Rectification combined with stripping

Delayed coking is the only main process in a modern petroleum refinery that is a batch-continuous process. The flow through the tube oven is continuous. The feed stream is switched between two drums. One drum is filling with coke while the other drum is being steam-stripped, cooled, decoked and warmed up (see chapter 6). The overhead vapors from the coke drums flow to a rectification unit. The rectification tower has a reservoir in the bottom where the fresh feed is combined with condensed product vapors (recycle) to make up the feed to the coker heater. 

For the sake of simplicity, column circuits can be used in which a main column is combined with a side column that operates as a rectification or stripping column. This arrangement saves an evaporator or a condenser (Figure 2.3.2-20). 

Furthermore, by definition, the feedstream combines with and has the same composition as the reject phase from the feed stage. Also, the permeate phase flow rate V has the same constant value for both stripping and rectification that is,... 

The third column (118) was a simple rectification column in which decane was separated from THF/ ethylacetate. Decane was recycled into the extraction column 116. Compared to different alternatives, which were simulated, this process has the following advantages. Water was eliminated from the ethylacetate/ THF-mixtures before their rectification. This approach takes advantage of the fact that the VLE-data of ethylacetate/ THF are more favorable than the ones of ethylacetate/ THF/ water. The counter current extraction with decane allows an efficient separation of the polar impurities such as methanol, ethanol, and acetic acid. Furthermore decane eliminated the water from the recovered solvent mixture (extractive rectification in column 117). Methylacetate posed a further problem and a rectification column was necessary to separate it from THF. The stripping column 117 combined the dewatering and the elimination of methylacetate. 

Thin film distillation and rectification may be combined in such a way that the thin film evaporator is connected to a column with both an enrichment and a stripping zone, or just an enrichment zone [2.17]. Such a combination is particularly useful for separation of sensitive mixtures in a vacuum. 

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