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Random Packing Hold-Downs

How much do hold-downs block vapor flow

Petroleum refinery FCC main fractionator

 

 

Subject: Packing Hold-down
Date: Mon, 9 Jan 2001 11:23:32 AM +1000

Andrew,

I am writing to ask a quick question. We're putting a new hold-down grid in the top of our FCC fractionator column and the open area will drop from 90.5% (existing grid) to 83.0% (new grid). I've been hunting through the texts I have on distillation design to try and find something on recommended, or typical, open area.

Unfortunately, I haven't found anything specific to this. Can you provide some advice? My thought is that the open area hasn't really changed significantly (<10%) so it shouldn't be an issue in terms of limiting capacity but now I've looked into it I'm interested to find out what typical values are.

L., Australian Refiner

Subject: Packing Hold-down
01/08/2001 8:59 PM -600

L.,

I assume you are talking about the FCC main fractionator. The answer for open area depends upon many factors. In general, hold-downs limiting tower capacity are very rare. However, in one case being presented by two engineers from Westlake Petrochemicals and I at the next AIChE meeting, blocking 8% of the tower area caused a major capacity problem.
Evidently, if 90.5% open area is working, then it's not a problem today. However moving to an 83% open hold-down may create a problem depending upon the packing size and tower loading.

If you have additional information, it would help give a more precise answer. The major questions are:

  1. It is the top of the FCC main fractionator?
  2. Why are you replacing the existing one?
  3. Is the bed a fractionation bed or pumparound?
  4. What type of distributor is above the bed (spray, pan-orifice, trough)?
  5. Is the packing structured, random, or grid?
  6. What size and brand of packing is in use?

Andrew Sloley
DGI

Subject: Packing Hold-down
01/09/2001 04:57:49 PM +1000

Andrew,

I have answered your questions below:

  1. It is the top of the FCC main fractionator? Yes - the hold down gird replacement is for the top of the FCC main fractionator.
  2. Why are you replacing the existing one? We are replacing the existing one as it has failed in the past (spot welds joining the "mesh" material together failed and also rings escaped around the periphery of the grid) and resulted in rings in the overhead fin-fans (a nightmare cleanup job!). The vendor that supplied the original grid and has recommended the new grid design (which is their standard hold down grid now).
  3. Is the bed a fractionation bed or pumparound? The bed is the top bed which is the top reflux (liquid from the overhead drum).
  4. What type of distributor is above the bed (spray, pan-orifice, trough) Trough distributor.
  5. Is the packing structured, random, or grid? The packing is random.
  6. What size and brand of packing is in use? The drawing shows this.

I have attached a copy of the vendor drawing sent me for approval. They also supplied the open area values (I didn't calculate them myself).

(See attached file: xxx.tif)

Regards,

L., Australian Refiner

Subject: Packing Hold-down
01/09/2001 2:10 PM -600

L.,

Summary

The best hold-down you can build depends upon if the packing is random packing or structured packing. Random packing requires some sort of mesh to hold the packing in the bed. For random packing, about the best hold-down you can get is a 90% open area.

For a structured packing hold-down you are looking at a maximum open area of around 95%. Special hold-down designs can increase the open area for structured packing.

Hold-downs limit capacity by restricting the flow of liquid down the tower and the flow of vapor up the tower. If the hold-down has lower capacity than the packing, it will limit tower capacity. Whether an 83% open area restricts the packing depends upon the packing size, vapor load, liquid load, and system.

Your system has relatively low liquid load, low operating pressure, low velocity liquid entering the packing and few foaming characteristics. All these improve the tolerance for low open areas. The vendor drawing shows the packing under the bed as a third-generation packing with a nominal size of two inches (50 mm). An 83% open area hold-down is unlikely to be more limiting than the packing under the bed.

Details

Random packing hold-downs keep the rings in place. One major reason is to keep the packing elements from getting into downstream process equipment. Other important reasons include preventing the bed from fluidizing at high vapor rates and to prevent the rings from plugging other internals inside the column.

Fluidized packing beds have reduced efficiency and capacity compared to stabilized beds. When high vapor rates fluidize a bed, the packing elements no longer contact each other. Liquid cannot trickle from one element to the next. The liquid has to free-fall the gap between elements. Since most liquids prefer to wet a solid surface rather than form drops, the liquid hold-up inside the packing increases when packing elements do not touch each other. This reduces capacity. Droplets of liquid moving from one packing element to another are re-entrained by the vapor, this reduces efficiency.

Loose packing inside columns is an often documented cause of unit failures. Additionally, loose packing clogs strainers and control valves and damages pumps and other downstream equipment.

Figure 1 shows a sketch of a typical random packing hold-down. A mesh holds the packing elements in place. A grid over the mesh holds the mesh in place. Depending on tower diameter, beams over the grid hold the entire assembly solid. The beams and/or grid attach to the vessel wall with clips or beam seats. Many different mechanical configurations are available. Figure 1 shows the grid and beams integrated together to hold the mesh down.

Figure 1
Random packing hold-down

Figure 2 shows a sketch of a typical packing hold-down for structured packing. An open grid holds the packing in place. Depending on tower diameter, beams over the grid hold the entire assembly solid. The beams and/or grid attach to the vessel wall with clips or beam seats. Figure 3 shows a single element of the grid sized to fit through a man-way.

Figure 2
Structured packing hold-down

Figure 3
Manway size element of a structured packing hold-down

The open area is the product of the open area of each layer. Starting with the mesh, Figure 4 shows a scale drawing of several different mesh layouts suitable for your service. Both a 1x1 (25x25) wire mesh and a 1.5x1.5 (38x38) wire mesh are shown. Roughly, the open area varies from 86% to 92% open. Figure 5 shows a scale drawing of an often used grid layout. It is 96.5% open. Combining the open values from the mesh plus the grid gives a range of 83% to 89% open. Required beams would block even more of the tower cross section.

Figure 4
Scale mesh drawing

Figure 5
Scale grid drawing

Adding in extra members for support and the need to make parts that fit through a manway decreases the open area. As a practical matter, it is very difficult to make a hold-down that has an open area above 90%. In fact, 78% to 83% is a much more realistic range for most applications.

Andrew Sloley
DGI

Images have been sized for full screen display on an 800x600 monitor.

This page updated 30 January 2001.
© 2001 Andrew W. Sloley. All rights reserved.