Adiabatic heat exchanger

The method of supplying and removing heat (adiabatic, heat exchange mechanism, etc.)... 

Adiabatic heat exchange with cooling media and heat losses through reactor walls are absent. 

Propane gas enters a continuous adiabatic heat exchanger at 40X and 250 kPa and exits at 240 C Superheated steam at 300 C and 5.0 bar enters the exchanger flowing countercurrenily to the propane and exits as a saturated liquid at the same pressure. 

Saturated steam at 300°C is used to heat a countercurrently flowing stream of methanol vapor from 65°C to 260 C in an adiabatic heat exchanger. The flow rate of the methanol is 5500 standard liters per minute, and the steam condenses and leaves the heat exchanger as liquid water at 90°C. 

Effectiveness H of the heat exchanger as a function of the ratio Ca/Cf must be known for the calculation. For an adiabatic heat exchanger. 

Heat transfer through a finite temperature difference is irreversible. Figure 2 shows the temperature profiles for two streams passing through an adiabatic heat exchanger. The following expression for the exergy destmction E due to heat transfer from the hot stream 3 to the cold... 

FIGURE 2 Temperature profiles and thermodynamic average temperatures for two streams passing through an adiabatic heat exchanger at constant pressure. 

For this case, the total heat transfer may be obtained from the energy balance equation considering adiabatic heat exchanger. 

Adl b tic Converters. The adiabatic converter system employs heat exchangers rather than quench gas for interbed cooling (Fig. 7b). Because the beds are adiabatic, the temperature profile stiU exhibits the same sawtooth approach to the maximum reaction rate, but catalyst productivity is somewhat improved because all of the gas passes through the entire catalyst volume. Costs for vessels and exchangers are generally higher than for quench converter systems. 

If there is no heat exchange on passing the plug, i.e.f the process is adiabatic, as Joule and Kelvin assumed,... 

Response time constant 403 Rkster. S. 6-13,655 Return bends, heat exchanger 505 Reversed flow 668 Reversibility, isothermal flow 143 Reversible adiabatic, isentropic flow 148... 

The heat transfer term envisions convection to an external surface, and U is an overall heat transfer coefficient. The heat transfer area could be the reactor jacket, coils inside the reactor, cooled baffles, or an external heat exchanger. Other forms of heat transfer or heat generation can be added to this term e.g, mechanical power input from an agitator or radiative heat transfer. The reactor is adiabatic when 7 = 0. 

Solution There are several theoretical ways of stabilizing the reactor, but temperature control is the normal choice. The reactor in Example 5.7 was adiabatic. Some form of heat exchange must be added. Possibilities are to control the inlet temperature, to control the pressure in the vapor space thereby allowing reflux of styrene monomer at the desired temperature, or to control the jacket or external heat exchanger temperature. The following example regulates the jacket temperature. Refer to Example 5.7. The component balance on styrene is unchanged from Equation (5.29) ... 

Figure 8.22. Schematic drawing of an adiabatic two-bed radial flow reactor. There are three inlets and one outlet. The major inlet comes in from the top (left) and follows the high-pressure shell (which it cools) to the bottom, where it is heated by the gas leaving the reactor bottom (left). Additional gas is added at this point (bottom right) and it then flows along the center, where even more gas is added. The gas is then let into the first bed (A) where it flows radially inward and reacts adiabatically whereby it is heated and approaches equilibrium (B). It is then cooled in the upper heat exchanger and move on to the second bed (C) where it again reacts adiabatically, leading to a temperature rise, and makes a new approach to equilibrium (D). (Courtesy of Haldor Topspe AS.)...

Figure 8.24. Left schematic diagram of an adiabatical three-bed, indirectly cooled reactor with two heat exchangers. Right a diagram showing the equilibrium curve to the upper right, the optimal operating line and the operation line for the reactor are to the left. [Adapted from C.J.H. Jacobsen, S. Dahl, A. Boisen, B.S. Clausen,...

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

Substantial heat-transfer intensification was also described for a special micro heat exchanger reactor [104]. By appropriate distribution of the gas-coolant stream, the axial temperature gradient can be decreased considerably, even under conditions corresponding to very large adiabatic temperature rises, e.g. of about 1400 °C. 

A reactor is run adiabatically when no heat is exchanged between the reaction zone and the surroundings. The reaction temperature can then only be controlled by quenching with a cold stream of the reaction mixture or by inter-stage heat exchangers. For highly thermally sensitive large molecules treated in the fine chemicals sector this is a very impractical mode of operation. Therefore, adiabatic reactors will not be discussed here. 

For reversible adiabatic expansion (no heat exchange with the surroundings) ... 

Differential scanning calorimetry (DSC) can be performed in heat compensating calorimeters (as the adiabatic calorimetry), and heat-exchanging calorimeters (Hemminger, 1989 Speyer, 1994 Brown, 1998). 

Techniques for approaching optimum temperature profiles for exothermic reaction, (a) Adiabatic operation of reactors with interstage cooling, (b) Countercurrent heat exchange. (Adapted from Chemical Reaction Engineering, Second Edition, by O. Levenspiel. Copyright 1972. Reprinted by permission of John Wiley and Sons, Inc.)... 

When there is no heat exchange between the inner vessel and its surroundings (adiabatic calorimeter, 1 in Fig. 1), the temperature of the calorimeter vessel varies when heat is liberated or absorbed. The quantity of heat produced or absorbed may be calculated from this temperature change, if the heat capacity of the inner vessel and of its contents is known. 

Several simplifying assumptions are adopted (1) the flow in the tube is considered to be one dimensional without turbulence or mixing (2) the processes in the tube are adiabatic. There is no heat conduction in the tube and no heat exchange between the gas and the tube walls (3) each heat exchanger is isothermal. 

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