The University of Texas at Austin
College of Engineering

Nonisothermal PFR Problem

THE DESIGN EQUATION FOR A PFR

For nonisothermal problems involving gas phase reactants and products,the molar flows of each component and the temperature are the dependent variables when the pressure drop is assumed negligible. The molar flowrates are found by solving the set of component material balances

where

and the temperature is found with

This set of ordinary differential equations is solved subject to the initial conditions, when they are known, as in the example that follows.

THE PROBLEM STATEMENT

Allyl chloride is to be produced in a 25-ft long 2 in-ID tube operating as a PFR. The feed is a 4:1 molar ratio of propylene to chlorine and it enters at a feed rate of 0.85 lbmole/hr, 2 atm of pressure, and 392°F. The reactor pressure may be assumed constant.

The rate constants have units of lbmoles/(hr-ft3-atm2) and are

where T is in degrees Rankine and R is in Btu/(lbmole-R). The rate expressions are

The thermodynamic data for this reaction are listed below. The component heat capacities in cal/gmole-K can be calculated using

Component
kcal/gmole		 A
 B
(x 102)		 C
(x105)		 D
(x109)
Cl2 0.00 6.432 0.8082 -0.9241 3.695
C3H8 4.88 0.866 5.602 -2.771 5.266
C3H5Cl -0.15 0.604 7.277 -5.442 17.42
HCl -22.06 7.235 -0.172 0.2976 -0.931
1,2-C3H6Cl2 -39.60 2.496 8.729 -6.219 18.49

The reactions are exothermic and the reactor is nonisothermal. A constant wall temperature of 392°F can be realized by boiling ethylene glycol on the outer surface of the reactor wall. The inside heat transfer coefficient is 5 Btu/hr-ft2-°F for a feed rate of 0.85 lbmole/hr and 2 atm total pressure.

THE QUESTIONS

1. Run the program to examine the effect of operating this reactor adiabatically and nonadiabatically. Before viewing the plots be able to answer parts a and b.

  1. Which case should have the highest temperature and why?
  2. Which case should lead to the highest production of allyl chloride?
  3. Why do you think the production of allyl chloride reaches an asymptotic limit for the adiabatic case?

2.Examine the effect of increasing the reactor wall temperature.

  1. Notice that the location of the maximum temperature changes. Why?
  2. Notice that the production of allyl chloride increases with increasing wall temperature. Why?

3.Change the propylene:chlorine feed rate, holding the chlorine fixed at 0.17 lbmoles/hr. The calculations assume the flow remains turbulent, the pressure is negligible, and the overall heat transfer coefficient is unchanged.

  1. What has happened to the maximum temperature for the adiabatic case and the location of the maximum temperature for the nonadiabatic case? Why?