Maybe, the kettle reboiler is a better choice if it can be replaced or refitted; certainly, it would be better to have two reboilers if fouling is an issue.
Another problem is the fractionation bottoms pump. From the description of lost head and vibration, it seems that some kind of erosion/corrosion is at work. Take this pump out of service immediately for non-destructive and destructive testing. A lined replacement should help; however, a closed impeller may be a better choice because it provides better protection to the leading edge than an open impeller. A good design practice is to maintain a velocity in the 9–12-ft/s range in all exchanger tubes to prevent fouling; this unfortunately requires hardening the material of construction to survive the erosion. Erosion is proportional to the cube of velocity.
Dirk Willard, senior process engineer
Superior Engineering, Hammond, Ind.
We're trying to settle a dispute between our on-site contractor and the company designing a vent scrubber for our alkylation spent acid tanks (see Figure 1). The design basis is a flow of 3,500 pounds per hour (pph) of a stream containing H2S (about 60 pph), N2, water, O2, 1,3 butadiene, 1,4 pentadiene, acetaldehyde, formaldehyde and other components from the tanks to a caustic scrubber and activated carbon bed (ACB) before going to a stack. This composition seems inaccurate but it's what we have. The gas molecular weight is 29.7 going into the scrubber and 29.1 exiting it. The initial pressure is only 3.2 in. water column (WC); the designer claims the pressure drop through the scrubber will be only 1 in. WC and that the drop through the ACB will be 0.5 in. WC. The designer proposes using a 3-in. schedule-40 carbon steel vent to convey vapor from three spent H2SO4 tanks, a distance of 175 ft.; the same 3-in. pipe will carry fumes 250 ft to the scrubber and then to the ACB 80 ft away; the stack is 120-ft high and 24-in. in diameter. The design doesn't include a blower or fan because of the fire risk, which also is why the firm chose steel rather than fiberglass pipe. It also lacks a flame arrestor before the scrubber; the designer says API-2210 doesn't require one because the existing conservation vent will provide sufficient protection. However, the contractor contends the conservation vent must be re-specified for the proposed design changes, the 3-in. pipe won't allow 3,500 pph, carbon steel will corrode and is, in fact, unnecessary, i.e., fiberglass will do because the vent isolates the vent system from flash back (agreeing with API-2210). The contractor also argues the pressure drops in the scrubber and ACB are too low. What do you think? Does the design need changing? If so, how?
Send us your comments, suggestions or solutions for this question by July 12, 2013. We'll include as many of them as possible in the August 2013 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.
And, of course, if you have a process problem you'd like to pose to our readers, send it along and we'll be pleased to consider it for publication.