September 2008 Process Puzzler: Kayo a Drum Problem

Readers suggest ways to improve the performance of a pot

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If solids, corrosion or service conditions prevent multiple level instruments, then a nuclear (gamma ray) device may work well. While it is much more expensive, the cost may be justified. Properly set up, gamma ray level instruments can separately show liquid level and foam level.

Finally, in severe cases, weight cells have been used to infer levels in extremely foaming systems. This is a last ditch effort as weight cells on large vessels can be complex to set up. They can also give false readings due to reactions from piping stresses. You are no longer measuring level but system mass inventory directly. From an assumed foam generation rate, you set a maximum system weight acceptable. I prefer nuclear devices to these but in some situations they do work.
Andrew Sloley, principal engineer
CH2M HILL, Bellingham, Wash.

Examine the design carefully
There are a number of problems in this design starting with the k. A k of 0.27 is probably too high for good separation at 560 psig. The Gas Processors Suppliers Association (GPSA) Engineering Data Book suggests de-rating k by 0.01 per 100 psig above 100 psig. This would provide a k of about 0.21 not 0.27. Decreasing k will increase the area needed for separation.

The velocity allowance (de-rating) of 0.15 seems correct for a foam problem; this term is multiplied by the allowable velocity to calculate the area. A barometric pressure of 12 psia corresponds roughly to an altitude of 4,200 ft above sea level. A low barometric pressure increases the required separation area less than 1%.

The residence time seems a little short. Given the flow involved, 519 gal/min, additional capacity might be desired since this drum appears to be the center of the desulfurization process.
Corrosion with H2S, HCN and other compounds may be a problem. A corrosion engineer should review the selection because an attack on the metal might be anticipated at the phase interface. Review NACE International standards for H2S service.

The selection of an L/D of only 3.4 seems wrong. “Coulson and Richardson’s Chemical Engineering” recommends a ratio of three for pressure up to 165 psig; above this pressure the ratio should be five. The reason for the selection is the high cost of a large diameter at high pressure. At 560 psig, a 13-ft diameter head would be more than four-in thick. Welding a thick plate is more of a challenge than welding a thin plate. If length is a problem because of space, a vertical separator, though less efficient than a horizontal separator, can work in most applications. Here’s another reason for a longer vessel: the capture droplet size is smaller. The vapor enters one end of a horizontal drum and exits the other after passing along its length. The longer it is, the more efficient it will be in settling liquid.

Now that the design has been critiqued, let’s consider the three process problems mentioned: de-pressurizing the liquid drained from the drum; reducing the effects of foaming; and avoiding the necessity of cleaning the mesh pad.

De-pressurization can be dangerous, especially at high pressure. Another concern is the wear on the orifice plate, the cheapest device available to reduce the pressure. One solution may be a reducing pot. Certainly more than one plate should be used for safety.

Foaming could be addressed with an additive but perhaps the best approach is to work with a reliable alternative like nuclear or a bubbler. Bubblers are the cheapest option and can work satisfactorily in foams and slurries if you can find a high pressure gas.

The best solution for the mesh pad is probably to be conservative on the tank design, allowing more settling time, and replacing the pad with a chevron. Chevrons can serve for many months without cleaning or plugging.

Dirk Willard, senior process engineer,

Ambitech Engineering, Hammond, Ind.

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