(492a) Active Vapor Split Control for Fully Coupled Columns: Experimental Studies
AIChE Annual Meeting
2011
2011 Annual Meeting
Separations Division
Advances In Distillation & Absorption II
Wednesday, October 19, 2011 - 12:30pm to 12:55pm
Abstract
The idea of a 4-product fully coupled distillation column has
existed in literature but has not been adopted in industry for the reasons of
complex design and operation. A sole exception is an installation by BASF (Dejanovic, Matijasevic et al.
2010). The potential energy saving in
4-product fully coupled columns like 4-product extended Petlyuk arrangement can
be about ~50% and ~30% in a Kaibel arrangement (Halvorsen and Skogestad 2003). In previous works, a 4 point temperature control structure for the
regulatory layer has been recommended (Strandberg 2006) and was experimentally demonstrated (Dwivedi, Halvorsen et al. 2011) using a pilot plant.
The full realization of energy saving potential of thermally coupled
columns may require on-line adjustment of the vapor split in order to handle
expected feed property variations and still maintain minimum energy operation.
In particular for cases where the optimal operating window with a fixed vapor
split is narrow, like in a 4-product Kaibel column, and also in some 3-product
DWCs where high purity is required in the side product. Ability to adjust the
vapor split also adds to the operational flexibility, which is important since
limitation in flexibility has been seen as a disadvantage in commercialization
of thermally coupled columns. Although some experimental work for 3- product
fully coupled columns have been reported (Mutalib, Zeglam et al. 1998;
Niggemann, Hiller et al. 2010), the use of an active vapor split valve for control as an during
has not been shown due interest or discouraged in the fully coupled columns (Agrawal and Fidkowski 1998).
This paper demonstrates the use of a vapor split valve in a 4 point
temperature regulatory layer of a four product experimental Kaibel column. The
excess degree of freedom that is, the liquid split valve that divides liquid
reflux between the prefractionator and the main column can be used as a degree
of freedom for economic objectives such as, for minimization the energy
consumption for a given purity specification or, for maximizing product purity
for fixed boil up.
Additional advantage of using the vapor split in the regulatory
layer is that, it is very fast input, which make set point tracking much
faster. Also in a multivariable system like a four product Kaibel column, the
interaction effects between the loops can be checked, as the vapor split valve
operates in a much faster time scale.
The design of the vapor split valve used, is rather rudimentary.
However, even with such a simple valve design, we can ensure a stable column
operation. The work emphasizes that with a simple temperature feedback action,
in spite of the input uncertainty i.e., here, uncertainty in vapor split
between prefractionator and the main column, the column can still be operated
and stabilized. As there were some limitations of the used prototype, more
innovations are required and can be a focus of interest for valve experts for
developing an effective and robust vapor split valves that can be operated for
such applications which allow low pressure drops.
References