Industry depends upon the data and correlations developed by Fractionation Research, Inc. (FRI) (see sidebar). FRI generates performance data on mass transfer equipment under various conditions in two commercial-size distillation towers (Figure 1).
Figure 1. FRI has both high- and low-pressure commercial-size distillation columns.
If a tray or packing isn’t properly installed, the data and any subsequent correlations won’t reflect the true nature of the device being tested. So, FRI’s installation personnel have learned how to install distillation equipment with the skill and care necessary to measure correct performance. In this article, we share some of that practical knowledge. It will help you make sure that the tower you are erecting will work as expected. The advice also applies to absorbers and strippers.
First, unless you or your installers are familiar with installing trays and packing, it is best to have the equipment vendor’s representative on hand during the equipment installation. At a minimum, the representative should be present to inspect critical items as they are installed. Now let’s look at what should be done both before and during installation.
For a new tower, start at the fabrication shop. There, check:
- tower internal diameter, especially around nozzles and manways;
- tangent-to-tangent measurement;
- location of wall clips;
- location of support rings;
- levelness of clips and rings; and
- removal of scale and dirt
After erection, check:
- roundness (large diameter towers can become elliptical);
- tower verticalness; and
- for scale and dirt (especially for packed towers with small-orifice liquid distributors).
A tower that is out-of-round by more than that allowed for by the trays’ and internals’ designs can make their installation difficult. It’s best to find this out before starting installation. Out-of-roundness occurs especially in the area of the manways and nozzles. Be prepared to do field modifications to some of the hardware during installation.
For an existing tower, do the same pre-installation checks. Tower roundness can change during years of operations and it’s best to know this during the equipment specifying stage.
Installation for packed towers
When dealing with packing, whether random or structured, you must pay close attention to the installation of five key elements:
- support plate;
- packing retainer;
- distributor; and
- feed pipe.
Support plate. Random packing support plates normally are multi-beam-type units (Figure 2) and usually are set on a continuous support ring.
Figure 2. This unit typifies the multi-beam design for mounting upon a support ring.
It’s important that the support beams fit the ring correctly so they don’t fall off the ring during vapor flow surges. If an upset is possible, it’s a good practice to clamp the support plate to the support ring. Because the top of the random packing bed can be leveled to maintain the specified clearance between the top of the packing and the liquid distributor, levelness of the ring isn’t critical; so, normal construction tolerances are acceptable.
Structured-packing support grids usually are flat-bar-type units (Figure 3).
Figure 3. Structured packing commonly rests upon a flat-bar-style support.
The support grid may be set on a ring or attached to wall clips. Take care to make sure the ring is level to within vendor specifications — if it isn’t level, this error carries all the way to the top of the bed, which may interfere with liquid distributor installation above the bed. If the support grid is set on a ring, consider clamping it to the ring if severe vapor surges are possible.
Random packing. Metal random packing normally is poured into a tower from boxes or bags. The vertical distance a metal packing can be poured is about 20 ft. However, the exact distance depends upon the packing shape and the gauge of material from which it is made; so, always check with the vendor. Don’t crush the packing into the support plate. A good practice is to lower the first packing onto the support until the plate is covered with a foot or more of packing. Take care not to introduce dirt and particulates into the tower during installation because they may plug up the liquid distributors after startup.
Also, make sure to evenly fill the tower. The creation of void spaces in the packed bed or at the packing/tower-wall interface can lower the apparent packing efficiency. The packing can be leveled as you go by spreading the packing around as it is poured or by using brooms and rakes to level it. It’s best not to walk on the packing to install it. If unavoidable, use boards to spread the weight of the person. Check with the packing supplier about the proper way to stand or walk on the random packing.
Plastic random packing can be handled in much the same manner as metal. However, you must make allowances for the material. The allowable drop height can change with the ambient temperature because a plastic material’s brittleness increases with decreasing temperature. Plastic packing also has a lower upper operating temperature limit than metal packing. Consult the supplier about temperature limitations of the plastic being used and how that affects drop height, general handling, and operating temperature.