significant number of possible applications. As already mentioned, the countercurrent movement is promoted by sequentially switching the inlet and outlet valves of
interconnected columns in the direction of the fluid flow, keeping the solid fixed. The
inlets and outlets are switched one column downstream after each switching period,
t s . Considering the closed-loop SMB approach and according to the position of the
columns relative to the nodes, the process can be organized into four sections. The
flow rates are different in each section, and each section plays a different role in the
operation. A binary mixture consisting of a less retained and a more retained species,
A and B, respectively, is considered. If one considers a 4-2-1-1 configuration,
section I, between the desorbent inlet and the extract ports, consists then of four
columns. Section II, between the extract outlet and the feed inlet, consists of two
columns; and section III, between the feed inlet and the raffinate outlet, is comprised
of one column. At last, section IV, between the raffinate outlet and the recycle port, is
also constituted by one column. Section II and section III form the separation zone.
Section I and section IV work as regeneration zones: the desorbent is used to
regenerate the adsorbent by desorption of component B in section I, and component
A must be adsorbed in section IV to regenerate the desorbent. Component B is
recovered in the extract outlet stream, while component A is recovered in the
raffinate outlet stream. The SMB is designed to operate under a cyclic steady state
(CSS). The cyclic steady state is generally reached after a few cycles. There are two
main approaches to model the SMB operations: the approach of the true moving bed
(TMB) as an SMB equivalent and the direct approach of the SMB. The second
strategy was considered to model and simulate the SMB process in all studies
reported in the literature for the separation of light olefins/paraffins separation by
SMB. This approach implies the inclusion of the switching of the input/output
streams at the boundary conditions of each adsorption column. For the case study
considered, the authors modeled the complex SMB unit considering the
interconnected eight adsorption column models that correspond to the eight single
SMB columns. To complete the SMB model, a manifold interconnection model was
implemented between columns, also considering the cyclic switching operation. A
third “model” was built to link all the individual parts, to represent the physical SMB
unit, with the eight single adsorption columns and the eight manifolds.
The desorbent node is referred to the desorbent inlet point, between zones IV and
I; the node of the extract is the point of withdrawal of the extract stream between
zones I and II; the feed node is the feed supply point of the unit between zones II and
III; and the node of the raffinate is the node located between zones III and IV,
through which the raffinate stream is withdrawn. The balances to the nodes used in
the SMB simulation can be found in the literature [9].
The cyclic port switching is performed, to represent the switching of the eight
single columns simultaneously. So, the feed, desorbent, raffinate, and extract streams
are switched one column downstream after each switching time. It is important to
mention that each column has a specific function according to the section in which it
is positioned and the initial conditions for each column change after each switching
time. From a mathematical point of view, the switching time is precisely the shifting
of the initial conditions of every single column. Each simulation started with the
Perspectives of Scaling Up the Use of Zeolites for Selective Separations from. . .
179
interconnected columns in the direction of the fluid flow, keeping the solid fixed. The
inlets and outlets are switched one column downstream after each switching period,
t s . Considering the closed-loop SMB approach and according to the position of the
columns relative to the nodes, the process can be organized into four sections. The
flow rates are different in each section, and each section plays a different role in the
operation. A binary mixture consisting of a less retained and a more retained species,
A and B, respectively, is considered. If one considers a 4-2-1-1 configuration,
section I, between the desorbent inlet and the extract ports, consists then of four
columns. Section II, between the extract outlet and the feed inlet, consists of two
columns; and section III, between the feed inlet and the raffinate outlet, is comprised
of one column. At last, section IV, between the raffinate outlet and the recycle port, is
also constituted by one column. Section II and section III form the separation zone.
Section I and section IV work as regeneration zones: the desorbent is used to
regenerate the adsorbent by desorption of component B in section I, and component
A must be adsorbed in section IV to regenerate the desorbent. Component B is
recovered in the extract outlet stream, while component A is recovered in the
raffinate outlet stream. The SMB is designed to operate under a cyclic steady state
(CSS). The cyclic steady state is generally reached after a few cycles. There are two
main approaches to model the SMB operations: the approach of the true moving bed
(TMB) as an SMB equivalent and the direct approach of the SMB. The second
strategy was considered to model and simulate the SMB process in all studies
reported in the literature for the separation of light olefins/paraffins separation by
SMB. This approach implies the inclusion of the switching of the input/output
streams at the boundary conditions of each adsorption column. For the case study
considered, the authors modeled the complex SMB unit considering the
interconnected eight adsorption column models that correspond to the eight single
SMB columns. To complete the SMB model, a manifold interconnection model was
implemented between columns, also considering the cyclic switching operation. A
third “model” was built to link all the individual parts, to represent the physical SMB
unit, with the eight single adsorption columns and the eight manifolds.
The desorbent node is referred to the desorbent inlet point, between zones IV and
I; the node of the extract is the point of withdrawal of the extract stream between
zones I and II; the feed node is the feed supply point of the unit between zones II and
III; and the node of the raffinate is the node located between zones III and IV,
through which the raffinate stream is withdrawn. The balances to the nodes used in
the SMB simulation can be found in the literature [9].
The cyclic port switching is performed, to represent the switching of the eight
single columns simultaneously. So, the feed, desorbent, raffinate, and extract streams
are switched one column downstream after each switching time. It is important to
mention that each column has a specific function according to the section in which it
is positioned and the initial conditions for each column change after each switching
time. From a mathematical point of view, the switching time is precisely the shifting
of the initial conditions of every single column. Each simulation started with the
Perspectives of Scaling Up the Use of Zeolites for Selective Separations from. . .
179