PP-Lined Static Mixer

Product Overview

The ywmixing PP-lined static mixer fills the temperature gap between the PVC-lined and PTFE-lined versions. The construction is the same — a carbon-steel pressure shell with a vented annular gap and a corrosion-resistant polymer liner and elements — but the liner material is polypropylene (PP), which raises the continuous service temperature from 60°C (PVC) to 95°C. This 35°C difference opens up several important services that the PVC-lined version cannot cover, including hot water-treatment chemistry, warm flue-gas desulfurization, and most fertilizer-process applications.

PP is the second-most-common liner material in the lined-mixer family after PTFE. It has a wider chemical-resistance envelope than PVC (notably better against organic solvents and oxidizers) and a significantly lower cost than PTFE (typically 1/5 to 1/8 the cost per kg). The trade-off is the 95°C temperature ceiling and the limited resistance to concentrated oxidizing acids. Where the chemistry is hot and oxidizing — for example, hot 70% HNO₃ — the PTFE-lined mixer is still required.

Internally, the mixer uses a left-right helical element set of solid PP rods, in 4 to 8 elements depending on the required coefficient of variation. Standard element twist is 180°; 270° twist is available for higher-viscosity media up to 1,500 cP. The L/D ratio of the housing is 6-12, with the higher end used for tight-coefficient applications such as pH adjustment in the presence of slow-reacting acids. End connections are PP-faced flanges (ANSI 150, DIN PN10, JIS 10K) with EPDM gaskets; FKM (Viton) is available for mildly aggressive organic service.

Material Properties

Polypropylene (PP) is a semi-crystalline thermoplastic polymer of the polyolefin family. Its chemical resistance is broad: it resists all dilute mineral acids (HCl, H₂SO₄, H₃PO₄, HNO₃ up to 30%), all organic acids (acetic, citric, lactic, formic), all alkalis (NaOH, KOH, NH₄OH), all salt solutions, and many organic solvents (alcohols, glycols, ketones, esters — at ambient temperature). The temperature limit is set by the heat-distortion temperature of 100°C at 0.45 MPa, which limits the continuous service to 95°C. Short-term excursions to 105°C are tolerable; sustained service above 100°C will cause creep and eventual distortion.

The mechanical properties of PP are good for a liner material. Tensile strength is 30-35 MPa, density 0.91 g/cm³ (the lowest of all common liner materials — PP liners float if the shell ever fills with water), and elastic modulus 1.3-1.8 GPa. The thermal expansion coefficient is high (100-150 × 10⁻⁶/°C, roughly 8× that of carbon steel), so the liner must be designed to accommodate differential expansion — typically a 2-3 mm radial gap, vented to atmosphere.

PP has several known chemical limitations. It is NOT suitable for concentrated sulfuric acid above 70% (the acid attacks the polymer), concentrated nitric acid above 30% (oxidative attack), aromatic and chlorinated hydrocarbons (swelling and dissolution), strong oxidizers (concentrated H₂O₂, ozone, fuming nitric acid), or any service above 95°C. For these services, the PTFE-lined or all-PVDF mixer must be used. PP is also unsuitable for service that sees UV exposure for extended periods — the polymer will degrade and crack. Black-pigmented PP is available for outdoor service.

PP offers one important advantage over PVC: it does not contain chlorine in the polymer backbone, so it does not release HCl when incinerated. This makes PP the preferred liner for pharmaceutical and food-processing applications where end-of-life disposal is a concern. PP is also FDA-compliant for food contact (21 CFR 177.1520) and is accepted by the European Food Safety Authority (EFSA) for food-contact materials.

Construction Design

The construction of the PP-lined mixer is the same three-layer design as the PVC- and PTFE-lined versions. The outer layer is the carbon-steel (or 316L stainless) pressure shell, hydrostatically tested to 1.5× design pressure before liner installation. The middle layer is a 2-3 mm vented annular gap with a tell-tale port for leak detection. The inner layer is the PP liner, a 4-6 mm thick tube extruded or sheet-formed into a cylindrical shape and locked at the flanges by trapezoidal spacer rings.

The PP liner is attached using a flared-end construction, similar to the PVC-lined version. The ends of the PP tube are belled outward against a steel backing flange, then clamped in place by a steel retaining ring. This construction is mechanically robust and is field-proven in lined-piping service. The vented annular gap accommodates the high thermal expansion of PP (100-150 × 10⁻⁶/°C, roughly 8× that of carbon steel) and prevents liner buckling during heat-up.

Internal elements are solid PP rods, twisted 180° (default) or 270° (option for higher viscosity) in alternating pairs. Elements are mechanically fastened to the liner at each end using PP element holders that are hot-plate welded to the liner. The 4-8 element set is selected based on the target σX, with 4 elements sufficient for simple mixing duty (σX ≤ 5-7%) and 6-8 elements required for tight-coefficient applications (σX ≤ 1-3%).

End connections are PP-faced steel flanges with EPDM gaskets. The PP face protects the gasket from the process fluid and provides a smooth, crevice-free transition. EPDM is the standard gasket material for water and dilute chemistry; FKM (Viton) is available for higher-temperature or mildly aggressive organic service. Bolting is galvanized carbon steel or 316L stainless; PTFE washers prevent galling and ensure even gasket loading.