Reactive Absorption Technology - Process Intensification for Green Chemistry - page 297

In the high-strength nitric acid process, a concentration of 98-99% is obtained
using extractive distillation. Weak nitric acid is introduced into a distillation column
with a dehydrating agent (e.g. sulfuric acid, 60%). Both acids are fed into the top of
the column, at atmospheric pressure, in counter-current to the ascending vapours. The
vapour leaving the column is concentrated nitric acid (99%), which is passed to a bleacher
and condenser. Concentrated nitric acid is obtained in the condenser. The byproducts (O 2
and NO) are introduced into an absorption column, which produces weak nitric acid.
11.5.3 Biodiesel and Fatty Esters Synthesis
Fatty esters are important specialty chemicals used mainly in cosmetics, pharmaceuticals,
cleaning products, food and, more recently, biofuels. Biodiesel is a biodegradable and
renewable alternative fuel with properties similar to petroleum diesel [35-37]. Unlike
petroleum diesel, which is a mixture of hydrocarbons, biodiesel consists of FAME that is
produced from green sources such as vegetable oils, animal fat or even waste cooking oils
from the food industry [38,39]. However, note that cheap waste oils have a substantial
amount of free fatty acids (FFAs), up to 100%.
Fatty esters are currently produced by acid/base-catalysed trans-esterification with
methanol or ethanol [40], followed by several neutralization and product purification
steps [41]. Nevertheless, all conventional methods suffer from problems related to the use
of liquid catalysts, leading to severe economic and environmental penalties. At present, the
most common biodiesel technologies employ homogeneous catalysts, in batch or continu-
ous processes, where both reaction and separation steps can create bottlenecks. There are
several processes currently in use at the pilot or industrial scale: batch, continuous,
supercritical, enzymatic and two-step [9]. The recent literature is quite abundant in studies
on integrated processes such as RD [4-6,18,42,43] and RA [7,9]. Several reactive
separation processes based on fatty acid esterification have been reported, aiming at
high performance and productivity and low energy requirements [2]:
RD: 191.2 kW.hour/ton biodiesel [6];
Dual RD: 166.8 kW.hour/ton biodiesel [18];
RA: 138.4 kW.hour/ton biodiesel [7];
Heat-integrated RD: 108.8 kW.hour/ton biodiesel [6];
Heat-integrated RA: 21.6 kW.hour/ton biodiesel [9].
Figure 11.7 shows a comparison of the energy requirements for a conventional two-step
process - acid and base catalysis [44] - versus recently reported reactive separation
processes [2,6,7,9,18]. It is worth noting here the difference in scale.
This section presents a novel energy-efficient RA process for biodiesel production that is
very easily controlled despite the high degree of integration. The integration of reaction
and separation into one unit, corroborated with the use of a heterogeneous catalyst, offers
major advantages, such as: reduced capital investment, low operating costs, simplified
downstream processing steps and zero catalyst-related waste streams and soap formation.
Rigorous process simulations have been used to design and control a plant producing
10 ktpy biodiesel (1250 kg/hour) by esterification of methanol with FFA, using sulfated
zirconia as a solid acid catalyst [7,9].
The conceptual design of the process is based on an RA column that integrates the
reaction and separation steps into one operating unit. The chemical equilibrium is shifted