The program htxcc2 simulates an exchanger in which one fluid has a constant temperature throughout the exchanger. It is particularly useful in the design of boilers and condenser (such as those associated with a distillation column) if the fluid that is boiling or condensing is essentially pure and enters the unit saturated. If, for example, we use saturated steam vapor to heat a fluid, the steam will condense at the saturation temperature corresponding to the pressure in the unit.
Here is what help tells us about the htxcc2 program:
>>help htxcc2 Htxcc2 - heat exchanger for constant temp outer fluid function [Q,A]=htxcc2(Ti0,TiL,To,Fcpi,U,dTmin) Constant Outer Fluid Heat Exchanger Argument List Ti0 Inlet temperature of inner fluid TiL Exit temperature of inner fluid To Temperature of outer fluid Fcpi Flow*heat capacity of inner fluid U Overall Heat Transfer coefficient dTmin Minimum approach temperature Return List Q Heat Transfer Rate from outer to inner fluid A Required Heat Transfer Area Example: >> [Qx,Area]=htxcc2(60,150,160,25,50,10)
Suppose we need to heat the fresh toluene feed in the HDA process from 298K to 409.75K, which is the temperature that we found to be the dew point of the feed to the reactor. We will assume that the toluene is always a liquid in the exchanger. If the approach temperature is to be 5K, the steam will have to be at 414.75K. The toluene and steam pressures will then have to be:
>>cnms cnms = hydrogen methane water benzene toluene biphenyl >>vappr(409.75,5) ans = 202.1834 <-- Toluene pressure must be > 2 atm. >>vappr(414.75,3) ans = 375.9907 <-- Steam pressure must be about 3.76 bar.
The flow rate of toluene is 190.4 kg mol/hr. An estimate of its Fcp is then:
>>Ntoluene=190400; <-- mols toluene/hr >>dHtol=HinkJ(409.75,'l',5)-HinkJ(298,'l',5) dHtol = 19.2516 <-- kJ/mol >>Fcpi=Ntoluene*dHtol/(409.75-298) Fcpi = 3.2801e+04
Here is our simulation of the heating unit:
>>[Q,A]=htxcc2(298,409.75,414.75,Fcpi,1,5) Q = 3.6655e+06 A = 1.0334e+05
Note that if we then want to find the additional area to vaporize the toluene using the same steam, the problem is quite simple since both fluids have constant temperatures throughout the exchanger. Thus:
Q = U*A*(To-Ti)
could be used to find the additional area.