Distillation sequences. The feed stream in the table below is to be separated in
ID: 702041 • Letter: D
Question
Distillation sequences. The feed stream in the table below is to be separated into four nearly pure products. None of the components is corrosive and, based (b) he boiling points nogne of the thre separations is dificuit. in Figure 1.9, five distillation sequences are possible. (a) Determine a suitable sequence of three columns using the heuristics of S17. (b) If a th component were added to give five products, Table 1.8 indicates fif that 14 alternative distillation sequences are possible. Draw, in a man ner similar to Figure 1.9, all 14 of these sequences. 1. the Feed rate, kmol/h Normal boiling point, K % Component Methane Benzene Toluene Ethylbenzene 19 263 85 23 112 353 384 409 in op (a un
Explanation / Answer
I assumed that these column are non-integrated i.e they have there independent utilities for reboiler and condenser.
1st heuristic-- Remove the light component which required very low temperature ( i.e refrigeration ) or high pressure to condensed . This will help to operate the remaining column at low pressure and also the cooling water which is available at 30-40 °C can be used. This will reduce the capital and operating cost.
Based on this , Methane which has very low condensing temperature need to be remove first .
2nd heuristic- Do the difficult separation at last.
From the boling point data, it seems separation of ethylBenzene and toulene will be difficult as the temperature difference is just 25°C compare to temperature differnce of benzene and toulene ( 31°C)
3rd heurisitc-- component comprising of large fraction of the feed should be removed first . In this way the net amount of liquid that you are handling or net amount of mixture that you are handling will be lesser and thus the cost of operation will go down.
Based on above , benzene need to remove as top product from 2nd distillation column. The bottom will have toulene and ethylbenzne.
Also, The above selected sequence represent the direct sequencing of the column ( ie remove the lighter first). Please note direct sequencing is always favoured.
Now 4th Heuristic says-- favour near equimolar splits between top and bottom product in individual columns.
As per this, seqeunce will be separate methane and benzene first column as top product. then individual component will be separated in remaining two column.
This heuristic conflict with above 3 heuristic .
I think 4th heuristic will be cost effective based on assumation that methane is recovered in vapor form instead of liquid.
Fig.19 is not given in the problem hence the result can not shown as per fig.19.
if 5th component is added. then sequence need to be evaluated based on heuristic metioned above.
PLease note these heuristic or guideline are for non-integrated column. In order to optimize the cost of operation and capital cost, it is always good to do engery integration, If the complexity in control / design is not an issue.
Also, there are few practical limitation, such as high design pressure , low temperature and expensive material of construction need to be consider while doing sequencing. Low temperature will required Stainless steel material instead of carbon steel , which is very expensive.
Direct sequence is based on removing the lighter first.