2.2 Atmospheric distillation of crude oil...A crude-oil atmospheric-distillation (or topping) plant makes it possible to obtain distillates (made up of the overhead product and the

The purpose of atmospheric distillation, which iscarried out at slightly above atmospheric pressure, isto separate (fractionate) the feedstock (crude oil) intodifferent products with characteristics aimed atsatisfying the demands of the market for motor fuels(gasoline, kerosene and diesel), combustible fuels(LPG, kerosene, heating gas oil and fuel oil) andfeedstocks for the petrochemical industry. Theproducts obtained can be used as they are or, in themajority of instances, must be subjected to furtherprocessing in a refinery (isomerization, catalyticreforming, desulphuration, vacuum distillation, etc.) inorder to make them usable as finished products.

In addition to hydrogen and carbon, hetero-atomssuch as sulphur, nitrogen and oxygen may be presentin the molecules making up the crude oil, along withorganometallic compounds containing iron, vanadium,nickel, sodium, arsenic, etc. Moreover, even thoughthe crude oil may be treated at the wellhead todehydrate and stabilize it, part of the reservoir waterstill remains in it; similarly, when transported by shipthe crude oil can be ‘polluted’ by the residual waterremaining in the tanks. This is generally watercontaining salts (chlorides) which can cause problemsof corrosion in the subsequent operations of heatingand condensation. Finally, the crude oil may containdissolved gases (hydrogen sulphide, nitrogen, carbondioxide) in different percentages depending on thetype of crude oil and the effectiveness of thestabilization treatment carried out, as well as on themethod adopted for transportation (via pipeline orship) from the production site to the place of use.Therefore, in addition to the distillation-basedfractionating operations, special treatment processesmust be carried out: for example, desalination iscarried out when it is necessary to remove water, saltsand sediment present in the crude oils. The dissolvedgases, such as hydrogen sulphide, nitrogen and carbon

dioxide, are separated by heating, while other gases(hydrogen chloride and hydrogen sulphide), which canbe produced as a result of the heating anddecomposition, must be neutralized in thecondensation phase by the addition of specialadditives, in order to avoid problems of corrosion.

A crude-oil atmospheric-distillation (or topping)plant makes it possible to obtain distillates (made up ofthe overhead product and the side fractions) and theresidue, by the physical separation of a mixture ofhomologous components. This separation, which makesuse of the differing distributions of the componentsbetween the vapour and the liquid phases, takes place instages operating in conditions close to equilibrium. Atypical scheme for such a plant can be found in Fig. 1.

The separation of the various fractions of thedistillate is achieved by fractional condensation of thevapours of the distillate, which is an operationrequiring heat removal. In the case of a distillationcolumn (or still) this heat removal is carried out bymeans of a series of refluxes: external reflux,consisting of part of the condensed overhead product,and intermediate refluxes, consisting of liquidwithdrawn from the column and, after cooling,returned to it at a point above that from which it waswithdrawn. Intermediate refluxes are commonly calledcirculating refluxes or pumparound.

The feed, coming from the storage tanks, ispumped to the heater, having been preheated with heatrecovered, by means of a heat exchanger, from theoverhead vapours, side fractions, intermediate refluxesand the atmospheric residue. After having been heatedin the heater to the temperature required for theoperating conditions, the feed is transferred to theflash zone of the atmospheric column by means of atransfer line, where the separation takes place into thevaporized fractions (equivalent to the total of thedistillates) and the liquid residue.

89VOLUME II / REFINING AND PETROCHEMICALS

2.2

Atmospheric distillationof crude oil

Separation of the feed into the side fractions andthe overhead product is carried out in the sections ofthe column between the flash zone and the column’soverhead, by partial cooling and condensation. Theside fractions and the bottom products are extractedfrom the column in the liquid phase while the topproducts are extracted in vapour phase andsubsequently cooled, condensed and, in part, recycledto the top of the column as an external reflux (seebelow).

The side fractions are sent to the side strippingcolumn (stripper), where the light components areremoved by the injection of steam, in order to improvetheir flammability characteristics; the lightest parts arereturned to the column. After being cooled by heatexchange with the feed and by means of availablecoolers, the side fractions are sent to storage or areused as the feedstock for downstream units. Thecirculating intermediate refluxes return to the columnafter having been cooled by heat exchange with thefeed and by available chillers. The atmospheric residueis sent to storage, after heat exchange with the feed

and on final cooling with available coolers, or else isused directly as feedstock for the vacuum distillationplant.

In stationary conditions, the enthalpy balance of anatmospheric distillation column reflects the parity ofthe inlet enthalpy with the outlet enthalpy. The inletenthalpy is given by the sum of the enthalpy of theincoming feed and the enthalpy of the steam sent tothe strippers. The outlet enthalpy is obtained by addingto the enthalpy of the distillates and of the bottomresidue, the heat removed in the heat exchangers thatare deployed in the circuit of the external reflux andcirculating refluxes.

The vapours exiting from the top of the column arecooled and condensed in different ways depending onthe number of overhead accumulators provided; a partof them constitutes the overhead reflux and a part is sentto the stabilization unit (Fig. 2). If there are twoaccumulators, there is an initial condensation and theliquid phase obtained is used as a hot reflux and sent tothe top of the column, at an intermediate temperaturebetween that at which it came out of the column and

90 ENCYCLOPAEDIA OF HYDROCARBONS

DISTILLATION PROCESSES

lightnaphtha

LPG

fuel gas

heavynaphtha

water

water

steam

causticsolution

causticsolution

oilywater

demulsifier

crudeoil

kerosene

light gas oil

heavy gas oil

corrosion inhibitor

neutralization additive

C-1C-2

D-2

D-1

D-3

C-3

C-4

C-5

C-6

H-1

atmospheric residue

Fig. 1. Simplified layout of a crude oil atmospheric distillation unit. D-1, desalter; C-1, principal column; C-2-C-5,stripping columns; C-6, stabilization unit; D-2 and D-3, reflux accumulators; H-1, heater.

that maintained in the second accumulator. The residualvapours coming out of the first accumulator aresubsequently condensed to obtain the top product andwater. In the second accumulator condensation may betotal or partial depending on the temperature andpressure conditions adopted and the nature of the crudeoil being used. In fact, in the case of crude oils withparticularly high contents of light hydrocarbons and inthe presence of incondensables, total condensation mayrequire temperatures which are too low to be achievedwith the available cooling systems. Alternatively, thepressure in the accumulator would have to be increasedwith a consequent increase in the pressure profile of theentire column and the need to increase the outlettemperature level of the heater in order to obtain therequired quantity of distillates. When a singleaccumulator is used, the vapours at the top of thecolumn must be condensed in order to achievecondensation of the reflux, the top distillate and thewater. The liquid hydrocarbon phase is in part used as acold reflux, while the remaining part is sent asfeedstock to the stabilization and splitter columns forfractionating into combustible gas, LPG, and light and

heavy naphthas. The temperature in the finalaccumulator is generally between 40 and 50°C, to becompatible with the available cooling systems (coolingwater or air).

Even in the case of a single accumulator it ispossible to have partial or total condensation of the topproduct, depending on the operating conditions of thetop product separator itself. The vapour phase, madeup predominantly of light hydrocarbons (C1-C4) alongwith any incondensable gases such as nitrogen,hydrogen sulphide, etc., is sent to the blow downheader and subsequently to the flare or, if required, isburnt as fuel or, alternatively, recovered bycompression. The condensed water is generally sentfor treatment to remove the polluting elements, inorder to allow its reuse.

2.2.1 Desalination

Desalination of the crude oil is a process whosepurpose is to eliminate the salts normally present inthe water phase which is generally present within it.


ATMOSPHERIC DISTILLATION OF CRUDE OIL

lightunstabilized

naphtha

fuel gas

heavynaphtha

water

water

steam

causticsolution

causticsolution

oilywater

demulsifier

crudeoil

kerosene

light gas oil

heavy gas oil

corrosion inhibitor

neutralization additive

C-1C-2

D-2

D-1

D-3

D-4

C-3

C-4

C-5

H-1

atmospheric residue

Fig. 2. Simplified layout of a topping unit with insertion of a pre-flash (downstream of the heater) and a double accumulator in the overhead of the principal column. D-1, desalter; C-1, principal column; C-2-C-5, strippingcolumns; D-2, total reflux accumulator; D-3, final cooling accumulator; D-4, pre-flash vessel; H-1, heater.

The salt contents of certain crudes, expressed in termsof NaCl equivalents, are summarised in Table 1. Thesalts typically contained in crude are sodium,magnesium and calcium chlorides. The need for thisprocessing is due to the fact that the salts, if notremoved, could cause deposits to form within theequipment (for example, on the surfaces of theexchangers, reducing the efficiency of the heatexchange) and the forming of hydrogen chloride (HCl)by hydrolysis of MgCl2 and CaCl2, leading tocorrosion, especially in the section containing theoverhead product condensers of the atmosphericdistillation column. Moreover, desalination treatment,which is performed generally in two stages (Fig. 3) tominimize the residual salt and/or solid content, isadvisable in order to avoid problems in thedownstream plant, in the event that the residue(atmospheric or vacuum) is to be sent for thermal orcatalytic conversion processing.

The water and salt content in a crude is dependenton the characteristics of the reservoir and on thetreatment which the crude itself has undergone at thewellhead (normally involving dehydration processeswhich reduce the water content to about 0.5-2% by

volume). Water can also come from transport by ship,if ballast H2O remains in the compartments used fortransporting the crude oil. The water is present in theform of dispersed particles with varying dimensions ofup to 1 µm in diameter. The characteristics of the waterand of the crude oil (density, surface tension, chemicalcomponents) and the method of production determinethe sizes of the particles and the stability of theemulsion, whose formation is assisted among otherthings, by the presence of natural emulsifying agentsin the crude, such as asphaltenes, resins, paraffins,organic acids and particles of inorganic solids.

First of all, the desalination treatment requires theaddition of water to the crude oil in order to form anemulsion of H2O droplets dispersed in the oil. Thewater dissolves the salts contained in the crude, hencethe greater the water dosage, the lower is the saltcontent of the output. Nevertheless, an excess ofwashing water can lead to crude being drawn into thewater and vice versa. A typical amount is 5-8% of H2Oby volume with respect to the input crude oil.

The formation of the water/oil emulsion is assistedby the presence of a mixing valve which, by means of apressure drop normally between 0.5 and 3 bar, ensuresa close oil/water contact. This pressure drop caused bythe mixing valve is the primary element in the correctoperation of the desalter. The ideal value of thispressure drop is dependent on a number of factors,such as the type of crude oil, the operating temperatureof the desalter, the oil/water mixing velocity, thepresence of emulsifying agents in the crude and thequality of the washing water. Therefore, it is onlythrough analysis of the actual operation of the desalterthat the correct pressure drop can be determined.Nevertheless, it is important not to employ pressuredrops in the mixing valve that are too high, since, whenusing excessive levels the removal of salt is drasticallyreduced due to the formation of a stable emulsion



Table 1. Salt content of some crude oils, expressed in terms of NaCl equivalent

Crude oil Source Salt content(mg/l)

Marlim Brazil 100

Cerro Negro Venezuela 655

Iranian Heavy Iran 102

Rospo Italy 388

Ural Russia 72

Athabasca Bitumen Canada 15-45

desaltedcrude

electricpower

electricpower

mixingvalve

mixingvalve

inletcrude

mixer mixer

dilutionwater

salt water recycle

Fig. 3. Operationaldiagram for a two stagedesalter:dLC, differentiallevel controller;PC, pressurecontroller.

which hinders separation of the aqueous phase. Theoperating pressure of the desalter must be maintainedabout 2-3 bar above the pressure corresponding to theboiling temperature of the crude oil at values such as toavoid the formation of vapours which could prevent thedesalter from operating properly.

Next it is necessary to break the emulsion formedin this way by sedimentation of the heavierdiscontinuous phase (the droplets of water). Inconditions of free fall and in the absence ofturbulence, the velocity of sedimentation followsStokes’ law; hence the falling velocity increases(therefore encouraging the separation) if the particlesize grows, if the viscosity of the continuous phasediminishes and if the difference between the densitiesof the two phases increases.

In order to promote an increase in particle sizesduring the desalination treatment of a crude oil, the oil(non-conductive) containing the water (conductive) issubjected to the effects of an electrical field. Thiseffect causes the conductive particles to aggregatetogether, because they are subjected to the influence ofelectrostatic induction and have a tendency to alignthemselves along the lines of force of the electricalfield. Hence, as they attracted by the electrodes, theyare subjected to forces of coalescence while in motionand form increasingly bigger particles, until they fallby force of gravity.

In a light crude (39°API) the velocity ofsedimentation of the water varies depending on thediameter of the droplets, as indicated in Table 2. Areduction in viscosity of the continuous phase, whichtakes place through an increase in operatingtemperature and which promotes coalescence anddecantation, also improves the contact efficiency ofthe crude oil and the injected water. However, anincrease in temperature causes an increase inconductivity of the oil, which becomes very rapidabove 120°C.

In the presence of particularly stable emulsions, itmay be necessary to use a de-emulsifier which, by

modifying the properties of the oil/water interfaceincreases the ability of the stabilizing agents todissolve in oil and reduces the surface tension of thewater droplets, so as to enable them to coalesce. In thisway the time needed for separation is reduced,enhancing the performance of the desalter.

In essence, a desalter consists of a vesselcontaining two electrodes (three in someconfigurations), one on the earthed side of the vessel,the other suspended on insulators; an electricitygenerator delivers a suitable electrical potential to thesuspended electrode. The positioning of and thedistance between the electrodes depend on thecharacteristics of the feed, the substances to beeliminated and the processing conditions. Thealternate current system is based on the application ofa high-voltage electrical field which generates anelectrical charge on the drops of water present in thehydrocarbon phase, causing them to oscillate as theypass between the electrodes. During this oscillation,the drops are first elongated and then compressed by the alternations of the electrical field. Through theeffect of this agitation the particles tend to aggregatetogether into particles of increasing size, untilsegregation under effects of gravity takes place and awater layer forms at the bottom of the desalter.

2.2.2 Vaporization

After the desalination treatment, the crude oil must beheated up to the inlet temperature required by thefractionating column. This is performed by means of aseries of heat exchangers, in which the heat of the hotstreams coming out of the fractionating column is usedto pre-heat the crude oil. Typically, this recoveryenables temperatures of between 240 and 280°C to beachieved. The remaining part of the energy is suppliedby a heater which raises the feed entering the columnto the temperature required to vaporize both theproducts which will then be extracted as side cuts anda part equal to about 10-20% of the bottoms product.This last fraction, after condensation in the trays of thecolumn immediately above the feeding area (flashzone), forms the internal liquid reflux.

The liquid fraction leaving the heater and theinternal liquid reflux are used to feed a steam strippingsection, provided in the bottom of the atmosphericcolumn, in order to remove the lightest componentsand hence to obtain a higher gas oil yield.

The required heater outlet temperature, to which aspecific enthalpy corresponds, depends on the type ofcrude oil treated (light, medium or heavy crude) and, ata given pressure, is correlated to the yield of distillatesrequired. The maximum permissible column feeding



Table 2. Rate of sedimentation of water, in a typical light crude, as a function of the diameter

of the droplets

Diameter of the droplets(mm)

Rate of sedimentation(cm/h)

2 0.033

10 0.813

30 7.366

100 82.55

400 133.10

temperature depends, therefore, on the characteristics ofthe feed and normally it does not exceed 370-380°C. Infact, if this limit is exceeded, cracking can take place inthe last part of the heater’s coil and coke can form,

resulting in an increase in the pressure drop in theheater and a reduction in the overall performance of theplant. Whenever excessive deposits of coke occur, it isnecessary to shutdown the plant and remove the cokewhich has been deposited (a decoking operation).

A variation of the traditional scheme involves theaddition of a pre-flash vessel (see again Fig. 2) which,operating at a temperature of 170-200°C and at apressure of 4-6 bar, removes the light part of the crudeand sends it directly to the main fractionating column,while the liquid fraction is subsequently heated up tothe temperature of the flash zone. As an alternative, apre-flash column could be inserted which enables gasand non-stabilized light naphtha to be separated as topproducts. The bottom of the pre-flash column may beequipped with steam stripping or a reboiler heater. Thebottoms product of the pre-flash column is furtherheated, by the use of exchangers and a heater, up to thetemperature required for the flash zone. This solutionis adopted when it is desired to reduce the diameter ofthe distillation column.

A key consideration for the vaporization section isthat of maximizing the recovery of heat from the hotstreams for heating the crude oil. The methodologycommonly used to optimize heat recovery and at thesame time to reduce the surface area of the heatexchangers thereby minimizing the investment cost,makes use of what is called pinch technology, which is arigorous and structured approach to determine theminimum amount of energy necessary in a process, andwhich drives the design in this respect. In applicationssuch as those being described, pinch technology iscapable of establishing the most appropriateconfiguration of a series of exchangers, seeking tominimize the costs of the heating (fuel for the heater)and cooling (air and water) utilities. This aim is achievedby analysing the composite curve which, in the deltaenthalpy/temperature representation of the streams,provides the graphical profiles of the energy availablefrom the process (hot composite curve) and of the energyrequirements of the system (cold composite curve).

Fig. 4 B shows the construction of a hot compositecurve, from two hot streams (1 and 2), where CP (kJ/°Cs) indicates the thermal flow rate of the stream, which isthe product of the specific heat Cp (kJ/°C kg) and theflow rate in weight M (kg/s). A hot composite curveshowing variations of enthalpy in time-temperatureunits, on the other hand, is shown in Fig. 4 A.

Composite curves provide a counterflowrepresentation of the heat exchange and are used toidentify the minimum energy requirement of thesystem. It is obtained by trying to superimpose the hotand cold composite curves, until the minimumtemperature difference obtainable in an economic wayis reached. Fig. 5 shows the degree of superimposition



CP�40

CP�40

CP�20

CP�60

0

80

120

160

200

40

02,000 4,000 6,000

tem

pera

ture

(°C

)

enthalpy change (kW)

1

2

0

80

120

160

200

40

02,000 4,000 6,000

tem

pera

ture

(°C

)


Fig. 4. Representation (A) and construction of a hot composite curve (B).

0

80

120

160

200

40

02,000 4,000 6,000

tem

pera

ture

(°C

)


heat recovery

QHmin�960

QCmin�120

Fig. 5. Use of hot and cold composite curves for determining heat recovery.

A

B

which allows the greatest possible heat recovery,highlighting the residual parts of the heating (QHmin)and cooling (QCmin) which must be ensured by externalfluids.

2.2.3 Fractionating

Fractionating is the basic operation carried out oncrude oil in every refinery, from which the productsare obtained which feed the downstream plant. Everyhydrocarbon has a boiling point which, for the lightestcompounds (from C1 to C4) at atmospheric pressure, isbelow 0°C, while for heavier compounds it can reachsuch high temperatures that it causes the molecules tobreak and cracking products to form. The crude oilwhich has been suitably heated to obtain a partialevaporation is sent to the bottom section of a column,in which, as a result of the effect of the material andheat exchange which takes place on the individualtrays, a temperature profile is established whichexhibits a decrease towards the top of the column. Thematerial and heat exchange is caused by contactbetween the vapours which rise towards the top and theliquid at a lower temperature which falls towards the bottom or the cold liquid (reflux) sent to the top of the column. At each point of the column, mixtures ofhydrocarbon liquids are collected whose boilingtemperature at the specific pressure at that pointcorresponds to the temperature of equilibrium on thetray itself. The lightest products remain in vapourphase and continue to rise towards the top of thecolumn. The temperature and pressure profilescorresponding to the theoretical stages in afractionating column are shown in Table 3. Thetheoretical stage 1 represents the overhead condenser,while stages 30 and 31 simulate the stripping trays.

Fractionating is ensured by the presence in thecolumn of devices (trays) which favour contactbetween the liquid and the vapour phases. In fact, athermal and material exchange takes place on the traysof the columns, between the vapours which are risingand the liquid which is descending and which, in thecase of a theoretical tray, allows a condition ofequilibrium to be reached between the vapours and theliquid coming out of an individual stage. In thematerial and thermal exchange which takes place, thevapours are enriched by the fractions with a lowerboiling temperature which evaporate from the liquidand are depleted by the fractions with a higher boilingtemperature which condense. In stable operatingconditions it is therefore possible to recover, atdifferent heights of the column, side cuts with thecompositions and characteristics required by thedownstream processes or which can be sent to storage.

Typical cuts from an atmospheric fractionatingcolumn are:• Top product: a mixture containing the lightest part

of the hydrocarbons (up to C4), light and heavynaphtha (in other set-ups, the heavy naphtha couldconstitute the first side cut).

• First side fraction: heavy naphtha or kerosene. • Second side fraction: light gas oil (typically used

as transportation fuel).• Third side fraction: heavy gas oil (typically used as

heating oil or fluxant for fuel oil).



Table 3. Temperature and pressure trends for an atmospheric distillation column

Tray Temperature (°C) Pressure (bar)

1 127 2.2

2 150 2.22

3 166 2.24

4 174 2.25

5 178 2.27

6 181 2.29

7 184 2.31

8 187 2.32

9 190 2.34

10 194 2.36

11 197 2.38

12 201 2.40

13 206 2.41

14 213 2.43

15 223 2.45

16 235 2.47

17 255 2.49

18 266 2.50

19 273 2.52

20 277 2.54

21 281 2.56

22 284 2.57

23 288 2.59

24 294 2.61

25 307 2.63

26 328 2.65

27 341 2.66

28 349 2.68

29 357 2.70

30 363 2.75

31 359 2.80

• Residue: product intended either to produce fueloil, after the possible addition of a fluxant, or asfeedstock for the vacuum distillation plant.The boiling ranges of the different fractions and the

topping operating conditions must be established apriori. To do this a number of combinations of yieldsare provided for, for which the plant must allow acertain degree of flexibility, such as to enableadjustments to be made quickly in line with the qualityof the crude oil and/or the products demanded by themarket. The starting point for determining theprovisional quantity and characteristics of the productsis the distillation TBP (True Boiling Point); based onthe specifications of the finished products andexperience, the yields and the principal characteristicsof the fractions can be calculated by making an arbitraryassumption about the final boiling points of the productsthat could be obtained from the crude. These products are found between boundary zones whichrepresent fractions common to two adjacent products.The boundary zones allow, when necessary, for the finalpoints originally specified to be shifted, subtracting allor a part of a certain border fraction from this or thatproduct and adding it to the adjacent product.

Once the characteristics of the desired productshave been established, it remains to establish theefficiency with which the fractions are to be separatedfrom each other; in the refining industry this is doneusing conventional methods, given the complexity ofthe composition of the mixtures involved. In practice, the starting point is the ASTM D 86 curve

(see Chapter 2.1) of the fractions, noting the differencebetween the temperature at which 5% of a fraction hasdistilled and that at which 95% of the fraction with aboiling point immediately below it has distilled; if thisdifference is positive there is a gap and thefractionating is considered to be good, whereas if it isnegative there is an overlap and the fractionating isdeemed to be unsatisfactory. By way of an example,Fig. 6 shows that heavy naphtha and kerosene are wellfractionated, that is, they are well separated from oneanother given that there is a gap of �13°C; on theother hand, light gas oil and heavy gas oil are not wellseparated, as there is an overlap of �42°C, since thelight gas oil contains about 30% of product whichshould be part of the heavy gas oil, while the lattercontains about 20% of product which should be part ofthe light gas oil.

Based on the value of the internal reflux ratio,which depends on other processing variables(characteristics of the fractions, overflash,intermediate refluxes), the number of theoretical traysnecessary is defined so as to achieve the value of theproduct of the internal reflux ratio by the number oftrays, which is, in turn, determined on the basis of thedeviation from the ideal fractionating (Fenske, 1932;Gilliland, 1940; Colburn, 1941). The number of actualtrays is determined by dividing the number oftheoretical trays by an efficiency coefficient whichdepends on both the physical characteristics of thefluid in question and the typology of the trays selected(Gunness, 1936; Ballast […] 1974).

Features of the contact devices

Fractionating traysThe most commonly used fractionating trays are

the valve, sieve and bubble cap types.Valve trays are based on the principle of the check

valve (Fig. 7). When the vapour flow increases, the



460

420

380

340

300

260

220

180

140

1000 10 20

overlap:318°C�360°C��42°C

30 40 50 60 70 80 90 100volume (% distillate)

dist

illa

tion

tem

pera

ture

(°C

)

gap: 164°C�151°C�13°C

heavy gas oillight gas oil

keroseneheavy naphtha

Fig. 6. ASTM distillation curves for petroleumfractions showing the fractionatingefficiency through gap or overlap (Giavarini, 1999).

Fig. 7. Detail of a valve tray(courtesy of Koch-Glitsch).

float begins to rise and the vapour passes through thevalve. Some floats begin to rise at 20-30% of thevapour load and at 50-70% they are completely raised.This allows for a wide range of operating loads (highflexibility), while maintaining high efficiency of thetray. Valve trays require special attention when they areused in columns which operate under vacuum becausein the event of pressure variations, the valves canbecome ‘stuck’ at the level of the tray or undergoerosion. Valve trays are not suitable for use in thepresence of fluids which can cause scaling, corrosionor the formation of carbon residues. On the other hand,they are recommended in situations which call forhigh flexibility, not operating under vacuum. Some

examples of their use relate to: vapour flow-rateswhich vary significantly and unpredictably in a sectionof the column; fractionating columns which are usedin batch mode with feeding loads and compositionswhich can also vary markedly; and columns which canoperate with wide variations of flow-rate, up to about30% from the design value.

Sieve trays (Fig. 8) are characterized by their lowcost, excellent efficiency, good capacity andreasonable flexibility (where flexibility is defined bythe ratio between the maximum load that still allowssatisfactory performance and the design load). Sievetrays can be used in almost all working conditions andare designed to operate satisfactorily in a wide rangeof operating conditions. The maximum capacity of asieve tray is at least equal to, if not greater than, that ofa valve tray; its flexibility can reach a maximum ofabout 3. They can also be used in the presence offouling fluids provided that the holes have a diameterof 20-25 mm. Sieve trays are not suited to workingconditions which call for high flexibility, for whichbubble cap or valve trays should be used, even thoughthey are more costly. At very low vapour loads (suchas during start-up) sieve trays have difficultymaintaining a sufficient head of liquid on the tray andexperience weeping to the point where they lose all ofthe liquid. Therefore, when a heated reboiler isprovided and the liquid is collected into a seal paninstalled below the downcomer of the sieve tray, it isnecessary to ensure satisfactory feeding by installingan auxiliary line from the bottom of the column to theinlet of the reboiler, or else a chimney tray under thesieve tray, as a draw-off tray.

Bubble cap trays (Fig. 9) are particularly expensive,from 50 to 100% more than the cost of valve trays.



Fig. 8. Sieve tray (courtesy of Jaeger).

Fig. 9. Bubble cap tray (courtesy of Rashig).

They should therefore be considered only when highflexibility is required and where there are problems ofscaling or the formation of carbon residues.

PackingPacking is used in a fractionating column for the

same reason as trays, that is, to ensure good contactbetween the liquid and the vapour phases so as topermit a material transfer (Tsai, 1985; Oglebay NortonCo., 1987). From an economics point of view, trays arebest used for processing large volumes of vapour andliquid.

Packing columns are normally considered for usewith corrosive, but not scale depositing, fluids in theheat exchange zone, in columns which operate undervacuum (where a low pressure drop is called for) andin columns with a diameter of less than 1,000 mm, asan alternative to cartridge trays.

When choosing the best packing, the factors to beevaluated include capacity, efficiency, resistance tocorrosion and cost. Fig. 10 shows some types ofpacking. Steel Pall rings should be considered as afirst choice, since they offer the advantage of highcapacity, are unbreakable and of all the various typesof packing have the greatest flexibility. Packings usingPall rings are considered to have a very long lifespanprovided there is no corrosion. Pall rings made fromother materials are also available: aluminium rings canbe used in pumparound zones for heavy hydrocarbons;plastic rings can be used at temperatures up to 120°C;while ceramic rings are preferable in corrosiveenvironments but have a limited lifespan, even thoughin well operated columns they can last as long as tenyears. Ceramic rings are resistant to acids, alkalis andsolvents, with the exception of caustic solutions andhydrofluoric acid. In designing a fractionating column,consideration should also be given to the possibility ofinstalling the structured type packing, illustrated inFig. 11. Generally this type of packing is moreexpensive, but it is normally used when highseparation performance with low pressure drop isrequired, for example in super-fractionating or vacuumcolumns.

2.2.4 Stripping and stabilization

In this section of the atmospheric distillation plant, theside and top cuts are treated in order to bring them upto the specifications required.

The cut that comes out of the top of the column,generally composed of LPG (Liquefied Petroleum Gas)and naphtha, is sent to a naphtha re-distillation column,that normally works at pressures of between 8 and 10bar and at a temperature of about 180°C at the bottom.The temperature at the bottom of this column isdependent on the pressure and on the final point ofdistillation of the naphtha. In this column, thosecomponents that remained dissolved in the naphtha areeliminated: this operation is called stabilization. Themixture, composed mainly of methane and ethane, isobtained in a gaseous form and is sent to the refinery’sfuel gas network. Furthermore this gas stream containsminor quantities of propane and butane (normal-butaneand iso-butane) with variable concentrations dependingon the pressure of the overhead accumulator and on thetemperature. A liquid blend is also obtained that iscalled LPG and is used domestically and as motor fuel.



Pall rings Berl saddlesRashig rings

Fig. 10. Types ofpacking.

Fig. 11. Detail of structured type packing(courtesy of Koch-Glitsch).

LPG is liquid at ambient temperature and at a sufficientpressure that depends on the relative ratio of theconcentrations of propane and butane. The naphthacoming out from the bottom of the stabilizing columncan, in turn, be sent to a column where the variousfractions (light, medium and heavy) are separated; thisinstance is what is referred to as a splitting column.

The side cuts of the principal fractionating column(kerosene, light and heavy gas oil) are instead each sentto feed a column containing 4 to 6 fractionating trayswhere, with the help of an injection of superheatedsteam, all traces of the lightest components areremoved. The operation of treating with superheatedsteam is called stripping and, as already mentioned, thecolumns where it is carried out are called strippers. Thefraction at the top of the stripper (strip out) is then sentback to the main column. This operation makes itpossible to increase the flash point of the side cut. Insome cases, especially for the kerosene fraction, it isdifficult to obtain the required flash point value simplyby stripping with steam: in this case a reboiler is usedto treat the liquid at the bottom of the stripper. In othercases, for example for the gas oil fraction that is to beused as fuel for diesel motors, it is necessary to removethe traces of water, that are normally found dissolved inthe fraction when stripping has been performed usingsteam. Elimination of traces of water to reach aresidual value of a few tenths of ppm, is obtained bysending the product at the bottom of the stripper to avacuum drying column. Alternatively, the gas oilfraction can be sent to a salt drier, followed by acoalescer. Once this refining has been carried out, theproducts are sent to their relevant tanks, after havingbeen appropriately cooled. The operations ofstabilization and stripping complete the processing ofan atmospheric distillation plant. Table 4 shows theyields and characteristics (density, sulphur content)obtainable by means of this process, when fed with atypical crude coming from the Middle East.

2.2.5 Problems of corrosion and materials

In choosing the materials to use in an atmosphericdistillation plant an evaluation must first be made of

the quality and quantity of the corrosive componentsin the crude. A rough evaluation of the corrosivepotential of the crude is provided by the totalquantities of heavy acids present, of which anapproximate indication is provided by theneutralization number, expressed in mg of KOHneeded to neutralize one gram of crude. This valuerelates to the neutralization of all the acids present inthe crude oil, organic and inorganic, strong and weak,of the sulphur containing compounds such ashydrogen sulphide, mercaptans and thiophenols, ofhydrolysable esters, and of salts formed by strongacids and weak alkalis (MgCl2). More specifically, theproducts present in the crude that influence the choiceof materials are the salts, the sulphur and thenaphthenic acids. In fact, the first two are alwayspresent in varying quantities.

Corrosion by saltsDepending on the reservoir from which it has come

and the method of transportation, salts are present inthe crude in varying quantities in the form of NaCl,MgCl2, CaCl2 and NaHCO3 in a ratio more or less thesame as that found in sea water. If the salt content islower than 8.5 g/m3 the crude oil is not considered tobe corrosive. The corrosiveness of salts is due to thedecomposition of MgCl2 and CaCl2 that takes place inthe heater coils at high temperature. In such a caseHCl is produced, which is not corrosive attemperatures above the dew point of water, whereas itbecomes extremely aggressive at lower temperatures.Corrosive phenomena appear when there iscondensation of water (column overhead zone,condensers and collecting accumulators of the refluxand/or of the top product). Normally the salts areremoved in the desalination section.

Corrosion by sulphurSulphur is present in the form of hydrogen

sulphide (H2S) dissolved in the crude or as organicsulphur combined in the molecules of thehydrocarbons; in the latter case it can appear assulphide, mercaptanic sulphur and thiophenic sulphur.The sulphur content limit that separates a corrosivefrom a non-corrosive crude oil is between 0.5 and 1%by weight. Nevertheless, it sometimes happens that



Table 4. Yields and characteristics of the fractions obtained from the stripping and stabilization operations

Light naphtha Heavy naphtha Kerosene Gas oil Residue

Yield (as % of volume of crude) 4.9 12.1 16.5 24.1 40.2

Density (kg/dm3) at 15°C 0.66 0.720 0.788 0.865 0.958

Sulphur content (% by weight) 0.02 0.03 0.1 1.4 3.2

crude oils with a sulphur content below 0.5% are morecorrosive than others that contain more than 1%. Thisdepends on the form in which the sulphur is presentand the temperature at which the hydrocarbonfractions are found. The corrosion is caused bydecomposition of the sulphur-containing molecule as aresult of heating, with the production of H2S; thesemolecules start to decompose at 260°C. Thedecomposition proceeds fairly quickly between 340and 400°C and is almost complete at 480°C.

Corrosion by naphthenic acidsThe definition of naphthenic acids in the

petroleum industry includes all the organic acidspresent in the crude oil. Their name is derived from thefirst acids discovered in crude, derived from thestructure of cyclopentane. However, crude oils contain agreat variety of organic acids, that include both low molecular-weight acids, such as fatty acids, andsaturated and unsaturated acids made up of single ormultiple ring structures. Consequently, naphthenic acidsinclude a vast family of hydrocarbon compounds that allcontain the acid radical group �COOH. Generally, inatmospheric distillation these acids are concentrated inthe cuts of heavy gas oils and residue; theircorrosiveness shows up at operating temperatures above240°C. Once the acid base of the crude oil has beenestablished, a crude is considered aggressive if theneutralization number is greater than 0.5 mg KOH/g.The aggressiveness of these acids is apparent mainlywhere there is high turbulence, for example in centrifugalpumps, in heaters (especially in the bends of the coils),in the connecting line between the heater and thefractionating column and in the inlet section where thepartially vaporized crude oil enters the column.

Choice of materialsThe choice of materials for the following areas of

an atmospheric distillation plant is critical: thevaporization section and the fractionation section.

In the hottest part of the vaporization section,above 250°C, where sulphidation of the material cantake place owing to the presence of sulphur, steelswith a high chromium content are used. The heatercoils (high temperature and high internal velocities)are usually made of 5Cr-0.5Mo or of 9Cr-1Mo. In thecase of naphthenic crude oils with a neutralizationnumber above 0.5 mg KOH/g, the heater coils aremanufactured from AISI 316L type stainless steel, atleast for the end part where vaporization occurs and asa consequence, there are high velocities.

In the fractionating section the column is normallymade from carbon steel with the lower part clad in12Cr; when there are naphthenic acids present in thefeed crude oil, AISI 316L stainless steel is used. In the

sections in which the shell of the column is clad in12Cr or in AISI 316L, the trays are also made of thesame material. The top section of the atmosphericcolumn is the part at highest risk of corrosion, due tothe presence of HCl which concentrates in thecondensing steam. Besides particular materials likemonel metal, used both as cladding material for theshell and for the 3-4 top trays, provision is normallymade for injection of corrosion inhibitors andneutralizing agents to ensure that the pH of the water,that is condensed and removed from the columnoverhead receiver, is kept between 5.5 and 6.5. Theparts that are primarily subject to corrosion are madethicker. Moreover, corrosion tests are also carried outor ultrasonic measurements are taken to check that theremaining thickness is always above the safety limits.

2.2.6 Operating variables

For the correct operation of an atmospheric distillationunit it is essential to consider the effect on both thequality and the yield of the products obtained, and onthe operating costs, of changes in the adopted valuesfor the principal operating variables (described below).

Temperature leaving the heater and entering thedistillation column

This variable must be precisely controlled becauseit determines the degree of vaporization of the crudeand the amount of heat supplied to the heater. Theinput temperature of the crude into the column iscorrelated to the characteristics of the crude itself andthe quantity and the quality of the fractions to beproduced; because all the energy is provided throughpreheating of the crude oil, any variation intemperature creates a disturbance in the equilibriumprofile between the phases and of the temperaturethroughout the column, such as to cause a variation inthe characteristics of the products if no action is takenusing the control systems.

A low exit temperature from the heater produces asmaller quantity of vaporized material andconsequently a reduced quantity of distillates, while anexcessively high temperature causes cracking reactionswith the formation of gas and carbon deposits, inparticular on the heater coils, making it necessary toshutdown the plant more often to clean/decoke theheater itself.

Operating pressure of the distillation column Once the pressure in the overhead accumulator has

been specified based on the characteristics of thecrude, the conditions of the cooling systems (coolingwater or air) and the possibility to obtain the greatest



degree of condensation to avoid loss of product, theoperating pressure of the atmospheric column isdetermined by the quantity of vaporized material inthe column.

The quantity of vaporized material, at a constantpressure, depends on the composition of the feedstock,the outlet temperature of the heater, on the degree ofsteam stripping injected in the bottom section, on theheat removed from the side refluxes (pumparound), by the flow rate of side fractions extracted from the column and the strip-out flow rates entering thecolumn from the side strippers.

Whenever the temperature in the column is used asan indirect measure of the composition of the products,the pressure in the column must be checked carefully.Typical operational pressure values are 2.1 bar in thebottom section and 1.4 bar in the overhead section ofthe column. For the same production of distillates,higher pressures require a higher outlet temperaturefrom the heater, and therefore a greater consumption ofenergy and leads to the possibility of formation ofcarbon deposits on the coils of the heater itself. In fact,working at higher pressures, other conditions beingequal, the fractionating of contiguous fractions is moredifficult. On the other hand, working at higher pressuresreduces the volumetric loads, with the consequentpossibility of reducing the diameter of the column.

Controlling the pressure in the overhead section of afractionating column can be carried out using twoprincipal typologies: partial condensation systems andtotal condensation systems. Some of the possibleconfigurations of pressure control by means of partialcondensation are shown in Fig.12. The layout in Fig. 12 Aapplies to systems in which the top product iscondensed but, because of the presence of inert gases orof significant quantities of methane and ethane, totalcondensation is not possible, so their continuousremoval in the form of gas must be provided for so as tokeep the pressure constant at the desired value. Thelayout in Fig. 12 B applies to columns where it isnecessary to maintain a constant pressure at the top ofthe column, acting on the flow of uncondensed materialthat is present in the accumulator and that is sent to theunit downstream. This layout is applied mainly tostabilizing columns in which there is always a fractionof gas present as a product. The layout in Fig. 12 C, alsocalled total reflux, is applied to columns in which theoverhead distillate is present only in the vapour phaseand the condensed liquid is used only as reflux. Thepressure controller keeps the overhead pressure of thecolumn constant, while a differential pressure controller,acting on the condenser bypass, also keeps the pressureconstant in the overhead receiver.

There are several configurations in totalcondensation systems for controlling the operating

pressure in fractionating columns without producinggas in the overhead section (overcooled or bubblepoint distillate). It is important, in choosing the mostsuitable system, to keep in mind the requiredflexibility of the system itself, the absolute value of theoperating pressure, the availability of inert gas (forexample nitrogen) at an adequate pressure, and thepossibility of contamination due to the blanketing gas.

In the configuration shown in Fig. 13 (distillate atbubble point, that is at the temperature at which, in aliquid made up of two or more components, the first



2 2 Montanari Ricci fig 12

Fig. 12. Partial condensation with purgingof inert gases (A); partial condensation where the non-condensed part is used for controllingpressure (B); partial condensation of the refluxonly in the column (C). PC, pressure controller;dPC, differential pressure controller; LC, level controller; FC, flow controller.

A

B

C

bubble of gas is formed), the pressure controller variesthe efficiency of the condenser, acting both on thevalve installed downstream from it, by modification ofthe flow rate of vapour sent for condensation andtherefore the flow of condensate obtained, and on thevalve installed on the hot bypass line. If the pressure in

the column overhead rises, valve A opens and valve Bcloses; if the pressure goes down, the oppositehappens. A hand control is also included in order todischarge any incondensable materials that mightaccumulate. This method of control is adopted to takeaccount of the performance differences of thecondenser caused by variations in the temperature ofthe incoming cooling water or air, according to thetype of condenser used, and depending on the seasonof the year or the day/night cycle.

In the configuration in Fig. 14 (overcooleddistillate, in other words, zero flow rate of vapourproduced) the action of the pressure controller onthe control valve has the effect of varying the flowrate through the condenser. If the pressure in thecolumn goes down, the control valve isprogressively closed and the liquid in theaccumulator is brought back to the bubble pointtemperature with a partial flow of vapour throughthe equalizing line; the opposite happens if thepressure rises.

The configuration depicted in Fig. 15 applieswhen total condensation is required with overcoolingof the liquid product; therefore the pressure in theaccumulator must be maintained by sending in anincondensable gas (nitrogen), since the vapourpressure of the liquid produced would be lower thanthe pressure value to be maintained. The pressurecontroller operates the appropriate control valve ifentry of nitrogen is called for to maintain therequired pressure, or alternatively on the other valve,causing a discharge to the flare, if there is a tendencyfor the pressure to increase, following theaccumulation of incondensables or of an increase inthe level that would tend to compress the nitrogenpresent in the space above the liquid contained in theaccumulator.

Flow and temperature of reflux in the columnThe heat which is not removed by means of the

products leaving the column must be extracted byremoving the liquid in appropriate sections of thecolumn and sending it back to the column aftercooling. In an atmospheric distillation column,generally there are three intermediate refluxespresent: overhead, kerosene and gas oil. Obviouslythe amounts of heat taken out by the pumparoundsystem vary depending on the temperature of theliquid draw-off, the return temperature and theflow rate; these amounts of heat are determiningfactors for the heat balance of the column and forthe assumed values of the flow rate of the liquidand the vapour in the various sections of thecolumn itself. The distribution of the extracted heatalong the column is determined by the degree of



A

hotby-pass

to fuel gasor flare

B

Fig. 13. Total condensation with hot by-pass. TI, temperature indicator; HIC, hand incondensablecontroller.

equalizingline

to fuel gasor flare

Fig. 14. Total condensation with overcooling.

nitrogen

to flare

Fig. 15. Total condensation with an inert gas blanketing.

fractionation required, as well as by the thermallevel to which the heat is removed. In addition tothe temperature, the flow rates of the variousrefluxes in the column also have an effect on thefractionating. High flow rates of reflux in thecolumn can cause flooding of the return zone andweeping onto the trays below, thus causing areduction in the efficiency of the trays themselves.Low reflux flow rates, instead, can induceentrainment of the vapour (blowing) towards thetrays above the return point into the column, againcausing a reduction in the fractionating efficiency.

Stripping steam in the columnSuper-heated steam is injected into the lower part

of the atmospheric distillation column in order toremove the lighter components from the bottomsproduct, that is those with a lower boilingtemperature. The effect of the steam is such as toreduce the partial pressure of the hydrocarbon phase,in this way causing the components with a lowerboiling temperature to pass from the liquid phase tothe gas phase. A typical flow rate for these injectionsis 2% by weight, in relation to the flow rate ofatmospheric residue coming out from the bottom ofthe column. A flow rate which is too low tends toleave a part of the light distillate in the bottomsresidue, while a rate which is too high can cause thehigher distillates to be contaminated with traces ofheavy product.

Flow rates of side draw-offs from the distillationcolumn

At each point along the atmospheric column, andespecially at the extraction point on the column of astream or a side cut, the temperature of the liquidreflects the boiling temperature of the liquid at thatexact point. However, an increase in the flow rate ofliquid extracted from the column leads to a reductionof the internal reflux; as a result, the liquid becomesricher in less volatile components and, as aconsequence, raises its boiling temperature in thesection concerned. Conversely, a reduction in the flowrate of the draw-off fraction causes more liquid toremain in the section of the column immediatelybelow the extraction point, with a consequentreduction in temperature.

As a general rule it can be stated that, as the extraction flow rate is increased, the boiling point of the extracted liquid rises, whereasas it is reduced, the boiling point decreases,therefore causing it to become richer in the morevolatile components.

Bibliography

Bolles W.L., Fair J.R. (1963) Tray hydraulics. Perforated trays,in: Smith B.D. (editor in chief) Design of equilibrium stageprocesses, New York-London, McGraw-Hill, Chapter 14.

Bolles W.L., Fair J.R. (1982) Improved mass-transfer modelenhances packed-column design, «Chemical Engineering»,July, 109-116.

Chin T.G. (1979) Guide to distillation pressure control methods,«Hydrocarbon Processing», 10, 145-153.

Fair J.R. (1963) Tray hydraulics. Bubble-cap trays, in: SmithB.D. (editor in chief) Design of equilibrium stage processes,New York-London, McGraw-Hill, Chapter 15.

Fonseca S. et al. (1997) Refinery energy and yield improvementsusing pinch technology, in: AspenWorld97. Proceedings ofthe conference, Boston (MA), 12-16 October.

Giavarini C. (1999) Guida allo studio dei processi diraffinazione e petrolchimici, Roma, Siderea.

Hatch L.F., Matar S. (1981) From hydrocarbons topetrochemicals, Houston (TX), Gulf, 8-17.

Perry’s chemical engineers handbook (1999), New York,McGraw-Hill, 18-22.

Ricci G., Bealing C. (2003) Using an integrated approachconserves energy and hydrogen, «Hydrocarbon Processing»,12, 76-81.

Snamprogetti (1975) Guida alla progettazione degli impiantipetrolchimici e di raffinazione, Milano, Pirola.

Spiegel L., Meier W. (1987) Correlations of performancecharacteristics of the various mellapack types (capacity,pressure drop and efficiency). Proceedings of the 4th

International symposium on distillation and absorption,Brighton, 7-9 September, Sulzer Chemtech Document22.54.06.40.

Vital T.J. et al. (1984) Estimating separation efficiency,«Hydrocarbon Processing», 12, 75.

White R.A., Ehmke E.F. (1991) Materials selection forrefineries and associated facilities, Houston (TX), NationalAssociation of Corrosion Engineers, 5-7.

References

Ballast tray design manual. Bulletin 4900 (1974), Dallas (TX),Glitsch.

Colburn A.P. (1941), «American Institute of ChemicalEngineering. Transaction», 37, 805.

Fenske M.R. (1932), «Industrial and Engineering Chemistry»,24, 482.

Gilliland E.R. (1940), «Industrial and Engineering Chemistry»,32, 1101.

Gunness R.C. (1936) Sc.D. Thesis, Cambridge (MA),Massachusets Institute of Technology.

Oglebay Norton Co. (1987) Bulletin 1HP-1.Tsai T.C. (1985) Packed-tower program has special features,

«Oil & Gas Journal», September, 77.

Paolo RicciRomolo Montanari

SnamprogettiSan Donato Milanese, Milano, Italy



Documents

2.2 Atmospheric distillation of crude oil...A crude-oil atmospheric-distillation (or topping) plant makes it possible to obtain distillates (made up of the overhead product and the