Nitrogen Inert Gas Systems based on Membrane technology by Iver Iversen Unitor ASA

Transcription

1 Nitrogen Inert Gas Systems based on Membrane technology by Iver Iversen Unitor ASA. Introduction Separation of air in a nitrogen stream and an oxygen enriched air stream using hollow fibre membranes has been commercially available since the early 98 s. For marine use, this technology was initially introduced to replace the high pressure nitrogen cylinders used onboard chemical tankers for maintaining cargo padding during voyages. Today the Membrane Nitrogen Inert Gas Systems are increasingly replacing the traditional fuel fired type of inert gas systems on all tankers where clean and dry gas is required for maintaining the quality of the cargo as well as protecting the ship from flammable conditions in the cargo area. Membrane Nitrogen Systems with capacities of 4-5 Nm3/h at 95% purity and high purity capability at reduced capacity are being installed on chemical tankers routinely. Small LPG / ethylene carriers have until today predominately been using nitrogen generators based on the Pressure Swing Adsorption (PSA) technology due to requirements for very high purity of 99.9% or better. However, membrane systems are technically attractive and increasingly cost competitive in the high purity range. Unitor has delivered more than 5 systems using MG Generon membranes where about 5 of these systems have been installed onboard LPG carriers as Inert Gas Systems or for the void space to protect the steel construction from corrosion. 2. Principle of Membrane Air Separation The system works by separating air into its component gases by passing compressed air through a bundle of hollow fibre, semi-permeable membranes. The membrane divide the air into two streams, one is essentially nitrogen and the other oxygen plus carbon dioxide and other trace gases. (Fig. )

2 2 Nitrogen Oxygen Water Vapor Nitrogen Feed Air Oxygen and water vapor are fast gases which quickly permeate the membrane, allowing nitrogen to flow through the fiber bores as the product stream. Fig. While oxygen, carbon dioxide and water vapour quickly permeate the membrane surface, most of the nitrogen flow inside the membrane fibre as a separate product stream. Millions of fibre, about the size of a human hair, can be packed into a single module. This provides a very large membrane surface area that efficiently produces large quantities of a high nitrogen purity product stream. (Fig. 2) End Plate Epoxy Tube Sheet Support Core Enriched Nitrogen Product Gas Epoxy Tube Sheet Oxygen- Enriched Air Feed Air O-Rings Hollow Fibers Fig. 2

3 3 3. The Membrane type Inert Gas System To make up a complete Membrane Inert Gas System, compressed air has to be provided normally from dedicated air compressor(s). The productivity of the membranes improves at increased pressure and a typical membrane system will operate from 4 bar(g) air supply. The 4 bar air can be provided by normal industrial type oil lubricated screw compressors. By using compressors with modulating control the pressure can be maintained constant, independent on the air flow requirement. This air will be saturated with water and contaminated by traces of oil and particulates. A feed air treatment system is required to provide clean air free from particles and oil. This clean air will allow for a high quality nitrogen product, free of contaminates, and will ensure long life for the membrane system. The nitrogen generator feed air system is the most critical part of the of the nitrogen membrane generator. A properly designed filter arrangement is of utmost importance to prevent contamination of the membranes, with the resultant loss of performance over time. A well designed filtering system (Fig. 3) incorporate: Water separator with a automatic drain and high level alarm. Electric heater to make the compressed air sub saturated. One coarse and one fine coalescing filter for oil and particle removal. Specially designed carbon bed filter to trap oil vapour. Heater for providing constant feed air to temperature at the membranes. Sub-micron particle filter to trap any possible dust particles from the carbon filter.this filter is the last protection of the membranes, and will have a high differential pressure alarm and shut down. A slow opening valve prevents pressure shock in the filter system. This arrangement prevents possible droplets to be expelled from the coalescing filters and dust from the carbon filter. The air separation system consists of parallel sets of membrane modules. The product gas (nitrogen) leaves at essentially the same pressure as the entering air. The oxygen-enriched flow is directed to permeate discharge at atmospheric pressure and is to be vented to a safe area. Down stream of the membranes, a pressure control valve is to keep the membrane system at a pre-adjusted constant pressure. This control valve is operated using the analogue signal from the oxygen analyser(s) that will provide automatic oxygen control. To obtain a good oxygen control in a dual purity system, one oxygen analyser for the high purity control and a second analyser for low purity should be incorporated. During start up of the system, and if the oxygen level in the nitrogen outlet exceeds the alarm setting of the oxygen analyser, a valve will blow-off the off spec. product. After achieving the desired nitrogen purity during the initial start up period, this valve will close, and the main supply valve will open for nitrogen supply.

4 4 Fig. 3 For LPG tanker systems, the design point will normally be at the high purity (99,9%) range. The membrane performance can be improved (Fig. 4) by 2-25% by changing ½ of the membrane modules in the system to be connected in series (Chart ) by a simple change over of some valves. Fig. 4 A further improvements by about % can be obtained by re-circulating the 2nd stage permeate to the suction side of the compressors. The product recovery will improve about 25% by parallel/serial mode, and additional about 5% by re-circulating (Chart 2).

5 5 Impact of Series Operation on Product Flow Normalized Product Flow Standard Series Series/Recycle %O2 in Product Chart Impact of Series Operation on Product Recovery Normalized Product Recovery.5 Standard Series Series/Recycle %O2 in Product Chart 2 A system arranged for both serial and parallel operation of the membranes will then provide optimum performance at both high and low purity.

6 6 4. Inert Gas Systems in use on LPG carriers The traditional fuel fired inert gas generator system (Fig. 5) will include a fan for the feed air, the fuel system, a burner and combustion chamber, a sea water pump for cooling water to be sprayed into the hot exhaust, an inert gas compressor to remove water by compressing the inert gas to about bar, a refrigerated dryer to reduce the dew point to 45 ºC and possibly a CO 2 stripper to remove the about 4% of CO 2 in the inert gas. Fig. 5 By mixing the exhaust gas with sea water, we create a very corrosive atmosphere, and all kind of precautions needs to be taken to avoid massive corrosion in the down stream piping and components. The principle of the PSA (Fig. 6) technology is based on air separated in a bed with carbon molecular sieve where the oxygen molecules are adsorbed much faster than the nitrogen molecules. When pressurising one carbon bed the gas flowing out will be pure nitrogen. The carbon bed will be saturated with oxygen in about min., and has to be de-pressurised and purged by some nitrogen product to remove the oxygen, the same time a second carbon bed is pressurised to maintain a continuos nitrogen flow. Fig. 6

7 7 A complete PSA system (Fig. 7), will include a feed air compressor (preferably dry/oil free), an air receiver to equalise the swinging air requirement, a refrigerated dryer, filter system, the two carbon bed towers and a nitrogen receiver to equalise the nitrogen purity. For these systems the quality and the timing and control of the automatic valve operation of the inlet and outlet valves of the carbon beds is very critical to maintain the high purity of nitrogen. Fig. 7 Compared with the traditional fuel fired inert gas generators or the PSA type of nitrogen generators, the simplicity of the Membrane nitrogen systems (Fig. 8) is remarkable. Fig. 8 Clean and dry nitrogen is provided by a standard screw compressor as one unit and a second unit in which the feed air preteatment, membranes, and control systems are contained. The entire system consists of only these two components. The installation is simple as both units will be pre-wired and thoroughly tested before delivery. Onboard it is only to provide the power supply and the interconnecting piping and wiring. Technical as well as due to physical size and weight the membrane system is an attractive solution for new buildings as well as for retrofitting on sailing vessels. The weight and size of the Membrane System of 4 Nm3/h at 99,9% is less than 5% compared with PSA system of same capacity (Fig. 9), a saving that can be used for added cargo carrying capacity.

8 8 Fig. 9 Pictures (Fig. & ) show the installation of a membrane system including the compressor in a small about 3 x 3 m store room onboard the four 73 m3 Lauritzen Kosan L-class vessels. This system has a design capacity of about 2 Nm3/h at 99,8% purity and exceeding 4 Nm3/h at 96% purity. The installation on the Lauritzen Kosan tankers, including the final commissioning, was done within 3 days. The first installed system has been in operation for more than a year and the owner reports good performance and operational savings by having the nitrogen systems installed. Fig.

9 9 Fig. 5. Membrane technology development All above data is based on the MG Generon membrane used in the Unitor / Generon nitrogen generators that has been available for the last 8 - years. The membrane technology, however, is in a constant development, and a new generation of membrane modules will be introduced to the market within the next few months. The new membrane material has a substantial higher productivity. The following charts are giving the predicted performance of the new MG Generon membrane relative to the current 72 High Performance membranes. Chart 3 shows at low purity and 2 bar feed air pressure, the membrane area will be reduced 65% when system design is for 95% purity. The feed air requirement (power consumption) will be about the same as today. Impact of Using High Flow Modules Relative to Current 72 Operated at 4 Bar and 4C (Production of 95% N2).2.2 Relative Membrane Area Relative Power Requirement Membrane Area Power.2.2 Chart Operating Pressure (Bar)

10 Chart 4 shows at low purity and 2 bar feed air pressure, the weight of the membrane modules will be reduced 85% when the system design is for 95% purity. Impact of Using High Flow Modules Relative to Current 72 Operated at 4 Bar and 4C (Production of 95% N2) Relative Membrane Area Relative Module Weight Membrane Area Module Weight Operating Pressure (Bar) Chart 4 Chart 5 shows at high purity and 2 bar feed air pressure, the membrane area will be reduced 6% when the design is for 99,9% purity. The feed air requirement (power consumption) will increase by about 2 %. Impact of High Flow Modules Relative to Current 72 Operated at 4 bar and 25C (Production of 99.9% N2) Relative Membrane Area Relative Power Requirement Membrane Area Power Feed Pressure (bar).9 Chart 5

11 The high productivity of the new membrane material results in reduced size and weight of the systems (Fig.2) as well as improved overall costs. Space and Weight Requirements for Different Technologies for Producing 99.9% N2 (4 NM3/hr Plant without Compression Equipment) 35 3 Space Requirement (M3) or Weight (tons) Space Requirement Weight 5 PSA Current Membranes New Membranes Fig. 2 The membrane technology will continue developing and new and improved membrane materials can be expected. The cost of the development is high so the development can be expected go by steps with 5 year intervals.