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Professional Degree Theses Student Theses and Dissertations

1927

Relationship between temperature and vapor pressure of light oils Relationship between temperature and vapor pressure of light oils

William Orval Keeling

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Recommended Citation Recommended Citation Keeling, William Orval, "Relationship between temperature and vapor pressure of light oils" (1927). Professional Degree Theses. 210. https://scholarsmine.mst.edu/professional_theses/210

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RELATIONSHIP BETWEEN TEMPERA.TURE

AND VAPOR PRESSURES

OF LIGHT OILS

BY- W. O. Keeling.

A THESIS

IN PARTIAL FULFILLMENT OF THE

REQ,UlREMENTS FOR THE DEGREE OF

CHEMICAL ENGINEER

June 24, ~927

Thesis: Relations~iP betw4en tempe~ature an~vapor preFsures ot light O~lS. 1\'Keellng. 1 1927 \ I I

- 1 -

mTRODUCTION

There are a few data scattered through the lit­

erature on the vapor pressures of pure hydrooarbons, but

the author has never seen a systematic study of the rela­

tionship between temperature and vapor pressures of oompli­

cated mixtures such as the various light oila encountered

in refinery praotioe.

suoh data would be useful in designing equipment

for distillation, steam stilling, vaouum distillation and

in oaloulating heat balanoe, etc.

An attempt has been made to link the vapor pres­

sure o£ a light oil, at various temperatures, to the aver­

age boiling point of the oil.

Apparatus Used

The apparatus used was an ordinary mercury baro­

meter, jaoketed by a two inch Pyrex tube. An electric heat­

ing coil was placed in the space between the barometer an4

pyrex tubing. A meter stick was placed behind the barometer.

One end of & small metal rod was inserted into one ena of

this stiok" the other end 'of the rod passing through the cork

whiah held in the bath oil. !He distanoe from the end of the

meter stiok to the end of the rod was measured. The free end

of the rod was made to barely touch the surfaoe of the mer­

cury in the barometer well by raising or lowering the latter.

Agitation of the bath oil was acoomplished by in-

- 2 -

troducing air into the lower part of the annular space.

By regulating the electrio current in the heater and the

amount of air used for stirring, the temperature was main8

tained at the desired point.

Kind o~ Oils Used

The oils seleoted for the experimental work were:-

1. Light, Straight-Run gasoline

2. Heavy, Straight-Run gasoline

3. 40-60 Blend of Cracked andStraight-Run gasoline

4. Pressure Distillate

5. Beluslne

6. Crude Oil

These oila represent most of the light oils encountered in

the average refinery praotioe. The vapor pressure of each

oil was first measured at different temperatures. Each 011

was then fraotionated into 10% cute and the vapor pressures

of each cut were determined at different temperatures.

Procedure

The prooedure used was to introduoe the oil to be

tested into the barometer by means of a ourved medicine

dropper. A suffioient amount of oil was introduoed into the

barometer as to insure some of the lighter ends remaining

in a liqUid form at the higher temperatures. At least 10 00

were used for eaoh test. After the 1ntroduotion of the 011,

- 3 -

the eleotrio current was turned on and the air "jet start­

ed. As the bath warmed up, the height of the barometer

was read at 20~F intervals. The resulta are tabulated in

the following tables.

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90

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- 14 -

DISCUSSION

]Tom the very nature of, the substances worked

with, the regularity of results oan not equal the results

to be obtained from pure substances. Gasolinea of exact­

ly the same gravity. or even the same initial and end points

may di~far from eaoh other in vapor pressures at any given

temperature. beoause of differenoes in oomposition. !he

author recognizes this and simply offers the data obtained

in the hope that it will be useful for average engineering

oomputations.

It will be observed that the first two or three

cuts from eaoh oil. partioular1y pressure gasoline. gives

vapor pressures that seem to be 8 little out of line with

the remainder of the outs. This is probably due to one or

two things ,- dissolved permanent gases. or wide variation

in composition. This is shORn also in the A.S.T.M. distil­

lations of the l~~ outs from eaoh 011. The tendenoy 1s for

the differenoe in temperature between the initial point and

end point to be greatest for the lightest outs and least for

the heaviest outs even though the same fractionating oolumn

was used and the same rate of distillation maintained. Then.

too. thQ're is a tendenoy for the overlap between the end

point of the first fraotion and the in1tial point of the

seoond to be larger than the overlap between two sucoessive

fractions of higher boiling cute, although the differenoe

- 15 -

shows lass regularity than the difference in temperature

between the initial and end points.

Rejecting the figures obtained for the first

three cuts of eaoh oil, excepting benzine and crude oil,

the remaining data seems to anow a general relationship

between temperature and vapor pressure. By use of this

relationship, this data can be extended from the partic­

ular produots tested (with the inoorrect figures thrown

out in any case) to general use on all usual light oils

and possibly the heavier. This relationship can be .%~

pressed mathematioally &s:

P • 600 f~ ~ tg8J 9Equation No. 1

p : Vapor pressure in m.m. of mercury at

temperature t in degrees Fahrenheit

t =-Temperature Fahrenheit

T =Average boiling point of fraction in

degreeS 19hrenheit

The following comments are in order about this empirical

equation:

1. It gives average 'figures for this data. On any

one fraction from a particular oil it may show some error.

Much of our engineering work covers average con4itions,

wi th the probable error within the aoouraoy of other fac­

tors, so this equation is available for use with judgement.

Further work would probably permit the uee of the equation

for any kind of oonditions with aooeptable aocuraoy.

- 16 -

2. The equation applies with greates~ accuracy to

olose cut fractions. Plant practioe rarely has to do

with olose outs. henoe a modification is necessary for

fractions of wide boiling point ranges.

Let t

T'a Value of T to be used in Equation No.1

T =Actual mean boiling point '

D : Distillation range, or differenoe be­

tween initial and end points in degrees

Fahrenheit

then,T' • T - ~

-!;;....(T-'"''''''''''4-60-)Equation No. n

3. EVen this modifioation .ill not give correctly

the vapor pressure if there is an appreciable quantity ot

dissolved gas in the oil. This dissolved gas adds to the

vapor pressure of the out itself. In this case the vapor

pressure of the gas mnst be oons'idered separately and mast

then be aided to that of the oil itself.

Use of the Kqaatlons Illustrated

Assume a 010S8 out naphtha fraction wi tb. an '&'.13 .P.

of 3750F. The vapor pressure is required at 2QOoF.

t II 2000'.

T • 3750 )1,

P • 600 (200 i 46019(316 /1601

• 600 %(.79)9 • 600 % .121 =73 m••• Kg.

- 17 -

~-

OheCking Over the data, temperatures at whlan this vapor

pressure will be exerted by outs of about this mean boil­

ing point seem to vary within 100F of 2000F.

If, instead of a close cut fraction, we have a

naphtha of about 200o~ I.P_, 4500 F E.P., 3750 FA.B.Pe, as

would be met with in actual praotice, the correction for

temperature will be required. Here t and T are as in above

problem.

D =450 - 200 • 250oF. Then

T r =T - D2 =375 - 25022(Tr460) 2(375r460)

• 375 - 62600 =375 - 37 • 33SoF, and2x835

P =600 (t ~ 460)9(T r 460)

= 600 (200 ~ 460)9 : 600 i;;g}933B 460)

: 600 %(.82,)9 =600 x.182 : 115 m.m. Kg.

instead of 73 m.m. for olose cut fraction

Qalcn1ationa for cl at ermining the degree of vacuum

required in vaouum distillation of light oils would be de­

termined by the same oaleula't,ions as above t but it should

be borne in mind tha't these data and equatioM should be

used only for "flash" distillstion and not for batoh.

Use of Data ~or steam Distillation caloulations

For Light Gasoline

The £ollowing formula w111 giva the theeret10al

- 18 -

amount of steam to use in steam distillation.

p -Px 18IS t al

or a - (P-p) 18- PMwhere p Partial ~reasure of oil.

P Absolute ~ressur8 on surfaoe of 011.

l{ lloleoular weight of oil.

a Quanti ty of steam to one part of 011.

18 Molecular weight of water.

Take t~e light gasoline. ~h1s has an average boiling point

of 2590~. ~his correspOnds to Ootane Whioh has a boiling

point of 259 and a molecular weight of 114.

Assume the all 1s to be distilled with steam at

atmospheric pressure at 240°1'. Required the amount of steam

to 11se.

P 760 K.K•• atmospheric pressure.

M 114, moleoular weight of oil.

18 Molecular weight of water.

a Unknown.

a =(760-675) 18 = .15 Ibs. steam, to one part676x114

of 011. Actual praotioe using open ani olosed 0011s shows

about double the oalculated amount of steam. Then .04 Ibs.

of steam per pound of oil, or 12 pounds of steam per barrel

of gasoline will 'be necessary when steam 8tll1in~ at 840°)'.

- 19-

In practioe,distillation is usually oarried on

under pressures of from 5 to 10 pounds per square inch.

Suppose wa steam still under 10 pounda per square inoh.

gage pressure the other factor's remaining the same. What

amount of steam will be necessary?

a =(1340 ... 675) x 18 • .16, 1bs. steam per lb.675x114 of 011 theoretioally.

Aotually .3 pounds of at eam per pound o-L oil oX' 90 pounds

of steam per barrel of oil.

Now let us take a heavier gas and sea What the

steam consumption w111 be. Let us take the 90-100 per aent

cut of heav7 straight-run gasoline. The average boiling

point is 4150:r. whioh corresponds to the boiling point of

Dodeoane whiah has a molecular weight of 1'70. SUppose the

distillation 1a to be carried out at 280o~. and atmospherio

pressure.

p =110 X.M.

P =760 Y.U.

K =170

a : {'760 - 110~ 18 :.6 pounds of steam per pound of Oil110 % 1 0

theoretlcaD3 or 1.20 pounds of steam or 360 pounds of steam

per barrel of oil actually.

Now let us distill this oil at 10 pounds gage

pressure at t.he same temperature.

a : (1340 - 1106 18 =1.88 pounds o~ steam per pound of, 110 x 1'1

011 theoretioally or 3.~6 pounds of steam per pound of oil

- 20 -

or 1128 pounds of steam per barrel of oil under actual

conditions.

Suppose that the distillation is carrie4 on under

a vacuum of 300 M.M., than.

a = (300 - 110) 18 = .18 pound of steam per pound of 011110 x 170

theoretically or .36 pounds of steam per barrel of oil or

108 pounds of steam per barrel of Oil, actually.

CONCLUSIONS

1. There is a definite relationship between the

vapor pressure of a light oil, at any given temperature,

and its average boiling point.

8. This relationship is expressed in the Equation No.

I for close aut oila, and in Equation No. II for oils o~

wide boiling point range.

3. An aOQuraoy of about 5% oan be had by use of theee

equations, Whioh is aoourat e enough tor most engineering

oaloulations.

4. Examples have been given showing the useo! the

equations.