Rice University - CENG 403 - Distillation - Matlab

Rice University - CENG 403 - Distillation - Matlab - System Overview

Modeling Binary Distillation Using Matlab

After flashing the reactor effluent, the liquid phase must be separated into streams that are primarily benzene, toluene and biphenyl. We will use Matlab programs to simulate one such distillation. The programs have the following restrictions:

There are only two compounds being separated,
VLE can be approximated by assuming ideal behavior of both phases,
Constant molar overflow of the liquid occurs throughout the column,
Ideal stages are set in the simulation.

We will have to neglect the effects of both the light compounds methane and hydrogen as well as the heavy compound biphenyl on our separation. We will emphasize the influence of various operating parameters on the performance of the column and the costs associated with running the column. Cost data for the example are taken from Peters and Timmerhaus pages 371 to 376 . The liquid out of the flash at 2000 kPa and 300K will be taken as the product to be split. It was shown as stream 3 in the table:


             Inlet     |                Outlet                 
   Stream            1 |            2          3   Total     
  Tmp K         929.00 |       300.00     300.00             
  State         vapor  |       vapor     liquid              
  Enthalpy -50020930.3 | -176426148.5  7900057.8 -168526090.7
Compound    Stream Flows                                       
 toluene       28631.5 |       1039.2    27592.3      28631.5
 hydrogen     795956.6 |     795431.9      524.7     795956.6
 benzene      150568.2 |      16569.3   133998.8     150568.2
 methane     2387533.0 |    2378826.1     8706.9    2387533.0
 biphenyl       5726.3 |          0.2     5726.1       5726.3
Total        3368415.7 |    3191866.8   176548.9    3368415.7

Since we can only simulate binary fluids in the distillation modules, we will neglect the hydrogen and methane in this product and "lump" the biphenyl with the toluene. Thus our binary feed will have the composition: 0.8009 mol faction benzene and the rest toluene . We wish to produce 265 lb mol of benzene at a purity of 0.9997 and will assume we can do so with a feed of about 170 kg mols/hr.

The main operating parameters that we can set in our distillation tower are:

pressure
number of ideal stages
location of the feed stage
column size
type of condenser: total or partial
reflux ratio

Some of these parameters will be seen to be interrelated with our specifications. Thus we will find that in order to obtain the required purity, we may need to increase the reflux ratio or the number of stages. In fact we can achieve the desired separation by doing either. Economics will then settle on how much to increase each parameter.

So far the specifications do not set the amount of benzene allowed in the bottom product from our column. As the mol fraction benzene in the bottom product increases, we will either lose benzene to waste streams or have to recycle it with recovered toluene back to our reactor. Such effects can only be taken into account in the final design of the process. We will check to see how different bottom product purities can influence the other column design parameters.

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