THIS MONTH'S PUZZLER
We bought a used parallel (cocurrent) direct rotary dryer to dry beet pulp. The heating medium is oil passed through a furnace. The feed comes in at 82% water by weight and is supposed to leave at 10%. The nameplate capacity of the dryer is 42,500 pounds per hour (pph); the operating rate at present is only 40,000 pph. A 100-hp fan supplies 45,000 standard cubic feet of air, which is heated to 1,560°F. The fan is taxed, although just 70 hp should be required. The pulp is only dried to 15% when we operate the dryer at full capacity, so we're forced to recycle pulp some of the time. The efficiency, i.e., BTUs required per pound of water evaporated, is supposed to be 1,420 but is about 1,600! Can you suggest why we're using so much heat and what we can do to allow us to limp through the next few weeks until a planned shutdown? What is the long-term solution?
INCREASE THE AIR FLOW
You haven't mentioned whether the beet-pulp flow rate is on a dry basis or a wet basis and the lower heating value of oil isn't specified, so I can't say how much oil you require. But you definitely will need more oil than required for evaporating the water because some energy is being lost in the form of hot air and from the insulation. If the dryer's capacity is as given, then there could be two reasons for the problems:
1. Oil flow rate has decreased. You can verify this by looking at the air temperature after the air heater. If air temperature has decreased, it means oil flow is lower. If your oil flow meter shows the normal value, then check this flow meter and increase the air flow.
2. Air flow rate has decreased. If air flow is lower, then the air will be saturated and won't have the capacity to absorb more moisture. You can verify this by measuring the relative humidity at the dryer outlet or measuring the wet bulb temperature. Air humidity should be less than 90%. If this is the case, then try to increase the air flow. If boosting the air flow isn't possible, then raise the air outlet temperature. This will increase your air inlet temperature, so take care that you are not exceeding the mechanical design temperature limit of the equipment.
Manoj Kumar, manager, central technical services
Reliance Industries Limited, Gujarat, India
CHECK FOR TRAMP AIR
One likely cause could be tramp air. It affects the efficiency of not only dryers but also furnaces. One symptom supporting tramp air as the likely cause is the high power draw by the fan. Get an infrared gun or thermal camera and look at the air ducts and the feed trough. Calculate the actual air flow through the blower from its curve and power draw readings. A dryer should be operating with a slight negative pressure to keep hot gases in; you might also use an anemometer to detect air flow from the dryer. Look especially at seals.
Unfortunately, this may not be the only cause. Too bad the dryer is used equipment. Its history might be helpful. Talk to the commissioning engineers to find out what steps they took to evaluate its initial performance. Hopefully, the commissioning involved an accurate operational acceptance test. Next, talk to the engineer who purchased the dryer, the seller and even the original maker. There's no "Carfax" on used process equipment but historical information may be available on such websites as www.eng-tips.com. You also could google the model and application to see if similar problems have been reported. Beyond that, you're pretty much on your own with used equipment.
There are other potential problems. Is the current feed material the same as what's supposed to be running through the dryer? Was this dryer even designed for this feed? If the flights (ridges) inside the dryer drum are not designed for a material, then the wet solid won't be distributed evenly around the circumference. Solids could clump, producing an inconsistent product or requiring additional dryer residence time and, perhaps, a recycle pass through the dryer.
Is the incoming air dry? If moist air is recirculating back into the dryer from a cold spot, condensation could occur.
Are the temperatures for the incoming and exiting air and for the feed and product correct? Could the oil be fouled? Was the oil flushed during commissioning? Is it the correct oil? Was an oil stabilizer added to reduce corrosion or fouling? What is the oil temperature? If the oil temperature isn't right, look at the furnace. Can you sample the oil? If the oil has broken down or there's sludge, then this could be the source of your mass balance problem.
Some short-term solutions may be of use. High-temperature caulk and insulation may seal the air leaks. Increasing the oil temperature may reduce the dryer performance enough to permit operating at a lower-than-nameplate feed rate; this must be done cautiously because sugar could caramelize, i.e., turn into a burnt or melted oxidized residue that can foul screw flights and sensors like resistance temperature detectors. Another concern is that the sugar is cooled downstream and you don't want to overwhelm the coolers. Adding a defouling agent to the oil may improve heat transfer in the furnace and the dryer. Also, consider some changes upstream. Dehumidifying the air used in the dryers may help. Predrying the feed also may make sense, provided this doesn't adversely affect the feed rate into the dryer. The flow should be steady to produce an even flow in the dryer flights.
Dirk Willard, consultant
We're having trouble with the vertical thermosiphon reboiler of the debutanizer column of our fluid catalytic cracker (FCC). The heating medium is the fractionation column bottoms, which arrive at the reboiler shell at a nominal 297°F, following cooling from 682°F in a steam-generating kettle reboiler at the discharge of the fractionation bottoms pump. During startup and shutdown, the discharge from the bottoms pump briefly spikes to 700–715°F. Last year, we abandoned in-place a corroded debutanizer kettle reboiler and have had problems ever since with the thermosiphon. We pull its tube bundle every 8–12 weeks to clean the fouling inside the tubes; now, we're having trouble pulling the bundle. The fractionation column bottoms pump, which was upgraded with a larger impeller and motor at the same time the kettle reboiler was abandoned, is seeing a drop in head, running loud and tripping vibration alarms. What's going on here? What can we do to improve the cycle time of the FCC?
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