Abstract:
A polymerization reactor for exothermic liquid phase reactions comprises a reaction zone which is divided into a plurality of channels by thermally conductive heat transfer fins which are conductively mounted on one or more heat pipes for the removal of heat of reaction from reactants and reaction products flowing between the heat transfer fins. The reactor of the invention is capable of maintaining essentially isothermal conditions without the use of complicated and maintenance intensive agitators. The reactor is particularly useful when viscosity of the reactants and/or reaction products is high, when the reaction conducted has a fast reaction rate and when consistent polymer properties are desired.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improved chemical reaction apparatus which is capable of removing large heat fluxes from a viscous reaction mixture while maintaining the reaction mixture at essentially isothermal conditions. The invention also relates to a method of conducting chemical reactions, and in particular, polymerization reactions, at essentially isothermal conditions using the novel reactor of the invention. 
     2. Description of Related Art 
     A variety of commercially important chemical reactions, and in particular polymerization reactions, require that reactants be maintained within a narrow temperature range to achieve desired product properties. 
     In the case of certain polymerization reactions, due to the low conductivity and high viscosity of the reaction mixture, heat transfer is a limiting factor in reactor design. Further, it is frequently not possible to compensate for the low conductivity of a polymer reaction mixture by using lower coolant temperatures because low coolant temperatures cause polymer solidification in the reactor. In many polymer reactors, the poor heat transfer characteristics of polymer reaction mixture results in poor reaction temperature control resulting in the formation of undesirable products in the reactor. For example, temperature variations in polymer reactors can lead to the formation of polymer products having lower molecular weight than desired. This negatively affects the flow and mechanical properties of the desired polymer end product. Since it is frequently the case that undesirable polymer reaction products are not easily separated from desired polymer products, many attempts have been made to produce polymer reactors which are capable of controlling, within a narrow temperature range, the reaction temperature of highly viscous reactants with poor thermal conductivity. 
     A wide variety of designs have been developed for continuous flow polymerization reactors which can handle viscous process liquids with poor thermal conductivity. 
     U.S. Pat. No. 2,727,884 to McDonald describes a polymer reactor which uses forced convection heat transfer. In this reactor, banks of cooling tubes in which a heat transfer fluid is circulated are gently agitated in the polymer reaction mixture. The agitation improves heat transfer, and at the same time preventing channeling of less viscous material in the reactor. Examples of another type of mechanically agitated, convection-type reactor, known as a wiped-film reactor, are disclosed in U.S. Pat. No. 3,513,145 to Crawford, U.S. Pat. No. 3,679,651 to Kii and U.S. Pat. No. 4,011,284 to Gawne. The construction of the internal coils required by such reactors is extremely labor intensive and therefore very costly. Further, the internal heat transfer coils employed in such reactors have a history of failure causing polymer to be contaminated with heat transfer oil. Despite the agitation in these reactors, the heat transfer characteristic of wiped-film reactors is nevertheless poor and hot spots frequently develop in such reactors. 
     Another example of a polymer reactor designed to provide high heat removal capability is disclosed in U.S. Pat. No. 3,838,139 to Latinen. This patent describes a horizontal cylindrical reactor vessel equipped with an agitator consisting of a plurality of discs with small clearance with respect to the cylindrical vessel. The discs divide the reactor vessel into compartments. The heat of reaction is removed from the reactor by the direct evaporation of a volatile monomer from the reaction mixture. Although this form of heat transfer is generally accepted as more efficient than convection, hot spots with temperature difference as high as 5–10° C. are still experienced. Further, the temperature in the various compartments of the reactor are not necessarily the same due to different polymer concentrations and reaction rates in the compartments. 
     Using the vaporization of volatile monomer reactants to remove the heat reaction from a polymer reaction mixture is generally not workable when the polymer reaction involves the co-polymerization of more than one monomer. In such cases, the vaporization of the different monomers is generally not equal causing uncontrolled concentration of the different co-monomers. Consequentially, the use of direct evaporation is to be avoided in such cases. 
     U.S. Pat. No. 4,419,488 to Fukumoto discloses another direct evaporation-type polymerization reactor. The inclusion of a mechanical agitator in a polymer reactor is often an unwanted necessity to improve heat transfer and homogeneity. These devices are costly, require much maintenance and can cause quality problems because the agitator shaft seal is often a source of air ingress to a reactor, which can generate undesirable oxidation by-products such as aldehydes and ketones. These compounds can retard a polymerization reaction and can cause product discoloration. 
     Several designs have eliminated the use of mechanical agitators in polymer reactors. U.S. Pat. No. 4,421,162 to Tollar and European Patent No. 0150225 A1 describe the use of flat annular plates disposed coaxially within a reactor shell. This concept can be applied to a polymerization reactor with a viscous reaction mixture by causing the heat of reaction to first be absorbed by conduction through the flat annular plates and then by conductive tubes which are in contact with the annular plates and then by convection into an appropriate heat transfer liquid flowing through the conductive tubes. The temperature in such reactor is usually well controlled, however, the volume occupied by the annular plates and tubes in the reactor reduce available reactor volume significantly. Variations of this heat transfer mechanism has been proposed by other inventors such as Oldershaw in U.S. Pat. No. 3,014,702, Brassie in U.S. Pat. No. 3,280,899, Aneja in U.S. Pat. No. 4,808,262 and Mattiussi in U.S. Pat. No. 5,084,134. 
     Anionic polymerization reactions have also been conducted in continuous stirred tank reactors. However, such reactions must generally proceed at low temperature due to the extreme reactivity of the reactants. Because it is desirable to operate continuous flow reactors hydraulically full to enable simple process control, heat removal in these reactors cannot depend on evaporation. Consequently, such reactors most often rely upon cooling jackets for heat transfer. However, the effectiveness of cooling jackets on anionic polymerization reactors is constrained by the low heat transfer coefficients applying of the convection mechanism and by the limited range of coolant temperature imposed by polymer solidification temperatures. 
     It would be desirable if there were available a continuous reactor for viscous polymer reaction mixtures with improved heat removal capability and with the capability of maintaining an essentially isothermal temperature profile throughout the reactor regardless of any varying heat loads associated with different reaction rates or reaction products. It would also be desirable if such a reactor were to be easy to construct, operate and maintain. It would further be desirable if the reactor were to have a relatively large void fraction for the conduct of polymerization reactions in as small a vessel as possible. 
     These benefits and other advantages are achieved with the present invention. 
     SUMMARY OF THE INVENTION 
     A stratified flow reactor of the invention consists of a shell, a heat transfer fluid channel, at least one heat pipe and a plurality of fins. The reaction zone is the shell side of the reactor and the heat pipe or heat pipes with fins mounted thereon extend through the reaction zone. The heat pipe or pipes are in fluid communication with the heat transfer fluid channel. The reactor vessel is closed at one end with at the other end being a cooling chamber through which the heat pipe or multiple heat pipes protrude. The heat pipe or pipes act as super heat conductors from the fins to the heat transfer fluid channel. In a preferred embodiment, the stratified flow react if the invention is a continuous flow polymerization reactor. 
     As described in U.S. Pat. No. 2,350,348 to Gaugler, heat pipes utilize evaporation of a heat transfer fluid from a porous medium affixed to a heat transfer surface to absorb heat. In the present invention, the heat pipe removes the heat of reaction from the reaction mixture by evaporative cooling from the heat transfer surface of the heat pipe system. The porous medium on the heat transfer surface is commonly referred to as a “wick”. The evaporation of the heat transfer fluid from the porous medium or wick enjoys extremely good heat transfer coefficients and enables extremely high heat flux at essentially isothermal conditions. If desired, the evaporated heat transfer fluid is condensed and returned to the heat transfer zone of the reactor. Since heat transfer coefficients associated with condensation are also high, both the heat absorption and heat release segments of the heat pipe equipped reactor of the invention enjoy very high heat flux rates. 
     The benefits of utilizing a heat pipe heat transfer device in the reactor of the invention as described are derived from its converting what would otherwise be convection heat transfer or submerged heat transfer surface evaporative cooling to evaporative cooling of a thin film from a porous surface from which the evaporated heat transfer fluid can quickly and easily escape. Convection heat transfer is limited by many factors, including the velocity of the heat transfer fluid, the temperature differential between the reaction mixture and the cooling fluid, the viscosity of the heat transfer fluids, the surface area available for heat transfer, the materials of construction of the heat transfer device and the condition of the heat transfer surfaces, i.e., whether they are fouled. Conventional evaporative cooling from a submerged heat transfer surface enjoys higher heat transfer coefficients than convection cooling, but is limited by the liquid phase surrounding the submerged tubes. The heat pipe substitutes thin film evaporation for submerged heat transfer surface boiling with a corresponding improvement of the tube side heat transfer coefficient of up to 10 times. Further, the heat release segment of the reactor of the invention relies upon the condensation of a heat transfer fluid which can take place in a condenser which is remote from the reactor, so that the surface area available for cooling need not be limited to the area of the heat pipe. Accordingly, condenser(s) with sufficient surface area to handle the required heat flux can be located away from the reactor of the invention while still being in close proximity to it. 
     Because the evaporation of a pure heat transfer fluid occurs at a single temperature and the heat transfer coefficients for the heat pipe heat transfer system of the present invention are very good, a stratified flow reactor equipped with a heat pipe heat exchange device according to the present invention can be operated at essentially isothermal conditions. 
     As described by Faghri (“Heat Pipe Science and Technology”, Taylor and Francis, 1995) and by Peterson (“An Introduction to Heat Pipes”, John Wesley &amp; Sons, 1994), the choice of the material of construction, the choice of the heat transfer fluid and the design of the wick structure for the heat pipe apparatus of the invention are within the capability of those skilled in the art. The materials of construction in contact with the heat transfer fluid are commonly selected from copper and copper alloys, aluminum and its alloys and stainless steels. 
     Although the term heat “pipe” is used in the description of this invention, innumerable configurations are possible, some of which are far from the cylindrical shape of a conventional pipe. For example, possible shapes could be, but are not limited to, flat, rectangular, annular, polygonal or tubular. When tubular heat pipe design is used, tube size can vary from less than 1 mm to several cm in diameter. 
     The heat pipe heat transfer system of the present invention is comprised of two or three sections: (1) an evaporator section where heat is absorbed by vaporizing a liquid heat transfer fluid, (2) an adiabatic section where the vaporized heat transfer fluid flows without changing state, and optionally, (3) a condenser section where the vaporized heat transfer fluid is condensed using an external source of cooling. The heat transfer fluid condensate can be returned to the evaporator section of the reactor by gravity or by pumping. The evaporator section of the heat pipe heat transfer system of the invention is comprised of a heat transfer tube having a porous surface or wicked internal surface. The heat transfer fluid is supplied to the porous or wicked heat pipe surface where the wicking action of the porous surface or wick wets the heat pipe with a thin film of heat transfer fluid. Because wicking is a surface tension phenomenon which can be limited in long heat pipes by liquid head, it is sometimes preferred for a reactor according to the invention to be comprised of multi-reactor sections, each having heat pipe transfer zones. 
     The heat pipe of the invention may be 1) sealed or 2) of the thermosyphon type. 
     When a reaction mixture is viscous such as a polymer syrup, heat transfer through the reactor of the invention is superior to convection, direct evaporation or conduction. The invention utilizes all three of these methods of heat transfer with maximum effectiveness without causing detrimental side effects. Heat of reaction is evenly extracted from the reacting viscous polymer syrup as it flows, in a laminar fashion, past the plurality of fins in the reactor shell. The fins can be made of a multitude of geometric shapes and materials of construction. Using conductive metals such as copper and aluminum alloys enhances thermal performance when these the materials are compatible with the process fluid. Conductance is further increased by constructing flat heat pipes as fins or embedding mini heat pipes within conventional fins. Conductance over long distance is avoided by locating the fins a short distance from the heat pipe or heat pipes. The heat pipe or pipes of the reactor act as collecting header(s) where the heat of reaction is transferred very quickly through indirect evaporation. No evaporation of monomers is involved. The use of a suitable internal fluid with a high latent heat of evaporation, such as water, can even improve heat transfer over the direct evaporation of the monomers themselves. 
     The design of a heat pipe for use a reactor according the invention depends on such factors as chemical resistance and compatibility, temperature range, operating pressure and desired heat flux. 
     In the sealed heat pipe embodiment of the invention, at the condenser end of the reactor, the heat of reaction is transferred to a condenser heat transfer fluid using forced convection turbulent flow heat transfer. Preferably, the condenser heat transfer fluid has relatively low viscosity and enables high heat transfer coefficients. The reactor of the invention does not require the use of any rotating equipment. 
     If a heat pipe in the reactor fails, the reactor is minimally affected as small and temporary contamination with the heat transfer fluid does not require a plant outage and heat load is taken over by other heat pipes in the vicinity of the failed heat pipe. The improved reliability means extended service life and low maintenance cost. 
     In the thermosyphon heat pipe embodiment of the invention, gravity or a pump is used to return reactor heat transfer fluid condensate to the reactor heat pipe(s) through separate piping. In a variant of this embodiment, the heat pipe(s) can communicate with a low-pressure vapor (steam) header serving a network of vapor (steam) users. In such case, a source of clean heat transfer fluid, such as boiler feed water, is required. The advantages of using thermosyphon heat pipe(s) in the reactor of the invention over sealed heat pipes include: 1) the cogeneration of low-pressure steam from reactor waste heat and elimination of reactor coolers, and 2) enabling the remote location of condensers with more surface area and multiple forms of cooling. 
     In the reactor of the invention, the heat transfer fluid is chosen to assure trouble free heat pipe operation depending on the temperature of operation. It can be selected from liquids having the desired boiling point at a selected operating pressure. Common heat transfer fluids are water, acetone, alkanes, ammonia, fluorocarbons, aromatic solvents and even pure liquid metals. 
     The wick utilized in the invention can be comprised of fiber mats, sintered metal powders of spherical or non-spherical shape, of single size or multiple sizes, and metal screens in single or multiple layers, all with or without external surface enhancements, such as fins. In a preferred embodiment of the invention the porous surface or wick is applied to the interior surface of the heat pipe by preparing a paste of metal powder with a liquid binder, applying the paste to the inside surface of the heat pipe and then heating the heat pipe to evaporate the liquid binder and sinter the metal powder to the interior surface of the heat pipe. Alternatively, a dummy core is inserted into the heat pipe and metal powder is sifted into the space between the core and the interior of the heat pipe. The heat pipe is then heated to sinter the metal powder to the surface of the heat pipe and the core then removed. A metal powder which may be advantageously used in the invention is copper powder. 
     In a preferred practice, two or more reactors of the invention are operated in series so as to provide multiple temperature zones suitable for manufacturing polymers of the desired properties. Isothermal reactors are particularly required when the rate of reaction is high (such as in anionic polymerization) or when the rate of heat removal is low (due to high viscosity, poor heat conductivity or low velocity). 
     In the case of thermosyphons, heat pipe temperature is regulated by the pressure of the heat transfer fluid. In the case of sealed heat pipes, heat pipes operating temperature is regulated by the circulating condenser heat transfer medium temperature. Temperature over the entire reactor can be controlled well within 1 degree C. 
     The reactor of the invention can be operated hydraulically full, meaning without vapor space. In this way, for any given flow rate of reactants the residence time in the reactor is known. This flow scheme simplifies process control enormously. A pump is used to push the material through the reactor or a series of reactors. Flow measurement can be made simply and accurately at the beginning of the process when viscosity is low, and there is no need to control the level or net weight of the reaction vessel, as is required with partially full reactors. Simplified instrumentation and control translates into very predictable and consistent polymer properties and ease of operation. 
     Any polymerization reaction system in the liquid phase may be used in the reactor of the present invention. The polymer reaction mixtures or syrups can be solutions of polymer or co-polymers in their respective monomers (mass polymerization) and solutions of polymers in solvents and their monomers (solution polymerization) generally having viscosities ranging from 1,000 cp to 500,000 cp and preferably ranging from 10,000 cp to 200,000 cp. Examples of monomers or co-monomers usable with this invention are:
         Ethylene (PE)   Propylene (PP)   Styrene (PS, ABS, SAN, SIBS)   Butadiene (PBR)   Acrylonitrile (PAN)   Acrylamide (Polyacrylamidej Dimethyl Terephtalate (PET)   Terephtalic acid (PET)   Methyl Methacrylate (PMMA)   Caprolactam (PA)   Naphtalene Dicarboxylate (PEN)   Maleic anhydride (SMA)       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present invention will become apparent from the following description taken in connection with the annexed drawings, in which: 
         FIG. 1  is a longitudinal cross section of a reactor according with the present invention with a sealed heat pipe. 
         FIG. 2  is a longitudinal cross section of a reactor according to the invention with a thermosyphon heat pipe. 
         FIG. 3  is a sectional view a corrugated fin arrangement which can be used in reactors according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Chemical reactors with heat pipe heat transfer devices and methods of using such devices to perform chemical reactions are disclosed. In the following detailed description of the invention, for purposes of explanation, specific features, materials, dimensions and the like may be set forth to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. In some instances, well known devices are shown in simplified or block diagram form so as not to obscure the invention unnecessarily. 
     With reference to  FIG. 1 , a preferred embodiment of a chemical reactor  10  constructed according to the present invention is illustrated. For purposes of simplified illustration, reactor  10  is shown with a single heat pipe  20  in reactor shell  30 . Reaction zone  31  is in the interior of reactor shell  30 . A commercial reactor  10  could contain hundreds of heat pipes  20 . Heat pipe  20  is equipped with wick surface  21  in the area of reaction zone  31 . Reactants are fed into reactor  10  through input nozzle  11 . Product from reactor  10  flows through output nozzle  12 . Reactor shell  30  is sealed with inlet head  32  and outlet head  33 . 
     Heat pipe  20  is equipped with fins  22  which are mounted on heat pipe  20  so as to enable good thermal conductivity between fins  22  and heat pipe  20 . Fins  22  are spaced apart along the length of heat pipe  20  using spacers  23 . Heat pipe  20  extends through outlet head  33  into condenser  40 . Condenser section  44  of heat pipe  20  in condenser  40  can optionally be equipped with fins on its external surface. Heat pipe  20  is mounted on outlet head  33  in sealed fashion to prevent the flow of reactants from reaction zone  31  into condenser  40  or coolant from condenser  40  into reaction zone  31 . Condenser  40  is comprised of closed shell  41 . Condenser  40  is mounted on reactor  10  at outlet head  33 . 
     Liquid coolant is fed into condenser  40  through coolant feed nozzle  42  and exits condenser  40  through coolant outlet nozzle  43 . Heat pipe  20  contains a liquid heat transfer fluid (“HTF”) having a boiling point which is the same as the design operating temperature of reactor  10 . Heat pipe  20  contains an amount of HTF which is sufficient to fully wet wick surface  21  within heat pipe  20  and to fill the remaining space of heat pipe  20  with vaporized HTF. In a preferred embodiment of the invention, wick surface  21  extends into a pool liquid HTF below level L in heat pipe  20  so that capillary action can draw liquid HTF into wick surface  21 . The heat of reaction released by the reactants in reaction zone  31  is conducted through fin  22  and heat pipe  20  and causes the evaporation of the HTF on wick surface  21 , which draws more wetting of wick surface  21  by capillary pumping. The HTF evaporated from wick surface  21  flows through the center of heat pipe  20  into condenser section  44  of heat pipe  20 . The vaporized HTF transfers its heat of vaporization to the coolant in condenser  40  by conduction through the wall of heat pipe  20 . This causes the evaporated HTF in heat pipe section  24  to condense and then flow down heat pipe  20 , so that it is available for the rewetting of wick surface  21 . 
     In the vertical reactor orientation of the invention depicted in  FIG. 1 , the length of heat pipe  20  is limited by the maximum capillary height. If necessary, several heat pipe zones may be present to cover a long section of a vertical reactor  10 . Alternatively, reactor  10  can be arranged in a horizontal orientation so that the length of heat pipe  20  is not limited by the capillary height. 
     Reactor  10  is equipped with baffles  34  to cause the reaction mixture to flow between fins  22 . 
     In operation, upon entry of a reaction mixture into reaction zone  31  an exothermic chemical reaction commences or continues and releases its heat of reaction to fins  22 . The heat of reaction is conducted through fins  22  to heat pipe  20 , and then by conduction through heat pipe  20  to the HTF on wick surface  21  on the interior surface of heat pipe  20 . The heat of reaction causes the liquid HTF to vaporize and flow through the center of heat pipe  20  to condenser  40  where the vaporized HTF gives up its heat of vaporization to the condenser coolant and condenses to form liquid HTF. The liquid HTF flows down wick surface  21  from condenser  40  rewetting wick surface  21  from the top and also flows down the center of heat pipe  20  to form a liquid pool extending to elevation L at the bottom of heat pipe  20 . The liquid pool at the bottom of heat pipe  20  provides HTF to rewet wick surface  21  by capillary pumping. 
     In reaction zone  31 , as the reaction mixture flows between fins  22  following the path defined by baffles  34 , it continues to react to form the desire product at essentially isothermal conditions because of the intimate contact of the reaction mixture with fins  32  and the excellent isothermal heat transfer enabled by heat pipe  20 . 
     With reference to  FIG. 2 , a preferred embodiment of a chemical reactor  110  with a thermosyphon type heat pipe  120  constructed according to the present invention is illustrated. For purposes of simplified illustration, reactor  110  is shown with a single heat pipe  120  in reactor shell  130 . Reaction zone  131  is in the interior of reactor shell  130 . A commercial reactor  110  could contain hundreds of heat pipes  120 . Heat pipe  120  is equipped with wick surface  121  in the area of reaction zone  131 . Reactants are fed into reactor  110  through input nozzle  111 . Product from reactor  110  flows through output nozzle  112 . Reactor shell  130  is sealed with inlet head  132  and outlet head  133 . 
     Heat pipe  120  is equipped with fins  122  which are mounted on heat pipe  120  so as to enable good thermal conductivity between fins  122  and heat pipe  120 . Fins  122  are spaced apart along the length of heat pipe  120  using spacers  123 . Heat pipe  120  extends through inlet head  132  and outlet head  133  in sealed fashion to prevent the leaking of reactants from reaction zone  131 . Heat pipe outlet  124  communicates with condenser feed line  141 . Heat pipe inlet  125  communicates with condenser outlet line  142 . Condenser feed line  141  carries vaporized HTF to condenser  140  and condenser outlet line  142  carries condensed liquid HTF from condenser  140  to heat pipe inlet  125 . 
     Heat pipe  120  contains, but is not filled with liquid heat transfer fluid (“HTF”) having a boiling point which is the same as the design operating temperature of reactor  110 . The HTF wets wick surface  121 . The heat of reaction causes the evaporation of the HTF on wick surface  121 . The HTF evaporated from wick surface  121  flows through the center of heat pipe  120  to heat pipe outlet  124  and then to condenser feed line  141  and condenser  140 . The vaporized HTF transfers its heat of vaporization to a coolant in condenser  140  by conduction. This causes the evaporated HTF to condense and then to flow through condenser outlet line  142  from which position the liquid HTF wets wick surface  121 . 
     Reactor  10  is equipped with baffles  134  to cause the reaction mixture to flow between fins  122 . 
     In operation, upon entry of a reaction mixture into reaction zone  131  an exothermic chemical reaction commences or continues and releases its heat of reaction to fins  122 , the heat of reaction is conducted through fins  122  to heat pipe  120 , and then by conduction through heat pipe  120  to the HTF wetting wick surface  121  on the interior surface of heat pipe  120 . The heat of reaction causes the liquid HTF to vaporize and flow through the center of heat pipe  120  to condenser  140  where the vaporized HTF gives up its heat of vaporization to the condenser coolant and condenses to form liquid HTF. The liquid HTF flows by gravity or pumping to heat pipe inlet  125  and wets wick surface  121  by capillary pumping. 
     In reaction zone  131 , as the reaction mixture flows between fins  122  following the path defined by baffles  134 , it continues to react to form the desire product at essentially isothermal conditions because of the intimate contact of the reaction mixture with fins  132  and the excellent isothermal heat transfer enabled by heat pipe  120 . Product exits reactor  110  at outlet  112 . 
     Because reactor  110  is positioned with heat pipe  120  in a horizontal orientation, the wicked length of heat pipe  120  can be longer than the maximum vertical capillarity height of wick surface  122 . 
     Reactors  10  and  110  can be equipped with fins  22  or  122  having mini sealed heat pipe embedded therein or soldered to the surface of fins  22  or  122 . Mini heat pipes in or on fins  22  or  122  act as enhanced heat transfer devices within fins  22  or  122  conveying heat from sections of fins  22  or  122  which are remote from heat pipe  20  or  120  to the heat pipe. The length of the mini sealed heat pipe is controlled by the distance from heat pipe  20  or  120  to the outer most extent of fins  22  or  122 . The mini heat pipe preferably has an interior diameter in the range of 2 mm to 5 mm. The wick is applied to the interior surface of the mini heat pipes in the same fashion as aforedescribed for principal heat pipes  20  or  120 . Heat is absorbed at the evaporator end of the mini sealed heat pipes in sections of the fins which a remote from a heat pipe and is conveyed to condenser section of the mini sealed heat pipe proximate to the major heat pipe  20  or  120 . 
     In an alternate embodiment of the invention, fins  22  or  122  can be flat heat pipes comprised of two flat conductive sheets having a porous layer applied to one side. The conductive sheets are bonded together with the porous layer and a liquid heat transfer fluid inside. The heat transfer fluid should have a boiling point which is the design operating temperature of the reactor and should not completely fill the void space between the conductive sheets. The flat heat pipe can be made by applying a porous surface to a flat sheet and folding it so that the porous surface is inside. The periphery of the sheet is sealed by stamping or welding. Where necessary to prevent bulging caused by the pressure of the vaporized heat transfer fluid in the flat heat pipe, the fold sheet can be secured with welds or by stamping at one or more interior positions. The flat heat pipe can be folded into corrugated fins  200 , as shown in  FIG. 3 . Corrugated fins  200  can be equipped with perforation  201  through which process fluid may pass to promote mixing and to avoid channeling. Perforations  201  are preferably sealed by stamping or welding. 
     EXAMPLE 
     The following example illustrates the efficacy of the reactors of the invention to maintain essentially isothermal conditions in chemicals and especially polymerization reactions. 
     Polystyrene mass polymerization technologies are differentiated by the configuration of the main polymerization reactors used to bring conversion from 30% to 45% solids to 65% to 85% solids. During the course of the polymerization reaction large amounts of heat is evolved. If this heat of reaction is not removed, the reactor temperature will increase causing an unwanted and uncontrolled spread of the polymer molecular weight which adversely affects polymer properties. 
     A polystyrene mass polymerization is conducted in a reactor according to the invention consisting of a jacketed vertical pipe containing several straight heat pipes onto which are fitted a number of fins. The inside of the heat pipes is covered with a porous medium from which a heat transfer fluid is vaporized to provide cooling. This vaporized heat transfer fluid is continually replaced by the capillary action of the porous medium on a pool of heat transfer fluid below. The heat transfer fluid is chosen to provide low surface tension, high heat of evaporation and stability over the operating temperature range. The vaporized heat transfer fluid is condensed in an external heat exchanger. The condensate is returned to the bottom of the heat pipes by gravity. For startup the jacket is heated with hot oil. The tubes are drained and vented to atmosphere to prevent over pressurization. 
     The anionic polymerization of styrene using the organo-lithium catalyst, normal butyl lithium, is considered. The polymerization rate is very fast and the reaction goes to completion in approximately 2 hours of residence time. Toluene, or alternatively ethylbenzene, is used to keep the reaction mixture in a liquid state at low temperature. 
     In the example, a stream of 70% styrene in toluene flowing at 10,000 kg/hr is reacted. It is desired to maintain a temperature of 100° C. throughout the reactor, which consists of a shell 8 ft diameter by 16 ft long, giving a residence time of 2 hours. The fins are made of flat aluminum heat pipes corrugated in a triangular pattern. The fins are made from sheets of aluminum with a layer of very fine copper powder sintered on one side. The sheets are bonded together with the porous layer and liquid water on the inside. The double sheets are perforated with 1″ diameter and ⅜″ diameter holes with a fluid tight seal being maintained all around the double sheets and in the positions of the perforations. The 1″ holes are aligned and one hundred 1″ heat pipes are inserted and pressure expanded to provide intimate bonding with the fins and mechanical strength. The 1″ heat pipes are made of copper tubing in which a layer of fine copper powder has been sintered. The outside is clad with aluminum to make the pipes corrosion-compatible with styrene. The corrugated fins are stacked together in layers after being rotated by 90° with respect to each other as illustrated in  FIG. 3 . The process fluid flows within the triangular channels and through the ⅜″ holes to communicate with neighboring channels and thus are in intimate contact with the fins. This flow pattern creates a status mixing effect, which prevents channels from plugging. 
     The heat pipes are mounted between tube sheets so that the outlet end of the heat pipe communicates with the condenser inlet and the condenser outlet communicates with the inlet end of the heat pipes. The heat pipes and fins form a bundle that is inserted in the tubular reactor shell, such as that depicted in  FIG. 2 . Reactants flow through the reactor shell as depicted in  FIG. 2 . 
     Polymer is recovered by subjecting the reactor effluent to 240° C. and devolatilization of the solvent under vacuum. The solvent is recycled. The remaining viscous melt is then pelletized by strand bath or underwater technique as commonly practiced. 
     The above description of the invention is intended to be illustrative and not limiting. Various changes or modifications in the embodiments described may occur to those skilled in the art. These can be made without department from the spirit or scope of the invention.