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Big_Orrin @Big_Orrin
, 12 tweets, 4 min read Read on Twitter
@anasalhajji #Crude_Quality_Matters
1. I wrote a couple of weeks ago the following tweet thread and said i would write one with regard to the heater/reboiler at the bottom and why running more light crude is not the best idea.

2. In a distillation column separation of the products is done by temperature. The temperature at the top is much lower than the temperature at the bottom. This difference causes a temperature curve along the column.
3. Each degree of temp corresponds to a particular composition of liquid/gas. So at particular points along the length of a column an outlet is placed. this is where the composition of the liquid/gas corresponds to a product. The higher in the column the lighter the composition
4. But the problem is that the liquid entering is a crude oil and all the various products are mixed up. So how do you separate them? It is done by heat transfer between a gas phase moving up and a liquid phase moving down. They come in contact on plates where they mix.
5. the mixture is at a certain temperature where some of the products in the gas turn to liquid and some of the liquid products turn to gas. this happens over a number of plates which separates the compositions out correctly so they can be withdrawn as the necessary products.
6. But to do this the column needs to have heat continually added. IT can be done either directly by steam into the base of the column where the heaviest liquid and highest temperature is , or done through a reboiler, where the heavy liquid is indirectly heated by steam
7. For the column to work well it needs to have a consistent amount of vapour/gas going up the column and a consistent amount of liquid coming down. Therefore, the column is designed with particular crude oils in mind to gain the best separation.
8. Now, if you start to add more light crude it changes the column dynamic. Light crude has more lighter ends and less heavier ends. Therefore, there is more gas going up the column and less heavier liquid coming down. So the column needs to re-balance.
9. What happens is that less liquid starts to reach the bottom of the column. Which mean there is less heat in the column, so the temperature curve starts to change. it now means that the compositions where the products are taken out change and could mean they are out of spec.
10. The reboiler than starts to over heat the liquid in the bottom as there is less and it is lighter. It creates more gas from the liquid at the bottom of the column which creates pressure up the column which also affects the composition of the products.
11. The design of a reboiler means that it has a very small range that it is effective over to control temperature, so changes to the composition of the bottom liquid can have significant effect that the reboiler cannot deal with.
12. With the reboiler and the overhead condenser in the first tweet storm, you can see the CDU is sensitive to change. It is flexible to a point but anymore and you end up with a lot of off-spec material That is why laws of physics/chemistry will always beat the laws of price
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