Hydrocarbon Processing - August 2021 - 45

Process
Optimization
J. SAHA and S. CHAUDHURI, Bechtel India,
Gurugram, India
Aromatic complex: A case study for relief load
estimation of a heat integrated refinery unit
Refineries are moving from fuels to chemical production as
the demand for fuel derived from crude oil declines in the near
future. Conventional fuel-producing refineries are being targeted
to reconfigure to produce more chemicals. Integration with
an aromatic complex is one way to produce the chemicals for
an oil-to-chemicals complex and will likely see wide industrial
application in the coming years.
In an aromatic complex, aromatic compound producing
units are closely integrated with one another to increase the energy
efficiency of the entire complex. In the case of a common
utility failure like power or cooling water, when the relief load
is generated from the entire refinery, the calculation of the relief
load from an aromatic complex must be performed carefully
to avoid double counting of the relief load to optimize the flare
header sizing. This article discusses the various approaches of
calculating relief loads from an integrated aromatic complex and
outlines a way to calculate an optimized relief load.
System overview. The principal products from an aromatic
complex are mainly benzene, para-xylene and ortho-xylene-
sometimes, a fraction of toluene and C10
aromatics are also
produced. A simple block flow diagram of a modern integrated
aromatic complex (excluding a reformer) is shown in FIG. 1. Reformed
naphtha from the reformer unit is split in the reformate
splitter (part of Unit B in FIG. 1) as light and heavy reformate
product. Aromatic-rich components like benzene and toluene
in light reformate are separated in the solvent extraction block
(Unit D in FIG. 1) by extractive distillation; they are then further
separated as pure benzene and toluene in the benzene toluene
fractionation block (Unit C of FIG. 1). Heavy aromatic components
(C9
) and sometimes toluene are sent to the trans-alkylation
block (Unit E in FIG. 1) to recover more benzene and xylenes.
The heavy reformate from the reformate splitter is sent to
the xylene fractionation section of Unit B (FIG. 1)-from here,
the mixed xylene stream containing meta and para isomers is
sent to the paraxylene separation block (Unit A in FIG. 1) to
separate p-xylene as product. The meta isomer is isomerized in
Unit F (FIG. 1) to recover more para isomers from it.
Relief system design: Steady-state approach. In an aromatic
complex, the major contributors of relief loads are the
columns from various process units. Generally, the unbalanced
heat load (UBH) method is used to estimate the relief loads from
individual columns. Here, the UBH method has been progressively
applied in evaluating relief loads from individual columns
of various units, then on each process unit, and finally on the entire
aromatic complex. Unbalanced mass and energy or enthalpy
are calculated for a specified relief envelope, and the relief load
is evaluated from that using the latent heat of vaporization. Generally,
the top tray liquid composition of the column is used to
estimate the latent heat of vaporization. Since multiple columns
are involved in the present case study, latent heat of vaporization
from the largest contributing column to relief is used to derive
the relief load from unbalanced heat for the entire complex.
Case definition and methodology. In an energy integrated
aromatic complex, a number of heat integration networks
(HENs) exist among the units. This case study discusses the
relief generated from the columns associated with the HEN
among Units A-E, shown in FIG. 1. The detailed network diagram
and the associated relief devices are shown in FIG. 2. Each
unit inside the complex has multiple fractionating towers involved;
however, for simplicity, a representative column from
each unit is shown in the diagram.
In this network, mainly two streams from Unit A (paraxylene
separation block), the column overhead vapor (dark solid
blue line in FIG. 2) and side draw stream (ochre line in FIG. 2), are
used as the reboiler heating medium of other columns in other
Light reformate (C7)
Reformed
naphtha
Reformate
splitter
(part of Unit B)
Transalkylation
(Unit E)
C9
p-xylene
Light ends
Xylene
fractionation
(Unit B)
o-xylene, C7
FIG. 1. Block flow diagram of a typical aromatic complex.
Hydrocarbon Processing | AUGUST 2021 45
p-xylene
m-xylene
Paraxylene
separation
(Unit A)
Isomerisation
(Unit F)
Solvent
extraction
(Unit D)
Benzene
toluene
fractionation
(Unit C)
Raffinate
Benzene
Toluene
C10
+
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