Indian Journal of Chemical Technology Vol. 8, July 2001, pp. 260-272

Development of a simple method for separation of a-pinene and ~-pinene from turpentine oil and designing multicomponent batch distillation columns

J K Sarna*" & P Bandopadhyay b

•chemical Engineering and Design Division, Regional Research Laboratory, Canal Road, Jammu Tawi 180 016, India

bFuel and Mineral Engineering Department, Indian School of Mines, Dhanbad 826 004, India

Received 25 May 2000; accepted 26 February 2001

A simple graphical method has been developed for designing batch fractionating towers for the fractionation of multicomponent mixtures. It can predict with accuracy the amount of high purity cut one can have, with optimum number of plates at optimum reflux ratio. This eliminates the rigorous determinations by computer. The method is equally good in predicting the amount and composition of subsequent cuts of high purity. The accuracy of the method has been established with the practical results from a pilot scale batch fractionating column.

Oil of turpentine is an important raw material for the production of various terpene chemicals. The production of these derivatives is based on chemical transformation of its components such as a-pinene, ~­pinene, !l3 -carene, longifolene to its useful products. Many of these transformations require isolation of its components in high purity, which is carried out by fractionation.

The main source of turpentine in India is the oleoresin from Pinus roxburgii Sarg. The average composition of Indian turpentine is,

a-pinene f)-pinene !l3-carene Longifolene Other terpenes

20-30% 5-10%

55-65% 2-10% 2-3%

a and f3-pinenes are isolated from the turpentine oil by fractionation in a batch fractionating column. The purity of a & f3-pinenes and their quantities isolated, depends on the number of theoretical plates present in the column, the feed composition and the reflux ratio kept during operation.

However, in case of multicomponent batch fractionation, it is not easy to calculate the number of theoretical plates required for getting certain quantity of a component in high purity.

* For correspondence (E-mail: rrlj @nde. vsnl.net.i n: Fax: (0191)548607;543829)

The first comprehensive model 1 for multi­component batch distillation was presented in which heat and material balances together with volume balances were employed. The assumptions involved ideal plates, constant volume hold-up, adiabatic operation and negligible vapour hold up.

Various methods, numerically integrating the differential equations of a comprehensive mathe­matical model of multi-component batch distillation were studied2

•3

• Rigorous treatment on separation of multi-component mixture have been reported4

.

Computer methods for solving dynamic separation problems5 and validating the models developed by comparing calculated distillation curves with the experimental curves developed6 have also been reported. The best modee reported in literature on simulation of batch distillation operations used the Gear method for integrating stiff differential equation available as computer software package such as LSODE. This package was also used for developing and applying a computer software package for batch distillation8

• Recently the work published on simulation, optimal design and control in batch distillation, involved direct solution of the time­dependent descriptive differential equation using the Gear alogrithm9

•

Various short cut methods have also been used for knowing the minimum reflux, minimum number of plates, along with operating reflux and corresponding theoretical plates required in continuous columns 10

-17

•

SAMA & BANDOPADHYAY: SEPARATION OF a-PINENE AND !3-PINENE FROM TURPENTINE OIL 261

A simple graphical method was also reported 18, for

solving problems on separation of multi--component mixtures in continuous columns. However these were not applicable in batch columns.

It is thus clear that all the methods developed so far require rigorous, tedius and laborious computer methods of calculations for designing multi-com­ponent batch distillation columns. To overcome these difficulties, a simple method for calculation has been developed for designing such columns. It has been tried on pilot scale batch distillation column for the isolation of a-pinene and 13-pinene from turpentine oil in the present studies. It has the following advantages:

i) Knowing the number of plates present in the column it can predict the quantity of high purity a-pinene that can be obtained from a feed of known composition at different reflux ratios.

ii) It can also show that if two cuts A&B of some composition are obtained at Reflux ratios R 1 & R2 respectively then it is possible to have a single combined cut (A+B) at a reflux ratio R3,

such that the time taken for the cuts A&B at R 1

and R2 is same as that of (A+B) at R3•

iii) It is also possible to predict the composition of the second cut for recovery of 13-pinene and its operating reflux ratio.

Experimental Procedure

Materials and Methods The present studies were carried out in batch type

packed fractionating column made of S.S. 304, packed with Hyflux packing made out of SS-304 wire, by M/s Evergreen Wire Cloth Factory Pvt. Ltd., 3-D, Maker Bhawan No.2, Merine Lines, Mumbai (Fig. 1). The column has a total packed height of 4200 mm packed in 4 sections. Column dia. is 175 mm. Each section is packed with 7 nos of Hyflux packing rolls of 175 mm dia x 150 mm height. It has a re­boiler of 200 L capacity, heated by hot oil circulation in its coil. It is operated under vacuum, created by piston type Ingersoll Rand Vacuum Pump. Each section of the column is provided with a distributor. The column is provided with a vertical condenser at the top, a solenoid valve for reflux distribution, a product cooler, two receivers of 50 L capacity each, a vapour trap, chilled by circulating cold water from a chilling unit.

Turpentine oil used (i) J & K state Turpentine oil having average

composition

a-pinene 13-pinene !:!3 -carene Longifolene

30% 10% 55-60% 2-3%

(ii) Mixture of a-pinene and 13-pinene obtained by pooling distillation cuts from the J&K Turpentine, having composition :

a-pinene 13-pinene

19.8% 80.2%

Gas liquid chromatography-Gas liquid chromato­graphy was used for getting composition of feed and product samples.

Vapour Pressure Data-Vapour pressure data of various components was obtained from (i) Chemical Engineers Hand Book, 6th. edition by R.H. Perry and C.H. Chilton, Me Graw Hill., Kogakusha Ltd., (ii) Jordan Earl T, Vapour pressure of organic compounds, Interscience Publishers Inc., New York (1954), Fig. 2.

Column evaluation-The column was first operated by charging 100 L of binary mixture in the reboiler of the column containing

a-pinene 13-pinene

19.8% 80.2%

The column was operated at 100 mm Hg absolute at the top of the column. The boiling points of the components at 100 mm Hg are as under.

a-pinene 13-pinene

The column was operated to find (i) the number of plates present in it at total reflux (ii) HETP of the packing (iii) its boil-up rate at flooding (iv) pressure drop across the column height (v) column hold-up (vi) working boil-up rate.

The column was operated at 100 mm Hg at the top. The column flooding rate at!:! P of 100 mm Hg was found to be 145.2 L/h. Its working boil up rate at 15+ 1 mm Hg was found to be 108.0 L/h. This corresponds to about 74% of the flooding rate. The GLC of the samples drawn at working boil rate were as

262

Component

a-pinene P-pinene

Feed % 19.8 80.2

T 1050

t 1200

1

INDIAN J. CHEM. TECHNOL., JULY 200 I

r;=.==========U::::::tP--+ TO VACUUM P U M P

,coNDENSER

REFLux_9iVID.E.R

Fig.1. All S .S. Batch frac,ion ating column ,packed

with hyflux packing

PRODUCT COOLER

RECEIVERS

Fig. I-AII S S Batch fractionating column, packed wth hyflux packing

Top product % 95.8 4.2

Bottom Product %

18.7 81.3

a at 100 mm Hg=1.1388 a at 115 mm Hg=1.1422 aavg=(1.1388 X 1.1422) 112=1.14

Now the average relative volatility aavg between a­pinene, ~-pinene was calculated as

Substituting the values in Fenske's equation for number of plates at total reflux

SAMA & BANDOPADHY A Y: SEPARATION OF a.-PINENE AND ~-PINENE FROM TURPENTINE OIL 263

1. o<.- Pinene

2. ~- Pinene

3. Camphene

4 113- Care ne

5. Myreene-300 6. Dipentene

m 200 7· Terpinolene

:I:

E E c

100 <ll 90 ... 80 ::J \1\ 70 ""' <ll 60 ... a. 50 ... ::J 40 0 a. .g 30

20

10~-------L~------~------~--------~------~ 2.0 2.25 2.5 2.75 3.0

- 1 - X103

T(°K)

Fig. 2-Vapour pressure of terpene hydrocarbons

Nm+ 1=log[(XuJXHK)o(XHiifXLK)s]/ log ( au<tHK)avg=35 .08

:. Nm=34.08 or 34 plates

Height equivalent to a theoretical plate (HETP) =4200/34=123.5 mm However taking the contribution of reboiler and condenser, the number of plates will be taken as 35 only.

Column hold up The column hold up His given by, H=Mo (1-Yc/Y,), where M0= Initial volume of mixture taken =100 L

f 0 =Volume fraction of high boiling component in the feed mixture =80.2%

Y1=Volume fraction of high boiling component under total reflux condition in the reboiler =81.3%

:. H=100 (1-0.802/0.813)=100 (1- 0.9865) =1.35 L

Summary of Results

No. of plates in the column

HETP

Column flooding rate

Working boil up rate

Column hold up

= 35

= 123.5 mm

= 145.2 L/h

= 108 L/h

= 1.35 L

264 INDIAN J. CHEM. TECHNOL., JULY 2001

Development of method for separation of a-pinene and {3-pinene from turpentine oil The method involves the use of following equations.

(i) Fenske equation for minimum number of plates

Nm=log [(XLK/XHK)o (XHK/XLK)s]/ log (au<JHK)avg ... (1)

(ii) Underwood equation for minimum reflux

Fore

aAXrA +as Xm + acXrc ...... = 1-q a A -e a 8 -e ac-e

.. . (2)

ForRm

ax ax ax A ctA + B dB + c ctc • • • • •• = Rm + 1

a A -e a 8 -e ac-e . . . (3)

where aLK > 8 > aHK

(iii) Distribution of Non-key components

By combining the Fenske equation with a component material balance, the fractionation of the non-key component can be predicted. The relationships to be used are the component material balance

fi= di +hi ... (4)

and the original form of the Fenske equation written in terms of an arbitrary component i and a reference component r.

(d/b)i=(a; avfCXravtm (d/b)r ... (5)

In determining the product composition values using the combination of these two equations, one takes advantage of whether a component is very volatile or not very volatile. The decision as to the degree of volatility is aided by defining, arbitrarily, a mean a value

a mean av = (a LKav + aHkav )/2 ... (6)

The following set of equations can be used to revise the estimate of the distillate and bottoms products:

Light component (di > bi) : a;av > a mean av

Reference component r is the heavy key (HK).

bi= fli[J +(d!b )HK ( a;avfaHKavtmJ · · · (7)

di=fi-bi ... (8)

Heavy components (hi> di): a;av<a mean av

Reference component r is the light key (LK)

di=fi/{ } +(b/d)LK ( aLkavfa; av)Nm}

bi=fi- di

... (9)

... (10)

In this way by using the revised product distri­butions, the appropriate specifications can be veJified. If necessary the distillate and bottom products are adjusted, and the equilibrium data/column operating conditions are verified. The resulting cyclic computa­tional procedure is repeated until the minimum number of theoretical stages for two trials agrees to be with in 5%.

Procedure-! 00 L of feed is assumed to be present in the reboiler of a batch column .

(i) Knowing the feed composition and the desired composition of the product, a certain cut (% of feed) of the distillate is assumed, the amount of components in the distillate cut is calculated.

(ii) The amount of components present in the distillate cut is deducted from those present in the feed. Composition of the balance left in the reboiler is determined.

(iii) The light and heavy key components are identified. au<JHK at the average column temperature is calculated.

(iv) Value of log [(XLK/XHK)o (XHK/XLK)sl is calcula­ted then using ( au<JHK)avg· Nm from Fig. 3 is determined.

(v) In a similar manner, NapiNm is determined from Fig. 4 and Napt is calculated.

(vi) The value of log [(XuJXHK)o (XHiviXLK)s. (XLK/XHK)F o.ssa] is calculated, and using CXavg· the ratio of Rapt !Rm from Fig. 5 is determined.

(vii) e from Fig. 6 is determined, using the ratio (XLK/XHK)F and a. If a is less than 1.5, it is recommended that Nm be calculated by the Fenske equation, and e be checked by Underwood Eqs 2 and 3, where q= 1 for feed at boiling point.

(viii) The value of a-8/a for each component is calculated and each component composition in the distillate is divided by its corresponding a­e/a. The resulting values are summed to get Rm+l.

(ix) Calculate Rm. Using Rapr!RffiJ determined from Fig. 5, calculate Rapt·

SAMA & BANDOPADHY A Y: SEPARATION OF a-PINENE AND ~-PINENE FROM TURPENTINE OIL 265

:::E z C'l 0

Fig. 3-Fenske equation for minimum plates expressed in graph form.

;E z ...... c. 0

z

2.8

2.7

2.6

2.5

2.4

2.3

2.2

\ \\

\ \ ~ 1'-. -'!-'.;

\ y_i('o 1'-..~a~ '\.

i\. ~ ~ i\. 2.1

2.0 ~0 " "' ~ 1.9

1.8

1.7

1.6

~~ /~ ~ ~ ........ ,, ~ -.::..::._

1.5

1.4 , I

"" """ "'-........._ ...............

"-., ['-........., ..........____ ~

0,_ .............. ----~ ~ t---. r--- --t--'-

I I I I I I

2 3 4 5 6 7 a \og [I XLI~/ XHK )D ( XHK/XLK)8]

Fig. 4---Relation between optimum-to-minimum ratio and Fenske separation factor of O:avg·

(x) Now knowing NITIJ Nap~o RITIJ Rapt and the number of plates present in the column N, the value of R may be calculated from the Gilliland graph (Fig. 7). Thus knowing N&R, the existing column can be utilized for the purpose or a suitable new column can be designed.

Restrictions for getting most accurate results :

i) ii)

"a" should be accurate. "a" should be ~ 1.5.

iii) Recovery of keys 95% or better i.e. the ratio of light and heavy keys in the distillate should be ~95:5. However close approximations are obtainable outside these specified limits.

Fig. 5--0ptimum-minimum reflux ratio relationship to the column's feed, distillate, and bottoms composition.

iv) For recoveries at 99% or better, "a" could be as low as 1.1.

v) The feed should be saturated liquid at its boiling point.

Deviations (i) In case the calculated values. of NITIJ Nap11 RITIJ Rapt

are small compared to the number of plates available in the column to be used, a bigger cut of same composition or a cut of higher purity is possible. So if desired a bigger cut or a cut of higher purity may be chosen and calculations repeated for new values of NITIJ Nap11 RITIJ Rapt·

(ii) If the values of NITIJ Nap11 RITIJ Rap~> obtained are high for economic operation, a smaller cut of the distillate is assumed.

(iii) Calculations are repeated with cuts of different sizes or purity till it suits the plates available in the column. Otherwise values of Napt may be found matching Rapt from the graph (Fig.7) or vice versa to suit the column.

(iv) Sometimes the size of the cut chosen or its purity expected is too high. In that case the values of Abscissa calculated for the graphs (Figs 3 to 6) will not fit in the graphs to get values of NITIJ Nap11 RITIJ Rapt· This shows the case is impractical for the size or purity of the cut chosen. In such a case more suitable cut size or purity is chosen so that the values of Abscissa fit in the graphs.

(v) Sometime if two cuts A&B of some composition are obtained at Reflux ratios R1 & R2 respec-

266 INDIAN J. CHEM. TECHNOL., JULY 2001

tively then it is possible to have a single combined cut (A+B) at a reflux ratio R3, such that the time taken for the cuts A&B at R 1 and R2

is same as that of (A+B) at R3.

Second cut (i) After taking the first cut in high purity, the light

and heavy key components present in the balance left in the reboiler are identified.

(ii) A cut is chosen, taking most of the lighters and the light key component, including a small amount of

2.6 r----r---r---r--.----.----,

1.0~~~~~~~ 0 0.5 1.0 1.5 2.0 2.5 3.0

( X.LK /xHK )F

Fig. 6--Underwood's 8 vs. key ratios in feed .

1·0

t 0.8

E - 0.6 '\ + c N 0-4 +

"r-... ""'-c - 0-2 .-

+ c -

"-... ........

............ ..........

b..

0.8 0.6

0.4

0.2

0.1 0.08 0.06

0.04

0.02

heavy key component. The light and heavy key component should be present in the ratio of ~95:5 .

The balance left after the cut should also have a small amount of light key component. The light and heavy key components in the balance should be present in the ratio~ 5:95. Percent composition of the cut is determined.

(iii) The quantities of the components present in the volume of the distillate cut chosen are worked out. These are subtract from their quantities present in the reboiler. Percent composition of the three streams is determined, i.e. new feed, distillate cut and balance in the reboiler after second cut. The distribution of non-key components is checked by the Fenskes method. NITIJ Nap~> RITIJ Rapt• is determined by the method developed.

(iv) In case at any stage the choice of light and heavy key components or the cut volume or its composition goes wrong, calculations will show that the method does not apply. In such cases the values calculated for Figs 3 to 6 may not fit in the graphs. Then, the fault is checked, rectified and calculations are repeated.

(v) Finally knowing the number of plates N available in the column, the corresponding value of R is determined from the Gilliland Graph (Fig.7).

Third cut--All the steps carried out for the second cut are repeated. In this way the whole feed can be worked out for all the cuts.

- ----- ........,

[', 1\

\

o o.2 o.4 0.5 o.a 1.0 o.o1 o.o2 o. 04 0.1 02 0.40-61-0 R Rm R+1

Fig. 7-Relation between reflux ratio and number of plates.

SAMA & BANDOPADHYAY: SEPARATION OF a-PINENE AND ~-PINENE FROM TURPENTINE OIL 267

Results and Discussion Problem

A sample of turpentine oil to be fractionated in the above column, is having the composition-a-pinene 32.6%, B-pinene 9.4%, t13-carene 58.0% (Table 2).

It is desired to have maximum distillate at reasonable reflux ratio, having average composition­a-pinene 95%, B-pinene 5%.

Solution Calculations by applying the new method developed Case-I

Assuming a cut of 20% of the distillate is obtained, then the amounts of a-, B-pinenes recovered will be

a-pinene 20 x 0.95=19.0 B-pinene 20 x 0.05=1.0

Balance left in the reboiler a-pinene 32.6-19.0= B-pinene 9.4-1.0= L13 -carene 58.0-0.0=

13.6 8.4 58.0 80.0

17.0% 10.5% 72.5% 100.0

Stream compositions are given in Table 1.

Temperatures : t 10p=90.1 °C; tbo1=93.1 °C; tavg=91.6°C Relative Volatilities: aLK=l.14, aHK=l.OO, aHK+I=0.69 Substitution in Eq. (1) gives Nm=log[95/5xl 0.5117 .0]/ log 1.14=1.0695/0.0569=18.79 From Fig. 4, NopiNm=l.79, N0p1=18.79xl.79=33.63

Now log [(XLK/XHK)o (XHK/XLK)s (XL~ XHK)o.ssaF] =log [(11.7353 X (32.6/9.4 )o.ssx1.1 4] =log 25.59=1.408 From Fig.5, Rop/Rm=l.436 Finding 8 from Underwood equation, by substitution in Eq. (2) (1.14 X 0.326)/(1-14-8)+(1.0 X 0.094)/ (1.0-8) +(0.69 X 0.58)/(0.69-8)=0, 8=1.038

To find Rm

Component Distillate% a a-8'a

a-pinene 95 1.14 0.0895 13-pinene 5 1.00 -0.038

axo~a-e

10.618 -1.316 9.302

:. Rm+ 1=9.3, Rm=8.3, Ropt=8.3 x 1.436=11.92 From Fig. 7 [(n+ 1}-(n+ l)m]/n+2=[36-19.79]/37=0.438 R-Rrr/R+ 1=0.22, R-8.3=0.220 R + 0.22, 0.780 R=8.520, R=10.92

Component

a-pinene (LK) 13-pinene (HK) t-3-carene (HK+I)

Table !-stream compositions

Feed Distillate % %

32.6 95 .0 9.4 5.0

58.0 0.0

Bottoms %

17.0 10.5 72.5

Table 2-New composition of Streams

Component Feed Distillate Bottoms % % %

a-pinene 32.6 95.00 17.00

13-pinene 9.4 4.97 10.51

t-3-carene 58.0 0.03 72.49

Since the distillate contains negligible amount of t-3-carene, it has not been considered in the calculations

Table 3-stream compositions

Component Feed %

a-pinene (LK) 32.6

13-pinene (HK) 9.4

t-3-carene (HK.., 1) 58.0

Summary Calculated

Nm =18.79 Nopt=33.63

Rm=8.3 Ropt=ll.92

100

Distillate %

95.0

5.0

0.0

100

Designed

Nm=18.79 N=35 Rm=8.3

Ropt=10.92

Bottoms %

11.80

10.87

77.33

100

Checking the distribution of Non-key component L13 -carene present in the distillate by the method given above.

Here Nm=18.79 a mean=(l.l4+ 1.0) /2=1.07 lXj <a mean lXj l13

- carene =0.69 f, t13 -carene =58 mol. d; l13-carene=58/[1+(17/95) (1.14/0.69) 1879

]

=0.0259===0.03 b; t13 carene=f, t13

- carene-d; t13 carene=58-0.03 = 57.97

New compositions of streams are given in Table 2.

Case-II Assuming a cut of 25% of the distillate is obtained,

then the amounts of a-, B-pinene recovered will be a-pinene 25x0.95 = 23.75 B-pinene 25x0.05 = 1.25

268 INDIAN J. CHEM. TECHNOL., JULY 2001

Balance left in the reboiler a-pinene 32.6-23.75 = 8.85 11.80% B-pinene 9.4-1.25= 8.15 10.87% .0.3-carene 58.0-0= 58.0 77.33%

75.00 100.0 Stream compositions are given in Table 3.

Temperatures : t1op=90.1 °C; tb01=93.1 °C; tavg=91.6°C Relative Volatilites : aLK=l.14, aHK=l.OO, aHK+I= 0.69 Substitution in Eq. (1) gives

Nm=log[95/5x10.87 /11.80]/ log 1.14=1.243110.0569=21.85

From Fig.4, Nop/Nm=l.77; N0p1=1.77x21.85=38.67

Now log [(Xu<.IXHK)o (XHdXLK)s (XLK/XHK) o.ssxa F] =log [(17.5025(32.6/9.4)0

·55

x l.l4]=1og 38.17066=1.582

From Fig. 5, Rop/Rm=1.44 Now from case I, 8= 1.038 Rm=8.3, :. Rop1=8.3xl.44=11.95

From Fig. 7, [(n+ 1)-(n+ 1)m]/n+2=[36-22.85]/37=0.3554 R-RdR+1=0.35, R-8.3=0.35 R + 0.35, or 0.65 R=8.65, :. R=l3.3

Summary Calculated Nm = 21.85 Nopt= 38.67

Rm = 8.3 Ropt=11.956

Designed Nm=2l.85

N=35 Rm=8.3 R=l3.3

Here again the Non-key component .0.3-carene present in the distillate comes to 0.008% by calculation, which is negligible.

Case-III

Assuming that a cut of 30% of the distillate is obtained, the amounts of a-, B-pinenes recovered will

Table 4-Stream compositions

Component Feed Distillate Bottoms % % %

a-pinene(LK) 32.6 95.0 5.86

B-pinene(HK) 9.4 5.0 11.28

!'13-Carene(H~ 1 ) 58.0 0.0 82.86

100 100 100

Nm=log(95/5x11.28/5.86)/ log 1.14=1.5632/0.0569=27.47

From Fig.4, Nop/Nm=l.726, N0p1=27.47x l.726=47.41

=log [36.5734 X (2.181)055xl.l

4]=1og 79.766=1.90 From Fig. 5, R0p11R=1.45, :. Rop1=8.3xl.45=12.035 From Fig. 7, [(n+ 1)-(n+ l)mYn+2=[36-28.47]/37=0.2035 R-Rm!R+l=0.58, R-8.3=0.085 R + 0.58, or 0.42 R=8.88, R=21.14 Here the reflux ratio required for a column having 35 plates is 21.44 which is high for a commercial column. Hence it is impracticable (uneconomical).

Case-IV Assuming that 20% cut is taken as per Case-1 at R=11:1 and if it is desired to take another 5% cut, following calculations are made to find the parameters. Now balance left in the still after taking 1st cut of 20%

%

a-pinene, 13.6 17.0

B-pinene 8.4 10.5

.0.3-carene 58.0 72.5

80.00 100.0

be a & B pinenes recovered in 5% cut a-pinene B-pinene

30x0.95=28.5 30x0.05=1.5

Balance left in the reboiler (Table 4)

a-pinene 32.6-28.5 =4.1

B-pinene 9.4-1.5 =7.9

.0.3-Carene 58.0-0 =58.0

70.00

Stream compositions are given in Table 4.

5.86%

11.28%

82.86%

100.00

a-pinene 5x0.95=4.75 B-pinene 5x0.05=0.25

Balance left in the reboiler

a-pinene 13.6-4.75=

B-pinene 8.4-0.25 =

.0.3-carene 58.0-0 =

8.85 11.80%

8. 15 10.87%

58.0 77.33%

75.0 100.00

Stream compositions are given in Table 5.

SAMA & BANDOPADHYAY: SEPARATION OF a -PINENE AND ~-PINENE FROM TURPENTINE OIL 269

Table 5---Stream compositions

Component Feed Distillate Bottom % %

a-pinene (LK) 17.0 95.0

B-pinene (HK) 10.5 5.0

L'.3-carene (HI(.1) 72.5 0.0

100 100

aLK=1.14, aHK=1.00, aHK+I= 0.69 Nm=log(95/5x10.87111.80)/

%

11.80

10.87

77.33

100

log 1.14=log 17 .5025/0.0569=21.85

From Fig. 4 Nop/Nm=l.77, N0r1=1.77x21.85=38.67

Now log [(XLK/XHK)o (XHK /XLK)B (Xu/XHK)o.ssa F] =log [17.5025 X (17.0/10.5)0

'55

xl.l4]=1.3743

From Fig. 5, Rop/R=1.435

Finding 8 from Underwood equation, substitution gives (1.14 X 0.17)/(1.14-8) + (1.0 X 0.105)/ (1.(~8)

+ (0.69 X 0.725)/(0.69-8)=0 8=1.07, To find Rm (Table 6)

Rm=l4.76-1=13.76, Ropr=13.76x1.435=19.75

From Fig.7,

[(n+ 1)-(n+ l)mVn+2=[36-22.85]/37=0.3554 R-Rrr!R+l=0.35, R-13.76=0.35 R + 0.35, or R=21.7

Summary

Calculated

Nm = 21.85 Nopt= 38.67 Rm = 13.76 Ropl= 19.75

Designed

Nm=21.85 N=35 Rm=13.76 R=21.7

This shows that it require a reflux ratio of 21.7 to get a cut of another 5%

Comparison of time required for getting 25% cut at one reflux ratio or two cuts of 20% and 5% at different reflux ratios

From Case ll

25% cut at R3=13.3: 1

Working boil up rate of the column=l08.0 L/h

Product rate at R3=13.3:1=108114.3=7.55 L/h.

Time required for getting 25 L product=2517.55=3.31 h

Table 6

Component Distillate a a-8/a %

a-pinene 95

B-pinene 5

From Case I & N

20% cut at R1=10.92: 1

1.14 ' 0.0614

1.00 -0.07

Product rate at R1=108/11.92=9.06 L/h

Time required for getting 20 L product

=25/9.06= 2.21 h.

For another 5% cut at R2=21.7: 1

Product rate at R2=108/22.7=4.76 L/h

aXr:/a-8

15.47

-0.71

14.76

Time required for getting 5 L product=5/4.76=1.05 h.

Total time for 25 L=2.21 + 1.05=3.26 h

Since time difference is hardly=0.05 h or

0.05x100/3.31=1.51 %

It is negligible. It is, therefore, advisable to go for single cut of 25% at R3=13.3

Practical results The column was operated with 100 L of turpentine oil charged in the reboiler, having composition :

a-pinene B-pinene 1'!3 -carene

32.6% 9.4% 58.0%

The pressure difference of 15±1 mm Hg was maintained across the column, during operation. The column was operated so as to have product rate of 7.5±0.2 L/h which correspond to a reflux ratio of 13.8:1 to 13:1. The total cut of 25 L was found to have composition as under

a-pinene B-pinene 1'!3 -carene

94.7% 4.8% 0.5%

which corresponded well with the assumption.

Second operation If the distillation was to be continued with the aim

to recover both a- and -B-pinene having 1'!3- arene in the still with about 2% B-pinene and 1% a-pinene and the top product may also contain some 2% 1'!3 -carene. The probable composition of top and bottom products will be as shown in Table 7.

270 INDIAN J. CHEM. TECHNOL., JULY 2001

Table 7

Component Feed % Top % Bottom %

a-pinene(LK.t) 8.85 11.80 8.25 52.89 0.60 1.01

B-pinene(LK) 8. 15 10.87 7.00 44.87 1.15 1.94

."13 -carene(HK) 58.00 77.33 0.35 2.24 57.65 97.05

75.00 100.00 15.60 100.00 59.40 100.00

Table 8

a Component Feed %

1.49 a-pinene(LK-1 ) 8.85 11.80

1.45 B-pinene(LK) 8.15 10.87

1.00 ."13 -carene(HK) 58.00 77.33

75.00 100.00

Nm=log(44.87/2.24x97.05/1.94)/logl.45=3.0009/ 0.161368=18.6

Checking the distribution of Non-key components for Nm=18.6 <Xmean=(aLK + aHK)/2=(1.45+1.0)/2=1.225 For a-pinene : OU-pinene > £Xmean OU-pinene= 1. 49 J a-pinene=8.85 ba.pinene=8.85/[1 +(0.35/57 .65) ( 1.49/1.00) 18

·6

]

=0.7967=0.8 da-pinene=8.85-0.8=8.05 Hence the final composition of all the streams will be as shown in Table 8.

From Fig. 4, Nop/Nm=l.8, Nopt=18.6 xl.8=33.48 For Rop/Rm=0.55 x 1.45 Log [1002.08 x (10.87177.33)]=log 209.574=2.32 From Fig. 5, Rop/Rm=l.35, For R!Tb Substitution in Eq. (2) gives ( 1.49 X 0.118)/( 1.49-8) +(1.45 X 0.1087)/ ( 1.45-8) +(1.0 X 0.773)/(1-8)=0, 8=1.33

From Table 9 Rm+l=10.3, Rm=9.3, Ropt=9.3 xl.35=12.555=12.6

From Fig. 7, [(n+ 1)-(n+ l)mVn+2=36-19.6137=0.443 R-Rrr!R+l= 0.21, R-9.3=0.21R + 0.2, R=l2.04=12

Summary

Calculated

Nm= 18.6

Nopt= 33.48

Rm=9.3 Ropr= 12.6 •

Designed

Nm=l8.6

N=35 Rm=9.3

R=!2

Top % Bottom %

8.05 52.27 0.80 1.34

7.00 45.46 1.15 1.93

0.35 2.27 57.65 96.73

15.40 100.00 59.60 100.00

Table 9

Component % a a-8/a <XXo/a-8

a-pinene 52.27 1.49 0.1074 4.8669

B-pinene 45.46 1.45 0.0828 5.4903

."13-carene 2.27 1.00 -0.33 -0.0688

10.2884

The column was then further operated at R=12:1, with product rate at 8.3±0.2 L/h, 15.4 L of product was collected and composition found as under:

a-pinene B-pinene ~3-carene

51.6% 46.0% 2.4%

which matches with the assumed composition. Time for the second cut of 15.4 L =15.4/8.3 =1.86 h. The rest was taken out at full rate, leaving a residue of about 5 Lin the still, the time taken was about 0.75h. Total time of distillation

1st cut =3.31 h 2nd cut =1.86 Rest =0.75 Total =5 .92 h (::: 6 h)

Summary of theoretical and practical results Summary of theoretical and practical results is given in Table 10 and Table 11 Feed: 100 L of turpentine oil, Working boil up rate: 108 L/h.

Objective The main objective at this stage was to establish

the simple graphical method for finding the maximum cut of a-pinene of 95% purity, at a reasonable reflux ratio and have second cut so as to obtain maximum

SAMA & BANDOPADHYAY: SEPARATION OF a.-PINENE AND ~-PINENE FROM TURPENTINE OIL 271

Table 10--Composition%

Product Component Feed Ist cut 25% lind cut 15.4%

Theoretical Practical Theoretical Practical

a.-pinene 32.6 95 94.7 52.27 51.6

~-pinene 9.4 5 4.8 45.46 46.0

~3-carene 58.0 0.5 02.27 02.4

Table 11--Reflux ratio for getting 1st cut of 95% a.-pinene and 5% ~-pinene, and 2"d cut of ~-pinene

Case %of feed

20

II 25

III 30

IV

1st cut 20

lind cut 5

Total 25

2nd cut

of ~-pinene 15.4

amount of a,f:3-pinenes with minimum amount of !l3-

carene. The feed mixture was purposely chosen of 3 components to find the suitability of method in multicomponent batch fractionation.

Conclusion The method developed is a simple, quick and a

useful tool for designing batch columns for fractionating multicomponent mixtures. The design calculations can be made with the help of a pocket scientific calculator in a few minutes. The method developed is quite accurate with in the limitations of ~95% purity of the light key component w.r.t. heavy key component in the distillate.

The method can predict the possibility of getting cuts from a multicomponent mixture. It can also predict, in case the choice of light and heavy key components, or the volume of the cut or its composition is wrong. In such cases the values calculated for Figs 3-6 do not fit in the graphs. It also indicates that it is of no benefit if a bigger cut at reflux ratio R is split into two or more smaller cuts at reflux ratios Rh R2, ••• etc. respectively. The total time required for recovery of full cut will be the same.

The method developed here is supported by the practical results from the pilot scale column. The practical results obtained match very well with the theoretical. Further work is continuing to design

Reflux ratio

10.92

13.30

21.14

10.92

21.70

12.00

Time for collection Hrs

2.21

3.31

2.21

1.05

3.26

1.86

commercial columns based on this method and comparison of the practical results with the results envisaged from design consideration by the method developed.

Nomenclature B = bi D = di F

fi GLC =

H = HETP

HK = HK+l Lo/D = (L/D)min (L/D)opt LK LK-1 Mo = n

N

Bottoms products, L/h Mole fraction of i in bottoms Distillate product Uh Mole fraction of i in distillate FeedUh Mole fraction of i in feed Gas liquid chromatogram Any component. Column hold up in L Height equivalent to a theoretical plate, mm Heavy key component Heavier than heavy key Reflux ratio Minimum reflux ratio Optimum reflux ratio Light key Lighter than light key Initial volume of mixture taken in L No. of plates in the column Minimum number of theoretical stages Number of theoretical stages Minimum of theoretical stages Optimum number of theoretical stages Heat required to bring 1 g-mole of feed to its boiling point and to vaporize it, divided by molal latent he!lt of vaporization of feed.

272

R 1.2.1

R .. R opt

r

twp

tbot

tavg.

X

X fA.fB. fC

X dA.dB,dC

Yo

a aavg·

!Xrnean avg

CXjav

a A,B,C

e

Subscripts avg. B bot D F HK

LK LKIHK M,min opt. r

=

=

= =

=

=

INDIAN J. CHEM. TECHNOL., JULY 2001

Reflux ratios Minimum reflux ratio Optimum reflux ratio Ref. component, usually heavy key Temperature, °C Column top temperature °C Column bottom temperature °C Column average temperature °C Mole fraction of component in liquid. Mole fraction of A,B,C in feed Mole fraction of A,B,C in distillate Volume fraction of high boiling component in the feed mixture Volume fraction of high boiling component under total reflux condition in the reboi ler. Relative volatility Average relative volatility Mean of average relative volatility Average relative volatility of any component Relative volatility of A,B,C Underwood's parameter.

Average Bottom Bottom Distillate Feed Heavy Key component Any component Light key component Relative, LK to HK Minimum Optimum Ref. component, usually heavy key

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