Exergetic optimization of phosphoric acid factory power plant

Abstract

Keywords: Condenser; Energy Efficiency; Exergy Efficiency; Phosphoric Acid Plant; Steam Turbine Generator

Introduction

The Tunisian production of Phosphoric Acid is among the five important ones in the world. Indeed the phosphate constitutes an important factor for the country economy balance. The annual production of Phosphoric Acid is about 500 000 tonnes. Despite the economic importance of the phosphate industry, the total annual cost of the energy production is very substantial. To overcome this problem, the Tunisian Chemical Group (TCG) established programs in the purpose to improve the quality of production and increase the performance of the different plants. Among these programs, a study is developed on the performance optimization of thermal power plant operated in phosphoric acid factory. This study is conducted by the Applied Thermodynamic Research Unit in collaboration with TCG. Furthermore, several investigations were conducted on energy and exergy optimization of chemical industrial factory power plants. S. Adibhatla et al. [1] carried out an energetic and exergetic analysis of 660 MWe thermal power plant at different load conditions and according to two operating modes: under constant pressure and under pure sliding pressure. The performance criteria are defined. Therefore, the exergy destruction rates are identified for each component. In the developed analysis, it has been shown that the boiler has the highest source of exergy losses followed by the turbine. Moreover, considering the two indicated operating modes, the results reveal a significant decrease in exergy destruction rate for the turbine and boiler feed pump when operating in sliding pressure mode. CJ Koroneos et al. [2] conducted an exergy analysis of a 300 MW lignite thermoelectric power plant. A comparative study is established between the actual plant and three proposed combined heat and power systems. Equal amount of fuel is used for the three considered systems working according to Rankine cogeneration cycle. Obtain results show that the cogeneration system design leads to an improvement at about 8.5% in energy production compared to other proposed configurations. A Thermodynamic and exergoeconomic analysis of thermal power plant is performed by A. Bolatturk et al. [3]. Using EES software the inlet and outlet thermodynamic properties of each component are determined. That permits to define energy and exergy efficiencies. Obtained results show that the main amounts of exergy losses are located in the boiler, in the turbine, in the condenser, in the heater and in the pump groups. While the This paper was recommended for publication in revised form by Regional Editor Jaap Hoffman 1 Department of Mechanical Engineering, National Engineering School of Gabes, Tunisia 2 Department of Chemical Engineering, National Engineering School of Gabes, Tunisia 3 Tunisian Chemical Group, Gabes, Tunisia *E-mail address: taherkhir@Yahoo.fr Manuscript Received 22 January 2016, Accepted 1 July 2016

Journal of Thermal Engineering, Research Article, Vol. 3, No. 5, pp. 1428-1441, October, 2017 highest amount of exergy loss costs are observed in the boiler, followed by the turbines and the condenser. Authors suggest that exergy and economic analysis of the thermal plants in project stage may be helpful to undertake future investigations and can minimize significantly the energy consumption of thermal systems. A. Atmaca. et al. [4] are developed a thermodynamic and exergoeconomic analysis of a cement plant placed in Gaziantep, Turkey. The considered plant has highest energy consumption and it is classified among the most industrial source of the CO2 emissions. In order to evaluate the performance of the factory, the authors are established mass, thermal and exergy balances for each component considering variation ranges of operating parameters. A set of performance criteria are defined in the aim to conduct this analysis. A general methodology for exergy balance in chemical and thermal process integrated in the ProSimPlus code was performed by A. Ghannadzadeh et al [5]. In the purpose to fully automate the exergy analysis, the authors established an exergy balance for the whole system using only one software. The adopted procedure permits not only to identify the exergy destruction source but it also to reduce the exergy losses. M. N. Khan et al. [6] developed a thermodynamic optimization for a four configurations of a combined steam and gas cycle with heat recovery steam generator. A parametric study is carried out in the purpose to determine the effect of the pressure ratio and the inlet turbine temperature (TIT) on the cycle performance. The results reveal that an increase in TIT leads to an increase in net output power of about 32.1% with 1500 K and

System Description

The diagram of the Phosphoric Acid Thermal Power Plant is presented in Figure 1. This plant is installed in the industrial area of the Tunisian Chemical Group (TCG) located in Gabes (South East - Tunisia). The main product of this factory is the Phosphoric Acid with about 1500 t as daily production. The thermal power plant of the indicated factory is mainly constituted by two steam turbine cycles STGI and STGII used to provide about 14 MW as total net electrical power required for the different units. The High Pressure steam (HP) mass flow rate, consumed by STGI and STGII is generated by an Evaporator Boiler Pre-superheater Superheater group (EBPS) at about 40 bars and 410 °C. The steam turbine cycle STGI is with extraction and condensation, while the second one STGII is with back pressure turbine.

Bl: Blower, CT: Condensate Tank, CTb:Turbo-blower Condenser, De: Deaerator, DU: Distillation Unit, EBPS: Evaporator_Boiler_Pre-superheater_Superheater, Ph.A. C.U: Phosphoric Acid Concentration Unit, SMM: Sulfur Melting and Maintenance, ST: Storage Tank, STGI/II: Steam Turbine Generator I/II, Tb: Turbine, SWP: Seawater Pump, TC: Turbine Condenser

Journal of Thermal Engineering, Research Article, Vol. 3, No. 5, pp. 1428-1441, October, 2017 The Medium Pressure steam (MP) at 12 bars and 280 °C and Low Pressure steam (LP) at 6 bars and 230°C, used to supply the other different units, are obtained by the expansion of HP stream through appropriate devices (8-9) and (13-14). For the steam turbine STGI the input steam mass flow rate (point 5) is expanded to reach the extraction level at Low pressure (point 6). The remained steam flow rate (called condensation rate) is extended through the last stage of the turbine to reach the condensation pressure level at point 7. The condensation occurs in the seawater turbine condenser (TC). The tank (CT) is used for condensate storage. In the second steam turbine cycle STGII, HP steam expands from point 10 to reach the medium pressure MP at the extraction level (point 11). The remained stream is expanded to low pressure LP (point 12). The Turbo-blower Tb is used to provide compressed air for the sulfur combustion process in a sulfuric unit not presented on the diagram. The steam from the Turbo blower is condensed in CTb and then transferred to the storage tank CT. The MP and LP streams are used in the units (SMM, Ph. A CU, DU, TC, CTb and De). The condensate issued from all the indicated units is transferred to the tank (CT) in order to feed two deaerators (De) working under the same conditions. A water treatment is performed inside the deaerators before boilers supply. Appropriate sensors are used to measure the operating parameters such as: temperatures, pressures and mass flow rates. The variation ranges of these parameters are given in Table 1. Table 1. Operating parameter ranges

Energy And Exergy Balances

In order to perform the energy and exergy analysis the following assumptions are considered [24]: - All process are assumed as steady-state and steady flow - The kinetic , potential and chemical exergy are neglected - The dead state was considered as P0=1.013 bar and T0= 293.15 K - No chemical reaction is occurred in the different processes For an open system and taking into account the indicated assumptions, the energetic and exergetic balances can be expressed as flow:

where the exergy transferred by heat is given by: 𝑇 𝑋̇ℎ𝑒𝑎𝑡 = ∑ (1 − 𝑇0 ) 𝑄̇

According to the stream numbering indicated in Figure 1, the energy and exergy balances for each component are given as follows: STG I

Journal of Thermal Engineering, Research Article, Vol. 3, No. 5, pp. 1428-1441, October, 2017 𝑊̇

Results And Interpretations

The thermal power plant was analyzed for real operating conditions during whole year. The main operating parameters are the turbine supply mass flow rate, HP steam temperature and pressure. In the other hand the seawater temperature varies sensibly for the different seasons in the local region. That may affect the 1433

Journal of Thermal Engineering, Research Article, Vol. 3, No. 5, pp. 1428-1441, October, 2017 performances of power plant components supplied by seawater. Hence all the indicated parameters will be taken into consideration for the following analytic study. A numerical code is established using EES software to perform the calculations required for the thermal and exergy plant analysis. The fluids properties of the different streams are given in Table 2. Table 2. Fluid properties in the different streams Stream

34. Water

34.17 15.55 3.33 3.33 18.05 12.5 5.55 0.84 0.84 22.77 NO 22.77 3.34 3.34 3.34 2.23 1.95 9.7 9.7 1.39 8.34 14.73 14.73 5.55 3.33 38.33 5 45.55 0.55 37.5 151.01 151.01 151.01 341.91 341.91 341.91 28.49 21.37 66.92 66.92

397 389 392 50 386 220 43 386 350 392 NO 250 392 360 190 99 165 165 54 165 54 120 97 41 40 97 49 104 104 104 27 29 37 27 29 33 29 50 27 61

38 39.65 37.7 0.12 37.5 5.64 0.09 37.5 5.8 37.7 NO 5.7 37.7 12 12 1 5.7 5.7 3 5.7 3 1.1 0.92 0.08 0.08 1.5 2.8 1.2 1.1 75 1.013 2.7 1.5 1.013 2.7 1.5 2.7 1.5 1.013 1.331 Losses Mass flow rate (kg/s) 1.39 1.11 1.39

3210 3188 3199 2542 3185 2895 2338 3185 3166 3199 NO 2958 3199 3186 2790 417.4 2773 2773 226.1 2773 226.1 2715 406.2 171.6 167.4 406.3 205.2 435.7 2683 443.5 113.2 121.7 155 113.2 121.7 138.3 121.7 209.3 67.26 245.9

6.786 6.734 6.772 7.937 6.754 7.086 7.422 6.754 7.562 6.772 NO 7.206 6.772 7.579 6.534 1.302 6.819 6.819 0.7543 6.281 0.7543 7.42 1.272 0.5852 0.5719 1.272 0.6901 1.350 7.336 1.351 0.3951 0.4228 0.5318 0.3951 0.4228 0.4777 0.4228 0.703 5.842 6.331

40699 18426 3946 601.9 21226 9031 720.07 979.7 763.1 26967 NO 18536 3946 3104 3920 168.8 1447 1447 48.72 1654 48.72 3542 219.1 9.35 4.906 1212 34.15 1178 278 1697 14162 42526 159782 14162 171100 171100 8024 91.64 5.27 1048

The Irreversibility rates of the different power plant components are presented in Figure 2. The minimum irreversibility rates are obtained for the condensers and deaerators followed by the blowers and turbines. The Steam Turbine Generator STGI presents the maximum irreversibility rates of about 4.1 MW

Journal of Thermal Engineering, Research Article, Vol. 3, No. 5, pp. 1428-1441, October, 2017

Figure 2. Irreversibility rates of thermal power plant components

The energy and exergy efficiencies are showed in Figure 3. The blower, turbines and deaerators present the better energy efficiencies. The minimum energy efficiencies are obtained for the condensers. The steam turbines and the deaerators present an exergy efficiencies above 62%. The minimum values are obtained for the condensers (20 – 25 %).

Figure 3. Energetic and exergetic efficiency of main components The variation of the net power generated by the steam turbine STGI according to HP steam mass flow rate is presented in Figure 4 for different value of condensation rate. The generated power increases gradually with HP steam mass flow rate. One can see that, for HP steam flow rate less than 35 t/h the generated power in not significantly affected by the condensation rate. While this parameter, has a sensible influence on the generated power for HP steam flow rate above 40 t/h. In fact, in this range, increasing the condensate rate leads to the enhancement of the generated power. A maximum net power of about 6 MW is obtained for 20 t/h of condensation rate. For the back pressure steam turbine STGII, the variation of the net generated power according to HP steam mass flow rate is presented in Figure 5. The generated power increases linearly to achieve about 7 MW for a HP steam mass flow rate of about 82 t/h. The total power generated by the two Steam turbines is widely sufficient for the plant requirements.

Journal of Thermal Engineering, Research Article, Vol. 3, No. 5, pp. 1428-1441, October, 2017

Figure 4. Variation of STGI net power with HP steam flow rate 8

Figure 5. Variation of steam turbine STGII power with HP steam flow rate The variation of the exergetic efficiency of steam turbine STGI according to HP steam flow rate is presented in Figure 6 for different values of condensate flow rate. The exergy efficiency increases with mHP to reach maximum values of about 49%, 51%, 52% and 54% for condensation flow rates of 8, 12, 18 and 20 t/h respectively. The optimum mHP values leading to the indicated maximum exergy effeciencies are respectively 55, 46, 52 and 54 t/h. For the back pressure steam turbine STGII, the variation of the exergetic efficiency according to HP steam mass flow rate is presented in Figure 7. The exergetic efficiency increases sensibly with mHP to reach a maximum value of about 75.5 % for a mass flow rate of about 73 t/h. That can be considerd as an optimum value for the STGII supply. Figure 8 illustrates the variation of energy and exergy efficiencies of steam turbines STGI and STGII according to HP steam pressure. For the explored ranges of HP steam pressure, the energy efficiencies of steam turbine generators STGI and STGII increase of about 1.37 % and 8.8 % respectively. While the exergy efficiencies increase of about 2.46 for STGI and 6.8 % for STGII.

Journal of Thermal Engineering, Research Article, Vol. 3, No. 5, pp. 1428-1441, October, 2017

Figure 6. Variation of exergetic efficiency of steam turbine STG I according to HP steam flow rate 0,8

Stgii

Figure 7. Variation of exergy efficiency of STGII according to HP steam flow rate 0,95 e,STG II

Figure 8. Energetic and exergetic efficiency variations of STGI and STGII according to P HP The influence of seawater temperature on the condenser irreversibility rates is presented in Figure 9. The irreversibility rates decrease with the increase of T sw. For a seawater temperature variation of 22 °C the 1437

Journal of Thermal Engineering, Research Article, Vol. 3, No. 5, pp. 1428-1441, October, 2017 irreversibility rates decrease of about 35 %. Indeed if the Tsw increases the temperature difference between the two streams through the condenser decreases too, therefore the irreversibility rate due to temperature gradient

I T decreases. That affects the condenser total irreversibility rate. 1000 950 900

Figure 9. Influence seawater temperature on irreversibility of condensers Figure 10 depicts the variation of the condenser exergy efficiency according to seawater temperature. It can be seen that increasing the seawater temperature from 12 to 24 °C leads to an increase of the exergy efficiency of about 4 times for the turbo blower condenser and 14 times for the turbine condenser. For T sw above 25 °C the exergy efficiencies increase slightly to reach maximum values of about 35 % and 45 % for the turbine condenser and the turbo-blower condenser respectively. Although the indicated rise of the exergetic efficiency the obtained values are very low especially in cold seasons when the seawater temperature is less than 15 °C. These results agree with I. H. Aljundi investigations [25] on energy and exergy analysis of a steam power plant. In fact the authors obtained the same values of condenser exergy efficiency in similar operating conditions. 0,5

Figure 10. Variation of exergy efficiency of condensers according to seawater temperature

Concluding Remarks

An Energetic and Exergetic Analysis is conducted on a Steam Turbine Power Plant used in existing Phosphoric Acid Factory. The heat recovery systems used in the different parts of the plant are also considered in the analysis. Mass, thermal and exergy balances are established on the main compounds of the factory. The effects of the key operating parameters such as seawater temperature, and mass flow rate on the cycle performances are investigated. The obtained results can be presented as follows. The minimum irreversibility rates are obtained for the condensers and deaerators followed by the blowers and turbines. The Steam Turbine Generator STGI presents the maximum irreversibility rates of about

Acknowledgments

Authors would like to thank the Tunisian Chemical Group for their support and collaboration.

Nomenclature

Bl Blower Cp Specific heat at constant pressure (kJ/kg.K) CU Concentration Phosphoric Acid Unit De Deaerator DU Distillation Unit Exergy (kW) Specific Enthalpy (kJ/kg) High Pressure steam Low Pressure Steam Medium Pressure Steam Mass flow rate (kg/s) m PAP Unit of Phosphoric Acid Ph. A Phosphoric Acid R Gas constant (kJ/kmol.K) SMM Sulfur Melting and Maintenance STGI Steam Turbine Generator I STGII Steam Turbine Generator II T Temperature (°C) Tb Turbine TC Turbine Condenser

Reference state Condenser of Turbine destruction Deaerator dry air energy exergy Gear humid air inlet outlet Pump seawater water vapor valve

Journal of Thermal Engineering, Research Article, Vol. 3, No. 5, pp. 1428-1441, October, 2017 Greek letters ε η

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