Page 184 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: GGY Final Pages
Encyclopedia of Physical Science and Technology EN004D-156 June 8, 2001 15:28

Cryogenic Process Engineering 27

refrigeration for the process. In expansion engines, this is ature must be revised or thermal efficiency modified
accomplished by converting part of the energy of the high- accordingly.
pressure gas stream into mechanical work. This work in
large cryogenic facilities is recovered and utilized to re- C. Centrifugal Expanders
duce the overall compression requirements of the process.
Turboexpanders are classified as either axial or radial.
On the other hand, cooling of a gas can also be achieved by
Most turboexpanders built today are of the radial type
expanding the gas through an expansion valve (provided
because of their generally lower cost and reduced stresses
that its initial temperature is below the inversion tempera-
for a given tip speed. This permits them to run at higher
ture of the gas). The cooling here is accomplished by con-
speeds with higher efficiencies and lower operating costs.
verting part of the energy of the high-pressure gas stream
On the other hand, axial flow expanders are more suitable
into kinetic energy. No mechanical work is obtained from
for multistage expanders since they permit a much easier
such an expansion.
flow path from one stage to the next. Where low flow rates
Expanders are of either the reciprocating or the centrifu-
and high enthalpy reductions are required, an axial flow
gal type. With the rapid growth of tonnage in cryogenic
two-stage expander is generally used with nozzle valves
processes, centrifugal expanders have gradually displaced
controlling the flow. For example, in the processing of
the reciprocating type in large plants. However, the re-
ethylene, gas leaving the demethanizer is normally satu-
ciprocating expander is still popular for those processes
rated, and expansion conditions result in a liquid product
where the inlet temperature is very low, such as for hydro-
coming out of the expander. Since up to 15 to 20% liquid
gen or helium gas. Units up to 3600 hp (2685 kW) have
at the isentropic end point can be handled in actual flow
been put in service for nitrogen expansion in liquid hydro-
impulse turbine expanders, recovery of ethylene is feasi-
gen plants, while nonlubricated expanders with exhausts
ble by the procedure. Depending on the initial temperature
well below 33.3 K are being used in liquid hydrogen plants
and pressure into the expander and the final exit pressure,
developed for the space program.
good flow expanders are capable of reducing the enthalpy
of an expanded fluid by between 175 and 350 kJ/kg. and
1. Reciprocating Expanders
this may be multistaged. The change in enthalpy drop can
Generally, reciprocating expanders are selected when the be automatically regulated by turbine speed. The develop-
inlet pressure and pressure ratio are high and when the ment of highly reliable and efficient turboexpanders has
volume of gas handled is low. The inlet pressure to expan- madetoday’slarge-tonnageair-separationplantsandbase-
sion engines used in air-separation plants varies between 4 load LNG facilities a reality. Notable advances in turboex-
3
and 20 MPa, while capacities range from 0.1 to 3 m /sec. pander design for these applications center on improved
Isentropic efficiencies achieved are from 70 to 80%. bearings, lubrication, and wheel and rotor design to per-
The design features of reciprocating expanders em- mit nearly ideal rotor assembly speeds with good relia-
ployed in low-temperature processes include rigid, guided bility. Pressurized labyrinth sealing systems use dry seal
cam-actuated valve gears, renewable hardened valve seats, gas under pressure mixed with cold gas from the process
helical steel or air-springs, and special valve packings that to provide seal output temperatures above the frost point.
eliminate leakage. Cylinders are normally steel forgings Seal systems for oxygen compressors are more complex
effectively insulated from the rest of the structure. Re- than for air or nitrogen and must prevent oil carryover to
movable cylinder liners of Micarta or similar nonmetallic the processed gas. By the combination of variable-area
material and floating piston design offer wear resistance nozzle grouping or partial admission of multiple nozzle
and good alignment in operation. Piston rider rings serve grouping, efficiencies up to 85% have been obtained with
as guides for the piston. Nonmetallic rings are used for radial turboexpanders.
nonlubricated service. Both horizontal and vertical de- Turboalternators were developed in the 1960s to im-
sign, and one and two cylinder versions, have been used prove the efficiency of small cryogenic refrigeration sys-
successfully. tems. This is accomplished by converting the kinetic en-
Reciprocating expanders, in normal operation, should ergy in the expanding fluid to electrical energy, which
not accept liquid in any form during the expansion cy- in turn is transferred outside of the system, where it can
cle. However, the reciprocating device can tolerate some be converted to heat and dissipated to an ambient heat
liquid for short periods of time provided that none of sink.
the constituents freeze out in the expander cylinder and
cause serious mechanical problems. If selected design
D. Expansion Valve
conditions indicate possibilities of entering the liquid
and especially the triple point range on expansion dur- The expansion valve or Joule–Thomson valve, as it is of-
ing normal operation, then inlet pressure and temper- ten called, is an important component in any liquefaction