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A Green Desuperheater for an Energetic Efficient Alternative to the Decompression Valve in Supercritical Water Hydrolysis Process. CFD Simulation.
Antonio Espuña, Moisès Graells and Luis Puigjaner (Editors), Proceedings of the 27th European Symposium on Computer Aided Process Engineering – ESCAPE 27 October 1st - 5th, 2017, Barcelona, Spain © 2017 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/B978-0-444-63965-3.50486-4
A Green Desuperheater for an Energetic Efficient Alternative to the Decompression Valve in Supercritical Water Hydrolysis Process. CFD Simulation. Luis Vaquerizo,a María José Cocero,a* a
High Pressure Processes Group, Department of Chemical Engineering and Environmental Technology, University of Valladolid, Calle Dr Mergelina S/N, Valladolid, 47011, Spain [email protected]
Abstract The supercritical water hydrolysis (SCWH) of biomass (P=250 bara & T=400 ºC) allows directly obtaining sugars, which are high value products in the chemical industry, in reaction times lower than 0.2 s. The process is characterized by the high selectivity values which can be obtained controlling the reaction time. Reaction kinetics show that glucose degradation is only retarded at temperatures below 250 ºC. Therefore, in the traditional SCWH process, degradation control is achieved expanding the hydrolysis stream in a valve which instantaneously cools down the products. Although the selectivity values obtained are greater than 96 %, the pressure is wasted on the valve expansion decreasing the global energetic efficiency of the process. In this paper a CFD simulation of a desuperheater which mixes the hydrolysis product with pressurized cooling water is presented. The temperature of the hydrolysis stream decreases below 250 ºC in cooling times lower than 20 ms maintaining the pressure at 250 bara and the selectivity value over 90 %. Keywords: Biomass, Hydrothermal Medium, CFD Model, Water Turbine.
1. Introduction The supercritical water hydrolysis (SCWH) of biomass allows obtaining glucose, a building block in the chemical industry, directly from biomass (Cantero et al., 2013). Supercritical water (SCW) (P>221 bara, T>374 ºC), which is the reaction medium, is an easily tunable fluid. Its characteristic properties can be modified just varying the pressure and temperature (Brunner, 2009). The main advantage of this environmental friendly process is the high selectivity values obtained in the hydrolysis due to the precise control of the residence time (Adschiri, 2014) achieved due to the instantaneous heating of the raw materials and cooling of the products. While in the inlet of the reactor, a suspension of biomass is mixed with supercritical water reaching instantaneously the reaction temperature, in the outlet of the reactor, the product stream is decompressed in a valve being instantaneously cooled, stopping the reaction and consequently avoiding the generation of byproducts. The product stream is expanded from the reaction conditions (T=400 ºC and P=250 bara) to a pressure lower than 40 bara which results in a temperature below 250 ºC, temperature at which the reaction is intensively retarded (Sasaki et al., 2000). Analyzing the downstream process, this sudden decompression in the valve results
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in high selectivity values, reaching maximums of 98 %, which avoids the necessity of further separation stages (Cantero et al., 2013). However, from an energetic point of view, a decompression is not efficient since it reduces the temperature and pressure levels avoiding further possibilities of heat and pressure integration. In the chemical industry steam desuperheaters are commonly implemented. The concept of operation is simple; a superheated steam stream is mixed inside the desuperheater with a cooling water stream. The cooling water stream vaporizes absorbing the sensible heat transferred by the superheated steam which becomes saturated (Rahimi et al, 2016). In this work a supercritical water green desuperheater is presented. The green attribute remarks that this cooling process is included in the green chemistry field since the only products involved are biomass hydrolysis products and water. The objective is the substitution of the decompression valve of the SCWH process by a desuperheater in order to increase the heat and pressure integration possibilities. The product stream of the hydrolysis reactor is mixed with a pressurized cooling water stream reducing its temperature to a value below 250 ºC. The cooling process is carried out in a reduced residence time avoiding selectivity losses and therefore the generation of byproducts. From an energetic point of view, the most efficient alternative will be the one which minimizes the energy consumption, which is the objective function defined as the sum of the electrical power and thermal energy required in the process.
2. Model description The desuperheater presented in this paper is based in the mixture of a hydrolysis product stream composed by 81.2 kg/h of water and 2.8 kg/h of hydrolysis products at 400 ºC and 250 bara and 141.8 kg/h of a pressurized cooling water stream at T=27 ºC and P=250 bara. A remarkable dilution effect is produced inside the desuperheater since the mass concentration of hydrolysis products varies from 3.3 % to 1.2 %. Consequently, in order to decrease the complexity of the simulation, only the water fraction has been considered and therefore, the influence of the hydrolysis products in the variation of the physical properties has been neglected. 2.1. CAD model and meshing
Figure 1. CAD representations of the desuperheater modeled in CFD. a) External contour. b) Internal contour.
The external and internal CAD models of the desuperheater which has been modeled in CFD are presented in Figure 1. As it can be seen from the figure, the design of the piece is not complex since simplicity is a required piece characteristic which facilitates further manufacturing and testing possibilities. While the total length of the desuperheater is equal to 170 mm, the inlets and outlet diameters are equal to 1/2 ”. On the left side, two streams, one of pressurized cooling water (the upper one) and other one of hydrolysis
A green desuperheater for an energetic efficient alternative to the decompression valve in biomass supercritical water hydrolysis process. CFD simulation.
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product stream are introduced in the desuperheater. Both streams are mixed in the narrowest part. The objective of this mixing section is to increase the velocity and favor the mixing effects in order to reduce the temperature of the hydrolysis stream as fast as possible. An almost instantaneous decrease in the temperature stops the hydrolysis reaction and avoids the generation of byproducts. In the last section, the diameter of the mixing section is increased in order to stabilize the mixture and obtain a uniform outlet stream. A 400,000 elements mesh has been generated. The independence between the final results and the mesh resolution has been verified considering that the difference between the average outlet temperatures in a simulation carried out with this mesh and in a second simulation in which the mesh elements are doubled is negligible. 2.2. Physical properties In CFD simulations of steam desuperheaters, the modelling of the physical properties is especially relevant due to the strong variations of their values between the liquid and vapor phases. Although in this desuperheater the inlet streams are in liquid and supercritical state, these variations are even more pronounced as a consequence of the proximity of the critical point. An imprecise modelling of the physical properties directly affects the accuracy of the CFD simulation. The simulation of the temperature distribution and the calculation of the required cooling water flow are directly functions of the physical properties values in each point of the desuperheater. An underestimation of the required cooling water flow results in an elevated outlet temperatures at which the hydrolysis reaction is not controlled and consequently byproducts are generated. The modelling of either the density or the specific heat is particularly complex since their values vary from 166 kg/m3 to 1,000 kg/m3 in the case of the density and from 76,300 J/kg·K to 4,115 J/kg·K in the case of the specific heat. Traditional CFD simulations in which supercritical fluids are involved have been always limited by the modelling of the physical properties in the vicinities of the critical point (Bermejo et al, 2010). For this reason, in order to overcome this limitation Ansys Fluent® has been directly connected with Aspen Plus® in which the IAPWS thermodynamic model calculates the values of the physical properties which are implemented in the CFD simulation. (Vaquerizo et al, 2017). 2.3. Turbulence model and boundary conditions The realizable k-İWXUEXOHQFHPRGHOwas selected due to its robustness, accuracy and the lower computational effort required compared with other turbulence models. The behaviour of the fluid in the proximities of the walls was simulated selecting the standard wall functions model. While the inlet mass flows and temperatures where specified in both inlets, the outflow boundary condition was selected in the outlet of the desuperheater.
3. CFD simulation results Figure 2 shows the pressure and temperature evolution profiles obtained in the CFD simulation. As it can be seen, the desuperheater pressure drop is lower than 0.5 bar. The pressure decreases from 250 bara which is the pressure in both inlets to 248.6 bara in the mixing point where the velocity reaches its maximum value. Finally in the stabilizing section, it increases up to 249.6 bara since the velocity is reduced when the section
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increases. Regarding to temperature, in the mixing section differences between the values in the central and in the lateral sections can be observed. While in the wall vicinities the temperature is approximately the same of the hydrolysis product stream, 400 ºC, in the central section the temperature varies from the cooling water stream temperature, 27 ºC, to 236 ºC. This fact proves that as the cooling water stream is injected in the central section, the points near the walls are not effectively mixed and maintain their inlet temperature. Later in the stabilizing section, the change of section produces a mixing effect and the temperature of the whole stream is equalized. The average temperature is equal to 236 ºC with maximum values of 250 ºC (so the degradation reactions are retarded) and minimum values of 232 ºC.
Figure 2. Mixture of the hydrolysis product stream and pressurized cooling water in the desuperheater. a) Pressure evolution profile (bar). b) Temperature evolution profile (K).
Secondly, it is necessary to analyze the required time to reduce the temperature of the hydrolysis product from 400 ºC to a value below 250 ºC at which the hydrolysis is considered to be retarded. As it can be seen from Figure 2, the temperature is stabilized at the beginning of the final section. Therefore, the cooling time is equal to the residence time from the mixing point of both inlet streams to the beginning of the final section. Figure 3 shows the evolution of the residence time along the desuperheater. As it can be seen from this figure, the cooling time is lower than 18 ms.
Figure 3. Mixture of the hydrolysis product stream and pressurized cooling water in the desuperheater. Cooling time evolution profile (ms).
A green desuperheater for an energetic efficient alternative to the decompression 2909 valve in biomass supercritical water hydrolysis process. CFD simulation.
Based in these results, the total hydrolysis time is equal to 33 ms, which is equal to the hydrolysis time to achieve a maximum selectivity value (15 ms) plus the cooling time (18 ms), following the studies of (Cantero et al, 2013) the selectivity value obtained would be higher than 90 %.
4. Downstream process alternatives analysis Once that the substitution of the expansion valve by a desuperheater has been validated, the different heat and pressure integrating possibilities of both downstream processes are presented in Figure 4 and Figure 5 and compared in Table 1. Regarding to the traditional alternative which implements an expansion valve to stop the hydrolysis reaction (Figure 4), the outlet stream of the hydrolysis reactor is expanded in an isoenthalpic valve in order to cool down the products stopping the hydrolysis reaction.
Figure 4. SCWH of biomass downstream process. Expansion valve alternative.
On the other hand, when the expansion valve is substituted by a desuperheater (Figure 5), the hydrolysis product stream and the pressurized cooling water stream are mixed reducing the temperature and consequently stopping the reaction.
Figure 5. SCWH of biomass downstream process. Desuperheater alternative.
The main advantage of the expansion valve alternative is the possibility of obtaining a concentrated product stream reaching a maximum value of 46.3 % w/w when the hydrolysis stream is expanded until atmospheric pressure and the generated vapor stream separated in a flash vessel. On the other hand, the main disadvantage of the implementation of a desuperheater is the intrinsic dilution of the product stream, reaching a maximum concentration value of 1.7 % w/w. Regarding to the heat integration possibilities, in both alternatives it is possible to heat up the raw materials with the product and vapor streams improving the energetic efficiency of the process. However, in the traditional alternative the heat integration possibilities are reduced when the outlet valve pressure is decreased in order to increase the final concentration. While the heat consumption is equal to 47.8 kW in the traditional process considering maximum product concentration, if a desuperheater is implemented it is possible to theoretically recover all the duty supplied (with a lower temperature level).
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Finally, regarding to the power consumption, the intrinsic inefficient characteristic of the traditional alternative is the waste of pressure in the expansion valve. If the hydrolysis stream is expanded until atmospheric pressure to reach maximum product concentration, no energy can be recovered and therefore the energy consumption is equal to the one of the feed pumps, 0.56 kW. On the other hand, the implementation of a desuperheater, although it requires an additional pump, it allows maintaining the pressure level after the hydrolysis reaction and consequently, the implementation of either water turbines or pressure recovery devices compensates that additional pumping energy required and increases the global efficiency of the process. In this example, considering that recovery devices with an efficiency of 90 % are implemented, only 0.15 kW are required which is equal to a reduction of 73.2 % in the power consumption with respect to the original case. Table 1. SCWH of biomass. Downstream process alternatives comparison.
Alternative
Power Balance (kW)
Thermal Balance (kW)
Product Concentration (% w/w)
Expansion Valve Desuperheater
0.56 0.15
47.8 0.0
46.3 1.7
5. Conclusions The substitution of the expansion valve by a desuperheater in the SCWH process has been presented. In order to precisely calculate the physical properties near the critical point, Ansys Fluent® was connected with Aspen Plus®. The results obtained from the simulation prove that the temperature is reduced below 250 ºC in 18 ms maintaining the pressure at 249 bara and the selectivity over 90 %. Comparing both alternatives, while the traditional process allows obtaining highly concentrated products reaching values up to 46 % weight, the implementation of a desuperheater increases the energetic efficiency of the process.
References Cantero, D.A., Bermejo M D, Cocero, 2013. High glucose selectivity in pressurized water hydrolysis of cellulose using ultra-fast reactors. Bioresour. Technol. 135, 697–703. Brunner, G., 2009. Near critical and supercritical water. Part I. Hydrolytic and hydrothermal processes. J. Supercrit. Fluids 47, 373–381. Adschiri, T., 2014. Biomass Conversion in Supercritical Water, in: Supercritical Fluid Technology for Energy and Environmental Applications. Amsterdam, The Netherlands. pp. 89–98. Rahimi, E., Torfeh, S., Kouhikamali, R., 2016. Numerical study of counter-current desuperheaters in thermal desalination units. Desalination 397, 140–150. Sasaki, M., Fang, Z., Fukushima, Y., Adschiri, T., Arai, K., 2000. Dissolution and Hydrolysis of Cellulose in Subcritical and Supercritical Water. Ind. Eng. Chem. Res. 39, 2883–2890. Bermejo MD, Martín Á, Queiroz JPS, Bielsa I, Ríos V, Cocero MJ., 2010. Computational fluid dynamics simulation of a transpiring wall reactor for supercritical water oxidation. Chem Eng J.; 158(3):431-440. Vaquerizo, L., Cocero, M.J., 2017. New tool to improve computational fluid dynamic simulations by improving thermodynamic models description, Chem Eng Science. (submitted).
A Green Desuperheater for an Energetic Efficient Alternative to the Decompression Valve in Supercritical Water Hydrolysis Process. CFD Simulation. Luis Vaquerizo,a María José Cocero,a* a
High Pressure Processes Group, Department of Chemical Engineering and Environmental Technology, University of Valladolid, Calle Dr Mergelina S/N, Valladolid, 47011, Spain [email protected]
Abstract The supercritical water hydrolysis (SCWH) of biomass (P=250 bara & T=400 ºC) allows directly obtaining sugars, which are high value products in the chemical industry, in reaction times lower than 0.2 s. The process is characterized by the high selectivity values which can be obtained controlling the reaction time. Reaction kinetics show that glucose degradation is only retarded at temperatures below 250 ºC. Therefore, in the traditional SCWH process, degradation control is achieved expanding the hydrolysis stream in a valve which instantaneously cools down the products. Although the selectivity values obtained are greater than 96 %, the pressure is wasted on the valve expansion decreasing the global energetic efficiency of the process. In this paper a CFD simulation of a desuperheater which mixes the hydrolysis product with pressurized cooling water is presented. The temperature of the hydrolysis stream decreases below 250 ºC in cooling times lower than 20 ms maintaining the pressure at 250 bara and the selectivity value over 90 %. Keywords: Biomass, Hydrothermal Medium, CFD Model, Water Turbine.
1. Introduction The supercritical water hydrolysis (SCWH) of biomass allows obtaining glucose, a building block in the chemical industry, directly from biomass (Cantero et al., 2013). Supercritical water (SCW) (P>221 bara, T>374 ºC), which is the reaction medium, is an easily tunable fluid. Its characteristic properties can be modified just varying the pressure and temperature (Brunner, 2009). The main advantage of this environmental friendly process is the high selectivity values obtained in the hydrolysis due to the precise control of the residence time (Adschiri, 2014) achieved due to the instantaneous heating of the raw materials and cooling of the products. While in the inlet of the reactor, a suspension of biomass is mixed with supercritical water reaching instantaneously the reaction temperature, in the outlet of the reactor, the product stream is decompressed in a valve being instantaneously cooled, stopping the reaction and consequently avoiding the generation of byproducts. The product stream is expanded from the reaction conditions (T=400 ºC and P=250 bara) to a pressure lower than 40 bara which results in a temperature below 250 ºC, temperature at which the reaction is intensively retarded (Sasaki et al., 2000). Analyzing the downstream process, this sudden decompression in the valve results
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in high selectivity values, reaching maximums of 98 %, which avoids the necessity of further separation stages (Cantero et al., 2013). However, from an energetic point of view, a decompression is not efficient since it reduces the temperature and pressure levels avoiding further possibilities of heat and pressure integration. In the chemical industry steam desuperheaters are commonly implemented. The concept of operation is simple; a superheated steam stream is mixed inside the desuperheater with a cooling water stream. The cooling water stream vaporizes absorbing the sensible heat transferred by the superheated steam which becomes saturated (Rahimi et al, 2016). In this work a supercritical water green desuperheater is presented. The green attribute remarks that this cooling process is included in the green chemistry field since the only products involved are biomass hydrolysis products and water. The objective is the substitution of the decompression valve of the SCWH process by a desuperheater in order to increase the heat and pressure integration possibilities. The product stream of the hydrolysis reactor is mixed with a pressurized cooling water stream reducing its temperature to a value below 250 ºC. The cooling process is carried out in a reduced residence time avoiding selectivity losses and therefore the generation of byproducts. From an energetic point of view, the most efficient alternative will be the one which minimizes the energy consumption, which is the objective function defined as the sum of the electrical power and thermal energy required in the process.
2. Model description The desuperheater presented in this paper is based in the mixture of a hydrolysis product stream composed by 81.2 kg/h of water and 2.8 kg/h of hydrolysis products at 400 ºC and 250 bara and 141.8 kg/h of a pressurized cooling water stream at T=27 ºC and P=250 bara. A remarkable dilution effect is produced inside the desuperheater since the mass concentration of hydrolysis products varies from 3.3 % to 1.2 %. Consequently, in order to decrease the complexity of the simulation, only the water fraction has been considered and therefore, the influence of the hydrolysis products in the variation of the physical properties has been neglected. 2.1. CAD model and meshing
Figure 1. CAD representations of the desuperheater modeled in CFD. a) External contour. b) Internal contour.
The external and internal CAD models of the desuperheater which has been modeled in CFD are presented in Figure 1. As it can be seen from the figure, the design of the piece is not complex since simplicity is a required piece characteristic which facilitates further manufacturing and testing possibilities. While the total length of the desuperheater is equal to 170 mm, the inlets and outlet diameters are equal to 1/2 ”. On the left side, two streams, one of pressurized cooling water (the upper one) and other one of hydrolysis
A green desuperheater for an energetic efficient alternative to the decompression valve in biomass supercritical water hydrolysis process. CFD simulation.
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product stream are introduced in the desuperheater. Both streams are mixed in the narrowest part. The objective of this mixing section is to increase the velocity and favor the mixing effects in order to reduce the temperature of the hydrolysis stream as fast as possible. An almost instantaneous decrease in the temperature stops the hydrolysis reaction and avoids the generation of byproducts. In the last section, the diameter of the mixing section is increased in order to stabilize the mixture and obtain a uniform outlet stream. A 400,000 elements mesh has been generated. The independence between the final results and the mesh resolution has been verified considering that the difference between the average outlet temperatures in a simulation carried out with this mesh and in a second simulation in which the mesh elements are doubled is negligible. 2.2. Physical properties In CFD simulations of steam desuperheaters, the modelling of the physical properties is especially relevant due to the strong variations of their values between the liquid and vapor phases. Although in this desuperheater the inlet streams are in liquid and supercritical state, these variations are even more pronounced as a consequence of the proximity of the critical point. An imprecise modelling of the physical properties directly affects the accuracy of the CFD simulation. The simulation of the temperature distribution and the calculation of the required cooling water flow are directly functions of the physical properties values in each point of the desuperheater. An underestimation of the required cooling water flow results in an elevated outlet temperatures at which the hydrolysis reaction is not controlled and consequently byproducts are generated. The modelling of either the density or the specific heat is particularly complex since their values vary from 166 kg/m3 to 1,000 kg/m3 in the case of the density and from 76,300 J/kg·K to 4,115 J/kg·K in the case of the specific heat. Traditional CFD simulations in which supercritical fluids are involved have been always limited by the modelling of the physical properties in the vicinities of the critical point (Bermejo et al, 2010). For this reason, in order to overcome this limitation Ansys Fluent® has been directly connected with Aspen Plus® in which the IAPWS thermodynamic model calculates the values of the physical properties which are implemented in the CFD simulation. (Vaquerizo et al, 2017). 2.3. Turbulence model and boundary conditions The realizable k-İWXUEXOHQFHPRGHOwas selected due to its robustness, accuracy and the lower computational effort required compared with other turbulence models. The behaviour of the fluid in the proximities of the walls was simulated selecting the standard wall functions model. While the inlet mass flows and temperatures where specified in both inlets, the outflow boundary condition was selected in the outlet of the desuperheater.
3. CFD simulation results Figure 2 shows the pressure and temperature evolution profiles obtained in the CFD simulation. As it can be seen, the desuperheater pressure drop is lower than 0.5 bar. The pressure decreases from 250 bara which is the pressure in both inlets to 248.6 bara in the mixing point where the velocity reaches its maximum value. Finally in the stabilizing section, it increases up to 249.6 bara since the velocity is reduced when the section
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increases. Regarding to temperature, in the mixing section differences between the values in the central and in the lateral sections can be observed. While in the wall vicinities the temperature is approximately the same of the hydrolysis product stream, 400 ºC, in the central section the temperature varies from the cooling water stream temperature, 27 ºC, to 236 ºC. This fact proves that as the cooling water stream is injected in the central section, the points near the walls are not effectively mixed and maintain their inlet temperature. Later in the stabilizing section, the change of section produces a mixing effect and the temperature of the whole stream is equalized. The average temperature is equal to 236 ºC with maximum values of 250 ºC (so the degradation reactions are retarded) and minimum values of 232 ºC.
Figure 2. Mixture of the hydrolysis product stream and pressurized cooling water in the desuperheater. a) Pressure evolution profile (bar). b) Temperature evolution profile (K).
Secondly, it is necessary to analyze the required time to reduce the temperature of the hydrolysis product from 400 ºC to a value below 250 ºC at which the hydrolysis is considered to be retarded. As it can be seen from Figure 2, the temperature is stabilized at the beginning of the final section. Therefore, the cooling time is equal to the residence time from the mixing point of both inlet streams to the beginning of the final section. Figure 3 shows the evolution of the residence time along the desuperheater. As it can be seen from this figure, the cooling time is lower than 18 ms.
Figure 3. Mixture of the hydrolysis product stream and pressurized cooling water in the desuperheater. Cooling time evolution profile (ms).
A green desuperheater for an energetic efficient alternative to the decompression 2909 valve in biomass supercritical water hydrolysis process. CFD simulation.
Based in these results, the total hydrolysis time is equal to 33 ms, which is equal to the hydrolysis time to achieve a maximum selectivity value (15 ms) plus the cooling time (18 ms), following the studies of (Cantero et al, 2013) the selectivity value obtained would be higher than 90 %.
4. Downstream process alternatives analysis Once that the substitution of the expansion valve by a desuperheater has been validated, the different heat and pressure integrating possibilities of both downstream processes are presented in Figure 4 and Figure 5 and compared in Table 1. Regarding to the traditional alternative which implements an expansion valve to stop the hydrolysis reaction (Figure 4), the outlet stream of the hydrolysis reactor is expanded in an isoenthalpic valve in order to cool down the products stopping the hydrolysis reaction.
Figure 4. SCWH of biomass downstream process. Expansion valve alternative.
On the other hand, when the expansion valve is substituted by a desuperheater (Figure 5), the hydrolysis product stream and the pressurized cooling water stream are mixed reducing the temperature and consequently stopping the reaction.
Figure 5. SCWH of biomass downstream process. Desuperheater alternative.
The main advantage of the expansion valve alternative is the possibility of obtaining a concentrated product stream reaching a maximum value of 46.3 % w/w when the hydrolysis stream is expanded until atmospheric pressure and the generated vapor stream separated in a flash vessel. On the other hand, the main disadvantage of the implementation of a desuperheater is the intrinsic dilution of the product stream, reaching a maximum concentration value of 1.7 % w/w. Regarding to the heat integration possibilities, in both alternatives it is possible to heat up the raw materials with the product and vapor streams improving the energetic efficiency of the process. However, in the traditional alternative the heat integration possibilities are reduced when the outlet valve pressure is decreased in order to increase the final concentration. While the heat consumption is equal to 47.8 kW in the traditional process considering maximum product concentration, if a desuperheater is implemented it is possible to theoretically recover all the duty supplied (with a lower temperature level).
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Finally, regarding to the power consumption, the intrinsic inefficient characteristic of the traditional alternative is the waste of pressure in the expansion valve. If the hydrolysis stream is expanded until atmospheric pressure to reach maximum product concentration, no energy can be recovered and therefore the energy consumption is equal to the one of the feed pumps, 0.56 kW. On the other hand, the implementation of a desuperheater, although it requires an additional pump, it allows maintaining the pressure level after the hydrolysis reaction and consequently, the implementation of either water turbines or pressure recovery devices compensates that additional pumping energy required and increases the global efficiency of the process. In this example, considering that recovery devices with an efficiency of 90 % are implemented, only 0.15 kW are required which is equal to a reduction of 73.2 % in the power consumption with respect to the original case. Table 1. SCWH of biomass. Downstream process alternatives comparison.
Alternative
Power Balance (kW)
Thermal Balance (kW)
Product Concentration (% w/w)
Expansion Valve Desuperheater
0.56 0.15
47.8 0.0
46.3 1.7
5. Conclusions The substitution of the expansion valve by a desuperheater in the SCWH process has been presented. In order to precisely calculate the physical properties near the critical point, Ansys Fluent® was connected with Aspen Plus®. The results obtained from the simulation prove that the temperature is reduced below 250 ºC in 18 ms maintaining the pressure at 249 bara and the selectivity over 90 %. Comparing both alternatives, while the traditional process allows obtaining highly concentrated products reaching values up to 46 % weight, the implementation of a desuperheater increases the energetic efficiency of the process.
References Cantero, D.A., Bermejo M D, Cocero, 2013. High glucose selectivity in pressurized water hydrolysis of cellulose using ultra-fast reactors. Bioresour. Technol. 135, 697–703. Brunner, G., 2009. Near critical and supercritical water. Part I. Hydrolytic and hydrothermal processes. J. Supercrit. Fluids 47, 373–381. Adschiri, T., 2014. Biomass Conversion in Supercritical Water, in: Supercritical Fluid Technology for Energy and Environmental Applications. Amsterdam, The Netherlands. pp. 89–98. Rahimi, E., Torfeh, S., Kouhikamali, R., 2016. Numerical study of counter-current desuperheaters in thermal desalination units. Desalination 397, 140–150. Sasaki, M., Fang, Z., Fukushima, Y., Adschiri, T., Arai, K., 2000. Dissolution and Hydrolysis of Cellulose in Subcritical and Supercritical Water. Ind. Eng. Chem. Res. 39, 2883–2890. Bermejo MD, Martín Á, Queiroz JPS, Bielsa I, Ríos V, Cocero MJ., 2010. Computational fluid dynamics simulation of a transpiring wall reactor for supercritical water oxidation. Chem Eng J.; 158(3):431-440. Vaquerizo, L., Cocero, M.J., 2017. New tool to improve computational fluid dynamic simulations by improving thermodynamic models description, Chem Eng Science. (submitted).