(604n) Economic Optimum Design and Control of the Monoisopropylamine Process

In this work, the steady state economic optimum design
and control of a process producing monoisopropyl amine (MIPA) via isopropyl
alcohol (IPA) amination is studied. The principal reactions are the MIPA
formation main reaction (IPA + NH₃ ® MIPA + H₂O)
and the di-isopropylamine (DIPA) formation side reaction (IPA + MIPA ®
DIPA
+ H₂O). The separation of MIPA, DIPA, H₂O, unreacted IPA
and NH₃, by the presence of IPA-H₂O and IPA-DIPA
homogeneous azeotropes is complicated.

A process flowsheet consisting of a high conversion packed
bed reactor, 2 distillation columns, a decanter and a stripper along with two
liquid recycles is shown to be capable of producing 99.5% pure MIPA. A high
conversion reactor must be used to ensure the reactor effluent has sufficiently
small amount of IPA. Post NH₃ recovery in the first column, the
decanter then exploits ternary liquid-liquid phase split to obtain nearly pure
water (some IPA) and DIPA (some water) phases. The water stream is stripped to
recover IPA and nearly pure water is discharged. The DIPA rich stream along
with the recovered IPA is recycled back to the reactor. The single pass reactor
conversion is a crucial design variable affecting the feasibility of product
separation as too much IPA (low conversion) causes the free reactor effluent
composition to be outside the liquid-liquid phase envelope. Even as the
economic optimum design attempts to minimize the reactor size to the extent
that liquid-liquid phase split just occurs in the decanter. Process operability
considerations dictate reactor size over design. With sufficient reactor
overdesign the hard operating constraint at maximum achievable throughput are
flooding limits for the two columns and not the decanter.

An effective plantwide regulatory control structure is
synthesized for the overdesigned reactor flowsheet and shown to provide robust
process regulations for large throughput changes. The case study provides an
interesting example of the ever-present conflict between economic design and
process operability.