Gas Purity vs Recovery: Two Different Measures

A gas separator can produce a very pure stream while losing a substantial share of the valuable gas elsewhere. It can also collect most of the target gas in a product that contains too many impurities for its intended use. Purity and recovery answer different questions, so a useful specification states both and defines where they are measured.

This distinction applies to helium purification and to the concentration and purification stages used for atmospheric rare gases. The starting material matters: a crude concentrate and a finished cylinder are different points in a supply chain. Our guide to where neon, krypton and xenon come from explains that context.

Two percentages, two denominators

Here, purity means the mole fraction of the target gas in the product: the amount of that gas divided by the total amount of gas in the same product. A mole is a unit of amount of substance. A product containing 99 mol of the target in 100 mol of mixture has a target mole fraction of 0.99, or 99 mol%.

Component recovery is the fraction of the target gas entering the chosen process boundary that leaves in the designated product. For steady operation with one feed and one product:

Use matching units, such as mol/h for both flows. Gas volumes can be used when converted to a consistent molar basis: specify the same reference temperature and absolute pressure, align wet or dry composition bases, and apply real-gas corrections where needed. An actual cubic metre at operating pressure is not interchangeable with a standard cubic metre. Gas Calc can support quantity comparisons, but the calculation basis still needs to be stated.

A balanced example: 99% purity, 80% recovery

Consider a hypothetical separation of 100 mol of feed containing 10 mol of a target gas and 90 mol of all other gases combined. Assume no chemical reaction, no inventory change and two outlet streams. The product contains 8 mol of target gas and has a purity of 99 mol%.

The total product is therefore 8 ÷ 0.99 = approximately 8.0808 mol. The remaining 91.9192 mol leaves through the other outlet. Tracking both the target and the other gases makes the balance visible:

The target balance is 10 = 8 + 2 mol, and the other-gas balance is 90 = 0.0808 + 89.9192 mol. Recovery is 8 ÷ 10 = 80%, even though product purity is 99 mol%. Dividing total product by total feed gives about 8.08%; that is the product's share of total feed, not target-gas recovery.

The reverse distinction is equally useful. If another hypothetical product captured all 10 mol of target together with 10 mol of other gases, its recovery would be 100% but its purity only 50 mol%. The rest of the feed would leave as 80 mol of other gases. High recovery alone would not make that product suitable for a high-purity application.

Where the remaining target gas goes

The valuable component may leave with rejected gas, purge or regeneration streams, diverted off-specification product, sampling flows or gas released during startup, warmup and shutdown. Some streams can be collected for further treatment. Calling a stream “waste” or “recycle” does not quantify its target-gas content or prove that it eventually becomes accepted product.

Draw the measurement boundary before calculating recovery. For the complete plant, fresh feed crosses that boundary once and accepted product crosses it once. A recycle loop entirely inside the boundary is neither additional fresh feed nor additional exported product. A separator's single-pass recovery and the overall plant's recovery can therefore differ.

Recycle can return valuable gas for another separation attempt, but it also returns whatever other components accompany it. Its effect depends on the stream composition, process design, purge strategy and equipment loading. A primary study by Weh and colleagues on helium recovery using an adsorption cascade (PDF) explicitly models two stages and a recycle connection. Its results belong to the studied feeds and configuration; they are not a general plant guarantee.

Measure a complete period and account for inventory

Gas held in vessels, piping or adsorbent beds is inventory. When its quantity changes during a test, the non-reacting target component must satisfy:

Target entering = target in accepted product + target in other outputs + increase in target inventory.

Filling the plant can temporarily reduce measured product recovery; drawing down stored gas can make product exceed the target entering during that same short interval. Neither observation alone establishes steady performance. MIT's material-balance notes explain the relationship between boundary flows and accumulation.

For a cyclic process, total the flows over complete, repeatable cycles. For a production campaign, agree how startup, regeneration, warmup and shutdown are counted and record starting and ending inventory. If flow and composition vary, calculate target quantity from their paired measurements over time; multiplying unrelated averages can misstate the result.

An instantaneous clean outlet reading is not the same as net production available for delivery. Ask how much accepted gas leaves the agreed boundary over the reporting period, after internal use and off-specification diversion. Energy consumption should use that same net output and stated delivery conditions as its denominator.

Does higher purity always reduce recovery?

No universal rule says that it must. Within a particular separator and operating mode, a stricter product cut or more purging may improve purity while carrying more target gas into another stream. A different arrangement, additional separation stages, suitable recycle or improved control may change that relationship. The effect must be demonstrated for the specified feed and operating range.

The helium cascade study linked above examines design and operating parameters affecting both measures. It combines laboratory work with modelling, so the two forms of evidence should remain distinguishable. A buyer should compare purity, recovery, net throughput and utility use together, supported by the proposed acceptance test.

Questions that make offers comparable

• Feed definition: what are the target concentration, full contaminant list, flow range, pressure, temperature and expected variability? Does “feed” mean fresh gas or fresh gas mixed with recycle?
• Product guarantee: which minimum purity and individual impurity limits apply, on what composition basis, at which sampling point and delivery pressure? Are purity, recovery and throughput guaranteed simultaneously?
• Recovery boundary: is the figure for one stage or the complete system? Are rejected batches, regeneration, startup and shutdown included, and how is inventory change treated?
• Analytical evidence: which contaminants are measured, with which methods, calibration, detection limits and uncertainty? Does the sample represent the complete delivered lot or a selected moment?
• Net output and utilities: how much accepted product is available per hour or campaign, and what electricity, cooling, heating, purge gas and other utilities are required for that output?

“Not detected” does not mean “absent”. NIST-hosted sample-description guidance calls for numerical purity and the analytical sensitivity limit when impurities are not detected. For procurement, identify the impurities that matter to the application and verify that the methods can address their specified limits. See our guide to PDD, laser and coulometric gas analysis and our quality and documentation approach.

For a technical enquiry, provide the feed composition and variability, required product and impurity limits, quantity, delivery conditions and intended operating schedule. Those details turn two headline percentages into a basis that suppliers can evaluate and buyers can verify.