Problem with your Dryer? Drying Plastics
Although pellet drying is a relatively simple process, it is occasionally a source of consternation. The pellets, at times, just will not dry. If you've been following the resin supplier’s recommended drying guidelines, the cause of poor drying can always be traced back to four root causes:
- Drying Temperature: Heat is the key to breaking the strong bond between water molecules and hygroscopic polymers. Above a certain temperature, the attraction the water molecules have for the polymer chains is greatly reduced, so that moisture can be drawn away by dry air
- Dewpoint: In the dryer, air is first dehumidified to a very low residual moisture content (dewpoint). This air is then heated to reduce its relative humidity. The result is drying air with a very low vapor pressure. Water molecules within the pellets, freed of their bonds by heating, will migrate toward the air around the pellets.
- Time: It takes time for heat from the air surrounding the pellets to be absorbed and for water molecules to migrate to the pellet surface. Resin suppliers specify how long a material must spend at the proper temperature and dewpoint for effective drying.
- Airflow: Hot, dry air delivers heat to the pellets in the drying hopper and strips moisture from the surface of the pellets and carries it back to the dryer. Airflow must be sufficient to heat the resin to its drying temperature and hold it at that temperature for the specified time.
Before looking for more sinister problems, it is always a good idea to visually inspect a malfunctioning dryer. Pay particular attention to air filters and hoses. Blinded filters or collapsed hoses will reduce airflow and compromise dryer performance. Ruptured filters will contaminate desiccant, inhibiting its ability to adsorb moisture. Frayed hoses may draw wet ambient air into the drying circuit, causing premature desiccant loading and high dewpoints. Poorly insulated hoses or hoppers may compromise the drying temperature.
Fig. 1—A return-air filter and heat exchanger/cooler is useful to prevent contamination and overheating of the desiccant, both of which prevent effective moisture removal.
Fig. 2—To verify that dryer airflow is adequate, measure the temperature profile in the hopper. In this example, assume the resin supplier recommends drying for 4 hr, and the processing rate is 400 lb/hr. You should note particularly whether the temperature at the 4-hr (400-lb) level is at the proper setpoint.
Fig. 3—Dewpoint monitoring at the dryer air inlet and exit can confirm proper conditions (A and B) or diagnose problems such as overheated desiccant (C).
Regeneration & cooling
Desiccant has a finite moisture capacity, so its adsorbed moisture must be purged by regeneration. As shown in Fig. 1, ambient air is drawn through a filter and into a blower that directs it through a set of heaters. Heated regeneration air is passed through the desiccant bed, which releases its adsorbed moisture as the desiccant’s temperature rises. Moisture-laden regeneration air is purged to the atmosphere. The hot regenerated desiccant must be cooled before being moved back into the drying circuit to ensure that it can remove moisture from the drying air.
Dewpoint readings can help diagnose several problems, so monitor the drying-air dewpoint throughout an entire dryer cycle. Normal dryer operation should produce a straight-line dewpoint reading in the –20 to –50 F range. Small fluctuations at desiccant changeover are acceptable. If your dryer is functioning properly, you should see a dewpoint at the dry-air inlet at least 30°F lower than at the return-air exit. (Fig. 3, lines A and B).
On the other hand, dewpoint spikes immediately after desiccant switchover (line C) indicate that the desiccant was not sufficiently cooled before being brought on-line. Hot desiccant will not adsorb moisture well until it cools down, after which the dewpoint should drop to the proper level. Improperly cooled desiccant can result in temperature spikes harmful to heat-sensitive materials like ionomers, amorphous polyesters, and some grades of nylon.
If you see good dewpoint readings immediately after desiccant beds are switched, but dewpoint rises rapidly before the desiccant cycle ends, this may indicate that ambient air is leaking into the closed-loop circuit and causing premature loading of the desiccant. Other possible causes are incompletely regenerated or contaminated desiccant. A dewpoint reading that is consistently near the return-air dewpoint reading indicates a totally inactive regeneration circuit or badly contaminated desiccant.
Common dryer problems
- Poor airflow caused by clogged filters
- Air passing through the middle of the load rather than dispersing through the pellets caused by unfilled hopper
- Supply/return dry air lines allowing ambient “wet” air to contaminate dry air
- Wet air contamination through loader on top of hopper
- Lack of cooldown on air returning to the bed in absorption process. (Air should be cooled below 65ºC (150ºF) to increase the desiccant’s affinity for - moisture, thus improving efficiency. An aftercooler is required when drying some polymers.)
- Reduced desiccant effectiveness caused by worn-out or contaminated desiccant
- Nonfunctioning regeneration heater and/or process heater
- Blower motor turning backwards
- The airflow is not being shifted when controls call for a dessicant bed change; i.e., one bed stays in process continuously and, thus, is not regenerated, resulting in poor or no drying.
Installing a Dew Point Transmitter to a Desiccant
Dryer Ideally, the dew point sensor should be installed before the heater and the hopper to measure and control the dew point of the air that is passed over the plastic material. If the transmitter is installed after the heater, a direct process installation is not always possible due to high air temperature, and a sampling system is needed. The sampling system cools and, if needed, filters the air sample before it flows to the sensor. If the process is operating in low or ambient pressure, a pump is required in the sampling system to draw the air sample from the process.
Installing a Dew Point Transmitter After the Hopper
The temperature inside the hopper may exceed 300 °C for several hours. For this reason, if dew point measurement from the hopper is needed, a sample air flow should be extracted and cooled prior to bringing it into contact with the dew point sensor. The air in the hopper may contain volatile compounds evaporating from the plastic granules. Typically their molecular size is relatively high so they do not affect the measurement. Monitoring dew point at the hopper outlet in addition to measurement at the dryer allows the operator to optimize the drying time. Once the measured dew point has stabilized to a pre-defined low enough level, the cycle is considered complete. Prolonging material delay in the hopper does not improve drying. Monitoring the dew point of incoming and outgoing air at the hopper ensures that conditions for the drying process remain optimal for the whole duration of the drying cycle.