【Weekly Topic】Production Technology: What are the key points of vacuum distillation? 2011.06.06~06.12

A common vacuum distillation system can be divided into three parts: the distillation unit, the pumping device, and the protection and pressure measurement devices. 1. Distillation section: This section is similar to ordinary distillation and can also be divided into three components. (1) The reduced-pressure distillation flask (Krebs flask) has two necks; the purpose of these necks is to prevent the liquid inside the flask from rushing into the condenser due to boiling during reduced-pressure distillation. A thermometer is inserted into one of the necks, while into the other neck is inserted a glass tube containing a capillary with its tip drawn into a fine thread, located at a distance of about 1–2 mm from the bottom of the flask. The upper end of the capillary is connected to a section of rubber tube equipped with a screw clamp; this clamp is used to regulate the amount of air that enters, allowing only a tiny amount of air to reach the liquid in the form of small bubbles. These bubbles serve as centers for vaporization as the liquid boils, thereby ensuring smooth distillation and also acting as a stirrer. (2) The condenser is the same as that in ordinary distillation. (3) Unlike ordinary distillation, the liquid receiving tube (tail pipe) is equipped with a small branch pipe that allows for connection to the pumping section. During distillation, if different fractions are to be collected without interrupting the process, two or more tail collectors can be used. By rotating the multi-tail liquid receiving tube, different fractions can be directed into the designated receivers. 2. Vacuum extraction section: Laboratories typically use water pumps or oil pumps to reduce pressure. Water pump (water circulation pump): The lowest pressure it can achieve is 0.1 Pa. Oil pump: The efficiency of an oil pump depends on its mechanical structure as well as the quality of the oil used in the vacuum pump. A good oil pump can achieve a vacuum level of 13.3 Pa. The oil pump has a complex structure and requires strict operating conditions. During distillation, the vapors of volatile organic solvents, water, or acids can damage the oil pump and reduce its vacuum level. Therefore, great care must be taken to protect the oil pump when in use. 3. Protection and pressure measurement device section: To protect the oil pump, a cold trap and several absorption towers must be installed in sequence between the distillate receiver and the oil pump. The choice of coolant in the cold trap depends on the requirements. Three desiccation towers are usually installed: the first one contains anhydrous CaCl2 or silica gel to absorb water vapor ； The second one contains granular NaOH and absorbs acidic gases ； The third one contains sliced paraffin to absorb hydrocarbon gases. Laboratories typically use mercury pressure gauges to measure the pressure in pressure reduction systems. Mercury pressure gauges are further divided into open-type mercury pressure gauges and closed-type mercury pressure gauges. 4. Key operating points: (1) When the liquid to be distilled contains substances with low boiling points, ordinary distillation is usually carried out first, followed by vacuum distillation using a water pump; vacuum distillation using an oil pump should be performed after the vacuum distillation with a water pump. (2) Once the apparatus is ready, first tighten the screw clamp on the rubber tube, then open the two-way piston on the safety bottle to allow the system to be in communication with the atmosphere. Start the oil pump (for vacuum pumps that have not been used for a long time, turn the belt pulley by hand before starting the pump; only start it once it rotates smoothly). Pump out air, gradually closing the two-way piston until it is fully closed. Pay attention to the bubbling activity inside the bottle – if the bubbling is too intense and there is a risk of material being ejected, immediately loosen the two-way piston slightly. Check the pressure inside the system using the pressure gauge to ensure it meets the required values. Then, carefully open the two-way piston while continuing to monitor the readings on the pressure gauge, adjusting the pressure inside the system to the desired value (based on the relationship between boiling point and pressure). (3) After the system has been thoroughly evacuated, condensate water is introduced, followed by heating (usually using an oil bath) for distillation. Once vacuum distillation begins, close attention must be paid to the distillation process; the pressure within the system should be adjusted regularly, and the pressure and corresponding boiling points should be recorded frequently. Different fractions are collected as required. (4) Once distillation is complete, remove the heat source, slowly unscrew the screw clamp (to prevent backflow), and gradually open the two-way piston to equalize the pressure inside and outside, allowing the mercury column in the manometer to return to its original level slowly. (If it is opened too quickly, the mercury column will rise rapidly, posing a risk of breaking the manometer.) Then turn off the oil pump and the cooling water.

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 types. 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. In order to maximize the pull-out 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. Trays with low pressure drop should be used in the vacuum distillation column; commonly used types include tongue trays and perforated trays. The precision requirements for fractional 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, thereby affecting 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 residue in the tower. Additionally, since no product is obtained from the top of the vacuum distillation column and the vapor load in the upper part of this column is low, a tapering design is often used to make the column a distillation column that is thicker in the middle and thinner at both ends.

1. Vacuum level 2. Bottom of tower temperature 3. Mid-stage reflux 4. Blowing at the bottom of the tower and gas injection into the furnace tubes 5. Liquid level at the bottom of the tower 6. Sealing oil system

This post was last edited by Refinery Operator on 2011-6-6 21:39. 1. Vacuum level 2. Bottom of tower temperature 3. Mid-stage reflux 4. Gas injection at the bottom of the tower and in the furnace tubes 5. Liquid level at the bottom of the tower 6. Oil sealing system. Points to note during equipment operation: 1. Control the temperature and pressure of the heating medium; these values should not be too high to prevent the material inside the tank from overheating and causing accidents. 2. Maintain a relatively stable heating temperature to enable the product to be distilled within its boiling range, thereby improving product quality. 3. Continuously monitor the temperatures at the bottom and top of the tower, keep records, and address any issues that arise promptly. 4. The fractionated products should be analyzed using gas chromatography. 5. The distilled finished product is sent to a storage tank for storage or packaged directly

1. When the liquid to be distilled contains substances with low boiling points, ordinary distillation is usually carried out first, followed by vacuum distillation using a water pump; vacuum distillation using an oil pump should be performed after the vacuum distillation with a water pump. 2. Once the device is ready, first tighten the screw clamp on the rubber tube, then open the two-way piston on the safety bottle to allow the system to be in contact with the atmosphere. Start the oil pump (for vacuum pumps that have not been used for a long time, turn the belt pulley by hand before starting the pump; only start it once it can rotate). Pump out air, gradually closing the two-way piston until it is fully closed. Pay attention to the bubbling activity inside the bottle – if the bubbling is too intense and there is a risk of material being forced out, immediately loosen the two-way piston slightly. Check the pressure inside the system using the pressure gauge to ensure it meets the required levels. Then, carefully open the two-way piston while continuing to monitor the readings on the pressure gauge, adjusting the pressure inside the system to the desired value (based on the relationship between boiling point and pressure). 3. After the system has been thoroughly evacuated, condensate water is introduced, followed by heating (usually using an oil bath) for distillation. Once vacuum distillation begins, close attention must be paid to the distillation process; the pressure within the system should be adjusted regularly, and the pressure and corresponding boiling points should be recorded frequently. Different fractions are collected as required. 4. Once distillation is complete, remove the heat source. Slowly unscrew the screw clamp (to prevent backflow) and gradually open the two-way piston to balance the pressure inside and outside, allowing the mercury column in the manometer to return to its normal level slowly. (If it is opened too quickly, the mercury column will rise rapidly, posing a risk of breaking the manometer.) Then turn off the oil pump and the cooling water.

This post was last edited by Refinery Operator on 2011-6-6 20:51. (1) Depending on the production requirements, vacuum distillation towers are divided into fuel-type and lubricating oil-type versions. 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. In order to maximize the pull-out 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. Trays with low pressure drop should be used in the vacuum distillation column; commonly used types include tongue trays and perforated trays. The precision requirements for fractional 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, thereby affecting 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 residue in the tower. Additionally, since no product is obtained from the top of the vacuum distillation column and the vapor load in the upper part of this column is low, a tapering design is often used to make the column a distillation column that is thicker in the middle and thinner at both ends.

1. First, it is necessary to consider the physicochemical properties of the mixture to determine whether vacuum distillation is indeed the appropriate choice, taking into account factors such as the necessity of the process, the characteristics of the materials, energy savings, environmental protection, processing time, and distillation efficiency, as well as data from pilot and scale-up tests； 2. The selection of vacuum level and temperature for vacuum distillation must take into account both the efficiency of material separation and the prevention of coking of the materials. It is also necessary to consider the overall energy consumption associated with the heating and cooling media; these parameters are usually determined through pilot-scale tests ； 3. Choose between continuous vacuum distillation and batch vacuum distillation based on the process requirements; for example, petrochemical processes typically use continuous vacuum distillation columns, while most fine chemical products are processed using batch vacuum distillation reactors ； 4. Vacuum distillation: The diameters of the vapor phase in the reboiler and at the top of the tower must be calculated accurately; otherwise the tower diameter will be too large, which can result in too high a gas velocity and affect product quality ； 5. In the case of pressure-reducing distillation using a packed tower, the type of packing and the design of the redistributor differ from those used under normal pressure conditions ； 6. Depending on the type of material to be separated, different types of vacuum pumps should be selected, such as water ring pumps or screw pumps ； 7. Since the temperature in vacuum distillation is usually high, the material tends to coking; therefore, thermal siphon and forced circulation types are chosen for the reboiler at the bottom of the tower, as these options have a significant impact on heating efficiency and production capacity ； 8. The vacuum level in vacuum distillation is subject to very strict control, and it is usually monitored using a mercury pressure gauge.

1. Vacuum level. Vacuum level is closely related to vaporization temperature; as the material vaporizes, the vacuum level may change, which in turn can cause changes in the boiling temperature. This aspect needs to be kept under constant attention. 2. Heating rate. The heating rate is the key factor; it determines the speed of distillation, which in turn affects the separation efficiency. In vacuum distillation, the most important thing is to maintain stability in the vacuum level of the tower, to prevent leaks in the vacuum system. It is also necessary to control the load properly, so as to avoid excessive load from causing fluctuations in the vacuum level. In operation, the reflux ratio should be adjusted promptly in response to changes in the vacuum level, in order to ensure stable quality of the product at the top of the tower. This is a challenging task, as changes in vacuum level affect the boiling point of substances, which in turn leads to changes in the temperature at the top of the tower and ultimately affects product quality. Therefore, maintaining vacuum stability is extremely important; the other requirements are similar to those for ordinary distillation towers. When designing vacuum distillation, it is important to ensure that the vacuum level is appropriate; the chosen vacuum degree must be suitable. If it is too high, many of the equipment components become vacuum-related components, increasing costs, while if it is too low, the purpose of vacuum distillation cannot be achieved. Additionally, the choice of vacuum method is very important – it depends on whether to use a water ring pump or steam injection. In general, in addition to considering factors related to normal-pressure design, more attention must be paid to vacuum design, with all relevant factors taken into account to avoid any mistakes.

Reply 1# sun-rock 1. The pressure needs to be stable； Any abnormalities in the vacuum pump should be addressed promptly. 2. The temperature of each tower section is related to the vacuum level. 3. The control of heat input is related not only to temperature but also to vacuum level

A common vacuum distillation system can be divided into three parts: the distillation unit, the pumping device, and the protection and pressure measurement devices. 1. Distillation section: This section is similar to ordinary distillation and can also be divided into three components. (1) The reduced-pressure distillation flask (Krebs flask) has two necks; the purpose of these necks is to prevent the liquid inside the flask from rushing into the condenser due to boiling during reduced-pressure distillation. A thermometer is inserted into one of the necks, while into the other neck is inserted a glass tube containing a capillary with its tip drawn into a fine thread, located at a distance of about 1–2 mm from the bottom of the flask. The upper end of the capillary is connected to a section of rubber tube equipped with a screw clamp; this clamp is used to regulate the amount of air that enters, allowing only a tiny amount of air to reach the liquid in the form of small bubbles. These bubbles serve as centers for vaporization as the liquid boils, thereby ensuring smooth distillation and also acting as a stirrer. (2) The condenser is the same as that in ordinary distillation. (3) Unlike ordinary distillation, the liquid receiving tube (tail pipe) is equipped with a small branch pipe that allows for connection to the pumping section. During distillation, if different fractions are to be collected without interrupting the process, two or more tail collectors can be used. By rotating the multi-tail liquid receiving tube, different fractions can be directed into the designated receivers. 2. Vacuum extraction section: Laboratories typically use water pumps or oil pumps to reduce pressure. Water pump (water circulation pump): The lowest pressure it can achieve is 0.1 Pa. Oil pump: The efficiency of an oil pump depends on its mechanical structure as well as the quality of the oil used in the vacuum pump. A good oil pump can achieve a vacuum level of 13.3 Pa. The oil pump has a complex structure and requires strict operating conditions. During distillation, the vapors of volatile organic solvents, water, or acids can damage the oil pump and reduce its vacuum level. Therefore, great care must be taken to protect the oil pump when in use. 3. Protection and pressure measurement device section: To protect the oil pump, a cold trap and several absorption towers must be installed in sequence between the distillate receiver and the oil pump. The choice of coolant in the cold trap depends on the requirements. Three desiccation towers are usually installed: the first one contains anhydrous CaCl2 or silica gel to absorb water vapor ； The second one contains granular NaOH and absorbs acidic gases ； The third one contains sliced paraffin to absorb hydrocarbon gases. Laboratories typically use mercury pressure gauges to measure the pressure in pressure reduction systems. Mercury pressure gauges are further divided into open-type mercury pressure gauges and closed-type mercury pressure gauges. 4. Key operating points: (1) When the liquid to be distilled contains substances with low boiling points, ordinary distillation is usually carried out first, followed by vacuum distillation using a water pump; vacuum distillation using an oil pump should be performed after the vacuum distillation with a water pump. (2) Once the apparatus is ready, first tighten the screw clamp on the rubber tube, then open the two-way piston on the safety bottle to allow the system to be in communication with the atmosphere. Start the oil pump (for vacuum pumps that have not been used for a long time, turn the belt pulley by hand before starting the pump; only start it once it rotates smoothly). Pump out air, gradually closing the two-way piston until it is fully closed. Pay attention to the bubbling activity inside the bottle – if the bubbling is too intense and there is a risk of material being ejected, immediately loosen the two-way piston slightly. Check the pressure inside the system using the pressure gauge to ensure it meets the required values. Then, carefully open the two-way piston while continuing to monitor the readings on the pressure gauge, adjusting the pressure inside the system to the desired value (based on the relationship between boiling point and pressure). (3) After the system has been thoroughly evacuated, condensate water is introduced, followed by heating (usually using an oil bath) for distillation. Once vacuum distillation begins, close attention must be paid to the distillation process; the pressure within the system should be adjusted regularly, and the pressure and corresponding boiling points should be recorded frequently. Different fractions are collected as required. (4) Once distillation is complete, remove the heat source, slowly unscrew the screw clamp (to prevent backflow), and gradually open the two-way piston to equalize the pressure inside and outside, allowing the mercury column in the manometer to return to its original level slowly. (If it is opened too quickly, the mercury column will rise rapidly, posing a risk of breaking the manometer.) Then turn off the oil pump and the cooling water.