Inert gas system technology continues to evolve

A new type of pressure swing absorption (PSA) system is extending industry options for purging cargo tanks

In the late 1960s and early 1970s a series of spectacular explosions on large tankers when in ballast concentrated industry minds on finding solutions for removing oxygen from the flammable gas mixtures that evaporate from cargo. Since that time a variety of systems have emerged offering different possibilities and benefits and are in common use.

The best way to maintain a non-explosive atmosphere is to purge and blanket cargo tanks with inert gas. Most vessels that employ inert gas systems use either exhaust gas from fuel burners or nitrogen. The approach taken will, of course, vary according to the ship type and size, cargo carried and required gas specification.

Modern crude oil tankers, which usually run on HFO, will typically use flue gas from the auxiliary boilers. This is a relatively cheap way to produce the inert gas as the boilers are often in operation anyway.

Most clean product tankers employ inert gas generators burning MDO; a clean inert gas can be produced that is suitable for most cargoes. Some chemicals require a very dry inert gas or do not allow CO2. In these cases, nitrogen generators can be used or an inert gas generator with a CO2 stripper.

Combined solutions are available for vessels that carry different cargoes. Currently, Novoship is installing a unified inert gas system consisting of a flue gas generator and a separate inert gas generator to protect the epoxy painted cargo tanks on four 112,000 dwt product tankers on order at Hyundai Heavy Industries. A Korean manufacturer has been nominated to supply the system.

Netherlands-headquartered Aalborg Industries Inert Gas Systems (formerly known as Smit Gas Systems) markets the Smit Gas FGin system for crude/product tankers. This can be operated in different modes according to the cargo being carried.

In inert gas mode, the FGin operates as a normal inert gas generator, burning clean fuel. The inert gas produced is said to be completely soot free, even under part load conditions. Inert gas can also still be produced when the boilers are idle. In flue gas mode, the FGin is said to remove up to 99 per cent of the sulphur and soot content.

The first FGin models, which delivered 4,000 m3/h, were introduced to the market in 1984. A larger variant, capable of supplying 8,700 m3/h, was launched in 2004. Over 100 FGin units across both ranges have been installed.

Where a ‘singular approach’ is preferred, installing nitrogen generating inert gas systems based on membrane technology is one option. Hollow fibre membrane modules have been commercially available since the early 1980s. For marine use this technology was initially introduced to replace the high pressure nitrogen cylinders used on board chemical tankers for maintaining cargo padding during voyages, and today this is industry’s preferred method for nitrogen generation.

As a company, Wilhelmsen Marine Systems has delivered well over 400 systems of this type for both chemical and product tankers, and reports that membrane-type systems with capacities of 4,000-5,000 Nm3/h at 95 per cent purity are finding increasing favour on chemical tankers. Unitor nitrogen systems delivered by Wilhelmsen Marine Systems can produce gas with a dew point of -60oC to -70oC without additional equipment.

The concept separates air into its constituent gases by passing compressed air through a bundle of hollow fibre semi-permeable membranes. The membrane divides the air into two streams: nitrogen plus argon and oxygen plus carbon dioxide and trace gases.

While oxygen, carbon dioxide and water vapour quickly permeate the membrane surface, most of the nitrogen flows inside the membrane fibre as a separate product stream. Millions of fibres, approximately the size of a human hair, can be packed into a single module. This gives a very large membrane surface area that produces large quantities of high purity nitrogen.

To make up a complete membrane inert gas system, compressed air has to be provided, normally from dedicated compressors. The productivity of a membrane improves at increased pressure, and a typical membrane system will operate from 14 bar g air supply. This 14 bar air can be provided by normal oil lubricated screw compressors.

By using compressors with modulating control, the pressure can be maintained at a constant level, independent of the air flow requirement. The air will, however, be saturated with water and contaminated by traces of oil and particulates; a feed air treatment system is therefore required. Cleaned air will result in a high quality nitrogen product, free of contaminates, and should ensure a long life for the membrane system.

An alternate method of nitrogen generation is the Pressure Swing Absorption (PSA) method. The PSA works just like a desiccant dryer. It has one tank producing nitrogen and a second tank regenerating. The pressure goes back and forth between the two tanks (hence ‘swing’) and particles are adsorbed by a special kind of active carbon.

The technique has found particular favour on applications that require large volumes of extremely pure nitrogen, such as gas carriers. To date, however, uptake of PSA systems outside the small LPG/ethylene segment has been limited. One of the key reasons cited for this is it its size.

A conventional PSA system has one generator for two tanks, necessitating a large buffer tank to equalise the flow between the two. For a 1,000m3/h per hour flow rate this typically equates to a 15m3 buffer tank, which obviously impacts a vessel’s cargo carrying/revenue generating capacity. Durability of the swing valves and potential purity fluctuations may also need to be given serious consideration.

However, a new range of ‘compact’ nitrogen generators – the X4 series – has been introduced by Danish company Oxymat, which uses the PSA principle and is said to offer comparable performance to membrane systems.

“With a nitrogen capacity of up to 10,000 m3/h, the X4 series system is extremely compact. It is built up around four generators and at any one time three will run and the other will regenerate. This allows for a constant air flow and for the buffer tank footprint to reduce in size from the conventional 15m3 to 0.5 m3,” says Thomas Billeschou-Hansen, a former chief officer on board Lauritzen LPG vessels, and now managing director of Oxymat’s Marine division.

If the flow of air to one of the generators is turned off, the three remaining will receive more air and produce
more nitrogen. “Even with a loss of 25 per cent of air, there is only a 10 per cent drop in nitrogen production. In this way the X4 series has inbuilt redundancy.”

“Membrane systems use amazingly large volumes of compressed air to produce high concentrations of nitrogen, and the compressed air is expensive to produce,” he adds. “Our system and a conventional membrane use the same amount of compressed air to produce nitrogen with five per cent oxygen; however, the X4 system can reduce electrical power consumption by 30 per cent. Where purer nitrogen is needed, the X4 series can produce six times more nitrogen per cubic metre of compressed air than a membrane system,” claims Mr Billeschou-Hansen.

“Our delivery term is 20-25 weeks and since our system consists of small parts, it can easily be transported to ships in small packages. If necessary, a service engineer will install the equipment at sea and we can have the system up and running within a week or two.” Many competing systems, he claims, require a shipyard docking of two to three weeks. Nevertheless, he concedes that his system remains larger then membrane designs and requires a higher initial outlay; however, he believes this investment pays for itself over the lifetime of the plant.

“Membranes typically need to be changed every three to five years, and they can comprise up to 60 per cent of the entire plant cost. The delivery time for membranes can run to two years, ours is 20 weeks or so.”

According to Mr Billeschou-Hansen, interest in the X4 series has been such that the company had to bring its market introduction forward three months ahead of schedule. “We had not budgeted on any sales this year; however have already sold around US$3 million worth of systems. Contracts have been signed with Ahrenkiel and Odfjell and a letter of intent has been formalised with Eitzen. The company has just completed an installation on the first of a series of five chemical tankers under construction in Turkey and is now quoting on the other four.

The company offers a 48,000 running hours guarantee on the active carbon and guarantees on the complete system are typically a year. Mr Billeschou-Hansen says that the systems are especially robust and the only thing that can destroy the carbon molecular sheave inside the system is water. The most likely source of failure is the compressors “so for this reason we source high quality units from Kaeser, Atlas Copco or TMC.” Factory acceptance tests, class approvals and shipment take place out of the company’s factory 40km north of Copenhagen, or its facility in Slovakia. TST