Yes - in many cases, an Air Separation Unit (ASU) designed primarily for gaseous products can also produce a certain amount of liquid oxygen (LOX) or liquid nitrogen (LIN).
However, liquid production is not simply a matter of "taking some gas out as liquid." The key issue is whether the ASU has enough refrigeration capacity to maintain the cold balance of the plant without significantly affecting gaseous product output.
Why Does Liquid Production Require More Refrigeration?
An ASU operates at cryogenic temperatures. During normal operation, the refrigeration generated inside the plant is carefully balanced with the refrigeration consumed by the process and equipment.
When a liquid product is withdrawn from the ASU, it carries a significant amount of refrigeration out of the system.
In simple terms, the plant has to provide additional refrigeration to compensate for the "cold" removed with the liquid product.
For example, at approximately 0.1 MPa, the refrigeration carried out with saturated liquid products is approximately:
Liquid ProductRefrigeration CapacityLiquid Oxygen (LOX)~407 kJ/kgLiquid Nitrogen (LIN)~435 kJ/kgLOX, on a volume basis~465 kJ/LLIN, on a volume basis~353 kJ/L
This is why even a relatively small liquid production rate can have a noticeable impact on the operating conditions of an ASU.
Where Does the Additional Refrigeration Come From?
The answer depends on the type and process configuration of the ASU.
During start-up, an ASU normally has significantly more refrigeration capacity than it needs for steady-state gaseous product production. This additional refrigeration is used to cool down the cold box and accumulate liquid inside the system.
Once the plant reaches normal operation, part of this refrigeration margin may potentially be used for liquid product production.
For different ASU configurations, the refrigeration capacity can be increased in different ways.
High-Pressure / Medium-Pressure ASUs
For small high-pressure or medium-pressure ASUs, increasing the operating air pressure can increase the available refrigeration capacity.
For example, practical experience shows that for a 50 m³/h small high-pressure ASU, producing approximately 4 L/h of liquid nitrogen may require an increase in air pressure of about 0.3–0.4 MPa compared with normal operating conditions.
Low-Pressure ASUs
For low-pressure ASUs, increasing the expansion flow through the expansion turbine can provide additional refrigeration.
However, this approach has an important limitation: expansion air is also an important part of the rectification process.
If too much air is expanded and bypassed from the distillation process, the separation efficiency can decrease, resulting in lower oxygen recovery and reduced gaseous oxygen production.
This is particularly important for smaller low-pressure ASUs.
A Practical Example: 6,000 m³/h ASU
Consider a 6,000 m³/h low-pressure ASU operating primarily to produce gaseous oxygen and nitrogen.
Under normal gaseous-product operation, the expansion flow may be approximately 19.6% of the total air processing flow.
If the customer also requires approximately 250 L/h of liquid oxygen, the expansion flow may need to increase to around 27% of the air processing flow in order to provide the additional refrigeration.
Under these conditions, the available refrigeration can also potentially support approximately 300 L/h of liquid nitrogen production, depending on the specific process configuration and operating conditions.
This illustrates an important point:
Liquid production capacity is closely linked to the available refrigeration margin of the ASU.
It cannot be determined simply by looking at the nominal oxygen or nitrogen production capacity.
Why Is Liquid Production More Difficult for Small Low-Pressure ASUs?
The situation is different for a 1,000 m³/h low-pressure ASU.
Under normal gaseous-product operation, the expansion air requirement may already be approximately 28% of the total air processing flow.
At this operating point, increasing the expansion flow further may not be practical.
A portion of the additional expanded air may have to bypass the distillation column and return through the main heat exchanger instead of contributing directly to the rectification process.
This reduces the effective air available for oxygen extraction and can therefore reduce gaseous oxygen production.
As a result:
For a 1,000 m³/h low-pressure ASU, producing additional liquid oxygen or liquid nitrogen while maintaining normal gaseous product output is generally not recommended without a detailed process evaluation.
What Are the Main Limitations?
When evaluating whether a gaseous-product ASU can also produce liquid products, several factors need to be considered:
1. Refrigeration Capacity
The first question is whether the plant has sufficient refrigeration capacity to compensate for the refrigeration carried away by the liquid product.
2. Expansion Flow
Increasing turbine expansion can generate additional refrigeration, but excessive expansion may negatively affect the distillation process.
3. Rectification Performance
The air separation columns require an appropriate balance of gas and liquid flows. Significant changes in expansion flow can affect column operating conditions and separation efficiency.
4. Gaseous Product Output
Liquid production should not be evaluated independently. The impact on gaseous oxygen and nitrogen production must also be considered.
5. ASU Process Configuration
The allowable liquid production rate varies significantly with ASU design, operating pressure, turbine configuration, heat exchanger design, column configuration, and product specifications.
Therefore, there is no universal liquid-production ratio that can be applied to every ASU.
Can Your Existing Gaseous ASU Be Modified to Produce Liquid Products?
Potentially, yes.
The feasibility should be evaluated based on the actual operating conditions and process design of the ASU.
A proper engineering assessment should normally consider:
Air processing capacity
Operating pressure
Expansion turbine capacity
Existing refrigeration margin
Oxygen and nitrogen product requirements
Required liquid production rate
Column operating conditions
Oxygen recovery
Impact on gaseous product output
Heat exchanger and cold box capacity
The key is to find the optimum balance between liquid production and gaseous product production.
Final Takeaway
A gaseous-product ASU can often produce a certain amount of liquid oxygen or liquid nitrogen, but the achievable liquid production rate is limited by the plant's refrigeration capacity and process configuration.
For larger ASUs with sufficient refrigeration margin, partial liquid production may be feasible with relatively limited impact on gaseous products.
For smaller low-pressure ASUs, however, increasing liquid production can require excessive expansion flow, which may reduce separation efficiency and gaseous oxygen output.
