For a packed stripping section not performing as well as required, check if you need better vapor distribution to the bed. If so, consider a steam sparger. Such a unit installed below an 8-ft.-diameter packed section improved effective packing performance from nearly 0% efficiency (zero stages) to two stages of separation.
How active are your trays?
Normally we assume tray flexibility ranges of 2:1 to 3:1 for sieve trays, and 3:1 and higher for valve trays. Many people think bubble cap trays have operable ranges of up to 10:1. These flexibility ranges generally do hold for single-pass trays with normal-to-high pressure drops, that is, a minimum of 0.05 psi to 0.06 psi per tray on a 2-ft. tray spacing. However, multiple-pass trays and those with low-pressure-drop designs have much lower vapor-handling flexibility. The clearest way to explain this is to look at multiple-pass trays.
Once bubbling starts on a specific spot on a tray deck, the fluid on the deck has a lower density than still liquid with the same depth next to it. So, the next increment of vapor flow tends to go through the tray where vapor bubbling has already started. If the vapor load is high enough, the entire tray surface is active. In contrast, at low vapor loads vapor tends to channel through one area rather than spreading out across the tray. Multiple flow passes for the liquid make the problem more severe. Table 2 shows standard values for minimum area of the tray that must be active to prevent severe bypassing .
Table 2. A greater number of flow paths poses greater demands for tray active area. (Click to enlarge). Source: Ref. 6.
Efficiency suffers if not enough liquid contacts the vapor. Small efficiency penalties might have been tolerable when energy was cheap but not now. So, even a modest gain in efficiency may justify some minor modifications to tray decks to improve activity. Consider blanking strips, blocking valves down, and even replacement tray sections to increase efficiency.
Have you checked basic heat integration?
Never presume that a plant’s current heat-integration configuration is logical, let alone optimal. Consider the quirky scheme used in a solvent recovery plant (Figure 6).
Figure 6. The scheme boasts an unusual element and the opportunity for significant improvement.
The overhead is heat integrated after the first cooling-water exchanger, which is really unusual and has never been adequately explained. Also, while the overhead is heat integrated, neither the hot vapor sidedraw nor the tower bottoms streams have heat recovery.
The fix (Figure 7) was to reconfigure the overhead system and add heat recovery.
Figure 7. Investment in reconfiguration and added heat recovery was paid back in a few months.
The sequence of overhead exchangers was reversed to heat-integrate before going to cooling water. One existing hot-oil exchanger was converted to feed preheat versus the vapor product draw. Finally, a new heat-recovery exchanger was added to the bottoms stream (10% of feed flow). One hot-oil exchanger was left on the feed stream (E3) to make the unit easier to start-up. Although the unit has a feed rate of only 360 gal/hr the heat duty was reduced by 3 million btu/hr. The changes involved a capital expenditure of less than $100,000 and yielded a savings of $280,000 per year.
Many other plants can take advantage of simple heat-integration steps between tower feed and tower bottoms. Don’t overlook these relatively easy ways to save money.
Simple steps can save energy
Regretfully, energy savings alone, despite today’s high energy prices, rarely justify major investment at plants. However, you can improve the energy efficiency of distillation columns without spending a lot of money. The small steps outlined here don’t involve much capital or process risk. This makes them more likely to get done.
Andrew Sloley is principal engineer for VECO U.S.A. in Bellingham, Wash. He also writes the Plant InSites column in Chemical Processing. E-mail him at ASloley@ putman.net.
- Tolliver, T.L. and L.C. McCune, “Finding the optimum temperature control trays for distillation columns,” InTech, 27 (9), p. 75 (Sept. 1980).
- Sloley, A.W. and G. Martin, “Process modeling for control system design and analysis,” presented at IASTED Conference on Modelling, Simulation, and Control in the Process Industry, Ottawa (May 1994).
- Shinskey, F.G., “Distillation control for productivity and energy conservation,” McGraw-Hill, New York (1977).
- Kister, H.Z. and I.D. Doig, “When would floating pressure strategy save energy?,” Chem. Eng. Prog., 77 (9), p. 55 (Sept. 1981).
- Sloley, A.W., “Sidestep side-draw control surprises,” Chem. Proc., 67 (7), p. 33 (July 2004).
- Sloley, A.W. and B. Fleming, “Successfully downsize trayed columns,” Chem. Eng. Prog., 90 (3), p. 39 (Mar. 1994).