ABSTRACT
Today the role of liquid hydrogen is largely limited to its use as a fuel for rockets and satellite launch vehicles. The next century, however is expected to see its use in much greater quantity as a medium of carrying energy from non conventional sources to our homes, factories, airplanes, trains and automobiles. Although hydrogen as a fuel will mostly be used in the gas phase, bulk storage, transportation and its utilisation in the mobile sector of the economy are expected to be done in the liquid form.
While the technology of liquid hydrogen is well developed, widely used in most developed countries, we in India are yet to make any progress in this vital subject. Funded by the Department of Non- Convetional Energy Sources of the Ministry of Energy, I IT, Kharagpur has initiated a small research programe on the production and storage of liquid hydrogen. This thesis forms a part of the overall development effort. The thesis has three main aspects:
(1) Analysis of hydrogen liquefaction cycles and systems and optimisation of process parameters,
(2) Design and construction of a small hydrogen liquefier based on precooled Linde-Hampson cycle,
and (3) Computer aided thermal design of cryogenic storage vessels.
ANALYSIS OF HYDROGEN LIQUEFACTION CYCLES
Economics of hydrogen liquefaction is dependent on the choice of the liquefaction cycle and the operating conditions. While a choice of liquefaction cycle is limited by the capacity of the liquefier, technology and component availability, optimum operating conditions can
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be selected by means of a proper thermodynamic analysis of the cycle. An attempt has been made to find out the performance and optimum operating conditions for different hydrogen liquefaction cycles and systems. These cycles are:
(a) Precooled Linde Hampson cycle (b) Dual pressure Linde cycle (c) Claude' cycle
(d) Helium-Hydrogen condensing cycle
Analysis is carried out by solving a set of system equations derived from the energy balance and performance of different components such as compressor, heat exchangers and expansion devices. The input p a rameters generally are: o p e r a ti ng pressure, heat e x c h a n g e r effectiveness, precooling temperature, mass fraction diverted to expansion machines, existance of ortho-para conversion units, efficiency of compressors and expansion machines etc. The output of the mathemetical models generally consists of yield, specific power consumption, liquid nitrogen consumption and figure of merit. Optimum design parameters and operating conditions have been derived based on the minimum specific power consumption.
DESIGN AND CONSTRUCTION OF A SMALL HYDROGEN LIQUEFIER
In spite of the current and future demand for liquid hydrogen many developing countries, including our own, are yet to make a beginning in this vital technology An attempt has been made at the Indian Inastitute of Technology, Kharagpur to construct an indigenous hydrogen liquefier. In the absence of an indigenous technology in expansion machines -reciprocating or turbine, our laboratory size liquefier is based on the precooled Linde-Hampson cycle. The liquefier
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consists of a gas holder, a compressor rated to deliver about 24 sm /hr 3 at 110 bar, a purification system, two liquid nitrogen baths: one at 77 K and the other at 65 K, the liquid hydrogen tank, six heat exchangers and the J.T. expansion valve. The compressor was available in the institute. The cold box containing heat exchangers, tanks, J.T valve and LH2 vessel is a stainless steel vessel 400 mm in diameter and 600 mm in height. The warm end heat exchanger is of coiled tube-in-shell
(Giaque Hampson construction) while the others are of tube in tube type.
Heat exchangers and the purifiers have been designed by us and fabricated in collaboration with a local industry. Other components such as LN^ and LH2 tanks, J.T valve and the necessary pipings and plumbings for the warm circuit have been designed and fabricated in house.
The warm circuit, apart from the gas holder and compressor, contains an elaborate purification system and a gas manifold section.
The purification system consists of (i) an oil separator, (ii) a charcoal bed for adsorption of oil vapour, and finally (iii) a cooled charcoal bed for adsorption of air impurities. The gas manifold section altogether contains eight cylinders: two for make up gas supply, four for recovery and two for nitrogen and helium purging gas supply.
The instrumentation consists of a thermal mass flowmeter and several pressure and temperature sensors. RTD and thermocouples have been used to measure the temperature at differnent locations in the flow stream.
Purity of the hydrogen stream is continuously monitored by comparing with a standrad sample in a binary gas analyser. Environment monitoring is provided by suitable sensors located on the ceiling. The system has been s u c c e s s f u l l y o pe r a t e d p r o d u c i n g l i q u id hydrogen. General indications are that the system will yield the designed output of about six liters/hour.
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COMPUTER AIDED DESIGN OF CRYOGENIC STORAGE VESSELS
Large storage vessels for liquid hydrogen and other c r y o g e n i c fluids is another area where a large technology gap exists between India and the western world. Adequate expertise and infrastructure are available in the country for manufacture of pressure vessel. The utilisation of their expertise in fabrication of large storage vessels for liquid hydrogen and other cryogenic fluids is hindered by the absence of proper design software.
CRYOTANK is a computer aided design software aimed at bridging the technology gap, at least partially. It is an user friendly programe, written in BASIC, to predict the thermal behavior of medium-to-large cryogenic storage vessels. The methemical model of the vessel computes the following:
(1) steady state heat inleak and boil off losses (2) initial cooldown and filling requirement
and (3) pressurant requirement for pressurisation to supercritical pressure
Four types of insulations have been considered: (a) high vacuum, (b) high vacuum with LN^ cooled radiation shield, (c) multilayer insulation and (d) multilayer insulation with LN2 cooled radiation shield. The data input is from the keyboard onto convenient forms appearing on the screen. The output information is presented both on the VDU and on the printer. It is hoped that this CAD package will be usefull to the Indian industry.
