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RefrigerationWritten by Norrie
Wednesday, 17 February 2010 20:42
INTRODUCTION
"Refrigeration is the Science of the Production of 'Coldness'"
Refrigeration is a cooling process used to remove heat energy from a substance by the evaporation principle, toproduce a temperature below that of its surroundings.
Simple examples of refrigeration are:
1. In desert locations, the Nomads throw water over their tents. As the water evaporates, the heat required for theevaporation process is removed from the interior of the tent thereby causing cooling.
2. If you dip your finger in alcohol, and then blow gently on it, your finger feels colder again this is due to theevaporation process which removes heat from the finger. In order to produce the cooling effect required, asubstance called a 'REFRIGERANT' is used.
The desirable properties of a refrigerant are listed below:
A Low Boiling Point below the temperature it is expected to maintain.A Low Freezing Point below the minimum temperature the system can reach to prevent solidification of therefrigerant.A High Latent Heat Will require more latent heat of vaporization which will remove more sensible heat fromthe substance or space being cooled.A High Critical Temperature Can be easily condensed. A Low Critical Pressure Will condense at relativelylow pressure, thereby needing low energy to produce the condensation pressure.NonCorrosive, NonToxic, NonFlammable.
However, some refrigerants used in industry do not meet all of these conditions.
For example:
Ammonia (BP 28 °F) is a good refrigerant, but it is toxic and, in the presence of water, it is corrosive.Propane (BP 44°F), is also a good refrigerant but, it is highly flammable.
Generally, refrigerants used in cooling systems are:
Arcton and Freon trade names for a group of refrigerants called 'ChloroFluoro Carbons' (CFC's), are used insome industries and particularly in domestic fridges, and meet the requirements listed above.CO2 (BP = 108 °F), can also be used as a refrigerant.
However, attempts to decrease the amount of 'Chlorofluorocarbons' (CFC's) are being made, in an effort to preventthe deterioration of the Ozone layer around the earth with helps to protect us from high levels of Ultraviolet light
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from the sun. Modern refrigerants are being developed which are not based on CFC's.
The purpose of a refrigeration system is to maintain a material or an inside environment (fridge, freezer, office,home, car, etc) at a desired temperature below that of the atmosphere.
To accomplish this, the refrigeration substance, (refrigerant), must continuously remove the heat from the material orthe space being cooled. The removed heat will then be dissipated into the atmosphere OUTSIDE of the space beingcooled.
When your fridge or freezer at home is running, you will note that there is quite a lot of heat coming from the backof the unit. This heat is due to the compression of the refrigerant gas PLUS the heat removed from the food in thefridge / freezer. Due to this heat, the room in which the fridge / freezer is kept, will be at a higher temperature thanthe rest of the building interior, (unless the building is air conditioned, which includes the room containing the fridge usually the kitchen).
REFRIGERATION PRINCIPLES
As stated earlier, water thrown over a tent in the hot desert sun will cause cooling. Evaporation of a liquid needs heatenergy. The latent heat required for the evaporation to take place is produced by the sensible heat from the tentinterior which, in turn causes cooling of the tent.
If you dip your fingers in a volatile liquid like gasoline or alcohol and then allowed the liquid to evaporate, you canfeel a cooling effect. The same thing will happen with water but, will not be as noticeable, as the evaporation processis slower. The cooling effect, again, comes from the heat being removed from your fingers to evaporate the liquid.
Large, modern refrigeration units use the evaporation principle to produce the low temperatures necessary to do thejob required. The process consists of a cycle of compression, cooling and condensation, then the expansion of theliquid, evaporation and recompression of the vapour.
THE VAPOUR COMPRESSION REFRIGERATION CYCLE
The principles used in a vapour compression refrigeration system are:
A. Compression of a gas causes its temperature to increase. When the gas is cooled and sensible and latent heat removed,the temperature decreases and the gas condenses to liquid which is also the boiling point of the liquid. (Thecompression also increases the temperature at which the liquid boils). The liquid is then further cooled to aroundatmospheric temperature.
B. When the liquid is expanded (volume increased) into a lower pressure system, it will boil and cause the liquidtemperature to decrease rapidly as it gives up sensible heat to provide the latent heat of partial vaporization of theliquid. The cold liquid and vapour, (the latent heat does not increase the vapour temperature), now pass through thecoils inside the ' Cold Box ' (or Evaporator) of the system.
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Exchange of heat between the refrigerant and the material or space being cooled, adds more heat to the refrigerantliquid which continues to evaporate. The refrigerant, on leaving the cooling system is now all cool vapour and passesto the suction of the compressor to begin the cycle again.
The sequence of the refrigeration cycle is as follows:
1. Compression of the refrigerant gas.2. Cooling and condensation of the refrigerant to liquid.3. Expansion and partial evaporation of the liquid into a lower pressure which causes cooling.4. Continued evaporation of the liquid in the 'cold box' further heating by removal of sensible heat from the
item being cooled, to provide the latent heat of vaporization of the refrigerant.5. Recompression of the vapour to begin the cycle again.
Figure: 7, shows a simple block diagram of the refrigeration thermodynamic cycle.
Figure: 7
The type of compressor used in refrigeration systems may be reciprocating or centrifugal in operation. Largeindustrial units may have multistage compression systems with interstage cooling, in order to achieve the requiredrefrigerant pressure without excessive temperature increases.
The cooling / condensing unit of a system can consist of cooling by natural convection as in a household fridge, orby forced air cooling i.e. a fan or fans to force the air over the cooling coils (similar to a car radiator), or by watercooled heat exchange equipment.
(Shown in Figure: 8).
Control of a refrigeration unit can be by a thermostatic system which will start and stop the compressor (a bimetalstrip switch or mercury switch), or, in large units, by control of the expansion valve. When the unit shuts down, the expansion valve will also close by activation of a solenoid valve.
Following are some typical uses of refrigeration in industry and everyday life.
To Reduce the Rate of Chemical Reactions and Storage of Food and OtherPerishable GoodsTo Store Flammable MaterialsTo Condense the Vapour of Low Boiling Point LiquidsFor Air ConditioningFor Freeze Drying of MaterialsFor Separation and Recovery of Process Fluids
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To Produce Cryogenic Temperatures
Figure: 8
CHEMICAL REACTION RATE
Some chemical reactions take place so rapidly that large releases of energy occur such as heat, fire and explosion. Inthese cases, the danger can be eliminated if the reactants are cooled before and during the reaction to a much lowertemperature, the reaction rate decreases and proceeds more slowly.
An example of this is the mixing of strong acids with strong alkalis; the heat produced may cause violent boiling andvaporization of the liquids giving rise to safety hazards. Cooling of the reactants will slow down the rate of reaction toa safer level.
Another good example of the need for cooling is the storage of food. A piece of meat at about 20 °C will begin todecompose in a day or two decomposition is a chemical reaction. By keeping the meat in a refrigerator, it will stayquite fresh for up to a week and, if stored in a freezer at about 25°C, it will keep for 6 months or more. Thedecomposition reaction is slowed down considerably.
A further example is the storage of drugs under refrigeration in order to protect them from deterioration. Figure: 9, Shows a typical household refrigerator. The thermostat simply starts and stops the compressor driver.
STORAGE OF FLAMMABLE MATERIALS
Some liquids are said to be 'Flammable' i.e. will burn. However, it is not the liquid that burns but its vapour. Theease with which a vapour from a liquid will catch fire, is known as the ' Flash Point ' of the liquid. The flash point isdefined as 'The Lowest Temperature at which a Material gives off a vapour that will ignite when exposed to a flame'.
Some liquids having a very low flash point i.e. BELOW the normal ambient temperature, and are likely to explodeif exposed to a spark electrically or mechanically produced. In this case, the liquids may have to be stored underrefrigeration to keep them safely below their flash point temperature.
Also, low BP liquids can 'Autoignite' if increased to the temperature at which this will occur such as leakage on to asteam line or other hot surface.
HOUSEHOLD REFRIGERATOR
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CONDENSATION OF THE VAPOUR OF LOW BP LIQUIDS
Normal Butane has a boiling point temperature of 0.5 °C (31 °F), therefore, at normal atmospheric temperatures,butane is a gas. Butane gas can be liquefied by compression and cooling to ambient or higher temperature or, it can becondensed by refrigeration cooling to below its boiling point. When the pressure of butane gas is raised to about 75psi, (around 5 bar), it will condense at about 42 °C (108 °F).
A further reason for condensing butane gas is that its volume as a liquid is about 1,000 times less than as a gas. Thestorage space needed is therefore also reduced. Cylinders of butane liquid are used for cooking and heating and otherpurposes, both domestically and in industry.
However, in cold winter conditions, the ambient temperature will possibly be too low for the butane liquid to boil andgive off its flammable vapour. In these cases, Propane liquid is used which boils at 42 °C ( 44 °F) which will giveoff vapour in very cold conditions.
Lighter gases, like Ethane and Methane can also be liquefied, (Liquid Natural Gas LNG), but, in these cases, thecondensation will not take place at ambient temperature, no matter what pressure is applied to them. However, theywill condense if cooled to a specific temperature called the 'Critical Temperature' of the gas concerned.
Critical Temperature: is defined as : 'The temperature ABOVE which a gas WILL NOT liquefy, irrespective of the pressure applied'.
Even at the critical temperature, very high pressure may be needed to condense the gas.
Associated with critical temperature, is 'Critical Pressure', which is defined as : 'The pressure required to condense a gas AT ITS CRITICAL TEMPERATURE'.
Table 1, below, gives typical Critical Temperatures and Pressures of some gases.
GAS CRITICAL TEMPERATURE CRITICAL PRESSURE
1. Nitrogen 147 °C ( 232 °F) 3.40 MPa (500 psi)
2. Oxygen 118 °C ( 180 °F) 5.00 MPa (730 psi)
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3. Methane 83 °C ( 117 °F) 4.60 MPa (670 psi)
4. Ethane 32 °C ( 90 °F ) 4.90 MPa (700 psi)
5. Propane 96 °C ( 208 °F ) 4.25 MPa (620 psi)
TABLE 1
In order to condense gases 1, 2 & 3, (in Table 1), they must be cooled to very low temperature in addition to being athigh pressure.
The process of compression and cooling to below critical temperature is known as the ' Liquefaction of Gases ' and isused to produce liquefied air from which liquid oxygen and nitrogen can be extracted by distillation. It is also used toproduce Liquid Natural Gas (LNG).
After a gas has been condensed at high pressure, the liquid can be stored at just above atmospheric pressure but, inthis case, the storage vessel must be specially constructed and insulated.
Some 'boiloff' of vapour will take place and the gas from the boil off may be utilised as fuel or simply vented to aflare system. The construction and insulation method will govern the amount of vapour boiloff which will occur dueto some heat energy finding its way into the liquid.
This boil off (evaporation), will remove heat from the liquid and therefore maintain the liquid temperature at itsboiling point, at the storage pressure involved, which is controlled.
An example of boiloff gases being utilised, is in the shipping of LNG. The boiloff gas is used as fuel to the ship'sturbines driving the propellers.
Table 2, below, shows the effects of pressure on the boiling point of some liquids.
Substance BP @ 0 Psig BP @ 50 Psig BP @ 100 Psig
Methane 259 °F 218 °F 200 °F
Ethane 124 °F 68 °F 38 °F
Propane 44 °F 28 °F 66 °F
Butane 31 °F 115 °F 156 °F
Pentane 97 °F 192 °F 237 °F
Hexane 156 °F 258 °F 307 °F
Water 212 °F 300 °F 338 °F
TABLE 2
4. AIR CONDITIONING (See Figure : 10)
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In hot climates and in summer time, we often feel uncomfortable due to the heat and humidity of the atmosphere.
Many modern work places, homes and vehicles, now have Air Conditioning (A/C) Units installed. These operate byhaving an 'Air Mover' (Fan) circulating air through a chamber which contains refrigerated cooling coils.
The actual refrigeration system is installed OUTSIDE the building or space being cooled. The refrigeration coils areset into the cooling chamber, also outside or in a separate room, through which air from the INSIDE of the space isbeing drawn by the fan (air mover).
The cooling coils are carrying a refrigerant, that has been expanded across an expansion valve and is being revaporised by removing heat from the circulating air passing over the coils and, after cooling by the refrigerant, the airis discharged back into the building.
As the cooled air exchanges heat with the contents of the space, the space and contents cool down, the warmer air thenreturns to the fan to repeat the cycle.
At the same time, the air gives up much of its moisture (humidity) by condensation on the cooling coils. Thecondensed water is collected in a driptray and disposed of.
The amount of work done by the system depends on the insulation of the building or space being cooled. Poorinsulation may result in the fridge unit not being able to handle the amount of heat to be removed.
Working, living and driving conditions are thereby made much more comfortable.
In summary, the process is as follows:
THE HEAT REMOVED FROM THE SPACE AND ITS CONTENTS, IS ABSORBED BY THEEVAPORATION PROCESS OF THE REFRIGERANT LIQUID AS IT PASSES THROUGH THE COILS INTHE HEAT EXCHANGE CHAMBER.
THIS HEAT, TOGETHER WITH THE SUBSEQUENT HEAT OF COMPRESSION, PASSES INTO THEATMOSPHERE (OR CONDENSER COOLANT), BY WAY OF THE OUTSIDE REFRIGERANTCONDENSER.
AIR CONDITIONING UNIT
Figure: 10
Refrigeration control systems vary depending upon the requirements. A household fridge or freezer has a simple, bimetallic strip type thermostat which starts and stops the compressor motor according to the temperature setting.
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If continuous operation is needed, it will be controlled by the automatic, thermostatic operation of the expansionvalve. (See Figure: 11)
REFRIGERATOR COLD BOX
Figure: 11
In an A/C system, the air circulation fan will be connected by electrical relays into the cooling system controls in that,as the fridge unit reaches its set temperature and switches off, the fan also stops.
A selector switch in the control system may, however, be set to keep the fan running when the fridge unit shuts down,This would be used to maintain air circulation through the building.
In winter time when the cooling system is not needed, the control system can be adapted by the addition of a heatingelement system which, if temperature drops too much, the heaters will come on to maintain a comfortableenvironment.
(See Figure : 12)
Heating can also be achieved by operating the fridge unit as a 'Heat Pump'.
This is done by an automatic valving system which bypasses the refrigerant cooler / condenser and the expansionvalve thus using the heat of compression to heat the coils in the exchanger chamber, thereby heating the circulatingair.
In this system, when the temperature rises to the set point of the thermostat, the compressor will shut down and also,if set to do so, the fan will stop.
As the air is circulating, a proportion of the discharge air from the fan, (HP), before cooling (or heating), is allowedto escape to atmosphere and an equal amount of fresh (LP) air allowed to enter the system at the fan inlet.
In this way, the air is being refreshed over a period of time. (The opening and closing of doors as people enter andleave the building, will also help to renew the air).
However, too much fresh air being allowed to enter, will decrease and even destroy the cooling or heating effect ofthe system causing it to run continuously.
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Figure: 12
In the above example, the fan, heating and cooling are set to 'AUTO'. The heating is set at 65 °F the heating willcontrol at this temp. Should the temp. rise to 70 °F, the cooling will switch on and, in each case the fan will operateautomatically.
5. FREEZE DRYING
The usual way of drying (dehydrating) substances or objects, is to use a desiccant solid or liquid to adsorb or absorbwater, or to drive out the moisture by addition of heat i.e. by evaporation.
Some substances cannot be dried by heating as they may be sensitive to heat or they cannot be contacted with adesiccant in case of contamination.
In these cases, a system called 'Freeze Drying' is used to remove the moisture.
The process consists of freezing the material to be dried, to between 20 °C and 25 °C. It is then placed inside achamber where the temperature is allowed to rise slowly.
The slow temperature increase causes 'Sublimation' of the ice particles to take place. This is a process wherein thesolid ice changes to water vapour directly with no liquid phase.
Dry air or inert gas passed through the chamber, carries away the water vapour leaving a freezedried product.
6. SEPARATION AND RECOVERY OF PROCESS FLUIDS
In some processes, refrigeration is needed to separate a mixture of components for use as refrigerants or in otherprocesses. In addition, the cooling also recovers other process liquids from the mixture.
An example of this is used in a Gas Plant where PROPANE is extracted from the Natural gas feed by fractionation ofgas condensate produced when the gas is cooled, first in a gas/gas exchanger and then in the inlet gas chiller andfinally in the fractionation overhead gas chiller.
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The propane is used as a refrigerant to cool the inlet gas stream and condense the water vapour. Glycol is injected intothe gas stream solely to prevent freezing of the water when the gas is cooled. The gas, condensate and water/glycolmixture are then separated.
The gas and condensate pass on to the other processes (from which the propane is produced for the refrigerationprocess). The water / glycol mixture goes to the glycol regeneration process.
See Figure : 13
PROPANE REFRIGERATION PROCESS Figure: 13
C R Y O G E N I C S
This section is purely for interest in the process of 'SUPER REFRIGERATION' as applied to one method ofproducing Liquid Natural Gas.
'CRYOGENICS' is 'THE SCIENCE OF THE SUPERCOLD'
i.e. Temperatures COLDER than 150 °F. Cryogenic temperatures can only be achieved by using special methods as in the production of Liquid Air (which canthen be separated to produce Liquid Nitrogen, Oxygen and other (rare) liquefied gases.
Cryogenic operations are used to produce Liquid Natural Gas (LNG), by a number of methods.
L.N.G PRODUCTION INTRODUCTION
Following is a very simplified explanation of one process of LNG production by cryogenics, as used in someoperations. (This is an example of a MULTIREFRIGERANT SYSTEM (MCR).
Liquid Natural Gas is a mixture of, mainly, Methane and Ethane which is liquefied at a temperature of 262 °F (163°C). This is achieved by compressing the natural gas to a high level (depending on operational requirements). The gasis first processed to remove Hydrogen Sulphide (H2S), Carbon Dioxide (CO2) and water vapour (H2O), to desirable
levels by absorption, adsorption and separation processes. (These contaminants are undesirable as they will freeze inthe very low temperature cooling processes). H2S is also a toxic, corrosive, acid (sour) gas; CO2 is also acidic (sour)
and corrosive in the presence of water).
(Other available booklets give detailed accounts of some dehydration and purification methods used).
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The HP gas is first cooled to about ambient temperature and, at high pressure, much of the water vapour in the gaswill be condensed and drop out in a separator vessel together with most of the condensed heavier hydrocarbon mixtureof Propane and Butane and heavier components.
The water is drained away to a disposal pit and, the hydrocarbon liquids go to dehydration units and then passed on toa Fractionation Unit for separation of the components. These components are then used to produce a Multicomponentrefrigerant (MCR) with excess going to LPG production and/or fuel.
After the separation of the above liquids, the Natural Gas Feed is then passed through dehydration units and on to aseries of cooling coils in a cryogenic exchanger.
Cooling is carried out in stages by the 'Multicomponent Refrigerant' (MCR), which consists of a mixture of: Nitrogen (N2), Methane (CH4 or C1), Ethane (C2H6 or C2), Propane (C3H8 or C3) and Butane (C4H10 or C4),. The
Nitrogen is added in order to achieve a final component liquid mixture of N2 and C1 with a Boiling Point (BP)
BELOW the temperature required to condense the natural gas at 262 °F (163 °C).
The nitrogen will bring the MCR final BP down to about 275 °F ( 170 °C). This provides the necessary differentialtemperature (.T) to fully condense the feed gas in the final stages of cooling. The different, progressively colderboiling points of the components, enables the refrigeration, as mentioned, to take place in stages across the cryogenicexchanger.
Butane being the highest BP at 31°F, begins the cooling process of the feed gas AND the other components of therefrigerant.
Passing upwards through the cryogenic exchanger, as each refrigerant component condenses, it is collected in a vesseland separated from the lighter component vapours. The liquid is then passed through a subcooling coil then anexpansion valve into a refrigerant drum and the resultant colder liquid is sprayed over tube bundles carrying therefrigerant mixture both vapour and liquid phases. The sprays also cool the tube bundles carrying the natural gasfeed.
This process is repeated for each component of the refrigerant. At the top of the cryogenic exchanger, thenitrogen/methane mixture condenses and is expanded to produce a liquid refrigerant at about 275 °F. (Methane itselfcondenses at about 259 °F and therefore cannot condense the feed gas without help from the nitrogen. The processnot only involves cooling but also the removal of Latent Heat).
The refrigerant at 275 °F will have enough 'cold' to give the necessary heat exchange to bring the final cooling andcondensation stage of the feed gas down to 262 °F, at which temperature it will liquefy and can then be stored inspecial, insulated tanks.
Between the last two cooling stages, the pressure of the feed gas is decreased. This is to begin the pressure drop priorto going to the LNG storage tanks at 262 °F and 0.5 Psi just above atmospheric pressure.
The process of heat exchange between the refrigerant liquid, vapour and feed gas coils, results in a cool, mixed MCRvapour which passes out of the cryogenic exchanger bottom and returns to the MCR compressors to begin the cycleagain.
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The MCR is compressed in two stages by two multistage compressors and has inter and after coolers using sea wateras cooling medium to give a final discharge at around 100 °F, at which temperature the butane and some of thepropane will condense to begin the cooling process described above.
Lighter gases, like Nitrogen, Methane and Ethane can be liquefied but, in these cases, the condensation will not takeplace at ambient temperature, no matter what pressure is applied to them. However, they will condense if cooled to aspecific temperature called the 'Critical Temperature' of the gas concerned.
Critical Temperature: is defined as: 'The temperature ABOVE which a gas WILL NOT liquefy, irrespective of thepressure applied'.
Even at the critical temperature, very high pressure may be needed to condense the gas.
Associated with critical temperature, is 'Critical Pressure', which is defined as: 'The pressure required to condense agas AT ITS CRITICAL TEMPERATURE'.
Table 3 below, gives typical Critical Temperatures and Pressures of some gases.
GAS CRITICAL TEMPERATURE CRITICAL PRESSURE
1. Nitrogen 147 °C ( 232 °F) 3.40 MPa (500 psi)
2. Oxygen 118 °C ( 180 °F) 5.00 MPa (730 psi)
3. Methane 83 °C ( 117 °F) 4.60 MPa (670 psi)
4. Ethane 32 °C ( 90 °F ) 4.90 MPa (700 psi)
5. Propane 96 °C ( 208 °F ) 4.25 MPa (620 psi)
However, once a gas has been condensed, the pressure may be decreased resulting in a decrease in temperature (arefrigeration principle). The lower the pressure, the lower the boiling point temperature of the liquid. Table 4 asfollows, shows these effects:
Table 4
Substance BP @ 0 Psig BP @ 50 Psig BP @ 100 Psig
Methane 259 °F 218 °F 200 °F
Ethane 124 °F 68 °F 38 °F
Propane 44 °F 28 °F 66 °F
Butane 31 °F 115 °F 156 °F
Pentane 97 °F 192 °F 237 °F
Hexane 156 °F 258 °F 307 °F
Water 212 °F 300 °F 338 °F
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Temperature Figures will be in degrees Fahrenheit (°F) should °C be required, use the following simple conversionformula:
All future pressure Figures will be in Pounds/Square Inch Gauge (psig). If 'Pascals' are required, use the followingconversion calculation:
Pounds/Square Inch (psi) x 6894.757 = Pascal (Pa)
e.g. 1. Atmospheric pressure (14.7 psi) x 6894.757 = 101,352.9279 Pa = 101.3 kPa e.g. 2. 620 psig x 6894.757 = 4,274,749.3 Pa This can be expressed in MegaPascal as: 4.27 MPa
Where pressure is indicated as 'psig' (gauge pressure), it denotes pressure that disregards atmospheric pressure. ForAbsolute Figures (psia) we have to add 14.7 to the gauge reading to convert to psia.
e.g. Atmospheric pressure is Zero psig. To change to psia we add 14.7. i.e. 0 psig + 14.7 = 14.7 psia etc.
LIQUEFIED NATURAL GAS (LNG)
Where large Oil Companies have a number of producing fields, some formations produce Crude oil and Natural gas.This gas is called 'Associated Gas' i.e. produced with crude oil.
In this case the fluids will normally be separated in the field and the oil sent by pipeline to the main centre for exportor processing. Separated water will be drained away to disposal pits. Other fields may produce gas only. This is called'Nonassociated gas'.
The gas from each field will generally be dehydrated and, depending on the H2S content, may be put through a
sweetening process before entering a main pipeline to the processing (LNG) plant.
Removal of water and H2S is carried out to protect the pipelines from corrosion. A chemical corrosion inhibitor will
also be injected into the gas flow at various locations for the same reason. In some cases the producing field will be ata high enough pressure to perform the necessary processes involved in the field. In others, the gas will need to be compressed to the required pressure. Throughout processing, the gas pressure will becontrolled at the desired level. Treated gas is then injected into the main pipeline as feed gas to the central processingplant the LNG plant.
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On arrival, due to friction and retrograde condensation of heavy components, the pressure decreases during thejourney and gas condensate and water a formed. In order to sweep these liquids along to the processing plant, thepipeline will be 'Sphered' or 'Pigged'.
Also on arrival at the LNG facility and before recompression, the liquid components are separated from the gas. Theseparated liquids are piped to water separation units and the hydrocarbon liquid is pumped to a drying unit and on to aNaphtha & LPG production plant.
In order to liquefy the Natural gas, it will be recompressed to the pressure required for the process used.
GAS TREATMENT UNIT
Prior to liquefaction, the gas at high pressure and near ambient temperature, is passed through a sweetening process toremove H2S and CO2 by absorption into an absorbent solution. (The process used depends on the requirements set
down by the Company Process and Chemical Engineers).
The 'Rich' absorbent is then regenerated (stripped) in a regeneration tower at reduced pressure and elevatedtemperature. Heat is provided by a reboiler using superheated steam as heating medium or a fuel fired reboiler.
The sour gases removed from the absorbent in the regeneration system are sent to the flares for disposal.
From the sweetening process, the gas is cooled and separated from liquids, The liquid hydrocarbons are piped to theliquid drying section together with liquid from other plant areas. Gas goes to the vapour drying section and the wateris drained away. In the drying sections, a solid desiccant is used. (Other systems use a glycol circulation fordehydration of the gas). The dry gas passes on to the LNG plant (or other process).
The dry liquid goes to the Fractionation Section together with dry liquids from other areas. Here, the components ofthe liquid are separated by distillation into: Methane (C1), Ethane (C2), Propane (C3), Butane (C4) and (C5 +
heavier) components.
These components are used mainly to maintain the inventory of the MCR in the Cryogenics process. Excess liquidsare used to produce LPG (Liquid Petroleum Gas) for domestic use in cylinders.
Any further excess may go to Naphtha recovery and storage or to the plant fuel system.
Figure: 14 is a simplified diagram of a gas and liquid treatment unit.
Figure: 15 shows a drier system using a solid desiccant.
SIMPLIFIED GAS TREATING UNIT
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Figure: 14
DRIER SYSTEM USING A SOLID DESICCANT
Figure: 15
In Figure 15, driers 1 & 2 are receiving the wet gas and discharging to the dry gas system. Some of this dry gas ispassing to the regeneration furnace where it is heated to 500 °F and piped to drier #4 to regenerate the desiccant.(Notice the reverse flow).
When regeneration is complete, the hot gas will be shut off and replaced by the cooling gas for four hours.
Drier #4 will then go on standby and #3 will go into service. The saturated drier #1 or 2 will then go on toregeneration, Etc.
THE CRYOGENIC REFRIGERATION PROCESS
MCR REFRIGERANT COMPOSITION
The Multi Component Refrigerant (MCR) has the following approximate composition:
Nitrogen (N2) 5%
Methane (CH4) (C1) 35%
Ethane (C2H6) (C2) 45%
Propane (C3H8) (C3) 10%
Butane (C4H10) (C4) 5%
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The above MCR inventory is maintained by the controlled injection of required components obtained from thefractionation unit. (Except for the Nitrogen which is provided by the Nitrogen generation unit). Excess inventory canbe vented to flare as needed.
The volume of MCR (as vapour) required to liquefy the feed gas is FOUR times the volume of feed gas processed.
MCR COMPRESSION AND COOLING SYSTEM
(See Figure: 16)
Beginning at the MCR vapour return line from the Cryogenic towers EC.1 (Main) and EC.2 (Sub). The cold MCRvapour flow through the subcryogenic tower is achieved by passing the MCR from the tower bottom into a Venturitube placed in the main tower MCR outlet line. The pressure drop across the Venturi gives sufficient DP across EC. 2.to provide the required MCR flow. The 'A' bundle feed gas outlet temperature is controlled by a TCV placed in theoutlet line.
The combined refrigerant flow at about 20 psig now passes to the MCR compression units. It first enters a suctionknockout drum to prevent any liquid from entering the 1st stage compressor. Any liquid which may separate out hereis revaporised by a hot sparge gas injection into the vessel bottom via a perforated pipe. This is done to preserve thetotal inventory of the MCR vapour.
On leaving the KO drum, the makeup components are added to the MCR flow via control systems. The MCR nowpasses into the 1st stage MCR compressor. This is a multistage, centrifugal compressor drive by a condensing steamturbine. (A combustion Gas Turbine may be used in some locations).
The first stage of compression increases the MCR to 125 psi and about 225 °F. It is then cooled to about 110 °F in theintercoolers and passed into the interstage drum where again, any liquid dropout is revaporised by hot sparge gas.
A computerised flow controller (FRC) is placed in the compressor discharge line. Should the Mass Flow drop below apreset point, the controller will begin to open the recycle valve and take discharge gas back to the compressor suctionto maintain a preset Minimum Flow through the compressor. This is necessary to prevent 'Surging' in thecompressor.
(Surging in this type of machine must be avoided in order to prevent damage to the compressor, its internals andassociated equipment and piping caused by high vibrations set up by the surging action).
The 2nd stage MCR compressor takes suction from the interstage drum and raises the gas pressure to 450 psi andabout 250 °F. The MCR is discharged through the salt water cooled aftercoolers and piped to the first MCR separatorD.1 on the main cryogenic tower EC. 1.
Again, as in the first stage, a computerised flow element meters the mass flow and controls a recycle system forminimum flow protection against surging.
Both MCR compressors may be driven by condensing steam turbines or by combustion gas turbines as required by theCompany. Each machine also has overpressure protection by safety valves to flare, installed in the discharge line.Suction and discharge lines are fitted with electric motor 'Remote Operated Valves' (ROV's) for quick operation if
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emergency operation is required.
The following simplified diagram (Figure: 16) shows the layout of the MCR compression system.
Figure: 16
CRYOGENIC EXCHANGER BASIC OPERATION
(See Figure: 17)
The MCR from the 2nd stage compression and cooling process enters the first separator D.1. Here, the butane (C4)
and some propane (C3) separate out as liquid.
REFRIGERANT VAPOUR FLOW
The MCR Vapour from D.1. is piped through the 'B' vapour coils of the main cryogenic tower EC.1. (cooled by thesprays from D.5.), and into separator D.2. where C3 and some C4 separate as liquid.
Vapour from D.2. is piped through the 'C' vapour coils (cooled by the sprays from D.6.), and into separator D.3.where C2 and some C1 separate as liquid.
From D.3, the vapour is cooled in the 'D' bundles by sprays from D.7. then passes through 'E' bundle cooled to 262°F.
From the 'E' bundle the MCR, now mainly liquid Methane (C1) and Nitrogen (N2) passes into D.4. via an expansion
valve where the pressure gives a final refrigerant temperature of about 275°F which feeds the 'E' bundle sprays. Thevapour from D.4. is piped directly into the main tower top.
The expansion valve at D.4. maintains the upstream MCR at high pressure some pressure loss from the originalpressure occurs through the system, due to the liquids formed in the preceding separators and friction due to therestriction to flow through the tube bundles and other fittings in the system.
2. REFRIGERANT LIQUID FLOW
The C4/C3 liquid from D.1. is passed through the 'B' liquid coils and piped to Refrigerant drum D.5. via an expansion
valve (Temperature Control Valve) which controls the temperature of the MCR vapour leaving the 'B' bundle. Theexpansion valve causes a sudden pressure drop in D.5. and, due to the 'JoulesThomson' effect the expansion of the
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high pressure liquid causes its partial vaporisation and therefore a large decrease in its temperature. The cold vapourfrom D.5. is passed directly into the main cryogenic tower. The subcooled liquid is sprayed over the 'B' bundles i.e.the refrigerant liquid and vapour coils, and the Feed Gas coils, cooling the bundles to the operation requirement. Thisbegins the cryogenic process.
The MCR liquid (C3/C2), that separates out in D.2. passes through the 'C' MCR liquid bundles before passing into D.
6. via the expansion valve. The pressure drop further reduces this liquid temperature. This liquid is sprayed over the'C' bundles thus further decreasing the temperature of the three streams. (Upstream of the D.6. expansion valve, somerefrigerant is piped to the fractionation unit for the cooling process in the recovery of Methane & Ethane. This MCRis returned to the system downstream of the D.5. expansion valve).
The MCR vapour from the 'C' bundles passes into D. 3. where C2/C1 liquids drop out to be piped through the 'D'
MCR liquid bundle, through the expansion valve into 'D. 7.' where the pressure drop produces a much lower liquidMCR temperature.
This is sprayed over the 'D' bundles to give their required outlet temperatures.
The liquid refrigerant formed in D. 4. consists mainly of methane and some nitrogen after the expansion valveproduces a final MCR temperature of about 275 °F.
This liquid is sprayed over the 'E' MCR vapour bundle and feed gas bundle to give an 'E' bundle outlet temperature of 262 °F.
The LNG leaving the 'E' bundle at 262 °F and reduced pressure, passes through a TCV which, due to its throttlingaction, will decrease the pressure further before the LNG finally enters the rundown line to the LNG storage tanks.
The tanks' pressure is maintained at 0.5 psi (just above atmospheric pressure).
From storage the LNG is pumped into special cryogenic tankers for shipment abroad. Provision is made to send offspec LNG to burnpit during startup and shutdown operations.
Figure: 17
About the Author
Norrie is a retired professional who has been working in Oil and Gas and LNG production in MarsaelBrega, Libya
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for 30 years.
Norrie used to be in the Training Dept. and prepared Programmes for Libyan Traine
Last Updated on Tuesday, 16 March 2010 08:49