Foreword

The purpose of the low temperature waste heat recovery system (referred to as waste heat recovery) is to improve the energy recovery of the sulfuric acid plant.

The key of the system is to increase the circulating acid operating temperature of the waste heat recovery tower to more than 200℃, replacing the traditional process of a suction tower, so as to improve the thermal temperature level and recover it [1].

However, the waste heat recovery process also faces a problem: When the temperature of a suction cycle acid reaches 200~220℃ and the production of 0.6 ~ 1.0MPa low pressure saturated steam, the cycle acid temperature must be maintained above 200℃, and the concentrated sulfuric acid at 200℃ is highly corrosive, so qualified stainless steel should be strictly used in material selection to ensure the safe operation of waste heat recovery.

When the sulfuric acid concentration of waste heat recovery is not qualified, sulfuric acid corrosion will be stronger at 200℃. Once it leaks, it will leak water (steam) to the acid side, and if the system cannot be stopped safely in time, serious equipment corrosion will occur, resulting in waste heat recovery not being used for a long time. Therefore, the reliable safety interlock will effectively prevent the waste heat recovery process parameters from deviating from the safe value, and even a small amount of water can be found in time and quickly restore the device to a safe state.

1, security interlock control logic

The purpose of the design of safety interlock control logic is that when the device operating process parameters deviate from the safe value, the corresponding safety interlock protection will be triggered, and can be quickly restored to the safety state in the fault state, and the corresponding control equipment can be restored to the safety state, to avoid expanding the fault due to human error, resulting in greater safety accidents.

The following is the safety interlocking logic designed according to the operating process parameters of the low-temperature waste heat recovery system and the conditions required for safety protection. (For reference only, the practical application needs to be designed according to the actual situation of low temperature waste heat recovery device safety interlock)

Serial number	Interlocking name	Conditions that trigger interlocks
1	Diluter Protection (41)	Main fan shutdown	Fan stop signal
		Low boiler water level	Less than the safe value; Recommended value 30%
		Boiler acid inlet temperature is high	Greater than the safe value; Recommended value 240℃
		Boiler acid outlet temperature is high	Greater than the safe value; Recommended value 220℃
		Dilute water supply pressure is low	Less than circulating acid pump outlet pressure
		Low acid concentration 3 minutes after starting the circulating pump	Acid concentrator A\B <97%
		Low primary acid flow	Less than the safe value of circulating flow
		Emergency stop	Manual stop
		Acid water inversion difference	Temperature difference of heat exchange equipment
2	Secondary absorption protection (42)	Main fan shutdown	Fan stop signal
		Low primary acid flow	Less than the safe value of circulating flow
		Secondary acid concentration low	Absorbed acid concentration less than97.5%
		Emergency stop	Manual stop
		Acid water inversion difference	Temperature difference of heat exchange equipment
3	Preheater protection (43)	Low acid 5 minutes after starting the circulating pump	Acid concentrator A\B is less than 97.5%
		The outlet water conductivity of preheater is high	The preheating water outlet is greater than100us/cm
		The outlet water temperature of the preheater is high	Outlet temperature greater than120°
		Emergency stop	Manual stop
		Acid water inversion difference	Temperature difference of heat exchange equipment
4	Boiler Protection (44)	Low acid 5 minutes after starting the circulating pump	Acid concentrator A\B is less than 97.5%
		The deaerator liquid level is low	Liquid level less than 30%
		Emergency stop	Manual stop
		Acid water inversion difference	Temperature difference of heat exchange equipment
		Circulating tank acid concentration	Less than99%
5	Heater Protection (45)	Circulating pump starts 3 minutes after acid, low - low	Acid concentrator A\B ＜97.5%
		Tower pump tank level, high - high	Liquid level less than 90%
		Heater outlet water conductivity, high - high	The heating water outlet is greater than 100us/cm
		Heater outlet water temperature, high - high	Outlet temperature greater than 120℃
		Emergency stop	Manual stop
		Acid water inversion difference	Temperature difference of heat exchange equipment
6	System Protection (46)	Main fan shutdown	Fan stop signal
		Boiler water level,	Less than the safe value; Recommended value 30%
		Boiler acid inlet temperature is high	Greater than the safe value; Recommended value 240℃
		Boiler acid outlet temperature is high	Greater than the safe value; Recommended value 220℃
		Dilution supply pressure	Less than circulating acid pump outlet pressure
		Low acid concentration 3 minutes after starting the circulating pump	Acid concentrator A\B<97.5%
		Low primary acid flow	Less than the safe value of circulating flow
		Emergency stop	Manual stop
		Circulating tank acid concentration	Less than 99%
		Factory wind pressure is low	Less than circulating acid pump outlet pressure

2, acid water inversion logic

The acid water inversion difference is the temperature difference between the inlet and outlet of acid water in the heat exchanger, so as to determine whether the heat exchanger is leaking.

When the heat exchanger acid leakage into the water or water leakage into the acid situation, because there is no stirring, acid and water mixing will react violently, so that the inlet and outlet of the heat exchanger acid and water reverse deviation phenomenon, at this time can be through the acid and water reverse deviation of the device safety interlock protection, to avoid serious equipment corrosion.

Serial number	Acid water inversion name	Logical condition
1	Preheater acid water inversion difference	Inlet water temperature < Import acid temperature and export acid temperature - import acid temperature is less than 4℃
		Imported water temperature > Imported acid temperature and imported water temperature > Outlet acid temperature
2	Heater acid water inversion difference	Inlet water temperature < Import acid temperature and export acid temperature - import acid temperature is less than 4℃
		Imported water temperature > Imported acid temperature and imported water temperature > Outlet acid temperature
3	Boiler acid water inversion difference	Inlet water temperature < Import acid temperature and export acid temperature - import acid temperature is less than 7℃
		Imported water temperature > Import acid temperature and export acid temperature > Boiler water temperature

3. Summary

If the waste heat recovery system wants to design a perfect safety interlock control logic, it also needs to put forward higher requirements for the measuring instruments involved in safety interlock control. Such as the concentration of circulating acid, the concentration of circulating tank, the measurement temperature of the acid water inversion difference, etc., are the core measuring instruments to protect the waste heat recovery system from corrosion, and are also the safety bottom line to protect the waste heat recovery system.

References

1. Liu Jizhong, "Heat Energy Recovery and Utilization of Large sulfur acid Making Plant" Sulfuric acid industry [J] 2008 (6) : 49 ~ 52

2. MICHAEL KEMMERICH, Waste Heat Recovery and Digitization in Sulfuric Acid Production Plant, Sulfuric Acid Industry [J] 2018 (2) : 22 ~ 26