Production Technology [Weekly Topic]: Discussion on Vacuum Distillation (2011-03-14~20)

The last edit to this post was made by supperstone001 on 2011-3-14 at 13:42. Advantages of the vacuum deep drawing technique: 1. Increases the yield of oil extraction; 2. Lowers the temperature at the bottom of the tower, reducing the generation of pyrolysis gases; 3. Improves the separation efficiency in the flashing section of the vacuum tower, reducing mist entrainment； 4. Reduces pressure drop and increases the gas-liquid contact area. Disadvantages: 1. The equipment structure is complex, increasing investment costs; 2. The process is complicated, raising operating costs; 3. Steam injection for vacuum creation leads to increased wastewater discharge

Suitable for the purification of materials that become liquid under pressure and can turn into a gas when the pressure is reduced. It can reduce energy consumption

Advantages: Vacuum distillation can carry out distillation processes that cannot be achieved under normal pressure； Some components have high boiling points at atmospheric pressure and are prone to cracking; reducing the operating pressure can lower their boiling points and prevent cracking reactions. Disadvantages: Difficult to operate, particularly while driving, and it has a large number of trays.

This post was last edited by shmily327 on 2011-3-14 at 17:56. Vacuum distillation is suitable for heat-sensitive substances, allowing operations at lower boiling points without decomposition. It is possible to reduce the required level of performance by using a lower heat source for heating. The pressure in a vacuum distillation column is low and difficult to control, making bumping more likely to occur. The temperature of the product collected at the top of the column is low, and a large amount of cooling energy is required for reflux.

This post was last edited and replied to by shmily327 on 2011-3-14 at 17:57. Reply 1# shmily327 Advantages of vacuum distillation: 1. Due to the reduced pressure, the boiling point decreases, resulting in energy savings and reduced consumption. 2. Vacuum distillation can lower the bubble point of mixtures, thereby reducing the separation temperature. Therefore, it is possible to reduce steam consumption for heating and utilize steam at lower pressures. Particularly when water vapor is used as the heat source, an increase in the heating temperature results in a much greater increase in the required saturated steam pressure ； 3. It can improve the separation capability; the greater the relative volatility between the mixtures to be separated, the easier it is to separate them. Under reduced pressure, generally speaking, the relative volatility between components increases ； 4. For toxic substances, vacuum distillation can prevent their leakage and reduce environmental pollution. Vacuum distillation is one of the commonly used methods for separating purifiable organic compounds. It is particularly suitable for those substances that undergo thermal decomposition, oxidation, or polymerization before reaching their boiling point during normal pressure distillation. Disadvantages of vacuum distillation: 1. The efficiency of the tray under negative pressure is not high. On the contrary, the tray efficiency is higher at normal and even positive pressures, because there are more opportunities for molecular contact ； 2. It also cannot save energy, as the latent heat of vaporization of substances under negative pressure is not significantly different from that under normal pressure. 3. Vacuum operations require high levels of sealing for the equipment; stringent requirements are imposed on such equipment, especially when dealing with flammable and explosive substances. If oxygen gets into the equipment, there is a risk of explosion.

This post was last edited by shmily327 on 2011-3-14 at 19:37. (1) For certain substances that tend to decompose or polymerize under high-temperature distillation, thus failing to achieve the desired separation effect (also known as heat-sensitive materials), vacuum distillation must be employed. (2) Vacuum distillation can lower the bubble point of the mixture, thereby reducing the separation temperature. Thus, the steam consumption for heating can be reduced, and heating steam at a lower pressure can be used. Especially when water vapor is used as the heat source, as the heating temperature increases, the required saturated vapor pressure must increase even more, which imposes new requirements on both the equipment and the source of steam. (3) It can improve the separation capability. We know that the greater the relative volatility between the components in a mixture to be separated, the easier it is to separate them. Under reduced pressure, generally speaking, the relative volatility between components increases. (4) For the separation of toxic substances, vacuum distillation can prevent the leakage of highly toxic materials and reduce environmental pollution, which is significant in terms of protecting people’s health.

The advantages and disadvantages of vacuum distillation should be analyzed from the perspective of why vacuum distillation is chosen. There are several reasons for using vacuum distillation: 1. The system contains heat-sensitive substances; when operating at normal pressure or under pressure, the operating temperature of the tower becomes too high, preventing normal operation. 2. For distillation operations involving highly toxic or extremely hazardous substances, reduced-pressure operations can prevent the leakage of such materials. 3. For certain special systems, applying negative pressure can increase the relative volatility of the substances, which facilitates separation. 4. Other reasons: It is precisely for these reasons that vacuum distillation is employed, and these can also be considered advantages of vacuum distillation. Disadvantages: 1. Vacuum distillation requires the creation of a vacuum, which consumes additional energy; it also involves a vacuum system, making the process relatively complex. Investments also increased. 2. Objections were raised regarding energy savings in the upper floors; after reducing the pressure, no energy savings were achieved – on the contrary, energy consumption increased. Because the latent heat of vaporization at low pressure is greater. 3. Vacuum distillation: For substances of the same mass, the actual volume of the gas condensed at the top of the tower is larger, which requires a larger heat exchange area in the condenser; as a result, the equipment investment increases as well. More advantages and disadvantages are open for discussion. Large-scale distillation operations

Compared with ordinary distillation towers and atmospheric distillation towers for crude oil, vacuum distillation towers have the following characteristics: (1) Depending on the production requirements, vacuum distillation towers are divided into fuel-type and lubricating oil-type. Lubricant-type vacuum distillation towers are primarily used to produce lubricant feedstocks, which are further processed to manufacture various types of lubricants. Fuel-type vacuum distillation towers primarily produce raw materials for further processing, such as those for catalytic cracking or hydrocracking. ⑵ The vacuum distillation column has a small number of trays, low pressure drop, high vacuum level, and large diameter. To maximize the extraction depth while avoiding decomposition, it is required that the vacuum level in the vaporization section of the pressure reduction tower be increased as much as possible under economically reasonable conditions. Therefore, on the one hand, powerful vacuum pumping equipment must be installed at the top of the tower, while simultaneously reducing the pressure drop across the tower trays. In vacuum distillation columns, trays with low pressure drop should be used; commonly used ones include tongue-type trays and perforated trays. The precision requirements for distillation between reduced-pressure fractions are generally lower than those for atmospheric distillation; therefore, usually only 3 to 5 distillation trays are installed between the two side-stream fractions in a reduced-pressure column. Under reduced pressure, the volumes of oil vapor, water vapor, and non-condensable gases in the tower increase, causing the diameter of the vacuum tower to grow. ⑶ Reducing the residence time of residue in the vacuum distillation tower: The vacuum residue at the bottom of the tower is the heaviest material; if it remains there for too long at high temperatures, reactions such as decomposition and condensation occur more significantly, leading to an increase in non-condensable gases, a decrease in the vacuum level of the tower, and coking in the bottom section of the tower, which affects the normal operation of the tower. Therefore, the diameter at the bottom of the vacuum tower is often reduced to shorten the residence time of the residue in the tower. Additionally, since no product is discharged from the top of the vacuum distillation column and the vapor load in the upper part of the column is low, a reduction in diameter is often employed, thereby turning the vacuum distillation column into a distillation column that is thick in the middle and narrow at both ends.

Advantages of vacuum distillation: For certain substances that tend to decompose or polymerize during distillation at high temperatures, making separation impossible, vacuum distillation must be employed； Vacuum distillation must be employed ； Vacuum distillation can lower the bubble point of the mixture, thereby reducing the separation temperature. Therefore, the steam consumption for heating can be reduced, and heating steam at a lower pressure can be used. Heated steam consumption and the use of heated steam at lower pressures. Especially when water vapor is used as the heat source and the heating temperature is increased, the required saturated vapor pressure must increase even further; this imposes new demands on both the equipment and the steam source. The greater the relative volatility between the separated mixtures, the higher the separation efficiency. The greater the relative volatility between the separated mixtures, the easier it is to separate them. At reduced pressure, generally speaking, the relative volatility between components increases, making separation easier. At reduced pressure, generally speaking, the relative volatility between components increases, which facilitates separation. Easy to separate. For the separation of toxic substances, vacuum distillation can prevent the leakage of highly toxic materials, reduce environmental pollution, and is thus of some significance in protecting human health. Environmental pollution has certain significance in protecting human health. Disadvantages: The disadvantage of vacuum distillation is that vacuum operation requires high standards for equipment sealing, which presents certain technical challenges. Especially for flammable and explosive substances, there is a risk of explosion when air gets into the equipment ； The production capacity of vacuum distillation is lower than that of atmospheric and pressurized distillation equipment.

Main advantages: reduces the decomposition of heat-sensitive substances and lowers the operating temperature. Main disadvantages: Vacuum distillation is carried out under a vacuum system, which requires high-quality equipment; in particular, the joints must be well-sealed to minimize leaks. The piping design for vacuum systems is also complex, especially in the case of high-vacuum systems, which demands high skills from designers – skills that are determined by their practical experience in designing vacuum systems. Selecting vacuum pumps is more complicated than selecting ordinary pumps, and there are few technicians specialized in vacuum systems.