Daily question in the tower equipment technology exchange area – participate in the prize-winning activity～(02.13)

The main drawback of wet vacuum distillation is the need to inject a large amount of water vapor into the tower system, which not only increases steam consumption but also raises the load on the primary condenser at the top of the tower. This leads to high consumption of cooling water, as well as the generation of industrial wastewater containing oil and sulfur. In traditional wet distillation, large amounts of steam are injected at the bottom of the tower to reduce the vapor pressure of the oil and gas in the flashing section; this not only wastes energy but also increases the load on the pumping system. It also requires a large amount of cooling water. In contrast, dry vacuum distillation uses packing or trays with low pressure drops, along with efficient vacuum pumps, to achieve a high vacuum in the flash section in order to meet the required vapor-liquid partial pressures. This overcomes the disadvantages of wet vacuum distillation; as a result, the steam consumption is **lower than that in wet vacuum distillation, and the cooling load on the overhead vacuum system is also reduced. Furthermore, under the same pull-out rate, the furnace outlet temperature for dry pressure reduction is lower than that for wet pressure reduction. For the above reasons, the energy consumption of dry vacuum distillation is lower than that of wet vacuum distillation. Currently, dry vacuum distillation is mostly used in fuel-type systems due to its slightly poorer fractionation efficiency.

Wet vacuum operation involves injecting superheated steam at the bottom of the tower; vacuum distillation also employs steam injection at the convective radiant tubes or the outlet tubes of the vacuum furnace. Dry vacuum distillation is defined as a process in which no steam is injected into any part of the vacuum system.

In wet vacuum operation, superheated steam is introduced at the bottom of the tower, whereas in dry vacuum distillation, no superheated steam is introduced at the bottom of the tower. Since wet vacuum distillation introduces a certain amount of superheated steam at the bottom of the tower, thereby reducing the vapor pressure of the oil and gas in the flashing section, its energy consumption is higher than that of dry vacuum distillation; however, its fractionation efficiency is better than that of dry vacuum distillation. Therefore, when vacuum distillation is carried out in a dry operation mode, a washing tank equipped with a vacuum booster at its top is connected in parallel to the vacuum distillation tower in order to increase the recovery rate. The feed section and the stripping section of the vacuum distillation tower are separated by a liquid seal distributor; the gas-liquid mixture from the stripping section enters the lower part of the washing tank through connecting pipes. The fraction oil taken from the third outlet line of the vacuum tower is cooled before entering the upper part of the washing tank, where it is sprayed downward to undergo counter-current heat and mass transfer with the upward-moving gas-liquid mixture. The gas-liquid mixture at the top of the washing tank is returned to the upper part of the vacuum tower via the vacuum booster, while the output from the bottom of the tank serves as washing oil and is returned to the vacuum tower. The additional washing tank enables the residue in the stripping section of the vacuum tower to undergo a thorough stripping process, which generally increases the recovery rate of vacuum residue by 3% to 6% compared to the original level.

In a dry vacuum distillation column, packing is used in place of trays, which reduces the overall pressure drop across the column. The vacuum system typically employs a three-stage steam ejector with a booster, enabling the residual pressure in the flash zone to be reduced to below 4 kPa (30 mmHg), which is lower than the hydrocarbon partial pressure in wet distillation; as a result, there is no need to introduce water into the column. Compared to traditional wet vacuum distillation, its main advantages include lower pressure drop due to the use of packing, higher vacuum levels within the column, a lower temperature at the outlet of the heating furnace that reduces the amount of non-condensable gas, thereby reducing the cooling load on the overhead condenser, decreasing the amount of cooling water required, and lowering energy consumption.

It is called wet-type because steam is added to the bottom of the tower； In the dry type, no steam is added to the tower bottom. In the dry type, due to the lack of water vapor addition, the operating pressure is relatively high and the pumping capacity of the vacuum system is low; however, its performance is not necessarily better than that of the wet type.

In wet vacuum operation, superheated steam is introduced at the bottom of the tower, whereas in dry vacuum distillation, no superheated steam is introduced at the bottom of the tower. Since wet vacuum distillation introduces a certain amount of superheated steam at the bottom of the tower, thereby reducing the vapor pressure of the oil and gas in the flashing section, its energy consumption is higher than that of dry vacuum distillation; however, its fractionation efficiency is better than that of dry vacuum distillation. Therefore, when vacuum distillation is carried out in a dry operation mode, a washing tank equipped with a vacuum booster at its top is connected in parallel to the vacuum distillation tower in order to increase the recovery rate. The feed section and the stripping section of the vacuum distillation tower are separated by a liquid seal distributor; the gas-liquid mixture from the stripping section enters the lower part of the washing tank through connecting pipes. The fraction oil taken from the third outlet line of the vacuum tower is cooled before entering the upper part of the washing tank, where it is sprayed downward to undergo counter-current heat and mass transfer with the upward-moving gas-liquid mixture. The gas-liquid mixture at the top of the washing tank is returned to the upper part of the vacuum tower via the vacuum booster, while the output from the bottom of the tank serves as washing oil and is returned to the vacuum tower. The additional washing tank enables the residue in the stripping section of the vacuum tower to undergo a thorough stripping process, which generally increases the recovery rate of vacuum residue by 3% to 6% compared to the original level. In a dry vacuum distillation column, packing is used in place of trays, which reduces the overall pressure drop across the column. The vacuum system typically employs a three-stage steam ejector equipped with a booster, enabling the residual pressure in the flash zone to be reduced to below 4 kPa (30 mmHg), which is lower than the hydrocarbon partial pressure in wet distillation. As a result, there is no need to introduce water into the column. Compared to traditional wet vacuum distillation, its main advantages include lower pressure drop due to the use of packing, higher vacuum levels within the column, a lower temperature at the outlet of the heating furnace that reduces the amount of non-condensable gas, thereby reducing the cooling load on the overhead condenser, decreasing the amount of cooling water required, and lowering energy consumption.