Multipurpose separation and purification facility.
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Date
2001
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Abstract
A waste acid stream is being produced by a local petrochemical company (SASOL) at a rate of 10 000 -12 000 tons per annum and contains approximately 44-mole % butyric acid, 20 % isobutyric
acid and 10 % valeric acid. Whilst this stream is currently being incinerated, SASOL has requested
an investigation into the possibility of separating and purifying butyric acid and isobutyric acid
from this waste acid stream.
The goal of this project was to determine a separation and purification route for butyric acid and
isobutyric acid from SASOL'S waste acid stream. In order to achieve this, vacuum distillation and
freeze crystallization were chosen for the recovery and purification of the acids respectively.
Vapour-liquid equilibrium data for key component pairs present in the waste acid stream (propionic
acid + butyric acid, isobutyric acid + butyric acid, butyric acid + isovaleric acid and butyric acid +
hexanoic acid) were experimentally determined in a dynamic VLE still. The measured VLE data
was successfully correlated us ing the gamma-phi approach. with the NRTL activity coefficient
model representing the liquid phase and the virial equation of state describing the vapour phase.
Using these equations. the VLE data obtained from the experimental work was then regressed to
provide interaction coefficients for the NRTL model. which were then used in the Hysys process
simulator to explore a range of design alternatives for distillation.
Hysys simulations showed that greater than 80 % butyric acid and isobutyric acid can be recovered
from the waste acid stream in a single distillation column containing 18 theoretical stages and an
optimum reflux ratio of 3.8. The simulation was performed at a pressure of 58kPa and a maximum
operating tempe rature of 150°C.
Batch distillation experiments performed in a batch rectification column at 250kPa recovered more
than 90% of both the butyric acid and isobutyric acid from a 450ml sample of the waste acid
stream. A subsequent batch experiment concentrated the recovered acids into a distillate containing
more than 95 % butyric acid and isobutyric acid combined.
To investigate freeze crystallization as a suitable operation for purifying butyric acid and isobutyric
acid a solid-liquid phase equilibrium curve for the system was generated us ing the Van Hoft
equation. The generated curve showed that butyric acid and isobutyric acid could be theoretically purified (>98%) by operating two crystallizers at -20°C and -55°C respectively. A simple freeze
crystallization experiment produced butyric acid with greater than 94% purity.
An economic feasibility study conducted on the process showed that separation and purification of
the acids by this process (distillation and crystallization) could create a business opportunity with
revenue of approximately R47 million per annum. Preliminary estimates for capital investment
amounted to approximately R5.4 million. for which the payback period was estimated at less than one year.
Description
Thesis (M.Sc.Eng.)-University of Natal, Durban, 2001.
Keywords
Butyric Acid., Valeric Acid., Separation (Technology), Crystallization., Acids--Purification--Distillation process., Vapour-liquid equilibrium., Theses--Chemical engineering.