1. Introduction
Existing submarines are equipped with diesel propulsion systems, in which the energy for submerged operation is stored in lead-acid batteries. The capacity of these batteries limits the underwater range. The batteries are charged by a diesel generator during snorkeling, during which the submarine is exposed to a higher risk of detection [
1]. Basically, the submarine is propelled by an electric motor and so the objective of the air-independent propulsion system is to silently generate electric energy when under the water. Electric energy can be produced with a diesel generator engine when the submarine is on the water surface or with a fuel cell with a reformer when the submarine is in military operation under the water. The steam reformer generates H
2 as fuel for the fuel cell and CO
2 vapor as a byproduct. However, CO
2 vapor cannot be discharged under the water during military operation because undissolved CO
2 will explode when rising up, which will be detected by others [
2,
3,
4]. The current technology used still requires considerable cost and high energy consumption to recover or separate CO
2 discharged from vessels. Thus, technological innovation is required.
The aim of this research was to understand how CO
2 generated from a reformer fully dissolves in seawater in a short time period. Regarding studies on dissolution rate, Cavenati et al. [
5] studied the adsorption of methane, CO
2, and N
2 by zeolite 13X under various pressures (0–5 MPa) and temperatures (298, 308, and 323 K). The CO
2 adsorption capacity of zeolite 13X was much higher than its adsorption capacity for other gases and the adsorption capacity increased with the pressure. Dash et al. [
6] conducted research on the solubility of CO
2 according to the reaction temperature (303–328 K) and CO
2 partial pressure (0.2–1500 kPa) in an aqueous solution of 2-amino-2-methyl-1-propanol and piperazine. Increasing the concentration of 2-amino-2-methyl-1-propanol and adding piperazine affected the absorption capacity, where the CO
2 absorption capacity of the solvent increased under a high operation pressure and low operating temperature. Manic et al. [
7] measured the solubility of CO
2 in seven ionic liquids in the 8–22 MPa pressure range and at temperatures of 313.2 and 323.2 K. They found that the solubility of CO
2 increased when the pressure increased and the temperature decreased.
With regard to dissolution velocity, Hirai et al. [
8] measured the CO
2 dissolution velocity while the flow velocity, temperature, and pressure conditions were varied. It was found that the dissolution velocity increased when the temperature, pressure, and flow velocity increased. Dang and Rochelle [
9] measured the solubility of CO
2 and its absorption rate in monoethanolamine (MEA)/piperazine/water at 40–60 °C. The absorption rate of MEA with 0.6–1.2 M piperazine added was 1.5–2.5-times higher than that when MEA was used alone. Faiz and Al-Marzouqi [
10] developed a model for absorbing CO
2 from natural gas at high pressures of up to 5000 kPa. They showed that the CO
2 removal rate increased with pressure, whereas the CO
2 absorption rate increased when chemical solvents, such as MEA, were used.
To understand the dynamic characteristics of CO
2, Lawal et al. [
11] investigated the effects of operating conditions in CO
2 capture facilities and performed dynamic modeling to solve possible issues proactively. With regard to the modelling, the analysis was conducted using equilibrium- and velocity-based approaches. The authors mentioned that the velocity-based model provided better predictions of the chemical absorption process than the equilibrium-based model with regard to understanding the dynamic behavior of the absorber and the failure of the stripper. Munusamy et al. [
12] measured the volumes of CO
2, N
2, CH
4, and CO and conducted research on their dynamic adsorption characteristics. The equilibrium adsorption capacities of each composition in the powder and granules of MIL-101(Cr) were measured at 288, 303, and 313 K. The CO
2 and CH
4 absorption of MIL-101(Cr) increased with the adsorption temperature.
Breault and Huckaby [
13] simulated adsorber performance using multiphase computational fluid dynamics (CFD) that included heat transfer. They investigated the behavior and performance of the riser adsorber for the solid circulation rate, gas flow rate, and heat removal. CO
2 adsorption increased with the solid flow and it decreased with the gas flow. Zhang et al. [
14] performed CFD for CO
2 capture at a membrane using methyldiethaneolamine and 2-(1-piperazinyl)-ethylamine. They intensively studied the membrane gas absorption (MGA) method for CO
2 separation, developed a 2D mass transfer model, and examined the effect of membrane performance on adsorption performance. Giammanco et al. [
15] investigated the dynamic characteristics of CO
2 gas for an optimal ionic liquid system. They suggested that a large amount of gas can be dissolved in imidazolium-based ionic liquids. Kim et al. [
16] examined the fluid dynamics and CO
2 removal efficiency of amine absorbents on a pilot scale with structured packing using a gas–liquid multiphase Euler CFD model.
Many studies have been conducted that focus on CO2 capture technologies from the perspective of power generation efficiency. Although research and development have been performed and continuous investments in such CO2 capture technologies have been made, studies conducted under actual operating conditions of underwater weapon systems are insufficient and it remains difficult to conduct research on the development and application of technologies in underwater weapon systems to design and optimize CO2 dissolution in seawater. Although processes with high-pressure and high-temperature absorbents exhibit high CO2 recovery efficiencies, the absorbents are costly and the conditions are different from the actual operating conditions of underwater weapon systems. In this study, seawater was used as a solvent for CO2 absorption. In addition, the actual operating conditions of underwater weapon systems were simulated by performing the absorption process under normal pressure instead of high pressure.
In this study, dissolution experiments were performed and dynamic characteristics were analyzed according to pressure and temperature. These experiments were carried out by applying dissolution technology using seawater as a solvent without an absorbent to improve the treatment of CO2 after hydrocarbon reforming. The experiments were performed at equilibrium pressures of 2–5 bar and the results were compared to those of the MATLAB numerical analysis. In addition, the seawater–CO2 diffusion coefficient was derived under experimental conditions.
The remainder of this paper is organized as follows:
Section 2 presents the experimental setup and procedure used in the study,
Section 3 explains the modeling equations used in this work as well as the sensitivity analysis, while
Section 4 discusses the results of this study.
Section 5 concludes this paper.
2. Experimental Setup and Procedure
2.1. Experimental Setup
CO
2 generated after hydrocarbon reformation was injected using an absorption and equilibrium system, which employed a batch-type reactor (
Figure 1).
The flow rate of each gas was maintained at a constant level using a mass flow controller (Line Tech, Daejeon, Korea) (MFC) allowing for gas of the desired composition to be introduced into the buffer tank and the reactor. The buffer tank supplied gases of each composition to the reactor after maintaining it at a constant temperature and pressure. The volumes of the buffer tank and reactor were both 9 L. The temperature and pressure of the buffer tank were measured using a thermocouple (Sunil Instrument, Busan, Korea) (TT) and pressure transmitter (Wika, Germany) (PI, PT) with 99.5% accuracy. Thermocouples (TT) and pressure transmitter (PT) are used to measure pressure and temperature, which are variables used to calculate the gas phase moles of a reactor.
During the test, the temperature and pressure were measured respectively and the measured values were recorded in the data logger.
2.2. Experimental Procedure
In the experiments, the impurities retained in the buffer tank, reactor, and piping were removed using a vacuum pump. After the reactor temperature was increased to the operating temperatures of 25 and 32 °C, 99.8% pure CO
2 was injected into the buffer tank until the operating pressure was reached. When the reactor pressure was stabilized to a constant level, the gas in the buffer tank was supplied to the reactor. The experimental conditions used to focus on the effects of solvent type, temperature, and pressure on CO
2 concentration are summarized in
Table 1.
All the solutions were tested in 99.9% pure distilled water (6 L) and seawater. For seawater, artificial seawater was fabricated and used in accordance with ASTM-D1141-98.
Table 2 lists the composition of the artificial seawater [
17].
At the initial stage, the buffer tank and reactor were isolated from each other and the empty buffer tank was pressurized with CO2 and the reactor filled with solution in vacuum state. After, the pressure and temperature were stabilized in both chambers. The gas in the buffer tank was fed into the reactor via an isolation valve that, when open, resulted in a pressure decrease in buffer tank and increase in the reactor. The pressure and temperature were data logged until both tank pressures were stabilized, which was assumed to be equilibrium state.
2.3. Amount of Gas Dissolved as a Function of Pressure and Temperature
The CO
2 pressure was estimated from the pressure change in the system. To obtain the removal rate of the injected gas, the actual gas state equation was used. The number of moles of gas dissolved in the solvent in the reactor was calculated using the pressure and temperature of the buffer tank and those of the reactor, as shown in Equations (1) and (2) [
18,
19,
20].
The initial number of moles of injected CO
2 (
) was calculated by substituting the pressure (
) and volume (
) of the buffer tank into Equation (1):
where
.
The compressibility factor (z) was adopted from HYSYS program, generally used in chemical process engineering, at a given pressure and temperature. The value varied from 0.975 to 0.99 in our research conditions.
The number of moles of CO
2 (
) at the equilibrium state between the buffer tank and the reactor could be calculated using the equilibrium pressure (
) and the sum of the volumes of the buffer tank and the reactor (
). The number of moles absorbed
) was calculated as follows:
Here, pressure is given in , temperature is given in , volume is given in , z is the compressibility factor and is the number of moles of the gas. The gas constant has the value 8.3144·10−5.