THIS MONTH'S PUZZLER
The horizontal counter-current-flow induced-draft finned air condenser on our debutanizer column can't keep up in the summer whenever the air temperature rises above 105°F. Instead of an outlet temperature of 115°F, which would ensure the exiting liquefied petroleum gas (LPG) is a saturated liquid, we get 125°F and the LPG leaves as a mixed gas/liquid. We're looking for ways to limp through until the next turnaround. We must get as much as possible out of the condenser because we are limited by space and load on top of the column. The column takes a feed from the fluid catalytic cracker at about 260°F. Naphtha is withdrawn from the bottom at about 360°F. The LPG — a mixture of propylene, propane, butanes, butenes and trace C5+ — leaves the top of the tower as a saturated gas at 155 psig and 142°F and goes to the air condenser. Under design conditions, the condenser should provide condensed liquid at 120°F to a water cooler, which then should discharge the condensate at 90°F. Our water cooler is designed for liquid LPG and is stressed at high temperature. Do you have any ideas on how we can improve the performance of this condenser?
SPRAY THE INLET AIR WITH WATER
First, check to see that the exteriors of the finned tubes are clean. If not, clean them. At a minimum, check exchanger performance to confirm that it is meeting design heat-transfer specifications. If not and the outside finned tubes are clean, consider possible fouling/corrosion on inside of the tubes.
Second, consider using water spray/evaporative cooling of the inlet air to the exchanger. The water source must be clean — use the quality produced by reverse osmosis (RO) or deionization (DI); and the water must be in vapor form when it gets to the tubes. Small drop size, spray or good separation will help ensure no free water gets to the tubes, motor or fan mechanism. Since the feed stream is coming from a fluid catalytic cracking unit, it's likely that the refinery will have a source of clean RO or DI water associated with its steam system. If not, RO units are available in many sizes almost off the shelf. The economics of this option will need to be evaluated, but it can be installed fairly quickly — especially if a clean water source is already available.
Lastly, look at the air flow path to the exchanger. Verify that the air entering the exchanger is at ambient temperature. It's possible that something has changed since original design that's causing the air source to be hotter. If this is the case, you may be able to make modifications to minimize heating the inlet air more than nature already has.
Fred E. Lewis, mid-con facility engineer
BP America, Houston, Texas
CONSIDER VARIOUS FACTORS
While hot air recirculation should be less of a concern for induced-draft-type air coolers, nevertheless check the possibility of this happening around the intake of the condenser. You mentioned there's a space limitation where the condenser is installed. Does the layout impose any constraint on the working of the equipment? Was a compromise made during the design stage?
It would be rational to assume that the thermal design of the condenser had considered the worst-case design scenario by using the hottest recorded summer air temperature with a reasonable design margin. There could be other factors worth considering:
1. Limited cooling points to exchanger fouling. Both the exchangers should be scrutinized. Finned-tubes can be fouled by several things including dust, leaves insects, etc. For the water-cooled type, check the "health" by looking at what the last inspection report said. If fouling is present, can you do something about de-fouling your exchangers and improving efficiency until your next big turnaround?
2. Does your condenser use louvers? Do they suffer from any mechanical defects that restrict air flow?
3. Check the problem history; use your control system historian. Did this problem (of limited heat transfer) come up only on hot summer days or does it persist?
Make sure that all basic process control instrumentation on this system is working as intended.
Muhammad F. Ghilzai, process engineer
Jacobs, Al Khobar, Saudi Arabia
EMPLOY A SPRAY BAR
Consider adding a water-injection system to provide some additional cooling to the air side of the condenser. The horizontal arrangement may make this a bit tricky, but one could experiment with a fine mist of demineralized water injected into the air inlet.
I had occasion several years ago to design a "spray bar cooler" on a Friday morning that could be put in service before the end of the day on an air-cooled fin-fan exchanger. The design consisted of three sections of ¾-in. pipe with the smallest holes we could drill (~1/16–3/32 in.) spaced about 3 in. apart. Each section of pipe was 8-ft long to span the length and width of the exchanger bundle and was connected with 90° elbows to form a U-shaped spray bar. We hung the spray bar from the bottom of the fin-fan below the finned coils, surrounding the fan motor, and connected the supply end to a hose from the demineralized water header that ran at about 100 psig. When we turned on the water, it began to rain from below in the fin-fan structure and we managed to pick up approximately 10° of additional cooling in the outlet of the process gas. I'm not sure if the system provided evaporative cooling or just improved cooling duty from the mass of water versus air, but it worked well enough to be used several more times when needed.
The induced-draft fan must be capable of handling the water vapor/mist that will be pulled through it. Thermal shock to the tubes themselves should also be considered to determine the risk of pulling a tube out of the tube sheet. In our case, the fin-fans were subject to heavy rain events that rapidly changed operating temperatures (which gave us the idea in the first place) so the reliability risks of making it rain were deemed acceptable.