Simulation Results

DISTILLA TION

CHAPTER 4 DISTILLATION

4.5 Simulation Results

Results obtained by the converged Hysys simulation me shown in Table 4-3. The feed enters the column as !'libcooled liquid. The product compositions are given in Table 4-4. Figure 4-3 shows changes of vapour composition from stage to stage for the main components in the feed. Figure 4-4 shows the composition profile in the liquid phnse.

Table 4-3: Hysys simulation results

Average operating pressure 45

kPa)

Maximum operating temperature

tc)

146 Number of theoretical

staqes 18

Optimum reflux ratio 3.8

Stage efficiency 60%

Recovery of Butyric Acid 89%

Recovery of Isobutyric Acid 98%

Feed Flow-Waste acid stream (m"/h) 1.33 Distillate Flow -Recovered stream (m Ih) 0.82

Bottoms Flow- Heavy acids (m Ih) 0.51

Tahle 4-4: Feeu nnd prouuct flow rate for main components Volume Flows (m Ih)

Feed Distillate Bottoms

Propionic acid (n-CJ ) 0.02 0.02 0.00

Isobutyric acid (i-C4 ) 0.28 0.27 0.01

Butyric acid (n-C4 ) 0.51 0.45 0.06

Isovaleric acid (i-Cs) 0.14 0.02 0.13

Valeric acid (n-Cs) 0.14 0.00 0.14

CHAPTER 4 DISTILLATION

0.1,---:--=-~~---r==:==~

~ ""'"

______ i·04

0.6 - . -n-CS

---l+--i-CS

05 ^--)l<-n·C3

- - - n-C6

.. +. ·nO

/ '

/ .

0. '6:~=~~ ^. ;---.--._._. _ 0 ^_~ /~ · sJ

o

Condeo nser ²

•

⁶

,

'0

" ^"

^{16 18 20}

Tray location Rcboiler

Figure 4-3: Vapour composition profile of distillation column

Q8

Q7

Q6

~

^- .- n { l ;

c Q5

0

U Jg Q4

~

"0 E 03

~

02 0.'

0---.:. F

• ^{• • '*} *

. -- ^{'" ... .} --. - . - ^---

_ 0 - ' ____

^{,r.- - - - - _ _ .}

$: -p- q -·y· ..

·;r: .. ·,..· .. .. · .. ·i· .. ·i· .. ·

0 2 ⁴ 6 8 W • M • • ~

Condenser Traylocaticn Rcboilcr

-

- - - -

Figure 4-4: Liquid composition profile of distillation column

73

CHAPTER 4 DISTILLATION

4 .6 Analysis of Simulation Results

The profiles obtained in Figures 4·3 nnd 4-4 nre best interpreted by the volatilities of the vanous species. Average relative volntilities are as follows:

Table 4-5: Volatility of main components Component Volatility (relative to n-C4 )

n-e₃ 1.9

i-C₄ 1.3

n-C4 1.0

i-Cs 0.7

n-C:; 0.5

n-Cs 0.3

n·C7 0.2

The sl.!par:nion is being achieved primarily between butyric acid and isovaleric acid, since, as shown in Tuble 4-4. most of the butyric acid and almost all of the more volatile components appear in the distillate. while Illost of the isovaleric acid and almost all of the less volatile componems appear in the bottoms. Butyric acid and isovaleric acid arc therefore called the key components. the more volatile (butyric acid) being the light key and isovaleric being the heavy key. These components appear in both the distillate and bottoms. while the other components (called non keys) arc present almOSI exclusively in either the bottoms or distillate. Propionic acid and isobutyric acid are called light non keys since they are more volatile than the light key. while valeric acid. hexanoic ucid and hcptanoic acid are considered heavy non keys as they are less volatile than the heavy key component.

Considering Figures 4-3 and 4-4, it is apparent that all components are present in their significant amounts 0.11 the feed stage. This is logical since all components are present in the feed which is

CHAPTER 4 DISTILLATION

light non keys , propionic acid and isobutyric acid, are so volatile that they do nOt enter the liquid to any great extent and thus ar~ unable to now down the column to any substantial extent. Therefore.

thes!! components drop to very low concentrations a few stages below the feed.

Next, it should be noted that the light non keys, propionic acid and isobutyric acid, have relatively constalll mole fractions in the liquid itnd vapour above the fecd until a point some twO to threc stages from the top of the column. These two components appear more significalllly in the distillate product. The same applies for the heavy non keys, with valeric acid, hexanoic acid and heptnlloic acid being present in the bottom product at more significalll amaUrHs. King (1971] derived a simple equation far estimating the limiting male fraction of the nankey companellls in a zone where it has a constant mole fraClion below the feed. For the heavy non keys he proposed Equations 4-6 and 4-7 for limiting vapour and liquid compositions respectively.

B

V ·

.rI/x/<, .hm = _ _ _

--'--'--1

a U;IIINK - [

where: YIINKhn, = limiting vapor mole fraction of heavy non key

XU"".II = bottoms I iquid mole fraction of heavy non key

OI.""IN" = relative volatility of light key with respecl ¹⁰ heavy nonkcy

B. V' = bottoms liquid and vapor now respectively

-"IINK.hm

x^lI"";.I"" =

KIf"K

(4-6)

(4- 7)

wherL' XI!"'':''.I"" is the limiting liquid mole fraction of tbc heavy non key and KHS " is an equilibrium

\';duc ( = YIN"/xIlS").

For thc light Ilonkcys King [1971] propOSCi- Equ:llions 4-8 and 4-9 for limiting liquid and vapor compositions respectively.

\' ( D J

• lfiA".D

L )

ex

^{I.m.: I m: - I}

, -

• /."./<; .1"" (4-8)

where: XL.NK.I"" = limiting liquid mole fraction or"heavy nonkey

YLNK.U= distillate vapor mole fraction of heavy non key

75

CHAPTER 4

ClI.NKlHI( = relative volatility of light non key key with respect to heavy key D, L = Distillate and reOux now respectively

.vu",; .l"" ::= x u "'; .I,," X K u;

DISTILLATION

(4-9)

where YtNK.h", is the limiting vapor mole fraction of the light nonkey and KLI( is an equilibrium value ( = yu;lxu.J. Table 4-6 below shows values obtained using equations propose? by King [1971} and those from the Hysys simulation. C:lIculated values compare reasonably with those from profiles in Figures 4-3 and 4-4.

T:lblc 4-6: Limiting compositions for nonkc)' components.

Limiting liquid composition Limiting vapour composition

Light Keys a b a b

Propionic acid (n-C_{3 )} 0.012 0.090 0.027 0.02

Isobutyric acid (i·C4 ) 0.243 0.194 0.263 0.23

Heavy Keys

Valeric acid (n-Cs) 0.056 0.050 0.048 0.04

Hexanoic acid (n-C_{6 )} 0.032 0.025 0.022 0.02

Heptanoic acid (n-C7 ) 0.029 0.022 0.009 0.001

Notes

a denotes values calclllated by King's [1971] equations.

b denotes ,'alues estimated from profiles in Figures 4-3 and 4-4.

The mole fraction curves for the two keys. butyric acid and valeric acid. in Figures 4-3 and 4-4 may be understood in light of the nbove discussion of the non key profiles. The keys adjust in mole fmction so as to accommodate fractionation against the non keys as well liS against one another.

Therefore, the mole fraction of butyric acid tends to increase upward. and the mole fraction of iSQ"nleric acid tend:-. to increase downward in the column as ¹¹ renection of the frnctionation between the two keys. At the very bottom

or

^{the column the composition of both the keys tend to}

stnrt decreasing downward. This is the result of fractionation of the keys with the heavy nonkeys.

CHAPTER 4 DISTILLATION

.70 ,---,

.60

u .so

•

,

" "

^E

•

^~ "0

~

130

.20 >---__ - - - -______________________ - - - -__ - - - -__ - - - -__ - - - -- -__ ^----~

o

Condonser

2

,

6

,

10

Stages

Figure 4-5: Temperature profile for distillation column

12

"

^{.6 Reboiler}

"

Composition prorile plots often sho\V the buildup of components in the column, forced there by the heavier or lighter components. These profiles do not sho\V the relative separmion bet\Veen the key components and therefore x-y diagrams are better suited. Hengstebeck (196 I

J

suggested analyzing the performance of a Illulticomponent distillation in terms of an equivalent binary distillation based upon the keys alone. This procedure has the feature of providing a familinr graphicnl representation of the distillntion process which assists in understanding through visualiz~ltion.

I'knp.lcbcck·s method suggests trc~uing a multicomponent distillation as a binary involving the k!.!ys if the flows and compositiom are based on the two keys alone. Therefore the composition of the light key ^(y' ".C-I. X· n.C-I) is related to the actual composition and are calculated using Equations 4- 9 and 4-10 respectively.

(4-9)

(4-10)

77

20

CHAPTER 4 DISTILLATION

Figure 4-6 shows the vapour liquid equilibrium curve for the equivalent binary system obtained

u~ing Hengstebeck's method. If the non keys do not affect the volalilit), of the key components, it is important thnt the curve obtained by this method compares favorably with the equilibrium curve obtained experimentally in Chapter 3.

0.9 0.8 0.7 . 0.6

't

^0.5

->

0.' 0.3 0.2 0.1 0

0 0.1 0.2 0.3 0 .. 0.5

x'n-C.

Figure 4-{,: "LE cur\'C obtaincd by Hcngstebcck's method

0.6 0.7 0.8

o Experimental VLEdata

0.9

As can be seen in Figure 4-6, thl!fc is good ngreemcnt between the curw produced by the simulator (shown by the bold line) and thC' experimental curve (denoted by circles). It !TIust be noted, how\:!ver. that the simulnlion in this instance was nm at an operating pressure of 14kPa for comp:lrison with experimentally measured VLE data, Jf the relative volatility of the key components were constant, there would not be a sudden shift in the CUf\'e as shown in Figure 4-6.

CHAPTER 4 DISTI LLATION

in the VLE clnt,} (Nelson et <11 [1983]). For this project. simulations with an incorrect representation of the VLE data of the key components drastically changed the simulation results. Therefore. the VLE data or the key components were experimentally detennined.

Ultilll<ltely. the silllul<ltion results show that butyric acid and isobutyric acid can be economically separated rrom the rest of the waste acid stream in a single distillation column. Since distillation is widely practiced and well documented. no experimental work was done to verify the simulation results and continuous distillation experiments should be considered at a pilot pl.ant stage. However.

batch work was undel1aken 10 verify the relative separation betwcen the key components. This is discusscd in Clmpler 5.

79

CHAPTER