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 The column takes a feed from the fluid catalytic cracker at about 260°F. Naphtha is withdrawn from the bottom at about 360°F. The LPG — a mixture of propylene, propane, butanes, butenes and trace C5+ — leaves the top of the tower as a saturated gas at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.
Other potential cooling methods might be vaporized nitrogen injection into the air inlet to lower air temperatures (beware of the asphyxiation hazard) or chilling the inlet air to the condenser. This wouldn't be cheap but might pay out in process throughput. It appears you only need to cool the inlet air to less than 105°F to maintain full production capability.
As always, thoroughly consider the process safety management risks associated with making changes before implementing any changes to your process equipment.
Pete Bisila, system reliability engineer-utilities
Flint Hills Resources, Inver Grove Heights, Minn.
CHECK INTO SEVERAL CHANGES
Here are a couple of suggestions that may help improve condenser performance until you can shutdown:
1. For a fixed UA, raising the column pressure increases the logarithmic mean temperature differential (LMTD) for the condenser and improves the ability to condense. Make sure your re-boiler has some margin for over-design because at a higher column pressure the re-boiler will become stressed due to an LMTD pinch.
2. If possible, add a cooling water supply, via spray nozzle distributor system to the condenser. Spray cooling water over the fins under the fan housing as the air is moving up and over the fins. This will provide an additional heat sink for the overhead stream of the column. You may need to collect this water into a portable sump that may need to get pumped on some frequency back to the cooling tower basin or the return headers.
3. Can your fan speed be adjusted higher to improve your heat transfer coefficient? If so, adjust higher if you have enough motor to do this.
When you have the ability to shut down, check the condenser for fouling. Have you been able to get the desired performance of the condenser in the summer months in the past or is this always the case in the summer? If the condenser performed reliably in the past, it may be more fouled now than before.
If the condenser has never performed, then it may be an issue with the design. Check the percent relative humidity you are currently experiencing versus the original design. If your relative humidity is higher than what was specified for a given rate, then the condenser may be under-sized for the duty required.
From an overall standpoint, check all current operating parameters with original design parameters and if something stands out that can significantly impact performance, e.g., flow, LMTD, blade tip angle, speed, etc., you may need to redesign the air cooler for your current conditions at the next available outage.
Eric M. Roy, principal engineer
Westlake Chemical, Houston, Texas
ASSESS THREE OPTIONS
Based on the description of the problem, the condenser seems to be working well. For a commercial unit to get a 20°F approach temperature on an air-fin is pretty good performance. Upgrading a unit with good performance to one with excellent performance is a challenge. For a conventional air-fin the temperature pinch normally occurs at the process outlet of the exchanger. Getting better performance comes down to three options: improve the approach; increase the saturation temperature of the process; or decrease the air temperature in.
Air-side fouling starts at the bottom of the air-fin where the air enters. Make sure that the bottom tube rows are as clean as possible. Pollen, fibers and dirt all increase approach temperatures.
Use a thermal imaging camera to look over surface temperatures. Cold spots show areas where fouling insulates the tubes. Additionally, the thermal imaging scan may show areas of low air flow (the tubes will be hot there). Look for what might cause this. Does the exchanger have air bypass due to leaks at the side, plenum damage or other factors? When pushing the equipment to the limit, good maintenance practices count for a lot.
Raising the tower operating pressure will increase the saturation temperature. This shifts the latent heat out of the cooling-water exchanger and into the air-fin. However, raising the pressure has other consequences as well. The higher operating pressure will increase the reboiler temperature. This may boost reboiler fouling or cause a pinch against available heat. At 155–160 psig, tower capacity may either increase or decrease. For hydrocarbons in this range, the balance of typical tray designs coupled with vapor and liquid density can create situations where capacity changes aren't obvious. If you do raise pressure, watch tower hydraulic capacity carefully.
If the tower is over-reboiled, see how much the bottoms duty can be turned down. Any extra duty into the bottom must be removed from the top as well. This is less likely to be successful than changing the tower pressure, but may still help.
On the air side, check local air currents with a ribbon strip — basically a streamer attached to a PVC pipe. Look for recirculation of air around the exchanger. Strategically placed barriers can dramatically lower air recirculation and keep air temperatures down.
There's one option to decrease the air temperature in – using a high pressure drop across a spray nozzle, mist underneath the exchanger. This isn't intended to put bulk water on the exchanger. The object is to create humidification cooling when the water evaporates. The water needs to evaporate before it gets to the exchanger. Pressure drops of up to 400 psig may be required on the spray nozzles. The target is to get roughly 50–100-micron water particles. This gives quick evaporation. The evaporation then cools the air. Depending upon local humidity, a temperature drop of 5–10°F may be possible. Unless the water is used for only short periods of time, very clean water is required. The exchanger will collect much of the solids created by material in the water.
Brute force increases in air capacity are less likely to be successful. Fan changes, speed changes, more motor power and other modifications may increase air rates. However, a 20°F approach temperature is already pretty good for a large commercial air-fin. Extra air will not move the outlet pinch very much under these conditions.
Andrew W. Sloley, principal engineer
CH2M HILL, Bellingham, Wash.
FAN POWER WON'T DO IT
There are few ways to improve the performance of air condensers in your situation. Looking at the difference between how the condenser is supposed to work and how it works now poses a real problem for the water cooler.
Let's assume you've already considered increasing the fan speed to maximize the air flow. In turbulent flow, air heat transfer is proportional to ΔT0.34; in laminar flow or natural convection, it is proportional to ΔT0.25. That's nice theory, but let's consider the practical. The trouble is that changing the air heat-transfer coefficient, hair, is difficult and not rewarding. Working with a few values from Figure 45, p. 225, of Kays and London's "Compact Heat Exchangers," McGraw-Hill, 1984, shows the following: Δhair = (Δfan power)0.173 and ΔP = (Δhair)3.81. This says that increasing the fan power increases hair only modestly with a huge increase in pressure drop. If we assume that heat transfer across a flat plate applies, Nu = 0.0292×Re0.8×Pr0.62, where Nu is the Nusselt number (which indicates heat transfer at a boundary), Re is the Reynolds number and Pr is the Prandtl number. Pr represents only immutable physical properties. Because Reynolds number is the dominant means of improving heat transfer, and this is an expensive approach, it may be possible to change coefficient by changing the fluid: spray the fins of the air condenser with cold water. A fine spray works best; perhaps cooling the shell of the column could help.
Also, it may be a good time to look at the air flow into and out of the fans. Recirculation can reduce exchanger efficiency significantly. Consider erecting walls to direct the air flow as desired.
There is another option: change the feed characteristics. At 260°F, the feed is a superheated vapor. A rough check using McCabe-Thiele for a typical relative volatility shows a couple of possibilities if the feed is changed from superheat to a subcooled or saturated liquid; even reducing the superheat might help. Lowering the feed temperature or changing the state would require raising the feed tray and reducing the condenser recycle rate; the minimum recycle rate decreases by going from a superheated vapor to a subcooled liquid. If this change is possible, it might be another way to deal with the air condenser problem. Unfortunately, this will put the load on another heat exchanger downstream of the debutanizer column. A refinery is a well-balanced machine; reduce the burden in one place and it must be taken up by another exchanger or tower.
Dirk Willard, lead process engineer
Fluor Global Services, Inver Grove Heights, Minn.
The seals of the lean amine pump serving our fluid catalytic cracker unit suffer a very short mean time between failure (MTBF). We are using tandem seals with an API Plan 52. The buffer fluid is a light lube oil recommended by our pump salesman. The amine is 21% by weight monoethanolamine. The 3×2 centrifugal pump was designed for 190 gpm at about 160 psig. The impeller is a mixed-flow type. The nominal suction specific speed is about 7,500 rpm; the suction diameter is 4 in., schedule 40. The difference between the net positive suction head available and required is about 6 ft water at nominal. Unfortunately, the pump usually runs above the nominal rate, at around 205 gpm at about 145 psi head. Inspection after failure shows severe crystallization and scoring of the shaft seal. In addition, the pump seems to run rough even when it's operating at 60–70% of the best efficiency point, which is at around 185 gpm. The cavitation grows worse over time so we pull the pump after six months to avoid potential failure. We followed the pump manufacturer's recommendation and installed a minimum flow loop that operates continuously with an orifice; the flow is about 60% of the nominal flow. Any suggestions on how we can improve the MTBF?
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