BATCH DISTILLA TIO N
CHAPTER 5 BATCH DISTILLATION
5.1 . l'
Batch Distillation ApparatusThere me variOlls types of batch distillation column configurations. Figure 5-2 shows the variOllS types of column configurations that are most commonly used.
The simplest form of a batch still (Figure 5-2a) consis\.') of a heated \'essel (pOt or reboiler) and a condenser. No trays or packing are used; therefore these stills do not pro\'idc separation to <lny great extent. For;J more detailed discussion on these types of columns the readl!r is rderred to the text of Scadel' & Henley 11998).
By nnalogy with continuous distillation columns, the conventional batch distillation column is known as a "batch rectifier" (Figure 5-2b). This column consists of a h~nted vessel (pot or reboiler), a condenser and a tray or packed column (rectifying column). The rectifying column concentrates the more volatile components in the vapol'. The feed mixture is introduced ill the bottom and the desired product is removed through the top of the column (distillnte).
In contrast to a "batch rectifier", an invcned batch distillntion column (Figure 5-2c) concentrates the less \'olatilc components in the liCJuid. This column is also known as "batch stripper", where the feed mixture is introduced at the top of the column and the desired product is removed from the I'cboiler (bOHOIllS). The reader is referred to work by Bernot et al (1991) for a comprehensive rcview on this Iype of column.
The most recent de\,elopment in Ihe configuration of batch distil!;\tioll columns is the middle vessel b;\tch distillation column (Figun: 5-3d). This type of column was first proposed by D~vyalith ct al 1198 I] and is extensively reviewed by Snfrit [1996] and most recently by Barlo & Bottean [1997].
In this type of column the feed is split between the condenser and the reboiler with the more volatile product., beillg removed as distillate and the less volatile products being removed as bottoms si multaneously.
CHAPTER 5
L
y
(0)
(c)
Notes
X-Iiqllid flow y- vapour flo\\' L-product flow
L
(h)
y x
~
YCC '" d
x(d)
BATCH DISTILLATION
L
L
Figure 5-2: D:1tch distillntion - (:1) simple distillntion, (b) batch reclifier, (c) batch stripper, (d) middle vessel column.
83
CHAPTERS DATCH D1STILLAT10:--l
T
Figure 5-3: Batch Distillation apparatus.
For tht! purposes of this project, a batch rectification still was used (Figure 5-3). This still was used in experimentation since the aim of batch distillation in this project was to recover butyric and isobmyric ncids, which nre the more volatile components. The still consists of a rectifying column (RC) with a condenser (C), reboiler (R) and distillate collecting equipment (D). The rectifying column is n vncuum jacketed glnss nppnratlls equipped with stainless steel packing providing an equivalent of four theoretical trays. The reboiler is electrically heated and n glycol-water mixture cools the condenser. A thermometer (T) is also located in the reboiler. The flow of liquid from the condenser can be split into (a) reflux to the column and (b) overhead distillate product, which fills
CHAPTER 5 BATCH DISTILLATION
5.1.2. Temperature and pressure measurement
A thennometer present in the reboiler mensured the temperature. The pressure was monitored with a Fischer pressure transducer. Tht: <lCCUnlCY in temperature and pressure measurements was not investigated in the batch distillation experiments. as the main aim of the experimcnts was to determine if separation of the aeids is possible.
5.1.3. Pressure control
For isobari..: operation the pressure was maintained at a constant set~point that wns (';olltrolled by a Fischer pressure controller which either vents to atlllosphere through needle valve V 2 or connects to the vacuum pUIllP through solenoid valve VI (These valves arc shown in Figure 5-1). By actuation of the solenoid valve. leading to a vacuum pllmp or atmosphere. thc pressure in the still was controlled. Control of the still pressure was estimated to be within ± 0.1 % of the sel~point pressure.
5.1.4. Composition analysis
The compositions of" the distillate samples were determined using a Varian. model 3300. gas chromatograph (GC). The column used was a 30·m Illcgabore capillary column of O.5J~l1lm
diameter with 007~FFAP on fused silica. The name ionisation detector was used.
CHAPTERS BATCH DiSTILLATION
5.2 Experimental Procedure
The npparatus was first cleaned and tested for leaks in a similar procedure described in Chapter 3.
Once this was successfully completed the actual batch distillation experiment was conducted.
5.2.1 Start-up
A sample of the waste Sll"l;:am as obtained from SASOL was first analysed by the gas chromatograph to determine the composition of the waste acid stream. The reboiler was then filled with 4501111 of this sample. The pressure controller was then switched on and a pressure of 250kPa was then set. At this point the vacuum pump was switched on, afler which the solenoid valve (VI in
Figur~ 5-1) was open. Thi:; decre'lsed the pressure towards the set-pain! pressure. The cold finger (Figure 5-1) wns then turned on to maintain a temperature of ISoC in the water bath. The cooling liquid pump (P"2 in f"igure 5-1) was also switched 011.
5.2.2 Heating of the fluid in the Reboiler
Once the system had reached the set-point pressure, the power input to the reboiler was switched all. The power input was then gradually increased to prevent the liquid in the reboiler from boiling too fast and therefore flooding the column. The liquid was brought to boil with the system under total reflux. This meant that the reflux valve Vs (Figure 5-3) had 10 be closed. A cycling procedure was then lIsed to .set the pattern for column operation. Operating procedures have been developed by Bogan [1937J, Smoker & Rose [1940J and Scrodth et al [1967J. For this project the method developed by Scrodth et <'11 [1967J was adoptcd a:; it is recommended for difficult industrial
~epariltions. The unit was then operated at total reflux \1I1til cquilibrium was established.
5.2.3 Sampling and analysis
CHAPTER 5 BATCH DISTILLATION
The distillate samples were then analysed using a gas chromatograph. The procedure for operating the gas chromatograph is discllssed in Chapter 3 and the operating conditions are shown in Table 5·1. This cycle was repeated until the desired recovery and separation was obtained.
Tablc 5-1: Gas chromatograph operating conditions Gas chromatograph
condition
Nitrogen carricr gas flow rate
[ml/min] 30
Oven temperature Profile
Initial temperature [oC] 130 I-Iold Time [min] 30 Temperalllre ramp [oC/min] 5
FinallemperalUre [oC] 180 I-Iold Time [min] 15
FID dctector
Tcmpernlure [oC) 200
Attenuation 8
Range 10
Hydrogen flowrale 1ml/min) 30 Air now mic Iml/min] 300 Injector temperature 220
87
CHAPTER 5 BATel1 DISTILLATION
5.3 Results and Discussion
Two separate batch distillation experiments were conducted. each with different objectives.
The first batch distillation (batch I) was an all em pt to recover more th,m 80% of the butyric and isobutyric acids from the waste acid stream. The opcrnting conditions for distillation of this bateh me shown in Table 5-2. The lowest pressure al which the system showed no nuctuations was 25kPa and was therefore set as the operating pressurc.
Table 5-2: Operating conditions fOl-diSlilllltion of first batch
Pressure 25 kPa
Average reboiler temperature 130C
Reboiler size 500 Illl
Temperature -cooling liquid 15 C
The sample from SASOL'S waste acid stream is a dnrk. viscous liquid. The experiment was run over six hours during which time a colorless. less viscous liquid was produced from the initial sample of Sasol's waste stream. However. at the cnd of the run some fon11 of blaek tar had lomled in the reboilcr. Systems containing materials that fon11 tars plug or foul continuous distillation columns thus rendering them inoperable. The I-Iysys simulations discussed in Chapter 4 could not have predicted this important factor. However. it is believed that these tars could have been fonned by the decomposition of cenain materials present in the waste acid stream in smaller compositions and can be eliminated by running a continuous distillation column at a lower pressure.
Due to tile Illany components present in the waste stream. it was 1101 possible to calibrate the gas chromatograph as discllssed in Chapter 3. Therefore. relative concentrations were used to delennine compositiolls. Results atlhe cnd of the first nm (batch I) arc shown in Table 5-3.