Propylene oxide (PO; CAS 75-56-9) is a liquid intermediate purchased under long-term agreements to support continuous alkoxylation, propylene glycol, and glycol ether production. The product is a colourless, low-boiling liquid with a boiling point of 34°C at 101.3 kPa, a closed-cup flash point of -37°C, and a vapour pressure of 59.6 kPa at 20°C. Because the molecule is flammable, reactive, and prone to acid- or base-catalysed oligomerisation, the purchase contract controls more than price: it fixes nitrogen padding, oxygen content, water content, and analytical release procedures. The term sheet often uses the title Propylene Oxide Long-Term Supply Agreement: Price Mechanism & Volume Tier Table to identify the two most contested commercial instruments: the quarterly pricing formula and the annual volume rebate schedule.

Propylene Oxide Long-Term Supply Agreement: Price Mechanism & Volume Tier Table

The price mechanism in a PO long-term supply agreement normally uses a feedstock-linked formula rather than a fixed price. A representative contract price is calculated as:

P_PO = (C3H6_monthly × F) + E + A + M − R_t

In this structure, F is the propylene factor. In publicly available contract descriptions for chlorohydrin-based PO, F falls between 0.78 and 0.82 kg propylene per kg PO; for hydrogen peroxide-to-propylene oxide production, the factor is lower because peroxide and solvent recovery are treated as fixed per-tonne adders. The energy term E is typically indexed to a natural gas or power marker; a representative range is 0.12–0.18 MMBtu/kg. The adder A covers logistics, analytical release, isotank or terminal handling, and drum depreciation. The margin M is often a negotiated constant or a contract-year step change. The rebate R_t is derived from the volume tier table.

Index sources are limited in the term sheet to a named publication such as ICIS, Argus, or OPIS. The averaging period is usually the last 3 monthly contract settlements before the quarter of delivery. A price collar of ±18% per contract year and a dead band of €15–25/mt are common in European contracts; outside the dead band the formula moves in full. For a buyer converting PO to polyether polyols, the fixed margin component is sometimes split into a fixed adder and a co-product credit if propylene glycol or polyglycols are returned to the supplier. If the buyer requests a lower aldehyde specification than the seller’s standard release, a premium of $4–6/mt is applied. These values are illustrative and vary with terminal location, shipment size, and co-product handling.

Which Feedstock Marker Arrangements Survive a PO Margin Squeeze?

Margin squeeze occurs when propylene and energy prices rise faster than downstream polyol spot prices. In this condition the propylene factor is the dominant variable. Buyers with high operating rates may prefer a wider dead band and a lower collar; suppliers with fixed chlor-alkali costs may instead demand a pass-through for chlorine and caustic soda co-products. A chlorohydrin plant produces approximately 2.0–2.2 metric tons of calcium chloride or salt effluent per ton of PO, depending on brine recycling; therefore the non-propylene adder is not small. Hydrogen peroxide-based production economics are sensitive to hydrogen peroxide and methanol or solvent recovery. In practice, the feedstock marker that survives a margin squeeze is the one that both sides can observe without manipulation: a published monthly propylene contract price with a named delivery basis, e.g., CIF NWE or FD USG, and a clearly defined freight differential. The term sheet should define fallback to a second publication if the first index is discontinued. A supplier may accept an index collar of ±12% for an annual take-or-pay volume of at least 40,000 mt; below that threshold the collar widens to ±18%.

At a continuous polyether polyol train with a nameplate capacity of 120,000 mt/year, PO feed rates approach 8.5–10.5 mt/h during normal operation. The alkoxylation reactor is typically a 30 m³ stainless steel vessel with an agitator power input of 75–110 kW, maintaining a temperature of 115–125°C and a pressure of 3–5 barg. Heat removal from the ring-opening reaction is approximately 1,600 kJ/kg PO, requiring a pump-around cooler with a temperature differential of 8–12°C. In this operating window, unreacted PO concentration in the headspace is held below 10,000 ppmv to avoid runaway acceleration. The buyer’s annual PO offtake for such a train is likely to fall in the 40,000–74,999 mt tier. The table below sets out the commercial logic of tier discounts and retroactive rebates. It is not a supplier quotation; it is a representative structure used in contract negotiations.

If actual offtake crosses a tier boundary, the rebate applies to all volume in that contract year, not only the marginal volume. The tier table is often renegotiated every 3 years with a volume ring range of ±10%. A too-steep top tier encourages oversupply and above-ground inventory risks; a flat rebate curve leaves no incentive for strategic offtake.

When Propylene Glycol Hydration Runs at 190°C and 16 barg

Direct hydration of PO to monopropylene glycol is carried out with a water-to-PO mole ratio between 12:1 and 20:1 to limit dipropylene glycol and tripropylene glycol formation. A continuous reactor at 180–200°C and 12–16 barg achieves high conversion, but the consecutive kinetics require a narrow residence time distribution. At a water-to-PO ratio of 15:1 and a temperature of 190°C, the reactor effluent typically contains 88–92 mol% monopropylene glycol on a water-free basis, with the balance split between dipropylene glycol and tripropylene glycol. The adiabatic temperature rise is roughly 70°C, so a quench cooler is located after each reaction zone.

PO quality affects this process: water above 100 mg/kg or acidity above 20 mg/kg as acetic acid reduces selectivity and increases coloured impurities. Purchase specifications for hydration service therefore add a narrower water release limit of ≤50 mg/kg and a total aldehyde limit of ≤30 mg/kg, with analysis by ASTM E203 and ASTM D1613 respectively. A terminal that cannot meet the narrower limit may be downgraded from hydration service to polyol service, which has a wider aldehyde tolerance.

Vapor Pressure, Stabilizer Chemistry, and ISO Tank Requirements

PO is stored in carbon steel tanks designed for a working pressure of 4 barg and tested at 6 barg. The blanketing gas is nitrogen with an oxygen content below 5% v/v. The lower and upper flammability limits of PO in air are 2.3% and 37% v/v, respectively, so headspace monitoring is continuous. A nitrogen pad of 0.5 barg is maintained during loading and unloading; dry-disconnect couplings and vapour return lines prevent loss. Flame arresters conform to ISO 16852:2016. Road tanker and isotank unloading equipment is selected under EN 14432:2014 for liquid chemical product discharge valves. Wetted pump seals are stainless steel or PTFE; copper alloys are excluded from PO service because metal-catalysed polymer formation can occur. Since PO has a low autoignition temperature of approximately 430°C, local exhaust ventilation is installed at drumming stations and sampling points. Commercial PO is usually supplied without a radical inhibitor; instead, water and acidity are controlled because protic acids and alkalis catalyse ring-opening oligomerisation. This is why the purchase term sheet treats moisture and acidity as release criteria rather than as ordinary contaminants.

Each lot is released against a certificate of analysis. Density at 20°C is measured by ASTM D4052 and is expected at 0.830–0.832 g/cm³; water is determined by ASTM E203; acidity as acetic acid by ASTM D1613; colour by ASTM D1209; and distillation by ASTM D1078. A typical distillation range is 33.5–35.0°C at 760 mmHg. In a production-scale batch, a water deviation of 20 mg/kg above the cap may not alter the initial polyether discharge, but it can produce a detectable shift in the reaction induction period and reduce the reactor cooling margin. Consequently, the receiving tank at a polyol plant is sampled at three levels: top, middle, and bottom. If the bottom sample exceeds the top sample by more than 30 mg/kg water, the tank is recirculated through a nitrogen-stripping loop and re-sampled before use. Lot retention is 24 months, and the laboratory retains a small headspace vial for aldehyde trend review.

Retroactive rebates are settled on the anniversary date of the contract. If the buyer lifts 18,000 mt in a higher tier than the annual target of 15,000 mt, the higher-tier rebate is applied to all invoices issued during the contract year. The supplier issues a credit note within 30 days after the independent surveyor verifies the annual cumulative offtake. If actual offtake falls below the take-or-pay threshold, the shortfall is charged at 35% of the formula price. A volume ring of ±10% is often attached to the nomination; inside the ring there is no penalty; outside the ring the supplier may require a rescheduling premium.