Article pubs.acs.org/IECR
Optimal Design and Operating Conditions of the CO2 Liquefaction Process, Considering Variations in Cooling Water Temperature Seok Goo Lee, Go Bong Choi, and Jong Min Lee* School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea ABSTRACT: Ship transportation of liquid CO2 is now considered as an alternative transport option to pipelines, in the absence of any suitable onshore storage locations. The compressors for a liquefaction process in the transport chain account for a significant portion of the total energy consumption. The temperature of seawater as an intercooling medium has a very significant effect on the energy consumption of the multistage compressors. Although several studies for the optimal design and operation of liquefaction process have been proposed, they do not consider the seasonal and locational variations in the cooling water temperature; therefore, further improvement in the energy efficiency in the CO2 liquefaction process is necessary. In this study, the variations in the operational energy and other operational issues of the CO2 liquefaction process, according to the actual seawater temperature in the range of 5−30 °C was investigated. Moreover, the optimal discharge pressure of the final stage compressor before the Joule−Thomson (J-T) expansion and the pressure ratio of multistage compressors are provided, given the seawater temperature, to minimize the total compressor power consumption. Furthermore, the optimal number and position of multistream heat exchangers are investigated. The total compressor power consumption is ∼90−140 kWh/ton CO2 in the seawater temperature range of 5−30 °C. ∼77% of the energy of the entire transport stage, as well as ∼10% of the energy in the entire CCS chain.2,3 Because most of the CO2 liquefaction processes are located near the seashore, seawater can be used as the coolant for the CO2 liquefaction process for the power plants along the shoreline does. Because the compressor power consumption is very sensitive to the coolant temperature, the seawater temperature becomes a critical factor in decreasing the operational energy. Furthermore, the seawater temperature varies seasonally and even daily, also differs depending on the location of the seashore. Therefore, the optimal operating conditions for minimizing compressor power consumption must be determined according to the variation of seawater temperature. However, the previous studies do not consider the variation in the cooling water temperature. Some studies focus on decreasing the minimum compressor power consumption using low temperature of seawater, whereas others use high temperature and do not provide details about the optimization.2−5 Aspelund et al.2 reported a CO2 liquefaction process aimed at ship transport, involving an open cycle using CO2 as the refrigerant and only considering the seawater temperature at 10 °C. Romeo et al.6 investigated the interactions among the intercooling temperature, pressure ratio, and efficiency of the compressor in multistage compressors. Moreover, economic optimization is performed in terms of capital investment and net compressor power consumption with energy integration. Decarre et al.4 analyzed four types of liquefaction process with different liquefying conditions and refrigerants. However, this
1. INTRODUCTION In recent years, a large amount of CO2 emitted from the fossilfuel-based power plants has become an important issue, because the rapidly growing concentration of CO2 is a main cause of global warming. Among the various technologies for reducing CO2 emission, the carbon capture and sequestration (CCS) has been attracting significant attentions as the most promising alternative. The CCS technology consists of capture, transport, and sequestration processes. Generally, the capture process is considered as the most important part, because it requires ∼70% of the energy in the entire CCS chain and is also typically expected to reduce the electrical output of coal-fired power plant by 30%−40%.1 Although many studies on the CO2 capture process have been reported to decrease the energy consumption, only a few systematic studies on improving the energy efficiency of the transportation process exist. Because the amount of vapor CO2 released from the power plants is tremendous, it should be pressurized to a higher density form such as liquid, solid, or supercritical phase. This study is concerned with the liquid phase, because comparable shipping technologies for the pressurized liquid are well-established for commercial liquefied petroleum gas carriers.2 Moreover, there are several difficulties in handling solid or supercritical phases, e.g., loading or unloading operation. The pressurized CO2 can be transported to a storage site via pipelines or shuttle ship. The focus of this study is on ship transport, because of its flexibility, compared to the regional constraints of pipelines, scattered capture, and storage sites, and diverse applications. The ship transport of liquid CO2 consists of five stages: liquefaction, intermediate storage, loading, ship transport, and unloading. Among them, the liquefaction stage is considered as the most important part, because it requires the largest amount of operational energy. The CO2 liquefaction process consumes © 2015 American Chemical Society
Received: Revised: Accepted: Published: 12855
July 1, 2015 October 19, 2015 December 10, 2015 December 11, 2015 DOI: 10.1021/acs.iecr.5b02391 Ind. Eng. Chem. Res. 2015, 54, 12855−12866
Article
Industrial & Engineering Chemistry Research
consumption were assumed to be the same as the values presented by Ung et al.3 2.2. Captured and Liquid CO2 Conditions. The inlet stream condition was assumed to be from a post-combustion capture process using the amine absorption as listed in Table 2.
study is mainly concerned with the CO2 transport without rigorous investigation on the liquefaction process. Alabdulkarem et al.5 compared various CO2 liquefaction processes, such as simple multistage compression, single refrigerant cycle using ammonia, cascade cycle, and open cycle. Although many process designs are systematically proposed, the outlet conditions of the liquid CO2 are not for ship transport but rather for enhanced oil recovery. Ung et al.3 suggested a novel design for decreasing the compressor power consumption; however, they only considered the seawater temperature to be 10 °C. Although many CO2 liquefaction process models have been suggested to minimize the compressor power consumption, no study considering the variations in the seawater temperature for the optimal design and operation has been reported. This work proposes an optimal CO2 liquefaction process for ship transport considering actual variations of seawater temperature near the power plant as well as other operational constraints. One critical issue with seawater coolant is that the compressor power consumption may ramp up suddenly when the seawater temperature is near 30 °C, because of the low critical temperature of CO2. We propose design and operational strategies that can counteract this critical issue and minimize the total compressor power consumption in accordance with the varying seawater temperature.
Table 2. Inlet (Captured) and Outlet (Liquid) CO2 Conditions Composition
a
thermodynamic mixing rule molecular binary interaction parameter, kij pressure drop of seawater heat exchanger pressure drop of multistream heat exchanger minimum approach temperature of seawater heat exchanger minimum approach temperature of multistream heat exchanger isentropic efficiency of compressor seawater (cooling water) temperature, TSW
1.0
TSW + ΔTmin
Captured CO2 323.0
6.5
−50.6
component
mole fraction
CO2 H2O impurities
0.9439 0.0561 negligible
CO2 H2O
0.9999 ≤0.5 × 10−5
TSW, seawater temperature; ΔTmin, minimum approach temperature.
Notably, the pressure of the captured CO2 may affect the total compression power. For example, if an inlet stream of 1 bar passes through four-stage compressors at a pressure ratio of 3, the final stage pressure is 81 bar. However, when the inlet stream pressure is increased to 1.1 bar, the final stage pressure becomes 98 bar. Although the difference of the inlet pressure is 0.1 bar, the final stage pressure and the vapor-to-liquid ratio after the expansion will change significantly, thus significantly changing the total compressor power consumption. The pressure of the captured CO2 as an inlet in the liquefaction process is related to the reboiler energy of the stripper in the capture process. The optimal pressure of the captured CO2 should be determined on the capture part, although it affects the liquefaction energy. Since 1 bar is the typical value, the inlet pressure is chosen to be 1 bar in this study. The vapor CO2 should be pressurized and refrigerated in the liquid phase for transport. Specifically, as shown in Figure 1, the captured vapor CO2 must be compressed above the triple point (TP) pressure (5.2 bar, denoted by the red horizontal dashdotted line in Figure 1) and cooled below the critical point (CP) temperature (31.1 °C, blue vertical dashed line in Figure 1). Transporting a greater amount of CO2 is possible as the conditions of liquid CO2 become more similar to those of the TP pressure, because the density of saturated liquid CO2 is ∼1200 kg/m3 at the TP and 600 kg/m3 at the CP. The pressure close to the TP may not be desirable, considering the safety margin for possible abrupt phase change. In the previous study, Barrio et al.7 used liquid CO2 under a pressure of 14−20 bar for a semipressurized vessel ship. Ung et al.3 and Aspelund et al.8 selected a pressure of 6.5 bar and a temperature of −54 °C. Decarre et al.4 compared the liquid CO2 conditions at 7 and 15 bar. In this study, a pressure of 6.5 bar and a saturated temperature of approximately −50 °C, respectively, are selected to compare with the results of other recent studies. The mass flow rate of the inlet is selected to be 323 ton CO2/h, based on a 600 MW coal power plant.9 The mass flow rate of the liquid CO2 cannot be proposed exactly, because it is dependent on various variables such as cooling water temperature, pressure ratio, and expansion pressure. The mass flow rate of liquid CO2 should not be determined exactly, because the liquid CO2 is stored in a large intermediate storage tank, since shipping is a discontinuous event.
Table 1. Conditions and Assumptions for the Simulation parameter
mass flow (ton/h)
Liquid CO2
2. MODEL DEVELOPMENT 2.1. Basis of Process Simulation. In this study, a commercial process simulator Aspen HYSYS (version 8.4) is used. Soave−Redlich−Kwong and van der Waals equations are used for the equation of state and mixing rule, respectively, because they appropriately reflect the solubility and binary interactions between CO2 and H2O. The pressure drop and minimum approach temperature for seawater heat exchanger were assumed to be 0.5 bar and 5 °C, respectively. In the case of the multistream heat exchanger, the pressure drop and minimum approach temperature were assumed to be 0.1 bar and 3 °C, respectively. The isentropic efficiency of the compressor was assumed to be 82%. These conditions and assumptions are summarized in Table 1.
commercial process simulator thermodynamic equation of state
temperature (°C)a
pressure (bar)
value Aspen HYSYS V8.4 Soave−Redlich−Kwong (SRK) van der Waals (vdW) 0.193 0.5 bar 0.1 bar 5 °C 3 °C 82% 5−30 °C
Because the proposed model has several recycle streams, it can introduce calculation errors through several iterations. The sensitivity value of the recycle block was selected to be 0.01, and those of the flow and composition were 0.001 (default values are 10). The absolute tolerance of the flow and composition was 0.01. Most of the values affecting the total compressor power 12856
DOI: 10.1021/acs.iecr.5b02391 Ind. Eng. Chem. Res. 2015, 54, 12855−12866
Article
Industrial & Engineering Chemistry Research
Figure 1. CO2 phase diagram and required minimum pressure and temperature conditions needed for liquefaction.
Figure 2. Solubility of water in liquid CO2 for (a) simulation results, (b) reference data-1,12 and (c) reference data-2.13
reference data. The simulation results are quite accurate, compared to the literature data in Figures 2b and 2c. The lowest value of water contents ∼500 ppm at 10 °C is necessary to meet the operational constraints for preventing the formation of hydrates. The dehydration part should be added to separate the remaining water, to reduce the water content level to
