The fluids handled within the adhesives and sealant industries are amongst the most difficult to handle of any in the main process industries globally. The characteristics which make them particularly difficult to handle include:
- High viscosities (typically 1-2000 Pa.s, or 1000 to 2 million cP).
- Often non-Newtonian (viscosity is not constant and varies with shear rate).
- Some must be processed at temperatures close to their solidification/hardening points where they are close to changing phase from liquid to solid (e.g. hot melt adhesives).
- Almost all have low thermal conductivities, often intentionally. This makes them particularly resistant to being heated or cooled.
- Their high viscosity requires that they be pumped at high pressures to be moved from A to B and through all required process equipment and process stages.
- To keep fluids flowing, they must often be processed at temperatures well above ambient.
- Some of the fluids processed are for food related use and require the alloy and non-alloy materials in contact with the fluids to be FDA approved (e.g. gaskets).
- Equipment can be difficult to clean safely, causing shutdowns and maintenance delays.
- Many fluids processed are susceptible to degradation if “cooked” for too long.
- Controlling the “thermal history” of a product by avoiding excessive ranges of residence times (and thereby reducing the potential for degradation) becomes more difficult as soon as flow becomes laminar.
- Process lines are often designed to handle a variety of process fluids and flow rates, which adds further complication.
Heating and Cooling
Heating and cooling are key processes in the adhesives and sealants industry and ones into which most of the above challenges inevitably fall. Many products need to be processed or formed at high temperatures (e.g., within a reactor or mix vessel) but later cooled to facilitate further processing or handling.
Heating may be required to:
- Enable solvents to be flashed off and recovered (devolatilization)
- Reduce product viscosity to reduce energy losses through screens, filters and other equipment
- Reduce viscosity to optimize belt cooler or underwater pelletizer operation
- Heat reactor contents during a pump around to shorten heat up and overall batch processing time
Cooling may be required to:
- Reduce product temperature to enable safe packaging to occur
- Reduce temperature and increase viscosity after reactor to optimize belt cooler or underwater pelletizer operation
- Reduce temperature before product extrusion
In the adhesives and sealants industries, the most common source of indirect heat is thermal oil. This is provided from a stand-alone package or central boiler system if thermal oil is used on other parts of the plant. The thermal oil is heated using gas or electrical heaters.
Typical coolants are thermal oil (with its temperature set below the target cooled product temperature) and tempered water. Cold water is rarely used because many products used in this industry have solidification temperatures higher than typical mains or cooling tower water temperatures. Additionally there is danger of water being vaporized into steam if its flow is insufficient to prevent its temperature rising to above its flash point at operating pressures (100 degrees C or 212 degrees F at atmospheric pressure).
In cooling applications the thermal oil returns from the heat transfer device hotter than when it was fed to it. At some point, it will be necessary to cool the oil. A plate and frame heat exchanger is connected to the hot oil feed tank using water as a coolant. It is important to note that this device will need to remove the same amount of heat from the oil as is being removed from the product.