Automatic Oxygen/Sulfur Ratio Control

Sulfur acid making plant is the upstream device and energy supply of chemical enterprises, its production efficiency and cost control are directly related to the economic benefits of enterprises. In the process of sulfuric acid production, oxygen/sulfur directly affects the productivity, energy efficiency and stability of process parameters. At present, the manual control method used in sulfur acid making equipment is often difficult to accurately regulate the oxygen sulfur ratio, resulting in low production efficiency, high energy consumption and unstable process parameters, which brings safety to the equipment. Therefore, it is of great significance to explore and realize the automatic control scheme for the optimal oxygen/sulfur ratio of sulfur acid making plant to improve the operation efficiency, reduce the production cost and enhance the market competitiveness of enterprises.

Effect Of Oxygen - Sulfur Ratio On Biogenesis

The concentration of SO₂ at the outlet of sulfur-burning furnace was determined at the time of design of each sulfur acid making device, and the domestic design was basically determined at f (SO₂) 11.5%; Because f (O₂) (dry basis) in the air is 20.948%. The sum of the initial SO2 concentration a and the initial oxygen concentration b in the flue gas of sulfur combustion is also constant.

b=20.948-a: a— — Initial SO₂ volume fraction, %; b— — Initial oxygen volume fraction, %.
Therefore, the oxygen/sulfur ratio R is also a function of the initial SO₂ volume fraction. R=20.948/11.5-1=0.824 The oxygen/sulfur ratio affects production at multiple levels.

When the oxygen/sulfur ratio is too low, it means that there is an oversupply of sulfur, there will be a decrease in conversion rate and insufficient sulfur combustion to produce sublimed sulfur, which will lead to excessive exhaust gas and accidents. Although a high oxygen to sulfur ratio can ensure the complete combustion of sulfur, excess oxygen not only consumes more energy to drive, but also increases the heat taken away by the flue gas and reduces the cost of steam recovery. (SO₂ concentration did not decrease 0.1% tons of acid production cost increase) Therefore, the oxygen/sulfur ratio control in the best balance point is very critical, not only to ensure that the sulfur completely combustion yield, but also to avoid excessive consumption of oxygen waste energy, is the key to reduce production costs.

Logic Of Automatic Control Of Oxygen Sulfur Ratio

When the SO₂ concentration at the outlet of the sulfur burner is confirmed, the device will synthesize the conversion efficiency to confirm the optimal oxygen/sulfur ratio during operation to achieve the best device efficiency. The two variables of the oxygen/sulfur ratio are sulfur and air, and the amount of sulfur is determined due to the load of the unit and is set as a fixed variable in the automatic control. Oxygen is the air flow provided by the fan (referred to as the air volume), in the automatic control of the air volume will change with the ambient atmospheric pressure, temperature, humidity and fan speed, so the air volume is set as a dynamic variable. The oxygen/sulfur ratio is controlled to stabilize the air volume, and the fan speed is automatically adjusted to achieve the optimal oxygen/sulfur ratio when the environmental parameters and load change.

Optimum Oxygen-Sulfur Ratio Determined

The design of domestic sulfur acid making device is basically determined in f (SO2) 11.5%, and the corresponding oxygen-sulfur ratio is R=0.824. However, the optimal oxygen and sulfur value of each unit is determined according to the characteristics of the unit (installed catalyst, converter gas speed, fan pressure head and process flow). These properties will eventually be reflected in the f (SO₂) emission concentration at the exit of the secondary tower. The emission concentration is affected by the conversion rate, and the main variable affecting the conversion rate is the lower oxygen content entering the converter. Therefore, the intersection point of the optimal oxygen-sulfur ratio is the best economic characteristics of the device, and the ratio of the minimum oxygen (air) entering the device to sulfur is the best oxygen-sulfur ratio. Table 1 shows the correspondence between the environmental protection economy and the energy efficiency of the device. Each device can choose the best oxygen/sulfur ratio R according to its own characteristics.

Oxygen Sulfur Ratio Automatic Control

The process variables that can be used for automatic control of oxygen/sulfur ratio are SO₂ concentration at the exit of sulfur burning furnace and O₂ concentration at the exit of secondary suction tower. The liquid sulfur flow rate is a fixed value of the load and measured by a mass flowmeter, and the measurement results are accurate and stable. Because the concentration measurement of SO₂ at the outlet of the sulfur burner is difficult at present, mainly because the flue gas contains SO₃ (actual value 0.2-0.5%), it is very easy to damage the analyzer. The flue gas at the outlet of the second suction tower only contains trace acid fog (30mg/m³) SO₂ content of 500ppm, so the flue gas is very friendly to the oxygen analyzer at the outlet of the second suction tower and can achieve stable measurement. Therefore, the oxygen and sulfur ratio automatic control process variable is selected as the concentration value of O₂ at the exit of the second suction tower as the process variable value.

Safety Automatic Control

Oxygen sulfur ratio control is the core to achieve the stability of the device process index, but also related to the safety of the device operation, perfect safety automatic control is the guarantee of the implementation of oxygen sulfur ratio automatic control. Therefore, the core of safety automatic control is that when a certain instrument in the control loop fails or the process system fails, it can alarm in time to exit the automatic control or return to the safety state.

Load Adjustment Strategy

When the device is to perform load reduction adjustment, set the load target value, the system automatically reduces the liquid sulfur flow first, the oxygen sulfur ratio increases, the O₂ quantity increases, and the system automatically controls the fan speed to achieve the best oxygen sulfur ratio. When the device is to implement load increase adjustment, set the load target value, the system automatically controls the fan speed to the predetermined load speed value, the oxygen and sulfur ratio increases, the O₂ quantity increases, the system allows to increase the frequency conversion of the fine sulfur pump to make the liquid sulfur reach the load target value, and the system automatically controls the extension speed to reach the optimal oxygen and sulfur ratio.

Sublimed Sulfur Prevention Strategy

In order to prevent the generation of sublimed sulfur, the fan air volume will be calculated according to the fan running power curve, corresponding to the system resistance and the fan air pressure, and the corresponding device load line function will be designed according to the wind pressure value. When the function is compared with the liquid sulfur flow rate, when the ratio exceeds the set safety value, the alarm will be paid, the fan speed adjustment will be automatically stopped, and the current fan speed will be maintained. When the fan comes in and the temperature rises, the safe value is set and the safety interlock is triggered to stop.

Load Sudden Change Strategy

When the device is in the automatic control state of oxygen and sulfur ratio, there is no external load adjustment, the system will not be affected by external interference caused by sudden load changes, therefore, the system in the absence of load adjustment, fan speed is only slightly adjusted to adapt to the environment and liquid sulfur trace changes. When there is a sudden change in the system, it can only be caused by the failure of the measuring instrument on the control circuit. Therefore, different degrees of freedom of automatic load control adjustment are designed in the system to cope with sudden load changes.

Warm Reminder:

Li Zheng Lai Technology (Chongqing) Co., Ltd. is committed to the intelligent and digital sulfuric acid equipment is willing to provide the sulfuric acid industry with automatic, intelligent and autonomous operation to provide the best oxygen/sulfur ratio control consulting and implementation of technical services.