In cargo ships, such as gas carrier or chemical tanker, depending on the characteristics of transportation, inert gas has been used in storage tank and freight docks for separation from the atmosphere and prevention of explosion, and it is used for such uses as inerting, padding, purging, and blanketing. The use of inert gas is for safe transportation and storage, and in general carbon dioxide(CO2) is used for oil tanker or chemical tanker, while nitrogen gas(N2) is used for liquefied gas carrier that transports LNG.
In previous large-scale chemical tankers, boiler combustion gas with abundant CO2, generated from the large-capacity IGG(Inert Gas Generator) and IGS(Inert Gas System) has been used as inert gas. However, due to the issues like the contamination of freight by SOx or NOx oxides generated from boiler combustion gas and global warming by the increase of CO2, the application of large-capacity Nitrogen generator to produce inert and clean gas, nitrogen, has become a growing trend.
Nitrogen generators that are currently used in cargo ships all use the membrane type because the cryogenic type requires to be a large-scale plant including cooling device and the PSA type has limited usage due to vulnerability of the absorbent to the vibration of freighters. Therefore, all nitrogen generators that are currently used in cargo ships choose the membrane type which is strong against vibration.
Except for large-scale gas carrier and chemical tanker, small- and medium-sized LPG or chemical tankers in thousands of tons use 10~30 high-pressure nitrogen bottles for inerting or padding depending on volume. However, these high-pressure nitrogen bottles should be transported to the shore and recharged, after nitrogen gas is exhausted, so it is disadvantageous in terms of cost, time and manpower. Thus, a demand for nitrogen supply method is growing.
This study is a preliminary stage in the development of recharging system by nitrogen generators which replace nitrogen-bottle supply method for small and medium-sized LPG or chemical tankers. It aims to examine hydraulic characteristics on gas membrane module for PARKER ST6010 for nitrogen generation which is used for land use and to evaluate validity of its nitrogen-generation processing.
First, the volume of nitrogen usage from nitrogen bottles for cargo ship was calculated to select gas membrane module in the same volume. And then, performance test was performed by the numbers and connection of the selected module. In order to understand hydraulic properties of modules in actual products, a prototype of nitrogen generator was fabricated and tested.
As a result of the study, gas membrane module was selected based on the survey of nitrogen charging capacity and using flow rate in the high-pressure bottle nitrogen supply method for small and medium-sized LPG or chemical tanker, and a testing system was built to measure complex signals of pressure, temperature, flow, and oxygen purity. Flows of two modules showed slight differences by pressure and purity. When two modules are connected in parallel, nitrogen flow was a little lower than the numerically doubled value of the flow in one module, but its efficiency of nitrogen generation was high. By measuring air quality, the validity of nitrogen-generation process was supported and the flow of the prototype was found to be higher than in module testing, due to processing optimization. And, with the capacity of the compressor used in the test, the ranges of pressure and purity which could allow continuous production of nitrogen flow were determined.