Distillation Control Philosophy

8/20/2019 Distillation Control Philosophy

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Distillation Control Philosophy

Some of the general guidelines are noted below:

column pressure normally controlled at a constant value

feed flow rate often set by the level controller on a preceding column

feed flow rate is independently controlled if fed from storage tank or surge tank

feed temperature controlled by a feed preheater. Prior to preheater, feed may be heated by bottom product via feed/bottom e changer

top temperature usually controlled by varying the reflu

bottom temperature controlled by varying the steam to reboiler

differential pressure control used in packed columns to monitor packing condition, also usedin tray columns to indicate foaming

the compositions controlled by regulating the reflu flow and boiled!up "reboiler vapour#

Pressure is often considered the prime distillation control variable, as it affects temperature,condensation, vapourisation, compositions, volatilities and almost any process that takes placeinside the column. Column pressure control is fre$uently integrated with the condenser controlsystem .

%eboilers and condensers are integral part of a distillation system. &hey regulate the energyinflow and outflow in a distillation column.

&he ' main methods of pressure and condensation control are:

"(# vapour flow variation,")# flooded condenser, and"'# cooling medium flow variation.

*apour +low *ariation

&he simplest and direct method for column producing a vapour product. &he pressure controller

regulates the vapour inventory and therefore the column pressure. See the +igure below.

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n important consideration here is the proper piping of the vapour line to avoid li$uid pockets.

+looded Condenser

&his method is used with total condensers generating li$uid product. Part of the condensersurface is flooded with li$uid at all times. &he flow of condensate from the condenser iscontrolled by varying the flooded area. -ncreasing the flooded area "by reducing flow# increasesthe column pressure "less surface area for condensation#.

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Cooling 1edium +low *ariation

Pressure can also be controlled by ad2usting the flow of coolant to the condenser " see +igure below#. 3peration using cooling water can cause fouling problems at low flow condition, whencooling water velocity is low and outlet temperature is high.

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+or air!fin condensers, the controller varies the fan speed or fan pitch to control pressure "see+igure below#. &his arrangement is energy!efficient as it minimises fan power consumption, butre$uires the use of variable!pitch fan or variable speed motor.

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3ther method: pressure control using inerts "see +igure below#.

4hen column pressure falls, an inert gas is admitted to raise the column pressure.

3r: split!range pressure control venting e cess gas to flare "see +igure below#

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-n most instances, both vapour and li$uid phase are present in the column overhead. &he vapourcontains components that can condense out but are undesirable in the li$uid, i.e. e cessivecondensation may lead to off!specification li$uid product. -n addition, it is also undesirable tolose li$uid product "through insufficient condensation# to the vapour. -t is therefore important tocontrol the rate of condensation to obtain the desired vapour!li$uid split.

&his is usually done by controlling the temperature of the li$uid product 2ust downstream of thecondenser. 3ne common scheme used is shown the +igure below.

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column is controlled by regulating its material and energy balances. Click here for moreinformation and an e ample .

&he following controls are briefly discussed in this Section:

eboiler Control

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&his is re$uired to provide good response to column disturbances, and to protect the columnfrom disturbances occurring in the heating medium. &he reboiler boil!up is regulated either: "(#to achieve desired product purity, or ")# to maintain a constant boil!up rate.

-n a typical reboiler control "see +igure below#, the control valve is located in the reboiler steam

inlet line.

+or inlet steam controlled reboiler, the heat transfer rate is regulated by varying the steam controlvalve opening, thereby changing the steam condensing pressure and temperature.

4hen an additional boil!up is re$uired, the valve opens and raises the reboiler pressure, whichincrease the temperature, and in turn increases the boil!up rate. &his scheme has the disadvantageof non!linear relationship between pressure and boil!up, and is affected by fouling in the reboiler.

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n alternative is to control the condensate flow, i.e. by putting the control valve on thecondensate line " see +igure below#. &he main disadvantage is that this scheme has poorerdynamic response than the previous scheme. 1anipulating the inlet valve immediately changesthe vapour flow, giving faster dynamic response. 3n the other hand, the condensate outlet valvehas no direct effect on vapour flow. &he response time varies with the condensate level in thee changer.

&he other main disadvantage is the si5ing of the condensate valve. -f condensate cannot bedrained in time, vapour flow may be restricted as much of the reboiler remains flooded. 3n the

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other hand, too fast of condensate draining "faster than vapour condensation in the reboiler# asresult in loss of li$uid seal in the reboiler and steam will pass into the plant6s condensate recoverysystem.

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Some reboiler control features the use of condensate pot. &his is particularly important in foulingor corrosive services "where leakage is a serious problem#. n e ample is shown in the +igure

below.

-n the system shown, by varying the level control set point, the tube surface area in the reboilerthat is e posed for vapour condensation can be ad2usted, thus changing the available heat transfer area. &he heat transfer rate can therefore be ad2usted.

&his arrangement also automatically minimise the condensation "and therefore, tube wall#temperature. pressure!balancing line is provided to maintain a steady pressure and level in thecondensate pot.

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&emperature Control

Column temperature control is perhaps the most popular way of controlling productcompositions. -n this case, the control temperature is used as a substitute to product compositionanalysis.

-deally, both top and bottom compositions should be controlled to maintain each within itsspecifications. See the +igure below.

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-n practice, simultaneous composition control of both products suffer from serious 7coupling7"interaction# between the ) controllers, resulting in column instability. -n the system shown,suppose that there are concentration changes in the feed conditions that result in lower columntemperature. &he top and bottom temperature controllers will respond by decreasing reflu andincreasing boil!up respectively.

-f the actions of the ) controllers are perfectly matched, and response is instantaneous, bothcontrol temperatures will return to their set points without interaction.

8owever, the ) actions are rarely perfectly matched, and their dynamics are dissimilar ! usuallythe boil!up response is faster. &he reflu and boil!up will 7cycle7 as shown in the +igure above.

&he interaction can be avoided by controlling only ( of the ) product compositions.

3n!line analyser can be used together with temperature control to control product composition.&he principal control action is rapidly performed by the temperature controller, while theanalyser slowly ad2usts the temperature set point to prevent off!specification product purity. setup is shown in the +igure below.

-n the above set!up, delayed analyser response is acceptable, as its time lags become a secondaryconsideration. &he fast temperature controller action renders this control method less sensitive toupsets and step changes in an analyser!only control system.

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nother advantage is that, should the analyser become inoperative, the temperature controllerwill maintain automatic control of the process.

+eed Preheat Control

+eed preheat is usually practised for heat recovery or to attain the desired vapour and li$uidtraffic above and below the feed tray. &he ob2ective of the preheat control system is to supply thecolumn with a feed of consistent specific enthalpy. 4ith a single!phase feed, this becomes aconstant feed temperature control9 with a partially vapourised feed, a constant fractionalvapourisation is re$uired.

s an e ample, consider case "a# as shown in the +igure below whereby the feed is a cold li$uid.-n this case, all the li$uid feed will go to the stripping section. -n addition, because the feed iscold, it will also condense some of the rising vapour.

s a result, the amount of li$uid flow in the stripping section is much larger than the li$uid flowin the rectifying section. &he vapour flow in the rectifying section is lower than the vapour flow

in the stripping section because of the condensation into the li$uid. ack on &op 0

&he following +igures showed ) other feed conditions: case "b# for saturated li$uid " left # andcase "c# for vapour!li$uid mi ture " right #:

nd the following +igures showed ) other feed conditions: case "d# for saturated vapour " left #and case "e# for superheated vapour " right #:

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Subcooled feed or superheated feed can be controlled "see +igure below# by preheating "left# ordesuperheating "right# the feed prior to column entry:

superheated bottom feed can be cooled by in2ecting a $uench stream as shown in the +igure

below.

istillation Control ! 1aterial ;nergy alance

distillation column is controlled by regulating its material balance and the energy balance.

-n essence, a material balance means that the sum of the products leaving the column must bee$ual "appro imately# to the feed entering the column9 and an energy balance means that the heatinput to the column must e$ual "appro imately# to heat removed from the system.

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4hen a column is in material and energy balance, there is no accumulation or generation ofmaterial or heat within the column, i.e. the column is 7stable7.

&he control system is dynamic, i.e. if a process variable changes, the control system reacts byad2usting the affected process variables until the system returns to normal condition.

&he term 7steady state operation7 describes the condition in a column when the process variablesare changing in small amounts within prescribed limits.

4hen a column is in steady!state operation, the changes to the column6s material balance andenergy balance variables are minimal and are handled by the control system. s mentioned in the

start of this chapter, one of the ob2ectives of control is to maintain the products within there$uired specifications, or simply 7specs7. 7spec7 is a value, or a range of values, for a physical

property or a set of physical properties that is re$uired for a product or products. sample of typical properties of interest in petroleum refining is shown in ppendi . %eturn to

ppendi (: ; amples of &ypical Petroluem Cut or +raction Properties


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-n the >.S. an arbitrary scale known as the P- degree is used for reporting the gravity of a petroleum product. &he degree P- is related to the specific gravity scale "(?oC / (?oC# by theformula:

*iscosity

&he dynamic viscosity of a li$uid is a measure of its resistance to flow. &he kinematic viscosityis e$ual to the dynamic viscosity divided by the density of the li$uid.

Cloud Point

&he temperature at which a fuel, when cooled, begins to congeal and present a cloudyappearance owing to the formation of minute crystals of wa .

+lash Point

&he lowest temperature under closely specified conditions at which a combustible material willgive off sufficient vapour to form an inflammable mi ture with air in a standardised vessel. +lash

point tests are used to assess the volatilities of petroleum products.

+ree5ing Point

&he temperature at which crystals first appear when a li$uid is cooled under specified conditions.-t is an important characteristic of aviation fuels.

Pour Point

&he temperature below which an oil tends to solidify and will no longer flow freely.

%eid *apour Pressure "%*P#

&he pressure caused by the vapourised part of a li$uid and the enclosed air and water vapour, asmeasured under standardised conditions in standardised apparatus: the result is given in psi at(@@ o+, although normally reported simply as 7%*P in lb7. %*P is not the same as the truevapour pressure of the li$uid, but gives some indication of the volatility of a li$uid, e.g. gasoline.

3ctane


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&he octane number of a fuel is a number e$ual to the percentage by volume of iso!octane in ami ture of iso!octane and normal heptane having the same resistance to detonation as the fuelunder consideration in a special test engine. -t is a measure of the 7anti!knock7 value of agasoline and the higher the octane number the higher the anti!knock $uality of the gasoline.

"7 nti!knock7 is an ad2ective signifying the resistance to detonation "pinking# in spark!ignitedinternal combustion engines#.

Smoke Point

&he ma imum height of flame measured in millimeters "mm# at which a kerosene will burnwithout smoking when tested in a standard lamp for this purpose.

Product specifications are set by the demands of downstream processes and by the marketplace.Products must meet certain $uality standards. +or a product to be saleable, it must comply withcertain pre!determined $uality.

Products are routinely tested to ensure that the specifications are met. &esting can be done bydirect composition measurement or by indirect measurement, according to prescribed standards,such as S&1.

Direct composition measurements are analysis that allow personnel to directly observe the percentages of components in a product. n e ample is the process chromatograph. -t provides adirect read!out of the component percentages. &he readings of the chromatograph can becompared against the specifications to see if any ad2ustments are needed to ensure that the

product meets specifications.

-ndirect composition measurements are analysis in which one measured property is used as anindicator of another property. 3ne common indirect measurement is the boiling temperature. +ore ample, since the boiling points of the components in a feed mi ture are known, thecomponents in a product can be indirectly identified by their boiling points. 4hen the product istested, its composition can be indirectly measured by recording the temperatures at which thedifferent components in the product boil.

-f the composition of a product is outside of the normal limits for that product, the product isreferred to as 7off!specification7.


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; ceeding product specifications or producing better $uality product than is re$uired is known as product giveaway.

%eturn to

n ; ample of Distillation Column Control

typical distillation column has a combination of different control loops . &he control system ofa particular column is designed to meet that column6s particular process re$uirements. ne ample is shown in the +igure below.

&here are several control loops associated with the distillation column:

&emperature:

(. 3verhead condensation "+in!fan#). 3verhead column "%eflu #

'. +eed preheatA. Column bottom "%eboiler steam#

Pressure:

(. 3verhead accumulator "3ff gas#

Bevel:

(. 3verhead accumulator "Distillate product#). Column bottom " ottoms product#

+low:

(. Column feed

-n this distillation column, the material balance "1 # loops consisted of the following:

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feed flow control loop "which sets the throughput, i.e. production rate# bottom level control loop "which controls the column level#accumulator level control loop "which regulates the product flow by regulating the overhead

accumulator level#

off gas pressure control loop "which controls the column pressure#

&he energy balance "; # control loops are the following:

reboiler temperature control loop "which control the column bottom temperature bycontrolling the steam input to the reboiler#

feed preheater temperature control loop "which controls the feed inlet temperature#overhead condenser temperature control loop "which regulates amount of cooling in the

column#e ternal reflu temperature control loop "which controls the temperature at the top of the

column#

-n this e ample, the main influence on the heat input to the column is the steam flow to thereboiler. 8eat also enters the system via the preheater. 8eat balance is achieved when the heatinput from the reboiler and preheater is removed by the condenser.

"


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controller responds by increasing the outflow of top product. &his increased outflow of materialsfrom the top will offset the decreased in outflow from the bottom, hence the 1 is restored.

Concentrations of the top and bottom product streams are affected as well ! higher bottomtemperature will results in more heavy components being vapourised from the bottoms product.

&his can be illustrated using a multi!component separation of products: C(, C), C', CA, C?,C , CE and C F. &he main separation is between ) key components: the light key "CA# andheavy key "C?#. &his is shown in the +igure below.

-f the bottom temperature is too high, more of the heavy key "8G# will be vapourised from the bottom product. &he vapour thus had become heavier due to the presence of the 8G. &he final boiling point "+ P# of the top product will be higher but the initial boiling point "- P# did notchange.

3n the other hand, the - P of the bottoms product will be higher, because the bottoms producthas been depleted of the 8G and become heavier. &he + P of the bottoms product is not affected

by the bottom temperature increase.

3ther possible disturbances

&his e ample illustrated 2ust one of the many disturbances that can upset the smooth operation of a distillation column. esides the reboiler e ample, which could be due to controllermalfunctioning, other disturbances can also occur. &he following list is not e haustive, but onlyserves as a reference of what possible events that can disrupt the smooth operation of a plant.

reboiler and other heat e changers: fouling of heat transfer surfaces, tube leaks, etc

charge heater: loss of fuel gas and/or fuel oil "e.g. due to low fuel gas pressure trip#overhead condenser: loss of cooling water or loss of power supply "for air!fin coolers#

pumps: overload trip, loss of power, cavitation, etc

control valves failure: e.g. loss of instrument air, 2ammed valve, faulty positioners, etc.

faulty instruments: wrong signals transmitted, false alarms, etc.

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feed changes: feed rate, more lower boiling components, contaminations, etc.

tower internals: e.g. flooding, weeping, channelling, etc.

&hough distillation column is designed and installed as per some sophisticated mathematicalcalculations and computer software model it not so easy while operating it in the field. 3perationof distillation column ultimately works by the hands of the distillation operator. ;ven tons of

books e plain how to design the distillation column and at last the erected column works basedon the operators and engineers in the field. ased on the e perience and knowledge of distillationoperation of senior personals could help but in course of time due to the shift of manpower theknowledge may shatter for particular operation and control of distillation column.

&he following are some of ma2or problems arise during running distillation column in a chemical plant.

• -nstrument devices and control systems problems

• column internal valves and packing damages

• Startup and shutdown operation problems

• %e!boiler and condenser fouling and inefficiency

• +oaming, entarinment, flooding problems

• Column tray, weir and down comer layout


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>p on main failure cases the column operation is disturbed or ad2usted based on startup and shutdown operation practices some of the hint for start up and shut down of distillation would comeas given below which are established by e perience.

8ow to start a distillation column:

• fter commissioning the column it pressure test to find out the leak at fitting and 2oints.

• &han the column is flushed with nitrogen the operation is called as line blowing.

• &he column is purged out to remove the o ygen content and then cooling system isstarted.

• ll vent and drains are kept in loop control systems.

• %eboilers is filled with feed mi ture till the level glass mark, heating media such as steamis introduced to heat the mi ture and produce the vapors till they reach the condenser.

• %eflu valve is opened to maintain the top temperature and pressure drop in column.+eedis introduced proportional to the vapors produced in the column.

• +inally column is operated at constant reflu till the pressure and temperature indicator show the designed process values.

• +eed flow rate is increased to the designed value and all controls valves are switchedfrom manual mode to auto mode.

8ow to shut down a distillation column:

• +low rates of the feed is reduced with controlling the reflu flow rate proportionally.

• 8eating system is stopped first following cooling system which is working until vapor produced in the bottom of the column.

• &han feed to stop with reflu valve open which will make the entire vapor to condenseand collected in the reflu drum and bottom of distillation column.

• ll the li$uid is than drained out from the column and reflu drum ring the column toroom temperature by flush out with inert nitrogen.

• +lush out the off gas valve to remove the remaining vapor mi tures to the flare +inallythe column should be check for o ygen level and opened to atmosphere for maintenance.


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Distillation Control Philosophy

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