Heat loss effect on bitumen ageing in tanks

. The article is devoted to laboratory research on the influence of convective motion as a result of thermal losses and their compensation in tanks on the physical and chemical ageing of bitumen BND 60/90 and BND 90/130. Laboratory tests of bitumen samples confirmed the hypothesis about the key role of free movement of bitumen in tanks on its degradation under conditions of maintaining operating temperatures and free air access. Thus, bitumen degradation has a functional relationship with thermal losses from the tank surface and the total energy input for bitumen heating.


Introduction
Bitumens and bituminous binders, regardless of their nature and composition of the components, degrade irreversibly under the influence of external factors. This process is called ageing and leads to cracking of asphalt pavements, flaking, and eventually to potholes and complete failure.
Bitumen ageing is generally understood as two processes [1]: evaporation of volatile compounds (physical ageing) and oxidation by oxygen (chemical ageing). The latter is the most significant in degradation processes and, at elevated temperatures, leads to an acceleration of irreversible changes in physical and mechanical properties. The duration of heating and the influence of temperature is generally proportional to the magnitude of changes in quality characteristics [2][3][4]. In addition, moisture, UV light and pressure act as activators of ageing. Even in the absence of oxygen at high temperature, degradation of the binder due to polymerisation can occur.
The greatest changes in the properties of bitumen occur in the technological process of the preparation of asphalt mixtures [5]. First of all, it concerns mixing bitumen with aggregates in asphalt-mixer, transporting mixtures to the place of paving and process of spreading and compaction of asphalt-concrete mixtures, when bitumen has high temperature and in thin film interacts with air oxygen on the surface of mineral grains. Ageing inhibitors are used to slow down ageing processes [6,7] and the temperatures of preparation and installation of asphalt mixtures (warm asphalt concrete) are reduced.
At present, studies of chemical composition changes, physical-mechanical, and rheological properties of bitumen and polymer-bitumen binder in the thin layer are of most interest to researchers. The RTFOT (Rolling Thin Film Oven Test) is the most commonly used method to model short-term ageing in the laboratory.
The ageing of bitumen in asphalt concrete (long term ageing) is less intensive and in the laboratory, it is simulated and studied by PAV (Pressure Aging Vessel) and several other methods [8][9][10].
The slowing down of the ageing of bitumen in the coating during the operational phase is ensured by rejuvenating compositions, including those made of biomaterials [11][12][13].
Technological ageing in tanks is very high, especially in small volume tanks. In their study of bitumen ageing in the supply chain from refineries to road asphalt mixtures production Emery et al. [14] found that the ageing index change per day (Ageing Index change per day) during long-term storage at temperatures of 120-165℃ in flow tanks of 25 tonnes reaches 0.017 Pa-s per day. However, in large volume tanks (more than 250 tonnes) this index is 5.7 times lower and is about 0.03 Pa-s per day.
Laboratory simulation of bitumen aging during storage in tanks (in volume) is usually carried out by temperature control in an oven, and to accelerate the tests the temperature is increased, increasing the ratio of the contact area with air to the mass of the sample.
The slowing down of ageing processes in tanks is ensured by reducing the contact area with oxygen by using vertical tanks, by replacing oxygen in the free space of the tank by inert gases (for example, nitrogen cushion device), by limiting temperatures during long-term storage and storage time at high temperatures. For example, based on these results the authors propose to limit the permissible period of bitumen storage at high temperatures (120-165 ℃) in tanks with a capacity of 25 tons not more than 15 days, a capacity of 100 tons not more than 30 days, and in tanks with a capacity of over 250 tons maximum storage time of 45 days.
Of great interest for research technologies of bitumen storage at high temperatures in the expenditure tanks of asphalt plants and storage tanks are also issues of energy conservation. The works of researchers concern the selection of optimal heating regimes, the use of alternative energy sources or more efficient heating methods [14][15][16][17][18][19].
In their study Amina Tahri et al. [16] found that energy costs to compensate for heat losses during long-term storage of bitumen at elevated temperatures can reach 345 kW / ton, which, for example, more than 3 times the required heat demand for heating bitumen from the amorphous state to operating temperatures and is associated with a duration of storage. In Russian practice the normative losses from thermal insulation surface are in the range 54 W/m² (for temperature inside the tank 150 ℃) which corresponds to 67-123 kW/day for tanks 25-50 m³, Zhong-Xin Xu [19] notes as requirements to thermal insulation of horizontal tanks the allowable losses not more than 3 ℃/day, which in terms of bitumen mass for tanks 25-50 m³ is 44-87 kW/day.
In studies technological ageing in volume is not associated with significant energy consumption to maintain the temperature regime during bitumen storage, although there are theoretical prerequisites for this.

Storage of bitumen in tanks under temperature-controlled conditions
Bitumen storage tanks (both consumable tanks in asphalt plants and large volume storage tanks) are usually vertical or horizontal tanks with internal heating elements (most often electric or thermal oil heaters), having one or two layers of thermal insulation made of nonflammable material. The process of bitumen storage under thermostatic conditions is accompanied by a continuous loss of heat through the bottom and walls of the tank and through the bitumen mirror through the roof [15][16][17]. The maximum losses occur from the tank wall where the bitumen is in direct contact with it (wetted perimeter). As a consequence, bitumen in the near-wall layer loses temperature (cools down) and sinks into the lower (bottom) layers of the tank / cistern, and in its place rises the hot product, heated by heating elements. Circulation flows are created as a result of the free convection of bitumen, as a consequence the bitumen surface in the tank is constantly renewed and, since for safety reasons air access to the tank is not restricted, oxidation of the constantly renewed surface takes place (Fig. 1). This process is most noticeable in small tanks where the bitumen surface area-mass ratio is higher and the heat stock is lower than in large tanks at the same temperature. This is indirectly confirmed by the results of Emery et al. [14].

Methods
To confirm the hypothesis about the key role of heat loss in bitumen degradation processes in tanks, two bitumen grades were tested: BND 60/90 and BND 90/130. Samples weighing 200 g were subjected to temperature control for 6 hours at 150 ℃. Two samples (with open lid and closed lid) during the experiment were in the oven, i.e. under conditions where there was no heat loss to the environment and no convective movement of bitumen in the test jar, and the other two samples (with open and closed lid) were kept at a constant temperature on the cooker. Switching the cooker on and off was done by reading the thermometer in automatic mode, ensuring that the set temperature of the bitumen (150 ℃) was maintained stable. Bitumen samples on the slab in a jar with an open lid were kept in conditions close to those of bitumen in storages, tanks and tanks with heating, when the set temperature regime is provided by the heating system, which emits heat to compensate for heat losses. The room temperature was +23 ℃.
The initial bitumen and asphalt after testing in different temperature and air access conditions to determine the following physical, mechanical and rheological properties: penetration depth of penetrometer needle at +25 ℃, softening temperature and brittleness, dynamic viscosity at temperatures of 60 ℃, 135 ℃, 165 ℃ and 180 ℃. The results for BND 60/90 bitumen are shown in Table 1 and for BND 90/130 bitumen in Table 2.

Results
In all heating modes there are changes in physical-mechanical and rheological properties, indicating the aging processes. At the same time, the greatest changes are observed when bitumen is thermostated on a plate with an open lid, providing free access to air and the exit (evaporation) of light fractions, in conditions of high heat losses. This is characteristic for both grades of bitumen tested and confirms the hypothesis of the key influence of free convection in bitumen as a result of heating to compensate for heat losses. It should be noted that for viscous bitumen BND 90/130, the dynamic viscosity at 60 ° C has changed by 60% relative to the viscosity of the original bitumen and for viscous bitumen BND 60/90 only by 30%. This is largely due to the release (evaporation) of light fractions in high heat loss conditions for less viscous bitumen, which in turn significantly affects the change in their group composition and leads to a loss of quality of low-temperature properties and hence a reduction in their operational durability. For the other test conditions, the nature of the changes is different for the bitumen grades studied. Thus, for example, for the less viscous bitumen BND 90/130 the main quality degradation processes are evaporation and polymerisation without air access; for bitumen BND 60/90 the main degradation process is oxidation: ageing in the samples without lids is more visible than in the samples without air access.

Conclusion
The results obtained show the necessity of reconsidering the processes of technological bitumen ageing in technological tanks, storages, and tanks of transport vehicles. In fact, the dependence of bitumen degradation in conditions of free air access on the value of heat loss from the surface of the tank is traced. In the future it may be possible to derive a mathematical connection between the excess energy input in the preparation process and the bitumen ageing rates.
Possibly, the obtained results will also extend the existing approaches to the modeling of bitumen technological aging at the stage of transportation and long-term storage.
Also, the results of bitumen degradation in conditions of zero temperature difference in relation to the environment create the idea of the necessity to design systems for heating technological tanks with an external supply of thermal energy (external heating), which is a thermal barrier for temperature reduction and the emergence of convective motion.