Glass lining protects the base metal of a reactor so effectively that corrosion will usually occur in the reactor jacket. Although the
heat transfer fluids used in the jacket space are relatively mild (especially in comparison to the contents inside the reactor itself), they will generally attack the jacket as well as the reactor exterior long before the glass-lined interior becomes damaged.
The most common form of jacket fouling is iron oxide corrosion. This results in prolonged heating and cooling times, a reduction in product quality, and equipment deterioration.
Corrosion can result from steam condensate. Materials like calcium, magnesium, silica, and iron are carried in the plant service water that is frequently used for cooling fluid.
These steps will extend the life of your vessel:
1. Select the right type of jacket and thermal fluids based on the heating and cooling requirements of your application.
2. Take proper care of your jacket interior to avoid scale buildup (among other problems)which can limit the flow and effectiveness of your heating/cooling fluid.
Selecting Thermal Fluids
While choosing heating and cooling fluids consider these practices to help offset corrosion:
• Water should be treated to remove impurities and should contain a corrosive inhibitor.
• Brine may be used for cooling, but must be kept at a neutral pH (the recommended brine concentrations should have a specific gravity of 1.2 and a pH of 8.0 – 8.5). Sodium dichromate in the amount of 1 lb/100 gal should be added.
• Never use brine alternately with steam or hot water. This can result in a highly corrosive condition.
• Ammonia, glycerol, ethylene glycol, etc., may be used as alternate coolants or heat transfer fluids
• Never use ammonia alternatively with steam or hot water
• Some coolants may decompose to acid component when exposed to heat/steam, leading to corrosion of both the vessel and jacket steel. This type of attack could potentially cause a glass lining failure due to nascent hydrogen dispersion.
Periodic inspection of the jacket is an important examination to add in your maintenance regimen. A regularly scheduled analysis will help to extend the life of your vessel
Glass lined jacketed vessels are equipped with a vent coupling at the top of the jacket shell and a drain coupling in the lower jacket expansion diaphragm near the vessel outlet.
The recommended practice is to connect the top vent line with a valve and vent whenever liquid is first injected into the jacket. This removes all air and permits full heat transfer. It also prevents corrosion of the upper closure. The diaphragm drain should be opened at least once a week during operation to clean out all accumulated deposits and periodically “blown down” to remove sludge from the expansion ring and to prevent premature failure due to corrosion.
When it is determined that the jacket must be cleaned due to mild fouling (especially due to brine usage), we recommend a 15% solution of sodium hypochlorite
After cleaning, all jacket-agitating nozzles should be inspected and replaced if needed. There are cases, however, when fouling is moderate to severe and more aggressive action needs to be taken.
Acid-based cleaning methods is the conventional way to treat jacket space corrosion. But this technique does not solve the problem with fouled reactors. Acids can easily damage or destroy the internal glass lining of reactors. During acid cleaning hydrogen atoms can permeate the steel from the jacket site and react to form molecular hydrogen. This reaction results in “fish scaling”, a term used to describe the crescent-shaped chips that are created by the pressure put on the glass lining. This fish scale damage extends from the substrate metal through the glass coating to the surface completely compromising the corrosion resistance integrity of the glass lining.
Never use acid solution, inhibited or otherwise for cleaning jackets of glass-lined equipment.
Alkaline cleaning can be performed and is much safer on a vessel than acid cleaning. However alkaline cleaning products have shown in practice to have little effect on the mineral scale which needs to be removed to restore heat transfer efficiency. Additionally, many other common cleaning chemicals have been proven to be ineffective against iron deposits.
Cleaning and Neutralization Process
The cleaning compound most frequently used for flushing the jacket of the reactor is the alkaline sodium hypochlorite solution.
- A solution de 2.5 gallons of commercial sodium hypochlorite (10%-15%) is added to each 200 gallons of water, record the pH, Color and solids in the solution.
- The solution is circulated in the jacket for 3 hours to 5 hours at room temperature (Max Temperature 130F)
- Drain the reactor jacket and measure the pH, color and solids in the solucion and compare
- Based on the pH reading, color and solids determine whether sufficient cleaning has taken place or whether an additional charge of solution is required. If the parameters shows of the cleaning solution has increased by at least 5% after recirculation, then sufficient cleaning will have taken place.