Chemical processing tower equipment: [Weekly topic] Please discuss the principle of flashing and its applications in the petrochemical industry (2011.07.17-07.30)

The flash distillation method involves reducing the system pressure at a certain temperature, and taking advantage of the difference in vaporization rates of various components at the same temperature to evaporate large amounts of C1–C4 light hydrocarbons, thereby separating them from crude oil. 　　The flash evaporation method can operate under negative pressure. After crude oil is dehydrated, it is generally subjected to negative-pressure flash evaporation at 0.06–0.08 MPa (absolute) and 55°C–65°C. It can also be heating flash evaporation. Generally, flash evaporation is carried out at 0.25~0.3 MPa (absolute) and 120°C.

Reply to 1# wu_pb: According to Henry’s law P=EX, the solubility of gas-phase solutes in the liquid solvent varies at different temperatures and partial pressures. When the solvent pressure decreases, the solute in the solvent is rapidly desorbed and released automatically, resulting in flashing. The energy for flashing is provided by the solvent itself, so the temperature of the solvent decreases during the flashing process. From a higher constant pressure to a lower constant pressure, when desorption equilibrium is reached, the amount of solute desorbed remains constant, and the amount of solute remaining in the solvent also remains constant. Therefore, there is only one control target for flashing, and that is the flashing pressure. Henry’s law is one of the fundamental laws in physical chemistry; it was discovered by the British scientist W. Henry in 1803 while studying the laws governing the solubility of gases in liquids. It can be expressed as follows: “At a constant temperature, the concentration of a certain gas in a solution is directly proportional to the equilibrium pressure of that gas above the liquid surface.” ”Experiments show that this law holds true only when the solubility of the gas in the liquid is not very high; in such cases, the gas acts as a volatile solute in a dilute solution, and the gas pressure corresponds to the vapor pressure of the solute. Therefore, Henry’s law can also be expressed as follows: at a constant temperature, the vapor pressure of a solute in a dilute solution is proportional to the concentration of the solution: pB = kxB, where pB represents the vapor pressure of the solute in the dilute solution ； xB is the mole fraction of the solute ； k is the Henry constant, whose value depends on temperature, pressure, and the nature of the solute and solvent. Since the various concentrations in a dilute solution are proportional to each other, xB in the above equation can also be mB (mass molar concentration) or cB (mole concentration), etc.; in such cases, the value of k will change accordingly. 　　Henry’s law applies only when the molecular states of the solute in the gas phase and the liquid phase are the same. If the solute molecules undergo dissociation, association, etc. in the solution, then xB (or mB, cB, etc.) in the above equation should refer to the concentration of the portion that is in the same molecular state as that in the gas phase ； When the total pressure is not high, if multiple gases are dissolved in the same liquid simultaneously, Henry’s law can be applied separately to each of these gases ； Generally speaking, the weaker the solution, the more accurate Henry’s law becomes; as xB→0, the solute can strictly obey the law. Principle: It mainly relies on the fact that when pressure is applied, the vapor pressure decreases, allowing more solvent (usually water) to flash into a gaseous state, thereby achieving concentration. Structure: Very simple. The diameter should be a bit larger; it’s a bit like a cyclone separator. Of course, a certain height must be maintained; otherwise, the liquid will also flow out. Vaporization occurs when saturated water under high pressure is introduced into a container at lower pressure; the sudden drop in pressure causes this saturated water to turn into both saturated steam and saturated water at the pressure of that container. Reason for formation: When water is heated under atmospheric pressure, 100°C is the highest temperature that liquid water can reach at that pressure. Reheating cannot increase the temperature of water; it can only turn the water into steam. The heat absorbed by water as it is heated to its boiling point is called \"sensible heat,\" or specific heat of saturated water. The heat required to convert saturated water into steam at the same atmospheric pressure is called \"latent heat\". However, if water is heated under certain pressure, its boiling point will be higher than 100°C, which requires more sensible heat. The higher the pressure, the higher the boiling point of water, and the greater its heat content as well. As the pressure decreases, some of the sensible heat is released; this excess heat is then absorbed in the form of latent heat, causing some of the water to be \"vaporized\" into steam. Actual situation: Flashing occurs in pipeline systems, which can easily cause cavitation damage to valves. Anti-cavitation high-pressure valves can be used, as they feature multiple stages of throttling to distribute the pressure difference; materials resistant to cavitation erosion can also be selected. Flash evaporation can also be used as an energy source, in the recovery of boiler wastewater in thermal power plants and in geothermal power generation. When it comes to flashing, attention must also be paid to the flashing steam. What is flashing steam? When hot condensate water or boiler water under certain pressure is depressurized, part of the water will evaporate again, and the resulting steam is known as flash steam. Why is flash steam important? Because it contains heat that enables the factory to operate economically, failing to utilize it means energy will be wasted needlessly. How is flash vapor formed? When water is heated under atmospheric pressure, 100°C is the highest temperature that liquid water can reach at that pressure. Reheating cannot increase the temperature of water; it can only turn the water into steam. The heat absorbed by water as it is heated to its boiling point is called \"sensible heat,\" or specific heat of saturated water. The heat required to convert saturated water into steam at the same atmospheric pressure is called \"latent heat\". However, if water is heated under certain pressure, its boiling point will be higher than 100°C, which requires more sensible heat. The higher the pressure, the higher the boiling point of water, and the greater its heat content as well. As the pressure decreases, some of the sensible heat is released; this excess heat is then absorbed in the form of latent heat, causing some of the water to be \"vaporized\" into steam. Flash evaporation also has valuable applications, such as flash condensation technology: vacuum is applied to lower the boiling point, allowing boiling at temperatures below 100°, followed by the removal of the condensed water. The advantage is that the materials are not damaged by high temperatures, and it is generally used in food and pharmaceuticals. Working principle: It is a method of separation that takes advantage of the differences in volatility among the various components in a liquid mixture; thermal energy is used to cause some of these components to vaporize, thereby concentrating the lighter components in the vapor phase while the heavier components remain in the liquid phase. In fact, in the distillation calculations for columns, such flash calculations must be performed for each theoretical plate; however, this is generally referred to as “equilibrium stage” calculations. Theoretical approaches are more rigorous, but the content of the calculations remains exactly the same. Every rigorous calculation, whether it’s a flash calculation or an equilibrium stage calculation, must satisfy three fundamental balances: mass balance, energy balance, and phase balance. Therefore, a standard flash calculation module can handle both changes in pressure (increase or decrease), as well as heat and work exchanges. The exchange of internal and external work, along with heat exchange, is reflected in the energy balance, while mass balance and phase balance determine the vaporization fraction and the composition of the vapor and liquid phases. Additionally, flash calculations have another very important and challenging task: determining whether the specified state is a two-phase mixture of vapor and liquid, or merely a vapor phase or a liquid phase. Moreover, the calculations related to liquid-liquid equilibrium also rely on flash calculations; of course, some software offers separate modules for these calculations due to their greater complexity. The description of flash calculations provided above is just one perspective and not entirely rigorous, but it is relatively more accurate. In short, whether you use PROII, ASPEN, or HYSIS, flash calculations truly constitute their most fundamental core component.

Vaporization occurs when saturated water under high pressure is introduced into a container at lower pressure; the sudden drop in pressure causes this saturated water to turn into both saturated steam and saturated water at the pressure of that container.

Vaporization occurs when saturated water under high pressure is introduced into a container at lower pressure; the sudden drop in pressure causes this saturated water to turn into both saturated steam and saturated water at the pressure of that container.

The process by which high-pressure, high-temperature steam enters an empty tank is a process of separating certain components, and the theoretical principle governing this is Henry’s law!