Re: Column overhead temp as an mv vesus reflux as an mv

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Posted by Graeme Taylor on August 21, 2000 at 14:30:21:

In Reply to: Column overhead temp as an mv vesus reflux as an mv posted by Nahum Lifshi-tz on June 28, 1999 at 10:12:08:

: Can anybody list his( hers )reasons ,in favor and against, opening the loop between a column overhead TIC to the top reflux flow controller when deciding on a DMC independent variable.
: When counting on internal measurments in the column, such as temperatures, does using the TIC as an independent introduce more noise?

Sorry I’m a little late for this discussion, but I have only just found this discussion board.

Has anyone experienced problems in opening the tower top temperature controller in a vacuum crude unit DMC, in particular when trying to control the viscosity and volatility of a lubes sidestream (or any other reasonably co-linear qualities)?

With the tower top temperature controller open all the handles appear to impact the cutpoints and the fractionation in the tower in a similar way, leading to the ratios of the volatility to the viscosity gains being similar for each MV. By choosing a design that requires a controller to rely on small differences in these ratios appears to produce a controller that is very sensitive to small errors in these gains.

Have people found that closing the tower top temperatures loop in these multiple product fractionators leads to a controller that has more distinct cutpoint and fractionation handles and as a result significant differences in the ratio of viscosity to volatility gains for different MVs? I would guess that having greater differences in these ratios tends to lead to a controller that is less sensitive to gain errors.

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: : Can anybody list his( hers )reasons ,in favor and against, opening the loop between a column overhead TIC to the top reflux flow controller when deciding on a DMC independent variable.
: : When counting on internal measurments in the column, such as temperatures, does using the TIC as an independent introduce more noise?

: Sorry I’m a little late for this discussion, but I have only just found this discussion board.

: Has anyone experienced problems in opening the tower top temperature controller in a vacuum crude unit DMC, in particular when trying to control the viscosity and volatility of a lubes sidestream (or any other reasonably co-linear qualities)?  

: With the tower top temperature controller open all the handles appear to impact the cutpoints and the fractionation in the tower in a similar way, leading to the ratios of the volatility to the viscosity gains being similar for each MV.  By choosing a design that requires a controller to rely on small differences in these ratios appears to produce a controller that is very sensitive to small errors in these gains.  

: Have people found that closing the tower top temperatures loop in these multiple product fractionators leads to a controller that has more distinct cutpoint and fractionation handles and as a result significant differences in the ratio of viscosity to volatility gains for different MVs?  I would guess that having greater differences in these ratios tends to lead to a controller that is less sensitive to gain errors.

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