Anti-scaling circulating heat exchange method in high-salinity wastewater MVR (mechanical vapor recompression) evaporative crystallization process

Abstract

The invention discloses an anti-scaling circulating heat exchange method for a high-salinity wastewater MVR (mechanical vapor recompression) evaporative crystallization process, which comprises the following steps of: arranging a seed crystal induction area in a pipeline from an outlet of a circulating pump to an inlet of an evaporator, and finishing initial nucleation of mother liquor on the surface of a seed crystal in advance under the conditions of high turbulence and moderate supersaturation; fine grains with the grain size of 20-30 microns are continuously added in combination with a metering pump, so that the nucleation position is controlled. An online turbidity monitoring point is arranged at an outlet of the induction area, and mother liquor is selectively fed into the heat exchange section or bypass backflow through a three-way valve according to turbidity, so that the content of solids entering the heat exchange section is stably controlled, and heterogeneous nucleation of a heat exchange surface is avoided. The evaporated feed liquid continues to grow in the crystallizing pond and is subjected to solid-liquid separation through a centrifugal machine, and the obtained mother liquid flows back to a system to form a seed crystal closed-loop cycle. According to the method, the scaling rate of the heat exchange tube bundle can be remarkably reduced, the cleaning period is prolonged, and the continuous and stable operation capacity of the MVR system is improved.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, and in particular to a method for preventing scaling and circulating heat exchange in the MVR evaporation and crystallization process of high-salt wastewater. Background Technology

[0002] High-salinity wastewater has a wide range of sources, including separation waste liquids and concentrated mother liquors from industries such as chemical, pharmaceutical, food, and mining. This type of wastewater typically contains a high mass fraction of inorganic salts, which easily reach a supersaturated state during evaporation and concentration, leading to the formation of crystal nuclei on the heat transfer surface and the gradual formation of a scale layer. With increasing national requirements for industrial water conservation and zero-discharge (ZLD), mechanical vapor recompression (MVR) evaporation crystallization technology is widely used in the treatment of high-salinity wastewater due to its low energy consumption and low operating costs.

[0003] In MVR evaporation crystallization systems, the mother liquor circulates repeatedly between the heat exchange section and the evaporator separator at a high circulation rate using a circulating pump, absorbing heat from the compressed steam within the heat exchange tubes to achieve evaporation. However, high-salt systems are prone to localized supersaturation during evaporation, causing primary crystal nuclei to preferentially form and adhere to the heat exchange tube walls, leading to intensified heterogeneous nucleation. Once attached to the heat exchange surface, the crystal nuclei continue to grow, accelerating the scaling rate of the heat exchange tube bundle. This results in decreased heat exchange efficiency, increased energy consumption, reduced evaporation rate, and a significantly shortened operating cycle.

[0004] To mitigate the impact of scaling, existing engineering methods typically employ increases in circulation volume, physical flushing, periodic shutdowns for cleaning, and the addition of scale inhibitors. However, increasing circulation volume only partially improves fluid shear forces and cannot fundamentally inhibit heterogeneous nucleation on the heat exchange surface; chemical cleaning or physical flushing requires frequent shutdowns, affecting system stability; and the addition of scale inhibitors may introduce secondary pollution, with limited scale inhibition effectiveness in high-salt and high-temperature environments. Furthermore, current technologies generally cannot effectively control the particle composition of the mother liquor before it enters the heat exchange section, and lack a method that can control the initial nucleation site, predict scaling risk, and allow for real-time adjustment. Summary of the Invention

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for preventing scaling and circulating heat exchange in the MVR evaporation crystallization process of high-salt wastewater is applied to an MVR forced circulation evaporation crystallization system comprising a mother liquor tank, a circulating pump, an evaporator, an evaporator separator, a crystallization tank, and a centrifuge. The method includes the following steps: (1) Set up a seed crystal induction zone in the circulation pipeline from the outlet of the circulation pump to the inlet of the evaporator; (2) Fine seed crystals with a particle size of 20–30 μm are continuously added to the inlet of the seed induction zone, so that the mother liquor can fully contact the seed crystals in the seed induction zone with the help of turbulence, and complete the initial nucleation or secondary crystal growth on the seed crystal surface. (3) Set up an online turbidity monitoring point at the outlet of the seed crystal induction zone to monitor the particle concentration of the mother liquor in the induction zone in real time; (4) Selectively circulate the mother liquor according to the turbidity monitoring results. When the turbidity is higher than the preset upper limit, the high particle concentration mother liquor is returned to the inlet of the circulation pump and re-enters the seed crystal induction zone. When the turbidity is lower than the preset lower limit, the mother liquor is transported to the inlet of the heat exchange section of the evaporator. (5) The low-particle-concentration mother liquor after diversion and screening is sent to the heat exchange section of the evaporator for evaporation; (6) The vapor-liquid mixture after evaporation is fed into an evaporator for vapor-liquid separation. The liquid phase enters the crystallization pool to continue crystal growth. The resulting crystals are separated into solid and liquid by a centrifugal mechanism. The separated mother liquor is returned to the inlet of the circulation pump and used as a supplementary source of micro-crystal seeds to supply the inlet of the seed induction zone, thereby forming a closed-loop seed supply system.

[0006] Furthermore, in step (1), the seed induction zone is set in a straight pipe section between the outlet of the circulating pump and the inlet of the evaporator. By reducing the effective flow channel size of the straight pipe section, the linear velocity of the mother liquor in the induction zone is increased, so that it reaches the flow velocity range that forms high turbulence conditions.

[0007] Furthermore, a heating device is installed outside the seed induction zone to maintain the operating temperature of the induction zone at 60–70°C.

[0008] Further, in step (2), the seed crystals are continuously added by a metering pump set at the inlet of the seed crystal induction zone, the amount added is 0.3–0.8 wt% of the mother liquor mass, and the seed crystal particle size is 20–30 μm.

[0009] Further, step (4) is achieved by a three-way valve set at the outlet of the seed induction zone. The three-way valve has a discharge passage for the main evaporator heat exchange section and a bypass passage. The bypass passage is connected to the return pipeline from the mother liquor tank to the inlet of the circulating pump. The opening degree of the three-way valve is automatically adjusted according to the turbidity signal output by the online turbidity meter so that the mother liquor with turbidity higher than the preset upper limit value is returned through the bypass and the mother liquor with turbidity lower than the preset lower limit value enters the evaporator heat exchange section.

[0010] Furthermore, when the turbidity of the mother liquor in the induction zone is higher than 1000 NTU, bypass reflux is performed; when the turbidity is lower than 700 NTU, the mother liquor is transported to the heat exchange section of the evaporator.

[0011] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention sets up a seed crystal induction zone between the outlet of the circulating pump and the inlet of the evaporator, and puts this zone in a state of high turbulence and moderate supersaturation. This invention enables the supersaturation in the mother liquor to be preferentially released in the induction zone, so that the primary crystal nuclei are generated on the surface of the seed crystals, rather than heterogeneous nucleation on the heat exchange tube wall, thereby reducing the scaling rate of the heat exchange section from the source.

[0012] After treatment with this invention, the solid content of the liquid entering the heat exchange section is low, and heterogeneous nucleation is reduced, resulting in a significant decrease in the scaling rate of the evaporator heat exchange tube bundle. Actual operation shows that the heat exchanger's heat transfer coefficient decreases by less than 5% within 24 hours, and the cleaning cycle is significantly extended from the original 0.5–1 day to 4–6 days, greatly reducing downtime frequency and improving system availability. Attached Figure Description

[0013] Figure 1 This is a process flow diagram of an embodiment of the present invention. Detailed Implementation

[0014] The technologies in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0015] The high-salt wastewater treated in this embodiment has a high salt concentration and complex composition, making it prone to initial scaling in the heat exchange tubes during evaporation and concentration, thus shortening the evaporator's operating cycle. The system employs mechanical vapor recompression (MVR) to force circulation and drive the mother liquor through the heat exchange tube bundle, thereby increasing turbulence and slowing down crystallization. However, relying solely on high circulation volume is insufficient to effectively suppress heterogeneous nucleation on the heat exchange tube walls; therefore, the anti-scaling circulating heat exchange method of this invention is necessary.

[0016] The original system process included: pretreated high-salt wastewater entering a mother liquor tank and then being pumped into the evaporation system; a circulating pump transported the mother liquor to the heat exchange section of the evaporator, where the mother liquor absorbed heat from the compressed steam and evaporated; the vapor-liquid mixture exiting the evaporator flowed into the evaporator separator, where the steam was compressed and reused, and the mother liquor entered the crystallization tank for further cooling and crystallization; the crystallized slurry then entered a centrifuge for solid-liquid separation, and the resulting mother liquor was returned to the inlet of the circulating pump and mixed with the newly entered wastewater for the next cycle. Since the centrifuge could not completely remove fine crystals, the returned mother liquor contained microcrystalline seeds with a particle size of 10–50 μm, providing a stable source of seed crystals for this method.

[0017] Based on the existing process, this embodiment introduces the anti-scaling circulating heat exchange method proposed in this invention, which specifically includes the following steps: Step (1): Establish a seed-inducing zone on the straight pipe section between the circulating pump outlet and the evaporator inlet. By reducing the diameter of this pipe section from DN65 to DN40, the flow velocity inside the pipe is increased to approximately 3.5–4.0 m / s. This increased flow velocity allows the Reynolds number in the induction zone to reach 2 × 10⁻⁶. 4 The above describes a highly turbulent state, which significantly increases the collision frequency between the mother liquor and the seed crystals, thereby promoting secondary nucleation on the seed crystal surface. Furthermore, installing a heating device outside the induction zone maintains the temperature in this section at 60–70°C, lower than the heat exchange section temperature but sufficient to maintain a moderately supersaturated environment, which helps control the nucleation rate.

[0018] Step (2): Fine seed crystals are continuously added to the induction zone inlet via a metering pump. These seed crystals originate from the centrifuged mother liquor in the crystallization pool and can be obtained as fine crystals with a particle size of 20–30 μm through simple clarification or sedimentation. The amount added is controlled at 0.3–0.8 wt% of the mother liquor mass. Due to the highly turbulent mixing environment in the induction zone, the seed crystals and mother liquor can come into full contact, causing the supersaturation in the mother liquor to be preferentially released on the seed crystal surface, thereby completing most of the initial nucleation within the induction zone.

[0019] Step (3): To ensure that the solid content of the liquid at the inlet of the heat exchange section remains at a low level, an online turbidity meter needs to be installed at the outlet of the induction zone to monitor the particle concentration in real time. The turbidity range at the outlet of the induction zone is generally stable at 900–1500 NTU, with different turbidities representing different seed concentration levels. A diversion node is set after the turbidity monitoring point, connecting two different material flows through a three-way valve: one to the inlet of the evaporator heat exchange section, and the other back to the main pipe at the inlet of the circulating pump. The opening of the three-way valve is automatically adjusted according to the turbidity to achieve selective circulation of particles. When the turbidity is higher than 1000 NTU, it indicates that a large number of seed crystals have been generated in the induction zone. If this part of the high-particle material is directly sent into the heat exchange section, it will increase the risk of deposition on the heat exchange tube wall. Therefore, the system guides this part of the liquid back to the inlet of the circulating pump through the return branch, so that it re-enters the induction zone to participate in further nucleation or growth. When the turbidity is lower than 700 NTU, it indicates that the particle concentration in the induction zone is not high, and this part of the mother liquor can safely enter the evaporator heat exchange section for evaporation. After the flow control is applied, the solid volume fraction of the liquid feed sent to the heat exchange section can be stabilized below 0.02 vol%, which significantly reduces the probability of heterogeneous nucleation on the heat exchange surface.

[0020] Step (5) The low-particle-concentration mother liquor after diversion and screening is sent to the heat exchange section of the evaporator for evaporation; Step (6) involves feeding the evaporated vapor-liquid mixture into an evaporator for vapor-liquid separation. The liquid phase enters a crystallization tank for further crystal growth. The resulting crystals undergo solid-liquid separation via a centrifuge. The separated mother liquor is returned to the inlet of the circulation pump and used as a supplementary source of fine seed crystals to supply the inlet of the seed induction zone, thus forming a closed-loop seed supply system. In this embodiment, the crystallization tank employs a combination of evaporative cooling and natural sedimentation, allowing large crystals to gradually form and ultimately enter the centrifuge for solid-liquid separation. The separated mother liquor is returned to the inlet of the circulation pump. Since the centrifuge cannot completely remove the fine crystals from the mother liquor, the returned mother liquor naturally contains fine seed crystals with a particle size of 10–50 μm, providing a stable source for seed induction in this method.

[0021] The implementation of this method significantly improved the scaling situation in the evaporator heat exchange section. Within 12 hours of continuous operation, almost no visible scale formed on the heat exchange tubes, and the heat transfer coefficient decreased by less than 5% within 24 hours. The cleaning cycle was increased from 0.5–1 days to 4–6 days, and the operational stability of the heat exchange section was significantly improved. The crystallization behavior of the entire system became more uniform, with no large crystal clusters or overflow in the crystallization pool, and the solid-liquid separation process remained smooth. The implementation of this method did not alter the existing equipment structure; by simply adding seed induction and selective circulation control, this project achieved a major improvement in scale prevention in the heat exchange section.

Claims

1. A method for preventing scaling and circulating heat exchange in the MVR evaporation and crystallization process of high-salt wastewater, applied to an MVR forced circulation evaporation and crystallization system comprising a mother liquor tank, a circulating pump, an evaporator, an evaporator separator, a crystallization tank, and a centrifuge, characterized in that, Includes the following steps: (1) Set up a seed crystal induction zone in the circulation pipeline from the outlet of the circulation pump to the inlet of the evaporator; (2) Fine seed crystals with a particle size of 20–30 μm are continuously added to the inlet of the seed induction zone, so that the mother liquor can fully contact the seed crystals in the seed induction zone with the help of turbulence, and complete the initial nucleation or secondary crystal growth on the seed crystal surface. (3) Set up an online turbidity monitoring point at the outlet of the seed induction zone to monitor the particle concentration of the mother liquor in the induction zone in real time; (4) Based on the turbidity monitoring results, the mother liquor is diverted. When the turbidity is higher than the preset upper limit, the high particle concentration mother liquor is returned to the inlet of the circulation pump to re-enter the seed crystal induction zone. When the turbidity is lower than the preset lower limit, the mother liquor is transported to the inlet of the heat exchange section of the evaporator. (5) The low-particle-concentration mother liquor after diversion and screening is sent to the heat exchange section of the evaporator for evaporation; (6) The vapor-liquid mixture after evaporation is fed into an evaporator for vapor-liquid separation. The liquid phase enters the crystallization pool to continue crystal growth. The resulting crystals are separated into solid and liquid by a centrifugal mechanism. The separated mother liquor is returned to the inlet of the circulation pump and used as a supplementary source of micro-crystal seeds to supply the inlet of the seed induction zone, thereby forming a closed-loop seed supply system.

2. The method according to claim 1, characterized in that, In step (1), the seed crystal induction zone is set in a straight pipe section between the outlet of the circulating pump and the inlet of the evaporator. By reducing the effective flow channel size of the straight pipe section, the linear velocity of the mother liquor in the induction zone is increased, so that it reaches the flow velocity range that forms high turbulence conditions.

3. The method according to claim 1, characterized in that, A heating device is installed outside the seed induction zone to maintain the operating temperature of the induction zone at 60–70°C.

4. The method according to claim 1, characterized in that, In step (2), the seed crystals are continuously added by a metering pump set at the inlet of the seed crystal induction zone. The amount added is 0.3–0.8 wt% of the mother liquor mass, and the seed crystal particle size is 20–30 μm.

5. The method according to claim 1, characterized in that, Step (4) is achieved by a three-way valve located at the outlet of the seed crystal induction zone. The three-way valve has a discharge passage for the main evaporator heat exchange section and a bypass passage. The bypass passage is connected to the return pipeline from the mother liquor tank to the inlet of the circulating pump. The opening of the three-way valve is automatically adjusted according to the turbidity signal output by the online turbidity meter so that the mother liquor with turbidity higher than the preset upper limit value is returned via the bypass, and the mother liquor with turbidity lower than the preset lower limit value enters the evaporator heat exchange section.

6. The method according to claim 5, characterized in that, When the turbidity of the mother liquor in the induction zone is higher than 1000 NTU, bypass reflux is performed; when the turbidity is lower than 700 NTU, the mother liquor is transported to the heat exchange section of the evaporator.

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