Solids drying is a complex process of simultaneous heat and mass transfer. The measurement and control of gas and solids properties is important for efficient operation. The majority of solid drying applications are convective in nature, involving heated air to transport heat to the product and remove evaporated water. Air and product property measurements can be part of a manual control or optimization plan. They also can be developed into a continuous control or optimization application by using online instrumentation.
A common machinery configuration for solids drying in many industries is a conveyor dryer. The conveyor dryer offers control of residence time and provides for varying drying conditions to meet production requirements. It uses air to provide heat to a product through convection and to remove evaporated water from the system (figure 1). Flow rate, temperature and humidity are important process parameters. Most conveyor dryer applications also recycle a large percentage of the drying air in an effort to conserve the energy applied to the system and maximize the evaporative capacity of the process air.
Air Temperature MeasurementAir temperature measurement in drying processes can be conducted using either a thermocouple or a resistance temperature device (RTD). These devices provide a single spot measurement. Care must be given to placement of the probes so the measured temperature is indicative of the average air properties. Probes should be installed in areas of high airflow to ensure a fast response and representative process temperature readings.
Manipulating the input to the heat source can control process air temperature. For fuel-burning systems, the flow of fuel to the burner is directly related to the process air temperature. For steam-heated systems, the pressure to the heat transfer coils can be adjusted.
The control of process air temperature normally is accomplished using a standard feedback controller. Programmable logic controllers (PLC) or standard single-loop controllers also will maintain a constant process air temperature.
The measurement of the mass ratio of water to air also is important to the dryer's efficient operation. Because the air is used to provide heat to the product, for evaporation of water and to remove water vapor, the level of water vapor in the air will determine the drying rate and process efficiency. Several methods may be used to determine water loading in an airstream.
Dry and wet bulb temperature probes commonly are used to measure humidity. The temperature reading of the wet bulb probe will be lower than the dry bulb probe due to evaporative cooling. The difference between the two readings can be related to the water loading of the air. Wet bulb humidity measurements typically are performed as spot manual readings because the consistent wetting of a temperature probe can be difficult.
Humidity transmitters also are available to measure water loading. They typically measure the electrical characteristics of a layered polymer film in contact with the air. The thin polymer film either absorbs or exudes water vapor as the relative humidity of the drying air rises or drops. The dielectric properties of the polymer film depend on the amount of water contained in it. As the relative humidity changes, the dielectric properties of the film and the capacitance of the sensor changes. The electronics of the instrument measure the capacitance of the sensor and convert it into a humidity reading. In the past, these probes were limited to lower temperatures and typically were used in heating and ventilation systems. The probes now have upper ranges of approximately 390oF (200oC), which allows them to be used in food and industrial drying applications.
Chilled mirror systems can be employed to measure the dewpoint of an airstream. A sample of the process air is pumped through an external analysis stream, where it comes in contact with the chilled mirror system. A temperature controller maintains the mirror at the maximum temperature, which causes condensation on the mirror. Typically these systems are used on low humidity air and not in drying systems, where the air has high humidity.
Control of Process Air HumidityThe control of air humidity for a recirculating air dryer typically involves changing the ratio of exhaust and recycle airflows. For a properly balanced dryer, the massflow of the makeup air matches the exhaust flow rate. An increase in the exhaust airflow will tend to lower the process air humidity.
Dampers can be installed in the exit air duct to adjust exiting airflow, although the adjustment of fan speed also could achieve a range of exhaust flow rates. In typical operation, dampers are adjusted manually as part of dryer performance audits. However, humidity control can be automated using an online sensor and positioning motors.
The adjustment of the exhaust flow rate assumes the evaporation load on the dryer is large enough to generate the humidity level. In some cases, it may be necessary to use water or steam nozzles to add additional water to the process airstream and achieve higher humidity levels.
Airflow RatesThe measurement of the airflow rate will determine the drying rate as well as overall process efficiency. In a conveyor dryer, the ratio of the exhaust airflow rate to the recycled airflow rate will determine the humidity level of the air moving through the product and the exhaust. The exhaust airflow through a duct can be easy to measure providing a suitable straight length is available.
For conveyor dryers with internal recirculation of air, it is not normally possible to use equipment to measure airflow because the flow contains excess turbulence for accurate measurements. Special dryer designs are available with external ducting to allow the use of instrumentation. However, measuring the pressure increase through the recirculating fans and fan speed will allow calculation of the recirculated airflow based on a calibrated fan curve available from the fan manufacturer. In cases where multiple fans are operating in parallel, the amount of airflow can be changed by turning the fans on and off. This does not allow any fine-tuning but may be sufficient for successful dryer operation.
Most dryers are designed to operate with an internal negative pressure. Slight negative pressure ensures that process air will not leak into the surroundings, but some air may leak into the dryer. Manometers and gauges are available to accurately measure the pressure.
The absolute pressure of the air used for drying also will affect dryer performance. As absolute pressure decreases, the mass and heat capacity per volume of air will decrease. Therefore, more air volume is required to provide the heat and evaporative properties required for successful dryer operation. This phenomenon has important design considerations for dryers operating in high altitudes or under low vacuums.
With the exception of dryers that are designed to operate under a strong vacuum, operating air pressure in a dryer normally is not controlled on a continuous basis. A balance between the exhaust and makeup airflow rates will allow the dryer to operate at a neutral or slight negative pressure.
Because dryer performance is measured by its ability to remove water from a product, measurement of the product mass flow and moisture content into the dryer is important in performance assessment. Bulk density and particle density will influence the resistance to airflow and the diffusion of water internally. The size of the individual particles also will influence the drying rate because internal diffusion will become a rate-limiting factor for large particles (>10 mm). The temperature of the incoming product will affect dryer performance because part of the energy input to the drying process may be required to heat the product before drying can begin.
The solids flow rate into the drying system and the range of moisture content to be removed define the work the dryer must perform. Therefore, the solids flow rate is a necessary requirement for any system that is going to control or evaluate the efficiency of the dryer. There are a number of measurement systems available to measure the flow rate of solids entering a dryer, including:
- Gamma Ray Absorption. This system is a rod-shaped gamma source whose type, length and activity distribution is matched to the conveyor system. Material loading is installed below the mass flow in a lockable shielding with a collimated radiation opening. It is used to measure the flow rate of a stream of solids.
- Displacement Sensors. The measuring principle of displacement sensors consists of a powered turbine wheel through which measured material flows. Material flow will be forced to accelerate through the turbine, thereby causing a braking effect, which is measured as torque on the turbine shaft. Measurement is made with a special torque transducer and a turbine speed pickup.
- Angular Momentum Sensors. These sensors can be used to measure flow during free-fall. Dry bulk materials enter the flow sensor and produce a mechanical deflection as the flow strikes the sensing plate. The horizontal force of this deflection is converted into an electrical signal and processed by associated electronics.
- Weigh Belts. Used in many solids flow applications and for simple flow rate indication to certified precision weighing, weighing is performed by load cells supporting part of the conveying mechanism and a speed measurement of the conveying equipment. The accuracy can be improved by weighing larger areas or length of the conveying system.
Control of the solids flow rate can be achieved by adjusting the operation of the equipment feeding the dryer. In cases where the dryer is processing bulk materials from a holding bin, a constant flow rate to the dryer may not affect overall plant efficiencies. However, many dryers operate following production equipment that is forming the product to be dried. These machines, such as extruders for food processing, can be complicated to operate making it difficult to maintain a constant flow-rate to the dryer.
It is important to measure and control the solids height on the dryer conveyor bed because it will determine the airflow's pressure drop. A change in pressure drop will result in a change in airflow rate.
Measuring Moisture ContentThe main reason for operating a dryer is to reduce the moisture content of the product. Measurement and control of the moisture content is critical for successful dryer operation. Typically samples are taken from the dryer's discharge and are analyzed off-line either using a 'quick' bench-top device or more defined laboratory procedure.
The control of the moisture content of the solids entering and exiting the dryer can be the most important parameter to successful dryer operation. Typically, if all inlet parameters stay constant during dryer operations, there should not be any significant change in the outlet moisture content of the product. However, in many situations, changes will occur in the inlet parameters that will affect the drying rate. These changes may occur at a low frequency and measuring the product moisture content using the laboratory or bench top methods can be used as information to change the drying process conditions to achieve constant outlet moisture content.
In many processing plants, the frequency of change may be too high and exceed the ability to measure the moisture content and make changes to the drying process manually. In these cases, using online moisture sensors and a continuous control system may make economic sense to achieve constant product properties. Another strategy for maintaining constant-drying rates is to model the heat and mass transfer rates within the dryer. Measuring changes in process temperatures (heat transfer) can allow the evaporation rate (mass transfer) to be predicted and the process changed in order to maintain constant outlet moisture content.
Numerous instruments facilitate the measuring and monitoring of dryer operating conditions. Obtaining air and product property measurements can lead to an increased understanding of the specific drying process as it relates to overall plant efficiency and product quality attributes.