Control scheme for hydrogen chloride synthesis

Operating Procedures for the “Two-in-One” Synthesis Furnace http://bbs.hcbbs.com/thread-120059-1-5.html

I am seeking information on process and instrument automation control solutions, such as automatic mixing control for hydrogen and chlorine gases, interlocks, etc. I was wondering if you could provide some details. Thank you

There are many control schemes, including complete designs. Simply put, chaining is important; there is chaining between hydrogen chloride and chlorohydrin treatment, as well as in the synthesis of PVC. Control of the chlorine and hydrogen flow rates at the inlet of the synthesis furnace, control of the pressure of chlorine and hydrogen, and so on.

Below is the three-in-one hydrochloric acid furnace with lower ignition program (PLC), a product from the German company SGL. 2.2 Ignition preparation procedure 1. Check and ensure that the covers of the mirrors and ignition ports are properly closed. 2. Add dilute alkali solution to the hydrochloric acid off-gas scrubber as needed. Open the isolation valve on the alkali solution supply line at the bottom of the hydrochloric acid exhaust scrubber tower. 3. Open the alkali supply and water supply isolation valves before opening the pressure relief valves (PCV-17034 and 17037). Adjust the rotameters (FI-17036 and 17039) to their normal operating setpoints. Adjust the rotameters (FI-17005, 17013, 17020) of each exhaust scrubber tower to their normal operating setpoints. 4. Start supplying cooling water to the synthesis unit: Open the exhaust valve to ensure that all air in the cooling water circuit is removed. Open the shut-off valve on the cooling water return pipe. Open the water inlet valve slowly to avoid water hammer. After confirming that all air in the pipe has been expelled, close the exhaust valve. 5. Confirm that the low cooling water flow alarm FSL-7008/7039 has been resolved. 6. Open the low-pressure steam supply valve leading to the exhaust stack of the hydrochloric acid synthesis unit. 2.3 Ignition procedure The startup sequence for the hydrochloric acid production plant is as follows: 1. Nitrogen purging 2. Ignition of the burner for burning air and hydrogen 3. Ignition of the main burner for burning hydrogen and chlorine 4. Turning off the pilot burner 5. Increasing the hydrochloric acid production rate on the DCS screen. 2.3.1 Automatic Ignition 1. Set the exhaust gas scrubber water flow controller (FIC-7013/7044) to AUTO mode, with a setpoint of 0.9 m3/h. 2. The soft water on/off valve XV-7095/7096 at the top of the exhaust gas scrubber tower will automatically open. Set the flow rate of the rotameter FI-17041/17042 to 0.3 m3/h. 3. Open the valve leading to the hydrochloric acid density tank and the discharge valve of the acid removal liquid collection tank. Check whether there is water flow through the transparent PVC pipe to verify that the sealed elbow of the hydrochloric acid product is filled. It is extremely important that the sealed elbow of the hydrochloric acid product always be filled with liquid, to ensure that hydrogen does not enter the hydrochloric acid storage tank. The presence of hydrogen in the hydrochloric acid storage tank can lead to an explosion. Once it is confirmed that the synthesis unit is ready for ignition (see Step 4 below), close the discharge valve of the acid-removal liquid collection tank, and open the discharge valve of the hydrochloric acid density tank. 4. Check the on-site ignition control panel. The indicator light indicating that ignition is ready should be ON, signifying that the following conditions are met: • The flow rate of absorbent (softening) water to the washing tower is sufficient • The flow rate of cooling water to the synthesis unit is sufficient • The hydrogen pressure is sufficient • The pressure of concentrated chlorine gas is sufficient • The flame detector shows no flame signal • The explosion-proof switch gives no alarm. The following indicator lights should be OFF: • Ignition failed • Ignition reset • Ignition completed. 5. Press button S1 “Ignition Reset” on the local control panel. 6. Press button S2 “Ignition” on the local control panel to start the following operations: • The setpoints and output values of the following controllers are set to zero and forced to MANUAL mode: FIC-7019, FIC-7023, FIC-7028 • Valves XV-7030/7061 will open, and nitrogen purging begins. If nitrogen flow is confirmed (FSL-7002/7033), the two-minute timer will start. 7. After 2 minutes, if the required amount of nitrogen has been supplied, the nitrogen purge valve (XV-7030/7061) will close, while the air valve for the ignition burner (XV-7068) will open. 8. 10 seconds later, the ignition transformer BX-7011/7042 is activated and begins to spark. 9. If the spark signal light is on, the hydrogen valve (XV-7004/7040) of the ignition burner will open, and after 2 seconds, the flame detector (BS-7010/7041) will be activated. If the flame detector fails to detect any flame within 6 seconds, the startup procedure will be terminated, and the ignition procedure must be restarted. 10. The ignition transformer turns OFF after 8 seconds. 11. 20 seconds after the ignition transformer is turned OFF, the hydrogen bypass valve (XV-7031/7062) and the on/off valve (XV-7029/7060) open. 12. Two seconds later, the concentrated chlorine bypass valve (XV-7021/7052) and the on/off valve (XV-7020/7051) open. At this time, the hydrochloric acid production plant will operate at about 10% of its normal capacity. 13. After 10 seconds, the air valves (XV-7006/7037) and hydrogen valves (XV-7004/7040) connecting to the igniter close, and the ignition burner goes out. 14. After another 10 seconds, the ignition procedure is completed, and the “Ignition Complete” indicator light turns ON. After receiving the signal that ignition is complete, the controller’s interlocks are released, allowing the production of hydrochloric acid to increase.

1. The molecular ratio of hydrogen to chlorine must be adjusted based on the purity of hydrogen chloride at the outlet of the synthesis furnace and the level of free chlorine, in order to find a ratio that is suitable for different flow rates and purities of hydrogen chloride. Our company has installed an online free chlorine monitor; although its accuracy isn’t perfect, it provides intuitive and continuous numerical readings. 2. The purity of hydrogen chloride is 92–95%; it can be increased slightly if the control system and interlock mechanisms are well-designed, and the level of operational management is high. 3. Regularly analyze the purity of hydrogen chloride to adjust the molecular ratio. 4. In addition to the display on the computer, an intuitive U-tube differential pressure gauge should also be installed in the operation window to show the molecular ratio. 5. The automatic control valves for hydrogen and chlorine combined into one unit, the switch valve for the acetylene main pipeline and the acetylene water ring pump are linked to the outlet temperature of the mixer where vinyl chloride is mixed and dehydrated, as well as to the outlet temperature of the vinyl chloride preheater. 6. The pressure in the combined hydrogen main pipe is interlocked with the hydrogen automatic control valve, the chlorine automatic control valve, the acetylene main pipe control valve, and the acetylene water ring pump. 7. The hydrogen transfer pump is interlocked with the electrolysis unit.

It seems that hydrogen chloride for vinyl chloride synthesis still doesn’t have automatic dosing yet. Interlocking of the chlorine and hydrogen control valves in the synthesis furnace with the control valves for hydrogen venting and chlorine discharge in case of emergencies.

I suggest you get in touch with Zhejiang University Zhongkong; the DCS systems they provide are compatible with the hydrogen chloride synthesis units manufactured by our company, and their performance is quite stable. This post was last edited by yzhms on 2009-3-8 00:26.]

We adopted a method of adjusting the hydrogen chloride flow rate, rather than using a method based on adjusting the purity ratio of hydrogen chloride. This was mainly due to concerns regarding the reliability of the online monitors for measuring hydrogen chloride purity, so we did not opt for that approach. In terms of interlock control, the programmable valve for hydrogen and chlorine fed into the furnace, as well as the chlorine return valve (part of the chlorine emergency handling system in the caustic soda plant to prevent overpressure in the chlorine system), are interconnected with the caustic soda plant; that is, when the caustic soda plant shuts down due to an interlock shutdown, the programmable valve for hydrogen and chlorine fed into the furnace is closed, while the chlorine return valve is opened.