Subject: Structured packing applications
Date: Sat, 6 Jan 2001 02:54:24 -0800 (PST)
Thank you for your previous e-mail about vacuum tower revamps. Also many thanks for the very interesting technical papers that have been sent to me. I have a question about structured packing application and performance in different industrial uses. Have you any experience of using of structured packing in light material separation (such as light and heavy straight run gasoline fractionation, LPG unit fractionation such as de-c2, de-c3, de-c4) ? Are there any structured packing applications for medium-to-high pressure distillation (for example 2-30 bar g)?
R., Asian Refiner
Subject: Structured packing applications
Date: Sat, 6 Jan 2001 03:39 -0600 (CST)
In general, the deciding issue for structured packing is more a question of vapor density and liquid density rather than system pressure. Three criteria control the selection of structured packing: (1) liquid load, (2) vapor density, and (3) liquid-to-vapor density ratio. For distillation systems, the vapor density and the liquid-to-vapor density ratio are strongly linked.
The lower the vapor density, the better structured packing performs compared to other devices. The higher the vapor density, the better trays or random packing perform compared to structured packing.
For hydrocarbon systems the selection criteria for vapor density and liquid-to-vapor density ratio where structured packing has few, if any, problems are:
Structured packing limitations imposed by high-liquid rates requires a more complex explanation than justified here. Briefly, the definition of high liquid load varies greatly with packing geometry. Liquid rates greater than 20 gpm per ft2 (~53 m3/m2-hr) for a one-inch (25 mm) crimp structured packing should be used with caution.
As a practical matter, a C4 separation at 165 psia (1,140 kpa) has a vapor density of 1.81 lb/ft3 (29.0 kg/m3) and a liquid-to-vapor density ratio of 17. Under these conditions, structured packing capacity has already fallen about five percent from its maximum level. For hydrocarbon systems, structured packing should be restricted to applications with system pressures below 165 psia (1,140 kpa, 10 bar g) or lower. DGI rarely recommends structured packing in hydrocarbon systems above 115 psia (690 kpa, 7 barg).
These limitations make structured packing a poor choice for most refinery gas plant and LNG gas plant towers. Field experience up to 400 psia (2,760 kpa) has shown actual capacities of only 50% of expected values. Efficiency also dropped dramatically under these conditions.
The high-pressure exception where structured packing has proven its value is when the liquid density is independent of system pressure and remains high at all pressures. Glycol dehydration and amine sour gas scrubbing are excellent structured packing applications.
While extensive operator experience has shown very poor performance with structured packing at high pressures, this has not stopped some vendors from continuing to propose its use under these conditions. A recent troubleshooting assignment with structured packing at high pressures was a sulfuric acid-catalyst alkylation unit with structured packing in the deisobutanizer.
Conventional trays were replaced with structured packing. In addition, the structured packing was in very deep beds (> 30 ft, 9 m). The design objective of the revamp was to increase tower capacity by 40% over conventional trays. Alternatives for reaching 28% with high-capacity trays were proposed. However, one vendor promised the 40% capacity increase with structured packing. Instead of achieving the increase, the capacity dropped by 15% with structured packing. The unit was shut down, packing removed, and high-capacity trays installed: at great cost.
Various reasons have been proposed for why structured packing fails at high vapor and low liquid densities. Figure 1 and Figure 2 show exploded views of some structured packing blocks.
The packing blocks are made of these layers nailed or welded to each other. Liquid flows down the surface of the packing and vapor rises through the channels of the packing (Figure 3).
Experimental work has shown that at high vapor densities and low vapor densities the rising vapor pushes the liquid back against the direction of liquid flow. The kinetic energy and momentum acting through the surface layer on the liquid film becomes significant.
Pushing back the liquid obviously hurts packing efficiency. Heavier liquid is pushed back and mixed with lighter liquid. Pushing back liquid also makes the liquid film on the packing thicker, reducing effective capacity.
High liquid loads make the liquid film on the structured packing thicker, reducing the area open for vapor flow. This increases vapor velocity. High vapor velocities make back-mixing more likely.
These affects were not well understood in the first installations of structured packing. Many tower failures resulted. Less justifiable are continued attempts to apply structured packing in industrial services where known failures have occurred. Before putting structured packing into any high-pressure service (> 165 psia, 1,140 kpa, 10 barg), the system should be tested and proven in a reasonable size test column. For these purposes, a reasonable size test column is a minimum of 12 in (30cm) in diameter and preferably 18 in (45 cm).
The Distillation Group.
 From Chen, G.; Kitterman, B. L.; Glaspie, D. L.; Axe, J.
R. Tower packing material and method. US patent 4,604,247. 5 August
 From Bucholz, M.; Yeoman, N.; Mattke, F. E. Nested packing for distillation column. US patent 5,413,741. 9 May 1995.
No performance, suitability for use, or lack of suitability for use for any given process service is implied to any particular model or brand of packing by these comments. Figures used have been used as illustrative of generic classes of equipment.