Experimental Study of the Dynamic Behavior of the Stripping Column

Article pubs.acs.org/EF

Experimental Study of the Dynamic Behavior of the Stripping Column for Post-combustion CO2 Capture with Monoethanolamine Xiaofei Li, Shujuan Wang,* and Changhe Chen Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Beijing Key Laboratory for CO2 Utilization and Reduction Technology, Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: CO2 capture from coal-fired power plants using a monoethanolamine (MEA) solution is one of the most promising technologies for CO2 abatement. The MEA absorption process for CO2 capture from power plants is an inherently dynamic system that is affected by the load variations in the upstream power plant because of fluctuations in electricity demand. This paper presents an experimental study of the dynamic behavior of a stripping column after various disturbances. Tests of negative and positive steps of different magnitudes in the reboiler heat duty were conducted to determine the process gain and time constant. The dynamic behavior of the desorption process was also investigated using ramp tests of different magnitude in the reboiler heat duties and feed solvent temperatures. The results show that the desorption process response to the heat duty could be approximated as a first-order time delay model. The solvent holdup time in the reboiler was the dominant factor controlling the system response time. The results also show that the CO2 capture is a nonlinear process.

1. INTRODUCTION Fossil-fuel-fired power plants are considered to be the largest source of worldwide CO2 emissions.1,2 Numerous capture technologies, such as post-combustion and pre-combustion capture and oxy-fuel combustion, have been developed to reduce CO2 emissions. Post-combustion capture by chemical absorption using aqueous monoethanolamine (MEA) is relatively mature and suitable for retrofitting existing power plants. Thus, it is considered to be the dominant technology for CO2 capture.3,4 However, the greatest challenge for the MEA-based CO2 capture system is high energy consumption in the stripping column.5−7 Much work has been focused on optimizing the operating parameters of the MEA-based CO 2 capture process.8−14 Alternative process configurations have also been proposed to reduce the energy consumption and operating costs.15−20 These studies were all steady-state investigations that assumed that the power plant operated continuously at a constant load, while the dynamic behavior of the capture system was not considered. A power plant may be operated at full load during the peak hours and partial load during the offpeak hours because of electricity demand fluctuations.21,22 The load variations in the upstream power plant then lead to changes in the exhaust gas flow rates, gas properties, and reboiler heat duty. These may cause operating challenges for the CO2 capture system.23 A better understanding of the dynamic behavior of the CO2 capture system is helpful for designing control strategies and optimizing the operation of the capture system in response to possible disturbances. Many researchers have developed dynamic models for simulations of amine-based CO2 capture plants. However, most researchers have only focused on proposing the dynamic model of individual units of the amine-based CO2 capture system. Kvamsdal et al.,24 Lawal et al.,25 Jayarathna et al.,26 and Posch et al.27 developed dynamic models for standalone absorbers. Ziaii et al.28 and Greer et al.29 described dynamic models for standalone stripper columns. Lawal et al.30,31 © 2014 American Chemical Society

proposed a dynamic model for describing the complete absorption/desorption process of CO2 capture from coal-fired power plants, so that the transient behavior of the complete process could be explored. Gaspar and Cormos32 presented and validated a dynamic model of a complete absorber/desorber system. They examined the effects of the power plant load and the temperature of the input-rich solution. Harun et al.21 developed a dynamic MEA absorption process model to predict the transient response of a MEA absorption process to changes in the flue gas flow rate and reboiler heat duty. Biliyok et al.33 introduced a dynamic model with dynamic validation. The validated model was then used to analyze the effect of increasing the flue gas moisture content and the impact of intercooling on the process performance. Recently, Jayarathna et al.34 and MacDowell et al.35 also developed dynamic, nonequilibrium models for CO2 capture plants. However, most of these models were only validated with steady-state experimental data at the University of Texas at Austin.36 Without dynamic validation, there is no guarantee that the dynamic models proposed in these references can accurately predict the dynamic response. Only two studies27,33 have presented dynamic validation studies, but the dynamic experimental data were limited to the absorber. There are few dynamic experimental and analytical studies of the dynamic behavior of the stripping column for the CO2 capture system in the literature. In this study, an experimental investigation at the laboratory scale was undertaken to describe the dynamic behavior of a stripping column. This study presents the transient response of the desorption process to changes in the reboiler heat duty and feed-rich solution temperature. Received: August 22, 2013 Revised: January 18, 2014 Published: January 20, 2014 1230

dx.doi.org/10.1021/ef402138m | Energy Fuels 2014, 28, 1230−1241

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Figure 1. Schematic diagram of the experimental setup. adjusting the lean solvent outlet valve. The system was operated until a steady state was reached. A steady state was achieved when the temperatures at all locations, the CO2 gas flow rate, and the stripper pressure were all stable. After 20 min of stable operation, a disturbance was introduced into the system. After the disturbance, the final conditions were maintained for another 60 min to ensure that the process reached a new steady state. Lean solution samples were taken to analyze the CO2 loading of the lean solution at various operating times. The temperatures, reboiler heat duty, gas flow rate, and stripper pressure were simultaneously recorded by the computer throughout the operation. 2.3. Measurements. The aqueous solution concentration was determined by titration with standard H2SO4. The CO2 loading of the rich solution was determined using precipitation titration. A pressure transmitter with an operating range of 0−0.2 MPa and an accuracy of ±0.2% was used to measure the stripper top pressure. Pt100 resistance temperature detectors (RTDs) with an accuracy of ±0.1 °C were used to record the temperatures. The positions of the RTDs attached to the stripper are given in Figure 2. The feedback real heat duty was record by the SCADA software in the computer. The temperature and pressure were monitored and recorded by an Agilent 34970A data logger. 2.4. Operating Conditions. A total of 10 tests were conducted in this experimental setup. Each test was begun with base conditions shown in Table 1. A 30 wt % MEA solution was used in each case. The CO2 loading of the amine solution was 0.5 mol of CO2/mol of MEA. The rich solution flow rate was 84.1 g/min. The temperature of the feed-rich solution was 85 °C. The stripper top pressure was kept at 150 kPa. The reboiler heat duty was 300 W.

2. EXPERIMENTAL SECTION 2.1. Materials. Reagent-grade MEA of 99% purity was used as the chemical solvent. The MEA solvent was diluted with deionized water to the desired concentration. The solution then absorbed pure CO2 gas (purity ≥ 99.99 mol %) until the desired CO2 loading was obtained. 2.2. Experimental Procedure. The objective of the present experiments was to determine the dynamic behavior of the stripping process after various disturbances and, thus, to provide fundamental information for designing control strategies and optimizing the operation of the capture system in response to possible disturbances. The schematic diagram of the experimental apparatus is shown in Figure 1. The apparatus consisted of a stripper, a preheater, a condenser, and associated control and measurement instrumentation. The sump at the bottom of the stripper is used as the reboiler with a 1.2 kW maximum capacity. The stripper contained a 1000 mm long, 60 mm internal diameter packed column filled with Dixon rings (2.5 × 2.5 mm). The stripper column, the reboiler, and all connection pipes were insulated to reduce heat losses. The stripper pressure was controlled by an overhead needle valve with a fixed opening. The reboiler heat duty was controlled by a power controller (Yingjie Electric Company, China), with a deviation of ±1 W. The feed temperature of the rich amine solution was achieved by letting the solution flow through coils in the oil bath. The feed-rich solvent temperature was controlled by the preheater. Each experimental run began by introducing about 800 mL of the rich solution into the reboiler. Then, the rich solution was heated at a constant reboiler heat duty. Once the preheater reached the desired set point and the reboiler temperature reached the boiling point, the rich amine solution was pumped continuously at the desired flow rate into the top of the stripper. The rich amine solution flow rate was controlled using a metering pump. As a result, the rich solution was regenerated in the stripper with a mixture of CO2 and water vapor released from the solution. The CO2 and water vapor mixture flowing out of the top of the stripper was then condensed in the condenser. The remaining mixture flowed through the drying pipe, allowing only pure CO2 to leave the system. The CO2 gas flow rate was measured by a wet gas flow meter. The liquid level in the stripper sump was kept constant by

3. RESULTS AND DISCUSSION 3.1. Step Changes in the Reboiler Heat Duty. The reboiler heat duty is the most important parameter for the stripping column. When the power plant operates at full load during peak hours and at partial load during the off-peak hours because of fluctuations in electricity demand, the steam supply to the reboiler and the flue gas flow rate will change. When the 1231


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control the CO2 removal efficiency.31,38−40 As shown in Figure 3, tests of negative and positive steps of different magnitude in the reboiler heat duty were conducted to investigate the dynamic characteristic of the stripping process. Figure 4 shows the stripper top pressure responses to positive and negative 10 and 30% step changes in the reboiler heat duty. As shown in Figure 4, a positive 10% step increase in the reboiler heat duty caused a 3.3% increase in the stripper pressure. While a positive 30% step increase in the reboiler heat duty casued a 12.0% pressure increase. When the reboiler heat duty was suddenly reduced by 10%, the stripper pressure decreased by 2.7%, while a 30% decrease caused a 7.4% stripper pressure decrease. The different magnitudes in pressure responses to the positive and negative step changes in the reboiler heat duty show the degree of nonlinearity in the desorption process. The stripper pressure response to the step changes in the reboiler heat duty can be approximated as a firstorder time delay model, so that the process dynamic characteristic can be determined using the transfer function G(s) = (Kp/(τps + 1))e−τds, where Kp represents the process gain for the parameter, τd represents the time delay, and τp represents the time constant. The process gain Kp was determined by calculating the ratio of the output change to the input step change. The time constant τp was estimated from the response of the controlled variable plot using the value of time at which the response is 63.2% complete.37 Table 2 shows the process gains, time delays, and time constants of the stripper top pressure response to step changes in the reboiler heat duty. As shown in Table 2, the pressure response to positive step changes in the reboiler heat duty has larger process gains and time constants than the negative step changes. The average process gain for the stripper top pressure was 0.16 Pa/W; the time delay was 0 min; and the time constant was 3.2 min. The transfer function can then be expressed as G(s) = 0.16/(3.2s + 1). Figure 5 shows the reboiler temperature responses to step changes in the reboiler heat duty. As shown in Figure 5, the reboiler temperature changes only a small amount for step changes in the reboiler heat duty, with 1.1 and 3.4% increases in the reboiler temperature observed for positive 10 and 30% step changes in the reboiler heat duty. Reductions of 1.0 and 3.7% in the reboiler temperature were observed when the heat duty was suddenly reduced by 10 and 30%. As seen from Figure 5, the reboiler temperature response to the step changes in the reboiler heat duty can also be approximated as a first-order time delay model. Table 2 shows the process gains, time delays, and time constants of the reboiler temperature response to the step changes in the reboiler heat duty. As shown in Table 2, the estimated process gain for the reboiler temperature because of the step changes in the reboiler heat duty was very small, only 0.05 K/W. The time delay between the output change and

Figure 2. Stripper temperature measurement elevations.

Table 1. Column Characteristics and Operational Parameters column characteristics column internal diameter (m) main packing height (m) packing (random) parameter liquid flow rate (g/min) MEA concentration (wt %) rich solution loading (mol of CO2/mol of MEA) inlet liquid temperature (°C) reboiler heat duty (W) desorber pressure (kPa) heat-transfer area of the reboiler (m2) liquid level in the reboiler (m) condenser temperature (°C)

value 0.06 1 Dixon rings value 84.1 30 0.5 85 300 150 0.038 0.040 4

flue gas flow rate changes, there are at least two options: one is to change the solvent flow rate to keep the L/G ratio constant, and the other is just to keep the solvent flow rate constant with a changed L/G ratio. This study focused on the reboiler heat duty changes to investigate the dynamic response characteristics of the stripping column. Fluctuations of the reboiler heat duty may be one of the main disturbances expected for the dynamic operation of the power plant. It can also be adjusted to

Figure 3. Step changes in the reboiler heat duty. 1232


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Figure 4. Stripper pressure responses to step changes in the reboiler heat duty.

where Treb,s is the reboiler temperature at steady state and Qreb,s is the reboiler heat duty at steady state. Subtracting eq 2 from eq 1 gives

Table 2. Process Gains, Time Constants, and Transfer Functions for the Stripper Top Pressure, Reboiler Temperature, and Stripper Packing Temperature variable

Kp

tp

τp

Pstripper Treb T106 T105 T104 T103 T102

0.16 0.05 0.04 0.08 0.09 0.08 0.08

0 0 0 0 0 0 0

3.2 5.5 5.5 6.5 6.8 6.7 6.5

transfer function, G(s) 0.16/(3.2s 0.05/(5.5s 0.04/(5.5s 0.08/(6.5s 0.09/(6.8s 0.08/(6.7s 0.08/(6.5s

+ + + + + + +

Vsumpρc p

1) 1) 1) 1) 1) 1) 1)

(3)

Define the following deviation variables: ′ = Treb − Treb,s , Treb

′ = Q reb − Q reb,s Q reb

(4)

where Treb ′ is the deviation variable of the reboiler temperature and Qreb,s is the deviation variable of the heat duty. Introducing the defined deviation variables, eq 3 can then be rewritten as ′ Vsumpρc pdTreb ṁ rich,inc p

′ = + Treb

′ Q reb ṁ rich,inc p

(5)

Then, the transfer function is 1 ṁ rich,inc p T ′ (s ) G(s) = reb = V ⎛ ′ Q reb(s) sumpρc p ⎞ ⎜ ⎟s + 1 ⎝ ṁ rich,incp ⎠

dTreb = −ṁ rich,inc p(Treb − Tfeed,in) − n H ̇ 2OΔH vap dt + Q reb

dt

= −ṁ rich,inc p(Treb − Treb,s) + (Q reb − Q reb,s)

input change was 0 min. The estimated time constant for the reboiler temperature was 5.5 min. Because the reboiler energy balance was directly influenced by the reboiler heat duty, the theoretical reboiler temperature can be estimated from the liquid-phase energy balance in the reboiler. The energy balance for the reboiler can be written as21,34 Vsumpρc p

d(Treb − Treb, s)

(1)

(6)

Then, the time constant and the process gain can be expressed by

where Vsump is the liquid volume in the reboiler drum, ρ is the aqueous solution density, cp is the specific heat of the rich solvent, Treb is the reboiler temperature, ṁ rich,in is the rich solvent mass flow rate, Tfeed,in is the feed-rich solvent temperature, ṅH2O is the condensate water molar flow rate, ΔHvap is the water latent heat of evaporation, and Qreb is the reboiler heat duty. The left side of eq 1 represents the energy accumulated inside the reboiler, while the right side terms are the liquid enthalpy, the vapor enthalpy leaving the reboiler, and the input heat duty. At steady-state conditions, the left side in eq 1 can be neglected. Therefore, the equation of state can be rewritten as

τp = Kp =

Vsump ̇ Vrich,in 1 ṁ rich,inc p

(7)

(8)

where Vsump is the liquid volume in the reboiler drum, V̇ rich,in is the rich solvent volumetric flow rate, ṁ rich,in is the rich solvent mass flow rate, and cp is the specific heat of the rich solvent. Equation 7 shows that the holdup time of the solvent in the reboiler will contribute to the time constant. The reboiler is sized to provide 5 min liquid holdup when half full, which agrees well with the experimental time constant.

0 = −ṁ rich,inc p(Treb,s − Tfeed,in) − n H ̇ 2OΔH vap + Q reb,s (2)

Figure 5. Reboiler temperature responses to step changes in the reboiler heat duty. 1233


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Figure 6. Temperature at different heights in the stripper column for step changes in the reboiler heat duty.

Figure 6 shows the temperature changes with time at different heights along the stripping column. The temperature profile varies smoothly with time. The temperatures at different heights in the stripper column change immediately with the step changes in the reboiler heat duty. The temperatures change because of the change in the amount of heat taken away by the regenerated vapor stream. The reboiler temperature increases with sudden increases in the reboiler heat duty, which leads to more vapor being regenerated. The vapor then increases the packing temperature. The reboiler temperature

then decreases with a sudden reduction in the reboiler heat duty, which leads to less vapor being regenerated, and the stripper packing temperature decreases with time. As seen in Figure 6, the stripper packing temperature response to the step changes in the reboiler heat duty can also be approximated as a first-order time delay model. Table 2 also lists the process gains, time delays, and time constants for the packing column temperatures. The calculated process gains for the packing column temperature response to the step changes in the heat duty were 0.04−0.08 K/W. The process gain and the time 1234


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Figure 7. Desorption rate of CO2 for step changes in the reboiler heat duty.

Figure 8. Regeneration energy variation for step changes in the reboiler heat duty.

Figure 9. Ramp changes in the reboiler heat duty.

Figure 10. Stripper pressure responses to ramp changes in the reboiler heat duty.

Figure 11. Reboiler temperature responses to ramp changes in the reboiler heat duty.

constant for the top packing column temperature are smaller than those for the bottom packing column temperature, which means that the bottom packing column temperature is more sensitive to changes in the reboiler heat duty than the top packing column temperature. No time delay was observed for the packing column temperature. The calculated time constants for the packing column temperature response to the step changes in the heat duty were 5.5−6.8 min. The liquid holdup

in the packing section is one critical factor determining the packing column dynamic characteristics. The rich solution was regenerated in the stripper with the gas stream out of the top of the stripper condensed in the condenser. Therefore, the CO2 concentration was around 100%. Figure 7 shows the desorption rate of CO2 for step changes in the reboiler heat duty. The CO2 desorption rate slightly increases (decreases) as the reboiler heat duty increases (decreases). The level controller keeps the liquid level in the 1235


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Figure 12. Temperatures at different heights along the stripper column for ramp changes in the reboiler heat duty.

Figure 13. Regeneration energy for ramp changes in the reboiler heat duty 1236


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Figure 14. Ramp changes in the feed-rich solvent temperature.

Figure 15. Stripper pressure responses to changes in the feed solvent temperature.

Figure 16. Reboiler temperature responses to changes in the feed solvent temperature.

stripper constant; therefore, the lean solvent flow rate will fluctuate. A material balance of the stripping system shows that the response of the lean solution flow rate was determined by the changes of the condensate water and regenerated CO2 flow rate. The experiments showed that the condensate water flow rate is negligible compared to the feed solvent flow rate. The changes in the lean solvent flow rate refer to changes in the amount of captured CO2, as shown in Figure 7. The changes in the lean solution flow rate were very small in this study. In this study, the regeneration energy is defined as the ratio of the energy supplied from the reboiler to the CO2 mass rate from the stripper. Figure 8 shows the dynamic response of the regeneration energy to step changes in the reboiler heat duty. The regeneration energy changes immediately after the disturbance, with sudden changes in the regeneration energy of approximately +4.9 and +14.3% when the reboiler heat duty was step increased by 10 and 30%. The decreases in the regeneration energy are approximately 9.8 and 12.1% when the reboiler heat duty was reduced by −10 and −30%. The heat of reaction, the sensible heat, and the latent heat of water vaporization are the three energy terms in the regeneration energy. When the heat duty increases, the temperature difference between the reboiler and the feed solvent increases; therefore, the sensible heat increases. The heat of reaction is also dependent upon the temperature; therefore, the changes in these two energy terms cause the changes in the regeneration energy. 3.2. Ramp Changes in the Reboiler Heat Duty. This section describes the effect of ramp changes in the reboiler heat duty on the stripping column performance. Ramp changes in the reboiler heat duty represent linear changes in the heat

supply from the power plant or changes in the heat-transfer coefficient in the reboiler. As shown in Figure 9, tests of positive and negative ramp change of different magnitude in the reboiler heat duty were conducted to investigate the dynamic behavior of the desorption process. In the ramp change tests, the base conditions were maintained for 20 min after the desorption process reached a steady state. The positive and negative 10 and 30% ramp changes in the reboiler heat duty were implemented within 20 min. The final conditions were maintained for another 60 min to ensure that the process had reached a new steady state. Figure 10 shows the stripper top pressure responses to the ramp changes in the reboiler heat duty. The nonlinearity of the desorption process can be seen from Figure 10. The stripper top pressure increased by 2.4% when the heat duty was increased by 10% and increased by 13.3% for a 30% increase in the heat duty. A negative 10% ramp change in the reboiler heat duty caused a 4.0% reduction in the stripper pressure, with a 30% decrease causing an 11.3% decrease. The proportional gain was defined as the ratio of the output change rate to the input change rate. The average proportional gain of the stripper pressure response to ramp changes in the reboiler heat duty was 0.37%/%. The stripper pressure shows a lag behind the new steady state of the reboiler heat duty. For positive ramp changes in the heat duty, the process took about 30 min for the stripper pressure to reach a new steady state. A much longer time was observed for the stripper pressure to reach a new steady state with negative changes in the reboiler heat duty. The stripper pressure then influences the temperature in the reboiler. Figure 11 shows the reboiler temperature responses to the ramp changes in the reboiler heat duty. As shown in Figure 1237


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Figure 17. Temperature at different heights along the stripper column for changes in the feed solvent temperature.

reach a new steady state, with 40 min needed for the reboiler temperature to reach a new steady state with negative changes in the reboiler heat duty. Figure 12 shows the changes in the temperatures with time at different positions in the stripping column for ramp changes in the reboiler heat duty. The temperature profiles vary smoothly with time, with the temperatures in different positions of the stripper column increasing (decreasing) with positive (neg-

11, the reboiler temperature increased by 1.0% for a 10% increase in the reboiler heat duty and by 3.9% for a 30% increase. A negative 10% reduction in the reboiler heat duty gave only a 0.7 reduction in reboiler temperature, while a 30% decrease gave a 4.9% reduction. The average proportional gain of the reboiler temperature response to the ramp changes in the reboiler heat duty was 0.12%/%. Positive ramp changes in the heat duty required about 31 min for the reboiler temperature to 1238


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Figure 18. Regeneration energy for changes in the feed solvent temperature.

ative) ramp changes in the reboiler heat duty because of changes in the amount of heat taken away by the vapor stream. The stripper packing temperatures take longer to reach a steady state for negative changes in the reboiler heat duty than that for positive changes because of the different heat-transfer rates. The temperatures at different heights along the stripper column take about 30−38 min to reach a new steady state. The average proportional gains of the packing column temperature response to ramp changes in the heat duty were 0.17−0.27%/%. Figure 13 shows the regeneration energy response to ramp changes in the reboiler heat duty. The regeneration energy is related to the ramp changes in the reboiler heat duty. A regeneration energy increase of approximately 14% was observed for positive 10 and 30% ramp changes in the reboiler heat duty. A total change in the regeneration energy of approximately −5.3% was observed when the reboiler heat duty was reduced by 10%, while the decrease was −10.1% for a 30% heat duty decrease. 3.3. Ramp Changes in the Feed-Rich Solvent Temperature. The feed solvent temperature can also change during CO2 capture. The feed-rich solvent is preheated by the hot lean solvent in the rich/lean heat exchanger. The feed-rich solvent temperature can change if the hot lean solvent temperature or the heat-transfer coefficient in the rich/lean heat exchanger changes. In these tests, the feed-rich solvent temperature was changed linearly, while the reboiler heat duty was maintained constant. As shown in Figure 14, two independent ramp change tests were conducted. In one test, the base conditions were maintained for 20 min and then the feed-rich solvent temperature was increased from 85 to 92 °C in 18 min. The final conditions were then maintained for other 60 min. In the second test, after 20 min of steady-state operation, the temperature was reduced from 85 to 79 °C in 28 min. The final conditions were then also maintained for 60 min to ensure that the system had reached a new steady state. Figure 15 shows the stripper top pressure responses to the ramp changes in the feed-rich solvent temperature. For the +8% ramp changes in the feed solvent temperature, the stripper top pressure increased by 14.7%, while a negative 7% ramp change in the feed solvent temperature lead to a −8.4% reduction in the stripper pressure. The average proportional gain of the stripper pressure because of the ramp changes in the feed solvent temperature was 1.52%/%. The stripper pressure then affects the reboiler temperature; therefore, as shown in Figure 16, the reboiler temperature is then sensitive to the feed-rich solvent temperature. As the feed solvent temperature increases (decreases), the stripper pressure increases (decreases), which increases (decreases) the reboiler temperature. A +3.8 change in the reboiler temperature was observed for a +8% ramp change in the feed solvent temperature, while a −2.9% change was observed for a −7% ramp change. The average proportional gain of the reboiler temperature response to the ramp

changes in the feed solvent temperature was 0.45%/%. The transition period behavior for the stripper pressure and reboiler temperature was 3−7 min after the feed solvent temperature reached a new steady state. The reboiler temperature and the feed solvent temperature both influence the temperature profile in the packing column. Figure 17 shows these combined effects on the stripper column temperature profile. As shown in Figure 17, the packing column temperatures are sensitive to the feed solvent temperature. The temperatures at different heights along the stripper column increase (decrease) for positive (negative) ramp changes in the feed solvent temperature because of changes in the amount of heat taken away by the feed solvent and regenerated vapor. Changes in the feed solvent temperature significantly change the vapor production in the reboiler. This change then changes the packing temperatures. The average proportional gains of the packing column temperature response to ramp changes in the feed solvent temperature were 0.44−1.10%/%. The transition period for the packing column was 4−12 min after the feed solvent temperature reached a new steady state. Figure 18 shows the dynamic response of the regeneration energy for changes in the temperature of the feed-rich solvent. For the +8% ramp change in the feed solvent temperature, the regenerated CO2 out of the stripper increased, which reduced the regeneration energy. For the −7% ramp change in the feed solvent temperature, the decrease in the regenerated CO2 increased the regeneration energy. Thus, to save energy, the feed-rich solvent temperature should be closer to its bubble point.

4. CONCLUSION Transient response tests were conducted on a MEA-based regeneration unit of a CO2 capture system to investigate the dynamic behavior of the stripping process. Various disturbances were introduced into the regeneration unit. The tests included step changes and ramp changes in the reboiler heat duty. Ramp changes in the feed solvent temperature were also investigated. The tests investigated the dynamic behavior of the key process variables, such as the stripper pressure, reboiler temperature, stripper packing column temperatures, and regeneration energy response to the changes in the reboiler heat duty and feed solvent temperature. The resulting responses of the desorption process variables obtained from the step tests were approximated as a first-order time delay model. The process gain for the stripper top pressure to a step change in the reboiler heat duty was 0.16 Pa/ W with a time constant of 3.2 min. The process gain of the reboiler temperature to a step change in the reboiler heat duty was only 0.05 K/W. The time constant for the reboiler temperature was 5.5 min because of the holdup time in the reboiler. The process gains for the stripper packing temperatures to step changes in the heat duty were in the range of 1239


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0.04−0.08 K/W with time constants of 5.5−6.8 min. The different magnitudes of the process variable responses to the positive and negative step changes in the reboiler heat duty show the nonlinearity in the desorption process. Negative ramp changes in the reboiler heat duty had much longer lag times for the process variables than positive ramp changes. The results for the ramp changes in the feed solvent temperature suggest that the process dynamic characteristics are more sensitive to the feed solvent temperature than the reboiler heat duty.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-62788668. Fax: +86-10-62770209. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Ch inese MOST Project 2013DFB60140 is greatly appreciated. NOMENCLATURE cp = specific heat (J kg−1 K−1) ΔHvap = heat of evaporation of water (kJ/mol of H2O) Kp = process gain (K/W; Pa/W) ṁ rich,in = mass flow rate of rich solvent (kg/s) ṅH2O = molar flow rate of water (mol/s) Pstripper = stripper pressure (kPa) Qreb = reboiler heat duty (W) Q′reb = deviation reboiler heat duty (W) Treb = reboiler temperature (°C) Treb ′ = deviation reboiler temperature (°C) Tfeed = feed-rich solvent temperature (°C) td = time delay (min) Vsump = volume of reboiler sump (m3) V̇ rich,in = volume flow rate of rich solvent (m3/s) ρ = density of aqueous solution (kg/m3) τp = time constant (min)

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REFERENCES

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Experimental Study of the Dynamic Behavior of the Stripping Column

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