Temperature Measurements in Distillation Processes | Alaqua INC

In industrial distillation equipment cooling and heating operations, temperature monitoring is a typical control parameter. Temperature control can be used to chill distillate to condense high volatility products into liquid phase, or it can be used to heat process fluid to evaporate high volatility components for better separation, depending on the application and process fluid.

Temperature control is linked to product quality, process optimization, and stability, which leads to increased plant safety and lower energy costs. Understanding the necessity of temperature measurement, despite its seeming simplicity, is important to running a distillation column at maximum efficiency.

Let's compare thermocouples with resistance temperature detectors (RTDs) in these and other applications, as well as highlight some recent sensor technology improvements.

Cooling Procedures for Distillation

Fin-fan cooling, refrigeration, and cooling towers are all common ways to chill distillate. The first two procedures cool process vapors, causing them to condense into liquids that can be recirculated for further processing. Cooling towers are commonly used in chemical processes to chill steam and heat water and return the condensate to plant utilities or reboilers, where it can be warmed and used for a variety of applications.

Process vapors are forced through a tube bundle in fin-fan cooling. Fins wrap around the outside of these bundles, increasing the tube's surface area and speeding up the cooling process. To circulate air over the tubing, large fans are required. A variable-speed motor controls the fan speed to ensure enough airflow to meet the appropriate cooling temperature while lowering energy expenses.

The temperature of the process fluid is monitored using thermocouple or RTD sensors. This temperature is then used as a PID process variable in the variable-frequency drive to control the fan motor speed.

A coolant (e.g., ammonia) is circulated through tubing in refrigerant systems. To cool the process of vapors flowing through the cooling region, compression and expansion cause substantial temperature decreases. This approach is usually utilized in applications that require extremely low cooling temperatures.

Temperature measurement is used in refrigerant cooling to control the flow of coolant through the system. The temperature of the process fluid inside the chilled area is monitored using thermocouple or RTD sensors. The thermocouple outputs are used to control the fan speed to allow the coolant to condense and expand to provide the desired cooling. The sensor outputs are also utilized to manage the flow of coolant by regulating the control valves.

Cooling towers are air-cooled systems that cool water by transferring heat directly from the air to the water. To reduce the temperature of the water, cooling towers bring air and water into close contact. A little amount of water evaporates as the temperature of the water drops, lowering the temperature of the water flowing through the tower. Water heated by other processes is pumped to the cooling tower and sprayed through nozzles to form minute droplets, exposing more of the water's surface area for maximum air-water interaction. The water can then be distributed throughout the plant after it has cooled. It can be used to power plant equipment or sent to boilers to generate steam for processes like distillation columns.

The temperature of the cooling water is monitored in cooling towers by thermocouples or RTD sensors. A controller regulates fan speed to expedite the cooling process by facilitating evaporation of the water droplets while ensuring that no electrical energy is wasted by running the fan faster than needed. Cooling towers are used in chemical processing since they don't require any particular coolants and are energy efficient.

In most cooling systems, thermocouples and RTDs can be interchanged. Because of their greater consistency and reproducibility, RTDs have become the de facto standard in most industrial cooling operations.

Distilling Hot Fractions

When treating distillates, in addition to chilling activities, heating processes are required. Chemical operations employ fractionating or distilling columns to separate mixtures into their constituent elements, or fractions. Differences in volatility are used to compute fractions (i.e., boiling point).

The temperature of the heating elements in distillation columns is monitored and controlled. The boiling point is controlled at specific heights inside the column by varying the temperature vertically, allowing fractions to be collected for further processing. The reboiler heats the process fluids inside the column. Steam jackets are also utilized in many columns to keep the vessel's temperature profile constant.

The temperature of the process fluid inside the column is measured using multiple temperature sensors installed in situ. By opening and closing control valves to inject additional heated process vapors from the reboiler or steam into the steam jacketing, the sensors' outputs are used to maintain the column's temperature profile and manage the heating of the process vapors.

RTDs have become the industry standard for distillation columns, much like they have for cooling systems. When temperatures are higher than 1,472°F [800°C], thermocouples are employed.

Conclusion

Finally, appropriate temperature measurement and control procedures are essential for distillation and fractionation processes to run efficiently, safely, and profitably. In cooling processes, thermocouples and RTDs can be interchanged, but at higher temperatures, it's important to use the proper sensor.

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