Standard operation specifications for CIP acid-base alternating cleaning of titanium heating tubes in fermentation tanks

The standard sterilisation and descaling procedure for food and pharmaceutical fermentation equipment is CIP online acid-base alternating cleansing. Surface thinning, microcracks, and local pitting corrosion will be accelerated by the continuous damage of the titanium dioxide passive film on titanium heating tubes due to improper cleaning parameters, liquid formula, and operation sequence. In order to safeguard the anti-corrosion barrier of titanium tubes during daily cleaning cycles, this specification establishes standardised cleaning fluid proportions, temperature, circulation duration, switching transition process, and prohibited operations.

1. Classification of standard CIP cleaning fluids and the corresponding limits for titanium
Alkaline cleaning solution (for the removal of organic grease, sugar, and carbon deposits)
The standard formula is an aqueous solution of food-grade sodium hydroxide that ranges from 3% to 5%. Limits of operation that can be controlled:

Temperature range: 40℃ to 60℃

Single circulation soaking time: 2–4 hours. Permissible mechanism: The loose outer layer of the TiO₂ film is only dissolved by a diluted mild alkali. The dense inner oxide layer remains unaffected and can be restored through oxygen-rich water passivation after washing. Forbidden alkaline conditions include a concentration of 10%, a temperature of 80℃, and continuous immersion for a period of 6 hours (which results in irreversible uniform matrix etching).

Acidic cleansing solution (for the removal of calcium magnesium salts and inorganic mineral scale)
First priority: a mild organic acid solution containing citric acid at a concentration of 2% to 4%. Limitations of operation: Minimum temperature of 45 degrees Celsius℃, solitary circulation time ≤ 2 hours Advantage: There is virtually no risk of titanium hydrogen embrittlement due to the low hydrogen evolution reaction. In the event that large scale necessitates inorganic acid (diluted sulfuric/hydrochloric acid, pH≥4):

Incorporate a corrosion inhibitor specifically designed for titanium pickling.

Adopt multiple short cycles with water flushing intervals, maintain a temperature of ≤40℃, and conduct a single soaking for no more than 1 hour.

Mandatory deionised water entire rinsing and 120~150℃ baking for hydrogen removal following cleaning. Hydrofluoric acid and cleaning agents containing ammonium fluoride are strictly prohibited, as fluoride completely dissolves the titanium passive film.

Flushing liquid with a neutral transition
Aerated deionised clean water (dissolved oxygen ≥6mg/L) is the standard medium. Function: To prevent direct mixing and abrupt pH jumps on the titanium tube surface, separate the acid and alkali liquids to reduce alternating film erosion damage.

2. The standard cleaning sequence for CIP (which is not reversible) is as follows:
Pre-rinse: For a period of 15 minutes, circulate clean, aerated water at normal temperature to remove any loose fermentation broth residues from the surfaces of the tubes.

Alkaline circulation cleaning: Dilute alkali is circulated at 40–60℃ for 2–4 hours to decompose organic carbon deposits.

Neutral intermediate flushing: The effluent pH is restored to neutral by continuously circulating pure water for 30 minutes, thereby eliminating any residual alkali.

Acid circulation descaling: Inorganic mineral scale is eliminated by circulating a citric acid solution for 1 to 2 hours.

Final full neutral flushing: Circulate tepid, oxygen-rich water (40~50℃) for 2 hours to completely remove residual acid and complete the preliminary passive film repair.

Static passivation standby: Before feeding the fermentation medium, maintain the tank filled with aerated clean water for 12–24 hours to regenerate the entire TiO₂ protective film.

3. Critical process control points for the mitigation of passive film damage
Ensure that the heating power interlock is strictly maintained during the cleansing process. To prevent local overheating that accelerates film dissolution, reduce the surface power density of the heating tube by 50% during acid/alkali circulation and prohibit full-power heating in the cleaning liquid.

Prevent the rapid generation of thermal stress microcracks on brittle passive film by controlling the rate of temperature rise and decline within a range of ≤5℃/min. Do not directly inject cold cleaning water onto hot titanium tubes.

Ensure that all tube surfaces are fully covered with continuous liquid. Establish an interlock protection for the liquid level; if the liquid level falls to the point where the tube sections are exposed, the heating power should be interrupted to prevent concentrated ion corrosion and dry-out.

Regulate the frequency of cleaning cycles in accordance with the production burden.

Low-load intermittent fermentation: Weekly single CIP acid-base cycle;

Biweekly alternating acid-base cleansing is implemented for medium-load 16-hour daily production.

High-load, continuous fermentation for 24 hours: daily mild CIP cleaning, monthly deep descaling acid cycle.

4. Corrosion hazards and common incorrect CIP operations
Input:

Inadequate Operation Titanium Heating Tube Damage Mechanism
Direct transition from strong alkali to acid without the need for neutral water cleansingPassive film is alternately dissolved and massive microcracks are accumulated as a result of sharp pH fluctuations.
Long-term circulation of heated, concentrated alkali exceeding 80℃Surface roughness and dullness, irreversible uniform etching of the entire wall
Continuous soaking in a diluted inorganic acid at a high temperature without an inhibitorThe matrix is penetrated by mass atomic hydrogen, which results in a concealed hydrogen embrittlement that poses a brittle fracture risk.
Acid cleaning circulation with full-power heatingPassive film erosion and pitting growth are expedited by local tube wall overheating.
Immediately cease flushing after acid cleaning in the absence of warm oxygen-rich water passivation.Damaged film is unable to self-repair due to a lack of oxygen supply, resulting in the expansion of defects into deep cavities during subsequent production.
For the removal of scale, employ fluoride-containing descaling compounds. The TiO₂ film is completely dissolved, resulting in rapid overall titanium matrix corrosion.
5. Supplementary steps for post-CIP inspection and maintenance
After passing through the complete CIP cycle and passivation standby:

Visual examination under high light to detect uniform grey discolouration or localised dark corrosion marks;

Quarterly major maintenance: To assess the integrity of the passive film, conduct an electrochemical potential test. If the surface potential is less than +150mV, perform a hydrogen peroxide chemical re-passivation.

The long-term corrosion loss from repetitive acid-base cycles is monitored through the half-yearly measurement of wall thickness in weld heat-affected zones.

Condensed Summary
A low-concentration, low-temperature mild cleaning fluid formula must be used for the acid-base alternating CIP cleaning of titanium heating tubes. Additionally, neutral water transition flushing should be incorporated between the alkali and acid procedures. Regulate the rate of temperature change and heating power during circulation, and ensure that there is an adequate amount of time for aerated water passivation following cleansing. In order to reduce the cumulative damage to the titanium dioxide passive film and extend the service life of heating tubes, it is recommended to refrain from using concentrated hot alkali, fluoride reagents, and long-term high-temperature inorganic acid soaking.