Causes of blockage in the water-cooled air coils of the high-pressure cylinder of a gas compressor
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Dear teachers: Our facility uses Endler furnaces to produce gas; sodium carbonate is used for desulfurization at atmospheric pressure, after which the gas is compressed using a gas compressor. The inlet pressure of the first stage of the compressor is 100 KPa with a temperature of 28 degrees Celsius. At the outlet of this stage, the pressure is 0.12 MPa and the temperature is 115 degrees Celsius; after passing through the water cooler, the temperature drops to 30 degrees Celsius before entering the second compression stage. The outlet pressure of the second stage is 0.34 MPa with a temperature of 110 degrees Celsius; again, after passing through the water cooler, the temperature falls to 26 degrees Celsius. In the third compression stage, the outlet pressure is 0.95 MPa and the temperature is 135 degrees Celsius; after passing through the water cooler, the temperature drops to 28 degrees Celsius before entering the fourth compression stage. The outlet pressure of this stage is 2.25 MPa with a temperature of 130 degrees Celsius, and after passing through the water cooler there, the temperature drops to 30 degrees Celsius before the gas is sent out. I would like to ask the experts: the tube bundles of the water coolers in stages 3 and 4 of our compressor’s high-pressure cylinder tend to get clogged by naphthalene and coal tar, whereas those in stages 1 and 2 of the low-pressure cylinder remain clean. I’m seeking an explanation for this phenomenon.
Is it because the concentrations of naphthalene and coal tar increase after compression, causing naphthalene to crystallize out at low temperatures, while the viscosity of coal tar increases and its fluidity decreases?
After the gas is pressurized at stages one, two, and three, the pressure of the gas coming out of the third pressure compressor reaches 0.95 MPa, with a temperature of 135°C; it is then cooled to 28°C through three-stage water coolers℃； The pressure of the gas after passing through the fourth-stage gas compressor reaches 2.25 MPa, while the temperature after passing through the fourth-stage water cooler is 30°C. The ratios of the outlet pressures at stages 3 and 4 to the inlet pressure at stage 1 are approximately 9.5 and 22.5, respectively; as the gas pressure increases significantly, the partial pressures of naphthalene and coal tar in the gas also increase substantially. According to the Clausius-Clapeyron equation in \"Physical Chemistry\" (edited by Fu Xiancai et al., Volume 1, PP277~278), it can be seen that the boiling points of naphthalene and coal tar also increase significantly accordingly. (For more complex two- and three-component phase transition temperatures, refer to sections 5.5 and 5.6 of the aforementioned textbooks.) The significant increase in the boiling points of naphthalene and coal tar means that cooling will lead to their precipitation, especially in the third and fourth stage water coolers, where the temperature of the gas is reduced to only 28 or 30 degrees. In stage one and stage two, little or no precipitation occurs because the concentrations of naphthalene and coal tar in the gas are very low, and the total pressure is also low; as a result, the corresponding partial pressures are low as well, leading to low boiling points. Since these boiling points are lower than the temperature of the gas, no precipitation takes place. After naphthalene precipitates, further phase changes occur, transforming it into solid naphthalene. This solid naphthalene mixes with the liquid coal tar to form a solid-liquid mixture. As the temperature drops, the viscosity of the coal tar increases, causing this solid-liquid mixture to adhere easily to the walls of the water cooler tubes. Additional note: The boiling point of naphthalene at atmospheric pressure is 217.9°C (the partial pressure of naphthalene is equal to atmospheric pressure). By combining this value with the enthalpy of phase change for naphthalene, and taking into account the pressure of naphthalene in the gas, it is possible to calculate the temperatures at which naphthaline precipitates at the outlets of stages 1, 2, 3, and 4.