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1. Moisture: Moisture content (g/m³) = W × 1000 / V0. In this formula, W represents the increase in weight after sampling using the phosphorus pentoxide drying tube, in grams； V0 – Sampled volume (at standard conditions), L. (Temperature during conversion) 1.1. Purpose of measurement: When the moisture content in the gas entering the conversion equipment exceeds the specified limit, acid mist condenses in the heat exchange equipment, causing corrosion. Check and adjust the operation of the drying tower based on the measurement results of the moisture content in the gas. Based on the measurement of the moisture content in the gas exiting the blower, check whether there are leaks in the gas pipeline before the blower. 1.2. Summary of the method: Moist gas passes through a drying tube filled with phosphorus pentoxide as a desiccant, whereby the moisture is absorbed. Based on the increase in weight of the drying tube after moisture absorption and the volume of gas passed through, the moisture content in the gas can be determined. The reactions proceed as follows: P2O5 + H2O = 2HPO3; P2O5 + 2H2O = H4P2O7; P2O5 + 3H2O = 2H3PO4. 1.3 Reagents and instruments. Figure: Apparatus for determining the moisture content in gases prior to drying. 1 – Gas pipeline ； 2—Sampling tube ； 3—Glass cyclone ； 4—Thermometer ； 5— Drying tube ； 6— Drying tube ； 7—Manometer ； 8—Flow meter ； 9—Screw clamp (1) Powdered phosphorus pentoxide (analytical grade) ； (2) Granular calcium sulfate: Pour anhydrous calcium sulfate into a porcelain dish, add water equivalent to one-third of its weight, and stir rapidly to form a paste. Spread this paste into a thin layer, then place it in an oven and bake at a constant temperature of 120–150°C for 2–3 hours. After cooling, remove it from the oven, crush it into small pieces, sprinkle a little more water on it, and bake again in the oven at 120–150°C for 2–3 hours. Once cooled, crush the material into granules with a size of 4–8 mesh. Finally, bake these granules at a constant temperature of 180–210°C before packaging them in bottles for later use ； (3) U-shaped drying tube ； (4) Cyclone separation tube ； (5) Sampling tube ； (6) Differential pressure flow meter or wet flow meter ； (7) Thermometer ； (8) Pressure gauge ； 1.4 Preparation for measurement (1) Preparation of phosphorus pentoxide drying tube: Cut clean, alkaline-free glass fibers into short strands about 6 mm long. Lay a layer of these fibers at the bottom of a measuring cup, then add powdered phosphorus pentoxide along with the glass fibers, and stir them together evenly using a glass rod. It is then quickly placed into a clean, dry U-shaped drying tube with a stopper; the openings of the tube and any connecting branches are wiped clean, and a stopcock is used to seal it. Before use, dry smoke gas is introduced into the tube for about 5 minutes to displace the air inside, after which the tube is placed in a desiccator for 20–30 minutes. It is then weighed and set aside, with the accuracy reaching 0.0002 g. (2) Preparation of neutral glass fibers: Take some glass fibers and place them in a beaker; soak them in concentrated hydrochloric acid for 24 hours, then wash until they reach a neutral pH. After drying at 105°C, they are ready for use. (3) If measuring the moisture content in the gas before drying, a dry cyclone separator must be installed in front of the U-shaped drying tube, and it must be weighed in advance (with accuracy to 0.0002 g). 1.5, Measurement Procedure (1) Ventilate for several minutes without connecting the drying tube. (2) Insert the sampling tube to 1/3 of the length of the pipe, connect it to the dryer, open the valve, and adjust the gas flow rate to around 2 L/min as indicated in the diagram. Collect 150 L of sample, then stop sampling, close the valve, remove the drying tube, place it in the dryer, and weigh it accurately after half an hour. 1.6 Calculation: In the formula, W represents the weight increase of the phosphorus pentoxide drying tube after sampling, in grams ； V0 — sampling volume (under standard conditions), in L. Precautions: (1) During sampling, the drying tube should be covered with a towel to prevent dust from entering the gaps in the stopcock of the drying tube. (2) When sampling at the inlet of the drying tower, the sampling tube should be tilted at a 30° angle to allow condensate to flow out and prevent it from flowing back into the gas pipeline. (3) Before sampling, check whether the entire device is leaking. (4) When sampling behind the drying tower, two layers of cotton 15 mm long can be placed inside the sampling tube to prevent a small amount of acid mist from entering the drying tube; the cotton should be dried for 24 hours, or a U-shaped glass fiber mist eliminator can be installed behind the sampling tube. (5) When measuring the moisture content in the gas prior to electrodesorption during the pickling process, the acidity caused by the moisture should be deducted. (6) When turning the stopcock on the U-shaped drying tube of the switch, it should be done in the same manner before and after sampling, so that the weight of the drying tube is not affected by the gas pressure inside it; if there is positive pressure, close the stopcock at the front first, and if there is negative pressure, close the stopcock at the back first. (7) Before each weighing, the drying tube should be placed in the dryer for 0.5 hours in advance. (8) When the moisture content at the outlet of the drying tower is low, the dried sample gas should be used for replacement for 5–10 minutes. (9) When measuring the moisture content in the gas before drying, it is necessary and possible to insert glass fiber triple-ball between the glass cyclone and the CaSO4 drying tube. 2. Calculation of acid mist concentration. In the formula: C1 – molar concentration of the NaOH standard solution ； V1 – Volume of NaOH standard solution used, ml ； V/1 – Volume of NaOH standard solution used in the blank test, ml ； C2 – Molar concentration of the iodine standard solution ； (V2 – V2 – volume of iodine standard solution used, ml) ； V/2 – Volume of iodine standard solution used in the blank test, ml ； V0 – Sampling volume (under standard conditions), L ； 0.0400 – millimolar mass of SO3, g/mmol/L ； 80 – Molar mass of SO3, g/mol ； 22.4 – Volume of 1 mol of SO3 at standard conditions, in L. 3. Calculation of SO2 content: C – molar concentration of the iodine solution, in mol/L ； V – Amount of iodine solution, ml ； V0 – Volume of residual gas (under standard conditions), ml ； 10.945 – Gas volume per mol of SO2, in ml. 3.1 Method Summary: Sulfur dioxide in the gas is oxidized to sulfuric acid and hydroiodic acid when it passes through a quantitative iodine solution. Using starch as an indicator, the concentration of sulfur dioxide in the gas can be determined based on the amount of iodine used and the volume of the remaining gas. The reaction proceeds according to the following equation: 3.2, Reagents and Instruments (1) 0.01 mol/L I2 standard solution ； (2) 0.1 mol/L I2 standard solution ； (3) 0.5% starch solution ； (4) Distilled water that does not consume iodine after treatment ； (5) Reaction tube; (6) Gas volumeter ； (7) Level bottle ； (8) Thermometer ； (9) Sampling tube. 3.3 Preparation for measurement: Check whether the gas measuring tube, level bottle, and instrument setup are leaking. Using a pipette, transfer 10 ml of the 0.01 mol/L or 0.1 mol/L I2 standard solution into the reaction tube, add water to fill the tube to 3/4 of its capacity, add 2 ml of 0.5% starch solution, and seal it with a rubber stopper for later use. Check whether each sampling point is unobstructed. When sampling under positive pressure, exhaust air for several minutes; when sampling under negative pressure, use the drainage and suction method to draw out the sample gas, thereby fully replacing the gas inside the reaction pipeline to facilitate measurement. 3.4 Connect the instrument properly, turn the valve so that air flow continuously produces bubbles; stop the airflow once the blue color of the solution just disappears. Then adjust the water level in the gas measuring tube to be level with the water level in the level bottle. Read the volume and temperature of the gas in the measuring tube, and determine the sulfur dioxide content by referring to a table or performing calculations based on the volume of the remaining gas. After the analysis is complete, raise the volumetric tube and remove the rubber stopper from the reaction tube to release air, allowing the water level in the volumetric tube to return to zero. 4. Derivation of the conversion rate formula: Let the SO2 concentration in the gas entering the converter be a%, and the SO2 concentration in the gas exiting the absorber be b%. Assume that the volume of gas entering the converter is 100 units, of which a units are SO2. Let the conversion rate be x. The reaction equation is: SO2 + 1/2O2 = SO3; therefore, the amount of SO3 produced during the reaction is ax/100 units ； O2 consumed simultaneously: ax/200 (parts) ； The remaining volume of gas after absorption is: 100 – (ax/100 + ax/200) units, that is, 100 – 3ax/200 units ； The remaining amount of SO2 is: a – ax/100 (parts) ； Thus, b = (a – ax/100) × 100 / (100 – 3ax/200) ； That is, b(100 – 3ax/200) = 100a – ax ； ax – 3abx/200 = 100a–100b ； That is, ax(1–3b/200) = 100(a–b) ； 5. Derivation of the absorption rate calculation formula: The same assumptions as for the conversion rate apply; in addition, it is assumed that the SO3 concentration in the gas exiting the absorption tower is C%, and the absorption rate is y% ； The amount of gas remaining after absorption (the remaining SO3 is not taken into account here, as its quantity is very small; it can be omitted for the sake of simplifying calculations) is: (100 – 3ax/200) units ； The remaining amount of SO3 is (ax/100)(1–y/100) parts ； C(100 – 3ax/200) = ax(1 – y/100). Therefore, C(100 – 3ax/200) = ax – axy/100. This gives y = (100 – 100C/ax)/(100 – 3ax/200) = 100 – 100C(100/ax – 3/200). Hence, 100/ax = (1 – 0.015b)/(a – b). Substituting this into the expression above yields 100/ax – 3/200 = (1 – 0.015b)/(a – b) – 3/200. Substituting this back into the original equation…