"Should we use packing or trays?" That's a critical early question when building a new distillation column. Specific services with special requirements may make a relatively unusual type of tray or packing the best choice. However, some general guidelines often apply. Table 1 summarizes these, with choices notably better for a given factor marked with a "+" and those particularly problem prone with a "-".
Let's now look at some of these factors:
Lowest cost, new tower. For a given capacity in a new unit, trays are nearly always the least expensive option if they are suitable for the service. The entire system, trays plus vessel, usually is the cheapest for sieve or valve trays with tray spacing in the 18–24-in. range.
Small diameter. Below 24-in. diameter, tray construction becomes difficult. You can use cartridge tray assemblies if trays absolutely are required. Liquid distributor fabrication also becomes a problem and can lead to compromises that reduce many of the benefits of structured packing. Random packing typically is a good choice.
Large diameter. Trays greater than 360-in. diameter can accumulate significant hydraulic gradients or may require complex multiple-pass designs. Opt for packing and ensure proper distribution.
High pressure. This gives high vapor density. Low liquid-density/vapor-density ratios tend to create backflow in packed beds. Capacity often is much lower than expected. Above a vapor density of 1.5 lb/ft3, problems can become extremely severe.
Vacuum and pressure-drop-sensitive systems. Pressure drop is nearly always critical in vacuum systems. In systems sensitive to pressure drop, structured packing is by far the best option.
Low liquid/vapor ratio. Such a ratio favors packing. In particular, packing is a much better choice if liquid loads decrease below 1 gal/min/inch of weir length.
High liquid/vapor ratio. Well sealed (often welded) bubble cap trays and random packing are the better options. At low vapor rates, the decreased turbulence in structured packing can reduce efficiency.
Flexibility. In general, packing can handle wide vapor-rate ranges. However, packing distributors are limited to fairly narrow liquid-rate ranges for effective performance. If more than a 2:1 range is required, strongly consider trays. Sieve trays can go to 3:1 ranges and a well-conceived valve or bubble cap tray can handle a 10:1 liquid-rate range (but at a cost in pressure drop).
Foaming. Often you have no good choices. Minimizing turbulence in vapor and liquid mixing keeps foam formation down. Gravity-flow distributors on structured packing tend to have the lowest foam-formation rates.
Expensive metallurgy. Economics favors devices with the least mass. Per unit of fractionation, structured packing uses the least metal. Bubble cap trays require significantly more than other choices.
Ceramic and plastic. Only non-metal packing is available — in both random (rings or saddles) and structured versions.
Solids, non-sticky. Hard solids favor sieve trays. The best solution is to keep the solids moving. You can use special designs such as trays without downcomers (dual flow trays) to eliminate dead spots in the flow patterns where solids can settle.
Solids, sticky. Specialty equipment such as baffle trays (often called disk-and-donut or shed trays) can cope with significant concentrations of sticky solids. Efficiency tends to be low and these trays work best at high liquid rates.
Multiple products and feeds. Situations calling for drawing or feeding vapor or liquid streams at the side of a column dramatically favor tray towers. By design, tray towers have vapor spaces often suitable for feed entry. You can easily add sumps and draw nozzles as well. In comparison, packed towers require special feed distributors, liquid collectors and other internals at every draw or feed location.
Low holdup. When you must minimize liquid holdup, such as in systems with unwanted reactions present, select structured packing or grids.
High holdup. Some systems, such as reactive distillation columns without solid catalyst beds, require a certain amount of liquid holdup. Here, bubble cap trays are the best option. With proper mechanical design, you can extend holdup to nearly any height.
Sacrificial internals. If a process destroys or irredeemably fouls equipment, you may need to consider the vessel internals as sacrificial. At this point, lowest overall cost (purchase, plus installation) starts to dominate selection. If the service permits their use, plastic rings, followed by aluminum rings, offer the best economics.
These guidelines give a reasonable starting point for internals selection. However, several factors may compete with each other. Experience and judgment in balancing competing factors usually dominate the decision.
ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can e-mail him at Asloley@putman.net