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Applications of jet vacuum pumps in the chemical industry Keywords: water jet vacuum pump, new type high-efficiency steam jet vacuum pump, steam-water series jet vacuum pump, steam-water combined jet vacuum pump, chemical processes, applications. Abstract: Nowadays, vacuum technology is increasingly used in industries such as petroleum, chemicals, and pharmaceuticals for processes such as reduced-pressure evaporation, crystallization, distillation, sublimation, drying, negative-pressure concentration, dehydration, chemical reaction absorption, and vacuum material transfer. There is a growing consensus that the use of vacuum technology in various processing steps can help save energy and reduce consumption, accelerate reaction rates, improve product quality, and enhance economic benefits. However, vacuum technology is an interdisciplinary field. China’s vacuum industry developed after the liberation by drawing on products from the former Soviet Union, the United States, Japan, and Germany. Although the number of vacuum equipment manufacturers in China has increased significantly in recent years, many of these manufacturers are not clear about how to select the appropriate vacuum pumps for different processes (let alone design new products based on the process parameters provided by customers). This poses significant difficulties for customers in making their selections. Based on our company’s over 30 years of experience in production, development, and related services, this article discusses how to properly select jet vacuum pumps for several typical processes in the chemical industry. I. Several Concepts Related to Vacuum Pumps To make good use of vacuum pumps in the processing stages of chemical production, it is necessary to first understand the following concepts: 1. The concept of \"vacuum\": \"Vacuum\" refers to a gas state in which the pressure is lower than one atmosphere within a given space; in other words, the density of gas molecules in that space is lower than that of the gas molecules at the atmospheric pressure in that area. A space state with absolutely no gas is called an absolute vacuum. 2. Vacuum pump: A device used to create, improve, and maintain a vacuum. 3. Pumping volume: The flow rate of gas pumped by the vacuum pump per unit time at the operating pressure. Unit: kg/h or m3/h ； 4. Limiting pressure (vacuum level): The absolute pressure that a vacuum pump can achieve when the steam extraction volume is zero. Units: Pa, KPa, MPa, or mmHg (the ultimate vacuum is the ultimate absolute pressure minus the local atmospheric pressure) ； 5. Operating pressure (vacuum level): The absolute pressure of the ejector pump at its rated pumping capacity. Units: Pa, KPa, MPa, or mmHg (the working vacuum is the working absolute pressure minus the local atmospheric pressure) ； 6. Working steam pressure: The working steam pressure of the steam jet vacuum pump. Unit: MPa or kgf/cm2 ； 7. Steam consumption: The amount of steam consumed per unit time by a steam-jet vacuum pump. Unit: kg/h ； 8. Cooling water temperature: the temperature of the circulating water in the water jet pump or the temperature of the circulating water in the condenser of the steam jet vacuum pump. Unit: OC ； 9. Cooling water circulation rate: The flow rate of cooling water that passes through the condenser via a water ejector or steam ejector per unit time. Unit: kg/h ； II. Several common vacuum pumps in the chemical industry and their advantages and disadvantages 1. Several vacuum pumps frequently used in the chemical industry – The vacuum pumps commonly seen in this industry can be simply divided into positive displacement vacuum pumps and jet vacuum pumps. Positive displacement vacuum pumps are devices that utilize periodic changes in the volume of the pump chamber to achieve suction and exhaust; reciprocating vacuum pumps, rotary vane vacuum pumps, slide valve vacuum pumps, water ring vacuum pumps, and Roots vacuum pumps all fall under the category of positive displacement vacuum pumps. A jet vacuum pump is a momentum transfer pump that uses the high-speed jet generated by the pressure drop due to the Venturi effect to convey gas to the outlet; water jet vacuum pumps, steam jet vacuum pumps, steam-water series jet vacuum pumps, and steam-water combined jet vacuum pumps all fall under the category of jet vacuum pumps. 2. Working principle, advantages, and disadvantages of variable-volume vacuum pumps: (1) Reciprocating vacuum pumps, rotary vane vacuum pumps, slide valve vacuum pumps, and Roots vacuum pumps draw in gas, compress it, and discharge it by means of the reciprocating motion or rotation of pistons. Their advantage is that the vacuum level of the pumps when they are first put into use is relatively high. However, since the piston is a moving component, its wear is inevitable; as the number of maintenance sessions increases, the working vacuum level will continue to decline until it can no longer meet the requirements of production. Such pumps have high operating noise, high fuel consumption, and a high failure rate, which are all serious drawbacks. Generally, one backup pump is required, which inadvertently increases the equipment investment and operating costs. Furthermore, if such pumps are used to draw in condensable gases such as water vapor, it will cause the lubricating oil to emulsify; therefore, they can only be used for drawing in non-condensable gases (such as air), and they cannot be used to draw in media containing particles, which limits their range of application. (2) The water ring vacuum pump relies on the rotation of an eccentric rotor with multiple blades mounted inside the pump casing; this rotation throws water against the casing, forming a water ring that is concentric with it. The interaction between this water ring and the rotor blades creates periodic changes in volume, which enables the suction, compression, and expulsion of gas. Its advantage is a large pumping capacity at low vacuum levels, and it can directly pump condensable gases such as water vapor. Its drawback is the low vacuum level ； It is not possible to suction media containing particles ； It is difficult to apply anti-corrosion treatment to rotors that rotate at high speeds; therefore, they cannot be used to pump corrosive media. 3. Working principle, advantages, and disadvantages of jet vacuum pumps: Jet vacuum pumps are momentum transfer pumps that use the high-speed jet generated by the pressure drop resulting from the Venturi effect to transport gas to the outlet. It is divided into water-jet vacuum pumps, steam-jet vacuum pumps, steam-water series-jet vacuum pumps, and steam-water combined-jet vacuum pumps. Jet vacuum pumps are increasingly used in various chemical processing operations due to their wide range of vacuum levels, ability to directly pump condensable gases such as water vapor as well as media containing particles, simple structure, ease of operation, few maintenance requirements for moving parts, and energy efficiency. Below is an explanation of the working principles of different types of jet vacuum pumps. (1) Working principle and structural diagram of the water jet vacuum pump. The components of a water jet vacuum pump are shown in its structural diagram. Its working principle is as follows: The water in the circulation tank gains certain pressure and flow rate due to the action of the circulation pump. Water with this pressure and flow rate enters the water collection chamber of the water jetter, where it is ejected through multiple Venturi nozzles located on the orifice plate. The high-speed jets generated thereby create a vacuum in the mixing chamber of the ejector. The medium to be pumped enters the mixing chamber under the effect of this vacuum, mixes thoroughly with the high-speed water flow, and then is discharged back into the circulation tank after its speed is reduced and pressure is increased via a Venturi tube. Non-condensable gases separate out, while condensable gases overflow from the tank’s outlet. This process repeats continuously. Its advantage lies in its low-level integral design; it achieves a higher vacuum level than water ring vacuum pumps. By replacing W-type reciprocating vacuum pumps, it eliminates the need for a pre-condenser, thereby saving on initial equipment investment and operating costs. (2) Working principle and structural diagram of the steam-water series jet vacuum pump: One or more stages of steam jet vacuum pumps are connected in series ahead of the ejector of a water jet vacuum pump, resulting in a steam-water series jet vacuum pump; its structure is shown in the diagram of the steam-water series jet vacuum pump. Its working principle is as follows: Saturated or superheated steam at a certain pressure passes through a Laval nozzle, where it loses pressure and gains speed, before entering the mixing chamber of the steam ejector. This creates a vacuum in the mixing chamber, which draws in the medium to be pumped into the chamber to mix with the working steam. The resulting mixture flows through a diffuser, where its speed decreases and its pressure increases, until it reaches the pressure at the inlet of the next stage, after which it is sent to the next stage of steam ejector or water ejector pump. It is of the low-stage integral type with a relatively high vacuum level; it can replace water ring-Rotary vane units and vane-Rotary vane units for directly pumping condensable gases. However, its capacity for pumping non-condensable gases at high vacuum levels is limited. (3) Working principle and process flow diagram of the new type of high-efficiency steam jet vacuum pump. The new type of high-efficiency steam jet vacuum pump consists of one or multiple stages of steam jet pumps together with high-efficiency condensers. Saturated or superheated steam at a certain pressure enters the mixing chamber of the steam ejector after being depressurized and accelerated through a Laval nozzle, thereby creating a vacuum in the mixing chamber. The medium to be evacuated is drawn into this mixing chamber and mixed with the working steam. The resulting fluid then passes through a diffuser, where its velocity decreases and its pressure increases, until it reaches the pressure at the inlet of the next stage, after which it is sent to the next steam ejector. The mixed fluid discharged from the steam ejector in front of the condenser enters the high-efficiency condenser, where the condensable gases are cooled into a liquid state; these liquids, along with the non-condensable gases, are then removed from the condenser via the cooling water (see the process flow diagram of the new high-efficiency steam ejector vacuum pump). It can be installed at any desired height, and is suitable for processes in which the amount of non-condensable gases in the medium to be evacuated is relatively small, with most of it being condensable vapors. By replacing traditional multi-stage steam jet vacuum pumps, it can save more than 60% of the working steam required. (4) Working principle and process flow diagram of the steam-water combined injection vacuum pump: It is composed of a traditional steam injection pump with an additional steam-water combined injection vacuum pump connected in series after its first-stage condenser; its working principle can be seen in the process flow diagram of the steam-water combined injection vacuum pump. It overcomes the respective disadvantages of steam jet vacuum pumps and water jet vacuum pumps while making use of their advantages; it can achieve a high operating vacuum level as well as a large pumping capacity. It saves more than 50% of the working steam and over 30% of the cooling water compared to traditional multi-stage steam jet vacuum pumps. No starting pump is required, and the time needed to evacuate the tower is short. (5) The advantages of multi-stage steam jet vacuum pumps are their simple structure, ease of use, and stable and reliable operation. The disadvantages include high steam consumption (resulting in high operating costs), the need to be installed at a height of over 11 meters, and difficulties in maintenance. III. Selection of a suitable vacuum pump based on the process requirements: In chemical production, a vacuum pump can be chosen appropriately according to the required operating pressure for the specific process as well as the properties of the gas to be evacuated. Generally, the required operating pressure is determined first; processes such as vacuum distillation, rectification, and sublimation require high vacuum levels (low operating pressure), while processes like vacuum evaporation, drying, concentration, crystallization, dehydration, decolorization, chemical reaction absorption, and vacuum transfer of materials require lower vacuum levels (higher operating pressure). Once the operating pressure is determined, a vacuum pump can be selected appropriately based on the volume of gas to be evacuated and the properties of the gas being evacuated. 1. List the applicable ranges of various vacuum pumps based on the operating pressures they can achieve. 2. Different jet vacuum pumps should be selected for pumping different gases. (1) Water-jet vacuum pumps are suitable for applications where the operating pressure is greater than 25 mmHg (3.3 KPa), and for pumping various types of gases. Their advantages are particularly evident when pumping condensable gases (i.e., gases that condense into water or upon contact with water), as well as gases containing dust or particulate matter. When a relatively low operating pressure is required and a large amount of non-condensable gas needs to be evacuated, steam jet pumps and steam-water series or combined steam-water jet vacuum pumps should be selected. (2) The operating pressure of a soda-water series jet vacuum pump can reach 2 mmHg (267 Pa), but its pumping capacity is lower than that of new high-efficiency steam jet vacuum pumps, soda-water combined jet vacuum pumps, and multi-stage steam jet vacuum pumps; it is suitable for intermittent production in small and medium-sized enterprises. (3) The operating pressure of the new type of high-efficiency steam jet vacuum pump can reach 0.5 mmHg (65 Pa); it is suitable for processes in which the gas to be evacuated consists mainly of condensable gases, with a relatively small proportion of non-condensable gases. (4) The operating pressure of the mixed steam-water jet vacuum pump and the multi-stage steam jet vacuum pump can reach below 10 Pa, allowing them to draw in various gases. The difference between them is that the mixed steam-water jet vacuum pump consumes less working steam compared to the multi-stage steam jet vacuum pump, thereby reducing operating costs. IV. Conclusion: With the continuous development of the commodity economy and the growing prominence of energy issues, it has become increasingly important for enterprises to reduce energy consumption, improve production efficiency, and enhance product quality as means to strengthen their core competitiveness. Choosing the appropriate jet vacuum pump for various processing processes in order to reduce operating costs is a effective way for enterprises to boost their core competitiveness. It is hoped that this article can serve as a reference for enterprises in making informed decisions regarding the selection of vacuum pumps.