A fractionating column requires proper mixing of the entering liquid and the liquid already inside. For columns with trays, this rarely poses a problem. Trays are excellent mixing devices. In contrast, columns with packing don’t always provide adequate liquid mixing and, as a result, can suffer performance and reliability problems.
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Fractionation depends upon heat transfer and mass transfer between the liquid and vapor phases. Heat transfer requires a temperature difference between the phases, while mass transfer requires a composition difference. The basic assumptions used to determine packing performance assume uniform flow distribution, temperature and composition across the tower cross-section.
The packing height to provide the equilibrium stages needed for a separation are determined using a height equivalent per theoretical plate or stage (HETP). Operating results from laboratory, pilot plant, and industrial equipment can generate data for determining HETPs. These data should account for the non-ideal performance of real packing and distributors, installation practices, and other factors.
Trouble arises when assumptions on equipment performance differ from reality. Fractionation calculations assume perfect mixing of the entering liquid and the liquid already in the column. Incomplete mixing or improper liquid distribution may cause many problems.
Different services may have many requirements. However, four common ones are:
• a subcooled liquid feed with heat transfer below the feed as the major objective;
• a subcooled liquid feed with fractionation below the feed as the major objective;
• a bubble-point liquid feed; and
• a mostly liquid feed containing a small amount (by mass) of vapor.
This list shows the services in order of increasing severity. A guideline offered for a more difficult service will work for the easier ones as well.
Heat transfer is a more forgiving service than mass transfer. By themselves, spray headers are effective heat transfer devices. If adequate pressure is available for sprays, allowing the liquid to enter the tower through a spray header may suffice. If the feed liquid rate is much higher (twice or more, for example) than the internal liquid rate, this system will meet the requirements for heat transfer. A reasonable amount of mixing will occur as internal liquid falls from the packed bed above into the feed liquid spray.
Fractionation requires more care in mixing the liquids. Never use sprays for mass transfer services. Instead, opt for a collector for the liquid inside the tower, a feed pipe for the liquid entering the tower, and an internal redistributor for the total liquid to the packing below. Fractionation works best with a smooth flow rate and even composition across the entire tower cross-section.
The piping arrangement must avoid having the feed liquid or internal liquid in only one spot across the tower cross-section. The external and internal liquids rarely have the same composition. Localized composition gradients will make fractionation less effective. In extreme cases, portions of the packed bed may dry out completely due to composition gradients.
If the feed is reliably subcooled, mixing it with the internal liquid will not form a vapor. This makes the equipment simpler as only liquid must be handled.
With bubble-point feeds, mixing the internal liquid with the feed liquid may cause some vaporization. Vapor inside gravity distributors may dramatically decrease their capacity. Because gravity liquid distributors have relatively narrow operating ranges, reduced capacity can seriously affect tower performance.
The most reliable way to de-gas a mixture is to mix the feed and internal liquid on a collector tray. The collector has sufficient volume to allow for de-gassing. An internal overflow then sends liquid to the distributor below the feed entry.
Increasing the amount of gas in the feed may mandate more complex approaches. If the feed contains gas before entering the tower, the best approach is to de-gas the feed, then mix it with the internal liquid. This may require a two-stage feed gallery.
Keep two key points in mind:
1. Pay attention to the internal liquid distribution device below the feed entry.
• Spray headers may be able to tolerate some reduced mixing.
• Gravity distributors require proper mixing of the internal and external streams.
2. Gas formation will mandate more-elaborate feed systems.
• Gas may form upon mixing the entering liquid with the internal liquid.
• The feed itself may contain gas.
