Page 19 - Industrial Plants 2014

MPC at the water pre-treatment unit
The MPC solution implemented in the water
pretreatment unit has been seamlessly integrated in
the existing automation hardware configuration, as
presented in figure 6. The application is based on
dynamical process models taking into account the
relation between controlled variables (CV),
manipulated variables (MV) and disturbances (FF).
Those relations have been extrapolated by observing
the historical process data gathered from plant DCS
as well as by mean of data collection campaign
coupled with devoted step test when possible. Step
test were performed in strict cooperation with plant
operation and always keeping into account process
stability and targets. Evaluation of relations between
controlled variables, manipulated variables and energy efficiency. In details, the purpose of the Fig. 6 - HW
disturbances gave the possibility to deploy models APC application is to automatically moving the architecture
representing the core of MPC. manipulated variables (i.e. soda and steam / load
The solution applied is the ABB Optimize Predict & ratio) in order to:
IT
Control (P&C), a software package for multivariable • keep the pH near the required setpoint,
control based on state-space modeling technology. avoiding the precipitation of carbonates that
The tool communicates with the existing plant may affect the overall process also due to the
control system, the ABB Melody DCS, through the deterioration of equipment and maximizing the
use of an OPC connection thanks to the existing efficiency of the stripping column;
OPC Server DA. • keep top stripping column pressure below a
The project represents a second step of APC minimum value in order to ensure stable
systems implementation strategy related to the operation;
entire IGCC plant. • keep the value of the residual ammonia below
The first step featured APC solutions in some of the a maximum, minimizing the need of the low
key process units, such as SDA Unit, AGR Unit and pressure (LP) steam for stripping;
Gasifier Units [3]. • keep the pH for the upstream measurement in
A third step was executed in 2012, covering an range when this measurement is available.
MPC acting as global IGCC coordinator to control When this variable is active, the controller uses
power export control and also a controller dedicated it as an additional feed forward in its state
to gasifiers water management. space model to predict future changes in the
downstream pH following changes in the
upstream pH.
Advanced controller

confguration and operation Below the configuration of the controller is reported,
covering the main controlled variables (CV) and
The APC technology was essential to achieve an control mode, manipulated variables (MV) and type
effective pH control in the wastewater stripping target where applicable (table 1).
column, so to simultaneously comply with In order to preserve safe operation and stable and
regulation, optimize operation and increase process conditions, at the controller configuration

Variable Description Type Table 1 – Controller
Controlled variables configuration
Residual ammonia from stripping column max limit
Downstream pH (downstream of stripping column) setpoint with band
Top column pressure min limit
Upstream pH measurement min/max (when available)
Manipulated variables
Soda al 10% inlet
Ratio steam/load at stripping column minimize

IndustrIal Plants - May 2014
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