BLEACH PLANT OPTIMIZATION UTILIZING NOVEL
MEASUREMENT TECHNOLOGIES COMPLEMENTED
WITH ADVANCED PROCESS CONTROL
O PAPEL vol. 79, num. 2, pp. 65 – 72 FEB 2018
Autors: Akhlesh Mathur1, Niclas Andersson2, Daniel B. Smith3, Renato Onofre4, Gabriel Morgan5
1 ASPAC Fiber Segment Director, BTG Instruments, Singapore
2 BTG Instruments Research and Development Director, Sweden
3 VP Automation, Capstone Technology, Seattle, WA, USA
4 South America Manager, Capstone Technology, São Paulo, Brazil
5 South America Fiberline Manager, BTG Americas, São Paulo, Brazil
ABSTRACT
Unit operations in a fiberline are designed to selectively remove
lignin to achieve a pulp with the desired brightness, cleanliness
and strength. Pulp being a commodity, there is always a pressure
to reduce costs. The primary focus is generally to reduce wood costs
but efforts to reduce bleaching chemicals, the second biggest cost
contributor, cannot be ignored.
Conventional bleach plant process control is based on the lignin
content of the pulp fibers expressed as kappa number, combined with
brightness measurement in the bleaching stages. There’s compelling
evidence that carry-over consumes bleaching chemicals (Chlorine
dioxide, in short ClO2). In the absence of carry-over measurement,
mills compensate by applying a high operator bias to cover peak
carry-over demand, which results in higher bleaching costs. To
overcome this challenge, the pulp industry is currently transitioning
from conventional fiber kappa number measurement using a
traditional multipoint analyzer to a new total kappa measurement
using an inline Bleach Load Transmitter for ClO2 charge control. This
way, the impact of carry-over lignin is also included in the control
of chemical charge. This continuous bleach load signal minimizes
operator bias and delivers significant bleaching chemical savings.
Manipulation of multiple process variables is required to achieve
effective bleach plant control and it’s extremely challenging
to optimize each of the controlled variables to maintain the
process close to target. While it’s of utmost importance of have
the sensors functioning accurately and regulatory control loops
in place, it is practically impossible for operators to manually
optimize bleaching chemicals to achieve final brightness at
minimal cost. To overcome this challenge, Multivariable Advanced
Control System (MACS), a proven advanced control platform,
has the potential to deliver large savings to mills. MACS uses
dynamic process models to account for the effect of bleach load
disturbances on downstream kappa number and brightness, and
manipulates the ClO2 dose to compensate for these disturbances.
MACS corrects for unmeasured disturbances via feedback control,
and accounts for varying process delays and non-linear bleaching
curves via real-time model adaptation. MACS also optimizes the
bleach load applied at each stage to minimize bleaching cost for
a given final brightness.
This paper highlights the advantages of a synergistic approach to
optimizing bleach plants by utilizing proven, novel and differentiated
measurement technologies with advanced process control strategies.
Some case histories taking this approach are also included.
Keywords: Kappa number, Lignin, Bleach Load Transmitter, Carryover
and Multivariable Advanced Control System
Corresponding author: Gabriel Silva Morgan da. BTG Americas. Rua Nicola Rollo, 151, apt 254-A, São Paulo, SP. CEP:05726-140. Brazil.
Phone: +55 11 99312 8898. E-mail: [email protected]
