A safe mechanical vapor recompression system

Abstract

The utility model provides a kind of safe mechanical vapor recompression system, including crystallizer, axial flow pump and heater sequentially communicated by circulation pipeline;And compressor is connected with crystallizer by gas phase pipeline;Gas phase pipeline includes steam exhaust pipeline and temperature and pressure increasing pipeline, remote pressure gauge, bursting disc and vent valve are sequentially provided in the steam exhaust pipeline, safety valve is provided on temperature and pressure increasing pipeline, and remote pressure gauge is interlocked with vent valve and steam regulating valve, compressor anti-surge and overpressure protection device are constituted.When overpressure occurs in system, steam regulating valve gradually closes, if system pressure further rises, then system automatically opens vent valve and discharges system gas, avoid system overpressure operation, prevent equipment damage.When the tube pass of heater breaks, steam directly enters crystallizer, system pressure sharply rises overpressure, bursting disc breaks, system gas is vented, prevent system equipment from explosion and other accidents due to overpressure.

Description

Technical Field

[0001] This utility model relates to the field of waste heat utilization technology, specifically a safe mechanical vapor recompression system. Background Technology

[0002] Mechanical vapor recompression (MVR) technology is an advanced process that has rapidly emerged and gained widespread recognition in the international evaporation and concentration field over the past two decades. Its basic principle is to use a high-efficiency centrifugal, Roots, or axial compressor to compress, heat, and pressurize the secondary vapor generated in the evaporator, then send it back to the shell side of the heater as a heat source to continuously heat the circulating liquid in the tube side, maintaining the liquid in a boiling evaporation state. After the compressed secondary vapor condenses and releases its latent heat, it forms condensate and is discharged, thus achieving a closed-loop cycle of "secondary vapor-mechanical compression-reheating". Since the compression process only consumes electricity to recover the latent heat of secondary steam, MVR systems typically only need to supplement a small amount of fresh steam (0.4 MPa to 1.0 MPa) during the start-up phase or when heat loss is significant. This can reduce the standard coal consumption per unit of evaporation to 10% to 30% of that of traditional multi-effect evaporators (MEE), while also significantly reducing cooling water consumption and CO2 emissions. Therefore, MVR systems are gradually replacing four-effect, five-effect, and even six-effect evaporators in industrial fields such as chemical, pharmaceutical, food, seawater desalination, landfill leachate treatment, and salt recovery.

[0003] A typical MVR forced circulation evaporation system consists of a crystallizer (or evaporator), a heater, a steam compressor, a condensate tank, a condensate pump, an axial flow (or circulating) pump, and online cleaning and automatic control instruments. During system operation, the circulating liquid flows at high speed through the heater tubes driven by the axial flow pump. After being heated by the shell-side steam, it enters the crystallizer for flash evaporation. The resulting secondary steam is defoamed and then enters the compressor. The compressor performs work on the steam through its impeller, increasing its temperature by 6°C to 20°C and its pressure by 20 kPa to 80 kPa. The steam then returns to the heater shell side for condensation and heat release, completing one thermodynamic cycle.

[0004] However, the safe and stable operation of an MVR system is extremely sensitive to the compressor's operating conditions. As the energy heart of the system, the compressor's stable operating range is limited by a combination of factors, including intake flow rate, pressure, temperature, and composition. Once the operating point deviates from the design curve and enters the "surge zone," the compressor will experience periodic backflow, pressure pulsation, and severe vibration. This can quickly lead to impeller fatigue cracks, bearing wear, and seal tearing, and in severe cases, even rotor breakage, casing cracking, and explosions. The heater, as the key pressure boundary coupling the steam and feed sides, is typically designed as a shell-and-tube structure: the shell side withstands a fresh supply steam pressure of 0.4 MPa to 1.0 MPa, while the tube side handles the circulating feed liquid, with an operating pressure generally only 0.02 MPa to 0.15 MPa. The design pressure difference between the two sides is as high as 0.3 MPa to 0.9 MPa. Under long-term high-speed erosion, chloride ion corrosion, salting-out crystallization wear, or alternating hot and cold stress, the heat exchange tubes are highly susceptible to pitting corrosion, intergranular corrosion, stress corrosion cracking, and even localized perforation and rupture.

[0005] If the heat exchanger tubes rupture, the high-pressure shell-side steam will instantly backflow into the low-pressure tube-side circulating liquid loop, causing the following chain of hazards: The system pressure suddenly increased: the heater tube side was connected to the crystallizer, circulation pipeline and compressor inlet. The high-pressure steam in the shell side rapidly increased the pressure of the entire circulation loop, resulting in a sudden increase in compressor outlet back pressure and a sudden drop in flow rate. The operating point instantly fell to the left of the surge line. Compressor surge damage: Under surge conditions, the compressor generates strong axial and radial vibrations, and periodic backflow impact occurs between the blades and the fixed guide vanes, inducing high-frequency fatigue cracks; at the same time, the bearing oil film becomes unstable, the sealing ring wears, leading to impeller damage, shaft imbalance, or even shaft breakage. Overpressure explosion risk: If the system's pressure relief device (rupture disc, safety valve) has insufficient discharge capacity, low-pressure components such as crystallizers, pipes, and flanges will bulge and tear due to overpressure, causing a high-temperature steam and liquid mixture explosion, resulting in casualties and significant property damage. Product contamination and shutdown: Fresh steam condensate seeps into the feed liquid, diluting the concentration of the mother liquor, damaging the crystal particle size, and causing a decrease in product purity. At the same time, the system needs to be shut down for emergency maintenance. The loss of a single large unit (evaporation capacity of 50t / h) can reach several million yuan each time it is shut down.

[0006] Existing technologies mostly focus on anti-surge control strategies for the compressor body, lacking a systematic and passive safety protection structure for the specific fault chain of "heat exchanger tube rupture, high-pressure steam intrusion, and compressor surge". Utility Model Content

[0007] Based on the above-mentioned problems, this utility model proposes a passive safety device with simple structure, rapid response, no need for external power source, and the ability to automatically isolate the high-pressure steam source and simultaneously depressurize at the moment of heat exchange tube rupture. It has urgent practical significance and broad prospects for promoting the improvement of the inherent safety of MVR system, the reduction of compressor surge damage risk, and the guarantee of continuous production.

[0008] The technical solution adopted by this utility model to solve the above problems is: A safe mechanical vapor recompression system includes a crystallizer, a heater, a condensate tank, a condensate pump, an axial flow pump, and a compressor; the crystallizer, axial flow pump, and heater are sequentially connected via a circulation pipeline; the compressor's inlet and outlet are connected to the top outlet of the crystallizer and the shell-side inlet of the heater, respectively, via vapor phase pipelines; the heater's shell-side outlet is connected to the condensate tank and the condensate pump via a condensate pipeline. The vapor phase pipeline includes a waste steam pipeline and a heating and pressurizing pipeline. The steam outlet at the top of the crystallizer is connected to the compressor inlet via the waste steam pipeline, and the compressor outlet is connected to the heater shell-side inlet via the heating and pressurizing pipeline. A remote pressure gauge, a rupture disc, and a vent valve are sequentially installed along the steam flow direction on the exhaust steam pipeline. An external pipeline, a fresh steam replenishment pipeline, and a safety valve are sequentially connected along the steam flow direction on the heating and pressurizing pipeline. A steam regulating valve is installed on the fresh steam replenishment pipeline. The heating and pressurizing pipeline is connected back to the exhaust steam pipeline at the compressor inlet via an external pipeline, and the external pipeline is equipped with a quick-cut-off switch valve; The remote pressure gauge, vent valve, and steam regulating valve are interlocked. The remote pressure gauge detects the pressure of the exhaust steam pipeline in real time, and its output signal is simultaneously connected to the control terminal of the steam regulating valve and the interlock terminal of the vent valve.

[0009] Furthermore, when the pressure detected by the remote pressure gauge is lower than the set lower limit, the opening of the steam regulating valve increases while the vent valve remains closed; when the detected pressure is higher than the set upper limit, the opening of the steam regulating valve decreases until it is fully closed. If the pressure continues to rise, the vent valve automatically opens to release pressure.

[0010] Furthermore, the bursting pressure of the rupture disc is ≤ 1.1 times the design pressure of the crystallizer.

[0011] Furthermore, the safety valve's opening pressure is 1.0-1.1 times the heater shell-side design pressure.

[0012] Furthermore, the circulation pipeline includes a first circulation pipeline, a second circulation pipeline, and a third circulation pipeline. The liquid phase output end at the bottom of the crystallizer is connected to the inlet of the axial flow pump through the first circulation pipeline. The outlet of the axial flow pump is connected to the liquid input end of the heater tube side through the second circulation pipeline. The liquid output end of the heater tube side is connected to the tangential liquid input end in the middle of the crystallizer through the third circulation pipeline.

[0013] Furthermore, the condensate outlet at the bottom of the heater shell is connected to the top of the condensate tank via a condensate pipeline, and the bottom of the condensate tank is connected to the outside via a condensate pump and an external discharge pipeline.

[0014] Compared with the prior art, the beneficial effects of this utility model are as follows: 1. Anti-surge pre-protection: The remote pressure gauge monitors the compressor inlet (exhaust steam line G1) pressure in real time. This is the most sensitive indicator of the surge boundary. Once the pressure exceeds the set upper limit (indicating downstream pressure buildup), the opening of the steam regulating valve is immediately interlocked and adjusted. When the system pressure rises further, the vent valve is then interlocked and opened to bypass the excess steam to the atmosphere, thereby increasing the compressor inlet flow instantaneously, avoiding system overpressure operation and preventing equipment damage.

[0015] 2. Dual redundancy overpressure relief: A rupture disc installed on the exhaust steam line G1 at the compressor inlet and a safety valve installed on the heating and pressurizing line G2 at the compressor outlet form a double safety barrier. When the heater tube ruptures, high-pressure fresh steam directly enters the crystallizer, causing a rapid increase in system pressure and resulting in overpressure, the rupture disc ruptures, venting the system gas and preventing accidents such as explosions due to overpressure.

[0016] 3. Fresh steam is precisely replenished automatically. The setpoint of the steam regulating valve is no longer manually set, but is cascaded with the opening of the pressure-steam regulating valve of the waste steam pipeline G1. This regulates the system pressure on the one hand, and reduces the waste of fresh steam on the other.

[0017] Therefore, the safe mechanical vapor recompression system adopted in this utility model solves the three functions of surge prevention, overpressure protection, and steam saving in one go with the same set of instruments and valves, which extends the compressor life and improves system safety. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the system of this utility model.

[0019] Among them, 1-crystallizer, 2-heater, 3-condensate tank, 4-condensate pump, 5-axial flow pump, 6-compressor, 7-remote pressure gauge, 8-rupture disc, 9-vent valve, 10-steam regulating valve, 11-safety valve, 12-on / off valve. Detailed Implementation

[0020] The present invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic diagrams, illustrating only the basic structure of the present invention, and therefore only show the components relevant to the present invention.

[0021] like Figure 1 As shown, a safe mechanical vapor recompression system includes a crystallizer 1, a heater 2, a condensate tank 3, a condensate pump 4, an axial flow pump 5, and a compressor 6.

[0022] The crystallizer 1 has a feed port at the top and a discharge port at the bottom of the cone. The discharge port is connected to a discharge pipeline to intermittently or continuously transport the liquid-solid mixture containing crystal particles to the downstream centrifuge for further processing.

[0023] The liquid output terminal at the bottom of crystallizer 1 is connected to the inlet of axial flow pump 5 via the first circulation pipeline L1. The outlet of axial flow pump 5 is connected to the liquid input terminal of heater 2 via the second circulation pipeline L2. The liquid output terminal of heater 2 is connected to the tangential liquid input terminal in the middle of crystallizer 1 via the third circulation pipeline L3. The liquid flows from crystallizer 1 through axial flow pump 5 and heater 2, and then returns to crystallizer 1, forming a forced circulation loop. The entire liquid is in the liquid phase, with a temperature increase of 6-20℃, and the flow rate is typically 30-200 times the feed rate.

[0024] The steam outlet at the top of the crystallizer 1 is connected to the inlet of the compressor 6 via the exhaust steam pipeline G1, which delivers the saturated wet steam from the crystallizer 1 to the compressor 6. The outlet of the compressor 6 is connected to the top of the shell side of the heater 2 via the heating and pressurizing pipeline G2, and the saturated wet steam is delivered to the shell side of the heater 2 via the compressor 6 to exchange heat with the liquid in the tube side of the heater 2.

[0025] The condensate outlet at the bottom of the shell side of heater 2 is connected to the top of condensate tank 3 via a condensate pipeline. After the saturated wet steam exchanges heat with the feed liquid, condensate is formed and collected in condensate tank 3. The bottom of condensate tank 3 is connected to the outside via condensate pump 4 and an external discharge pipeline to discharge the condensate.

[0026] The heating and pressurizing pipeline G2 at the outlet of compressor 6 is connected to the exhaust steam pipeline G1 at the inlet of compressor 6 via an external pipeline. A quick-shutdown switch valve 12 is installed on the external pipeline. A fresh steam replenishment pipeline is connected to the heating and pressurizing pipeline G2 at the outlet of compressor 6. A steam regulating valve 10 is installed on the fresh steam replenishment pipeline, and fresh steam is fed into the heating and pressurizing steam pipeline after passing through the steam regulating valve 10. A safety valve 11 is installed on the heating and pressurizing pipeline G2 at the outlet of compressor 6.

[0027] A remote pressure gauge 7, a rupture disc 8, and a vent valve 9 are sequentially installed on the exhaust steam pipeline G1 at the top of the crystallizer 1. The remote pressure gauge 7, the vent valve 9, and the steam regulating valve 10 form an interlock relationship. The remote pressure gauge 7 detects the pressure of the exhaust steam pipeline G1 in real time, and its output signal is simultaneously connected to the control terminal of the steam regulating valve 10 and the interlock terminal of the vent valve 9.

[0028] The entire mechanical vapor recompression system operates as follows: After entering the crystallizer, the feed liquid undergoes forced circulation via an axial flow pump, and after being heated by the heater, returns to the crystallizer. The heated feed liquid undergoes flash evaporation within the crystallizer to generate exhaust steam. This exhaust steam enters the compressor through the top exhaust steam line for compression, increasing its temperature and pressure. The heated exhaust steam then enters the shell side of the heater through a heating and pressurization line, heating the feed liquid in the heater tube side. The exhaust steam condenses to produce condensate, which enters the condensate tank and is then discharged to the outside via a condensate pump. The solution containing crystal particles at the bottom of the crystallizer enters downstream for further processing through the discharge line. To maintain normal system operation, a small amount of fresh steam is typically supplied from the outside. This fresh steam supply line enters the system through the heating and pressurization line at the compressor outlet.

[0029] The normal pressure range of the exhaust steam after compression by the compressor is 150–200 kPa·a, and the steam pressure is typically 0.4–1.0 MPa·a. To prevent overpressure of the exhaust steam entering the heater, a safety valve is installed on the heating and pressurization line at the compressor outlet. The design pressure of the heater shell side is approximately 0.4 MPa·a, therefore the safety valve's tripping pressure is set to 0.44 MPa·a. When the pressure in the heating and pressurization line at the compressor outlet exceeds 0.44 MPa·a, the safety valve trips, causing a drop in the pressure of the heating and pressurization line and the heater shell side, thus preventing damage to the equipment and pipelines.

[0030] The remote pressure gauge operates within a range of -10 to 20 kPa. When the pressure reading on the remote pressure gauge drops below the lower limit, the steam regulating valve opens wider until fully open, while the vent valve remains closed. Conversely, when the pressure on the remote pressure gauge rises above the upper limit, the steam regulating valve closes wider until fully closed. If the pressure rises further after the steam regulating valve is fully closed, the vent valve opens, causing the system pressure to drop. In extreme cases, such as a tube rupture in the heat exchanger, steam enters the tube side from the shell side and then the crystallizer. When the steam regulating valve is closed and the vent valve is opened, the system pressure rises further. When the pressure reaches the burst pressure of the rupture disc, the disc ruptures, releasing pressure and protecting the crystallization system from overpressure damage and safety accidents. The design pressure of the crystallizer is generally around 0.1 MPa·g, and the burst pressure of the rupture disc can be set to 0.1 MPa·g.

[0031] The compressor signal and the switching valve signal are respectively connected to the microcontroller (not shown in the diagram). When the system starts up, insufficient gas flow can cause compressor surge because the flow rate through the compressor is lower than the surge limit flow rate under that operating condition, i.e., lower than the minimum allowable flow rate at that speed. At this time, a pressure difference appears between the compressor inlet and outlet, and the compressor current becomes unstable. The microcontroller receives the signal and controls the switching valve to open, thereby connecting the compressor inlet and outlet and allowing gas to circulate between the compressor inlet and outlet. This eliminates the pressure difference between the compressor inlet and outlet, preventing compressor surge caused by insufficient gas flow. Once the system is running normally, the valve can be closed to improve compressor operating efficiency.

[0032] The above description, based on the preferred embodiments of this utility model, provides inspiration. Those skilled in the art can make various changes and modifications without departing from the technical concept of this utility model. The technical scope of this utility model is not limited to the contents of the specification but must be determined according to the claims.

Claims

1. A safe mechanical vapor recompression system, comprising a crystallizer (1), a heater (2), a condensate tank (3), a condensate pump (4), an axial flow pump (5), and a compressor (6); wherein the crystallizer (1), the axial flow pump (5), and the heater (2) are sequentially connected via a circulation pipeline; the inlet and outlet of the compressor (6) are respectively connected to the top outlet of the crystallizer (1) and the shell-side inlet of the heater (2) via a vapor phase pipeline; the shell-side outlet of the heater (2) is connected to the condensate tank (3) and the condensate pump (4) via a condensate pipeline, characterized in that, The vapor phase pipeline includes a waste steam pipeline (G1) and a heating and pressurizing pipeline (G2). The steam outlet at the top of the crystallizer (1) is connected to the inlet of the compressor (6) through the waste steam pipeline (G1). The outlet of the compressor (6) is connected to the shell-side inlet of the heater (2) through the heating and pressurizing pipeline (G2). A remote pressure gauge (7), a rupture disc (8), and a vent valve (9) are sequentially installed along the steam flow direction on the exhaust steam pipeline (G1). An external pipeline, a fresh steam replenishment pipeline, and a safety valve (11) are sequentially connected along the steam flow direction on the heating and pressurizing pipeline (G2). A steam regulating valve (10) is installed on the fresh steam replenishment pipeline. The heating and pressurizing pipeline (G2) is connected back to the exhaust steam pipeline (G1) at the inlet of the compressor (6) via an external pipeline, and a quick-cut-off switch valve (12) is provided on the external pipeline; The remote pressure gauge (7), the vent valve (9) and the steam regulating valve (10) are interlocked. The remote pressure gauge (7) detects the pressure of the exhaust steam pipeline (G1) in real time, and its output signal is simultaneously connected to the control terminal of the steam regulating valve (10) and the interlock terminal of the vent valve (9).

2. The system according to claim 1, characterized in that, When the pressure detected by the remote pressure gauge (7) is lower than the set lower limit, the opening of the steam regulating valve (10) increases and the vent valve (9) remains closed; when the detected pressure is higher than the set upper limit, the opening of the steam regulating valve (10) decreases until it is fully closed. If the pressure continues to rise, the vent valve (9) will automatically open to release pressure.

3. The system according to claim 1, characterized in that, The bursting pressure of the rupture disc (8) is ≤ 1.1 times the design pressure of the crystallizer (1).

4. The system according to claim 1, characterized in that, The safety valve (11) has an opening pressure that is 1.0-1.1 times the shell-side design pressure of the heater (2).

5. The system according to claim 1, characterized in that, The circulation pipeline includes a first circulation pipeline (L1), a second circulation pipeline (L2), and a third circulation pipeline (L3). The liquid output end at the bottom of the crystallizer (1) is connected to the inlet of the axial flow pump (5) through the first circulation pipeline (L1). The outlet of the axial flow pump (5) is connected to the liquid input end of the heater (2) through the second circulation pipeline (L2). The liquid output end of the heater (2) is connected to the tangential liquid input end in the middle of the crystallizer (1) through the third circulation pipeline (L3).

6. The system according to claim 1, characterized in that, The condensate outlet at the bottom of the shell side of the heater (2) is connected to the top of the condensate tank (3) via a condensate pipeline. The bottom of the condensate tank (3) is connected to the outside via a condensate pump (4) and an external discharge pipeline.

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