## 2 Experimental
### Instrumentation and Control Equipment of the Test Rig  
The test stand is equipped with various devices and control circuits installed to measure the required parameters of the absorption process and for safe operation. A pressure reducer is installed on each line of the gas mixer to set the maximum pressure of the gas entering the absorber. After the pressure reducer, a magnetic valve is installed, which allows opening or closing the gas supply and can be closed in case of emergency. An MFC is attached to each line of the gas mixer to control the volume flow. A temperature sensor is placed near each absorber tray to measure the temperature of the liquid on that tray. A Coriolis device is placed at the inlet of the fluid to measure the temperature and flow rate of the water entering the absorber. A pressure difference meter was attached to the absorber column to estimate the total pressure drop in the tray.  
The test rig is equipped with five control loops for control. The first control loop is used to regulate the pressure to the set point and to prevent the pressure from rising above 0.6 MPa (permissible internal pressure of the glass absorber). The pressure control circuit consists of a control valve and a pressure sensor. The control valve is attached to the gas outlet of the absorber, while the pressure sensor is attached to the column. The pressure control loop starts controlling the pressure after the gas enters the absorber, resulting in a pressure increase. The pressure sensor sends a signal with the actual value of the pressure to a PID controller. The PID controller compares the set point of the pressure with the actual value of the pressure and gives a signal to the control valve, which opens or closes with the percentage value to keep the pressure at the desired set point. At the outlet of the absorber, there is a safety pressure valve that releases the pressure in the absorber when it reaches the value of 0.45 MPa. This design protects the glass absorber from unexpected pressure development above 0.45 MPa.  
The second control circuit is used to control the liquid level at the bottom of the absorber. The liquid level control is necessary because it prevents the gas from flowing from the liquid outlet and prevents the accumulation of the liquid in the absorber to a high level. The level control circuit consists of a pressure differential device and a control valve. The pressure differential device is installed in the sum of the column, while the control valve is attached to the liquid outlet of the absorber. The third control circuit regulates the level of absorbent in the reboiler, since a certain loss of absorbent occurs due to the evaporation of water. The control circuit consists of a make-up pump and a level sensor. The level sensor sends a signal to the make-up pump when the absorbent level falls below the set point to pump fresh absorbent into the reboiler. The fourth control loop is used to control the temperature of the absorbent in the reboiler. The purpose of this control circuit is to regenerate the absorbent by heating it using the heating coil installed in the reboiler. Since the third control circuit may not work for unexpected reasons, a control circuit (the fifth control circuit) is installed in the reboiler to protect the heating coil, which switches off the heating element when the liquid level in the reboiler drops below the set value of the absorbent level.