Side Column Unit Operation - Technical Information

The side column unit operation is used to model side strippers and side rectifiers which are associated with a main column. The use of side columns is currently restricted to the Inside-Out, Enhanced I/O and Sure algorithms. See Column Algorithms for further information on these methods.

Side columns always use the same distillation algorithm as the main column. Multiple side columns attached to one main column are possible and, in fact, this is widely practiced in the petroleum refining industry.

The side columns are added to the flowsheet with the Side Column unit operation and attached to the main column with the feed and product streams. Every side column has at least one external product which exits the complex column arrangement.

Solution methods for side columns vary with the algorithm. The Inside-Out (and Enhanced I/O) algorithm merges the side column with the main column and solves the complex column arrangement simultaneously. There are three benefits to this approach:

1. The simultaneous method results in more precision in the solution

2. The simultaneous solution is more efficient and uses less computing time

3. The simultaneous solution provides more flexible product specifications.

For example, the latter benefit permits the use of both a D86 (5%) and a D86 (95%) specification for a side stripper product. To solve this same set of specifications with the Sure method requires the use of a multi-variable controller unit, wrapped around the main column/side column units.

The Sure method solves each side column separately from the main column and uses recycle streams to relate the side column and main column. While special recycle logic is used to converge the column/side column recycle problem, this method has three disadvantages when compared to the Inside-Out column simultaneous treatment :

1. The solution is less precise since a recycle stream tolerance is used in addition to the column equation tolerances

2. The recycle approach is much slower

3. Main column variables (except the main column draw rate) cannot be directly related to the side stripper products. This makes it necessary to use controllers to solve for more than one specification on a side product.

Additional Information on Side Strippers

Side strippers are widely used to control the front end volatility (flash point) of liquid products such as diesel fuel and kerosene. The liquid product is drawn from the main column and charged to the top tray of the side stripper which typically has 6 to 10 actual trays. A stripping medium (usually steam) is fed to the bottom tray of the side stripper to strip about ten percent of the liquid feed (the lightest material) which is then returned to the main column for further fractionation together with the stripping medium. The stripped liquid is withdrawn from the bottom tray of the stripper as a finished product. Steam side strippers have an overall tray efficiency of about 25 percent and can be represented with two theoretical trays.

A variation in side stripper design is the use of a reboiler on the bottom of the side stripper to "heat strip" the liquid feed. No stripping medium is used for reboiled side strippers. The advantage of this arrangement is a smaller stripped vapor return stream to the main column which reduces the vapor loading for the main column. Reboiled side strippers have higher tray efficiencies than those which use a stripping medium. Therefore, three to five theoretical trays are typically used to model these strippers.

Side strippers do not normally have any other items of equipment such as condensers, pumparounds, side heaters/coolers, etc. Only the Sure method permits the use of a condenser on a side stripper. This capability may find utility when modeling some unusual types of column configurations.

Additional Information on Side Rectifiers

Side rectifiers are used to remove heavy materials from vapor draw products by providing a rectification section. The vapor draw from the main column is fed to the bottom tray of the side rectifier which may have a large number of trays. The side rectifier must have a condenser or cooling duty at the top to condense the liquid reflux which is used to rectify the vapor product.

The overhead product from the side rectifier is removed as a finished product. The liquid from the bottom tray is returned to the main column for further fractionation.

The side rectifier corresponds to the rectification section of a conventional distillation column. An overall tray efficiency of 45 to 55 percent is reasonable for many applications.

Side rectifiers do not normally have other items of equipment such as pumparounds, side heaters/coolers, etc. Reboilers are never used for these columns.