Hydrocarbon Processing - April 2021 - GP-40

BACK TO BASICS
Simplicity of design and ease of implementation are the main advantages
offered by the BR cycle. However, since
only sensible heat is transferred, the BR
cycle may be economically advantageous
only for micro- and small-scale LNG
production, as increasing the cooling
duty would result in an increase of N2
flowrate and a disadvantageous increase
in plant dimensions.
N2 is not flammable, nontoxic, easy to
procure and does not need storage facilities. For these characteristics, the BR cycle
is often considered for offshore application
where safety is a major issue. Moreover, it
is not affected by wave-induced motion.
Compression refrigeration cycle. In

the CRC, heat is extracted from a process
stream by evaporating, at low pressure,
the refrigerant fluid in a kettle-type heat
exchanger and rejecting heat by condensing the refrigerant vapor at relatively
high temperature.
The rejection is accomplished by
transferring the extracted heat to an ex-

ternal utility or to a heat sink within the
process, or to another refrigeration system (cascade refrigeration). The simplest refrigeration closed cycle entails a
sequence of evaporation (heat extraction
at low pressure), compression, condensation (heat rejection at high pressure) and
expansion. " Closed cycle " means that the
working fluid of the refrigeration system
is permanently contained within the mechanical system.
In the context of the LNG industry, the
thermodynamic efficiency of the basic
closed cycle can be improved by increasing the number of refrigeration stages or
by using more than a single working fluid
(refrigerant) in a cascade arrangement.
FIG. 2 shows a dual-compression, dual-expansion refrigeration system. The
refrigerant vapor stream (5) at high pressure is cooled and expanded in a throttling valve. The resulting two-phase flow
is separated in the economizer. The vapor
is recycled back to the second stage of the
compressor after having been mixed with
the outlet stream from the compressor

(5)
Two-stage
compressor

Condenser
(9)

(3)

(4)

Throttling
valve
(7)

Intercooler
Economizer

Temperature

(6)

(5)

(6)
(9)

(8)
(7)

(8)
Throttling
valve

Evaporator

(2)

(1)

(1)

(3)

(4)

Enthalpy

(2)

FIG. 2. The dual-compression, dual-expansion refrigeration cycle.
Condenser
(8)
Throttling
valve

(7)

Evaporator
(6)

(4)

Temperature

(5)

(3)

(4)

(3)

Condenser

Throttling
valve

(5)

Evaporator
(1)

(2)

(6)
(2)

(1)
Process stream

FIG. 3. Cascade refrigeration cycle.

40

(5)

(8)

MARCH/APRIL 2021 | GasProcessingNews.com

Enthalpy

(5*)

intercooler. The pressure of the liquid
phase withdrawn from the economizer is
reduced by means of a second throttling
valve and sent to the evaporator, where it
provides the refrigeration duty by evaporating at low pressure.
The thermodynamic transformation
taking place in this refrigeration cycle is
illustrated in the right side of FIG. 2. The
power required to transfer heat from the
evaporator to the condenser is given by
the enthalpy (H) difference between
points (5) and (2). If the pressure were
raised with only one compression stage,
the final state of the gas would be in the
position (5*). Since ΔH(5-2) is less than
ΔH(5*-2), the power requirement of the
two-stage compression is less than the
power required by a single-stage compression system.
It should be noted that if the throttling
valves are replaced with expanders, the
efficiency of the refrigeration is further
increased because the expansion across
a process expander provides additional
cooling and power recovery. In most
LNG facilities, the precooling of natural
gas is accomplished with a tri-stage propane refrigeration cycle.
In the cascade arrangement, two or
more refrigerant fluids (generally propane and ethane) are used in two distinct
refrigeration cycles. The low-temperature cycle provides the cooling in the
evaporator and rejects heat to the other
cycle by means of the evaporator/condenser heat exchanger. This latter is common to both cycles. FIG. 3 illustrates the
process setup for a cascade refrigeration
cycle. The cascade refrigeration concept
was used in early LNG plants.
Mixed refrigerant. A single refrigerant
fluid must be compressed and expanded
to pressures low enough to reach a temperature colder than the process stream.
The lower the gas liquefaction temperature, the larger its duty, and generally
the more complex the refrigeration system becomes.
Since natural gas is a mixture of components, its condensation curve-the
plot of temperature against specific enthalpy ( J/kg) or cumulative heat rate-
is a monotonic function that decreases
over the entire enthalpy domain. An inherent loss of thermodynamic efficiency
occurs when attempting to match the
discrete single-refrigerant temperature
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Hydrocarbon Processing - April 2021

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