Hvordan eliminere luftboblestagnasjonskorrosjon på titanvarmerør i forseglede gjæringstanker

# Document: Technical Article No. 68 ## Title: Methods for preventing air bubble stagnation and corrosion on titanium heating tubes in sealed fermentation vessels When heated, dissolved air will separate into minute bubbles in fermentation containers that are completely sealed. A gas-liquid two-phase stagnant zone is formed when these droplets adhere to the outer wall of titanium heating tubes for an extended period. The continuous liquid film necessary for the self-repair of the titanium dioxide passive film is destroyed by this unique microenvironment, which induces localised concentrated pitting corrosion. This corrosion frequently manifests on the upper half of the tube body and the top of flanged heating assemblies. This article provides a comprehensive analysis of the corrosion mechanism of bubble stagnation and provides a comprehensive set of structural design and operation adjustment solutions to fundamentally eliminate bubble retention hazards. ## 1. Mechanism of corrosion for stagnant air bubbles on titanium surfaces 1. Passive film incomplete repair is a consequence of intermittent oxygen supply. The liquid phase dissolved oxygen is isolated from the bubble-covered area. The TiO₂ passive film in question is unable to be replenished with oxygen in order to heal scratches and thinning areas. The liquid-covered tube wall is an oxygen-rich intact cathode, whereas the bubble-covered area becomes an oxygen-deficient active anode, resulting in self-galvanic corrosion on the same titanium tube. Cycle after cycle, corrosion fissures continue to expand downward. 2. Enrichment of solute concentration at the margins of bubbles The gas-liquid boundary of bubbles is a site of continuous liquid evaporation. As a result, organic acid, chloride, and sugar substances precipitate and accumulate along the bubble ring, resulting in the formation of a highly concentrated microzone that is highly corrosive. This microzone accelerates localised matrix etching. 3. The passive film experiences thermal stress fractures that alternate between dry and wet conditions. The periodic alternation of dry gas contact and moist liquid immersion on the tube surface is caused by the expansion and contraction of bubbles in response to temperature fluctuations. The dense microcracks are generated by the repetitive application of tensile and compressive stress to the brittle passive film, which in turn creates channels for the penetration of corrosive ions. ## 2. Primary factors contributing to bubble stagnation 1. The layout of the heating conduit installation is unreasonable. The most severe scenario of bubble accumulation involves the horizontal installation of long titanium heating tubes. Bubbles ascend and accumulate on the top arc of the tube, rendering them incapable of escaping. Bubbles that rise above the assembly are also trapped by tubes that are installed too close to the tank top. 2. Inadequate medium circulation flow velocity The microbubbles produced by heating are not able to be removed by static or low-speed liquid flow. The scouring effect on the tube surface is lost when the flow velocity is less than 0.6 m/s. 3. Surface power density that is excessive The liquid flow's capacity to transport bubbles is significantly exceeded by the high heat load, which instantly generates a large number of small vapour bubbles, resulting in cumulative bubble coverage. 4. Low-pressure static operation in a sealed vessel In the absence of continuous aeration circulation, dissolved gas is unable to be discharged from the system, and new bubbles are continuously separated during heating. 5. Bubble capture blind areas are formed by recessed weld reinforcement and gaps between PTFE supports and tube walls, which are local dead zones. These areas are where small bubbles accumulate and merge into large persistent air pockets. ## 3. Optimisation of structural design to prevent bubble retention 1. Enhance the inclination of the heating conduit installation. Titanium heating tubes must be installed with a 10℃–15℃ upward inclination from the flange end to the free end; horizontal arrangement is prohibited. Bubbles ascend to the liquid surface of the tank for discharge by sliding along the inclined tube surface to the free end. The vertical tube layout is the optimal choice for small-volume containers, as it virtually eliminates bubble accumulation. 2. Widen the distance between the tank top and tube bundles. The vertical distance between the top of the heating tube group and the minimum operating liquid level must not be less than 300 mm, ensuring that there is sufficient space for bubble upward escape. 3. The weld and support contact surfaces are polished to a smooth finish. Grind all weld seams to a smooth arc transition without protruding reinforcement; choose PTFE supports with a wide-surface arc shape to prevent concave gaps that capture bubbles. 4. Install flow guide baffles around the heating tubing. To create a spiral scouring liquid flow around the tube wall, install diversion plates at the upstream side of the tube bundle. This will continuously remove attached microbubbles. ## 4. Standardised operation control to minimise bubble generation and retention 1. Regulate a reasonable surface power density In order to reduce the quantity of heating-separated microbubbles, the power density of sealed fermentation tanks is reduced by 20% in comparison to open tanks, as per the high-load low heat load design standard. 2. Maintain a medium circulating flow velocity of at least 1.0 m/s. Before heating, activate the circulating pump to ensure that a stable liquid scouring is formed; it is important to avoid a single static heating without circulation. 3. Continuous slight aeration during cultivation at a constant temperature The promotion of overall liquid mixing, the discharge of supersaturated dissolved gas from the tank, and the reduction of new bubble precipitation on the tube wall are all facilitated by trace compressed air aeration. 4. Operation of gradient temperature rise To prevent the separation of instantaneous mass bubbles as a result of a sudden temperature increase, it is recommended that the heating process be conducted at a rate of <=3℃/min during the startup phase. 5. Consistent deflation of the tank during maintenance To reduce dissolved gas saturation in the medium and discharge accumulated gas in the upper part of the tank, the tank exhaust valve should be opened for 10-15 minutes every three days. ## 5. Bubble stagnation corrosion inspection and judgement technique Upon completion of the tank emptying maintenance, utilise a bright light to examine the tube surface: - Typical feature: A dark grey pitting corrosion zone in the shape of a ring, distributed on the upper semicircle of horizontal tubes, in accordance with the long-term bubble coverage track. - Electrochemical potential test: The potential of the bubble coverage area is 80–120 mV lower than the lower liquid immersion area of the same tube, suggesting severe passive film damage. - Ultrasonic thickness measurement: The upper arc wall thickness loss is substantially greater than that of the tube's bottom.|Hazard Source of Bubble Stagnation|Targeted Elimination Measure|Core Effect|| ----|----|----|| Horizontal tube layout trapping bubbles|Install tubes with a 10℃–15℃ upward inclination or vertical layout|Bubbles slide and escape without long-term surface attachment|Low liquid flow is unable to strip microbubbles|Maintain a circulation flow of 1.0 m/s or higher and install flow guide baffles.|Continuous liquid scouring eliminates newly generated tiny bubbles.|High power density mass bubble precipitation.|Reduce surface power density by 20% for sealed tanks.|Suppress instantaneous large-scale bubble separation.|Static sealed tank supersaturated dissolved gas.|Continuous micro-aeration + regular tank exhaust.|Reduce dissolved gas content to reduce bubble generation.|Weld/support gap bubbl Gas-liquid two-phase oxygen-deficient microzones are formed by stagnant air pockets on titanium heating tubes, which results in self-galvanic localised pitting corrosion. In order to prevent the long-term attachment of bubbles to the tube wall, the fundamental solution comprises an inclined tube structural layout, high-velocity circulating scouring, low heat load design, and continuous aeration operation. The timely detection of early bubble corrosion defects and the prevention of through-wall leakage failure of titanium heating tubes can be achieved through regular visual inspection and potential testing during maintenance.