September's Puzzler | August Issue
We are designing a desulfurization process for a client overseas. The client produces a wet coke gas that must be treated for COS, H2S, and HCN before it can be used to make chemicals. A knock-out pot serves double duty — collecting the circulating spent slurry and vapor from the client’s gasification plant. The vapor flow is 50,000 lb/hr and the liquid flow is 519 gal/min. The pot operates at 200°F and 560 psig with a barometric pressure of only 12 psia. A k of 0.27 was used with a velocity allowance of 0.15 to size the tank for vapor separation. The residence time is set at 30 minutes. The calculated diameter is 13 ft with a length/diameter (L/D) ratio of 3.4. The vapor passes through a mesh pad demister that is only partially used because of low vapor flow. Several problems are anticipated with the design: the liquid drains to atmospheric; foaming sometimes occurs, disrupting level measurement; the mesh pad fouls every four months; and the process water used on a timer to keep the pad clean may contain particulates. How would you improve the design?
Address the process issues
For some problems, the best you can do is to figure out how to live with the least bad situation rather than being able to find a solution. Your options depend upon the cost the client is prepared to spend. Some ideas to check the economics of include: vortex tubes for foam reduction; staged clean water washes; vane mist eliminators in series with the mesh pads; multiple vapor exits; extra vessel elevation; overlapping level instruments; nuclear level instruments; and weight cells.
Vortex tube clusters (VTCs) take advantage of direction changes to help collapse foams in a multiple-phase stream. If the source of drum foam is the feed, using VTCs may aid in reducing the amount of foam in the drum.
If the process will tolerate occasionally using a clean wash, then putting in a second wash system to extend the time between plugging on the mist eliminator pads may be justified. Perhaps one wash in four or five might use clean water.
A minimum velocity is needed for wire and vane mist eliminators to effectively remove mist. The higher the pressure drop the smaller the droplet removed. Fouling occurs from the process liquid trapped by the eliminator or particulate in the wash water. The challenge is balancing pressure drop, fouling tendency and efficiency. You could consider using a correctly sized vane eliminator followed by the over-sized wire mist eliminator. The vane mist eliminator has a much greater tolerance for fouling because of its large passages. Of course, this will require a much fancier water-wash system. The purpose of the vane mist eliminator is to reduce the loading of process liquid in the wire mist eliminator. This may extend the mist eliminator run before it plugs.
Multiple vapor exits may also be usable. With the correct valves you could have two or three vapor outlets. If each has a run of four months or so, you have effectively extended the vessel service duration between shut down and cleaning cycles. A single feed in the center of the drum and a vapor outlet at each end would be a good first choice. Of course, if you have less risk exposure to premature shut-down, you may also consider making one end of the vessel correctly sized rather than over-sized for the mist eliminator used.
Extra vessel elevation may be critical in getting liquid out of the drum. If the contents are foam rather than clear liquid, then the static head available for liquid draining is much less. Extra height (and larger exit nozzles) will prevent many gravity fluid-draining problems.
Level indication with foam is notoriously difficult. The first method to deal with liquid level problems is to use something like a displacer or a differential pressure (dp) cell for level measurement and to use more than one of them on multiple, different ranges. If the different level instruments show multiple liquid levels, the lower instruments are probably completely flooded with foam. Instead of measuring level they are measuring an average liquid density across their range. The actual foam level will be in the range of the highest level indicator showing a level. While not perfect, this can be a vast improvement over a single level instrument.