The essential raw materials for the industrial gas industry are present all around us in air. The challenge for all industrial gas companies is to find effective and economical ways to separate and deliver them. In theory, gas separation is just a simple molecular sorting operation, but the devil is in the detail and the various separation process have different cost implications. Nina Morgan discusses developments in cryogenic air separation.

The basic principle of separating air into its component molecules has been well understood for over 200 years and commercial separation has been undertaken for over 100 years.

Air is made up of a mixture of gases, each composed of molecules of a specific size and with specific properties. In theory, all you need to do to obtain pure forms of the gases you want is to separate the different molecules out on the basis of their size and behaviour. In practice this can be done either at room temperature - using methods such as pressure swing adsorption (PSA), vacuum swing adsorption (VSA) or membranes - or cryogenically.

Choosing the appropriate method to produce gases depends on many factors such as the types, amounts and purities of gases needed. In general, cryogenic air separation units (ASUs) are the best choice for supplying large amounts of high purity oxygen or nitrogen. Less abundant gases such as argon, xenon, krypton and neon are now typically produced by means of a secondary distillation processes. Argon makes up around 1% of the air we breathe. Xenon, krypton and neon, along with helium, make up less than 0.1%.

The largest ASUs are capable of producing more than 3,600tpd of oxygen along with more than 10,000tpd of nitrogen and a possible 190tpd of argon. These products can be recovered as liquids, which are then distributed to customers for use in liquid or gaseous form, or produced as a gas, which is transported to major end-user customers via pipeline.

For customers who need more than 10tpd of oxygen or nitrogen, industrial gas companies may set up a small, dedicated plant on the customer's site. Alternatively, where there is a cluster of major customers, gases companies may opt to set up a single large ASU which supplies multiple end-users. Often the large plants are constructed in situ, although most are at least partially shop-built and then transported to the site. Air Liquide, for example, recently shipped a 3.9m diameter, 50m long argon column weighing 100 tons to a steel works in Asia.

Getting bigger and better all the time
Ten years ago a 3,600tpd oxygen unit was thought to be large and top of the range. These days larger plants with higher production rates are becoming more common. Air Liquide, for example, recently installed an ASU on a SASOL site in South Africa that produces 4,000 tpd of oxygen, while BOC/Linde has commissioned a series of four ASUs, each producing 10,000 tpd of nitrogen, for an oil field recovery application in Canterell, Mexico. A fith unit being supplied by Linde should be operational in 2007. While production capacity rates are going up in ASUs, the cost to customers of the gases they produce is generally coming down.

How did this come about? The goal in ASU design is to reduce both capital and running costs. The trend towards larger ASUs is because, as a general rule, the larger the plant, the lower the relative capital element of the product cost. In addition, better designs, along with the use of standard design platforms and standard components, mean that today's ASUs cost less to build and install. With construction costs running at about 30-40% of the total cost, making plants easier to install saves money right from the start. The streamlined design and construction process is also leading improvements in order-to-commissioning times. What used to take 24 months now takes under 18 months.

Enhancing efficiency
ASUs use large amounts of energy, so improvements to cut power consumption and enhance efficiency in every step of the air separation process (see text box: $quot;How it works$quot;) are key elements for cutting running costs - just so long as they don't lead to unacceptable increases in capital costs and are subject to rigorous economic evaluation. Techniques such as computational fluid dynamics are making it possible to design ways to allow liquids and gases to flow through the machines more smoothly. As a result, the new designs of compressors and expanders work more efficiently, use less power, and often cost less.

The development of new more effective adsorbents - molecules that act as sieves to filter out impurities from the incoming air - has also added to the efficiency of the process and led to reduced power costs. In addition, modern versions of the heat exchangers used to cool the air before separation and to warm the final products are produced with ever more thermodynamically efficient designs. As a result, they perform more effectively and offer greater flexibility when it comes to the heat optimisation of the fluids in the process cycle. They also reduce refrigeration demand without increasing pressure drop.

Improvements have also been made to enhance the efficiency of the distillation step. To work effectively, the liquid and vapour streams must be brought into contact with one another. The greater the contact area between the two phases, the more efficient the separation will be. In the past, this mixing took place in perforated distillation trays, where vapour was pushed through the perforations and mixed with liquid flowing over the trays to create a foam. But these days distillation trays are generally replaced with structured packing made up of carefully designed perforated sheets of metal wound round in a spiral or packed in blocks. The use of structured packing increases the surface area available for gas mixing. It also reduces power consumption because in structured packing the vapour flows over the surface of the liquid, rather than being forced through it; this requires less energy.

The use of structured packing is a key technology behind high purity nitrogen generators, and makes it possible to produce argon economically without the need for a further purification process. It also helps enhance production of rare gases - something a number of companies are keen to do. Linde, for example, recently announced the introduction of new ways to combine krypton-xenon and argon recovery for enhanced yield.

A key enabler
Advances in computer control systems, including linear model predictive control (LMPC), are also helping to cut air separation costs. For example, computer control systems make it easier to cope efficiently with variations in flows from the compressor while still producing gases of the exact purities customers require.

Computer modelling speeds up and improves the design process by making it possible for engineers to input operating requirements and then quickly examine many different options in order to choose the best solutions in different situations. The modelling programs can work out the complicated details of internal sizing and produce a scaled mechanical drawing in around thirty minutes from start to finish. In the early 1990s, producing a scaled drawing that could form the basis of a quote or a design process would have taken a week.

The computer programs can also carry out process simulations. These make it faster and easier to sort through the many different options involved in carrying out a complicated process and to choose the best ones for a particular situation - and at the best possible price.

Air composition
The air we breathe is made up of a mixture of gases. Air contains 78.1% nitrogen; 20.96% oxygen; 0.93% argon. The remaining 0.03% is made up of neon, helium, hydrogen, krypton and xenon.

How pure is pure?
The main contaminant in oxygen gas is argon. General purity of oxygen contains 99.9% or more oxygen. This is widely used in many industrial processes. Low purity oxygen contains 90-95% oxygen. This is typically used to enrich air to improve combustion. Medical grade oxygen is usually 99.99% pure.

The main contaminant in nitrogen gas is oxygen, so nitrogen gas purities are usually expressed in terms of how many parts per million (ppm) of oxygen are present. Nitrogen gas containing less that between 10 and 0.1 ppm oxygen is classed as high purity nitrogen. Low purity nitrogen, used for inerting and blanketing where there are no hydrocarbons present which might burn in the presence of oxygen, contains 98% N.

How air separation works
The basic cryogenic air separation process involves cooling air to reach the point of liquefaction, or boiling point, of the products, then allowing the liquid air to warm up and vaporise. The different gases in air boil, or vaporise, at different temperatures. This makes it possible to separate out the different gases in air using fractional distillation - the same process used to make products such as whisky and petrol or gasoline.

Step by step, here's what happens inside an ASU:

Step 1. Incoming air is compressed to a higher pressure in order to provide energy need for the separation process. The compression also heats the air. The air is then cooled back to ambient, or room, temperature using a water in a heat exchanger.

Step 2. The cooled compressed air is cleaned by passing it through a molecular sieve. This removes impurities such as carbon dioxide and water, which could freeze and cause blockages. It also removes hydrocarbons, which could burn in oxygen rich environments.

Step 3. Heat exchangers cool the air down to temperatures of around -170 to -180 ºC, close to the temperature where air becomes a liquid. Refrigeration is produced by expanding a high pressure stream of nitrogen or air through an expansion turbine. As it expands it cools. Although the temperatures involved are much lower, this is similar to the system used in domestic refrigerators.

Step 4. This cold air passes through a series of distillation columns. As the air evaporates in the columns it is separated into its components. The gases must pass through several distillation stages until they reach the right purities and flow rates. Argon is typically produced in a side column.

Step 5. The final products are either warmed using the heat exchangers, and are released from the ASU as a warm gas that can be sent down a pipeline to customers or are recovered as liquefied gases which are drawn off the distillation column to be transported to customers a liquid in tankers.