7 - distillation0.pdf

8/10/2019 7 - DISTILLATION0.pdf

1/58

Phase 2SEGAS SERVICES Training Program Distil lations Page 1 of 58

SEG S Services Train ing Cent r e

Dam ietta LNG Plant

DISTILLATION

DESIGN, OPERATION, TROUBLESHOOTING

COURSE OUTLINE

Course Description:

Topics include:

1. Introduction

2. Phase Behavior

3. Distillation Column, Components

4. Principles of Gas Processing Operation

5. Product Specifications

6. Distillation Column Operation

7. Distillation Column Troubleshooting


2/58

Phase 2SEGAS SERVICES Training Program Distil lations Page 2 of 58


Dam ietta LNG Plant

General Objectives:

Upon completion of this course, the student shall:

1. Distillation & of Terns Definition in Gas Processing

2. Have a good understanding of vapor-liquid phase behavior and how this relates to

distillation.

Be able to describe various components on a distillation column and explain their

functions.

Understand distillation column specifications and how these are controlled.

Understand the operation of a distillation column from startup to operation to

shutdown.

Be able to understand and recognize different distillation column upsets/problems

and how to correct for them.


3/58

Phase 2SEGAS SERVICES Training Program Storage Tanks Page 3 of 58


Dam ietta LNG Plant

An Introduction

Distillation is defined as:

a process in which a liquid or vapour mixture of two or more substances is separated into its

component fractions of desired purity, by the application and removal of heat.

Distillation is based on the fact that the

vapour of a boiling mixture will be richer in

the components that have lower boiling

points.

Therefore, when this vapour is cooled and

condensed, the condensate will contain

more volatile components. At the same

time, the original mixture will contain more

of the less volatile material.

Distillation columns are designed to achieve

this separation efficiently.

More of less Volatile

High P.P

More of Volatile

low P.P


4/58



Dam ietta LNG Plant

Distillation Principles

Separation of components from a liquid mixture via distillation depends on the differences inboiling points of the individual components. Also, depending on the concentrations of the

components present, the liquid mixture will have different boiling point characteristics.

Therefore, distillation processes depends on the vapour pressure characteristics 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:

o Energy Input Raises Vapour Pressure

o Vapour Pressure Is Related To Boiling

o A Liquid Is Said To Boil When Its Vapour Pressure Equals The Surrounding

Pressure

o The Ease With Which A Liquid Boils Depends On Its Volatility

o Liquids With High Vapour Pressures (Volatile Liquids) Will Boil At Lower

Temperatures

o The Vapour Pressure And Hence The Boiling Point Of A Liquid Mixture Depends On

The Relative Amounts Of The Components In The Mixture

o Distillation Occurs Because Of The Differences In The Volatility Of The Components

In The Liquid Mixture


5/58


6/58



Dam ietta LNG Plant

and liquid hydrocarbon stream

Associated Gas

Gaseous hydrocarbons occuring as a free-gas phase under original oil-reservoir conditions of

temperature and pressure.

Atmospheric Pressure

The pressure exerted on the earth by the earth's atmosphere, A pressure of 760 mm of

mercury or 101.3250 kPa is

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 at 60F and

Blanket Gas

A gas phase maintained in a vessel containing liquid to protect the liquid against air

contamination, to reduce the hazard of detonation, or to maintain pressure of the liquid. The

source of the gas is external to the vessel.

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.

Bottoms.

The liquid or residual matter which is withdrawn from the bottom of a fractionator or other

vessel during processing or while in storage.

B-P mix

A liquefied hydrocarbon product composed chiefly of butanes and propane. If it originates in

a refinery, it may also contain butylenes and propylene.

Breathing

The movement of vapor in or out of an atmospheric pressure storage tank because of a

change of level of the stored liquid, a change in the temperature of the vapor space above the


7/58



Dam ietta LNG Plant

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 products.

Bubble Point

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

Butane, Commercial

A liquefied hydrocarbon consisting predominately of butane and/or butylene

Butane, Normal

Normal butane is an aliphatic compound of the paraffin series having the chemical formula

C4HlO and having all of its carbon atoms joined in a straight chain.

Calorimeter

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

Casinghead Gas

Unprocessed natural gas produced from a reservoir containing oil. It contains heavier

hydrocarbon vapors and is usually produced under low pressure from a casing head on the

well.

Charcoal Test

A test standardized by the Gas Processors Association and the American Gas Association for

determining the natural gasoline content of a given natural gas. The gasoline is adsorbed from

the gas on activated charcoal and then recovered by distillation.

Chromatography

A technique for separating a mixture into individual components by repeated adsorption and

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.


8/58


9/58



Dam ietta LNG Plant

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.

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


Depropanizer

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


Desiccant

A substance used in a dehydrator to remove water and moisture. Also a material used to

remove moisture from the air.

Desulfurizationf

A process by which sulfur and sulfur compounds are removed from gases or liquid

hydrocarbon mixtures.

Dew Point

The temperature at any given pressure, or the pressure at any given temperature, at which

liquid initially condenses from a gas or vapor. It is specifically applied to the temperature 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 and then


10/58


11/58



Dam ietta LNG Plant

Gas Constant (R)

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

Gas Injection

The injection of natural gas into a reservoir to maintain or increase the reservoir pressure 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 the producing

well bore.

Gas-Oil Ratio (GOR)

The ratio of gas to liquid hydrocarbon produced from a well. This may be expressed as

standard cubic meters of gas per cubic meter of stock tank liquid.

Gas Processing T

The separation of constituents from natural gas for the purpose of making salable products

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 sometimes other

substances such as sulfur.

Gas-Well Gas

The gas produced or separated at surface conditions from the full well stream produced from

a gas reservoir.

Gas-Well Liquid

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

reservoir.

Gathering System

The network of pipelines which carry gas from the wells to the processing plant or other

separation equipment.


12/58



Dam ietta LNG Plant

Heat Media (Heating Media)

A material, whether flowing or static, used to transport heat from a primary source such ascombustion of fuel to another material. Heating oil, steam.

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 amount of 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 hexanes or

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 hydrocarbon analysis

which contains the hexanes (or heptanes) and all hydrocarbons heavier than the hexanes (or

heptanes).

Hydrate

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

immiscible

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

Inerts

Elements or compounds not acted chemically by the surrounding environment. Nitrogen and

helium are examples of inert constituents of natural gases.


13/58



Dam ietta LNG Plant

Isobutene

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 surrounds a

vessel or machine element in order to remove, add, or distribute heat in order to control the

temperature within the vessel or element.

Joule-Thomson Effect

The change in gas temperature which occurs when the gas is expanded at constant enthalpy

from a higher pressure to a lower pressure. The effect for most gases at normal pressure,

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 paper which has

been moistened with lead acetate solution.

Lean Gas

(1) The residue gas remaining after recovery of natural gas liquids in a gas processing plant.(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 absorbed

constituents 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.


14/58



Dam ietta LNG Plant

Lng (liquefied natural gas)

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

loading rack

A structural and piping installation alongside a railroad track or roadway used for the purpose

of filling railroad tank cars or transport trucks.

LP-Gas (Liquefied petroleum gas)

Predominately propane or butane, either separately or in mixtures, which is maintained in a

liquid state under pressure within the confining vessel.

LNG (liquefied refinery gas)

Liquid propane or butane produced by a crude oil refinery. It may differ from LP-gas in that

propylene and butylene may be present.

LTX (low temperature extraction unit)

A unit which uses the cooling of a constant enthalpy expansion to increase liquid recoveryfrom streams produced from high pressure gas condensate reservoirs. Also called LTS (low

temperature separation) unit.

Mercaptan,

Any compounds of the general formula RSH. All mercaptans possess a foul odor.

Miscible Flood

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

Natural Gas

Consisting predominately of mixtures of hydrocarbon gases. The most common component is

methane.


15/58



Dam ietta LNG Plant

Natural Gasoline

A mixture of hydrocarbons, mostly pentanes and heavier (C5+) extracted from natural gas,

Natural Gas Processing Plant

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

Odorant

An odoriferous compound added to natural or LP-gas to impart a distinctive odor for

detection of fugitive vapors. Ethyl mercaptan is the most widely used odorant for LP-gas,

Oil-Well Gas

Gas that is produced from an oil well.

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 as

ullage.

Packed Column

A fractionation or absorption column filled with packing designed to give the required

contact between the rising vapors and the descending liquid.

Pentane-Plus

A hydrocarbon mixture consisting of is opentane (C5H12) and heavier components with

higher boiling points.

Pigging

A procedure for forcing a device through a pipeline for cleaning purposes, separating

products, or inspecting the line.


16/58



Dam ietta LNG Plant

Raw Gas

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

Recovery

That percent or fraction of a given component in the plant feed which is recovered as plant

product.

Recycle

Return of part of a process stream to a point upstream from where it was removed to enhance

recovery or control.

Reflux

In fractionation, the portion of condensed overhead returned to the column to enhance

achievable purity of the overhead product.

Reflux Ratio

A way of giving a relative measurement to the volume of reflux. Usually referred either to the

feed or overhead product.

Relative Density

The ratio of the mass of a given volume of a substance to that of another equal volume of

another substance used as standard. Unless otherwise stated, air is used as the standard for

gases and water for liquids, with the volumes measured at 15.56C and atmospheric pressure

(101.325 kPa).

Relief System

The system for safely relieving excess pressure to avoid exceeding equipment design

pressure.

Residue

The material which remains after a separation process. (1) Residue gas is that gas remaining

after the recovery of liquid . products. (2) Residue may also be the heaviest liquid or solid

remaining after distillation or reclaiming process.


17/58


18/58



Dam ietta LNG Plant

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. In plants

using a low molecular weight absorption oil, the still is designed as a fractionation column. In

plants using a high molecular weight absorption oil, the still may use steam or other fluids as

stripping medium. Also used to refer to regenerators in amine treating and glycol dehydration

systems.

Strapping

A term applied to the process of calibrating liquid storage capacity of storage tanks in

increments 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 term isapplicable to columns using a stripping medium, such as steam or gas.

Sulfur

A yellow, non-metallic chemical element. In its elemental state, it exists in both crystalline

and amorphous forms. In many gas streams, sulfur may be found as volatile sulfur com-

pounds, such as hydrogen sulfide, sulfur oxides, mercaptans, and carbonyl sulfide. Reduction

of the concentration of these gaseous sulfur compounds is often necessary for corrosion con-

trol and possibly for health and safety reasons.

Sulfur Dioxide (SO2)

A heavy, colorless, suffocating gas that is chemically an oxide of sulfur. Conversion of the

gaseous sulfur oxides to sulfur is necessary for corrosion control, for health and safety

reasons, and for complying with governmental standards.

Sweet

Gas containing essentially no objectionable sulfur compounds. Also, treated gas leaving a

sweetening unit.


19/58



Dam ietta LNG Plant

Sweet Gat!

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

Tonne

A unit of mass measurement, commonly used in international petroleum commerce; an

expression for the metric ton, or 1000 kilograms.

Trayed Column

A vessel wherein gas and liquid, or two partially miscible liquids, are contacted, usually

concurrently on trays. Also refer to packed column.

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 because of

their relatively high chemical reactivity. Unsaturates are produced by a thermal cracking or

chemical reaction and can be found in synthetic gas (SNG) or light refinery gases (LRG).

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 or

vapors from some other source.

Weathering

The evaporation of liquid caused by exposing it to the conditions of atmospheric temperature

and pressure. Partial evaporation of liquid by use of heat may also be called weathering.

Weathering Test

A GPA test for LP-gas for the determination of heavy components in a sample by

evaporation under specified conditions.


20/58



Dam ietta LNG Plant

Wellhead

The assembly of fittings, valves, and controls located at the surface and connected to the flowlines, 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 term synonymous

with rich gas. Refer to definition of "rich gas".


21/58



Dam ietta LNG Plant

CHAPTER- 1

PHASE BEHAVIOR

Overview

This topic is aimed at all process operations staff wanting to expand their knowledge of the

behavior of gas/liquid mixtures and how this relates to the separation process of distillation.

Objectives

On completion of this topic, the trainee will:

- understand the definition and concepts for phase behavior

Contents

1.1 Vapor-Liquid Equilibrium


22/58



Dam ietta LNG Plant

Phase Behavior

Introduction:

In planning, design and operation of modem petrochemical processes, engineers,

technologists and process operators must know with- reasonable accuracy the properties of

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 in order to

determine the required volume of the vessel, the operating' pressure and thus the 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, or

temperature, or a characteristic that can be calculated or deduced, such as internal energy.

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 particular state. A

change in the state of a system results in a change -in at least one of its properties,

Phase- a completely homogeneous and uniform state of matter.

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

Ideal Gas- is an imaginary gas which obeys exactly certain simple laws such as the laws of

Boyle, Charles, and Dalton. No real gas obeys these laws exactly over all ranges of

temperature, although "lighter" gases (hydrogen, oxygen, air, etc.) under ordinary

circumstances obey the ideal gas laws with but negligible deviations.

Ideal Gas Law- from the work of Boyle and Charles, scientists developed the relationship

now called the Ideal Gas Law. The equation used is pV = n RT. This equation can relate the

volume, pressure, temperature, and the amount of a given gas.

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

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

Vapor Pressure - the pressure at which vaporization and condensation are at constant

temperature and pressure under equilibrium conditions for a pure substance or mixture.


23/58


24/58



Dam ietta LNG Plant

CHAPTER 2

DISTILLATION COLUMN COMPONENTS

Overview

This topic is aimed at all process operations staff wanting to expand their knowledge about

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)

- 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 mounted internally,

an overhead system and a reboiler system. In some cases, packing may be used rather than

individual trays. (Figure 2)


25/58


26/58



Dam ietta LNG Plant

Basic Distillation Equipment and Operation

Main Components of Distillation Columns

Distillation columns are made up of several components, each of which is used either to

tranfer heat energy or enhance materail transfer. A typical distillation contains several major

components:

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

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

o a reboiler to provide the necessary vaporisation for the distillation process

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

o 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 and reboiler,

constitute a distillation column. A schematic of a typical distillation unit with a single feed

and two product streams is shown below: Figure 3

Figure 3


27/58



Dam ietta LNG Plant

Basic Operation and Terminology

The liquid mixture that is to be processed is known as the feed and this is introduced usuallysomewhere near the middle of the column to a tray known as the feed tray. The feed 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 bottom in the reboiler.

Heat is supplied to the reboiler to generate vapour. The source of heat input can be any

suitable fluid, although in most chemical plants this is normally steam. In refineries, the

heating source may be the output streams of other columns. The vapour raised in the reboiler

is re-introduced into the unit at the bottom of the column. The liquid removed from the

reboiler is known as the bottoms product or simply, bottoms.

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

condenser. The condensed liquid is stored in a holding vessel known as the reflux drum.Some of this liquid is recycled back to the top of the column and this is called the reflux. The

condensed liquid that is removed from the system is known as the distillate or top product.

Figure 4

Liquid

Vapour

Reboiler

Bottoms

Tray 1

Tray 2


28/58



Dam ietta LNG Plant

Thus, there are internal flows of vapour and liquid within the column as well as external

flows of feeds and product streams, into and out of the column.

Figure 5


29/58



Dam ietta LNG Plant

Column Internals

Trays and Plates

The terms "trays" and "plates" are used interchangeably. There are many types of tray

designs, 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 the

riser. 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 the

cap, finally discharging through slots in the cap, and finally bubbling through theliquid on the tray.

2. Valve Trays

In valve trays, perforations are coveredby liftable caps. Vapour flows lifts the

caps, thus self creating a flow area for the

passage of vapour. The lifting cap directs

the vapour to flow horizontally into the

liquid, thus providing better mixing than

is possible in sieve trays.

Figure 6

Figure 7


30/58



Dam ietta LNG Plant

3. Sieve Trays

Sieve trays are simply metal plates with holes inthem. Vapour passes straight upward through the

liquid on the plate. The arrangement, number

and size of the holes are design parameters.

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 Flows in a Tray Column

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

column.

Figure 8

Figure 9


31/58


32/58


33/58



Dam ietta LNG Plant

These strangely shaped pieces are supposed to impart good vapour-liquid contact when a

particular type is placed together in numbers, without causing excessive pressure-drop across

a packed section. This is important because a high pressure drop would mean that moreenergy 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 replacing a

section of trays with packings. This is because:

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

o efficiency of separation is increased for the same column height

o packed columns are shorter than trayed columns

Packed columns are called continuous-contact columns while trayed columns are called

staged-contact columns because of the manner in which vapour and liquid are contacted.

Column Reboilers

There are a number of designs of reboilers. It is beyond the scope of this set of introductory

notes to delve into their design principles. However, they can be regarded as heat-exchangersthat are required to transfer enough energy to bring the liquid at the bottom of the column to

boiling boint. The following are examples of typical reboiler types.


34/58



Dam ietta LNG Plant

CHAPTER- 3

PRINCIPLES OF GAS PROCESSING OPERATION

Figure 13


35/58



Dam ietta LNG Plant

Overview

Natural gas liquids (or "NGLs), are hydrocarbons in a Liquid state. There are many reasonsfor removing NGLS from the gas stream, including:

1. The hydrocarbon components may be more valuable in their liquid state, either mixed

or separated into their different individual elements.

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

3. Heavier NGLs can separate from the gas stream in pipelines as temperatures drop,

thus restricting flow in the pipeline.

4. The NGLs may be used for re-injection on enhanced oil recovery projects. That is,

they will be injected into a formation to help "sweep" crude Oil through the reservoir

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 the natural gas

stream. The three most common are: Cryogenics, refrigeration and lean oil absorption.

1. Cryogenics

In the cryogenic process, a natural gas stream is cooled to extremely low temperatures to

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 heavier hydrocarbons are

then separated from the methane.

EXPANDER

COMPRESSORDEMETHANIZER

COMPRESSO

METHANE SEPARATO

NATURAL

GAS

HEAT

EXCHANGER

N. GAS

Cr o enics


36/58


37/58



Dam ietta LNG Plant

Chilling of the gas is the cornerstone of the cryogenic process. The gas stream can be chilled

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 obtain the lowest

temperatures, -100 F (-73 C) to -200 F (-129 C), pressure reduction with energy removal

is accomplished using an expander -compressor.

METHOD TEMPERATURE RANGETYPICAL PERCENT

RECOVERY

Refrigeration0 to 20 F

( - 18 to 29 C )

EthanePropane

Butanes

Heavier

2555

93

97

Pressure Reduction

( J.T Process )

- 50 to 70 F

( - 46 to 57 C )

Ethane

Propane

Butanes

Heavier

70

80

97

99

Expander Process- 125 to 150 F

( - 87 to 101 C )

Ethane

Propane

ButanesHeavier

80

96

99100

HIGH PRESSUREGAS

HIGH PRESSUREGAS

COMMONSHEET

LOW PRESSURE

GAS

LOW PRESSURE

GAS

COMPRESSOREXPANDER

Expander Process


38/58



Dam ietta LNG Plant

Cryogenics has relatively moderate energy requirements, and is considered the most efficient

method for removing NGLS from the gas stream. .

2. Refrigeration

Refrigeration is a separation process that has been in use for many years. The principle is to

chill the natural gas stream by passing the gas through a chiller. Temperatures in this process

range from 0F (-18 C) to -20 F (-29 C). Chilling causes the heavier hydrocarbons to

liquefy, and then these NGLS can be separated from the gas.

Refrigerant

The energy requirements of refrigeration are primarily for compressing the refrigerant and

driving the condenser. While the volume of heavier hydrocarbons recovered is less than with

lean oil absorption , the amount of equipment and energy used for recovery with refrigeration

is much less, making it a more economical method than lean oil absorption.

COMPRESSORCONDENSER

REFIRGERANT

VAPOR

SURGETANK

WARM NATURALGAS

REFIRGERANTLIQUID

CHILLER

COLD GAS ANDLI UIDS


39/58



Dam ietta LNG Plant

3. Lean Oil Absorption

In lean oil absorption, lean oil is flowed through a natural gas stream, absorbing the heavierhydrocarbons as it contacts them. The hydrocarbons are then recovered by distilling them out

of the now "rich" oil. Once the NGLs are distilled, the lean oil is recycled back through the

system.

High percentages (90-95%) of propane and heavier hydrocarbons (98-100%) can be

recovered 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 is gradually

fading from the scene as cryogenics takes its place.

LEAN OILGAS

CHILLER

NATURAL

GAS

LIQUIDS

NATURAL

GAS

METHAN

SEPARATOR

PUMP

STILL

RICH OIL

LEAN

OIL

ABSORBER

Lean Oil Absorption


40/58



Dam ietta LNG Plant

Fractionation

Flow


41/58



Dam ietta LNG Plant

4. Fractionation ( Deethanizer & Depropanizer & Debutanizer )

Fractionation is a. downstream treatment option for NGLs after they have been separated

from the gas stream. By definition, fractionation consists of separating two components in a

mixture of two or more components. For example, a depropanizer separates propane from a

stream that contains propane and one or more heavier hydrocarbons. The propane is cooked

out of the mixture and is the overhead product from the tower. The bottom stream is

practically free of propane.

Fractionation is essentially a distillation process. As you can see by the diagram on the left,

different NGLs are cooked out in various towers. There are usually two means of control.

ling the purity of the top and bottom streams of a given tower. vary the temperature at the top

of the tower, or vary the temperature at the bottom of the tower.

Generally speaking, pressure and feed rate cannot be changed, since this would change

conditions in the rest of the plant.

Whether to fractionate is a question of economics. Is there a profitable market for the

fractionated product? Because market conditions change rapidly and often, this question must

constantly be answered to determine if fractionation will be done. There are times when the

mixed NGLs will be more valuable than their separate components.


42/58



Dam ietta LNG Plant

CHAPTER 4

PRODUCT SPECIFICATIONS

Overview

This topic is aimed at all process operations staff wanting to expand their know about 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

4.1 Distillate and Bottoms Specifications

4.2 Control of Product Composition


43/58


44/58



Dam ietta LNG Plant

The thermal condition of the feed determines how much additional heat must be added to the

column by the reboiler. For efficient separation, it usually is desirable to ha\'e the feed at its

bubble point when it enter the column. If the feed composition varies, its bubble point alsovaries. It is common practice to set the temperature control at a point which is equivalent to

the bubble point of the heaviest feed,

Reboiler Control:

Since fixing the feed conditions and column pressure determines four of the variables listed

earlier, only two other. Variables remain to be controlled in order to fix the operation of the

distillation column. Frequently, the boil-up rateis chosen as one of

the two remaining independent variables. The boil-up rate is controlled by setting the flow of

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 last

independent variable to be controlled. The reflux furnishes the continuous supply of liquid 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 the accumulator.

Conclusion:

By controlling the six independent variables and assuming constant feed conditions the

separation efficiency of a distillation column can be controlled. The consistent and optimum

specification for distillate and bottoms products can hence be achieved.

4.2 Control of Product Purity & Full Filling ( demands ) :

A distillation unit operates between two extremes. In one case, insufficient separation isreflected in unacceptable product purity. In the other case, separation can be far in excess of

what is demanded so that utilities and unit capacity are wasted.

The goal of distillation then is to achieve specified product purity without causing waste. To

obtain this goal, some measure of product composition is needed.

Since distillation separates materials according to their difference in vapor pressure. and since

vapor pressure is a temperature controlled function, temperature measurement can be used to

indicate composition.


45/58


46/58



Dam ietta LNG Plant

Safety Equipment, Relief Devices:

Distillation columns requires safety systems in order to function the plant environmentwithout creating risk to plant personal and other process equipment. These safety 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

Reboile :high temperature alarm. high and low flow alarms (steam ) low flow

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:

a) Utility failure

- Loss of coolant

- Loss of electric power

- Loss of (hot oil)

- 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


47/58



Dam ietta LNG Plant

d) Internal sources

- Noncondensables- Chemical reaction

- Cosed column outlets

e) Transient sources

- Pockets of water flashing to steam

- Internal explosions

Methods of relieving overpressure which should be included in The column design are:

- 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

5.2 Standard Column Control:

Distillation column control schemes may be broken into two major types, material balance

systems and heat balance systems. Of the two, the material balance system or some variation

of it, is the most common,

Material Balance System:

In a material balance control system, product composition is controlled by controlling the

flow of material into and out of the column, the column pressure is controlled by the amount

of cooling in the overhead condenser while the distillate and bottoms product are both

controlled by level control. It is common practice to have both distillate and bottoms on levelcontrol, thus making accumulation in the column unlikely. The temperature of the bottoms is

controlled by the hot oil 0flow on temperature control while the reflux is on flow control.

This is referred to as indirect control as the tower top temperature is controlled by a flow

controller.

A variation of this is to use temperature control to manipulate the reflux. This is known as

direct control.


48/58



Dam ietta LNG Plant

Energy Balance System:

In energy balance control system, the energy balance controls the product composition whilethe free variable is one of the product flows. The shortcoming of this scheme is that

variations in the material balance interact with the control system. These controls are 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 satisfactory material balance system

cannot be implemented or in conjunction with an advanced computer control system.


49/58



Dam ietta LNG Plant

CHAPTER 6

DISTILLATION COLUMN OPERATION

Overview

This topic is aimed at all process operations staff wanting to expand their knowledge about

the operation of a distillation column.

Objectives

On completion of this topic, the Trainee will :

- Know the generic procedure of distillation column startup and shutdown

- Be knowledge in distillation column major operating variable adjustments

- Have some knowledge applicable to column energy conservation

Contents

5.1Startup / Shutdown

5.2Major Operating Variable Adjustments

5.3 Energy Conservation and Column Optimization

6.1 Startup / Shutdown:

Once a column is bolted up after initial construction or a shutdown, a number of operations

must occur to prepare the column for startup. These' steps, referred to as column

commissioning, are used to clear the system of undesirable materials, test the column and to

take preventative measures against performance deterioration.

Line Blowing:

Normally done with a utility service such as air and/or water. This facilitates the removal andcleaning of any pipe scale, rust, and loose slag from weld joints. Most blowing 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 must be

leak proof and can be checked by monitoring the column pressure [no drop in transmitter

signal] and also by "snooping" or checking for leaks in the field.


50/58



Dam ietta LNG Plant

Purging:

This is done using an inert gas utility such as nitrogen. This is extremely' important as do notuse air [especially on vacuum systems] on the column it cause explosive mixtures to occur.

The column should be kept under a nitrogen "pad" prior to startup as not to cause any

problems on startup.

Blinding and Unblinding:

Most columns under construction and during vessel entry may have piping blind flanges or

spades inserted as not to have any utility and/or process materials enter the equipment [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 the blinds and spades are connecting

equipment to piping. The removal may be done after blowing so as not to contaminant theequipment 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 the soapy water

solution. If any leaks occur, the solution will bubble vigorously.

Washing:

Some columns and equipment may need to be washed with water or a solvent prior to

operation 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 put through the

column itself. A "water run" can be done to help calibrate instruments and check for pump

seal problems, leaks, etc..

Steaming :

Some columns can be purged with steam to exclude air. This can be done especially in warm

climates ( no freezing problems). Also, steam makes a good leak check since it can "find"leaks better than any external testing.

Dryout:

Normally done with dry air or nitrogen utility. Can also be a solvent drying as well depending

on the application.

Once the column has been properly commissioned, the system is ready for startup. The

sequence of steps used for startup is listed below although deviations are often required for


51/58



Dam ietta LNG Plant

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 vacuum

pumps/ejectors [if vacuum column] or by allowing the pressure control system to

respond to column liquid boil up.

3) Column heating and cooling.

The condenser cooling medium should be on and functional. The reboiler / heatingsystem should be ready to run once the liquid feed is introduced and a column bottoms

level 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

started slowly and carefully due to the thermal stress created in the equipment during

the heatup.

6) Bringing column to desired operating rates.

Depending on desired operating set points, the feed should be put on a minimum

setting until the entire column is up to temperature, pressure, reflux ratio [if the

distillate/forwards can be recycled], If the forwards can not be recycled, then a total

reflux 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 the bottoms 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 reduce

column feed -rates, This will gradually reduce heat and cooling loads as well as

forwards and bottoms rates while still on spec.. The column may be put on total reflux

so as to keep the reboiler running on a liquid inventory [i.e. no . bottoms being


52/58



Dam ietta LNG Plant

pumped forward either].

2) Shutting Down Heating And Cooling Sources.

The feed can be stopped per step 3) below first if the heating and cooling sources are

ready to be shutdowl1. The heating source should be shutoff first as to help cool the

column with any remaining reflux.

3) Stopping Feed.

The feed can be stopped sooner than the heating source shutdown if level rise in the

column becomes too high [alarm point]. This will help minimize the pump-out of

excess 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

pumped out 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

further cooled 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

the column to atmospheric pressure.

7) Eliminating Undesirable Materials.

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

vessel work.

8) Prepare the Column for Opening to Atmosphere.

Hopefully the column has been purged correctly and all piping drained/blown to a

vapor/liquid recovery system.

Again, these steps may vary with each specific column or specific company procedure.


53/58



Dam ietta LNG Plant

Operations staff should be aware of any specific hazards or potential problems with the

specific column prior to startup/shutdown. Prior to startup/shutdown, there are several other

items that should be considered:

1) Prepare adequate procedures for operating, startup, shutdown and maintenance.

2) Ensure the startup/shutdown team consists of personnel who have all the required skills 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 and audits,

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 and ensure

these are well understood

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

6.2 Operation of Distillation Columns:

There are several fundamental process variables in a distillation column which must be

controlled in order to get proper separation of components:

Overhead Temperature:

The overhead temperature is determined by the composition of the liquid on the first tray, this

liquid must be at its' boiling point at the pressure of the column. The composition is

controlled by the reflux ratio. The higher the reflux ratio, the higher the concentration of thelighter 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 are limits to this which are

dependent on the condenser temperature and the phase characteristics of the material in the

column.

Temperature Profile:

The temperature profile of a column is a plot of the temperature on each tray from the top of

the column to the bottom. The lowest temperature should be at the top of the column,


54/58



Dam ietta LNG Plant

corresponding to the lightest components while the highest temperature should be at the

bottom, corresponding to the heaviest component. The profile between the top and bottom of

the column should be a smooth curve with no flat spots. This indicates that the composition isdifferent on each tray and efficient separation is occurring. If there is a section of the column

in which the temperature profile is flat, the composition on those trays is very close and

minimal separation is taking place. The exception to this is often the feed area where a very

cold (subcooled) or very hot (superheated) feed can cause to temperature profile to flatten.

Column Pressure:

The operating pressure in the column is determined 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 to

condense the lightest component in the overhead vapor to liquid. The pressure at which thisoccurs determines the operating pressure of the column. If the lightest component is not

condensed. The vapor in the overhead receiver will cause the system pressure to increase

until the temperature in the condenser is low enough to condense the light component. If this

results in a pressure which is too high for the 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 for a

distillation column. The definition of -the reflux ratio is the ratio of liquid returned to thecolumn divided by the distillate flow, The higher the value, the more liquid that is returned 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 tray to tray. As the

reflux ratio is so important, the column is designed for a specific value known as the

optimum reflux ratio. This value is different for each application. Easy separations may

required a reflux ratio as low as 0.5 while difficult separations may require as high as 8 or 9.

Once a column is running, the reflux ratio is fine tuned for the specific operating conditions

of the plant.

Bottoms Temperature:

The temperature at the bottom of the column determines the composition of the bottom

product, 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 of vapor will become

too great for the column and jet flooding will result. As the bottoms temperature is increased,

the reflux ratio may required adjustment to maintain the same overhead temperature. The

bottoms temp is usually controlled by the amount of heating medium, such as steam of hot

oil, flowing through the reboiler.


55/58


56/58



Dam ietta LNG Plant

7.1 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 spray from,

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 in entrained into the tray above. The

liquid accumulates on the tray above instead of flowing to the tray below.

Froth Entrainment flooding- At higher liquid rates, the dispersion of the tray is in the form

of a froth. When vapor flow rate is raised, froth height increases. When tray spacing is small,the froth envelope approaches the tray above. As this surface approaches the tray above,

entrainment rapidly increases, causing liquid accumulation above. For large tray spacing [18 -

24 inches], the froth envelope seldom approaches the tray above. Given enough vapor

velocity, the froth would turn into spray and cause spray entrainment flooding per above.

Downcomer Backup Flooding- Aerated liquid is backed up into the downcomer because of

tray pressure drop, liquid height on the tray, and frictional losses in the downcomer 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, friction losses inthe downcomer and downcomer entrance become excessive, and the frothy mixture cannot be

transported to the tray below. This causes liquid accumulation on the tray above.

Generally, at low tray spacing (less than 12 - 15 inches), froth entrainment flooding is

favored. At higher tray spacing, and when conditions do not favor vapor cross flow, the froth

regime will turn into a spray as vapor velocity increases, and spray entrainment flooding is

favored. Finally, when downcomers are small or downcomer backups are high, downcomer

flooding is favored.

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]


57/58



Dam ietta LNG Plant

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

7.2 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 trays will get damaged if vapor slugs are allowed to "bubble"

through the column. Reliable bottoms level indication is essential to safe.

column operation. .

Some techniques for preventing tray damage due to excessive liquid levels:

1.) Pump out liquid vs boil it out.

2.) Construct the bottom seal pan to be strong.

3.) Construct the bottom 25 percent of trays for extra mechanical strength.

4.) Provide a liquid level differential pressure measurement for the bottom

5.) Provide facilities for easy diversion of bottom liquid to either the feed tank or storage

tank so that liquid level can be readily reduced.

6.) Ensure smooth and stable automatic control of boil up to the tower.

Liquid Level in Reflux Accumulators:

An overflowing accumulator will usually backup liquid into the condenser, flooding some

tubes. Often, column pressure will rise and the relief valve may possibly lift

A low accumulator level can cause pump damage due to lack of liquid [cavitation].

Proper level indication with interlocks to pump operation or column operation should be

available.

Sources and Effects of Water Problems:

The main adverse effects of water in distillation column service are pressure surges, flooding,

cycling, corrosion, hydrates, and off-spec products.

Typical sources of water are:


58/58


Dam ietta LNG Plant

1.) The feed stream. Water may be found in the feed storage tank or from a leaking

heat exchanger.

2.) Undrained water in a stripping steam line. "Dry" steam is required prior to column

introduction.

3.) Chemical reaction. A condensation reaction forming water may occur between

organic chemicals

4.) A leaking heat exchanger [reboiler, condenser].

5.) A pre startup wash. leak -test, pr steam-water operation.

6 ) Condensate formed in previous operations. It could have remained trapped in pipelines

or pockets inside the equipment.

7 ) A water-containing stream that found its way to the column [a typical example is water

discharged from the desalter safety valve and ends up in a refinery crude column] .

Leaking Heat Exchangers:

Tube leaks may occur in the reboiler, condenser, pre heater, precooler. or any other heat

exchanger linked with the distillation column.

Effects of heat exchanger tube leaks into the column include off-spec products and/or

undesirable chemical reactions. In some cases, this reaction may lead to rapid corrosion or

plugging. It is important to realize that leaked material may cause vapor slugs and tray

damage if conditions permit.

Leaks can be detected by heat exchange equipment sampling and/or off-line drain/vent

checks. An leaking tube on an" exchanger will show the heating material on the other side of

the heat exchanger.

7 - distillation0.pdf

Documents