Reactive Distillation Technology - Process Intensification for Green Chemistry - page 261

Table 9.2 Design parameters simulating the heat-integrated RDC. Reprinted from Kiss
# 2011, with permission from Elsevier.

Parameter Value Units/Remarks

Total number of theoretical stages 15 Reactive from 3 to 12

Column diameter 0.4 m

HETP 0.5 m

Valid phases VLL —

Volume liquid holdup per stage 18 l

Mass catalyst per stage 6.1 kg

kg/m 3

Catalyst bulk density 1050

Fatty acid conversion

> 99.99

%

Fatty acid feed, on stage 3 (liquid, at 145 C)

1167 kg/hour

Methanol feed, on stage 10 (liquid, at 65.4 C) 188 kg/hour

Reboiler duty 136 kW

Condenser duty 72 kW

Reflux ratio (mass ratio R/D) 0.10 kg/kg
Boil-up ratio (mass ratio V/B) 0.12 kg/kg
Production of biodiesel (FAME) 1250 kg/hour
RD column productivity 20.4 kg FAME / kg cat hour
Specific energy requirements 108.8 kW hour / ton ester
FAME, fatty acid methyl ester; HETP, height equivalent to a theoretical plate; VLL, Vapor-Liquid-Liquid.

Although, heat-integrated RD offers major advantages, such as reduced capital invest-
ment and operating costs, no catalyst-related waste streams and no soap formation. The
controllability of the process is just as important as the savings in capital and operating
costs. In processes based on reactive separations, feeding the reactants according to their
stoichiometric ratio is essential to achieving high product purity [48,57]. This constraint
must be fulfilled not only during normal operation but also during the transitory regimes
that arise due to planned production rate changes or unexpected disturbances. In spite of
the high degree of integration, the heat-integrated RD process can well be controlled - a
key result being an efficient control structure that can ensure the reactants ratio
required for the total conversion of fatty acids and the prevention of difficult separations
(for details, see [33]).
Figure 9.8 (bottom) depicts the dynamic simulation results [33]. The simulation starts
from the steady state. At time t

1 hour, the acid flow rate is increased by 10%, from
1168.0 kg/hour to 1284.4 kg/hour. Then at time t

¼

5 hours the acid flow rate is decreased to
1051.2 kg/hour, representing a 10% decrease with respect to the nominal value. The new
production rate is achieved in about 2 hours. The purity of FAME remains practically
constant throughout the dynamic regime, with the main impurity being methanol. Notably,
the acid concentration stays below the 2000 ppm requirement of the ASTM D6751-08
standard (i.e. acid number

¼

0.50 mg KOH/g biodiesel).
It is worth noting that both design alternatives (base case and heat-integrated flow sheet)
presented here are suitable for a large range of fatty acids and alcohol feedstocks. These
processes based on RD have no additional separation steps and produce no waste salt
streams, as water is the only byproduct.

<