Control system upgrade of spray dryer improves product quality and profitability.
BY MICHAEL MCENERY December 20, 2019
A powdered food manufacturer faced the challenge of a major upgrade to the controls of an existing filter belt style spray dryer that had been in service for many years. The dryer’s controls and capabilities were falling behind the company’s requirements for developing custom solutions for clients. The company’s project engineer recognized the large capital expenditure (CAPEX) savings that would be achieved by working with technology partners to retrofit all existing controls as opposed to replacing the entire spray dryer.
A spray dryer is a process used in powdered food production. It transforms a constant feed of liquid into a dried powder by spraying the feed at high pressures through an atomizer nozzle into a flow of hot air. This particular dryer, a filter belt style spray dryer, is a hybrid that uses a mesh filter conveyor belt to transfer the powder through a second zone where additional hot air is blown through the mesh to provide additional drying time, and a third zone for cooling. This style of spray dryer provides gentle, flexible and economical drying for sticky food and dairy products with high levels of fats, carbohydrates or proteins.
The proper balance of multiple interacting process variables of a spray dryer can be difficult to achieve. The level of heat applied in the first zone is the most critical. If too much heat is applied, it can burn the product, remove too much moisture and even damage the mesh belt. Overheating also consumes more energy, which was a concern for the manufacturer. Insufficient heating can allow product to be too moist, which can cause it to stick to the equipment, causing significant downtime, or to cake during and after packaging, causing product quality issues.
Moisture content of powdered foods is also a critical parameter because they are often sold by weight, which causes the volume of the packaged product to vary. Air flow rates also affect air pressures in each zone, as well as the amount of heat energy required. Air pressures need to be negative enough to prevent dust intrusion into the process area, but not so negative to cause the burner flames to go out, resulting in significant downtime if wet product begins sticking to the dryer.
Diagram of a spray dryer system.
The primary goals of the control system upgrade included eliminating all manual adjustments made by operators with recipe-driven proportional-integral-derivative (PID) loop setpoints, increasing the accuracy of control for several critical process variables, reducing time to startup from ambient temperature, integrating multiple controllers into a central programmable logic controller (PLC), fully automated the existing clean-in-place (CIP) system, and provided historical process data for use in energy reduction and ongoing improvements in product quality.
Adjusting and optimizing the temperatures, air flow rates and pressures within the zones of the dryer simultaneously is quite a balancing act, and operators had been performing this by physically adjusting the position of multiple manual dampers throughout the dryer. Liquid product feed rate and pressure into the atomizing nozzles are also key process variables. Operators had been adjusting these with manual valves before.
To eliminate these manual adjustments, three variable speed drives (VSDs) replaced single-speed motors on fans to provide variable air flow control throughout the dryer. Existing VSDs were replaced match the variable frequency drives (VFDs), as well as to replace analog wiring to stand-alone controllers with Ethernet communications to a central PLC. A VSD was added to control the speed of the product feed pump. Process engineers were now able to determine the setpoints that optimized the operation, and engineers used PID loops in the PLC to maintain them.
Storing the setpoints as recipe parameters allowed for fine tuning of the dryer for characteristics of each product being made. The control system was configured for up to 50 recipe parameters, providing plenty of flexibility for increased instrumentation and control in the future.
Spray dryer performance improvements were made possible by increasing the accuracy of control for several critical process variables: in particular, the inlet air temperature, outlet product temperature and zone air pressures. New burner management systems were installed to provide more accurate control of inlet air temperature and were integrated with the PLC, which managed the PID loops and provided the temperature control variable (CV) to the burners. Three infrared sensors were installed and provided much greater accuracy and reliability of outlet product temperature measurements. New variable speed fans provided better control of air flow and pressures.
Minimizing the time required to heat the spray dryer from ambient temperature at the beginning of a production run was also a project priority. This was being done manually by operators and was easy to overshoot, activating high temperature safety switches and alarms, which then shut down the burners and required a manual restart. A more severe consequence of overshooting high temperatures was damage to the conveyor belt, which had resulted in significant costs and downtime to replace it. PLC logic was added to bring the dryer up to temperature as quickly as possible without overshooting. This increased system runtime and allowed the operator to focus on other tasks during the critical startup phase.
In addition to the investment in field control devices, an upgrade to control hardware was also implemented. Multiple existing standalone controllers and relay-based control panels were replaced with one central PLC and an operator terminal to provide a central point of control and data collection. This system in turn integrated with an existing plant-wide process historian and reporting system. Critical process data and alarms were able to be shared with other areas of the plant in real time.
By integrating all controls into the PLC, the plant benefitted by having control of and access to all data and parameters associated with the control functions throughout the process. For example, the main conveyor belt speed, a critical item, had used a stand-alone controller with a field mounted potentiometer for speed control with no feedback to the system. This was replaced with an Ethernet VSD and a speed tachometer input to the PLC. The belt speed was made a recipe setpoint, was accurately controlled by the PLC and all the related process data was captured in the plant’s process historian for evaluation by process engineers to assist with optimization.
The spray dryer takes up a large area and operators found it very beneficial for them to have access to all the dryer’s operational information no matter where they were working on the dryer. Wireless industrial tablets using virtual network computing (VNC) remote access were implemented to allow the operators to view the operator interface screens remotely. Two wireless access points were installed in the production area to support the wireless tablets.
Cleaning and sanitization also are critical to the spray dryer’s operation. The CIP process had been performed manually and was automated as part of this project. Six CIP zones were created to maintain a minimum pressure for velocity cleaning. Automated valves were added and integrated into the PLC along with existing pumps. Wash times were added as recipe setpoints. PID temperature control was added to the heating of CIP and wash water tanks. Automating the CIP operations helped ensure required cycle times were met, which allowed the operator to focus on other tasks and duties and captured process data for record keeping.
Providing historical process data to the company’s process engineers provided benefits in multiple ways. First it allowed quality control personnel to review the historical data and ensure all production requirements were met. It also allowed them to correlate manually measured quality data, such as product moisture content with process conditions. Process engineers can use the data to optimize setpoints to minimize energy consumption without negatively impacting product quality.
During the initial months of operation with the new controls, the spray dryer has delivered significant performance improvements. Energy savings was achieved by lowering heat setpoints for the primary heating zone and by reducing and coordinating the airflow rates through the dryer. Productivity was increased due to faster startups, fewer shutdowns due to flame loss caused by air pressure variances and almost eliminating the need to clean the dryer during a production run due to wet product plugging the equipment. Improved finished product moisture level tolerances have reduced over drying, thereby increasing total product output, which is measured in weight. Automating the CIP operation reduced product changeover time by well over 50%.
Spray dryers are complicated and difficult to optimize due to the interaction of multiple process control variables, including air flow, temperature and feed rate, and the tight operating ranges of other variables such as air pressure and product moisture content. By integrating all controls into a central PLC-based control system and adding new control devices, this food manufacturer was able to increase production rates, improve product quality, reduce energy costs and ensure cleaning was performed thoroughly. McEnery Automation’s expertise with food processing systems, spray dryers, CIP systems and PID loop control allowed them to guide the customer through design decisions, project implementation and programming and an effective and efficient startup.
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