Foreword

Low temperature waste heat recovery system uses w (H₂SO₄) more than 99% of the metal corrosion characteristics, the use of corrosion resistance special stainless steel series, In the process of sulfuric acid production, the concentration of sulfuric acid is strictly controlled within a small range w (H₂SO₄) (99.05~99.51%), and it is operated at a specific H₂SO₄ temperature and concentration. To balance heat recovery and maintain unit operating life.

The flue gas temperature and SO₃ concentration of the waste heat recovery system are basically not affected by the load of the device, so the temperature of the low-temperature waste heat recovery system remains stable. However, the low temperature waste heat recovery system also acts as a SO₃ absorber, and its outlet w (H₂SO₄) reaches 99.5%. In order to ensure the absorption efficiency of SO₃ in the waste heat recovery system, water must be added to dilute the primary circulating acid to control the concentration of circulating sulfuric acid between w (H₂SO₄) 99.05~99.2%. In the production process, in order to ensure the safety of waste heat recovery tower, the concentration of primary circulating acid must be controlled at w (H₂SO₄) ≥ 99%. In order to ensure the absorption rate of primary SO₃, the acid fog of the exit flue gas of the low-temperature waste heat system must be qualified, SO the concentration of primary circulating sulfuric acid should be controlled at w (H₂SO₄) ≤ 99.2~99.3%. Therefore, the accurate and stable measurement of the primary circulating acid concentration in the waste heat recovery system is very important for the safety of the waste heat recovery tower and the long-term stable operation of the sulfuric acid plant. As the key energy recovery and absorption system in the production of sulfuric acid plant, the stability of the acid concentration analyzer of the waste heat recovery tower directly affects the operation efficiency, equipment safety and automatic control investment. Therefore, it is very important to solve the problem of fluctuation of sulfuric acid concentration data in primary cycle in time. This paper will deeply discuss the reasons for the fluctuation of sulfuric acid concentration analyzer data and the corresponding solution strategy, in order to provide a valuable reference for relevant technical personnel.

Importance of measuring the concentration of first and primary circulating acid

1. The sulfuric acid concentration of the waste heat recovery system is automatically controlled by adding water, and the process variable of automatic control is the first-order circulating sulfuric acid concentration. Therefore, the primary circulating sulfuric acid concentration is both a safety instrument for the protection device and a measuring instrument for the automatic control of the water adding process. When the concentration of sulfuric acid in the primary cycle cannot be accurately measured, the load change in the production process inevitably causes the concentration of primary sulfuric acid to be lower than w (H₂SO₄) 99.00%, or even lower, and the corrosion is faster. It causes serious corrosion of waste heat recovery tower wall, evaporator, heat exchanger, pump and primary acid separator.

2. The fluctuation of sulfuric acid concentration data in the primary cycle will lead to the loss of safety protection of the low-temperature waste heat recovery system, which will seriously lead to the corrosion of the evaporator, heat exchanger, pump, acid distributor, grid, tower and other equipment, causing the device to stop for a long time.

3. The fluctuation of sulfuric acid concentration data makes automatic water adding lose process control variables, and the concentration of sulfuric acid in the primary cycle cannot be automatically controlled, thus increasing the working intensity of the operator. When the load changes, the concentration of sulfuric acid in the primary cycle cannot be automatically adjusted in time, which will lead to the concentration of acid in the primary cycle deviating from the allowable value of the process. Causing the system to be corroded or the absorption rate to decrease the acid fog exceeds the standard, corroding the cold and hot heat exchanger at the back end.

Analysis of the causes of data fluctuation

1. The concentration of primary circulating sulfuric acid is measured after the diluter, at which time the temperature of sulfuric acid is 180℃ and the pressure is 0.1MPa. In order to quickly achieve uniform mixing of water and sulfuric acid, the diluter adopts the technology of atomizing water with compressed air to enhance the disturbance effect. At present, the only sensors used to measure high temperature concentrated sulfuric acid are the AS118 model from Lilly Technology and the imported 222 conductivity sensor. However, because the conductivity sensor cannot measure sulfuric acid containing air medium, there is a large deviation between the measured value of the primary cycle acid concentration in the waste heat recovery system and the actual value, and the measured data fluctuates significantly, as shown in Figure 1. Therefore, these measurements cannot meet the requirements of safety interlocking and automatic water filling control.

Fluctuation picture of acid concentration (Figure 1)

2. In low-temperature waste heat recovery systems, the operating parameters of primary cycle sulfuric acid are set at a temperature between 180 and 190 degrees Celsius, a pressure in the range of 0.1 to 0.2 mpa, a flow rate between 800 and 1000 cubic meters per hour, a concentration between 99.05% and 99.15%, while the air content accounts for 5% to 8% of the sulfuric acid. The sampling point of the sulfuric acid concentration analyzer is located at the horizontal pipe where the primary circulating sulfuric acid main pipe enters the tower body. Since the main pipe enters the tower immediately after the acid separator, the sampling point pressure of the analyzer is basically equivalent to the static pressure of the acid separator level. As the diluter adopts the technology of atomizing desalted water with compressed air to enhance the disturbance, the circulating sulfuric acid from the diluter is mixed with air medium. Currently used conductivity sensors cannot accurately measure sulfuric acid concentrations in airborne media. When sulfuric acid containing air medium flows through the conductivity sensor, the measured conductivity is abnormally low and unstable because the air is not conductive. In view of the higher the concentration of sulfuric acid, the lower its conductivity characteristics, sulfuric acid containing air medium through the conductivity sensor, the measured concentration value will be higher than the actual value, and there is a large fluctuation, so that accurate measurement can not be achieved.

Solutions

Based on the physical and process characteristics of primary circulating sulfuric acid, Lilly & Lilly Technology (Chongqing) Co., Ltd. has designed a super clean steady-state gas-liquid separator for sulfuric acid, which is designed to completely separate the air in primary circulating sulfuric acid, thereby eliminating the interference of air on the conductivity sensor. The equipment consists of a two-stage high-efficiency gas-liquid separation device, a siphon balancing device and related accessories. When the high temperature concentrated sulfuric acid flows into the gas-liquid separator, it is first separated in the primary gas-liquid separator. Then, flow to the secondary gas-liquid separator, in this device, the thermal resistance installation sleeve is specially designed to meet the needs of the conductivity sensor for temperature measurement; Next, the installation line flows through the conductivity sensor and finally returns to the HRS cycle tank. The top of the gas-liquid separator is equipped with a siphon balancing device. By reducing the principle of pipe diameter, the negative pressure at the acid discharge port can effectively reduce the adverse influence on the front separation device and the conductivity measuring device. At the same time, the heat insulation plate and heat dissipation hole device are provided inside the gas-liquid separator, which significantly reduces the potential impact of heat emitted by high temperature sulfuric acid on the sensor.

Acid concentrator with ultra-clean steady-state gas-liquid separator

After transformation 1

After renovation 2