Processes with feed -effluent heat exchange

2 Processes with feed-effluent heat exchange 

For simplicity, we assume that the holdups of all the units are constant and that the temperature dependence of physical properties such as density and heat capacity is negligible. Under these assumptions, the energy-balance equations for the process can be written as 

1 Note that, since in this case the material flow rate is the same through all the units of the process, we can formulate most arguments in terms of specific stream enthalpies hi, rather than resorting to energy flows. 

Under these assumptions (and accounting for the fact that the energy flows Hi associated with the material streams can be written as products of the material flow rate F and a specific enthalpy, Hi = Fh ), the model of Equation (6.29) becomes 

Finally, the total enthalpy 0 + 02 + 83 can be used to derive a representation of the slow component of the process dynamics  

---

Figure 7.1 shows a typical chemical process in which a feed-effluent heat exchanger is coupled with an adiabatic exothermic reactor. The heat of reaction produces a reactor... 

FIGURE 1 Typical temperature profiles for several process heat exchanger applications (a) product cooler (b) feed heater with condensing stream (c) multicomponent feed heater with vaporization and superheating (d) pure-component product condenser (e) multicomponent product condenser (f) typical feed-effluent heat exchanger. 

Figure 6.2 A process with a feed-effluent heat exchanger.

The bottoms from the DIB contains most of the nC4, along with some iC4 impurity and all of the heavy isopentane impurity. Since this heavy component will build up in the process unless it is removed, a second distillation column is used to purge out a small stream that contains the isopentane. Some n C4 is lost in this purge stream. The purge column has 20 trays and is 6 ft in diameter. The distillate product from the second column is the recycle stream to the reactor, which is pumped up to the required pressure and sent through a feed-effluent heat exchanger and a furnace before entering the reactor in the vapor phase. 

The last example is a gas-phase process with a tubular reactor, gas recycle compressor, feed-effluent heat exchanger, condenser and separator. The steady-state design of this process leads to an uncontrollable system if the reactions are highly temperature sensitive. We demonstrate that changing the design produces a much more easily controlled process. We consider a complete plant, not just the reactor in isolation. 

Consider a recycle process where an irreversible, exothermic reaction A-t-B—>X occurs in a gas phase, adiabatic tubular reactor. The process flowsheet consists of one tubular reactor, one distillation column, one vaporizer, and one furnace with two heat exchangers which was first studied by Reyes and Luyben [10] (Fig. 1). Two fresh feed streams FoAand Fob are mixed with the liquid recycle stream D and sent to a steam-heated vaporizer. According to the requirement of reaction temperature, the vapor from the vaporizer outlet stream is preheated first in a feed-effluent heat exchanger followed by a furnace to get proper reactor temperature as well as for the start-up purpose. The exothermic reaction takes place in the tubular reactor and the reactor temperature increases monotonically along the axial direction with the following inlet and outlet temperatures, Tj and Tout- The hot gas from the reaetor preheats the... 

There are reactions where the heat of reaction can be employed to preheat the feed when an exothermic reaction is operated at a high temperature (e.g., ammonia N2 + 3H2 <-> 2NH3 or methanol CO + 2H2 CH3OH synthesis, water-gas shift reaction CO + H20 <-> H2 + C02). These processes may be performed in fixed-bed reactors with an external heat exchanger. The exchanger is primarily used to transfer the heat of reaction from the effluent to the feed stream. The combination of the heat transfer-reaction system is classified as autothermal. These reactors are self-sufficient in energy however, a high temperature is required for the reaction to proceed at a reasonable rate. 

Description The Isomar process re-establishes an equilibrium distribution of xylene isomers, essentially creating additional paraxylene from the remaining ortho- and meta-xylenes. The feed typically contains less than 1 wt% of paraxylene and is first combined with hydrogen-rich recycle gas and makeup gas. The combined feed is then preheated by an exchanger (1) with reactor effluent, heated in a fired heater (2) and raised to the reactor operating temperature. The hot feed vapor is then sent to the reactor (3), where it is passed radially through a fixed-bed catalyst. 

---