Distillation

7/15/2019 Distillation

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DISTILLATION

Introduction

Distillation is defined as:

a process in which a liquid or vapourmixture of two or more substances isseparated into its component fractions of

desired purity, by the application andremoval of heat.

Distillation is based on the fact that thevapour of a boiling mixture will be richerin the components that have lower

boiling points.

Therefore, when this vapour is cooledand condensed, the condensate willcontain more volatile components. At the

same time, the original mixture willcontain more of the less volatile material.

Distillation columns are designed toachieve this separation efficiently.

Integrated Training Program / Phase B Distillation Page 1 of60

Copyright 2004 International Human Resources Development Corporation


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Distillation Principles

Separation of components from a liquid mixture via distillation depends on thedifferences in boiling points of the individual components. Also, depending on theconcentrations of the components present, the liquid mixture will have different boiling

point characteristics. Therefore, distillation processes depends on the vapour pressurecharacteristics of liquid mixtures.

Vapour Pressure and Boiling

The vapour pressure of a liquid at a particular temperature is the equilibrium

pressure exerted by molecules leaving and entering the liquid surface. Here are

some important points regarding vapour pressure:

energy input raises vapour pressure

vapour pressure is related to boiling

a liquid is said to boil when its vapour pressure equals the surrounding pressure

the ease with which a liquid boils depends on its volatility

liquids with high vapour pressures (volatile liquids) will boil at lowertemperatures

the vapour pressure and hence the boiling point of a liquid mixture depends on therelative amounts of the components in the mixture

distillation occurs because of the differences in the volatility of the components in

the liquid mixture




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UNIT 1

DEFINITIONS OF WORDS AND TERMSUSED IN THE GAS PROCESSING INDUSTRY

Absorber

A tower or column that provides contact between natural gas being processed and aliquid solvent

Absorption

Absorption Factor

The operation in which one or more components in the gas phase are transferred to(absorbed into) a liquid solvent

A factor which is an indication of the tendency for a given gas phase component to betransferred to the liquid solvent. It is generally expressed as A = L/KKV where L and Vare the moles of liquid and vapor, and K is the average value of the vapor liquidequilibrium constant for the component of concern.

Absorption Oil

A hydrocarbon liquid used to absorb and recover components from the natural gas beingprocessed.

Acid Gas

The hydrogen sulfide and/or carbon dioxide contained in, or extracted from, gas or otherstreams.

Adiabatic Expansion

The expansion of a gas, vapor, or liquid stream from a higher pressure to a lowerpressure in which there is no heat transfer between the gas, vapor, or liquid and the

surroundings.

Adsorbent

A solid substance used to remove components from natural gas being processed.

Adsorption

The process by which gaseous components are adsorbed on solids because of theirmolecular attraction to the solid surface




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Amine (alkanolamine)

Any of several liquid compounds containing amino nitrogen generally used in watersolution to remove, by reversiblechemical reaction hydrogen sulfide and/or carbondioxide from gas and liquid hydrocarbon stream

Associated Gas

Gaseous hydrocarbons occuring as a free-gas phase under original oil-reservoirconditions of temperature and pressure.

Atmospheric Pressure

The pressure exerted on the earth by the earth's atmosphere. A pressure of 760 mm ofmercury or 101.3250 kPa is used as a standard for some measurements. State regulatory

bodies have set other standards for use in measuring the legal volume of gas.Atmospheric pressure may also refer tothe absolute ambient pressure at any givenlocation.

Barrel

common English - unit measure of liquid volume which, in the petroleum industry,equals 42 U.S. liquid gallons for petroleum or natural gas liquid products measured at60F and equilibrium vapor pressure. One barrel equals 0.159 cubic meters, or 6.29

barrels per cubic meter (See Fig, 1-2).

Blanket gas

A gas phase maintained in a vessel containing liquid to protect the liquid against aircontamination, to reduce the hazard of detonation, or to maintain pressure of the liquid.The source of the gas is external to the vessel.

Blow Case

A small tank in which liquid is accumulated and then forced from the tank by applyingas or air pressure above the liquid level.

Blowdown

The act of emptying or depressuring a vessel. This may also refer to discarded material,such as blow down water from a boiler or cooling tower.

Boilaway Test

Sometimes used to describe the GPA weathering test for LPgas. Refer to the definitionof weathering test




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Bottoms

The liquid or residual matter which is withdrawn from the bottom of a fractionator orother vessel during processing or while in storage.

B-P mix

A liquefied hydrocarbon product composed chiefly of butanes and propane. If itoriginates in a refinery, it may also contain butylenes and propylene. More specifically,it conforms to the GPA specifications for commercial B-P mixes as described in GPAStandard 2140.

Breathing

The movement of vapor in or out of an atmospheric pressure storage tank because of achange of level of the stored liquid, a change in the temperature of the vapor spaceabove the liquid, or a change of atmospheric pressure.

BS&W (basic sediment and water)

Waste that collects in the bottom of vessels and tanks containing petroleum orpetroleum products.

Bubble Point

The temperature at a specified pressure at which the first stable vapor forms above aliquid.

Commercial Butane,

A liquefied hydrocarbon consisting predominately of butane and/or butylene and whichconforms to the GPA specification for commercial butane defined in GPA Standard2140.

Normal Butane,

In commercial transactions, a product meeting the GPA specifications for commercialbutane and, in addition, containing a minimum of95 liquid volume percent normalbutane. Chemically, normal butane is an aliphatic compound of the paraffin serieshaving the chemical formula C4HlO and having all of its carbon atoms joined in astraight chain.

Calorimeter

An apparatus which is used to determine the heating value of a combustible material.




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Carbonyl Sulfide

A chemical compound of the aldehyde group containing a carbonyl group and sulfur(COS). Sometimes a contaminant in natural gas and NGL. It may need to be removed inorder to meet sulfur specifications.

Casinghead Gas

Unprocessed natural gas produced from a reservoir containing oil. It contains heavierhydrocarbon vapors and is usually produced under low pressure from a casing head onthe well.

Charcoal Test

A test standardized by the Gas Processors Association and the American GasAssociation for determining the natural gasoline content of a given natural gas. Thegasoline is adsorbed from the gas on activated charcoal and then recovered by dis-tillation. The test is prescribed in Testing Code 101-43, joint publication of AGA andGPA.

Chromatography

A technique for separating a mixture into individual components by repeated adsorptionand desorption on a confined solid bed. It is used for analysis of natural gas and NGL.

Claus Process

A process to convert hydrogen sulfide into elemental sulfur by selective oxidation.

Compressibility Factor

A factor, usually expressed as "Z," which gives the ratio of the actual volume of gas at agiven temperature and pressure to the volume of gas when calculated by the ideal gaslaw.

Compression Ratio

The ratio of the absolute discharge pressure from a compressor to the absolute intakepressure. Also applies to one cylinder of a reciprocating compressor and one or morestages of a rotating compressor.

Condensate

The liquid formed by the condensation of a vapor or gas; specifically, the hydrocarbonliquid separated from natural gas because of changes in temperature and pressure whenthe gas from the reservoir was delivered to the surface separators. In a steam system itmay be water that is condensed and returned to the boilers.




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Convergence Pressure

The pressure at a given temperature for a hydrocarbon system of fixed composition atwhich the vapor-liquid equilibrium K-values of the various components in the systembecome, or tend to become, unity. The convergence pressure is used to adjust vapor-liquid equilibrium K-values to the particular system under consideration.

Copper Strip Test

A test using a small strip of pure copper to determine qualitatively the hydrogen sulfidecorrosivity of a product. Refer to GPA LP-gas copper strip test (Copper Strip Method),ASTM D-1838 test procedure.

Critical Density

The density of a substance at its critical temperature and critical pressure.

Critical Pressure

The vapor pressure of a substance at its critical temperature.

Critical Temperature

For a pure component, the maximum temperature at which the component can exist as aliquid.

Cryogenic Plant

A gas processing plant which is capable of producing natural gas liquid products,including ethane, at very low operating temperatures, usually below minus 50C.

Cubic Meter

A unit of volume measurement commonly used in international commerce forpetroleum, petroleum products and natural gas. One cubic meter measured at 15.56C =264.172 U.S. gallons = 6.29 barrels = 35.315 cubic feet measured at 15.56C.

Deaerator

An item of equipment used for removing air or other non-condensible gases from aprocess stream or from steam condensate or boiler feed water.

Debutanizer

A fractionator designed to separate butane (and more volatile components if present)from a hydrocarbon mixture.




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Dehydration

The act or process of removing water from gases or liquids.

Demethanized Product

A product from which essentially all methane and lighter materials have been removed.

Demethanizer

A fractionator designed to separate methane (and more volatile components if present)from a hydrocarbon mixture.

Depropanizer

Afractionator designed to separate propane (and more volatile components if present)from a hydrocarbon mixture.

Desiccant

A substance used in a dehydrator to remove water and moisture. Also a material used toremove moisture from the air.

Desulfurizationf

A process by which sulfur and sulfur compounds are removed from gases or liquidhydrocarbon mixtures.

Dew Point

The temperature at any given pressure, or the pressure at any given temperature, atwhich liquid initially condenses from a gas or vapor. It is specifically applied to thetemperature at which water vapor starts to condense from a gas mixture (water dew

point), or at which hydrocarbons start to condense (hydrocarbon dew point).

Distillation

The process of separating materials by successively heating to vaporize a portion andthen cooling to liquefy a part of the vapor. Materials to be separated must differ in

boiling point and/or relative volatility.

Doctor Test

A qualitative method for detecting hydrogen sulfide and mercaptans in NGL. The testdistinguishes between "sour" and "sweet" products.




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Dry Gas

(1) Gas whose water content has been reduced by a dehydration process. (2) Gascontaining little or no hydrocarbons commercially recoverable as liquid product. Gas inthis second definition preferably should be called lean gas.

End Point

The maximum temperature observed on the thermometer during an ASTM distillationtest.

EP-Mix (ethane-propane mix)

A product consisting of a mixture of essentially ethane and propane.

Expansion Turbine

A device which converts part of the energy content of a gas or liquid stream intomechanical work by expanding the gas or liquid through a turbine from which work isextracted.

Extraction

The process of transferring one or more components from one liquid phase to anotherby virtue of different solubility in the two liquids. It is also used to indicate removal of

one or more constituents from a stream.

Field Separator

A vessel in the oil or gas field for separating gas, hydrocarbon liquid, and water fromeach other.

Flash Point

The lowest temperature at which vapors from a hydrocarbon liquid will ignite. SeeASTM D-56.

Fractionation

See definition of "distillation." Generally used to describe separation of a mixture ofhydrocarbons into individual products based on difference in boiling point and/orrelative volatility.

Freeze Valve

A specially constructed and calibrated valve designed and used solely for determiningthe water content in propane product. See ASTM D-2713.




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Gas Constant (R)

The constant multiplier in the Ideal Gas Law. Numerically, R=PV/T, if V is the volumeof one mole of an ideal gas at temperature T and pressure P.

Gas Hydrate

Refer to definition of "hydrate".

Gas Injection

The injection of natural gas into a reservoir to maintain or increase the reservoirpressure or reduce the rate of decline of the reservoir pressure.

Gas Lift

A method for bringing crude oil or water to the surface by injecting gas into theproducing well bore.

Gas-Oil Ratio (GOR)

The ratio of gas to liquid hydrocarbon produced from a well. This may be expressed asstandard cubic meters of gas per cubic meter of stock tank liquid.

Gas Processing

The separation of constituents from natural gas for the purpose of making salableproducts and also for treating the residue gas to meet required specifications.

Gas Processing Plant

A plant which processes natural gas for recovery of natural gas liquids and sometimesother substances such as sulfur.

Gas-Well Gas

The gas produced or separated at surface conditions from the full well stream producedfrom a gas reservoir.

Gas-Well Liquid

The liquid separated at surface conditions from the full well stream produced from a gasreservoir.

Gathering System

The network of pipelines which carry gas from the wells to the processing plant or otherseparation equipment.




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Heat Media (Heating Media)

A material, whether flowing or static, used to transport heat from a primary source suchas combustion of fuel to another material. Heating oil, steam, and an eutectic saltmixture are examples of heat media.

Heating Value (Heat Of Combustion)

The amount of heat obtained by the complete combustion of a unit quantity of material.The gross, or higher, heating value is the amount of heat obtained when the water

produced in the combustion is condensed. The net, or lower, heating value is the amountof heat obtained when the water produced in the combustion is not condensed.

Heavy End

The portion of a hydrocarbon mixture having the highest boiling point. Usually hexanesor heptanes and all heavier hydrocarbons are the heavy ends in a natural gas stream.

Hexanes Plus (Or Heptanes Plus)

The portion of a hydrocarbon fluid mixture or the last component of a hydrocarbonanalysis which contains the hexanes (or heptanes) and all hydrocarbons heavier than thehexanes (or heptanes).

Hydrate

A solid material resulting from the combination of a hydrocarbon with water underpressure.

Immiscible

Liquids that will not mix nor blend to give homogeneity are said to be immiscible.

Inerts

Elements or compounds not acted upon chemically by the surrounding environment.Nitrogen and helium are examples of inert constituents of natural gases.

Isobutene

In commercial transactions, a product meeting the GPA specification for commercialputane and, in addition, containing a minimum of 95 liquid volume percent isobutane.Chemically, a hydrocarbon of the paraffin series with the formula C4HlO and having itscarbon atoms branched.

Jacket Water

Water which fills, or is circulated through, a casing which partially or wholly surroundsa vessel or machine element in order to remove, add, or distribute heat in order tocontrol the temperature within the vessel or element.




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Joule-Thomson Effect

The change in gas temperature which occurs when the gas is expanded at constantenthalpy from a higher pressure to a lower pressure. The effect for most gases at normalpressure, except hydrogen and helium, is a cooling of the gas.

Lead Acetate Test

A method for detecting the presence of hydrogen sulfide by discoloration of paperwhich has been moistened with lead acetate solution. See ASTM D-2420.

Lean Gas

(1) The residue gas remaining after recovery of natural gas liquids in a gas processingplant. (2) Unprocessed gas containing little or no recoverable natural gas liquids.

Lean Oil

Absorption oil as purchased or recovered by the plant, or oil from which the absorbedconstituents have been removed.

Lift Gas

Gas used in a gas lift operation.

Light Ends

The low-boiling, easily evaporated components of a hydrocarbon liquid mixture.

Light Hydrocarbon

The low molecular weight hydrocarbons such as methane, ethane, propane and butanes.

LNG (liquefied natural gas)

The light hydrocarbon portion of natural gas, predominately methane, which has been

liquefied.

Loading Rack

A structural and piping installation alongside a railroad track or roadway used for thepurpose of filling railroad tank cars or transport trucks.

LPG (liquefied petroleum gas)

Refer to definition of "LP-gas".




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LP-gas (Liquefied petroleum gas)

Predominately propane or butane, either separately or in mixtures, which is maintainedin a liquid state under pressure within the confining vessel.

LRG (liquefied refinery gas)

Liquid propane or butane produced by a crude oil refinery. It may differ from LP-gas inthat propylene and butylene may be present.

LTX (low temperature extraction unit)

A unit which uses the cooling of a constant enthalpy expansion to increase liquidrecovery from streams produced from high pressure gas condensate reservoirs. Alsocalled LTS (low temperature separation) unit.

Mercaptan,

Any of a homologous series of compounds of the general formula RSH. All mercaptanspossess a foul odor.

Miscible Flood

A method of secondary recovery of fluids from a reservoir by injection of fluids that aremiscible with the reservoir fluids.

Natural Gas

Gaseous form of petroleum. Consisting predominately of mixtures of hydrocarbongases. The most common component is methane.

Natural Gasoline

A mixture of hydrocarbons, mostly pentanes and heavier, extracted from natural gas,which meets vapor pressure, end point, and other specifications for natural gasoline asadopted by the GPA. See GPA Standard 3132.

Natural Gas Processing Plant

Term used for gas processing plant, natural gasoline plant, gasoline plant, etc.

NGL (natural gas liquids)

Natural gas liquids are those hydrocarbons liquefied at the surface in field facilities or ingas processing plants. Natural gas liquids include ethane, propane, ,butanes, and naturalgasoline.




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Odorant

An odoriferous compound added to natural or LP-gas to impart a distinctive odor fordetection of fugitive vapors. Ethyl mercaptan is the most widely used odorant for LP-gas, while tertiary butyl mercaptan, usually mixed with small amounts of othercompounds, is the predominant odorant for natural gas.

Oil-Well Gas

Gas that is produced from an oil well.

On-Stream Factor

The percentage of time a unit is on-stream.

Operating Factor

The percentage of time a unit is performing the function for which it was designed.

Outage

The vapor volume in a liquid vessel left for liquid expansion. Sometimes referred to asullage.

Packaged Unit

A shop-assembled group of equipment and accessories which needs only foundations,inlet and outlet piping, and utility connections to make an operating unit.

Packed Column

A fractionation or absorption column filled with packing designed to give the requiredcontact between the rising vapors and the descending liquid.

Peak Shaving

The use of non-conventional fuels to supplement the normal supply of pipeline gasduring periods of extremely high demand.

Pentane-Plus

A hydrocarbon mixture consisting of isopentane (C5H12) and heavier components withhigher boiling points.

Pigging

A procedure for forcing a device through a pipeline for cleaning purposes, separatingproducts, or inspecting the line.




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Pipeline Gas

Gas which meets a transmission company's minimum specifications.

Propane

A normally gaseous paraffinic compound (C3H8). The term includes all productscovered by GPA specifications for commercial and HD-5 propane. See GPA Standard2140.

Commercial Propane,

A liquefied hydrocarbon product consisting predominately of propane and/or propyleneand which conforms to the GPA specification for commercial propane as defined inGPASt andard 2140.

Propane HD-5

A special grade of propane consisting predominately of propane and which conforms tothe GPA specification for HD-5 propane as defined in GPA Standard 2140.

Raw Gas

Unprocessed gas, or the inlet gas to a gas processing plant.

Raw Mix Liquid

A mixture of natural gas liquids prior to fractionation. Also called "raw make".

Recovery

That percent or fraction of a given component in the plant feed which is recovered asplant product.

Recycle

Return of part of a process stream to a point upstream from where it was removed toenhance recovery or control.

Reflux

In fractionation, the portion of condensed overhead returned to the column to enhanceachievable purity of the overhead product.

Reflux Ratio

A way of giving a relative measurement to the volume of reflux. Usually referred eitherto the feed or overhead product.




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Relative Density

The ratio of the mass of a given volume of a substance to that of another equal volumeof another substance used as standard. Unless otherwise stated, air is used as thestandard for gases and water for liquids, with the volumes measured at 15.56Candatmospheric pressure (101.325 kPa).

Relief System

The system for safely relieving excess pressure to avoid exceeding equipment designpressure.

Residue

The material which remains after a separation process. (1) Residue gas is that gasremaining after the recovery of liquid products. (2) Residue may also be the heaviestliquid or solid remaining after distillation or reclaiming process.

Retrograde Condensation (vaporization)

Condensation or vaporization that is the reverse of expected behavior. Condensationcaused by a decrease in pressure or an increase in temperature. Vaporization caused byan increase in pressure or a decrease in temperature.

Rich Gas

Gas feed to a gas processing plant for liquid recovery.

Rich Oil

The oil leaving the bottom of an absorber. It is the lean oil plus the absorbedconstituents.

RVP (Reid Vapor Pressure)

The vapor pressure of a material measured by the Reid Method and apparatus as

detailed in ASTM Test Procedure D-323.

S & W (See bs&w)

Saturated compounds hydrocarbon compounds having no unsaturated carbon valencebonds. Natural gas and natural gas liquids are saturated compounds.

Saturated Liquid

Liquid which is at its boiling point or is in equilibrium with a vapor phase in itscontaining vessel.




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Saturated Vapor

Vapor at its dew point.

shrinkage

The reduction in volume of a gas stream by removal of some of its constituents such asfor recovered products, fuel, or losses.

SNG (Synthetic or Substitute Natural Gas)

The gas product resulting from the gasification of coal and/or gas liquids or heavierhydrocarbons.

Solution Gas

Gas which originates from the liquid phase in an oil reservoir.

Sour

Liquids and gases are said to be "sour" if they contain hydrogen sulfide, carbon dioxide,and/or mercaptans above a specified level. It is also used to refer to the feed stream to asweetening unit.

Sour Gas

Gas containing undesirable quantities of hydrogen sulfide, mercaptans, and/or carbondioxide. It is also used to refer to the feed stream to a sweetening unit.

Splitter

A name applied to fractionators, particularly those separating isomers (e.g., butanesplitter refers to a tower producing most of the isobutane in the feed as overhead andmost of the normal butane in the feed as bottoms).

Sponge Absorbent

An absorbent for recovering vapors of a lighter absorbent that is used in the mainabsorption process of a gas processing plant.

Stabilized Condensate

Condensate that has been stabilized to a definite vapor pressure in a fractionationsystem.

Stabilizer

A fractionation column designed to reduce the vapor pressure of a liquid stream.




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Stage Separation System

A system of separators where the liquid portion of the well effluent is separated fromformation gas and flash vapors

Still

The column where the absorbed product is recovered from the lean absorption oil. Inplants using a low molecular weight absorption oil, the still is designed as afractionation column. In plants using a high molecular weight absorption oil, the stillmay use steam or other fluids as stripping medium. Also used to refer to regenerators inamine treating and glycol dehydration systems.

Strapping

A term applied to the process of calibrating liquid storage capacity of storage tanks inincrements of depth.

Stream Day

A continuous 24 hour period of plant operation.

Stripper

A column wherein absorbed constituents are stripped from the absorption oil. The termis applicable to columns using a stripping medium, such as steam or gas.

Stripping Factor

An expression used to describe the degree of stripping. Mathematically, it is KVfL, thereciprocal of the absorption factor.

Stripping Medium

As stated under "stripper", the medium may be steam, gas, or other material that willincrease the driving force for stripping.

Sulfur

A yellow, non-metallic chemical element. In its elemental state, it exists in bothcrystalline and amorphous forms. In many gas streams, sulfur may be found as volatilesulfur compounds, such as hydrogen sulfide, sulfur oxides, mercaptans, and carbonylsulfide. Reduction of the concentration of these gaseous sulfur compounds is oftennecessary for corrosion control and possibly for health and safety reasons.

Sulfur Dioxide (SO2)

A heavy, colorless, suffocating gas that is chemically an oxide of sulfur. Conversion ofthe gaseous sulfur oxides to sulfur is necessary for corrosion control, for health andsafety reasons, and for complying with governmental standards.




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Sweet

Gas containing essentially no objectionable sulfur compounds. Also, treated gas leavinga sweetening unit.

Sweet Gas

Gas which has no more than the maximum sulfur and/or CO2 content defined by (1) thespecifications for the sales gas from a plant; (2) the definition by a legal body. Also, thetreated gas leaving a sweetening unit.

Temperature Correction Factor

A factor for correcting volume at a given temperature to that at a specific referencetemperature. Reference temperature most commonly used in the petroleum industry is15.56C.

Therm

A unit of gross heating value equivalent to (1.055) X 107 kJ.

Tonne

A unit of mass measurement, commonly used in international petroleum commerce; anexpression for the metric ton, or 1000 kilograms.

Trayed Column

A vessel wherein gas and liquid, or two partially miscible liquids, are contacted, usuallyconcurrently on trays. Also refer to packed column.

Turboexpander

Refer to definition of "expansion turbine."

ullage (See outage)

Unsaturated Compounds

Hydrocarbon compounds having one or more unsaturated valence bonds, i.e., ethylene,propylene. These compounds are not found in natural gas streams or gas liquids becauseof their relatively high chemical reactivity. Unsaturates are produced by a thermalcracking or chemical reaction and can be found in synthetic gas (SNG) or light refinerygases (LRG).

Vapor Pressure (true vapor pressure)

The pressure exerted by the equilibrium vapor of a liquid when confined in a closedpreviously evacuated tank or test apparatus.




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Vapor Pressure Gasoline

A descriptive phrase for natural gasoline meeting a specified vapor pressure.

Vapor Pressure, GPA

Vapor pressure as specified by GPA procedures.

Vapor Recovery

Equipment or process for the recovery of desired components from stock tank vapors orvapors from some other source.

Volatile Sulfur

An obsolete term referring to sulfur compounds that will vaporize readily (See sulfur).

Weathering

The evaporation of liquid caused by exposing it to the conditions of atmospherictemperature and pressure. Partial evaporation of liquid by use of heat may also be calledweathering.

Weathering Test

A GPA test for LP-gas for the determination of heavy components in a sample byevaporation under specified conditions.

Weight In Air

Weight compared to a standard with no correction for air buoyancy.

Wellhead

The assembly of fittings, valves, and controls located at the surface and connected to theflow lines, tubing, and casing of the well so as to control the flow from the reservoir.

Wet Gas

(1) A gas containing water, or a gas which has not been dehydrated. (2) A termsynonymous with rich gas. Refer to definition of "rich gas".

Wobbe Number

A number proportional to the heat input to a burner at constant pressure. In Britishpractice, it is the gross heating value of a gas divided by the square root of its gravity.Widely used in Europe, together with a measured or calculated flame speed, todetermine interchangeability of fuel gases.




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UNIT 2

PHASE BEHAVIOR

PHASE BEHAVIOR

Introduction:

In planning, design and operation of modem petrochemical processes, engineers,technologists and process operators must know with- reasonable accuracy the propertiesof the fluids with which they deal. If a piece of process equipment is to be designed, the

properties of the fluid which will be contained in that vessel must be determined inorder to determine the required volume of the vessel, the operating' pressure and thusthe wall thickness and even the specifications for the control devices.

1.1 Vapor-Liquid Equilibrium :

Terms/Definitions:

Property - any measurable characteristic of a substance, such as pressure, volume, ortemperature, or a characteristic that can be calculated or deduced, such as internalenergy.

State - when a system possesses a unique set of properties, such as temperature,pressure, density, and so on, at a given time. Thus the system is said to be in a particularstate. A change in the state of a system results in a change -in at least one of its

properties.

Equilibrium - a state in which there is no tendency toward change,

Phase - a completely homogeneous and uniform state of matter.

Ideal Gas - is an imaginary gas which obeys exactly certain simple laws such as thelaws of Boyle, Charles, Dalton. No real gas obeys these laws exactly over all ranges of

temperature, although "lighter" gases (hydrogen, oxygen, air, etc.) under ordinarycircumstances obey the ideal gas laws with but negligible deviations.

Ideal Gas Law - from the work of Boyle and Charles, scientists developed therelationship now called the Ideal Gas Law. The equation used is pV = n RT. Thisequation can relate the volume, pressure, temperature, and the amount of a given gas.

Equations of State - relate the p- V - T properties of a pure substance (or mixture) bytheoretical or empirical relations. The Ideal Gas Law is a simple example of an equationof state.

Vapor - a gas below its critical point which can condense (i.e change its phase).




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Gas - a substance which is above its critical point and is noncondensable.

Vapor Pressure - the pressure at which vaporization and condensation are at constanttemperature and pressure under equilibrium conditions for a pure substance or mixture.

"Normal" Boiling Point - the temperature at which boiling will take place under apressure of 1 atmosphere [101.3 kPa, 760 mm Hg].

Dew Point - the temperature at which a vapor starts to form a liquid during the processof condensation.

Bubble Point - the temperature at which a liquid starts to form vapor during the processof vaporization.

Saturated - when a vapor or liquid is just about to condense a drop of liquid or vaporizea 'puff of vapor respectively.

Superheated - when a substance is above its saturated vapor region. The degrees ofsuperheat refer to the difference in temperature between the solution temperature andthe actual temperature of a substance above the saturation region.

Subcooled - when a substance is below its saturated liquid region.

Triple Point - when a substance is at a set of conditions in which the solid, liquid, andvapor phases are all in equilibrium.




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Multiphase Systems

Water can exist in many phases, solid, liquid and vapor. These states can change fromone to another at the proper conditions with 'the addition or removal of the correctamount of energy. For instance:

liquid to solid : freezing

solid to liquid : melting

liquid to vapor : boiling

vapor to liquid : condensation

sublimation : solid to vapor

At 101.325 kPa pressure, 1 atmosphere, the boiling point of water is 100 C. This is

usually referred to as the normal boiling point of a substance and the energy required toboil that water is referred to as the heat of vaporization. The boiling point of waterchanges with a change in pressure. For example; as pressure is lowered, the boiling

point of water decreases,

There are several points of specific interest on a P-T diagram. The first of these is thecritical point, Tc and Pc, critical temperature and critical pressure respectively. Abovethis point, the material no longer exhibits the properties of liquid or a gas, rather the

properties change slowly from those of a gas to those of a liquid. There is no line which

defines the difference between liquid and vapor. If a substance above Tc and Pc iscooled, there will be no condensation of vapor to liquid, rather there is a slow change inphysical properties. Critical pressures and temperatures can be found in the PhysicalProperties section .

The second point to mention is the triple point. This is defined as the one temperatureand pressure where solid, liquid and vapor can coexist in equilibrium, This is of limitedimportance to the processing industries.

The line running from the triple point to the critical point, the line representing theequilibrium between liquid and vapor, is the line of most interest to us, As water is

heated at 101.325 kPa, the temperature rises to 100 C, then remains constant at 100 Cuntil enough energy is added to vaporize the liquid, (heat of vaporization, hv) then thetemperature will begin to increase once more. It is important to realize that is true onlyfor pure components and not mixtures.

As the temperature increases in the above example, the liquid begins to exert a pressureon the vapor above it. This is referred to as the vapor pressure of that component. Vapor

pressure is very important for expressing the volatility of a substance at fixedconditions. For instance, the volatility of petrol is defined by the vapor pressure. Vapor

pressure is also very useful in performing calculations involving vapors and liquids. It isusually given the symbol p* and can be found in tables or charts.




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Multicomponent Systems:

Most systems in processing facilities are mixtures of components, not pure components.For our purposes, we will limit our discussion to two components to simplify mattersalthough the same principles apply for systems containing multiple components.

For mixtures, there are now three variables to plot, pressure, temperature andcomposition. For simplicity, only two of these variables are plotted at once and the thirdis kept constant:

For example:

Pressure versus temperature constant composition

Temperature versus composition - constant pressure

Pressure versus composition - constant temperature




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Critical Point for a Pure Substance ( Water )




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UNIT 3

DISTILLATION COLUMN COMPONENTS

Overview

This topic is aimed at all process operations staff wanting to expand their knowledgeabout the various internals and externals to a distillation column.

Objectives

On completion of this topic, the trainee will :

- know the components of a distillation column and understand their function

- understand the physical limitations of trays and packings (flooding and jetflooding)

- know the common process variables and understand their meaning

Contents

- notes and diagrams on distillation column components

Components of Distillation Columns:

The modem distillation column is comprised, of a tall column with trays mountedinternally, an overhead system and a reboiler system. In some cases, packing may beused rather than individual trays.




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Figure 2 : Distillation Column with Trays and Packing




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Sic Distillation Equipment And Operation

Main Components of Distillation Columns

Distillation columns are made up of several components, each of which is used either totransfer heat energy or enhance material transfer. A typical distillation contains severalmajor components:

a vertical shell where the separation of liquid components is carried out

column internals such as trays/plates and/or packings which are used to enhancecomponent separations

a reboiler to provide the necessary vaporisation for the distillation process

a condenser to cool and condense the vapour leaving the top of the column

a reflux drum to hold the condensed vapour from the top of the column so that

liquid (reflux) can be recycled back to the column

The vertical shell houses the column internals and together with the condenser andreboiler, constitute a distillation column. A schematic of a typical distillation unit with asingle feed and two product streams is shown below:




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Basic Operation and Terminology

The liquid mixture that is to be processed is known as the feed and this is introducedusually somewhere near the middle of the column to a tray known as the feed tray. Thefeed tray divides the column into a top (enriching or rectification) section and a bottom(stripping) section. The feed flows down the column where it is collected at the bottomin the reboiler.

Heat is supplied to the reboiler togenerate vapour. The source of heatinput can be any suitable fluid, althoughin most chemical plants this is normallysteam. In refineries, the heating source

may be the output streams of othercolumns. The vapour raised in thereboiler is re-introduced into the unit atthe bottom of the column. The liquidremoved from the reboiler is known asthe bottoms product or simply, bottoms.

The vapour moves up the column, and asit exits the top of the unit, it is cooled by

a condenser. The condensed liquid isstored in a holding vessel known as thereflux drum. Some of this liquid isrecycled back to the top of the columnand this is called the reflux. Thecondensed liquid that is removed fromthe system is known as the distillate ortop product.

Thus, there are internal flows of vapor and liquid within the column as well as externalflows of feeds and product streams, into and out of the column.




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Column Internals

Trays and Plates

The terms "trays" and "plates" are used interchangeably. There are many types of traydesigns, but the most common ones are :

1. Bubble cap trays

A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers theriser. The cap is mounted so that there is a space between riser and cap to allow the

passage of vapour. Vapour rises through the chimney and is directed downward by thecap, finally discharging through slots in the cap, and finally bubbling through the liquid

on the tray.




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2. Valve trays

In valve trays, perforations are covered byliftable caps. Vapour flows lifts the caps,thus creating a flow area for the passage ofvapour. The lifting cap directs the vapourto flow horizontally into the liquid, thus

providing better mixing than is possible insieve trays.




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3. Sieve trays

Sieve trays are simply metal plates with holes in them. Vapour passes straight upwardthrough the liquid on the plate. The arrangement, number and size of the holes aredesign parameters.




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Because of their efficiency, wide operating range, ease of maintenance and cost factors,sieve and valve trays have replaced the once highly thought of bubble cap trays in many

applications.

Liquid and Vapour Flow in a Tray Column

The next few figures show the direction of vapour and liquid flow across a tray, andacross a column.

Each tray has 2 conduits, one on each side, calleddowncomers. Liquid falls through thedowncomers by gravity from one tray to the one

below it. The flow across each plate is shown in theabove diagram on the right.

A weir on the tray ensures that there is always someliquid (holdup) on the tray and is designed such thatthe the holdup is at a suitable height, e.g. such thatthe bubble caps are covered by liquid.

Being lighter, vapour flows up the column and isforced to pass through the liquid, via the openingson each tray. The area allowed for the passage ofvapour on each tray is called the active tray area.




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As the hotter vapour passes through the liquid on the tray above, it transfers heat to theliquid. In doing so, some of the vapour condenses adding to the liquid on the tray. The

condensate, however, is richer in the less volatile components than is in the vapour.Additionally, because of the heat input from the vapour, the liquid on the tray boils,generating more vapour. This vapour, which moves up to the next tray in the column, isricher in the more volatile components. This continuous contacting between vapour andliquid occurs on each tray in the column and brings about the separation between low

boiling point components and those with higher boiling points.

Tray Designs

A tray essentially acts as a mini-column, each accomplishing a fraction of the separationtask. From this we can deduce that the more trays there are, the better the degree of

separation and that overall separation efficiency will depend significantly on the designof the tray. Trays are designed to maximise vapour-liquid contact by considering the

liquid distribution and

vapour distribution

on the tray. This is because better vapour-liquid contact means better separation at eachtray, translating to better column performance. Less trays will be required to achieve thesame degree of separation. Attendant benefits include less energy usage and lowerconstruction costs.

There is a clear trend to improve separations by supplementing the use of trays byadditions of packings.

Packings

Packings are passive devices that are designed to increase the interfacial area forvapour-liquid contact. The following pictures show 3 different types of packings.




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These strangely shaped pieces are supposed to impart good vapour-liquid contact whena particular type is placed together in numbers, without causing excessive pressure-dropacross a packed section. This is important because a high pressure drop would mean thatmore energy is required to drive the vapour up the distillation column.

Packings versus Trays

A tray column that is facing throughput problems may be de-bottlenecked by replacinga section of trays with packings. This is because:

packings provide extra inter-facial area for liquid-vapour contact

efficiency of separation is increased for the same column height

packed columns are shorter than trayed columns

Packed columns are called continuous-contact columns while trayed columns are calledstaged-contact columns because of the manner in which vapour and liquid arecontacted.




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Column Reboilers

There are a number of designs of reboilers. It is beyond the scope of this set ofintroductory notes to delve into their design principles. However, they can be regardedas heat-exchangers that are required to transfer enough energy to bring the liquid at the

bottom of the column to boiling point. The following are examples of typical reboilertypes.




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UNIT 4

PRINCIPLES OF GAS PROCESSING OPERATION

Overview

Natural gas liquids (or "NGLs), are hydrocarbons in a liquid state. There are manyreasons for removing NGL's from the gas stream, including:

1. The hydrocarbon components may be more valuable in their liquid state, eithermixed or separated into their different individual elements.

2. Pipeline gas quality specifications may restrict the amount of NGL is allowed.

3. Heavier NGLs can separate from the gas stream in pipelines as temperaturesdrop, thus restricting flow in the pipeline.

4. The NGLs may be used for re-injection on enhanced oil recovery projects. Thatis, they will be injected into a formation to help "sweep" crude oil through thereservoir by mixing with the oil and pushing the crude toward the well bore.

There are several different processes that can be used to separate NGLs from thenatural gas stream. The three most common are: Cryogenics, refrigeration and lean oil

absorption.




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1. Cryogenics

In the cryogenic process, a natural gas stream is cooled to extremely low temperaturesto liquefy the ethane and heavier hydrocarbons. When a gas stream is chilled to below-50 F (-45 C ), heavier gas components are easily liquefied. The ethane and heavierhydrocarbons are then separated from the methane.

Cryogenic processing of a natural gas stream involves three basic steps: dehydration,chilling and fractionation.

Because of the very cold temperatures in cryogenic processing, the gas stream must bealmost totally dehydrated to prevent the formation of Hydrates.

Dehydration is usually accomplished with either a liquid or solid desiccant. The methoddepends on the water content of the inlet gas at your facility. Some cryogenic processeswill use only dry-desiccant dehydration. But if the water content of the inlet gas is-high,first there would be liquid desiccant dehydration, followed by dry-desiccantdehydration.



EXPANDER

COMPRESSO

R

DEMETHANIZER

COMPRESS

ORMETHANE SEPARAT

OR

NATURAL

GAS

HEAT

EXCHANGER

N. GAS

LIQUID

CRYOGENICS


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DEHYDRATION WATER

NATURAL GAS STREAM

DRY GAS

GAS AND LIQUID

CHILLING

- 150 F ( - 101 C )

FRACTIONATIONGAS

( METHANE)

NATURAL GAS LIQUID

CRYOGENIC PROCESSING


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Chilling of the gas is the cornerstone of the cryogenic process. The gas stream can bechilled by heat exchange with cold gas, and by pressure reduction or pressure reduction

with energy removal. The pressure reduction method ( "J.T or Joule- Thompson),provides cold temperatures in the range of -50 F (-45 C) to -100 F (-73 C). To obtainthe lowest temperatures, -100 F (-73 C) to -200 F (-129 C), pressure reduction withenergy removal is accomplished using an expander -compressor.

Cryogenics has relatively moderate energy requirements, and is considered the mostefficient method for removing NGLS from the gas stream.

METHOD TEMPERATURE RANGETYPICAL PERCENT

RECOVERY

Refrigeration0 to 20 F( - 18 to 29 C )

EthanePropaneButanesHeavier

25559397

Pressure Reduction

( J.T Process )

- 50 to 70 F( - 46 to 57 C )


70809799

Expander Process- 125 to 150 F( - 87 to 101 C )


809699100



HIGH PRESSUREGAS

HIGH PRESSUREGAS

COMMONSHEET

LOW PRESSUREGAS

LOWPRESSURE GAS

COMPRESSOR EXPANDER

EXPANDER PROCESS


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2. Refrigeration

Refrigeration is a separation process that has been in use for many years. The principleis to chill the natural gas stream by passing the gas through a chiller. Temperatures inthis process range from 0F (-18 C) to -20 F (-29 C). Chilling causes the heavierhydrocarbons to liquefy, and then these NGLS can be separated from the gas.



COMPRESSORCONDENSER

REFIRGERANT

VAPOR

SURGETANK

WARM NATURALGAS

REFIRGERANTLIQUID

CHILLERCOLD GAS AND

LIQUIDS


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Refrigerant

The energy requirements of refrigeration are primarily for compressing the refrigerantand driving the condenser. While the volume of heavier hydrocarbons recovered is lessthan with lean oil absorption , the amount of equipment and energy used for recoverywith refrigeration is much less, making it a more economical method than lean oilabsorption.

3. Lean Oil Absorption

In lean oil absorption, lean oil is flows through a natural gas stream, absorbing theheavier hydrocarbons as it contacts them. The hydrocarbons are then recovered bydistilling them out of the now "rich" oil. Once the NGLs are distilled, the lean oil is

recycled back through the system.



LEAN OILGAS

CHILLER

NATURALGAS

LIQUIDS

NATURALgAS

METHANe

SEPARATOR

PUMP

STILL

RICH OIL

LEANOIL

ABSORBER

LEAN OIL ABSORPTION


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High percentages (90-95%) of propane and heavier hydrocarbons (98-100%) can berecovered using lean oil absorption.

Lean oil absorption is relatively expensive to operate because it requires a lot of energy/equipment compared to refrigeration or cryogenics.

This process was the mainstay of the industry for years. Although still in use, it isgradually fading from the scene as cryogenics takes its place.




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FRACTIONATIONFLOW


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4. Fractionation ( Deethanizer & Depropanizer & Debutanizer)

Fractionation is a. downstream treatment option for NGLs after they have beenseparated from the gas stream. By definition, fractionation consists of separating twocomponents in a mixture of two or more components. For example, a depropanizerseparates propane from a stream that contains propane and one or more heavierhydrocarbons. The propane is cooked out of the mixture and is the overhead productfrom the tower. The bottom stream is practically free of propane.

Fractionation is essentially a distillation process. As you can see on the perviousdiagram, different NGLs are cooked out in various towers. There are usually twomeans of control ling the purity of the top and bottom streams of a given tower byvarying the temperature at the top of the tower, or varying the temperature at the bottom

of the tower.

Generally speaking, pressure and feed rate cannot be changed, since this would changeconditions in the rest of the plant.

Whether to fractionate is a question of economics. Is there a profitable market for thefractionated product? Because market conditions change rapidly and often, this questionmust constantly be answered to determine if fractionation will be done. There are timeswhen the mixed NGLs will be more valuable than their separate components.




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UNIT 5

PRODUCT SPECIFICATIONS

Overview

This topic is aimed at all process operations staff wanting to expand their knowledgeabout the control of distillation columns and the impact to product separation.

Objectives

On completion of this topic, the trainee will:

- understand how a distillation column is controlled (the six independent variables)

Contents

5.1 Distillate and Bottoms Specifications

5.2 Control of Product Composition




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5.1 Distillation and Bottoms Specifications:

This topic is devoted to a discussion of measurement and control techniques common isdistillation.

For purposes of discussion, it will be assumed that:

1. A column already exists.

2. Many of the control variables will be fixed by the engineering design of thecolumn.

3. The column has a reboiler.

4. The feed point to the column is fixed and making only two products a

distillate and bottoms product.

If the assumptions are held for the above, there are eight (8) variables

1. Column Pressure 5. Heat Added (boil-up)

2. Feed Flow Rate 6. Bottom Product Flow Rate

3. Feed Composition 7. Heat Removed (reflux)

4. Feed Temperature 8. Distillate Product Flow Rate

only six (6) of these variables can be controlled independently. Compositions fordistillate and bottom products are called dependent variables since they are notcontrolled directly but rather through the other variables mentioned above.

Pressure Control

Most distillation columns are maintained at some kind of constant pressure. This helpsin keeping the condensation loading to the condenser as even as possible (dew point).The specific pressure is determined from economic considerations- For example, ifoperating pressure increases, the column temperature increases thus increasing thecondenser cooling requirements and decreasing the reboiler heating requirements.

Feed Control

The first variable fixed would be the column pressure (as discussed above). If the feedconditions were fixed, this would necessitate that three (3) more independent variableswould be determined - feed rate feed, composition, and feed temperature.

The feed flow rate can be maintained at a constant rate by using flow control.

The feed composition has a great influence upon the operation of a distillation unit.'Unfortunately, the feed composition is seldom subject to adjustment. For this reason, itis necessary to make changes elsewhere to the operation of the column in order to

compensate for variations in feed composition.




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The thermal condition of the feed determines how much additional heat must be addedto the column by the reboiler. For efficient separation, it usually is desirable to have the

feed at its bubble point when it's enters the column. If the feed composition varies, itsbubble point also varies. It is a common practice to set the temperature control at a pointwhich is equivalent to the bubble point of the heaviest feed.

Reboiler Control:

Since fixing the feed conditions and column pressure determines four of the variableslisted earlier, only two other. variables remain to be controlled in order to fix theoperation of the distillation column. Frequently, the boil-up rate is chosen as one of thetwo remaining independent variables. The boil-up rate is controlled by setting the flowof heat to the reboiler. In kettle reboilers, the heating medium (e.g. steam) is added as

required to try and achieve the bottom product specification

Reflux Control:

For total control of the distillation operation, reflux rate represents the sixth and lastindependent variable to be controlled. The reflux furnishes the continuous supply ofliquid to the top and down through the column just as the boil-up from the reboiler

provides a continuous supply of vapor up through the column.

The rate of reflux is regulated by a flow controller on the reflux line

The rate of distillate product withdrawal is controlled by the liquid level in theaccumulator.

Conclusion:

By controlling the six independent variables and assuming constant feed conditions theseparation efficiency of a distillation column can be controlled. The consistent andoptimum specification for distillate and bottoms products can hence be achieved.

5.2 Control of Product Purity & Fulfilling demands

A distillation unit operates between two extremes. In one case, insufficient separation is

reflected in unacceptable product purity. In the other case, separation can be far inexcess of what is demanded so that utilities and unit capacity are wasted.

The goal of distillation then is to achieve a specified product purity without causingwaste. To obtain this goal, some measure of product composition is needed.

Since distillation separates materials according to their difference in vapor pressure andsince vapor pressure is a temperature controlled function, temperature measurement can

be used to indicate composition.




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The important consideration is to measure temperature on a tray which most reflectschanges in composition. When composition of the bottom product is the important

consideration, it is desirable to maintain a constant temperature in the lower section ofthe column. When composition of the distillate product is the important consideration, itis desirable to maintain a constant temperature in the upper section of the column.

Measuring temperature in a column usually requires that the sensing device be in theliquid on the tray. Distillation temperature is an indication of composition only whencolumn pressure remains constant.

In most cases, the controlling of pressures and temperatures can approximate the controlof product ( the compositions should be close to specification with a given set point of

pressure and temperature(s) in the column. Laboratory analysis can be done to confirm

and/or adjust the pressure and temperature set points.

Analytical or specific composition control is another way to sidestep the problems oftemperature control. Several types of analytical instruments can be used for distillationcolumn control. The most common ones used are infrared and gas .chromatographs (G.c/ s). If the distillate and/or bottoms streams could be sampled continuously and fedthrough an analyzer, the stream compositions could be used for column control.




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UNIT 6

DISTILLATION COLUMN CONTROL & INSTRUMENTATION

Instrumentation -Measurement Points, Alarm Points. Safety Equipment, Relief

Devices:

Measurement Points, Alarm Points:

As discussed in topic #5, there are different variables that can be used to operate a givendistillation column. The different locations, control loops, and alarm points.

Notes:

a) Feed Control:b) Reboiler Control:

c) Bottoms Control:

d) Tower Pressure Control:

e) Reflux Control:

f) Distillate Control:

Safety Equipment, Relief Devices:

Distillation columns requires safety systems in order to function within the plantenvironment without creating risk to plant personal and other process equipment. Thesesafety systems may range in complexity from simple alarms to interlock systems.Examples of areas that should have alarms are:

Reflux Drum : high and low level alarms, high pressure alarm

Column Bottom : high and low level alarms

Feed Flow : high and low flow alarms

Reflux Flow : high and low flow alarms

Reboil : high temperature alarm, high and low flow alarms (steam ) lowflow alarms (process).

A major concern in the operation of distillation systems is overpressure of the

equipment and possible catastrophic failure , A partial list of the causes of

overpressure is found below:




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a) Utility failure

- loss of coolant

- loss of electric power

- loss of steam

- loss of instrument air

b) Controller failure ( human error in opening valve)

- failure of reboiler controller

- failure of pressure controller

- failure of feed controller

- failure of pump around controller

c) Extraneous sources

- valve opening to external pressure source

- upstream upset (change in feed composition)

- exchanger failure (tube rupture)

- exterior fire

d) Internal sources- noncondensables

- chemical reaction

- closed column outlets

e) Transient sources

- pockets of water flashing to steam

- internal explosions

Methods of relieving overpressure which should be included in the column designare:

- properly sized relief valves on column

- adequate vapor vent from reflux drum

- properly specified control valves with correct failure mode

- manual bypass around control valves




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Standard Column Control:

Distillation column control schemes may be broken into two major types, materialbalance systems and heat balance systems. Of the two, the material balance system orsome variation of it, is the most common.

Material Balance System:

In a material balance control system , product composition is controlled by controllingthe flow of material into and out of the column. The column pressure is controlled bythe amount of cooling in the overhead condenser while the distillate and bottoms

products are both controlled by level control. It is a common practice to have bothdistillate and bottoms on level control, thus making accumulation in the column

unlikely. The temperature of the bottom is controlled by the steam flow on temperaturecontrol while the reflux is on flow control. This is referred to as indirect control as thetower top temperature is controlled by a flow controller.

A variation of this is to use temperature control to manipulate the reflux. This is knownas direct control.

Energy Balance System:

In energy balance control system, the energy balance controls the product compositionwhile the free variable is one of the product flows. The shortcoming of this scheme isthat variations in the material balance interact with the control system. These controlsare more sensitive to the changes in the material balance than the changes in the energy

balance. For these reasons, energy balance controls are only used if a satisfactorymaterial balance system cannot be implemented or in conjunction with an advancedcomputer control system.




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UNIT 7

DISTILLATION COLUMN OPERATION

Startup / Shutdown:

Once a column is bolted up after initial construction or a shutdown, a number ofoperations must occur to prepare the column for startup. These steps, referred to ascolumn commissioning, are used to clear the system of undesirable materials, test thecolumn and to take preventative measures against performance deterioration.

Line Blowing:

Normally done with a utility service such as air and/or water. This facilitates theremoval and cleaning of any pipe scale, rust, and loose slag from weld joints. Mostblowing is done to atmosphere and not into the distillation column itself.

Pressuring and Depressuring:

This is done with the same utility mentioned above. The column, connecting equipment,piping, along with all instrumentation is checked for leakage. The column system mustbe leak proof and can be checked by monitoring the column pressure [no drop intransmitter signal] and also by "snooping" or checking for leaks in the field.

Purging:

This is done using an inert gas utility such as nitrogen. This is extremely' importantsince air can't be used [especially on vacuum systems] on the column as it allowsexplosive mixtures to occur. The column should be kept under a nitrogen "pad" prior tostartup so as not to cause any problems on startup.

Blinding and Unblinding:

Most columns under construction and during vessel entry may have piping blind flangesor spades inserted so as not to have any utility and/or process materials enter theequipment [safety concerns]. The removal of blinds and spades are required prior to the

purging step but may be left on during any line blowing activity if the location of theblinds and spades are connecting equipment to piping. The removal may be done afterblowing so as not to contaminate the equipment with the blown out rust, scale, slag,etc..

Leak Testing:

Per above "pressuring and depressuring " step. A soapy water solution can be used as a"snoop" test. Most times this solution is placed in a bottle [or can be purchased as"snoop"]. All flanges and equipment / instrument connections are squirted with thesoapy water solution. If any leaks occur, the solution will bubble vigorously.




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Washing:

Some columns and equipment may need to be washed with water or a solvent prior tooperation as to eliminate any concerns with fouling, reactivity with dirt, rust, carbon,etc., as well as any "deadspots" that cause problems with the process material to be putthrough the column itself. A "water run" can be done to help calibrate instruments andcheck for pump seal problems, leaks, etc..

Steaming:

Some columns can be purged with steam to exclude air. This can be done especially inwarm climates ( no freezing problems). Also, steam makes a good leak check since itcan "find" leaks better than any external testing.

Dryout:

Normally done with dry air or nitrogen utility. Can also be a solvent drying as welldepending on the application.

Once the column has been properly commissioned, the system is ready for startup. Thesequence of steps used for startup is listed below although deviations are often requiredfor specific distillation columns and the hardware availab1e

1) Final elimination of undesirable materials.

The distillation column system should be clean, leak checked, and ready to run.

2) Bringing column to normal operating pressure.

Depending on what the desired pressure is, this may be done by use of vacuumpumps/ejectors [if vacuum column] or by allowing the pressure control system torespond to column liquid boil up.

3) Column heating and cooling.

The condenser cooling medium should be on and functional. The reboiler/heating

system should be ready to run once the liquid feed is introduced and a column bottomlevel established.

4) Introduction of feed.

This is done once all the above are done.

5) Starting up heating and cooling sources.

The cooling medium should already be running, The heating medium should be startedslowly and carefully due to the thermal stress created in the equipment during the

heatup.




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6) Bringing column to desired operating rates.

Depending on desired operating set points, the feed should be put on a minimum settinguntil the entire column is up to temperature, pressure, reflux ratio [if thedistillate/forwards can be recycled], If the forwards can not be recycled, then a totalreflux can be done for a short time until the specifications are okay, then reflux ratio can

be set along with a feed increase and required reboiler/heating load increase [to keep thebottoms on spec],

Column shutdown also follows a similar series of steps, typically:

1) Reducing column rates.

Assuming all control systems are on line and at set point, the first step is to reducecolumn feed rates, This will gradually reduce heat and cooling loads as well as forwardsand bottoms rates while still on spec.. The column may be put on total reflux so as tokeep the reboiler running on a liquid inventory [i.e. no . bottoms being pumped forwardeither.

2) Shutting down heating and cooling sources.

The feed can be stopped per step (3) below first if the heating and cooling sources areready to be shutdown. The heating source should be shutoff first as to help cool thecolumn with any remaining reflux.

3) Stopping feed.

The feed can be stopped sooner than the heating source shutdown if the level rise in thecolumn becomes too high [alarm point]. This will help minimize the pump-out ofexcess liquid inventory from the bottom of the column if required for any shutdownactivity.

4) Draining liquids.

Both the condenser [reflux inventory] and reboiler [bottoms inventory] can be pumpedout or drained to storage inventory prior to any column decommissioning activities.

5) Cooling or heating column.

If required for maintenance and/or vessel entry work, the column may need to be furthercooled or heated using the process material and/or utility as required.

6) Bringing column to atmospheric pressure.

For vacuum systems, nitrogen is used to repressure the column. For pressure systems,the condenser system is often vented to a recovery and/or flare system in order to get thecolumn to atmospheric pressure.




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7) Eliminating undesirable materials.

In most cases, nitrogen is used to purge out any remaining vapors and liquids Prior tovessel work.

8) Prepare the column for opening to atmosphere.

Hopefully the column has been purged correctly and all piping drained/blown to avapor/liquid recovery system.

Again, these steps may vary with each specific column or specific company procedure.Operations staff should be aware of any specific hazards or potential problems with thespecific column prior to startup/shutdown. Prior to startup/shutdown, there are several

other items that should be considered:1) Prepare adequate procedures for operation, startup, shutdown and maintenance.

2) Ensure the startup/shutdown team consists of personnel who have all the requiredskills for the procedure.

3) Adequate training for the startup team, supervisors and operators is required.

4) Proper startup/shutdown planning must be complete.

5) Secure any raw materials, equipment or spares required.

6) Develop adequate procedures for last minute modifications, safety checks andaudits, inspections and recordkeeping.

7) Develop adequate procedures for emergency response in case of accident

8) Develop individual tasks and objectives for each member of the startup team andensure these are well understood

9) Ensure proper checklists are available for each phase of the startup/shutdown.

Operation of Distillation Columns:

There are several fundamental process variables in a distillation column which must becontrolled in order to get proper separation of components:

Overhead Temperature:

The overhead temperature is determined by the composition of the liquid on the firsttray, this liquid must be at its' boiling point at the pressure of the column. Thecomposition is controlled by the reflux ratio. The higher the reflux ratio, the higher theconcentration of the lighter component on the top tray. As the lighter component boils,at a lower temperature, the temperature at the top of the column will be lower. There arelimits to this which are dependent on the condenser temperature and the phasecharacteristics of the material in the column.




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Temperature Profile

The temperature profile of a column is a plot of the temperature on each tray from thetop of the column to the bottom. The lowest temperature should be at the top of thecolumn, corresponding to the lightest components while the highest temperature should

be at the bottom, corresponding to the heaviest component. The profile between the topand bottom of the column should be a smooth curve with no flat spots. This indicatesthat the composition is different on each tray and efficient separation is occurring. Ifthere is a section of the column in which the temperature profile is flat, the compositionon those trays is very close and minimal separation is taking place. The exception to thisis often the feed area where a very cold (subcooled) or very hot (superheated) feed cancause the temperature profile to flatten.

Column Pressure:

The operating pressure in the column is determined by two factors, the composition of

the overhead vapor being condensed and the cooling ( both amount and temperature)available in the overhead condenser. The condenser temperature must be cold enough tocondense the lightest component in the overhead vapor to liquid. The pressure at whichthis occurs determines the operating pressure of the column. If the lightest component isnot condensed, the vapor in the overhead receiver will cause the system pressure toincrease until the temperature in the condenser is low enough to condense the lightcomponent. If this results in a pressure which is too high forthe design of the column,the light end can be vented as vapor from the reflux drum.

Reflux Ratio:

The reflux ratio, along with the bottoms temperature, is the primary control variable fora distillation column. The definition of the reflux ratio is the ratio of liquid returned tothe column divided by the distillate flow, the higher the value, the more liquid that isreturned to the top of the column. The reflux ratio controls the tower top temperature,the degree of separation of the components and the amount of liquid flowing from trayto tray. As the reflux ratio is so important, the column is designed for a specific valueknown as the optimum reflux ratio. This value is different for each application. Easyseparations may require a reflux ratio as low as 0.5 while difficult separations mayrequire as high as 8 or 9. Once a column is running, the reflux ratio is fine tuned for thespecific operating conditions of the plant. .

Bottoms Temperature:

The temperature at the bottom of the column determines the composition of the bottomproduct, the higher the bottoms temperature, the less light ends in the bottom product.The rate of heating also determines the vapor return rate to the column. The higher the

bottoms temperature, the higher the vapor return rate. At some point the amount ofvapor will become too great for the column and jet flooding will result. As the bottomstemperature is increased, the reflux ratio may require adjustment to maintain the sameoverhead temperature. The bottoms temp is usually controlled by the amount of heatingmedium, such as steam of hot oil, flowing through the reboiler.




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UNIT 8

DISTILLA TION COLUMN TROUBLESHOOTING

Flooding:

Distillation column capacity is usually restricted by flooding.

Flooding is excessive accumulation of liquid inside the column. This accumulation

is generally caused by one of the following mechanisms:

Spray Entrainment Flooding - at low liquid flow rates, trays operate in the sprayform, where most of the liquid on the tray is in the form of liquid drops. As vapor

velocity is raised, a condition is reached where the bulk of these drops is entrained intothe tray above. The liquid accumulates on the tray above instead of flowing to the traybelow.

Froth Entrainment flooding - At higher liquid rates, the dispersion of the tray is in theform of a froth. When vapor flow rate is raised, froth height increases. When trayspacing is small, the froth envelope approaches the tray above. As this surfaceapproaches the tray above, entrainment rapidly increases, causing liquid accumulationabove. For large tray spacing [18 - 24 inches], the froth envelope seldom approaches thetray above. Given enough vapor velocity, the froth would turn into spray and causespray entrainment flooding per above.

Downcomer Backup Flooding - Aerated liquid is backed up into the downcomerbecause of tray pressure drop, liquid height on the tray, and frictional losses in thedowncomer apron.

Downcomer Choke Flooding - As liquid flow rate increases, so does the velocity of

aerated liquid in the downcomer. When this velocity exceeds a certain limit, frictionlosses in the downcomer and downcomer entrance become excessive, and the frothymixture cannot be transported to the tray below. This causes liquid accumulation on thetray above.

Generally, at low tray spacing (less than 12 - 15 inches), froth entrainment flooding isfavored. At higher tray spacing, and when conditions do not favor vapor cross flow, thefroth regime will turn into a spray as vapor velocity increases, and spray entrainmentflooding is favored. Finally, when downcomers are small or downcomer backups arehigh, downcomer flooding is favored.




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Flooding can be determined by one or more of the following symptoms:

1) Excessive column differential pressure [delta pressure is greater than 3 inches of

water per foot of packed bed]

2) Sharp rise in column differential pressure

3) Loss of bottoms

4) Rapid rise of entrainment from column top tray [large rise in reflux vs normal]

5) Loss of separation as can be detected by temperature profile or product analysis.

Operation Difficulties:

Dislodging and Damage of Trays:

Most tray or packing damage comes from excessive liquid in the bottom of the column.

In some cases, totally flooded tr

Distillation

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