Use an emulsifier
The problem with the clogging of the column parts could be solved by adding an emulsifying agent to the feed, so that it may retain some more of the components that keep it fluid at the end of the run. This emulsifying agent could be selected simply based on achieving this goal or on just modifying the surface tension of the bottoms so that they can be pumpable out of the system. There may be a drawback to this solution in that the tower may loose some of its efficiency to remove the desired distillate, as some of it will be retained with the emulsifier in the bottoms.
Luis Alejandro Betancur González, process engineer
Research on Industrial Process, Inc., Medellín, Colombia
Look at the exchangers and pre-heater
There are two classes of problems I can discern from the puzzle description: operating difficulties and maintenance issues. Of the two, the easiest to solve are those involving maintenance. Let’s consider maintenance first. The first problem is the choice in heat exchangers.
These appear to be TEMA Class BEM units, which are difficult to maintain and do not belong where steam is used. B-E-M stands for integral bonnet cover (B) with a single shell pass (E) and a fixed tube-sheet (M). As a rule of thumb, allowance should be made for thermal expansion where the temperature difference between the tube-side and shell-side fluids is greater than 70°F (39°C). This has been stretched to 100°F but it is better to err on the conservative side.
A better choice might be TEMA Classes BEP, BES or BET. It is worth noting that expansion joints should be installed on most equipment where there is a similar potential temperature differential but they could be missing due to an oversight by a lazy CAD-driver. There are other self-inflicted wounds festering in the design of this column.
If the oil is being burned and causing a clogging problem in the column it may be due to poor temperature control in the pre-heater. With the steam plant at a distance, it could be that the column is not receiving its share of saturated steam but is getting a mix of condensate. The temperature control valve on the pre-heater could then be swinging too far open and isn’t operating in its design range. An over-zealous operator running the loop in manual could have a similar detrimental effect. The venturi meter could be used as a tool if it’s reading high during start-up since it measures fluid velocity; condensate will partially block the tube. Another approach would be to do a material and energy balance around the pre-heater to determine the true enthalpy of the steam. A few process changes might eliminate the burning problem.
One approach might be to substitute the oil with another solvent that’s less temperature-sensitive. Another could be to add the oil directly to the column bottom. Mixing could be a problem, but this might be addressed by spraying or adding an in-line mixer or static mixer. Perhaps the reboiler pump could be used. The idea is to give the oil time to heat up rather than allowing it to be scalded by hot feed. A column simulation would make it easier to determine the optimum solution(s). There are a few more drastic measures for reducing fouling, at least in the column. Consider increasing the sieve size in the trays. Again, a simulation is recommended to evaluate the effect of this option on product quality.
Now, let’s look at the process problems. Clearly, the oil is doing its job. The impurity is being removed but the process is not designed to handle the quantity of solids collected in the reboiler feed tank. The evidence of a problem with solids is the way a control engineer chose to measure level in the reboiler feed tank: a nuclear level transmitter. This method is a favored choice when sludge build-up rules out most other modes.
Measuring the solid is easier than managing it. Years ago, when a similar situation came up I suggested an automated strainer; my boss went with a suction strainer. With the suction strainer, which was an integral part of the pump, the reboiler pump had to be replaced and up-sized. The automatic strainer provided some of its own pressure so it was debatable whether the reboiler pump needed to be replaced. Mechanical reliability was in doubt with the discharge strainer because of the severe corrosion.
Our choice of materials was usually Inconel 600. Neither pump solution would eliminate the need for collecting and land-filling a wet solid from the bottom of the feed tank. Unfortunately, little progress seems to have been made in automating drying technology especially in applications involving dirty or corrosive solids. My preference would be a vacuum dryer but it is only good for a maximum of 40% removal. Vacuum dryers require little operator attention, at least compared to other drying methods.
Dirk Willard, process engineer
GTP, Schaumburg, Ill.
A spray condenser tank uses a float as a level instrument. Metal chlorides settle in the tank and are eventually pumped to waste treatment. Unfortunately, solids build up, especially during start-up, causing failure of the float. Last year alone, eight floats were replaced. Two months ago, two floats were replaced during a single outage, causing more than 30 hours of unexpected downtime. The current instrument is loop-powered. There are no additional points left in the PLC for self-powered instruments. Can you propose alternatives for measuring level?
Send us your comments, suggestions or solutions for this question by September 7, 2007. We’ll include as many of them as possible in the October 2007 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to ProcessPuzzler, Chemical Processing, 555 W. Pierce Rd., 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.