first step is the reaction of the catalyst & cocatalyst for ex. water to form catalyst – cocatalyst complex that donates the proton to isobutylene molecule, to give carbonium ion, (CH3)3 C+. This ion then reacts with monomer with the reformation of a carbonium ion at the end of each step. The head to tail addition of monomer to ion is the only one possible for energetic reasons. Since the reaction is C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

generally carried out in a hydrocarbon medium of low dielectric constant, the anion & the growing cationic end form an ion pair. The termination of reaction can take place by the rearrangement of the ion pair to yield a polymer molecule with terminal unsaturation, plus the original complex. Initiation BF3 Catalyst

+

H+(BF3 OH)¯ +

(BF3 OH)¯ H +

H2O Lewis Base

H3C C = CH2 H3 C

CH3 C+ (BF3 OH)¯ CH3

CH3 Ion pair

Propagation :CH3 (CH3)3 C+(BF3 OH)¯

+ H2 C = C CH3

CH3 | (CH3)3 – C – CH2 – C + (BF3 OH)¯ | CH3 C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

Termination :(CH3)2 [(CH3)2 C CH2]xC(CH3)2+(BF3 OH)¯ CH2 +

¯

H (BF3 OH) + CH3 [(CH3)2 C CH2]xC CH3 Here catalyst cocatalyst complex is regenerated, and many kinetic chains can be produced from each catalyst – cocatalyst species. KINETICS :Although many cationic polymerizations proceed so rapidly that it is difficult to establish the steady state, the following kinetic scheme appears to be valid. Let the catalyst be A and the cocalalyst by RH. Initiation, propogation, termination and transfer may be represented as follows.

A + RH

K

H+AR¯

H+AR¯ + M

Ki

HM+ AR¯

HMx+AR¯ + M

Kp

HM+x+1 AR¯

HMx+AR¯

Kt

Mx + H+ AR¯

Kttr

Mx + HM+AR¯

HMx+AR¯ + M

Here the rate of inibaton is ri = Kki [A][RH][M] Where

[A] is catalyst concentration . [M] is monomer concentration.

Rate of propagation C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

rp = kp [HM+ AR¯] [M] &

Rate of Termination rt = kt [HM+ AR¯]

Applying steady state & rearranging overall rate of polymerization. ro = rp = K ki kp [A][RH][M]2 kt Polyolefins polymerization process can be divided into two main types. • CATALYTIC • HIGH PRESSURE FREE RADICAL CATALYTIC POLYMERISATION: In catalytic polymerization the polymerization, takes place on the surface of catalyst. The heat removal problem exists not only between the reaction fluid and coolant, but also between the catalyst & polymer particle, and the fluid, insufficient heat transfer between the particles and the fluid may lead to an increase in the particles temperature, called overheating or the hot spot formation. This may lead to heterogeneous product properties and more likely, unstable reactor operation. Sufficient Agitation is needed to prevent the polymer particles from fusing. The surface of the polymer particles may be soft and sticky due to overheating, swelling or low crystallinity of polymer. The agglomeration of polymer particles often leads to unstable process operation.

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

The particles may also be attracted by the electrostatic forces. This can cause the particles to stick to the walls of reactor. Again problems in process operation may follow with insufficient care.

TYPES OF CATALYTIC POLYMERIZATION Catalytic Polymerisation processes can be divided into.  Liquid Phase - Slurry processes.  Liquid Phase - Solution processes.  Gas Phase. LIQUID PHASE -SLURRY PROCESSES In Slurry polymerization the catalyst and the growing polymer particles are suspended in a liquid diluents. The diluents may not contain compounds that irreversibly react with the active sites of the catalyst (i.e. the catalyst poisons) and it should not dissolve the polymer. It should however dissolve the reactive compounds needed in the polymerization, that are monomers, Comonomers, hydrogen and isobutene or propane are commonly used. The special feature of the Borstar process which use propane in a supercritical state as a diluents. The

traditionally

slurry

polymerisation

reactors

are

continuously operating stirred tank reactors (CSTR). They are equipped with an agitator to ensure sufficient stirring. They also have cooling jackets to remove the heat of reaction. Since this alone does not provide sufficient cooling, they also have external heat exchangers through which slurry is circulated. A loop reactor is also used in a slurry polymerization. This reactor is a long circular pipe. Agitation is achieved by pumping the slurry at high C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

velocity through the reactor. The reactor has a cooling Jacket to remove the heat of reaction. Since a loop reactor has a high ratio of surface area to volume, the Jacket can provide sufficient cooling. The heat removal from the liquid is effective for this reason the slurry reactor has a high capacity per volume. This means that the average residence time in a slurry reactor is relatively small about an hour. In this respect loop reactors outperforms CSTRs. Also the heat transfer between the fluid and particles is effective enough in a slurry processes to prevent overheating of the particles. Solubility of polymer into diluents restricts the product range in slurry processes. The solubility depends on the density & MFR; the lower the density higher the solubility. The opposite is true for the MFR. Thus LDPE products with density lower than 930 Kg/m3 are not usually made by slurry processes. The MFR range that can be produced in a slurry process is limited by the solubility of hydrogen in the solvent selecting a catalyst with high hydrogen sensitivity can compensate this. Another way to overcome the problem is to operate the reactor under supercritical conditions. After the product is taken out of the reactor, the polymer needs to be separated from the fluid. If high boiling diluents (such as hexane) are used, separation is achieved by centrifuging. The polymer is pelletised & the fluid recovered & recycled. The recovery consists of separating the dissolved polymer, separating the different fractions (ethylene, comonomer, diluents) & purifying them, the purified material is then fed back into - the reactor. If a low boiling diluents (such as isobutane or propane) is used, the polymer is usually separated C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

from fluid by flashing. The polymer is then dried & pelletised. The flash gases are seperated & purified. After this, they are recycled back into the process. To summerise, the slurry process has a short residence time resulting in short transitions. The reaction medium is homogeneous (no hot spots). On the other hand, the solubility of the polymer into the diluents limits the product range. Also, the solubility may cause fauling of the reactor. The recovery of the unreacted material is sometimes complicated, especially in case of high boiling diluents. LIQUID PHASE SOLUTION POLYMERIZATION :In this kind of polymerization, the monomer is dissolved in a suitable inert solvent along with the chain transfer agent, whenever used. The free radical initiator is also dissolved in the solvent medium, while the ionic and co-ordination catalysts can either be dissolved or suspended. The presence of the inert solvent medium helps to control the viscosity increase & promote heat transfer. The major disadvantage of the solution polymerization technique is that however inert the selected solvent may be, chain transfer to the solvent, cannot be completely ruled out & hence it is difficult to get very high molecular wt. products. The polymer formed will also have to be isolated from the solution either by evaporation of solvent or by precipitation of non-solvent & removal

of their final

traces

is always

extremely

difficult. Solution

polymerization technique’s nevertheless, can advantageously be used where the polymer is used in it's solution form, as in case of certain adhesives and coating

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

compositions, or in system where the polymer formed is insoluble in it’s monomer or solvent and precipitates out as a slurry and is amenable for easy isolation. Practically, industrial production of polyacrylonitrile is manufactured by free radical polymerization and polyisobutylene by catalytic polymerization, use solution technique. GAS PHASE REACTOR: In gas phase reactor, catalyst and growing polymer particles are suspended in an upward gas flow. The gas velocity is high enough to fluidize the particles but not high enough to entrain the polymer from reactor. The particles then form a compact bed; so-called fluidized bed. The gas enters the reactor at the bottom through a metallic perforated plate, a gas distribution plate that evenly distributes the gas through the bed. The unreacted gas leaves the reactor at the top. The pressure of the exit gas is then raised in a recycle gas compressor and the gas is fed back into the bottom of the reactor. The catalyst (or polymer containing active catalyst) is fed directly into the bed from where the polymer is also collected. The gas flow carries the particles and provides an effective mixing of bed. As a rule, the bed may assumed to be perfectly mixed. The heat of polymerization is removed by cooling the recycle gas on a heat exchanger, after the compressor cooling a gas is not as effective as cooling a liquid. This leads to a lower capacity per volume than what can be obtained in a slurry reactors. This in turn means, a longer average residence time of about three hours. Adding a condensing component into the recycle- gas stream can increase the cooling capacity. This component is condensed, either in a recycle gas C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

cooler or in a separate cooling unit. When a liquid component is injected into the bed together with the gas, it evaporates. The evaporation cools the bed effectively. This type of operation is called condensing mode. Cooling may also cause other problems. If the catalyst is very active, it is possible that the heat of polymerization is not transferred sufficiently from the particle to the surrounding gas. Then the temperature of the particle increases and reaches the softening temperature of the polymer. Tills may lead to the agglomeration of particles and chunk formation. This kind of behavior is called hot spot formation. After the polymer is taken out of the reactor the gases are removed by reducing the pressure. The unreacted gases are fed back into the recycle gas stream. No further separation or purification steps are needed. The polymer is purged with nitrogen dried & palletized. As the polymer does not dissolve in the gas a very wide product range can be produced in a gas phase process. Densities down to 910 kg/m3 may be produced. Even lower densities are possible but then the reactor temperature must be selected so that it is lower than the softening temperature of the polymer. In theory, the MFR range produced in a gas phase reactor is unlimited. However, problem may arise because of the high hydrogen concentrations needed to produce very high MFR’s. Selecting a catalyst with high hydrogen sensitivity may compensate this. To summaries, gas phase processes are able to produce a very wide product range the process is simple & economical; the average residence time is long, which means long transitions.

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

POLYMERIZATION OF OLEFINS: [ UNIPOL PROCESS] Polyethylene, the worlds largest volume, plastic today, achieves position largely due to a remarkable catalyst in concert with a remarkable fluidized bed process. On coming up with this catalyst, which operates at relatively low pressure and temperature. Union carbide developed a unique and versatile fluidized bed process, called Unipol, for producing linear low density polyethylene, which is rapidly replacing conventional process from the world. In this process, (see fig.) reactant gas (ethylene with it's co monomers, butane & higher) is fed at a rate of three to six times the minimum fluidization velocity, into a bed of polyethylene particles kept at 75-1000C and at 20atm. Extremely small silica-supported catalyst particles are also fed into the bed continuously. Polymerization occurs on the surface of catalyst, causing the particles to grow into large granules of 100-250 µm. The height of reactor size is reported to be 2.6-4.7 times the bed diameter. One pass conversion of ethylene is rather low, about 2%, so large recycle flows are needed. Since the reaction is highly exothermic (3300 Kj/Kg of ethylene converted), it is important to avoid the hot-spot formation &

local accumulation of catalyst at the walls of reactor, from the

engineering point of view; this process is gas-solid reaction with growing solids. In the Unipol process, two types of catalyst are used: chromium titanium compounds on silica carrier & Ziegler. These catalysts are so active that more than 105 volumes of polymer can be produced by unit mass of active ingredient in the catalyst. Because of the great dilution of catalyst in the granules formed & their large, the raw product is ready for use without-palletizing it or removing the catalyst. In addition no solvent is used in the process, & one marks C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

the whole range of product from low to high-density polymers. All these factors contribute to make this a remarkably efficient attractive and economical process. Following the debut of Unipol process fluidized bed catalytic polymerization has been extensively investigated by many companies. For ex. Copolymerisation of ethylene with hexane-I and octane-I has been developed by Exxon and by union carbide. Mitsui petrochemical and Motedison have developed an ultrahigh performance MgCl2/ TiCl4 catalyst for the gas phase polymerization of propylene. Union carbide already developed their own fluidized bed polypropylene process.

EXPERIMENTAL STUDY OF CATALYTIC PYROLYSIS OF H.D.P.E. IN LABORATORY FLUIDIZED – BED The disposal of municipal and industrial waste is recognized to be a major environmental problem. Landfill is becoming much more expensive and of questionable desirability for many localities. The destruction of waste by incineration is predating, but this practice is expensive and often generates problems with unacceptable emissions. Now days we have a dual fluidized bed process for obtaining medium quality gases from municipal solid waste. Thermal cracking of waste using kilns or fluidized beds has been piloted on a significant scale. WHY CATALYTIC PYROLYSIS ? The thermal degradation of polymers to low molecular weight materials has a major drawback, in that a very broad product range is obtain. In addition to this, these process requires high temp., typically more than 5000 C and even up to 9000 C. C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

Catalytic pyrolysis provides a means to address these problems suitable catalysts can have the ability to control both the product yield and product distribution from polymer degradation as well as

to reduce scientifically the

reaction temp., potentially leading to a cheaper process with more valuable products. In contrast to thermal degradation research, catalytic pyrolysis has been carried out by considering a variety of catalysis with little emphasis on the reactor design, with only simple adiabatic batch and fixed bed reactors being used. Although catalysis has been used this is often done by thermally degrading the polymer and passing the degradation products through the catalyst. WHY FLUIDIZED BED ? The use of fixed beds where polymer and catalyst are contacted directly leads to problems of blockage and difficulty in obtaining intimate contact over a significant portion of the reactor volume without good contact the formation of large amounts of residue is likely and scale up to industrial scale is not feasible. Much less is known about the performance of catalyst in polymer degradation using a fluidized bed reactor. The objective of catalytic pyrolysis of High Density Polyethylene over a zeolite catalyst was to explore the capabilities of a laboratory catalytic fluidized bed reaction system using a zeolite catalyst. 1. For the study of product distributions 2. For identification of suitable reaction conditions for achieving waste polymer recycling.

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

EXPERIMENTS A process flow diagram of the experimental system is shown in fig. The reactor consists of Pyrex glass tube with sintered distributor in the middle section. The tube had an inverted bell shape and was divided into three parts 1. Upper Section 2. Middle Section 3. Lower Section A three zone heating furnace with digital controller's was used, and the temp. of the furnace in its upper, middle and bottom zones were measured using three thermocouples. By these means the temp. of the preheated nitrogen below the distributor and catalyst particles in the reaction volume could be effectively controlled to within ± C. An additional thermocouple on a movable joint connected to the reactor was used to measure the temp. at any position in the reactor. Another thermocouple was located close to the middle of the furnace and was coupled directly to a high temp. cutout. FLUIDIZING GAS FLOW High purity nitrogen was used as the fluidizing gas, the value and preheated in the bottom section of the reactor tube. Flow meters were used to measure the full range of gas velocities from incipient to fast fluidization. Before catalytic pyrolysis experiments were started, several fluidization runs were performed at ambient temp. and pressure. C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

1. To select suitable particles size (both catalyst and polymer ) and 2. To optimize the fluidizing gas flow rates to be used in the reaction. The particle sizes of both catalyst and polymer were chosen as to be large enough to avoid entrainment but not too large to be inadequately fluidized. Entrainment of polymer is only an issue, as the polymer enters the reactor, as once it is in the bed it is effectively trapped. High flow rates of the fluidizing stream improve catalyst – polymer mixing and external heat transfer between the hot bed and the cold bed catalyst. On the otherhand, an excessive flow rate could cause imperfect fluidization and considerable entrainment of fines. After selecting suitable particle

parameters, the minimum

fluidization velocity of catalyst Umf at different operating conditions was calculated using the Ergun equation. The velocity for Umf working at 3600C was found experimentally to be 1.03 cm/s, which is in good matching with that predicted by the Ergun equation. (1.05 cm/s) Fluidizing gas velocity. In the range 1.5-4 times the value of Umf were used in the course of this work.

Of course, during the

experiments, the actual particle density would vary according to the quantity of polymer present inside the catalyst particles. POLYMER ADDITION :The polymer feed system was designed to avoid plugging the inlet tube with melted polymer and to eliminate air in the feeder. The feed system was connected to a nitrogen supply to evacuate polymer into the fluidized bed catalyst. Thus the HDPE particles were purged under nitrogen into the top of the reactor and allowed to drop freely into the fluidized bed at time t = 0 min. C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

REACTOR PERFORMANCE On addition of the polymer the fluidized bed remains fluidized. The added polymer melts, wets the catalyst surface and is pulled into the catalyst macropores by capillary action. At sufficiently low polymer/catalyst ratio the outside of the catalyst particles are not wet with polymer, so the catalyst particles move freely. The volatile products leaving the reactor were passed through a glass fiber to capture the catalyst fines followed by ice-water condenser to collect condensable liquid product. A three-way value was used to route product either to sample gasbag or to automated sample value system. Sample bag is used to collect gaseous samples at intervals of 10 min. And this multiport sampling value allowed frequently rapid sampling of the product stream were required. Spot samples were collected and analysed at various time (0,1,2,3,.....15 min), The overall Hydrocarbon gas yield was calculated from both gas , avg. sample and spot sample. Rate of H.C. Sample can be calculated – Rgp = H.C. Prod x rate x 100 / (total H.C. product over whole run (g) Analysis of gaseous production is done by gas chromatograph. COMPARISON 1)

The yield from the pyrolysis of C1 to C4 gases.  Fixed Bed: 50 wt.%.  Fluidized Bed: 67.5 wt %.

2)

Fixed Bed: Higher aromatic yield 35 wt %

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

Fluidized Bed: Aromatic yield 3.0 wt %. 3)

Fluidized Bed: Higher olefins yield.

4)

Fixed Bed: Higher residence time.

5)

Fixed Bed: At same temp. Low pyrolysis yield.

CONCLUSION: • CLOGGING IS AVOIDED • CONSTANT TEMPERATURE THROUGH THE REACTOR • IMPROVED YIELD OF THE VOLATILE PRODUCTS • SELECTIVITY IN PRODUCT DISTRIBUTION • EXCELLENT HEAT AND MASS TRANSFER • REACTION IS ACHIEVED AT LOWER TEMP AND WITHIN SHORT TIME OF CONTACT.

STUDY OF MONOLITHIC REACTOR CONVERSION OF METHANOL TO GASOLINE RANGE HYDROCARBON The conversion of methanol to gasoline (MTG) over zeolite ZSM-5 catalysts has received considerable attention as an alternate route for the production of transportation fuels (Chang, 1983). The process is highly selective to hydrocarbons, and the general reaction pathway is represented as follows.

This reaction has been investigated in fixed and fluidized-bed

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

reactors. Both processes require that the finely synthesized zeolite crystals (1-10µm) are palletized for mechanical strength. Composite catalyst pellets are formed by molding or extruding mixtures of zeolites crystals and amorphous binders. Larger pellet dimensions are employed in fixed beds to maintain an acceptable pressure drop across the reactor. The exothermic MTG reaction may cause the temperature to rise in fixed beds and deactivate the catalyst. This problem is best overcome in fluidized beds, which are operated isothermally. Here, smaller pellet sizes are necessary for easy fluidization. However, fluidized beds report difficulties associated with catalyst attrition, and the entrainment of resultant fines. Novel reactor configuration featuring catalysts supported on monoliths or honeycomb structure may overcome the operational problems associated with other reactor types. A monolithic reactor can be imagined as a bundle of small, parallel tubes with catalytic material deposited in the walls. The main advantages of catalyst monolithic reactors are a very low-pressure drop across the reactor and a high exposed surface area. Moreover, these reactors can be operated in the horizontal mode without plugging or channeling. Further., the simple flow pattern and reactor structure make for more reliable scale-up. However, for most processes of interest, flow in channels of monolithic reactors is laminar. Monolithic reactors are employed extensively in controlling automobile and industrial NOx emissions. Other applications include methanization of carbon oxides ,cracking of naphtha , control of volatile organics on immobilized biofilm. It should be noted that most monolithic reactor applications listed above involve surface catalysis . Under laminar flow, mass transfer to the C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

catalyst-coated wall limits conversion. By contrast, in Zeolites most catalytic sites are thought to be located at the intersections of the molecule-sized pores inside the crystal. Here diffusion of reactants and products in and out of the intracrystalline pores is essential to the process configurational or intracrystalline diffusion orders of magnitude smaller than bulk or knudsen diffusion and exists in all zeolite processes regardless of the reactor type hence conversion in convention palletized reactor systems may be constrained by configuration and in the event of very fine intercrystaline spaces Knudsen diffusional resistances by comparision; a thin layer of pure zeolite on monoliths offers shorter diffusion distances. Traditional methods of coating zeolites on monolith require the use of binders to attach the catalyst crystal on the surface . Higher zeolite loads requires use of larger amounts of binder which induce additional intercrystalline mass transfer reaction traces. Recently, a method of forming zeioites in situ on the monolith surfaces been reported . This ensures that the monolith surface is covered only with the active catalyst. The potential of these reactors, however, not been adequately studied. In this paper, the application of zeolite ZMS - 5 coated monolithic reactors for the production of gasoline hydrocarbons from methanol is investigated. The facts of temperature, methanol flow rate, and the dilution conversion are studied. Experimental results obtained here are compared with literature of reported for fixed and fluid bed reactors. Mathematical modeling provides insight on the nature of the milted mass transfer resistance in these reactors. Pyrolysis of HDPE over HZSM-5 Influence of reaction temperature. The main product distributions for HDPE degradation over HZSM 5 at temperature in the range of 290-4300 C are illustrated in table. Overall, the liquid and residue fractions C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

decreased with increasing reaction temperature. Temperature and as expected faster rates were observed at higher temperatures. At 4300C, the maximum rate of hydrocarbon production was 54 wt % min-1 after only 0.5 min, with all the polymer degraded after approximately 3 min. As the temperature decreased, the initial rate of hydrocarbon production dropped and the time for the polymer to the completely degraded lengthened. At 2900C, the rate of hydrocarbon production was significantly lower throughout the whole reaction with the polymer being degraded over 15 min. Pyrolysis of HDPE over HZSM-5 . Influence of HDPE to Catalyst Ratio In the present study the amount of catalyst used in the degradation of HDPE remained constant and, therefore, as more polymer was added to the reactor, fewer catalytic sited per unit weight of catalyst were available for cracking. The overall effect of increasing the polymer to catalyst ratio from 1:10 to 1:1 on the rate of hydrocarbon generation was small but predictable. The maximum rate observed dropped slightly, and the time taken to generate the maximum rate extended from 1 to 3 min. The total product yield after 15 min showed only a slight downward trend even after a 10 fold increase in added polymer. This can be attributed to the high activity of HZSM-5 and excellent contact between HDPE and catalyst particles. Consequently as more polymers was added lower C1 - C4 hydrocarbon gases and coke yields but higher liquid yields. In addition more BTX (coke precursor) was produced, but increasing the polymer to catalyst ratio had virtually no effect on gasoline production.

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

As the polymer to catalyst ratio increases, the possibility of HDPE adhesion to the reactor was increases as the amount of unreacted polymer in the reactor rises. However, for the work carried out in this paper no such problems were observed. Pyrolysis of HDPE over HZSM-5. Influence of Fluidizing Gas Rate: The results shown in fig. illustrate that for efficient polymer degradation good mixing is required with a drastic dropoff in the rate of degradation observed only at the lowest fluidizing flow used. Good mixing will both favour rapid distribution of the polymer feed over the catalyst and reduce any mass transfer resistance to the escape of products from the catalyst surface. Thus good mixing will increase the rate of polymer degradation. Furthermore, changing the fluidizing flow rate influences the product distribution. Glass works: The uncoated monoliths have 62 chanllesl/cm2 a length of 7.7cm and a diameter of 2.5cm. The zeolite layer was formed on the cordierite surface. The ZSM-5 coating was converted to the hydrogen form via ammonium ion exchange followed by calcinations at 6000 C. Two HZSM-5 coated monolith samples were fabricated for this work. Feed:- Methanol obtained from Fisher Scientific (HPLC) grade was used as feedstock without further processing. In one set of experiments, pure methanol was employed. In the other, methanol was diluted with nitrogen. Apparatus and Procedure: A schematic diagram of the laboratory-scale setup, in which experiments were conducted, is shown in the fig. Before introduction of the methanol, the reactor was purged with nitrogen for at least 1 hr. After the nitrogen flow was shut off, methanol feed was introduced to the preheater by a syringe pump C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

at two rates: 0.075 and 0.15mL/min. All experiments reported here were conducted at 101.3kPa (1 atm) and 300-425°C. Temperature was measured by thermocouples located at the inlet and exit of the reactor. To ensure steady state, product samples were with-drawn 1 h after the methanol feed was introduced. Reactant and product analyses were conducted in a Hewlett Packard gas chromatograph (5890)-mass spectroscope (5970) system using ultrapure helium as carrier gas. A 150 m long capillary column was employed to separate the product components. The column temperature was initially maintained at 35°C for 125 min, then heated to 200°C at 2 °C/min, and maintained at 200°C for 10 min. Catalyst deactivation, by coke formation, was observed after 4 h of continuous operation. Most of the catalytic activity was restored by burning the coke in a stream of air (16-20 L/min) at 4300C for 10-12 hr. Hence, data reported in this work should be considered as fresh catalyst activity. Reactor Model: One-dimensional mathematic models have been successfully used to explain behavior of monolithic reactors for automobile catalyst converters, methanization of carbon, and selective catalytic reduction nitrogen oxides. A mathematical model is formulated below for conversion of methanol across a single monolith channel. The model accounts for diffusion and reaction the zeolite coating, as also possible axial dispersion the reactor. At any fixed axial position along the reactor, the mass balance for methanol (denoted by A) over a zeolite coated monolith substrate of thickness dx is represented as follows:

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

ASSUMPTIONS: The main assumptions made here are listed as follows: 1. Feed and product vapors behave like ideal, incompressible gases. 2. Volume flow of gas is equal in all monolith channels. 3. The pressure drop across the reactor, calculated by the Hagen-Poiseuille equation, is very low (
is isothermal. This is experimentally verified by measuring the

temperature at the reactor inlet and exit. The maximum temperature rise observed was 20 C. 7. The Zeolite is uniformly distributed on the monolith channel surface. 8. Reactions occur only in the zeolite layer, and the reaction is of first order. 9. At the gas velocities encountered in this work, the Bodenstein number = du/Db = 0.16-0.52. Hence axial dispersion is affected by axial molecular diffusion only.

RESULT AND DISCUSSION Experiments were first conducted on an uncoated monolith in the temperature range 360-4000 C. Low amounts of methanol were converted on uncoated cordierite (14%) and most of the product was dimethyl ether (70% by weight). This small conversion is to be expected as methanol dehydration is known C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

to be catalysed over amorphous silica

alumina, both of which are present in the

cordierite phase. For the zeolite-monolith system, the MTG reaction activity was measured by oxygenate conversion. There are two adjustable parameters in the mathematical model the preexponential kinetic factor, and the diffusivity, D (at 296K). The best fit of the experimental data for both monoliths was obtained with a preexponential factor of 8.8 x 10 cm/g and a diffusivity , at 296K, of 3.4 x 10 cm /sec. Table 2 lists single-component configurational diffusivities in ZSM-5 for some compounds involved in the MTG process. During the reaction, a number of product hydrocarbons diffuse out of the intracrystalline pores, thereby restricting the flow of methanol reactant to the active sites, experiments on counterdiffusion of benzene - toluene in ZSM-5 particles indicate that the configurational diffusivities of both components were lowered by an order of magnitude compared with single component measurements. Hence, the diffusivity (D) estimated here is of the lumped type which

reflects

the

overall

multicomponent

configurational

diffusion/counterdiffusion processes. This conclusion is also

supported by the

magnitude of the diffusivity estimate, which is within the range of reported values for MTG reaction product . The reaction rate calculated for the conversion of oxygenates at 585K is 2.0x10mol/g which compares favourably with the rate reported (2.4 x 10 ) mol/g at the same temperature.

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

Under similar operating conditions, zeolite, crystal dimension and characteristic catalyst length, effectiveness factor for palletized and monolithic systems are evaluated in . It is noted that diffusion effect are less severe for thin zeolite layers on monolithic substrates than for pellets. EFFECTS OF METHANOL PARTIAL PRESSURE. The influence of reactant partial pressure on conversion a different temperatures is presented in fig. The reactor model, which assumes firstorder kinetics, is in agreement with experimental data. The small slope the curves is probably due to different reactor residence times effected by diluting a fixed quantity of methanol with varying amounts of nitrogen. EFFECT OF TEMPERATURE Change in oxygenate conversion with temperature at two reactant feed rates shown in fig. The conversion rises rapidly start increasing temperatures up to about 380 C. Beyond this point, the conversion is relatively insensitive temperature and tends to level out. As operated under similar conditions, are also included in this table. Oxygenate conversion is uniformly high in all three reactor types (>99%). Further it is seen that the hydrocarbon product distribution in monolithic reactors is between that observed for fixed and fluid beds. This may be explained in part by the interaction of differing mass transfer resistances in the three reactor types. In fixed and fluid beds, the gas flow outside the pellets is normally turbulent and , hence , bulk diffusion resistance

is rarely present.

However , much of the zeolite is inside the pellet and access to the crystals is through pellet pores whose dimensions may be the same as or smaller than the mean C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

free path of gaseous reactant product molecules. Hence, the combined effect of inter and intracrystalline diffusional resistances in catalyst pellets affects the distribution of hydrocarbon products. Larger catalyst pellets employed in fixed beds result in longer diffusion paths. This might explain the lower amount of intermediate olefins and higher quantity of aromatics in fixed beds reactors compare to fluid beds. For

zeolite

monolithic

scheme

only

configurational

diffusional resistances exits. Like fluid beds, intercrystalline resistances, here are lower than fixed beds. Under similar process conditions, the product the reaction rate was increased, the limiting conversion was served to be lower. At all feed rates report, if kinetics alone were controlling, the limit conversion would be uniformly high (>99%). An indication that a mass transfer regime is encountered at high temperature (>400 C) is favoured by comparing experimental with predicted conversion values 4. Similar reactor behaviour has been reported investigators for other systems . This view is further supported by evaluating change in the Thiele modulus per characteristic catalyst length with temperature. At lower temperatures, chemical kinetics predominate, while at higher temperature, configurational diffusion is dominant. The optimal process temperature selected at the onset of mass transfer control, is about 370 C. The effect of temperature on product distribution is plotted in fig. As the temperature is raised increases in light olefins and methane are observed as a result of secondary cracking reactions. Higher temperatures are also reported to uncouple olefin formation reactions from aromatization reactions.

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

HYDROCARBON SELECTIVITY. The overall conversion and hydrocarbon product distribution obtained in the ZSM-5 coated monolithic reactors at 380 C and atmospheric pressure are presented. Results reported for fixed and fluid bed reactors selectivities in monolithic and fluid beds are comparable monolith 1 and fluid beds. However unlike fluid beds past the catalyst surface is laminar in monoliths. Thus product are transferred to the bulk gas by diffusion -slower process than convective transport. This may account for the lower olefin intermediates observed in monolith . 1. In monolith 2. Operated at a lower space velocity higher quantities of paraffins and aromatics (corresponding to lesser amounts of olefins) were formed. This is probably due to longer residence times in the reactor, whereby the olefins formed undergo further cracking and hydrogen transfer. AGING STUDIES Oxygenate content in (he project provided a convenient method of measuring catalytic activity. Aging experiments were conducted at 380C with a methanol feed rate of 0.075 mL/min. Changes in oxygenate conversion with time are plotted in Fig

. The Catalyst activity is observed to be unchanged for the

first 4 h of continuous operation.

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

CONCLUSION Important findings of this works are listed as follows 1. Zeolite coatings of high purity and crystallinity can be consistently formed on ceramic monolithic substrates. Catalyst loading up to 31% by weight of uncoated monoliths are obtained. 2. Methanol is converted to gasoline - range hydrocarbon with yields similar to those obtained in fixed and fluid bed reactors. The product distribution , though is somewhat different from standard reactor types. 3. The unique, one piece construction of monolithic reactors offer advantages of both fixed and fluid bed reactors. No problems of catalyst attrition or excessive pressure drops are observed. 4. At the crystal sizes synthesized configurational diffusion plays an important role in determining conversion levels. There have been suggestions that, at submicron particle sizes, the number of catalytic sites on the surface of the zeolite crystal becomes significant. However, at these reduced sizes surface reactions may dominate and the unique "shape-selective" aspect of zeolites may be considerably diminished.

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

CONCLUSION In this present work the fluidized bed catalytic polymerization was studied. It was compared with the various kinds of the other processes of polymerization. The

various

divisions

of the

catalytic polymerization

environment were discussed. Catalytic pyrolysis of HDPE over zeolite catalyst was studied under the fluidized bed environment. It was compared with the fixed bed batch process. It was shown that how fluidised bed process is more effective than both of fixed bed and Batch process. The exothermic reactions may cause the temp. to rise in fixed bed and deactivate the catalyst .This problem is overcomed in the fluidized beds, which arc operated isothermally. Here smaller pellet size are necessary for the easy fluidization, However, fluidized beds reports difficulties associated with the catalyst attrition and entrainment of the resultant fines. This problem can be overcomed with reactors featuring catalyst supported on monolith or honeycomb structure. This reactor offers advantages of both fixed and fluid bed reactors.

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

BIBLIOGRAPHY 1) Diazo Kunni; Octave levenspiel. Fluidizing Engineering 2) P.N. Sharratt and Y. H. Lin A.A. Garforth and J. Dwyer Investigation of catalytic pyrolysis of HDPE over a zeolite catalyst in Lab. fluidized bed reactor. ( Ind. Engg. Chem. Res. 1997,36, 5118-5124). 3) Jimmy E. Anita, Rakesh Govind. Conversion of Methanol to Gasoline range hydrocarbons in a ZSM-5 coated Monolith Reactor. (Ind. Engg. Chem. Res. 1995,34, 140-147 ) 4) J.H. Wilhemn. R.H. boundary conditions of the flow reactor (Ind. Engg. Sci. 1966 ). 5) John H. Perry. Chemical Engineer’s Handbook 6) Fried W. Billmeyer Text book of polymer science 7) Joel R. Fried. Polymer Science 8) Warren L Maccabe, Jullian Smith, Petter Harriot. Unit operation of Chemical Engineering. 9) www.google.com 10)www.nap.edu

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

Influence of Operating Conditions on Product Selectivity for the Catalyzed Pyrolysis of HDPE using HZSM-5: Experimental and Predicted Equilibrium Results

Reaction Conditions Reaction

Temperature

(0C)

a

Polymer to catalyst ratiob

Fluidizing N2 ratec (mL

(%wt)

min-1)

Ratio

290

360

430

10

60

100

270

500

720

i-butane /n-butane

5.4

4.8

2.5

4.8

4.8

4.9

4.2

4.8

5.0

i-butane /n-butaned

1.17

0.95

0.81

i-butene /∑-butenes

0.52

0.49

0.43

0.49

0.48

0.44

0.48

0.49

0.49

i-butene /∑butened

0.56

0.52

0.48

∑olefins

5.4

4.5

3.6

5.5

5.2

3.8

3.3

5.1

5.6

/∑

paraffinse

Represents a series of runs where polymer to catalyst ratio=40 wt% and fluidizing N2 rate=570 mL min-1, bReaction temperature=360 0C, fluidizing N2 rate=570 mL min-1, b Reaction temperature=360 0C, polymer to catalyst ratio=40wt%. dPredicted equilibrium data. d∑olefins denotes the sum of all olefinic products. ∑paraffins denotes the sum of all paraffinic products.

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization Comparison of Methanol Conversion to Hydrocarbons in Different Reactors (P=1 atm ) Reactor type Fixed bed

Monolith 1

Monolith 2

Fluid bed

(chang et al., 1978)

(this work)

(this work)

(Liederman et al., 1978)

0

Temperature ( C) WHSV (h-1) Si/AI (mol/mol) Conversion (%) Hydrocarbon distrib (wt%) Methane+ethane Propane n- butane Isobutane C2- C4 olefins C5+ nonaromatics Aromatics Total % durene in HCs a Based on weight of zeolite

C.O.E.& T., Akola

371 0.8 ~15.0 99+

380 0.95 12.2 98.93

380 0.64 17.7 99.08

371 1.0 ~15.0 99.7

4.1 13.7 4.1 16.8 2.6 14.3 44.4 100.0 0.8

0.89 4.91 1.69 6.55 15.29 31.64 39.03 100.00 4.10

1.40 7.49 2.67 8.49 12.27 13.94 53.74 100.00 3.61

1.03 2.98 1.28 12.98 17.90 37.17 26.66 100.00 3.03

Fluidized Bed Catalytic Polymerization

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

Figure : Sketch of Unipol process for making polyethylene Two Catalyst 1. Chromium – tantanium compound 2. Ziegler No solvent is used

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

 Temp increases & liquid & residue fraction decreases 430 – Miximum  Quantity of Polymer increases catalyst constant.  Optimum - 1. High activity 2. Excellent contact

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

 Temperature increases, rate increases  Quantity of polymer increases , rate decreases  Optimum – 1. High activity 2. Excellent contact

C.O.E.& T., Akola

Fluidized Bed Catalytic Polymerization

 Efficient polymer degradation – Good mixing Mixing  Rapid distribution of polymer feed over catalyst surface.  Reduces M.T.R. to escape product from catalyst surface Good mixing rate increases or decreases High flow rate increases or decreases

Effect of Fluidizing Gas flow rate

C.O.E.& T., Akola