Optimization of a Bitumen Binder Modification Process Under Continuous Operation Conditions

This paper presents an analysis of batch and continuous modification modes of BND 60/90 bitumen binder with petroleum resin. Optimal conditions, namely modification time, stirring rate, and the ratio of components were found. The conditions of continuous operation mode were further optimized. keywords: Bitumen, Petroleum resin, Asphalt, Protection coating, Organic solvent, Industrial oil, Continuous mode reactor.


INTROdUCTION
Asphalt concrete is the most practical and widespread paving material.Although it has many benefits, some drawbacks still exist: cracking upon time, pavement scaling, spalling etc. Heavy traffic, overloaded trucks and climatic factors speed up the degradation processes.Therefore, it is important to develop and implement efficient materials and technologies to protect the road pavement from workload and climatic factors in order to prolong their working lifespan.
A common method used for the prevention of asphalt concrete pavement degradation is the application of protective layers to the surface (thin layer coating of the pavement with bitumen emulsion or protective compositions) 1 .
Treatment with protective impregnation compositions does not require power-consuming or expensive equipment and large amounts of raw materials.
After the treatment of the surface of asphalt concrete, the impregnation composition forms a strong thin hydrophobic film on the whole surface of the pavement, providing high protective properties.Consumption of the material per square meter of the pavement is low.
In 2 we earlier described an optimal ratio of components for a protective impregnation composition based on road bitumen of BND 60/90 grade which is modified by a petroleum resin (PR).Physico-chemical, technological and working parameters of the prepared samples of the compositions were described.
All the samples of the impregnation composition were prepared in batch operation mode: the bitumen binder was modified by the introduction of PR (up to 15 mass %) in a steel reactor with an overhead stirrer.The initial temperature was 160°C, then it was maintained not higher than 180°C for 90 (industrial oil was used as a plasticized when needed).The modified binder was then combined with the solvent by pumping the latter to the reactor.
To evaluate the transfer of the technology to a pilot scale, it is important to consider the analysis of batch and continuous operation modes of binder modification, as well as to select the optimal conditions: time of modification, stirring rate, and ratio of the components.
The goal of the current research was to optimize the conditions of continuous mode modification of the bitumen binder.
Viscous road bitumen of the 60/90 grade is the most demanded bitumen for the production of asphalt concrete pavement, because its rheological properties match the requirements for roadwork to the greatest extent.It is used as a binder in the construction and reconstruction of road pavements.
Petroleum resin (PR) "Sibplast" was selected as the modifying agent, while industrial oils I-20 and I-30A were used as plasticizers (Tables 2  and 3).
The modification process under continuous operation mode conditions was performed as follows:    The bitumen (1000 g) was heated to 130°C and placed to a steel vessel (2000 mL) equipped with a hot plate (500 °C maximum temperature) with electric heat regulator, an overhead propeller stirrer and an electronic thermometer.The stirrer blade length was 70 mm.The hot plate was turned on and the stirring rate set to 500 rpm.The heat regulator was set to 450°C, when the inner temperature reached 160°C, petroleum resin was quickly added in one portion.When the temperature reached 170°C, the time count was started.When the inner temperature reached 175°C, the hot plate temperature was set to 320 -330°C.The heating time of the modified bitumen was 30 minutes.The time of modification was shortened by 7.5 min.Samples were taken after 30, 45, 60, and 90 minutes of modification time.
To evaluate the continuous operation mode of modification of the bitumen binder we constructed a laboratory model of a flow reactor for the preparation of the binding component.The summary of the process is in dispensing bitumen and petroleum resin, heating the mixture up to 180 °C and feeding it into the steady-flow reactor where the modification process occurs.The model has several entrance points for the introduction of industrial oil.Piston pumps were selected for the feeding of PR and bitumen.The technological parameters of the model can be managed either by a computer or from a control panel (Figure 1).

RESULT ANd dISCUSSION
To evaluate the transition of the technology to a pilot scale, an analysis of batch and continuous operation modes of modification of the bitumen binder was performed.Laboratory (up to 10 kg) and semi-industrial (up to 200 kg) conditions for the preparation of the product were considered.The parameters analyzed included: loading of the raw material, efficient stirring, heat retention of the reaction mass.The analysis of the modification schemes is given in Table 4.The most important difference of the continuous mode from the batch mode is the stable power consumption without any peak loads.The finish warming of the mixture is performed by a pipe furnace in less than 1 min prior to feeding into the reactor.Heating from 160 to 180°C in batch mode conditions should require not more than 30 min.For the 10 kg and especially 200 kg scale it would require heating equipment of increased power capacity.Due to the high heat retention of the reaction mass, any particular reactor size requires additional selection of modification conditions.
The continuous operation mode scheme has significant drawbacks in terms of loading of the raw materials (more expensive equipment is needed).In the batch scheme, the loading is performed by manual labor or using equipment with loading rate that can be regulated in a considerably wide range.In the case of the continuous mode, special dosage units are required that provide high accuracy and stability of operation.The cost of such equipment is at least 10 times greater than the cost of equipment for the batch process.However, in this case, automated control systems can be used, which would decrease labor costs and allow to automate the production.
The main difference of the continuous operation mode from the batch one is the material flow management.While in the batch scheme the weight or volume of the loaded components are preliminary measured, in the continuous process type the component ratios are determined by the dosage units.
Several dependencies of the binder properties were found out in the study of bitumen modification with PR: -From the reaction mass stirring rate; -From the modification time in combination with the operation mode (batch and continuous); -From the bitumen to PR ratio.

Study of the reaction mass stirring rate on the binding component properties
The modification of BND 60/90 grade bitumen was performed at 100, 200 500 and 1000 rpm of the propeller stirrer.
-Flow reactor temperature: 180 °C; -Stirring with a propeller stirrer; -Production capacity 1100 g/h; -Sampling was carried out from a reactor valve placed on the 0.5 L height which corresponds to 30 min process time.
The main controlled binder parameter: penetration at 25°C.The data on the dependency of penetration from the stirring rate are given in Table 5.
Analysis of the results shows that the value for penetration does not change for stirring rates above 500 rpm.So we selected this stirring rate for further experiments.-Sampling was carried out from reactor valves placed on the 0.5 L and 1 L height which corresponds to 30 min and 60 min process time for the 1000 g/h production capacity, or 45 and 90 min for 670 g/h capacity.
The following properties of the binder were compared: penetration at 25 °C, ball-and-ring softening temperature, Fraas brittle point.Experimental results are given in Tables 6-8.
Analysis of the data shows that for both the batch and the continuous operation modes the maximal decline of penetration values occurs during the first 30 minutes of modification.The most important difference is in the rate at which penetration values continue to decline after this time.While for the batch operation mode it is considerable -5-10 units in the time range of 30-90 min, for the continuous mode it is only 1-2 units.
The presented data demonstrate that the properties of the product are almost identical for both modification modes.However, Fraas brittle point values are significantly higher for the continuous mode compared to the batch mode.An exception are the results for MNPZ bitumen that has a difference of 1-2 degrees in this parameter.

Study of the bitumen to PR rate on the properties of the binder product
The bitumen -PR ratio is one of the most important parameters for the development of a continuous operation mode bitumen modification process.
While in the batch mode this ratio is easily controlled, in the continuous operation it is much more difficult to set the proper material flows.Of particular importance is to set acceptable limits for the ratio, as the dosage pumps have their own tolerable deviations.It is necessary for the bitumen to PR ratio to be within the set interval of deviation.

Process conditions -
Flow reactor temperature: 180 °C; -Stirring with a propeller stirrer; -Production capacity 1000 g/h; -Sampling was carried out from a reactor valve placed on the 0.5 L height which corresponds to 30 min process time at 1000 g/L production capacity.
The controlled parameters were: penetration at 25°C and Fraas brittle point.
The experimental data are given in Tables 9 and 10.
It is clear that the bitumen to PR ratios of 90 : 10 for the continuous process and 85 : 15 for the batch process give very close results.
Preparation of the binder in continuous mode using mineral oil as an additive Process conditions -Flow reaction temperature 180°C; -Stirring with a propeller stirrer; -Production capacity 1000 g/h; -Sampling was carried out from a reactor valve placed on the 0.5 L height which corresponds to 30 min process at 1000 g/h capacity.
The controlled parameters were penetration at 25 °C and Fraas brittle point.
The oil was fed by a peristaltic pump at the 0.5 L valve level through an entrance point for the oil.
The results are presented in Table 11.

CONCLUSIONS
An analysis of the batch and continuous technological schemes of a bitumen binder modification was performed.
The minimal reaction mass stirring rate to provide the perfect-mixing reactor under continuous operation was found out -500 rpm.
The maximal decline of penetration values occurs during the first 30 min for both operation modes.
The most important difference of the continuous mode from the batch one is the very slow rate of penetration decline after 30 min process time.
The Fraas brittle point in continuous mode is significantly higher compared to the batch mode.
Modification of bitumen with bitumen to PR ratios 90 : 10 and 85 : 15 (for the continuous and batch modes, correspondingly), give very close results.

Fig. 1 :
Fig. 1: Laboratory model of a flow reactor to produce a binder component