From Bag Price to Cost per Kilo of Water Absorbed: A Four-Line Model

Two quotes arrive on the same day. One is 2,080 dollars a tonne for a 30/60 mesh grade at 305 g/g in deionised water. The other is 2,410 dollars a tonne for a 20/40 mesh grade at 280 g/g. The instinct is to take the cheaper one. Run the numbers on the stream you actually have, and the expensive quote is 19 percent cheaper per kilo of water bound, because the coarse grade holds 120 g/g in your 1.8 percent brine against 74 for the fine one.

Bag price is the least useful number in an SAP purchase. This article builds a four-line model that turns any quote into a cost per kilo of water actually absorbed, and shows where each line comes from.

Line One: Divide by the Absorption Figure for Your Liquor

Start with the absorption number measured in the stream you will dose, not the datasheet figure. If you do not have it, the quick estimate is to take the DI value and multiply by 0.55 for a 1 percent salt stream, 0.35 for 3 percent, and 0.18 for a stream with 500 mg/L calcium. These are rough, but they are far closer than using 300 g/g for everything.

Then the cost per kilo of water is the price per tonne divided by the absorption in grams per gram, divided by 1,000. At 2,080 dollars and 74 g/g, that is 28.10 dollars per tonne of water. At 2,410 dollars and 120 g/g, it is 20.08 dollars. The gap is the whole argument.

Line Two: Moisture, the Six Percent You Paid For

Most polyacrylate granules ship at 4 to 8 percent volatile content. That water is sold at polymer prices. On a 20 tonne order at 6 percent, you have bought 1.2 tonnes of water for 2,496 dollars. Correct the price to a dry basis before comparing: divide the price by one minus the moisture fraction. A 2,080 dollar product at 8 percent moisture is 2,261 dollars dry; a 2,150 dollar product at 3 percent is 2,216 dollars dry, and the ranking flips.

Ask for the volatile content on the certificate, and if it is not there, ask why. A supplier who does not measure it either does not dry the product consistently or does not want you to know.

Line Three: Mesh Distribution and What the Coarse Fraction Is Worth

A 30/60 mesh cut and a 100/200 mesh cut are different products at similar prices, and the mesh distribution inside a bag varies by 10 to 15 percent between suppliers. Coarse granules absorb more slowly but hold more under load and generate less dust; fine granules are faster but lose capacity in a stacked or pumped system.

Value the difference in your own process. If fines cost you 40 minutes of line time a week and 3 percent product loss, a 60 dollar per tonne premium for a tighter coarse cut is trivial. If you are dosing into a fast-dissolving sachet, the fine cut is worth paying for. The mesh premium should be judged against a process cost, never against another quote in isolation.

Line Four: Crosslinker Density and the Strength Premium

Higher crosslink density gives a firmer gel that resists shear and load, at the cost of 15 to 25 percent of free-swell capacity. For a spill kit or a waste solidification job where the gel is stacked, that trade is worth it. For a product where the gel is never touched, it is money wasted.

Match the grade to the duty and stop paying for strength you do not use. A simple test: press a 30 mm gel cube with a thumb. If it holds shape under firm pressure, the grade is high-crosslink. If it squashes to a paste, it is low. Buy the one your process needs, and put the saving into a tighter mesh specification instead.

Run all four lines on a single spreadsheet and you will usually find that the cheapest bag is mid-table and the best value is one of the middle quotes. The SAP granule product data gives the moisture, mesh and brine figures needed to fill the sheet.