Caustic Soda Jumbo Bag Feed Systems in Low Temperature Chlorine Absorption Service

In low-temperature chlorine absorption duty, solid sodium hydroxide make-up from 500–1000 kg flexible intermediate bulk containers is a process interface rather than a warehousing task, because the hygroscopic solid, the chilled scrubbing liquor, and the exothermic dissolution step interact directly with absorber pH control. The discharge station for caustic soda flakes or prills must be designed as a closed system: a UN-certified FIBC is suspended by a jib crane over a sealing spout, the bag outlet is clamped to a glovebox entry, and a slitter blade opens the liner before a twin-screw conveyor transfers the solid to a loss-in-weight hopper. The hopper is mounted on three load cells so that the screw feeder speed can be adjusted against the chlorine mass flow entering the absorption column. In this service, the chlorine feed is often saturated with moisture and enters the absorber at 5–12 °C, while the circulating caustic solution is held at 10–15 °C to prevent sodium chlorate formation. The feed system therefore operates inside a cold enclosure where the solid caustic surface can fall below the ambient dew point, causing condensation. This condensation dissolves surface alkali and promotes a sodium carbonate hydrate crust that changes the flow function of the bulk solid and leads to bridging. The design of the unloading equipment is therefore governed by the need to exclude ambient moisture, to control the exotherm of dissolution, and to deliver an uninterrupted mass flow of solid NaOH into the chilled absorption loop without allowing free chlorine breakthrough or product degradation.

What Causes Discharge Bridging in Caustic Soda Jumbo Bag Stations Below 10°C?

Bridging in solid NaOH discharge at low temperature is caused by particle surface moisture adsorption, crystal bridging, and cohesive arching. Sodium hydroxide flakes and prills are hygroscopic and, when cooled below the enclosure dew point, form a surface film of concentrated sodium hydroxide solution. The film absorbs carbon dioxide from the purge air and precipitates sodium carbonate or sodium bicarbonate hydrates, which act as solid bridges between particles. Bulk flow properties must therefore be measured by shear cell testing per ASTM D6128-16, using the Jenike shear cell or a translational shear cell. The design data required include the cohesive strength as a function of consolidation stress, the flow function coefficient, and the angle of internal friction. For mass flow, the hopper cone half-angle is typically specified at ≥70° from vertical for NaOH flakes, with an outlet diameter not less than 300 mm; for prills, the outlet may be reduced to 200 mm if the surface is kept dry. The discharge chamber should be purged with dried, filtered air at a dew point of ≤-20 °C and a temperature slightly above the hopper metal temperature. Vibratory bin activators or inflatable liners are used only during the discharge cycle, not continuously, because prolonged vibration can consolidate the bed and worsen arching. A breaker bar located above the screw conveyor is required for flake service to fragment lumps formed by surface caking. The screw conveyor is configured with a decreasing pitch from the intake to the discharge flange to provide even drawdown over the full hopper outlet. Published test data for caustic soda flake flow at hopper temperatures below 0 °C is limited; therefore, site-specific shear cell tests and a pilot discharge trial are necessary before final hopper geometry is fixed.

Dissolution of the solid sodium hydroxide in the make-up tank releases approximately 44.5 kJ/mol, which must be removed from the chilled caustic loop to maintain the absorption liquor below 15 °C. The dissolving tank is usually a vertical cylindrical vessel with a conical bottom and an external recirculation loop. Solid caustic enters the tank through a rotary valve positioned above the liquor surface, but the preferred method for this service is an eductor or wetting cone that contacts the solid with recirculated dilute NaOH before the crystals fall into the bulk liquid. The eductor motive flow is taken from the absorber recycle pump, and the dissolving chamber is designed for a superficial velocity of 1.0–1.5 m/s in the outlet nozzle to prevent settling of undissolved flakes. Chilled water at 3–5 °C passes through a plate-and-frame heat exchanger downstream of the dissolver, and the cooled liquor is returned to the absorber suction header at 8–12 °C. The feeder is controlled by a cascade loop: a loss-in-weight signal from the hopper load cells is compared with the chlorine mass flow entering the absorber, and a pH trim adjusts the screw speed to maintain a scrubber liquor pH of 10.0–11.5. The free alkali in the sodium hypochlorite product is periodically verified by titration per EN 901, and the make-up rate is corrected for the small caustic consumption caused by carbon dioxide absorption from the purge air. A mismatch between the density and conductivity loops triggers an alarm and stops the screw feeder before the scrubber pH can swing outside the specified corridor.

Exothermic Dissolution Enthalpy Versus Hypochlorite Stability in the Chilled Scrubbing Loop

Because the scrubbing reaction consumes two moles of NaOH per mole of Cl₂, the make-up rate is directly proportional to the chlorine loading, but the dissolution enthalpy can locally raise the liquor temperature above the chlorate formation threshold if the feeder overshoots. Sodium hypochlorite decomposes by two competing routes: at pH below 9.5 or above 12.0, and at temperatures above 20 °C, the hypochlorite ion disproportionates to chlorate and chloride; the low-temperature absorber is specifically operated at 10–15 °C to suppress this route. The alkali feed system is therefore not only a solids handling problem but also a thermal regulation problem. A temporary loss of feed accuracy, caused by bridging or feeder dead zones, can reduce free NaOH concentration and allow acidic chlorine hydrolysis products to corrode the absorber internals. Conversely, an uncontrolled surge of solid caustic into the dissolver can raise the liquor pH above 12.0 and increase carryover of free NaOH into the product. The main equipment conflict is that the heat exchanger must simultaneously protect the scrubber from thermal runaway and avoid precipitation of sodium chloride crystals, which form when a high-strength hypochlorite solution is cooled below the sodium chloride liquidus. Wetted parts in the recirculation loop are specified as titanium Grade 2, PTFE-lined carbon steel, or PVDF; 316L stainless steel is limited to ambient or lower temperatures and must be fully drained and flushed to avoid crevice corrosion in chlorinated brine. The use of carbon steel is avoided because chlorine gas and hypochlorous acid cause rapid pitting. The dissolver vessel is sized on the basis of a minimum residence time of 30 min at the maximum caustic demand corresponding to full chlorine load, and the recirculation pump is specified for 10–15 times the make-up flow to ensure complete dissolution without localised hot spots.

At bag changeover, the dust extraction system and the operator access doors are interlocked so that the enclosure is purged with dry air at a dew point below -20 °C before the spout is untied. Sodium hydroxide occupational exposure limits are regulated by OSHA 29 CFR 1910.1000 at 2 mg/m³; the enclosure is maintained under negative pressure with a dust collector rated for corrosive dust and an alarm on the filter differential pressure. The slitting operation is the highest-risk manual task, because residual flakes can be ejected by stored stress in the FIBC liner and because the bag surfaces may carry trace sodium carbonate from contact with humid air. The station includes a polyethylene liner cutter mounted inside the glovebox, a bag spout stretching mechanism, and a washdown hose for spills. A jumbo bag lifting frame is required to prevent operators from cutting the bag while the bag is still connected to the hoist. The floor under the station is sealed and connected to a caustic-resistant drainage system, because sodium hydroxide solution spilled during bag changeover attacks concrete and can create a slip hazard. The discharge enclosure is fabricated from polypropylene panels with 316L stainless steel supports; carbon steel is not acceptable for wet caustic dust protection, though it may be used for the exterior structural frame if coated and inspected per ISO 12944. The feed system also includes an automatic flush valve that rinses the eductor and the dissolver level sensor with softened water at the end of each bag discharge cycle, because concentrated caustic left in dead legs can solidify when the plant is shut down in winter.

When Dilution Water Sources Switch to High-Hardness Groundwater Without Softening

If the make-up water for the caustic dissolving tank is changed from softened municipal water to raw borehole water containing calcium and magnesium bicarbonate, the exothermic dissolution front promotes conversion of bicarbonate to carbonate and precipitates calcium carbonate and magnesium hydroxide as scale on the eductor throat, heat exchanger plates, and absorber packing. The scale layer reduces heat transfer and increases pressure drop until the scrubber temperature exceeds the chlorate formation threshold, or until the packed bed requires washing with inhibited acid. To avoid this, the dilution water should be softened to less than 5 mg/L total hardness as CaCO₃ before entering the dissolver, or a side-stream scale inhibitor should be dosed with a corrosion inhibitor compatible with hypochlorite. The dissolution tank level and conductivity probes must also be redundant, because scale blinding of a single conductivity probe can cause over-concentrated caustic to enter the chilled scrubber and freeze in static tees at 12–15 °C for 50 wt% caustic. For this reason, the caustic storage and dosing lines are heat-traced only where 50 wt% liquid caustic is used; the solid jumbo bag feed system is located in an unheated but moisture-controlled area, and its transport piping to the dissolver is as short as practicable. The use of softened water also prevents precipitation of calcium carbonate in the absorber packing, which would otherwise reduce the available surface area for chlorine absorption and increase sodium hydroxide consumption.

Operational experience from chlor-alkali and drinking water installations shows that the highest unplanned downtime in caustic feed systems is associated not with chemical metering but with mechanical failure of flexible connectors and with liner fragments entering the screw conveyor. Jumbo bag liners made from low-density polyethylene are often specified to be removed entirely before spout connection, but when the station is exposed to temperatures below 5 °C, the liner becomes brittle and tears during withdrawal. The torn fragments can wrap around the feed screw shaft and trip the drive. To mitigate this, the unloading hopper is provided with a knife gate, a clear view port, and a hopper throat safety screen with 50 mm mesh; the screw conveyor drive is protected by a shear pin and a zero-speed switch. The feed screw is fabricated from 316L stainless steel or better, with polished flights and an abrasion-resistant coating at the discharge end. The flexible connection between the screw discharge and the dissolver eductor must be a lined braided hose with an electrically continuous design, because static charge can accumulate on the moving bulk solid even though sodium hydroxide itself is not combustible. The entire feed system is included in the plant preventive maintenance schedule for inspection of load cell calibration, knife gate seat leakage, and screw flight clearance at intervals not exceeding 6 months. A specific incompatibility in this service is the combination of solid caustic feed with acid-based cleaning systems. Acid washing of the absorption column or the dissolver must never be performed while the jumbo bag feed hopper is connected, because acidic mist can be drawn into the dry solids handling enclosure and cause rapid exothermic neutralisation at the product contact surfaces. The bag discharge station should be isolated by a lockable knife gate and a blind during acid cleaning. Similarly, the addition of ammonium-based or amine-based corrosion inhibitors to the scrubbing loop is inappropriate because hypochlorite reacts with ammonia to form chloramines, which can off-gas from the cold scrubber and pose a toxic exposure risk. If freeze protection is required for the instrument sensing lines, low-conductivity heat-transfer fluid is used rather than glycol containing inhibitor packages that may degrade hypochlorite. These operational boundaries are stated because failure to isolate the solid feed system from cleaning and maintenance activities has led to localised corrosion and operator exposure incidents in production-scale equipment.