https://doi.org/10.1016/j.ijggc.2015.12.009
2.1. Plant description
“The pilot plant used in this study, represented schematically in Fig. 1, is controlled and monitored using a combination of Labview software and discrete panel-mounted PID microcontrollers. Synthetic flue gas, composed of N2 and CO2, is used. Nitrogen is fed to the facility from a tank upstream of a gas blower. After the blower, a mass flow controller determines the rate at which CO2 is fed to the gas line. For this test campaign, an inlet CO2 concentration representative of NGCC exhaust (4.3%, v/v) is achieved with a cascade PID control system of the CO2 mass flow controller. The flue gas is heated to an inlet temperature of approx. 45 °C, before entering the absorber column. After exiting the absorber, flue gas is analysed for CO2 content in order to determine the capture rate, before being cooled and recycled by feeding back to the blower. While not truly representative of real PCC plant operation, this reduces nitrogen usage, and ensures that the majority of the gas stream becomes rapidly saturated, reducing water losses via evaporation in the absorber.”
Fig. 1. Absorption/desorption test facility process flow diagram.
The solvent tank can hold up to 500 l, and 116.8 kg neat monoethanolamine (MEA) and 270.5 kg deionised H2O were introduced to produce a solution of 30.16% (w/w) MEA. During plant operation, with solvent circulating in the pipework and vessels, the solvent tank inventory generally remained at around 10% of its maximum level. At the flow rates used in this experimental work, between 40 min and 113 min is required for the entire solvent inventory to circulate once through the plant. The associated effect on plant response is discussed in detail later in this paper.
The main solvent pump circulates liquid to the top of the absorber, where it is heated to approx. 40 °C. The intent is to reproduce the effect of a cross-flow solvent heat exchanger. The absorber sump level is maintained using a Varibell control valve. Upon exiting the absorber, a secondary pump circulates the liquid to the top of the desorber, which is held at a pressure of approx. 1.8 bar.
Steam at a pressure of 5 bar (absolute) is used for solvent regeneration and other heating, for example, mimicking the effect of a cross-flow heat exchanger. CO2 and water vapour exit the top of the desorber, where the water is condensed and CO2 vented to atmosphere.
The concentrations of CO2 at the absorber inlet and outlet are monitored using infra-red (IR) CO2 analysers. Lean and rich solvent loading are determined by manual sampling at the desorber and absorber outlet, followed by benchtop titration analysis. At standard operating conditions (Table 2) solvent takes approx. 40 min to circulate through the plant, leading to time delays between e.g. a lean sample being taken and this discrete “packet” of solvent reaching the absorber inlet. This effect is exacerbated further at lower flow rates and must be taken into consideration when relating variations in loading to those in capture rate.
The absorber has an inner diameter of 158 mm and contained 6.92 m of Sulzer Mellapak 250.X packing. The desorber has an inner diameter of 350 mm and contained 5.00 m of Sulzer Mellapak 500.X packing. “
”
The facility in this work is used for a range of applications and is not designed specifically for post-combustion capture at atmospheric pressure. Unlike other pilot plants reported in Table 1, which utilise a cross-heat exchanger to increase the temperature of the rich solvent, the pilot plant used in this study does not have this capability so the effect is simulated using separate heat exchangers. Using a setpoint of 20 °C below the outlet temperature, a rich solvent heater increases the liquid temperature at the desorber inlet. The solvent is cooled to room temperature before entering the main solvent tank, then heated to approx. 40 °C before entering the absorber. A PID control loop with steam supply is used to control the solvent temperature at absorber and desorber inlets. Temperature settings are selected to be representative of the typical temperature pinch of a cross flow heat exchanger (Table 2).
Table 2. Summary of baseload operating conditions.
| Controlled variable | Value |
|---|---|
| Gas flow rate at absorber inlet (N m3/h) | 120.5 |
| Gas inlet temperature (°C) | 46.14 |
| Inlet gas CO2 concentration (%, v/v) | 4.27 |
| Steam flow rate (kg/h) | 19.5 |
| Liquid flow rate (l/h) | 344.4 |
| Steam pressure (bara) | 4.0 |
| Desorber pressure (bara) | 1.80 |
| Liquid inlet temperature, absorber (°C) | 40.05 |
| Liquid inlet temperature, desorber (°C) | 104.07 |
| Measured parameter | Value |
|---|---|
| CO2 capture rate (%) | 89.7 |
| Reboiler duty (GJ/tCO2) | 3.96 |
| L/G ratio (l/m3) | 2.86 |
| Lean solvent loading (mol amine/mol CO2) | 0.232 |
| Rich solvent loading (mol amine/mol CO2) | 0.345 |
”