Latest developments
in Advanced Process
Control in evaporator/
spray drying
AUTHORS:
Simon Mazier / Keith Smith
Introduction
Spray drying is used in the
manufacture of a wide range of highvalue items, from dairy products to
food flavourings to pharmaceutical
ingredients. It is a complex and
energy-intensive process, requiring
high levels of manual intervention from
well-trained, experienced operators.
To remove water safely – without
damaging critical product properties –
requires significant capital investment.
But once the plant is running, it can be
difficult to increase capacity as demand
grows.
In the sectors where they are
deployed, spray dryers are the most
energy-hungry components of water
removal. A typical multi-effect
evaporator will extract as much as ten
times the amount of water from the
product for the same amount of energy
as the dryer. Therefore, it is vital to
fully optimise evaporator performance
as the first stage of process
improvement. This has to be done
carefully as changes to evaporator
operation will have an immediate effect
on dryer performance and – hence –
product quality.
Perceptive Engineering has developed
a suite of software and engineering
tools to increase the yield, throughput
and efficiency of the entire drying
process, resulting in lower energy
consumption, more consistent
product quality and less waste. This
software-based upgrade is a lowcost, low risk alternative to capital
expansion, typically offering a return
on investment of 3-6 months.
Both the dryer and evaporator have
three key phases of operation: the
crucial start-up phase, when product
feed is first introduced to the process
unit; normal operation, when
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PECM Issue 18
disturbances such as tank changes
and shifts in ambient temperature
or humidity give rise to process and
quality upsets; unit shut-down, when
the feed product transitions from
concentrate back to water.
This article will describe one of the
solutions developed to address the first
critical stage: process start-up.
Operational Challenges – Start-Up
Each production unit is brought online
through a series of small, carefullycontrolled step-changes. Feed solids
transition quickly from water to
product. With the Evaporator, the
challenge is to avoid conditions that
increase the risk of fouling. An overaggressive start-up on the evaporator
will increase fouling very early in
the production run; this will shorten
overall run-time before cleaning is
required, reducing overall equipment
effectiveness. Early fouling also affects
heat transfer, meaning higher energy
consumption throughout the cycle, or
lower throughput.
In the dryer, aggressive starts
frequently lead to either scorched
product which fails quality tests, or
over-moist product which blocks the
dryer. Either way, production capacity
is severely c ompromised and waste or
re-work rates are high.
Slow, carefully-managed start-ups can
avoid these issues, but they extend
the time delay between batch or grade
changes. As manufacturers look to
increase agility and reduce operating
costs, and as new formulations come
along that are more challenging to dry,
these issues must be addressed more
elegantly.
Model predictive control (MPC) has
been used in the optimisation of spray
dryers and evaporators for more
than twenty years. Through a series
of response step tests and using
sophisticated modelling techniques,
MPC can determine in advance the
sequence of control moves necessary
to push the evaporator or dryer closer
to their constraints, then keep them
there – even when disturbances like
tank or batch changes are upsetting
the process. MPC is able to execute
multiple coordinated control moves,
many times a minute, continuously
during production. This constant
fine tuning of the process could not
be achieved in the PLC and would be
impossible for an operator to replicate,
which is why more manufacturers are
exploiting the technology to gain a
competitive edge.
Perceptive are renowned experts in
deploying MPC programmes for a wide
range of global clients. They have
been selected by Abbott Nutrition,
Danone and Wyeth as APC provider for
the manufacture of high-value infant
formula in North America, Europe,
Singapore and China.
Perceptive Engineering has now
perfected the techniques for taking
control of both the evaporator and
dryer much earlier in the production
cycle, by modelling – then optimising
– each unit as it moves through
the start-up phase and on to stable
operation. This expertise has been
captured in the latest release of
Perceptive’s award-winning software.
The system runs within a standard
Windows® environment and interfaces
to client systems using standard
protocols such as OPC or ODBC.