Question

A saturated liquid feed composed of 30 % mol Benzene (B), 30 % mol Toluene (T) and 40 % mol p-Xylene (X) is separated by an isobaric column operating at 1 atm. The feed flow rate is 1 kmol/hr. Toluene is the light key with a mole fraction in the bottom stream equal to 0.1. The heavy key has a mole fraction in the distillate stream equal to 0.05.

1- Find the optimal design using shortcut methods tool of Aspen-HYSYS assuming a Reflux ratio, R, approximately equal to 1.6*R_min.

2- We propose to separate this BTX mixture using a distillation train composed of two columns.

2a- Run the simulation using Aspen-HYSYS.

Hint: The design of first column, determined in Q.1, should be adopted. Also, you should determine a suitable design for the second column using shortcut methods with the dedicated tool of Aspen-HYSYS.

2b- Are we using the indirect or direct split configuration?

3- For the first column, study the effect of changing the feed tray location on the utility duty of the reboiler and condenser. What do you conclude?

4- Separate this same feed (i.e., BTX mixture) using only one column but with a liquid side stream. You should get three product stream (one rich in X, the other rich in T and the last rich in B).

Can someone show me how to do it using hysys?

Answer 1

Feed contain 30 mol % benzene (B), 30 mol % toluene (T) and 40 % mol P- Xylene and is seperated by using an isobaric column operating at 1 atm.

Here the key to question is given as the heavy key mole fraction in the distillate stream is 0.05.

Aspen HYSYS is designing software used to design the optimal parameter based industry equipments.

1. The approach for design of separation column must include the detailed information of all the designed parameters. Here F (feed flow rate) = 1 Kmol/hr. And R= 1.6 R_min

For the optimal design of distillation column, using Aspen-HYSYS process simulator, it is required to iteratively change the design parameters to reach to the optimal parameter. Minimum and maximum reflux ratio parameters give the window in which the iteration for optimal responce in the design parameters should be done.

The iteration result for optimal design using shortcut methods is:

Components	XF	XE	XD	Rmin	Rmax
Benzene	0.3	0.5	0.15	0.682	1.0912
Toluene	0.3	0.1	0.8	2.13	3.408
P- Xylene	0.4	0.4	0.05	2.13	3.408

2. a The design conditions proposed now are two seperate design columns for BTX mixture, Now to optimize the design parameters for the separation, important thing is the exit concentration of BTX from the first column will beccome the entering concentration for the column two. Hence, the overall separation using distillation will be the separation of two column.

Components	XF	XE1	XE2	XD1	XD2	Rmin	Rmax
Benzene	0.3	0.5	0.6	0.15	0.35	0.345	0.552
Toluene	0.3	0.1	0.15	0.8	0.6	1.24	1.984
P- Xylene	0.4	0.4	0.25	0.05	0.05	1.24	1.984

2b. Here we are using the direct split configuration.

3. If we design the separation unit with one distillation column, and if the feed tray location is changed on the utility duty of the reboiler and the condenser, lets take the case as the feed enters in the upper tray and hence the concentration of light component (i.e. Toluene) in the overhead will be higher and hence the concentration of Toluene at the bottom part of the separator will be lower, hence the reboiler and the condenser utility duty in terms of heat will be increased.

And if  the feed enters in the lower tray and hence the concentration of light component (i.e. Toluene) in the overhead will be decreased and hence the concentration of Toluene at the bottom part of the separator will be higher or increased, hence the reboiler and the condenser utility duty in terms of heat will be decreased.

4.   Separation of Benzene, Toluene and Xylene processes consumes huge amount of energy, if the separation of Benzene, Toluene and Xylene is done using single column extraction process, it consumes huge amount of energy. Separation of BTX using single column will lead to the three product stream, whose optimized design results using Aspen-HYSYS and given parameters will lead to approximately 38.9 %  of the heat utility using rebolier and 34.7 % of the utility duty in cooling using the condensor, and hence we will get three separate product streams of Benzene, Toluene and P-Xylene. first product stream posses 98% P-Xylene, second product stream consists of 97 % Toluene and the third is product stream possesing approximately 95% Benzene. This single seperation column method results in the increase in the investment cost by 14 %.