Currently, many processes in the fields of petrochemicals, fine chemicals, pharmaceutical production, and environmental protection engineering involve evaporation crystallization systems [1]. During the commissioning of various related projects at home and abroad, some problems have been encountered in the evaporation crystallization system. This paper summarizes the common problems encountered and proposes corresponding solutions.

Process Flow of Single-Effect Evaporation Crystallization System

The evaporation crystallization system process is briefly described as follows: The material is heated to the required temperature by fresh steam and the condensate water of the tubular evaporator through a preheater, and then enters the crystallizer. After reaching a certain level, it is forced to evaporate and circulate between the tubular evaporator and the crystallizer by a circulation pump. After the material reaches saturation, crystals will be generated under the saturated driving force. When the material reaches the required solid content, it is discharged. The secondary steam generated by the material in the crystallizer enters the condenser for condensation, and the entire system maintains a negative pressure vacuum through a vacuum pump.

Common Problems and Solutions during the Commissioning of Evaporation Crystallization Systems

2.1 Pump Tripping Phenomenon

Pumps play a role in material transfer in the process system [2]. If the pump stops working, the entire system will be unable to operate. During commissioning, pump tripping is common. The causes include:

(1) Excessive load, including excessive material viscosity and exceeding the solid-liquid ratio, leading to motor seizure;

(2) Foreign objects in the pump cavity, jamming the impeller, etc.;

(3) Two-phase power supply is incorrectly connected to the motor.

Solutions:

(1) In the case of correct motor selection, increase the upper limit of current protection to solve the tripping phenomenon;

(2) When the pump is running, listen for any abnormal sounds in the pump to determine if there are any foreign objects in the pump cavity;

(3) Before starting up, carefully check the product specifications, model, power, voltage, and current parameters to avoid inconsistencies between the wiring and requirements.

2.2 Material Leakage Phenomenon

During commissioning, materials may appear in the condensate water, causing material loss and condensate water contamination. The causes include:

The liquid level in the crystallizer is too high, the gas-liquid separation space is small, and the material is carried away;

Severe entrainment of mist droplets during evaporation, and the demister does not achieve its working effect, causing the material to be carried away;

The system vacuum is too high, exceeding the design range, causing material leakage.

Solutions:

(1) At the design stage, fully consider the gas-liquid separation space, and strictly control the feed rate. Through liquid level control, avoid the occurrence of excessively high liquid levels;

(2) Strictly control the material concentration. For conditions with severe mist entrainment, a defoaming device or increased defoamer should be designed;

(3) Set a pressure transmitter in the separation chamber of the crystallizer to stabilize the vacuum through linkage control.

2.3 Heat Exchanger Scaling Phenomenon

The preheater, evaporator, and condenser [7] in the system all play a role in heat transfer. According to the principle of heat transfer, the thicker the scale layer, the lower the heat transfer efficiency, and the shorter the operating cycle of the heat exchanger [8]. During system commissioning, the main causes of scaling include:

(1) Ions in the material medium combine to form insoluble precipitates, and the precipitation scaling phenomenon becomes more serious as the temperature increases;

Solutions:

(1) Reduce the content of scaling ions in the material as much as possible;

(2) The equipment should be cleaned regularly through online chemical cleaning or offline chemical cleaning [9] to ensure that the heat exchanger surface is smooth and free of blockage.

2.4 Heat Exchanger Coking Phenomenon

The heat exchanger separates hot and cold materials through the heat transfer wall. If the heat transfer surface is damaged and the coke layer is increased, the heat transfer of the heat exchanger will fail. During the system commissioning operation, the causes of coking include:

(1) If the material is cut off or the feed rate is less than the sum of the discharge and secondary steam, and the steam is not stopped, the material level will decrease, causing the material to hang on the wall, resulting in dry burning and coking of the equipment wall;

(2) Excessive steam pressure or sudden changes in operating conditions may cause severe coking on the heat transfer surface.

Solutions:

(1) Strictly follow the operating manual for start-up and shutdown, and set a flow meter in the feed pipeline to monitor the system and avoid dry burning and coking due to material interruption or insufficient material;

(2) The steam pipeline should be equipped with a steam regulating valve and a pressure stabilizing valve to control the steam volume, which should not be too high. If the excess is serious, the inlet valve should be cut off as soon as possible, and the amount of cold material should be increased to absorb the residual heat.

2.5 Failure of Automatic Control of Electrical Components

At present, the evaporation crystallization systems used in engineering applications have all achieved automatic control. Among them, regulating valves, level sensors, temperature transmitters, and pressure transmitters play a decisive role in the linkage control. If they fail, it will affect the system from minor control disorders to major system damage. The causes of failure include:

(1) Failure of the regulating valve positioner, unable to achieve remote adjustment;

(2) Level meter failure, unable to determine the real-time level. The cause may be a wiring error in the level meter itself, or foaming of the material causing a false level;

(3) Temperature and pressure transmitters fail, unable to provide real-time feedback on the system temperature and pressure. The temperature probe may be in contact with the wall, and the pressure transmitter probe hole may be blocked, causing it to fail.

Solutions:

(1) Damaged regulating valves should be returned to the factory for repair in time, and inspection and testing should be carried out before leaving the factory. After passing the inspection, they should be sent to the site. If the problem cannot be solved in time, a backup plan can be used for local operation, but experienced operators are required.

Different types of level gauges should be installed strictly according to the installation instructions. For excessively long level probes, consider the impact of material flow and whether it may cause measurement failure. If false level readings are caused by material foaming, add defoaming agents or install a defoaming device in a timely manner.

For temperature and pressure changes, first check the wiring. If there are no errors, check whether the probe is wrapped or damaged. The system should have a local instrument for comparison and correction with the sensor.

2.6 Pipeline Blockage

During the system crystallization process, the pipeline transporting the crystal slurry may be blocked to varying degrees due to factors such as valve settings and temperature control. The main causes of blockage include:

Some crystalline materials will rapidly crystallize on the inner wall of the pipe as the temperature decreases, causing pipeline blockage.

Valves in the pipeline system serve to cut off the flow. When the valve is closed, the material flow stops. When the valve is fully open, the material flows. However, some valves may have mechanical dead angles even when fully open, causing crystal accumulation and pipeline blockage after prolonged operation.

Solutions:

To prevent large temperature changes during pipeline transmission, insulation measures or heating devices can be designed to ensure that the material does not crystallize in the pipeline and cause blockage.

Valves are generally selected based on simple structure and large flow area, such as ball valves. Avoid using globe valves or butterfly valves. Set up redundant valves in the pipeline for easy removal in case of severe blockage.

Conclusion

The evaporative crystallization system is an organic combination of multiple small units. Different problems are always encountered during commissioning. The above problems are common influencing factors and common problems found in multiple sites. Effectively solving and avoiding unstable factors or incorrect operations during commissioning will result in a longer system operating cycle, longer equipment life, and higher product quality.