By Clint Rybak
Due to the almost immeasurable array of complex hydrocarbon molecules, asphalt is uniquely viscoelastic—making it suitable for many specialty commercial applications.
Subsequently, one of the most costly elements associated with the usage of asphalt is the heat required to produce, distribute and store raw and modified binders.
The calculations used to design and analyze heat loss in storage and transportation systems can be complex and onerous, due to many associated variables. Some calculations involve convection, conduction and radiant equations while utilizing models of insulation R values, ambient temperatures, wind speeds, heat transfer coefficients of materials, surface area, etc.
However, there is an underlying basic principle that can simplify the analysis and decision making processes around heat optimization of asphalt handling and storage equipment.
The quantity of heat transferred from asphalt to the atmosphere is based on the temperature differences and the surface area between the two. Put simply, more heat will be lost in a given period of time from a tank of asphalt to the atmosphere when the tank is hotter rather than cooler, as the temperature difference between the asphalt and the ambient air is the key driver for heat transfer.
The more surface area of contact between the asphalt and ambient conditions, the faster heat loss will occur. In essence, what this means is that by keeping large asphalt storage tanks cooler (or closer to ambient temperatures) less heat will be lost during a given period of time.
This is primarily accomplished through the use of smaller day storage tanks that can be kept at loading temperatures or the use of inline heat exchangers during loading. The economics of this capital investment can be analyzed versus the heat savings by using some of the above mentioned complex calculations or by a relatively simple system specific methodology.
One can readily tell how much heat is being lost from storage equipment by simply turning off the heat and seeing how much the temperature of that storage system drops in a given amount of time, as this will also be the amount of heat that is required to maintain that temperature during that period of time as well.
We can equate the heat loss using the relatively simple equation for the heat required to change a given media?s temperature of Q=m*Cp*¿T. This says the quantity of heat required to change the asphalt?s temperature is equal to its mass times its heat capacity times the temperature change. For round numbers we can say the heat capacity of asphalt is approximately 0.52 BTU per pound per degree F. What this really means is that it takes 0.52 BTUs of energy to raise the temperature of 1 pound of liquid asphalt 1 degree Fahrenheit.
For example: If the heat is turned off on a 25,000 net barrel tank full of 64-22 asphalt and it loses 2 degrees of temperature per day, how much does that cost? We know the quantity of heat lost is: = (25,000 barrels) / (5.54 barrels per ton) * (2000 lbs per ton) * (0.52 BTU per Pound per Deg. F) * (2 Deg. F per Day):= 9.4 MMBTU per day.
At a natural gas price of $5 per MMBTU and an assumed heating system efficiency of 80 percent that equates to a heating cost of about $59 per day. This exercise can be done again with the tank temperature much lower and under various ambient conditions, to determine the heat loss at the lower tank temperature and potential savings with capital investments in heat exchangers or day tanks.
Similarly, this method can be used to check the heating system efficiency and make determinations about whether to maintain current tank temperature or let the tank temperature sag during inactive periods.
In summary, there is much engineering thought and complex calculation that goes into the design of an asphalt handling system. Once installed, analysis of these systems can be greatly simplified by paying attention to the heat loss information the system is telling you.
Clint Rybak is the Wholesale Asphalt Sales Director for ConocoPhillips Company in Illinois.