Analysis and Discussion on the Design Scheme of a Special Vane Separator for the Secondary Steam in the Concentration Evaporator of 151-Chlor-alkali Caustic Soda Production Plants

In the chlor-alkali industry, caustic soda plants are a relatively old topic compared to PVC plants. However, precisely because the technical processes are outdated, the upgrading and transformation of related technologies is often urgent. It is for this very reason that many owners have turned to our Nuwei Energy Technology Company, one after another, to request technical upgrades for the separation of the secondary steam generated in their caustic soda mother liquor concentration and evaporation processes.

In the traditional caustic soda concentration and evaporation process, a multi-effect evaporator with three effect stages is often used to gradually evaporate and concentrate the caustic soda mother liquor generated in the electrolysis process. In terms of separation technology itself, whether it is a three-effect evaporator, a two-effect evaporator, or a single-effect evaporator, a gas-liquid separator is required for the secondary steam in order to remove the large number of liquid droplets and foam carried by this steam, as well as the dissolved caustic soda. However, considering the priorities involved, owners tend to opt for technical upgrades to the separation system for the secondary steam from single-effect evaporators.

In the first-effect evaporator, the secondary steam enters the condenser directly and is connected to the vacuum pump pipeline; therefore, this evaporator operates at the highest vacuum level, resulting in the lowest evaporation temperature. Although the operating temperature of the first-effect evaporator is the lowest, its evaporation workload is not small. The caustic soda mother liquor from the electrolysis process is first fed into a first-effect evaporator for evaporation and concentration.

The concentrate from the first-stage evaporator, after being processed, has its heat recovered from the concentrates of the other stages of evaporators and reheated before being fed into the second-stage evaporator. The secondary steam generated by the second-effect evaporator is separated from the liquid in a gas-liquid separator; the condensed liquid flows into the condensate waste water storage tank, while the secondary steam is sent to the first-effect evaporator to be used as a heat source for that evaporator.

The second-stage concentrate, after being processed by the second-stage evaporator, has its heat from the third-stage evaporator recovered and reheated before being fed into the third-stage evaporator. Three-effect evaporators often use raw steam as the high-temperature heat medium; the condensate from this steam is temporarily stored in a condensate tank before being returned to the boiler water treatment system. The secondary steam generated by the three-effect evaporator has a high temperature, and it is fed into the two-effect evaporator to serve as its heat source. A two-effect evaporator can also use a small amount of raw steam as a supplementary heat source.

Due to the secondary steam condensate from the second-effect evaporator, which, just like that from the first-effect evaporator, is stored in a secondary steam condensate sludge water tank, it needs to be sent to a downstream sludge treatment unit for processing. The downstream slag water treatment unit has strict requirements regarding the sodium hydroxide content in the incoming slag water; it must be less than 15 ppm. Otherwise, the downstream slag water treatment unit will be overburdened and may even fail. To control the caustic soda content carried by the secondary steam condensate, it is necessary to install a gas-liquid separator on the secondary steam to separate and purify it.

The reason why owners pay special attention to the separation and purification of the secondary steam from the primary evaporator is also due to the unique properties of this secondary steam. The separation and purification of the secondary steam from the first-effect evaporator are of great importance for the following reasons: 1. The first-effect evaporator operates under high vacuum conditions, and the flow rate of the secondary steam is high, which makes it easy for drops and foam of caustic soda to be carried along. 2. The secondary steam from the first-effect evaporator needs to be condensed using circulating water, and the outlet pipeline of the condenser is connected to the vacuum system. 3. The secondary steam from the first-effect evaporator, along with the alkali solution it carries, is cooled and condensed, resulting in a large amount of secondary steam condensate that is discharged into the slag water tank. The sodium hydroxide droplets contained in this condensate can cause the alkali content in the slag water to exceed acceptable levels, overloading or even breaking down the downstream slag water treatment units. This, in turn, leads to backflow of slag water that disrupts normal operations. 4. The secondary steam from the first-effect evaporator carries liquid droplets and foam containing caustic soda into the circulating water condenser; as the temperature drops, caustic soda crystallizes and accumulates within the heat exchanger, causing blockages and corrosion. 5. The secondary steam from the first-effect evaporator carries liquid droplets and foam containing caustic soda into the circulating water condenser; as the caustic soda cools, crystals form and accumulate within the heat exchanger, causing blockages. This leads to poor airflow in the vacuum pipeline, high pressure drops, and a decrease in vacuum level. The owner also reported two specific operational issues: first, due to the accumulation of caustic soda crystals and condensate in the condenser and vacuum pipelines, the vacuum pumps used for direct evacuation ended up carrying a significant amount of caustic soda and liquid droplets, which were spilled on the ground around the exhaust pipe openings, causing noticeable environmental pollution. Secondly, the caustic soda crystals accumulated inside the heat exchanger cause electrochemical corrosion of the heat exchange tubes over time, leading to the penetration of these tubes. A large amount of circulating water enters the secondary steam condensate sludge tank, overloading the downstream sludge treatment units and even causing them to fail.

Here is a flowchart of the caustic soda concentration and evaporation unit for your reference:

In the earliest conventional caustic soda plant concentration and evaporation processes, no dedicated gas-liquid separation components were installed for either the secondary steam from the first-effect evaporator or that from the second-effect evaporator. In actual operation, the owner, the design institute, and the process package supplier have all found it highly necessary to install a gas-liquid separation unit for the secondary steam of the first-effect evaporator. Therefore, some owners collaborated with design firms to install wire mesh demisters inside the first-effect evaporator. The clean wire mesh demister, when it first started operating, did indeed play a role in separating the alkaline droplets carried by the secondary steam. However, as the operating time increases, caustic soda crystallizes and blocks the screen, causing the pressure drop to soar and resulting in poor pumping performance by the vacuum pump. Many wire mesh demisters installed on the cross-section of the evaporator, along with their supporting components, are deformed by the high-pressure airflow; as a result, the wire mesh elements shift, and large amounts of air carry liquid droplets and foam containing caustic soda straight into the heat exchanger or even the vacuum pumping equipment. This in turn leads to problems with the heat exchanger, vacuum pumping equipment, and the sludge water tank. Plant operators have to stop the operation and replace the screen internals within just a few months; the maintenance work is extensive and labor-intensive.