Page 190 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN004D-156 June 8, 2001 15:28

Cryogenic Process Engineering 33

the insulation space is evacuated. In small laboratory de- pumping. Self-pressurization involves removing some of
wars, the insulation is obtained by coating the two surfaces the fluid from the container, vaporizing the extracted fluid,
facing the insulation space with low-emissivity materials and then reintroducing the vapor into the ullage space,
and then evacuating the space to a pressure of 0.13 mPa thereby displacing the contents of the container. The ex-
or lower. In larger vessels, insulations such as powders, ternal gas pressurization method utilizes an external gas
fibrous materials, or multilayer insulations are used. to accomplish the desired displacement of the container
Several requirements must be met in the design of the contents. In the mechanical pumping method, the contents
inner vessel. The material of construction must be compat- of the stroage vessel are removed by a cryogenic pump lo-
ible with the stored cryogen. Nine percent nickel steels are cated in the liquid drain line.
acceptable for high-boiling cryogens (T > 75 K), while Several different types of pumps have been used with
many aluminum alloys and austenitic steels are usually cryogenic fluids. In general, the region of low flow rates
structurally acceptable throughout the entire temperature at high pressures is best suited to positive displacement
range. Economic and cooldown considerations dictate that pumps, while the high-flow applications are generally best
theinnershellbeasthinaspossible.Accordingly,theinner served by the use of centrifugal or axial flow pumps. The
container is designed to withstand only the internal pres- latter have been built and used for liquid hydrogen with
3
sure and bending forces, while stiffening rings are used flow rates of up to 3.8 m /sec and pressures of more than
to support the weight of the fluid. The minimum thick- 6.9 MPa. For successful operation, cryogen subcooling,
ness of the inner shell for a cylindrical vessel under such a thermal contraction, lubrication, and compatibility of ma-
design arrangement is given in Section VIII of the Ameri- terials must be carefully considered.
can Society of Mechanical Engineers’ (ASME) Boiler and Cryogenic fluid transfer lines are generally classified
Pressure Vessel Code. as one of three types: uninsulated, foam-insulated lines,
The outer shell of the stroage vessel, on the other hand, and vacuum-insulated lines. The latter may entail vacuum
is subjected to atmospheric pressure on the outside and insulation alone, evacuated powder insulation, or multi-
evacuated conditions on the inside. Such a pressure differ- layer insulation. A vapor barrier must be applied to the
ence requires an outer shell of sufficient material thickness outer surface of foam-insulated transfer lines to minimize
with appropriatelyplaced stiffeningringstowithstand col- the degradation of the insulation that occurs when wa-
lapsing or buckling. Here again, specific design charts ad- ter vapor and other condensables are permitted to diffuse
dressing this situation can be found in the ASME code. through the insulation to the cold surface of the lines.
Heat leak into a storage system for cryogens generally Two-phase flow is always involved in the cooldown of
occurs by radiation and conduction through the insulation a transfer line. Since this process is a transient one, several
and conduction through the inner shell supports, piping, different types of two-phase flow will exist simultaneously
instrumentation leads, and access ports. Conduction losses along the inlet of the transfer line. Severe pressure and
are reduced by introducing long heat-leak paths, by mak- flow oscillations occur as the cold liquid comes in contact
ing the cross-sections for heat flow small, and by using with successive warm sections of the line. Such instability
materials with low thermal conductivity. Radiation losses, continues until the entire transfer line is cooled down and
a major factor in the heat leak through insulations, are re- filled with liquid cryogen.
duced with the use of radiation shields, such as multilayer The transport of cryogens for more than a few hundred
insulation, boil-off vapor-cooled shields, and opacifiers in meters generally requires specially built transport systems
powder insulation. for truck, railroad, or airline delivery. Volumes from 0.02
3
Most storage vessels for cryogens are designed for a to more than 100 m have been transported successfully
90% liquid volume and a 10% vapor or ullage volume. by these carriers. The use of large barges and ships built
The latter permits reasonable vaporization of the liquid specifically for cryogen shipment has increased the vol-
contents due to heat leak without incurring too rapid a ume transported manyfold. This has been particularly true
buildup of pressure in the vessel. This, in turn, permits for the worldwide transport of LNG.
closure of the container for short periods either to avoid
partial loss of the contents or to permit the safe transport
of flammable or hazardous cryogens. VII. INSTRUMENTATION

Once low temperatures have been attained and cryogens
C. Transfer Systems
havebeenproduced,propertymeasurementsmustoftenbe
Three methods are commonly used to transfer a cryogen made at these temperatures. Such measurements as tem-
from the storage vessel. These are self-pressurization of perature and pressure are typically required for process
the container, external gas pressurization, and mechanical optimization and control. In addition, as cryogenic fluids