1-1 Refrigeration Refrigeration systems are common in the natural gas processing industry and processes

related to the petroleum refining, petrochemical, and chemical industries. Several applications

for refrigeration include NGL recovery, LPG recovery, hydrocarbon dew point control, reflux

condensation for light hydrocarbon fractionators, and LNG plants.

Selection of a refrigerant is generally based upon temperature requirements, availability,

economics, and previous experience. For instance, in a natural gas processing plant, ethane

and propane may be at hand; whereas in an olefins plant, ethylene and propylene are readily

available. Propane or propylene may not be suitable in an ammonia plant because of the risk

of contamination, while ammonia may very well serve the purpose. Halocarbons have been

used extensively because of their nonflammable characteristics

1.2 Fundamental principles and processes

Single-stage mixed refrigerant processes that can provide refrigeration at very low

temperatures were first proposed nearly 70 years ago by Podbielniak and were adopted for

large-scale liquefaction of natural gas after the pioneering work of Kleemenko of the former

Soviet Union in the 1960s. Today most base-load natural gas liquefaction plants operate on

mixed refrigerant processes. Mixed refrigerant processes have also been adopted for peak-

shaving natural gas liquefaction plants.

Mixed refrigerant processes were also studied in the early 1970s in the former Soviet Union

by Brodyanskii and his colleagues for small cryocooler applications [The interest in mixed

refrigerant cryocoolers was revived about 10 years ago when DARPA funded projects for the

development of low-cost cryocoolers Currently, there’s worldwide interest in using mixed

refrigerant processes for the liquefaction of nitrogen and separation of air Several U.S. patents

have been granted during the last five years on the liquefaction of nitrogen using mixed

refrigerant processes and two large plants have been built and tested Refrigeration processes

can be divided into two broad groups based on the variation of pressure with time at any

location of the process as follows [

• periodic refrigerators in which the pressure at any point of the cycle varies with time, and

• steady-state refrigerators inwhich the pressure at any point of the cycle is constant and does

not vary with time.

Steady-state processes have been used in cryogenic liquefiers and refrigerators for over a

century. Pure (single-component) fluids have traditionally been used in cryogenic

refrigerators, whereas the fluid being liquefied is itself used as the refrigerant in the traditional

liquefaction processes, the exception being the liquefaction of natural gas using mixed

refrigerant processes. There are several advantages in using zeotropic refrigerant mixtures in

cryogenic refrigerators and liquefaction systems:

• The exergy efficiency (figure of merit) of refrigeration and liquefaction systems

operating with refrigerant mixtures is many times that of systems operating with

pure fluids.

• The operating pressure is much lower when refrigerant mixtures are used, compared to pure

fluids.

• Refrigeration and liquefaction systems operating with pure fluids operate largely in the

superheated vapor region, whereas those operatingwith refrigerant mixtures operate largely in

the two-phase region. Consequently, the heat transfer coefficients in the heat exchangers are

much larger in systems operatingwith refrigerant mixtures compared to those operating with

pure fluids, resulting in smaller heat exchangers.

• The degradation of heat exchanger performance due to longitudinal (axial) heat conduction

is much smaller due to higher apparent specific heat of refrigerant mixtures

in the two-phase region compared to the specific heat at constant pressure (cp) of pure fluids

in the superheated (single-phase) region.

1.3 Properties of Pure substance A pure substance is one whose chemical composition does not change during

thermodynamic processes. Water and refrigerants are examples of pure substances. These

days emphasis is on the use mixture of refrigerants. The properties of mixtures also require

understanding of the properties of pure substances

Water is a substance of prime importance in refrigeration and air-conditioning. It exists in

three states namely, solid ice, liquid water and water vapor and undergoes transformation

from one state to another. Steam and hot water are used for heating of buildings while chilled

water is used for cooling of buildings. Hence, an understanding of its properties is essential

for air conditioning calculations. Substances, which absorb heat from other substances or

space, are called refrigerants. These substances also exist in three states. These also undergo

transformations usually from liquid to vapor and vice-versa during heat absorption and

rejection respectively. Hence, it is important to understand their properties also.

If a liquid (pure substance) is heated at constant pressure, the temperature at which it boils

is called saturation temperature. This temperature will remain constant during heating until all

the liquid boils off. At this temperature, the liquid and the associated vapor at same

temperature are in equilibrium and are called saturated liquid and vapor respectively. The

saturation temperature of a pure substance is a function of pressure only. At atmospheric

pressure, the saturation temperature is called normal boiling point. Similarly, if the vapour of

a pure substance is cooled at constant pressure, the temperature at which the condensation

starts, is called dew point temperature. For a pure substance, dew point and boiling point are

same at a given pressure.

Similarly, when a solid is heated at constant, it melts at a definite temperature called

melting point. Similarly cooling of a liquid causes freezing at the freezing point. The melting

point and freezing point are same at same pressure for a pure substance and the solid and

liquid are in equilibrium at this temperature. For all pure substances there is a temperature at

which all the three phases exist in equilibrium. This is called triple point

The liquid-vapor phase diagram of pure substance is conveniently shown in temperature

entropy diagram or pressure-enthalpy diagram or p-v diagram. Sometimes, three dimensional

p-v-t diagrams are also drawn to show the phase transformation. In most of the refrigeration

applications except dry ice manufacture, we encounter liquid and vapor phases only.

Thermodynamic properties of various pure substances are available in the form of charts and

tables

Fig. 1.1 P-h diagram for a pure substance.

The next figures show the different charts of the refrigerants and the other refrigeration

medium.

1.4 Ton of refrigeration (TOR) The cooling capacity of older refrigeration units is often indicated in "tons of refrigeration"

(TOR). A ton of refrigeration represents the heat energy absorbed when a ton (2000lbs.) of ice

melts during a 24-hour day. The ice assumed to be solid as 32 degrees F. (0 degrees C.)

initially and becomes water at 32 degrees F. (0 degrees C.). The energy absorbed by the ice is

the latent heat of ice times the total weight.

Today, refrigeration units are often rated in Btu/hr or KW instead of tons. The Btu

equivalent of one ton of refrigeration is easy to calculate. Multiply the weight of one ton

of ice (2000lbs.) by the latent heat of fusion (melting) of ice (144 Btu/lb.). Then divide by

24 hours to obtain Btu/hr.

TOR = 2000 X 144/24

TOR = 288,000 Btu/24 hours

TOR = 12,000 Btu/hr.

TOR = 3.51 kW

1.5 Standard vapor compression refrigeration system (VCRS) Vapor-compression refrigeration systems are the most common refrigeration systems in use

today. Figure 5.2 shows the different components of the vapor compression refrigeration

system. The cycle consists of four thermodynamically processes as follows :

Process 1-2: Isentropic compression of saturated vapour in compressor (S=const.)

Process 2-3: Isobaric heat rejection in condenser (Pc=const.)

Process 3-4: Isenthalpic expansion of saturated liquid in expansion device (h=const.)

Process 4-1: Isobaric heat extraction in the evaporator (Pe=const.)

Figure 1.3 shows the pressure enthalpy diagram of the standard vapor compression

refrigeration cycle, while figure 5.4 shows the (T-S) diagram.

Figure 1.2 Components of vapor compression refrigeration system.

By utilizing the Pressure-Enthalpy (P-H) diagram, the refrigeration cycle can be broken down

into four distinct steps:

1- Expansion

2- Evaporation

3- Compression

4- Condensation

The vapor-compression refrigeration cycle can be represented by the process flow and P-H

diagram shown in Fig. 1.3.

Fig. 1.3 (P-h) diagram of vapor compression refrigeration cycle.

Fig. 1.4.Schematic of (a) a reversible refrigerator and (b) reversible gas cooler for cooling/

liquefying a gas from state 1 to state 2.

Consider a refrigerator that provides refrigeration over a constant temperature and operates

on reversible thermodynamic processes. Such a refrigerator will henceforth be called a

reversible refrigerator [Fig. 1.4(a)]. Heat is rejected to the surroundings at a temperature To

and absorbed at a temperature T .T <To/. The heat transfer between the refrigerator and

source/sink is assumed to occur at a zero temperature difference

in all reversible refrigerators. The temperature of the refrigerant is therefore the same as that

of the ambient (To) during the heat rejection process and that of the load (T ) during the heat

absorption process. The first and second laws of thermodynamics can be written for a

reversible refrigerator as follows:

Substituting Eq. (1.2) into Eq. (1.1) gives the expression for the power required by a

reversible refrigerator as follows:

where T and To refer to the refrigeration and ambient temperatures, respectively. Q and _ Qo

are the heat absorbed and heat rejected, respectively

The coefficient of performance (COP) of any refrigerator is defined as follows:

where Q and _Wc refer to the heat absorbed and compressor work input in joules, and Q and

_ Wc refer to the heat transfer rate from the low-temperature source and the power supplied to

the compressor in watts.

The coefficient of performance (COP) of an ideal reversible refrigerator providing

refrigeration at constant temperature can be expressed in terms of the temperatures for the

heat source and heat sink using Eq. (1.3) as follows:

Figure 1.4(b) shows the schematic of a gas cooler in which the process fluid is cooled from a

temperature T1 to a temperature T2. The first and second laws of thermodynamics can be

written for the control volume of an ideal gas cooler [Fig. 1.4(b)] operating on reversible

processes and providing refrigeration over a range of temperatures as follows:

Where – W revl, refers to the power input to the reversible gas cooler and Pn is the mole flow

rate of the process fluid.

Substituting Eq. (1.7) into Eq. (1.6) gives the expression for the minimum power required

for cooling a gas from state 1 to state 2 as follows:

In the above expression, ex refers to the exergy of the fluid being cooled((eх=(h-ho)-To(s-

so)) and To is the ambient temperature. It is evident from Eq. (1.8) that the minimum work

required to cool a unit mole of a gas using an ideal gas cooler operating on reversible

processes is the same as the exergy change of the fluid being cooled and is independent of the

process used for cooling.

Since the entropy at state points 2 and 3 is the same(S2=S3), the above expression for the net

power required to cool the gas from temperatureT1 to T3 can be expressed as

1.6 Two stage of compression with flash intercooler and gas removal In this system the flash intercooler is used to cool the refrigerant between the two stages of

compression to reduce the total work and the compression temperature. As well as, to remove

the gases from the expansion process before interring to the evaporator to increase the

refrigeration effect. The mass flow rate of the refrigerant is not constant in the whole of the

cycle due to the heat and mass exchange in the flash tank. However the mass flow could be

divided into two levels, one for the lower cycle is called m_ _ and the other for the upper

cycle is called m_ _. Figures 1.5 & 1.6 show the flow diagram of the cycle and the (P-h)

diagram. Due to the variation of the mass flow rate, the cycle calculations should be related to

power not to work calculations.

The intermediate pressure

To find the mass flow rates through the cycle, the lower cycle mass flow rate could be

given from the refrigeration effect as;

After you can find the value of the upper cycle mass flow rate by applying the enrgy

equation on the flash tank as;

The total Power is

The refrigeration effect

The system coefficient of performance

Fig. 1.5 Two stage of compression with flash intercooler system.

Fig. 1.6 (P-h) diagram for two stage of compression with flash intercooler system.

1.7 Cascade refrigeration system A cascade system consists of two separate single-stage refrigeration systems: a lower

system that can better maintain lower evaporating temperatures and a higher system that

performs better at higher evaporating temperatures. These two systems are connected by

a cascade condenser in which the condenser of the lower system becomes the evaporator

of the higher system as the higher system’s evaporator takes on the heat released from the

lower system’s condenser. See figures 1.7 and 1.8.

It is often desirable to have a heat exchanger between the liquid refrigerant from the

cascade condenser and the vapor refrigerant leaving the evaporator of the lower system.

The liquid refrigerant can be sub-cooled to a lower temperature before entering the

evaporator of the lower system, as shown in the next figure. Because the evaporating

temperature is low, there is no danger of too high a discharge temperature after the

compression process of the lower system.

When a cascade system is shut down while the temperature of the ambient air is 25°C,

the saturated vapor pressure of the refrigerant increases. For a lower system using HFC-

125 as the refrigerant, this saturated pressure may increase to 1440 kPa abs. For safety

reasons, a relief valve at the cascade condenser connects to an expansion tank, designed

to store the refrigerant from the lower system in case of shutdown. For extremely low

evaporating temperatures, a multistage compression system may be used in either the

lower or higher system of a cascade system.

1.7.1 Advantages and Disadvantages The main advantage of a cascade system is that different refrigerants, equipment, and oils

can be used for the higher and the lower systems. This is especially helpful when the

evaporating temperature required in the lower system is less than -60°C.

One disadvantage of a cascade system is the overlap of the condensing temperature of the

lower system and the evaporating temperature of the higher system for heat transfer in the

condenser. The overlap results in higher energy consumption. Also a cascade system is

more complicated than a compound system.

The performance of the cascade system can be measured in terms of 1 kg of refrigerant in

the lower system, for the sake of convenience. If the heat transfer between the cascade

condenser and the surroundings is ignored, then the heat released by the condenser of the

lower system is equal to the refrigerating load on the evaporator of the higher system.

1.7.2 Assumptions

The following assumptions are made in all the examples provided in this chapter:

1-The pressure drop in all heat exchangers and phase separators is zero.

2-The ambient temperature is 300 K.

3-The minimum temperature approach between the hot and cold streams is 3 K in all cold

heat exchangers.

4-The adiabatic efficiency of all compressors is 80% and that of all pumps is 90%.

5-The heat inleak from ambient is negligible.

Figure 1.7 Refrigeration Cascade system layout.

Figure 1.8 (p-h) diagram for cascade refrigeration system

1.7.2 Applications of cascade systems

1-Liquefaction of natural gas and petroleum vapours

2-Liquefaction of industrial gases

3-Manufacturing of dry ice

4-Deep freezing etc.

5-Medical applications

1.7.3Optimum cascade temperature:

For a two-stage cascade system working on Carnot cycle, the optimum cascade

temperature at which the COP will be maximum, Tcc,opt is given by:

where Te and Tc are the evaporator temperature of low temperature cascade and

condenser temperature of high temperature cascade, respectively.

Refrigeration effect

Total Power

Heat balance of the cascade condenser

And the C.O.P

In a refrigeration cycle, energy is transferred from lower to higher temperature levels

economically by using water or ambient air as the ultimate heat sink. If ethane is used as a

refrigerant, the warmest temperature level to condense ethane is its critical temperature of

about 90°F. This temperature requires unusually high compression ratios — making an ethane

compressor for such service complicated and uneconomical. Also in order to condense ethane

at 90°F, a heat sink at 85°F or lower is necessary. This condensing temperature is a difficult

cooling water requirement in many locations. Thus a refrigerant such as propane is cascaded

with ethane to transfer the energy from the ethane system to cooling water or air.

One example of cascade cycle is ConocoPhillips currently has at least two trains in

operation: Atlantic LNG, and Egyptian LNG. More trains are being constructed since this

process is expanding to compete with the APCI. It shares about 5% of the world’s LNG

production and it has been in operation for more than 30 years.

The process uses a three stage pure component refrigerant cascade of propane, ethylene,

and methane .The pretreated natural gas enters the first cycle or cooling stage which uses

propane as a refrigerant. This stage cools the natural gas to about -35oC and it also cools the

other two refrigerants to the same temperature. Propane is chosen as the first stage refrigerant

because it is available in large quantities worldwide and it is one of the cheapest refrigerants.

The natural gas then enters the second cooling stage which uses ethylene as the refrigerant

and this stage cools the natural gas to about -95oC. At this stage the natural gas is converted

to a liquid phase (LNG) but the natural gas needs to be further sub cooled so the fuel gas

produced would not exceed 5% when the LNG stream is flashed. Ethylene is used as the

second stage refrigerant because it condenses methane at a pressure above atmospheric and it

could be also condensed by propane. After methane has been condensed by ethylene, it is sent

to the third stage where it sub cools the natural gas to about - 155oC then it is expanded

through a valve which drops down the LNG temperature to about - 160oC. Methane is sent

back to the first cooling stage and the LNG stream is flashed into about 95% LNG (which is

sent to storage tanks) and 5% fuel gas used as the liquefaction process fuel. Methane is used

as the sub cooling stage refrigerant because it could sub cools up to -155 oC and it is available

in the natural gas stream so it is available at all times and at lower costs.

Figure 1-9: ConocoPhillips simple cascade schematic

We will now attempt to perform a simulation of this simple process. We firs recognize that

the boiling points of each of the refrigerants will limit the temperatures at the outlet of each

exchanger.

Table 1.1: Refrigerants boiling points

The T-Q profile of the simple cascade is shown in the next figure:

Figure 1-10: Simple ConocoPhillips Cascade LNG Cooling Curve

1.8 Mixed Refrigerants

The MR process uses a mixture of hydrocarbons and nitrogen as a single refrigerant. Selecting

the correct composition of refrigerant to maintain proper temperatures and pressures during

separation and flashing is an important part of optimization.

Figure 1-11: Refrigerant vs. LNG cooling curve

One of the example of MR is APCI. This process accounts for a very significant proportion

of the world's baseload LNG production capacity. Train capacities of up to 4.7 million tpy

were built or are under construction. It's illustrated in Figure 4 as part of an overall LNG plant

flow schme.

There are tow main refrigeration cycles . The precooling cycle uses a pure component ,

propane. The liquefaction and sub-cooling cycle uses a mixed refrigerant (MR) made up of

nitrogen , methane, ethane and propane.

The precooling cycle uses propane at three or four pressure levels and can cool the

process gas down to -40° C. it's also used to cool and partially liquefy the MR. The cooling

is achieved in kettle-type exchangers with propane refrigerant boiling and evaporating in a

pool on the shell side, and with the process streams flowing in immersed tube passes.

A centrifugal compressor wih side streams recovers the evaporated C3 streams and

compresses the vapour to 15-25 Bara to be condensed against water or air and recycled to

the propane kettles.

In the MR cycles, the partially liquefied refrigerant is separated into vapour and liquid

streams that are used to liquefy and sub-cool the process streams from typically -35°C to

between -150°C-160°C. This is carried out in a proprietary spiral wound exchanger , the main

cryogenic heat exchanger (MCHE).

The MCHE consists of two or three tube bundles arranged in a vertical shell, with the

process gas and refrigeration entering the tubes at the bottom which then flow upward under

pressure.

The process gas passes through all the bundles to emerge liquefied at the top. The liquid MR

streams is extracted after the warm or middle bundle and is flashed across a joule Thomson

valve or hydraulic expander onto the shell side.It flows down wards and evaporates, providing

the bulk of cooling for the lower bundles. The vapour MR streams passes to the top(cold

bundle) and is liquefied and sub-cooled, and is flashed across a JT valve into the shell side

over the top of the cold bundle . It flows downwards to provide the cooling duty for the top

bundle and, after mixing with liquid MR, part of the duty for the lower bundles.

The overall vaporized MR streams from the bottom of the MCHE is recovered and

compressed by the MR compressor to 45-48 Bara . It's cooled and partially liquefied first by

water or air and then by the propane refrigerant, and recycled to the MCHE. In earlier plants

all stages of the MR compression were normally centrifugal, however, in some recent plants

axial compressors have been used for the LP stage and centrifugal for the HP stage.

Figre 1.12: APCI propane precooled mixed refrigerant process.