This paper reports the results of heat transfer characteristics of calcined petroleum coke in waste heat recovery process. The model of heat exchanger was set up. The model has been used to investigate the effects of porosity (0.58 to 0.79), equivalent heat conductivity coefficient (0.9 to 1.1), and equivalent specific heat (0.9 to 1.1). The calculated values of calcined petroleum coke temperature showed good agreement with the corresponding available experimental data. The temperature distribution of calcined petroleum coke, the calcined petroleum coke temperature at heat exchanger outlet, the average heat transfer coefficient, and the heat recovery efficiency were studied. It can also be used in deriving much needed data for heat exchanger designs when employed in industry.
With the rapid development of economy in China, energy consumption has been increasing on a vast scale in recent years. The equivalent coal consumption of China was 3.75 billion tons in 2013 [
Calcined petroleum coke is one of the important basic raw materials. It is widely used in producing anode of aluminum electrolytic, graphite electrode, recarburizer, industrial silicon, and other carbon products. The production capacity of calcined petroleum coke in China is the largest in the world, over 70% of which is produced in tank calcined furnaces. When calcined petroleum coke is discharged from tank calcined furnaces, the temperature is 1000°C. The heat of high temperature calcined petroleum coke is about 33.5% of the total heat of calcination process [
Waste heat reutilization of the solid particle has been studied in some field. Cai and Ding [
The investigations on the reutilization of calcined petroleum coke waste heat are lacking. In this paper, heat transfer characteristics of calcined petroleum coke in waste heat recovery process were studied.
The waste heat of calcined petroleum coke is recovered by a heat exchanger. The heat exchanger of calcined petroleum coke includes an internal heat exchanger and an external heat exchanger. Its structure is shown in Figure
The structure of heat exchanger ((
In this paper, the calculated model of calcined petroleum coke heat exchanger is equal to the experimental heat exchanger model in scale. Considering the structure characteristic of the heat exchanger, the calculation model is appropriately simplified. The lower ring pipe and the upper ring pipe of the internal heat exchanger and the external heat exchanger will not be calculated. The calculated model includes two external heat exchanger tubes, one internal heat exchanger tube, and the region between the tubes and thermal barriers. The tube pitch is equal from top to bottom. Additionally the symmetry plane of calculation model is the plane where the short axis of the oval heat exchanger exists. The calcined petroleum coke particles of high temperature and the gas among them are regarded as homogenous continuum. The physical parameters of calculation model are calculated as the porosity of calcined petroleum coke. Considering the thermal insulation layer outside the heat exchanger, it is assumed that there is no heat loss from the heat exchanger lateral.
According to the simplification above, the model has been set up by Gambit. The lengths of
Simplified calculation model ((
When heat exchanges between the high temperature calcined petroleum coke and water, the mass, momentum, and energy conservation exist. In this paper, the unsteady model is used for calculation. The mass, momentum, and energy conservation equations are shown as follows.
The mass conservation equation is as follows:
The momentum conservation equation is as follows:
The energy conservation equation is as follows:
The equations of the flow model in the pipe are as follows.
Turbulence intensity is
Turbulent kinetic energy is
Turbulence kinetic energy dissipation rate is
According to the simplified model, the heat transfer of the calcined petroleum coke in the heat exchanger mainly includes the heat transfer among calcined petroleum coke particles and the heat transfer between calcined petroleum coke and the wall of the heat exchanger. The heat transfer process in this paper is steady with no inner heat source, so the heat conduction differential equation is as follows:
The discrete points within the domain are solved. The implicit format of the unsteady heat conduction equation is used to solve problems in this paper:
The heat transfer equation of calcined petroleum coke and the wall of heat exchanger is as follows:
The calcined petroleum coke velocity, water velocity in the tube, and calcined petroleum coke temperature in the heat exchange inlet are constant. The calcined petroleum coke particles and the gas among them are regarded as homogenous continuum. The physical parameters of calculation model are calculated as the porosity of calcined petroleum coke. The effective heat conductivity coefficient of homogenous continuum is calculated using (
The effective specific heat of homogenous continuum is calculated using
When the heat exchanger model is meshed, the internal boundary is refined locally. The quantity and quality of the grid have a great influence on the calculation results. If the grids are much less, it cannot meet the precision request. If the grids are too dense, it will require sufficient computer memory. And the calculating time will increase. So it is critical to confirm the quantity of grids. The test of grid independence has been widely used to solve this problem. There are four schemes for meshing. The quantity of grids is 130442, 215817, 313721, and 467604. When calculation is completed, the temperature of calcined petroleum coke at outlet is used as the evaluation standard. The calculation results are shown as in Table
Comparison of the calculation results of different meshing schemes.
Grid quantity | The maximum temperature of calcined petroleum coke at outlet/K | Relative change/% (relative to the former scheme) | |
---|---|---|---|
Scheme |
130442 | 647.8571 | — |
Scheme |
215817 | 653.0132 | 0.796 |
Scheme |
313721 | 657.2785 | 0.653 |
Scheme |
467604 | 662.5237 | 0.798 |
Table
The velocity of water in tube is 1 m/s, the inlet temperature of water is 300 K, the inlet temperature of calcined petroleum coke is 1073 K, and the velocity of calcined petroleum coke in the heat exchanger is 6 × 10−5 m/s.
The heat recovery efficiency is calculated by
The definition of the equivalent heat conductivity coefficient is the ratio of the actual heat conductivity coefficient to the standard heat conductivity coefficient. The standard heat conductivity coefficient is the actual heat conductivity coefficient of the calcined petroleum coke porosity which is 0.7.
The definition of the equivalent specific heat is the ratio of the actual specific heat to the standard specific heat. The standard specific heat is the actual specific heat of the calcined petroleum coke porosity which is 0.7.
The experimental system of the waste heat utilization exchanger was built in Weifang Lianxing New Materials Technology Co., Ltd. The heat transfer experiments were carried out by the experimental system (Figure
Schematic of experimental system ((
The experimental system is installed in the tank calcined furnace. The high temperature calcined petroleum coke is supplied directly from the tank calcined furnace. The water circulation system is composed of cooling pond, pumps, valves, sight glass, a drum, and so on. There is a WSM-D two-phase flowmeter in the heat exchanger outlet, which is used to measure steam dryness and flow. There is a FLUXUS F601 ultrasonic flowmeter in the heat exchanger inlet, which is used to measure the water flow. Its measuring accuracy is ±0.5%. Its measuring velocity range is from 0.01 m/s to 25 m/s. Its repeatability is 0.15%.
The measurement system is composed of a temperature sensor, a pressure sensor, and a piece of data acquisition instrument. The temperature sensor is used to measure the temperature of water in the heat exchanger inlet, the temperature of calcined petroleum coke in heat exchanger, the temperature of steam in the heat exchanger outlet, and so on. The pressure sensor is used to measure the water pressure and the steam pressure. The data acquisition instrument is used to record the data. The temperature measuring points of calcined petroleum coke in the heat exchanger are shown in Figure
Temperature measuring points of calcined petroleum coke in the waste heat exchanger.
The experimental data were used to validate the developed model. Simulations were conducted for the same operational conditions as those employed in the experimental investigation. Figure
The temperatures of numerical and experimental result.
Figure
The temperature distribution of calcined petroleum coke.
Figure
Variations of temperature at the heat exchanger outlet with different calcined petroleum coke velocity porosity.
Figure
Variations of average heat transfer coefficient with different calcined petroleum coke velocity porosity.
Figure
Variations of heat recovery efficiency with different calcined petroleum coke velocity porosity.
The normal value of heat conductivity coefficient is the effective heat conductivity coefficient when the calcined petroleum coke porosity is 0.7. The definition of the equivalent heat conductivity coefficient is a ratio of the objective heat conductivity coefficient and the normal value.
Figures
Variations of the temperature at the heat exchanger outlet with different equivalent heat conductivity coefficients.
Variations of the average heat transfer coefficient with different equivalent heat conductivity coefficients.
Variations of the heat recovery efficiency with different equivalent heat conductivity coefficients.
The normal value of specific heat is the effective specific heat when the calcined petroleum coke porosity is 0.7. The definition of equivalent specific heat is a ratio of the objective specific heat and the normal value.
Figure
Variations of the temperature at heat exchanger outlet with different equivalent specific heat.
Figure
Variations of the average heat transfer coefficient with different equivalent specific heat.
Figure
Variations of the heat recovery efficiency with different equivalent specific heat.
The main conclusions of the present study are as follows. The temperature of calcined petroleum coke is low near the wall of the heat exchanger tube, and the temperature differences are small in different planes, while, in the center of the internal and external heat exchangers, the temperature of calcined petroleum coke is high and the temperature differences of different planes are great. When the porosity increases from 0.58 to 0.79, the average temperature of calcined petroleum coke at the outlet of the heat exchanger decreases from 541 K to 489 K and the maximum temperature decreases from 650 K to 582 K. The average heat transfer coefficients of the internal and external heat exchanger decrease by 6.4 and 5.6 W/(m2·K), respectively. The heat recovery efficiency increases by 5.2%. With the increase of the equivalent heat conductivity coefficient (from 0.9 to 1.1), the average temperature reduces by 35 K and the maximum temperature decreases by 50 K, the average heat transfer coefficients of the internal and external heat exchanger increase by 2.8 and 2.6 W/(m2·K), respectively, and the heat recovery efficiency increases by 4.3%. If the other conditions remain the same, when the equivalent specific heat increases from 0.9 to 1.1, the average temperature of calcined petroleum coke increases from 501 K to 535 K and the maximum temperature increases from 595 K to 644 K. The average heat transfer coefficient of the internal and external heat exchanger decreases by 0.16 and 0.28 W/(m2·K), respectively. The heat recovery efficiency decreases by 4.1%.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by Shandong Provincial Natural Science Foundation, China (ZR2013EEQ005), and Shandong Provincial Science and Technology Development Program, China (2013GGX10404).