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Factors Influencing Multiple-Effect Evaporation

Views: 0     Author: Site Editor     Publish Time: 2024-01-02      Origin: Site


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Factors Influencing Multiple-Effect Evaporation

Understanding the factors that influence multiple-effect evaporation is important for achieving optimal performance and efficiency in industrial evaporation. This comprehensive translation delves into the core aspects that shape the effectiveness of multiple-effect evaporation systems. From materials' inherent properties to equipment's operational nuances, this analysis provides an expert perspective on how each element contributes to the overall efficacy of the evaporation process. For professionals and beginners in industrial processing, we hope this article can be of help when exploring multiple-effect evaporation.

1. Material Characteristics

Key parameters of material properties include density, specific heat capacity, thermal conductivity, viscosity, boiling point elevation, enthalpy, surface tension, thermal sensitivity, and corrosivity.

  • Density, specific heat, thermal conductivity, and viscosity primarily affect the heat transfer coefficient on the material side, directly impacting the choice of evaporation area.

  • The surface tension of the material chiefly influences the vapor-liquid separation process and the selection of the separator's diameter and height.

  • Boiling point elevation mainly affects the choice of process flow, evaporation temperature, temperature gradient distribution, and the number of effects. Materials with a high boiling point elevation require a limited number of effects to ensure sufficient heat transfer temperature difference, considering mixed-flow or other processes in design.

  • Besides impacting the heat transfer coefficient, the viscosity of the material also influences the choice of evaporator type; materials with high concentration and viscosity require the selection of forced circulation or scraper-type evaporators to prevent coking due to slow material flow.

  • The thermal sensitivity of materials necessitates short residence times in the evaporator to prevent quality degradation, thus reducing the number of effects and the circulation time in the evaporator.

  • When evaporating materials have specific maximum or minimum temperature requirements, it is essential to consider evaporation temperature, evaporator type, and process flow in the design.

The corrosivity of materials, especially at high temperatures, is a critical factor in selecting materials for evaporation equipment.

2. Altitude

Altitude refers to the vertical height of a location above the sea level baseline, a physical quantity indicating terrain elevation. With increasing altitude, the local atmospheric pressure decreases, meaning the barometric pressure is lower and the corresponding boiling point of the solution decreases. The effect of altitude on multiple-effect evaporation is directly reflected in the system's vacuum level.

Multiple-effect evaporation usually operates under negative pressure, obtained through a vacuum system. Altitude affects the ultimate vacuum of the vacuum system.

For example, a vacuum pump at sea level may have an ultimate vacuum of -0.094MPa, while at an altitude of 1500m, its ultimate vacuum is -0.08MPa. However, altitude does not affect the final evaporation temperature, which remains 36.2℃ in both cases.

For heating steam, steam at 0.1MPa has a corresponding temperature of 99.6℃ at sea level and 95.1℃ at an altitude of 1500m. Therefore, in high-altitude areas, the overall heat transfer temperature difference in the evaporator is reduced, which must be fully considered in multiple-effect evaporation design.

3. Process Parameters

Process parameters are among the critical factors affecting multiple-effect evaporation. The throughput and evaporation capacity of the evaporator determine the size of the equipment, the number of effects, and the evaporation area, all of which directly influence the investment cost of the equipment.

The temperature of the heating steam is a crucial factor in selecting the number of effects. Adequate consideration of maintaining a reasonable heat transfer temperature difference for each effect determines the maximum number of effects for the evaporation equipment; the energy consumption ratio decides the minimum number of effects; the final effect cooling water issue affects the final evaporation temperature and the choice of the final effect condenser; the concentration ratio of the material influences the choice of evaporator type and area calculation.

4. Equipment Operation and Handling

The actual operating efficiency of a multiple-effect evaporation unit relates to both the initial optimization design and subsequent installation, debugging, and operation. Typically, more and more variable issues arise during operation and handling.

(1) Vacuum Level

The vacuum level is a crucial parameter in multiple-effect evaporation, affecting not only the evaporation temperature but also the evaporation capacity and energy consumption ratio. The main reasons influencing the vacuum level include:

  • Poor equipment sealing or leaks;

  • Ultimate vacuum or vacuum pump suction capacity not meeting design requirements;

  • Excessive non-condensable gases or failure to promptly discharge them, causing equipment blockage;

  • Insufficient cooling water in the condenser, preventing timely condensation of secondary steam, affecting the vacuum;

  • Inadequate condenser area or poor condensation effect;

  • Improper seal water quantity for the vacuum pump, high water temperature, etc.

(2) Temperature Difference Between Effects

A temperature difference mismatch between two effects can significantly reduce the evaporation capacity of the preceding effects, severely impacting the evaporation effect and increasing energy consumption. The main causes of this phenomenon include:

  • Secondary steam from the previous effect used as the heating source for the next effect is not consumed promptly. This can be due to a smaller area of the next effect, insufficient material volume in the next effect, structural phenomena in the next effect reducing the heat transfer coefficient, etc.

  • Air leakage between these two effects, disrupting the pressure difference between stages.

  • Inability to promptly discharge the condensate from the secondary steam generated by the previous effect, hindering steam condensation.

  • Small steam pipeline design between effects, impeding smooth steam flow and causing local blockages.

(3) Feed Parameters

  • Feed Volume: The evaporation system has inherent operational flexibility, meaning there is a range for the feed volume during normal operation. According to the law of energy conservation, the maximum heat transfer capacity of the equipment is fixed. Excessive feed volume affects the heat transfer coefficient, altering the evaporation capacity and reducing the concentration of the output. When the feed volume is too low, the flow of material on the material side of each effect significantly decreases, not only raising the evaporation temperature but also causing dry burning or coking in severe cases.

  • Feed Concentration: Variations in feed concentration cause changes in the density, viscosity, specific heat capacity, and thermal conductivity of the feed material, directly impacting the heat transfer coefficient and changes in boiling point elevation, subsequently affecting evaporation.

  • Feed Temperature: Feed temperature affects the physical properties of the material, thereby influencing the heat transfer coefficient on the material side. Generally, evaporators require bubble point feeding. If the feed temperature is too low, part of the evaporator area is sacrificed to raise the material's sensible heat rather than latent heat, reducing evaporation capacity.

(4) Ejector

In evaporation systems with ejectors, discrepancies between actual operating conditions and design parameters of the ejector can alter its performance. When the mixed fluid outlet pressure is within a certain range, the ejector's injection coefficient remains relatively constant. However, when the pressure exceeds a certain value, the performance of the ejector drops sharply. Ejectors are highly sensitive to pressure changes in the fluid being injected, and even minor changes in the injected pressure can significantly impact the ejector's performance.

Increasing the working steam pressure does not necessarily improve the ejector's performance. Increasing the working steam pressure within a small range can enhance the performance of the ejector, but when the pressure exceeds a certain value, it can decrease the performance. The primary reason is that increasing the working pressure of the ejector also increases the input of additional steam. The ejector coefficient slightly increases with the rise in working steam temperature and tends to increase linearly. Changing the temperature of one fluid (working steam or injected steam) only affects the flow rate of that fluid and does not impact the flow rate of the other fluid.

(5) Pump Influence

Pumps are the primary dynamic equipment in the evaporation system, mainly responsible for transporting liquids. Multiple-effect evaporation systems typically use pumps with double mechanical seals. The quality of the pump's mechanical seal, the actual flow rate, and head during operation all impact the evaporation system.

When material pumps have insufficient flow due to seal leakage or other reasons, it can cause liquid accumulation in the previous effect and material shortage in the next effect, increasing the temperature difference between stages and, in severe cases, causing dry burning or even coking blockages in the evaporator.

If the condensate pump cannot promptly extract condensate, it causes condensate accumulation in the evaporator, severely inhibiting heat transfer in the evaporator and changing the steam heating method in the evaporator to hot water heating, significantly reducing efficiency and even causing steam blockages, leading to an increase in the temperature of the previous effect.

(6) Other Influences

In addition to the operational issues mentioned above, the effects of piping, separator separation efficiency, valve quality issues, and operational standardization problems can all impact the evaporation system.

The multiple-effect evaporation is an essential process in industrial manufacturing. The article underscores the importance of considering every aspect, from material characteristics to equipment operation, to optimize the efficiency and effectiveness of evaporation systems. This detailed discussion offers insights that can lead to enhanced operational strategies and improved process outcomes. By comprehending these critical factors, industry professionals can make informed decisions, leading to more sustainable and cost-effective industrial processes.

If you are considering a multi-effect evaporator or other evaporation & crystallization projects, please do not hesitate to contact us. Vnor expert team is always ready to provide you with professional consultation and advice. Tell us your project needs and we will offer a complete customized solution for you.

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