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JOHN C. MORGAN, RALPH A. TROUPE, RICHARD D. ANDERSON, THOMAS CAVANAUGH, and ARUTUN MARANCI Northeastern University, Boston, Mass.
The Plate Heater
- a Continuous Flow-Type Reactor
This heater can be modified for use under extreme conditions which are imposed by slow reactions having low thermal demand CONTINUOUS FLOW REACTORS are widely used for industrial processes because large quantities of reactants may be handled in a relatively small reactor space and because of the advantages of continuous operation ( I ) . There are two basic types of flow reactors used by the chemical industry-the stirred vessel and the tubular. The stirred vessel or tank is similar to a batch reactor with reactants continuously added and products continuously withdrawn. I n fact a series of stirred vessels is sometimes used to produce the required reactor volume. In this case it is frequently a simple matter to convert from a batch operation to a continuous one and vice versa. Stirred vessels are generally restricted to liquid systems a t low or medium pressures, although sometimes used for liquid-solid and gas-liquid systems. The tubular reactor consists of a long, narrow tube or series of tubes through which the reaction mixture flows continuously as in a pipe line. Heat can be transferred through the tube wall to furnish or remove heat and to maintain the reaction temperature. One variation of the tubular reactor has the tube coiled to provide greater mixing and, in some cases, less floor space. Since the volume of a typical stirred vessel roughly increases as the cube of
Editor's Note I/EC has published a number of articles on the use of plate exchangers: in June 1960, a group of three articles on the theory, design, and application of heat exchange equipment in the Chemical Process Industry; in September, a two-part article on the heat transfer and flow characteristics of the plate exchanger. The reader will find these a valuable extension of the information presented in this article in developing uses for this type of heat, or exchange, equipment.
the diameter while the surface area increases only as the square of the diameter, it can be seen that a low ratio of heat transfer surface to reactor volume exists. This, together with unfavorable heat transfer coefficients, makes stirred vessels a poor choice for highly exothermic or endothermic reactions unless equipped with internal coils or an external circulating heat transfer system. By the same principle, the smaller the tube diameter the easier it is to maintain isothermal conditions in the reaction mixture. O n the other hand, the smaller the tube diameter the lower the capacity of the tube so that it may be necessary to operate with a number of tubes in parallel to obtain the desired throughput. Becauseof the largevolume represented, use of the stirred vessel is advantageous where the reaction time is long and the required heat transfer rate is low. Smith (2) states that from a design standpoint the essential difference between tubular and tank reactors is in the degree of mixing obtained. The mixing in turn affects other variables such as heat and mass transfer. In the stirred vessel essentially complete mixing is obtained by mechanical agitation. This results in uniform temperature, pressure, and composition throughout. On the other hand in most tubular reactors, because the length-diameter ratio is large, the mixture flows end to end with little or no mixing. Another result of the complete mixing obtained in the stirred vessel is the fact that the composition of the reactor contents is the same as that of the exit stream. Since reaction rate usually decreases as the conversion increases the vessel reactor operates a t the poorest position in the range of rates encountered. A tubular reactor is able to capitalize on the high initial rate at its entrance. Smith (2) gives an excellent comparison of the stirred reactor and the tubular reactor as well as a mathematical analysis of both.
The Plate Heater The plate heater is designed to cause the heat exchange fluids to flow in thin films over metal surfaces stamped out in undulating patterns. This type of flow pattern produces turbulence even at low velocities thereby increasing the effectiveness of heat transfer and mixing. The apparatus is assembled from these thin metal plates, separated by rubber
gaskets, by mounting them between a top carrying bar and a bottom guide bar. The group of plates is tightly clamped between a fixed headpiece and a movable tailpiece. This arrangement occupies a small floor space and permits the apparatus to be taken apart and assembled easily. Moreover, by means of connector plates a number of different heat exchange operations can be performed in the same frame and with variable heat transfer areas. A complete description and background for the plate heater is given by Troupe and others (6, 7). The characteristics of the plate heater, as outlined above, indicated that reacting fluids could be made to flow through the apparatus in thin films with excellent heat transfer and mixing. Further, in those cases where the reaction is both exothermic and endothermic at different times or where the heat evolved varies with time, the location of the area needed for heating or cooling or both could be established and the plates arranged to handle this situation. In addition to the advantages which recommend the plate heater as a flow type-reactor one must also examine the limitations to its use for this purpose. At present the upper limit of pressure for this equipment is about 200 p.s.i.gd and the limiting temperature about 300 F. No doubt improvements in the compounding of gasket materials will raise these limits and, in addition, will overcome the handicap of gasket solvency in certain liquids. Probably the most serious shorYcoming in this application is the limited throughput of the apparatus. The nature of the design of the equipment leaves little hope that this limitation can be improved.
T o determine the extent to which t h e plate heater may b e used as a flow reactor it should b e subjected to tests with four different types of reactions: A reaction involving only small heat effects b u t whose rate is comparatively slow. Since a rather long residence time is required the flow rates will b e low, probably in a range where there is little or no turbulence. Such a reaction will test such factors as extent ofmixing, stagnant areas, a n d heat losses. A highly exothermic or highly endothermic reaction. This type of reaction should test the ability to achieve isothermal operation u n d e r difficult conditions. A reaction having both exothermic and endothermic periods during the course of the reaction. Such a reaction, for example a polymerization, will test the ability to predict and design both heating a n d cooling areas within the same frame a n d to maintain conVOL. 52, NO. IO
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trol of the reaction with fixed heating and cooling sections. 0 A reaction involving two liquid phases. This will test the efficiency of the mixing within the reactor. This article covers tests for on1 . the first of these reaction types. The reaction selected was that between methanol and lactic acid. This reaction was selected because of the availability of kinetic data (f),t h , knowledge of techniques associated with it, and the fact that it would not cause corrosion problems with the materials of which the equipment was constructed. Further, literature references to pilotplant flow reactor tests on this and similar reactions (3, 4 ) are available.
HEAT-UP WATER OUT
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REACTION MIXTURE Ih
The basic unit of the flow reactor was a Model HTF Chester-Jensen plate heater with plates fabricated of Type 31 6 stainless steel
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Design of the Experimental Apparatus
The basic unit of the flow reactor was a Model H T F Chester-Jensen plate heater with plates fabricated of Type 316 stainless steel. The heater was arranged in three sections-a heat-up section, an isothermal section, and a cooling section. These sections were separated by connector plates and connections were originally made with creamery hose equipped with quick-opening fittings. Reaction Mixture Flow. The reaction mixture was fed from a tall, narrow glass tank which had been calibrated and marked so that the flow rate could be measured. The mixture was pumped from this tank to the heat-up section of the apparatus by a reciprocating-type of metering pump. In the heat-up section the mixture passed through a single plate passage to raise it to reaction temperature. The reactants next passed to the isothermal section which was arranged so that the fluid passed through five parallel
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The heater was arranged in three sections-i.e., heat-up, isothermal, and cooling. The sections were separated by connector plates and connections were originally made with creamery hose equipped with quick-opening fittings
plate passages. From the isothermal scction the mixture passed to the single plate passage of the cooling system. Upon emerging from the cooling section, the product passed through a needle valve used to control back pressure on the system, and was collected. Temperatures were measured by thermometers at the pump discharge and at the outlet from the cooling section and by iron-constanLan thermocouples at the inlet and outlet cf the heat-up section inlet and outlet of the isothermal section, and inlet to the cooling section. Pressure was measured at the discharge from the cooling section at a point upstream from the needle valve. Heat-up Fluid Circulation. The requirements for this section are good heat transfer to permit the reaction temperatilre to be reached in the shortest time, close control of the reaction mixture outlet temperature, and prevention of vaporization of the methanol. Originally it was planned to calculate the heat transfer area needed using available relationships ( 6 ) . However, no equations were available for calculating the individual film coefficient for flows at rates as low as that of the reaction mixture. Because of this, an arrangement was selected which had the reaction mixture flow in a single passage with the heating medium flowing countercurrently through passageson bothsidesof the reaction mixture passage (Figure 1). Hot water was chosen as the heating medium rather than steam because of the necessity for close control and prevention of vaporization (7). The hot water was continuously circulated from an 85-gallon tank through the plate heater and back into the tank using a centrifugal pump. The water in the tank was heated by a Platecoil tank heater using steam as the heating medium. A propeller-type agitator circulated the tank fluid past the Platecoil at high velocities.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Temperature control in the heat-up section was accomplished by controlling the temperature of the water in the tank. A very sensitive potentiometer and controller, actuated by a thermocouple immersed in the water in the tank, controlled the action of a solenoid valve in the steam line to the Platecoil heater. Temperatures of the heating water were measured by thermometers at the inlet and outlet of the plate heater. Flow rates were measured by a rotameter in the circulating line. In the experimental apparatus this section \vas subjected to a more severe heat transfer test than would be encountered in actual practice because the reactants were chilled to suppress any reaction prior to the time they entered the unit. I n actual practice the reactants would enter at room temperature. Isothermal Fluid Circulation. This section was designed to function in a manner similar to a constant temperature bath, Accordingly the hot water employed to maintain the reaction mixture at constant temperature was circulated through the isothermal section of the plate heater in five parallel passages. In this way each passage for reaction mixture alternated with one for hot water (Figure 1). Further, the hot water was circulated a t a rate sufficient to limit its temperature drop to a maximum of several degrees Fahrenheit. The hot water supply tank had a capacity of 30 gallons and was equipped with a steam sparger and a propeller type agitator. The sparger was designed with an orifice having a steam capacity approximately one and one half times that needed to maintain the proper temperature. The hot water was circulated with a rotary pump. Temperature control in the isothermal section was maintained by controlling the temperature of the hot water in the tank.
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P L A T E HEATER Table
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Summarized Data
Operational
Run No. 2 3 Reaction mixture Temp., a F. Feed tank 60.0 55.0 Inlet heat-up section 63.5 62,s" Outlet heat-up section 199.7 199.3 Inlet isothermal section 197.5 199.0 Outlet isothermal sec199.5 tion 199.7 Inlet cooling section 199.7" 199.6" 58.0 57.5 A t discharge Flow rate, ml./sec. At inlet 5.78 5.60 At reaction temp. 6.24 6.05 Outlet pressure, p.s.i.g. 35 35 Heat-up water Temp., O F. 209.0 208.0 Inlet 203.0 204.0 Outlet Flow rate, g.p.m. 3.18 4.78 Outlet pressure, p.s.i.g. 30 24 Isothermal water Temp., O F. 204.0 202.0 Inlet 201.0 200.5 Outlet 3.82 4.83 Flow rate, g.p.m. Outlet pressure, p.s.i.g. 35 35 Cooling water Temp., F. 52.5 Inlet 55.0 Outlet 5.90 Flow rate Outlet pressure 35 35 a Estimated average; readings variable.
A thermocouple immersed in the hot water actuated a sensitive potentiometer and controller which in turn controlled the operation of a solenoid valve in the steam sparger line. Temperatures of the circulating water were measured by thermometers located a t the entrance and exit of the isothermal section. Flow rates were measured by a rotameter in the circulating line. Cooling Water Circulation. Cooling water was piped directly from the service main to the cooling section of the plate heater. Since the object in this section was to lower the temperature of the products as quickly as possible the cooling water was directed through passages on both sides of the reaction product passage (Figure 1). T h e flow rate of the cooling water was maintained a t the maximum level possible. Inlet and outlet water temperatures for this section were measured by thermometers and the flow rate was measured by a rotameter.
flaws in the design and operation of the equipment. During these preliminary experiments data were collected in the same manner as planned for the runs with reactants. In the series of runs using water in place of the reaction mixture it was discovered that serious heat losses were occurring from the water as it passed through the hoses connecting the various sections of the apparatus. For example, in the hose connecting the outlet of the heat-up section to the inlet of the isothermal section, the water representing the reaction mixture dropped in temperature about 7 " F. The reason for this is obvious. Although the hose wall is not a good heat conductor, the volume of the hose is relatively large and the flow rate is extremely small. To combat these heat losses, the hoses were wrapped with electrical heating tapes whose outputs were controlled by variable voltage regulators. This method overcame the difficulty. When methanol-water mixtures, in the same ratio as the methanol-lactic acid mixtures to be used later (Table I), were passed through the apparatus it was found necessary, as expected, to maintain a back pressure on the stream to keep the mixture entirely in the liquid phase. When a back pressure of 35 to 40 p.s.i.g. was maintained on the methanol-water side of the plates it was discovered that the reaction volume of the apparatus increased. A series of experiments confirmed that this was due to a slight buckling of the plates due to unequal pressure on the two sides and to expansion of the hoses connecting the various sections. The operating procedure was changed to raise the pressure on the sides of the plates occupied by the heating media in order to equalize, as nearly as possible, the pressures and thus prevent plate buckling.
Kinetic Experiments Materials. Edible grade, 80% lactic acid, furnished by E. I. d u Font de Nemours t j , Co., Inc., was concentrated to 85.90% total acidity (as lactic acid) by evaporation under vacuum. Absolute methanol, also furnished by Du Pont, was the other reactant. T h e catalyst used was analytical reagent grade sulfuric acid analyzing 97.05% sulfuric acid. Procedure. T h e reactants were' measured and stored in separate bottles in a freezer chest. Water, mixtures of water and methanol, and finally some of the reaction mixture were pumped through the system while adjustments were made to stabilize the operating conditions. When the system was operating satisfactorily, the chilled reactants were combined, sampled for analysis, and passed through the system. During the experiment flow rates, temperatures a t inlet and outlet of each section of the equipment, and pressures were recorded a t approximately 2-minute intervals. At the same time, flow rates, inlet and outlet temperatures, and pressures were recorded for all heating and cooling streams. After about 1 1 / 2 volumes of reaction mixture had passed through the apparatus, samples of the effluent were taken, chilled, and analyzed. All samples were analyzed by the method of Troupe and Kobe (4).
H P L A T E HEATER
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