Stripper column control - variable feed composition

[old_author]

Hello,

I'm evaluating the performance of an existing stripper column with new feed conditions. The distillation column is used as part of a treatment process for a waste stream consisting of mixed organic solvents and water from a series of batch processes. The exact compositions of the top and bottom products is not critical - the objective is to remove as much of the low-boiling organic fraction from the bottom product as possible while keeping the water composition in the top product below a certain fraction (10 wt%).

The main challenge is the variability of the feed composition, which can range from 20 to 95% water. The relative compositions of the organic components can vary as well. This has a significant impact on the internal vapor flow rate in the column.

I've proposed the following control strategy for the column:
[ul]
[li]Reboiler steam flow rate controlled by reboiler temperature[/li]
[li]Distillate rate controlled by level in reflux drum[/li]
[li]Constant reflux rate (flow control loop). The reflux rate will be selected during design to maintain vapor flow in the column within recommended tray limits and to achieve the required distillate quality under worst-case conditions (high water fraction in feed). The product compositions will vary with the feed composition.[/li]
[li]Constant feed rate[/li]
[li]Pressure controlled by split range control of nitrogen gas inflow and vent gas outflow[/li]
[/ul]

In my experience, this is an unusual control strategy, but the application is also somewhat unusual. Is there something I'm missing that would give difficulties during operation, or has anyone had experience with a similar application?

 

[georgeverghese]

Presumably N2 is for pressure control at column top because operating temp is below 100degC, and there may be insufficient volatiles when feed water fraction is high to maintain top pressure. So this ovhd condensor can only be a partial condensor, and non condensible N2 will need to be vented off at the condensor to maintain heat transfer eff, together with unavoidable loss of organics.

Watch out for azeotropes where the water fraction may be in excess of 10% by wt. There may even be ternary (or worse) azeotropes.