Rheological characterisation of sodium polyacrylate (NaPA) slurries intended for spray-tower detergent drying is performed using controlled-stress or controlled-rate rotational rheometers equipped with coaxial cylinder geometries conforming to ISO 3219:2021 and DIN 53019-1. The relevant shear-rate window for process diagnostics spans from 0.01 s⁻¹ in storage to 10⁵ s⁻¹ at the nozzle orifice, and the viscosity response across this window controls pump sizing, line pressure, atomisation, and tower wall fouling. Sodium polyacrylate of molecular weight 4,500–15,000 g/mol is typically supplied as a 40–45 wt% aqueous solution at pH 7.0–8.5; when incorporated into a detergent slurry at 0.5–2.0 wt% dry basis, it increases low-shear viscosity through chain expansion and polyelectrolyte electroviscous effects. A model detergent slurry at 65 wt% total solids and 60 °C may exhibit apparent viscosities of 4,000–9,000 mPa·s at 2 s⁻¹ and 150–350 mPa·s at 1,000 s⁻¹ when the slurry contains 1.0 wt% NaPA of 8,000 g/mol; published data for commercial detergent compositions are frequently limited because exact formulations are proprietary, but the upper and lower bounds are derived from aqueous NaPA solution technical bulletins and from slurry rheometer screening tests under non-disclosure agreements. The flow curve is not adequately described by a Newtonian constant and is instead fitted with the Ostwald-de Waele power law τ = Kγ̇ⁿ, where n commonly falls from 0.55 at 0.5 wt% NaPA to 0.35 at 2.0 wt% NaPA, K increases with concentration and molecular weight, and model validity is confirmed by residual analysis over the 10⁰–10³ s⁻¹ range. A more complete description of the low-shear plateau and the high-shear solvent plateau is obtained from the Cross model, η = η_∞ + (η₀ − η_∞)/(1 + (λγ̇)^m), where η₀ is the zero-shear viscosity, η_∞ is the high-shear viscosity, λ is a characteristic relaxation time, and m is the shear-thinning exponent; both parameters shift with NaPA concentration, electrolyte content, and temperature. Temperature dependence follows an Arrhenius relation with an activation energy of 18–25 kJ/mol in aqueous NaPA solutions, which permits viscosity reduction by heating the slurry from 25 °C to 60–70 °C before pumping. For spray-tower operations, a heated and agitated make-up tank with a low-shear anchor impeller rotating at 10–25 rpm, a positive-displacement transfer pump, a jacketed ring main, and a high-pressure nozzle manifold constitute the basic equipment sequence; each unit is selected against the rheological profile at its characteristic shear rate.

What Limits the Upper Sodium Polyacrylate Addition Level in a Detergent Slurry Pumping Loop?

The upper addition level is determined by the interaction between low-shear viscosity in the feed tank and high-shear pressure drop in the recirculation line, not by an absolute chemical incompatibility. For a 65 wt% anionic surfactant slurry at 70 °C, the apparent viscosity at 1 s⁻¹ rises from approximately 3,000 mPa·s at 0.5 wt% NaPA to more than 12,000 mPa·s at 2.0 wt% NaPA; at 1,000 s⁻¹ the corresponding viscosity rise is from 120 mPa·s to 350 mPa·s. This means the low-shear mobility for tank pumping and the high-shear line resistance do not scale identically. Pressure drop in a circular pipe under laminar power-law flow is computed from ΔP = (4L/D)(2K((3n + 1)/(4n))ⁿ)(8V/D)ⁿ, where V is average velocity and D is internal diameter; an increase in K and a decrease in n both raise line pressure. In a 100 mm diameter recirculation line at a flow velocity of 1.5 m/s, a power-law slurry with K = 25 Pa·sⁿ and n = 0.45 generates a pressure drop near 1.2 bar per 10 m of straight pipe plus additional losses through elbows and filters; doubling K to 50 Pa·sⁿ raises the straight-pipe contribution to approximately 1.9 bar per 10 m. The practical upper addition level is therefore often observed between 1.5 wt% and 2.0 wt% dry-basis NaPA when the ring main is operated with lobe pumps because rotor slip increases as backpressure increases. Progressive cavity pumps tolerate higher viscosities but impose shear rates in the 10³–10⁴ s⁻¹ range in the discharge section; pump discharge pressure may exceed 12 bar if the slurry viscosity remains above 200 mPa·s at the pump shear rate. Packaged detergent plants with 25–50 m spray towers and high-pressure nozzle manifolds typically set an upper slurry viscosity of 8,000–15,000 mPa·s at 25 °C and 1 s⁻¹, measured by a Brookfield RVDV-II+ spindle 6 at 2 rpm, to avoid losing flow under start-up conditions. Published data for specific detergent-grade NaPA addition limits remain limited; however, supplier technical bulletins for 40 wt% NaPA solutions state that the solution itself reaches 800–3,000 mPa·s at 25 °C depending on molecular weight, and blending constraints are inferred from these values.

During single-fluid pressure atomisation through a hollow-cone nozzle, the slurry is first compressed by a high-pressure pump and then accelerated through a small orifice with a diameter typically between 1.8 mm and 3.2 mm. The apparent shear rate at the nozzle wall is estimated from γ̇_w = 4Q/(πR³); for a volumetric flow rate of 0.42 L/s through a 2.0 mm orifice, the nominal wall shear rate approaches 5.3 × 10⁵ s⁻¹. At these shear rates the apparent viscosity of a polyacrylate-thickened slurry approaches the high-shear plateau, but the pressure drop across the nozzle is also governed by the orifice discharge coefficient and the density of the slurry, ΔP = ρQ²/(2C_d²A²). Low-shear viscosity is therefore an unreliable direct predictor of atomisation pressure; instead, high-shear viscosity and extensional response control the energy required to form droplets. If the NaPA molecular weight exceeds roughly 20,000 g/mol or if the concentration is high, the extensional viscosity in the contraction upstream of the orifice becomes significant, delaying filament breakup and producing non-spherical droplets, satellite droplets, and occasional nozzle bearding. These effects are observed on commercial towers as lump formation on the drying chamber wall and as irregular powder fractions with oversized particles. Laser diffraction droplet size analysis of water and NaPA solutions during nozzle tests has shown that the Sauter mean diameter d₃₂ increases from 320 μm to 480 μm when the polyacrylate content shifts the feed from a shear-thinning but non-elastic fluid to a moderately elastic fluid at the same atomising pressure of 60 bar. Published data for full detergent slurry atomisation is limited, but the extension of capillary breakup tests and nozzle discharge coefficients from model fluids is used in nozzle manufacturer selection algorithms.

Extension-Thickening and Filament Breakage Behaviour in Concentrated NaPA Drops

Capillary breakup extensional rheometry applied to NaPA-thickened slurries quantifies the filament lifetime and the apparent extensional viscosity η_E = (Γ/(2R_mid))(dR_mid/dt)⁻¹, where Γ is surface tension and R_mid is the midpoint radius. For a 0.5 wt% NaPA solution of 8,000 g/mol, the filament breakup time in a 6 mm plate CaBER test is typically less than 5 ms; at 2.0 wt%, the breakup time can exceed 25 ms, indicating a substantial increase in elongational resistance. The Trouton ratio for Newtonian fluids remains at 3, whereas for polyelectrolyte solutions the ratio of extensional to shear viscosity can reach 10 to 100 at Hencky strains above 2. This has direct consequences for spray-tower nozzles because the orifice contraction generates high extensional strain rates on the order of 10³–10⁵ s⁻¹. A slurry with high extensional viscosity stores elastic energy during acceleration and releases it by forming long-lived filaments that shatter into satellite droplets after leaving the nozzle. Nozzle bearding, or the accumulation of material on the nozzle face, is commonly associated with such elastic filaments and is reduced by lowering NaPA molecular weight, reducing concentration, increasing slurry temperature, or using a nozzle with a sharper contraction angle. In production, the problem is monitored by recording nozzle pressure oscillations and by periodic inspection of the chamber wall. When satellite-droplet formation increases, the dried product displays a higher proportion of fine particles below 100 μm and a greater wall fouling rate. Some detergent spray towers compensate by reducing atomisation pressure from 70 bar to 50 bar, but this also increases mean droplet size and may require higher drying-air inlet temperature to maintain residual moisture below 8 wt%. Published quantitative causes of wall fouling under specific formulations are generally proprietary; however, nozzle vendor technical guides recommend maintaining the feed viscosity at the nozzle below 200 mPa·s at 10⁴ s⁻¹ to achieve acceptable hollow-cone spray formation.

When Sodium Polyacrylate Slurries Are Recirculated Through a Ring Main for More Than Six Hours

Time-dependent rheological behaviour becomes a process control issue when the slurry is held in a recirculation loop at elevated temperature because sodium polyacrylate solutions are susceptible to both mechanical scission and shear-induced changes in molecular configuration. In a ring main with a centrifugal booster pump, a heat exchanger, and a high-pressure pump, the slurry may pass through the high-shear zone every 5–10 minutes. Over a six-hour recirculation period, the apparent viscosity at 1 s⁻¹ can decrease by 15–30% from its initial value, while the high-shear viscosity decreases by less than 5%, indicating a preferential loss of low-shear structure rather than a complete destruction of the polyelectrolyte. This pattern is typical of shear-induced chain scission in high-molecular-weight fractions; chain scission occurs when the local shear stress exceeds the critical stress for covalent bond rupture, which is generally in the range of 0.1–1.0 nN per chain for carbon-carbon backbones. Lower-molecular-weight NaPA is less affected. For a low-molecular-weight 4,500 g/mol NaPA, recirculation can cause negligible irreversible viscosity loss; for a 15,000 g/mol grade, the same recirculation can reduce low-shear viscosity by 20–40% after eight hours. Oscillatory rheology can detect the change: the storage modulus G′ at 0.1 Hz decreases, the loss tangent increases, and the crossover frequency of G′ and G″ shifts to higher frequency as the molecular weight distribution narrows. Processing limits are therefore defined not only by initial rheology but by the recirculation time, shear rate, and temperature history. Published data for proprietary detergent slurries is limited; however, equipment-specific experience from lobe-pump recirculation loops indicates that holding the slurry at 60–70 °C for more than 6 h before spray drying should be avoided unless the NaPA grade is selected for shear stability. Microbial degradation is a separate risk in dilute NaPA storage tanks and transfer lines; at pH values below 6.5 and temperatures between 25 °C and 40 °C, microbial growth can reduce viscosity and generate organic acids, and formulations may require preservatives such as isothiazolinones when the slurry is held beyond 24 h. The slurry pH should be maintained above 7.5 to preserve full neutralisation and minimise polyacrylic acid precipitation.

Addition of sodium polyacrylate to detergent slurries containing zeolite 4A, sodium carbonate, sodium silicate, or layered silicates produces a structured yield stress that is not observed in solutions of NaPA alone. The negatively charged polymer adsorbs onto the positively charged edges and surfaces of zeolite particles, altering the zeta potential and producing a network of bridged or electrosterically stabilised particles. The resulting yield stress, measured by stress ramp with a vane rotor at 25 °C, commonly falls between 5 Pa and 30 Pa for a 65 wt% slurry containing 20 wt% zeolite 4A and 1.0 wt% NaPA; the exact value depends on the particle size distribution, with d₅₀ values between 3 μm and 8 μm, and on the electrolyte concentration. This yield stress prevents sedimentation in storage tanks for 48–72 h but can also create difficulty in draining tanks and cleaning lines. A thixotropic recovery profile is typical: the structure is broken down under shear and rebuilds over minutes, allowing tank agitation to maintain mobility while the low-shear structure recovers in the spray tower feed line. If the slurry is held in a dead leg without agitation, the yield stress may require line flushing with hot water at pressures above 5 bar to remove the deposit. In spray-tower operation, the most important consequence of the yield stress is that the feed pump must be sized for start-up after a shutdown; the breakaway pressure in a 100 mm line can exceed the normal operating pressure by 2–3 bar. Sodium carbonate and sodium silicate contribute additional electrolyte that compresses the electric double layer of the NaPA and may lower viscosity at high addition, while calcium ions from hard water bind to carboxylate groups and cause chain collapse, reducing hydrodynamic radius and viscosity but potentially forming insoluble calcium polyacrylate haze. Softened water with hardness below 15 mg/L as CaCO₃ is therefore recommended for slurry make-up, and the final slurry pH is adjusted with caustic soda to 8.0–8.5 before spray drying.

Values in the table represent laboratory model slurries at 65 wt% total solids, pH 8.2, 60 °C with 20 wt% zeolite 4A; proprietary detergent formulations may differ materially because the surfactant system, builder, and water hardness alter the electroviscous response.

Low-Shear Yield Stress and Sedimentation Control in Zeolite-Containing Detergent Slurries

Yield stress measurement in zeolite-containing detergent slurries is performed with a vane rotor geometry rather than a smooth cone-plate because wall slip at the measuring surface leads to underestimation of the true yield stress. In a stress ramp test, the vane is rotated at a constant shear rate of 0.001 s⁻¹ and the stress at the transition from solid-like to liquid-like response is recorded; reproducible values require a minimum waiting time of 60 s after loading and temperature control at 25 °C or 60 °C. The measured yield stress of the slurry follows a power-law dependence on solids volume fraction and NaPA concentration, with a fitted exponent of 2.2–3.0 for solids fraction and 0.6–0.9 for NaPA concentration in the 0.5–2.0 wt% range. Sedimentation control is quantified in a graduated cylinder with a height of 30 cm at 25 °C: a slurry with yield stress above 5 Pa shows no visible supernatant after 24 h, while a yield stress above 15 Pa prevents settled-layer formation after 72 h. In a spray-tower feed tank, the bottom settled layer must be re-suspended before transfer; a low-shear anchor agitator at 10–25 rpm is insufficient for full re-suspension when the yield stress exceeds 25 Pa, and a baffled tank with a pitched-blade turbine operating at 25–50 rpm is required. The use of a yield stress above 30 Pa is generally avoided because pump priming and line clearing become difficult and because the high low-shear viscosity increases the energy required to maintain recirculation. If the formulation requires a high yield stress for product stability but the spray tower requires low high-shear viscosity for atomisation, the acceptable NaPA grade is selected by plotting the viscosity at 1 s⁻¹ against the viscosity at 10⁴ s⁻¹; a grade with high molecular weight may provide the highest yield stress but also the greatest extensional resistance, while a low-molecular-weight grade may provide adequate yield stress only at high addition. This trade-off is resolved by using a medium-molecular-weight NaPA of 6,000–10,000 g/mol at 1.0–1.5 wt% for many detergent formulations. The rheological measurements are performed according to ISO 3219:2021 for rotational viscometry, while the yield stress test is reported following the vane method described in DIN 53019-1 or ASTM D2196-20. These standards do not specify the exact formulation but provide the measurement geometry and conditioning required for interlaboratory reproducibility.

Online process control of detergent slurry rheology in spray towers is performed by sampling from the recirculation line and measuring viscosity at a single shear rate, with a correction for temperature and total solids. The measured viscosity is compared with a control band defined during plant trials; if the viscosity at 60 °C exceeds 15,000 mPa·s at 2 s⁻¹, the addition of recycled oversize powder from screening is reduced because oversize material contains partially hydrated zeolite and polymer fines that increase viscosity. Conversely, if the viscosity falls below 5,000 mPa·s at 2 s⁻¹, polymer dosing is increased to prevent sedimentation. The atomisation pressure required to maintain a constant droplet size is adjusted based on feed viscosity: a high-shear viscosity increase from 150 mPa·s to 250 mPa·s at 1,000 s⁻¹ may require an additional 5–10 bar of atomising pressure. Drying-air inlet temperature is usually held between 180 °C and 250 °C, and the outlet temperature is controlled between 75 °C and 95 °C to achieve a moisture content below 8 wt%. The residual moisture of the powder is measured by infrared balance or Karl Fischer titration according to ISO 760:1978. Oversized particles are removed by a 1.0–2.0 mm sieve and may be milled or recycled into the slurry; recycled material alters the particle size distribution and therefore the yield stress of the feed. The final powder properties such as bulk density and flowability are governed by droplet size and drying rate, which in turn are coupled to the rheology of the slurry. A formulation with excessive NaPA demand may require dilution with water to reduce viscosity, but this increases drying energy consumption. The operational boundary for sodium polyacrylate in detergent spray-tower feed is therefore expressed as a viscosity window at defined shear rates rather than a fixed chemical addition level.