1. Direct answer summary

An air separation unit for liquid oxygen is a cryogenic plant. It cools filtered air to about −183 °C, separates oxygen by distillation, and draws the product off as liquid. It fits projects that need 99.5–99.8% oxygen, on-site storage, or liquid supply for cylinders, tankers and medical use. PSA and VSA plants cannot make liquid oxygen directly, because they stop near 93–95% purity and deliver gas only. This guide covers sizing, scope, cost drivers, safety rules and supplier checks for Southeast Asian projects.

2. What is an air separation unit for liquid oxygen?

It is a cryogenic distillation plant, not an adsorption unit. Liquid output is built into the cycle from the start. The unit takes in ambient air and removes water, carbon dioxide and hydrocarbons. It chills the air until it liquefies, then separates oxygen from nitrogen and argon in distillation columns. Oxygen leaves the cold box either as gas or as liquid at roughly −183 °C. NIST lists the normal boiling point of oxygen at 90.2 K. That is about −183 °C (NIST Chemistry WebBook, phase change data).

Air is roughly 78% nitrogen, 21% oxygen and 1% argon. Those components liquefy at different temperatures, which makes distillation practical. The column does the separation, and the refrigeration system pays for it in compressor power.

2.1. How does cryogenic distillation produce liquid oxygen?

1.  Compress and cool the air. A multistage compressor raises air pressure; an aftercooler removes the heat of compression.

2.  Clean the air. A molecular sieve purifier traps water, CO₂ and trace hydrocarbons. This step protects the cold sections from ice and dry-ice blockage.

3.  Cool the air to cryogenic temperature. The main heat exchanger brings the feed close to liquefaction using returning product streams.

4.  Distil the air. A high-pressure column and a low-pressure column separate nitrogen overhead from oxygen-rich liquid at the bottom.

5.  Draw and store the liquid. Oxygen-rich liquid is drawn from the low-pressure column. It is subcooled, then sent to an insulated storage tank.

The distillation step needs very low temperatures, so insulation quality decides how much liquid survives storage. A pumped-liquid cycle sends LOX to a cryogenic pump, then to a vaporizer, when the site needs high-pressure gas.

2.2. When is liquid oxygen the right product form?

Liquid oxygen makes sense when you must store oxygen rather than consume it immediately. One normal cubic metre of oxygen gas becomes roughly 1.25 litres of LOX. That ratio is why a small tank holds a large gas reserve. That density matters in Southeast Asia, where inter-island logistics are slow and cylinder freight is expensive.

Liquid also suits cylinder filling plants, medical gas distributors, and sites with fluctuating demand. A plant that runs a steady gas load and never sells liquid usually does not need liquefaction. Liquefaction adds real power demand, so specify it only when the business case supports it.

3. Cryogenic ASU vs PSA vs VSA vs delivered LOX

An air separation unit for liquid oxygen wins when you need liquid output or purity above 95%. Choose adsorption for small gas-only demand. Buy delivered LOX when demand is low or uncertain.

The purity and power comparisons come from a peer-reviewed review of air separation technologies published through NETL. That review states that adsorption systems typically deliver 93–95 vol% oxygen. It also shows that liquid product needs two to three times the power of gaseous product. The same review expects neither adsorption nor membranes to challenge cryogenic plants for large tonnage at high purity.

Buyers who only need gaseous oxygen can weigh the PSA oxygen generator route first. For a buyer in Vietnam, Indonesia or the Philippines, the practical question is usually volume and logistics, not technology preference. Delivered LOX is flexible but exposed to supplier pricing and tanker schedules. An on-site unit trades that flexibility for capital, power contracts and operating staff.

4. How to size capacity and purity

Size an air separation unit for liquid oxygen on peak simultaneous gas and liquid demand. Then add a margin you can justify. Most specification errors come from mixed units, not from wrong engineering.

4.1. Which capacity unit should you quote in?

Suppliers of an air separation unit for liquid oxygen quote in Nm³/h of gas, litres per hour of liquid, or tonnes per day. Those numbers are not interchangeable. Convert everything to one basis before you compare offers, and state the reference conditions. A quote of 500 Nm³/h gas and a quote of 500 L/h liquid describe very different plants.

4.2. Which purity grade do you actually need?

Standard cryogenic plants reach 99.5–99.8% oxygen. Air Liquide's published standard-plant data lists 99.6–99.8% pure oxygen for packaged units. Higher grades exist for electronics and specialty use. They need extra purification stages and cost more to run. Medical oxygen must meet the local pharmacopoeia, so confirm the analytical and documentation requirements before you fix the process design.

4.3. What inputs must be fixed before requesting quotes?

Capacity also decides which equipment class you buy. Small and mid-size liquid demand maps to the small and medium-sized air separation equipment range. Tonnage projects move to the large air separation equipment line.

5. What equipment sits inside the unit?

An air separation unit for liquid oxygen is a package of five systems. Quotes are only comparable when all five sit inside the same battery limit.

The five systems are air compression, air purification, cold box, product storage, and control and safety instrumentation. The cold box holds the main heat exchanger, the distillation columns, the subcooler and the turboexpander. Product storage means an insulated LOX tank, a vaporizer, and the transfer piping. Control means a DCS or PLC, plus oxygen analysers and trip logic.

Battery limits decide who supplies what. A quote that stops at the cold box outlet leaves you to buy storage, vaporizers, civil works and power distribution. Ask each supplier to mark the battery limit on a block diagram, and compare those diagrams side by side. This single step removes most of the price confusion in a tender.

Also confirm the utility list: power at motor voltage, cooling water quality and flow, instrument air, and nitrogen for purging. In tropical sites, add humidity and ambient temperature to the design basis. Compressors lose capacity as inlet air heats up.

6. Seven steps to specify an air separation unit for liquid oxygen

Fix the product slate first, then the interfaces, then the commercial terms. Buyers who start with price usually renegotiate scope later.

1.  Define the product slate. State gas flow, liquid flow, purity and pressure for each stream, including peak and normal cases.

2.  Set the design basis. Give ambient temperature, humidity, altitude, seismic and wind loads, and power quality data.

3.  Choose the cycle. Decide between low-pressure gas output and a pumped-liquid cycle with vaporizers. Base that choice on how you will deliver oxygen.

4.  Draw the battery limits. Mark every interface: power, cooling water, drain, vent, storage, and the tie-in to your existing plant.

5.  Agree codes and documentation. List the design codes, welding and inspection standards, and the documents you expect at handover.

6.  Define the performance test. Agree how capacity, purity and specific power will be measured. State what happens if they fall short.

7.  Plan logistics and commissioning. Confirm shipping splits, heavy-lift requirements, installation supervision, spare parts and operator training.

Each step produces a document you can attach to the inquiry. Suppliers quote faster and more accurately when the design basis is explicit. Vague inquiries invite optimistic offers that expand in price after contract signature.

7. What drives the cost?

Capacity, purity and the liquid fraction drive capital cost; compressor power drives operating cost. Neither number is meaningful without the other.

Operating cost deserves equal attention. Compressor power usually dominates the lifecycle cost of an air separation unit for liquid oxygen. Boil-off from the storage tank adds a smaller but permanent loss. It grows when insulation or vacuum quality is poor. Maintenance, spares and downtime complete the picture.

Ask every supplier for specific power in kWh per Nm³ or per tonne of oxygen, at a stated product mix. Then compare that figure against your electricity tariff. A cheaper plant with higher specific power can cost more within a few years of operation.

8. Safety and site requirements for liquid oxygen

Liquid oxygen is not flammable, but it makes everything else burn fiercely. Design an air separation unit for liquid oxygen around cleanliness, ventilation and separation distances.

OSHA 29 CFR 1910.104 sets a widely used benchmark for bulk oxygen systems. It covers storage and handling, not the ASU process plant itself. It requires noncombustible surfacing where liquid oxygen might spill, because asphalt and bitumen are treated as combustible. It also requires venting, relief devices, and oxygen-compatible piping and fittings. Parts must be cleaned to remove oil and grease. The storage area needs a permanent “OXYGEN – NO SMOKING – NO OPEN FLAMES” placard.

Those rules map onto four practical site decisions. Keep LOX storage outdoors or in a dedicated noncombustible room with good ventilation. Separate the tank from fuel storage, hot work and occupied buildings. Keep oil, grease and incompatible materials away from oxygen-wetted parts. Train staff on cold-burn first aid and on the fire risk created by oxygen enrichment.

Local regulations always override a foreign benchmark, so confirm the national gas code and the medical-gas rules in your country. In coastal Southeast Asia, add corrosion protection, typhoon anchoring and flood-safe tank elevation to the civil design.

9. Common mistakes first-time buyers make

Most costly errors are specification errors, and they surface after the contract is signed.

The first mistake is comparing quotes built on different units. A gas figure in Nm³/h and a liquid figure in L/h cannot be ranked without conversion. The second is an undefined battery limit, which hides storage, vaporizers and electrical scope in someone else's column.

The third is ignoring ambient conditions. A plant rated at a mild design temperature delivers less oxygen on a 38 °C afternoon. The shortfall appears exactly when demand peaks. The fourth is buying a PSA plant for liquid service, which no amount of ancillary equipment will fix.

The fifth is treating the performance test as a formality. Without agreed measurement methods and acceptance thresholds, a capacity shortfall becomes a commercial argument rather than a warranty claim. The sixth is under-planning spares and service. An air separation unit for liquid oxygen runs continuously. A two-week wait for a valve or analyser cell is a two-week outage.

A seventh, quieter mistake is buying more purity than the market pays for. Extra purity costs power every hour of every year.

10. How Nanjing Bangwin fits this specification

Nanjing Bangwin Gas Equipment Co., Ltd. is a Chinese supplier of complete air separation and gas liquefaction packages. It is one option to include in a competitive tender.

The company designs, manufactures and supplies industrial gas separation, liquefaction, and waste-gas recovery equipment. Its public product range covers small and medium-sized air separation equipment, large air separation equipment, and PSA oxygen generators. That range matters here, because buyers often compare a cryogenic plant against adsorption for the same project. The cryogenic line fits projects that need an air separation unit for liquid oxygen, not gaseous oxygen.

The company states that its equipment runs in the Philippines, Malaysia, Indonesia and other markets. Its units also operate across more than 30 Chinese provinces. Those references are relevant for Southeast Asian buyers who want regional operating experience. The company also states that its large units are designed for a service life above 25 years. It claims 10%–20% energy savings against conventional designs. Treat those figures as company claims, and verify them against guaranteed performance data in the contract.

For a liquid oxygen project, request the same six items from Bangwin that you would from any supplier. Start with the battery limit drawing and the specific power guarantee. Add a reference list, the inspection and test plan, the spare parts list, and the commissioning scope.

Who this suits: gas distributors and industrial users in Southeast Asia who want a complete cryogenic package. That includes factory testing before shipment and installation supervision on site. If you already have a design basis, send it through the contact page. Ask for a scope-by-scope comparison rather than a single lump sum.

11. FAQ

11.1. How much liquid oxygen can an air separation unit produce per day?

It depends on the liquefaction capacity built into the cycle, not on the gas rating alone. A unit rated mainly for gaseous oxygen may produce only a small liquid slipstream. Ask for the liquid figure in L/day at stated purity, and confirm the power consumption at that liquid rate.

11.2. Can a PSA plant fill liquid oxygen tanks?

No. PSA plants deliver gaseous oxygen at 93–95% purity, and they have no distillation column to liquefy the product. Filling a LOX tank needs a cryogenic plant, or delivered liquid from a merchant supplier.

11.3. What purity can an air separation unit for liquid oxygen reach?

Standard cryogenic plants reach 99.5–99.8% oxygen. Higher grades for electronics or medical use need additional purification stages, at higher capital and operating cost. Medical use adds pharmacopoeia compliance and analytical documentation on top of the purity figure.

11.4. How long does delivery and commissioning take?

Timing depends on capacity, cold box fabrication slots, and shipping route. Do not accept one total figure. Ask for a milestone schedule: design approval, procurement, fabrication, factory testing, shipment, cooldown and performance test. Penalise only the milestones the supplier actually controls.

11.5. What power supply does the plant need?

The main air compressor of an air separation unit for liquid oxygen dominates the electrical load. It usually runs on medium-voltage motors. The exact figure follows from capacity, pressure and cycle choice. Request a single-line diagram and a load list with the quotation. Confirm transformer capacity and grid stability before you commit.

11.6. Do I need a storage tank and vaporizer?

Yes, if you sell or distribute liquid oxygen. The tank buffers production against demand and covers shutdowns. A vaporizer converts LOX back to gas when you supply pipelines or cylinders. Both items sit outside many cold box quotations, so check the battery limit.

11.7. How should I compare suppliers from China?

Compare on identical scope, identical design basis and identical guarantees. Ask for specific power, purity, turndown, delivery schedule and spare parts pricing in the same format. Then verify references and inspection records. A low price with a narrow scope is not a lower price.

11.8. Which certificates should I request?

Ask for the quality management certificate and the pressure vessel approvals that apply in your country. Also request the electrical area classification for the site. Finally, add material certificates for oxygen-wetted parts and welder qualification records for cryogenic piping.

12. Conclusion

Fix the product slate, define the battery limit, and compare total cost rather than equipment price.

An air separation unit for liquid oxygen is a long-life asset, and its economics are decided before the purchase order. Capacity, purity and liquid fraction set the capital cost. Compressor power sets the operating cost. Battery limits decide which of those numbers you actually own.

Use the seven specification steps as your inquiry checklist. Ask every supplier for the same documents, the same guarantees and the same test method. Then compare delivered cost, specific power, service response and reference quality together.

If your project needs both gas and liquid, model the liquid demand honestly before you choose a cycle. Oversized liquefaction is one of the few mistakes that cannot be corrected later at low cost. When the design basis is ready, send it to two or three suppliers, including Nanjing Bangwin. Compare their battery limit drawings before you compare their prices.