recoveryoperations

32 F e b r u a r y 2 0 1 5 Pulp & Paper International (PPI)

By SÖREN BACK and MARK WILLIAMSON

A Metso whole-line measurement system that was instrumental in SCA Obbola’s causticizing plant startup includes a new reduction degree measurement

FROM SMELT TO WHITE LIQUOR – THE FINAL LINK

AUTOMATIC ON-LINE analyzers for control-

ling the kraft pulp mill causticizing operation

have been in operation for more than 15 years

now and their benefits have been well documented.

It’s established technology, as Metso counts more

than 160 causticizing line applications worldwide

with more than 600 sampling points. But as technol-

ogy progresses there is always a need and a challenge

to break new ground in measurement capabilities.

The new Metso Recovery Analyzer, named Metra,

does just that as it now completes the required chain

of quality control measurements of the causticizing

process from the smelt to white liquor and gives op-

erators the tools they need to stabilize and optimize

the entire process from the recovery boiler onward.

It’s the final link.

Now, with Metra, the first measurement point

has been moved as far back as possible to the smelt

dissolving tank where the reduction degree from the

recovery boiler is measured. This is accomplished

by a new titration module that measures sodium

sulfate, complementing the previous capability to

measure various alkalinity, sulfidity and causticizing

efficiency parameters in the rest of the line accord-

Green liquor sampling point before the slaker: from left: Kristina Jonsson, Annika Hedman and Thure Sandström.

recoveryoperations

33Pulp & Paper International (PPI) F e b r u a r y 2 0 1 5

ing to the established ABC titration methods. As well

as providing information for recovery boiler reduc-

tion management, this measurement also allows the

precise control of dissolving tank green liquor density

and TTA.

As with all new developments, the Metra analyz-

ers were first tested in host mills, those being two

kraft pulp mills in Finland. Those analyzers have now

been taken into regular production use and operators

follow reduction degree more frequently and make

adjustments to maintain it. The first full-production

unit was purchased by SCA’s Obbola, Sweden, kraft-

liner mill, where it has been performing well from day

one, helping the mill to optimize the operation of a

new causticizing plant since the startup in November

2013.

ROI POTENT IALWhat extra capability for control and process

optimization does this give operators and mill en-

gineers? Antti Kokkonen ,Metso’s product manager

for Metra, responds: “Since it does not react in the

causticizing process, the sulfide generated in the

boiler is the sulfide in the final white liquor. Therefore

higher reduction degree means that less white liquor

is needed to achieve the required alkali charge to the

digester. Also, the amount of dead load accumulation

in the recovery process can be decreased by improved

reduction efficiency and this will improve energy and

loading performance of the process equipment. This

also enables higher energy production in the recovery

boiler. In addition to that, there is lower lime demand

at a given production rate, and with higher Na2S con-

tent the theoretical maximum causticizing efficiency

is lower, which also decreases dead load if the mill is

achieving reasonable CE% target.”

He estimates that a possible return on investment

for a 1,000-tonne/day fiber line where reduction

degree was increased by 3% could add up to Euro 1

million/yr. These gains are due to debottlenecking

and production increases of over 1% in the causticiz-

ing plant, lime kiln, and evaporation plant and in

energy savings in these stages.

STARTUP “ONE OF THE BEST”While the other mills used the analyzer to im-

prove the existing processes, the Obbola mill had it its

own reasoning relating to the plant startup. “There

were a couple of reasons for us to consider an auto-

mated sampling and analyzing system,” says Thure

Sandström, manager of the recovery area. “As we were

investing in a completely new causticizing plant we

wanted to have a modern system improving work

safety, as there are always safety issues in manual

sampling, and more frequent process data to support

the process operators. An important parameter was

to have frequent data on the reduction degree.”

“Earlier, process operators took samples from

five points of the process and thereafter analyzed the

samples in a semi-automatic titration unit,” Annika

Hedman, laboratory engineer, adds. “We could see

that there were variations in the data obtained, varia-

tions that in many cases were due to who took the

samples and analyzed them, and not due to process

variations.”

The analyzer measures process parameters at five

points using a central titration and analyzer module.

The sampling points are green liquor after the recov-

ery boiler at the dissolving tank, green liquor before

the slaker, lime milk at the top of the slaker, white

liquor after the last causticizing vessel and white

liquor after the filtration. This final measurement is

critical for accurate digester charging. The samples

New module with optical detection for sodium sulfate

recoveryoperations

34 F e b r u a r y 2 0 1 5 Pulp & Paper International (PPI)

are automatically taken and fed to the central titra-

tion measurement unit. The unit runs through a

prioritized sequence of titrations each taking a few

minutes for the sample points in whole line. When

this sequence is completed the reduction degree

measurement is updated once per hour, meeting the

customer’s expectations.

Sandström explains, “We wanted an online

solution for measuring the reduction degree. Earlier

the sampling frequency of 6 to 8 hours was not good

enough as we needed reliable information quicker,

down to a level of every hour, or every second hour,

on the recovery boiler performance. This called for an

automated solution. After evaluating different auto-

mated analyzing methods of the reduction degree, we

felt most comfortable with the Metso Metra solution,

although it just had been introduced on the market.

“The start-up was one of the best I have ever

experienced,” Sandström continues. “We installed the

sampling and analyzing equipment during a produc-

tion shut down. A couple of weeks later (after the

shutdown) we took the system into operation and got

reliable measurement data from the very first day.”

STABLE ANALYS IS“We could immediately see that the stabil-

ity of the analysis results improved as we got rid of

the variations due to different ways of taking the

samples,” project leader Kristina Jonsson stresses. “It

is of course important where the sampling points are.

We managed to get the first sampling point immedi-

ately after the dissolving tank, which is as close to the

recovery boiler as you can get. This sampling point is

200 metres away from the analyzer unit, which means

that the sample stream is pumped outdoors in a

heated tube to the analyzer, where the actual sample

is taken for analysis.”

“The data achieved is displayed in our control

system showing reduction degree, TTA, causticizing

efficiency, sulfidity and EA, says Erik Henriksson, re-

covery boiler process operator. “If there is a deviation

we can make a comment in the system’s log-book so

that other shift teams can see what has happened and

what has been done to correct it. As we have seen that

the results from Metra system are very reliable, we are

now focussing on fine tuning target values as well as

limits for the reduction degree.”

“The system has proven to be very stable,” Hed-

man continues. “We add chemicals to the analyzer

unit once a week. The reduction degree is manually

checked once a month and we have found that to be

enough. The shift operators check the system once

every shift to make sure that there are e.g. no leakages

or any other visual problem.”

Metso’s Kokkonen adds,” High uptime- typically

more than 98% - for minimal maintenance is a core

concept of the Metra design. There are preventive

maintenance and troubleshooting information easily

available at the analyzer’s touch screen interface and,

more importantly, there is an intelligent self-diagnos-

tics feature built in to the analyzer. Our availability

guarantee is 98%. Furthermore, no calibration is

needed at any point.”

REL IABLE TOOL“With the new system we have learned more

about how the recovery process, and in particular

how the recovery boiler, performs,” Peter Olsson,

manager, recovery boiler operation, adds. “Now we

have a reliable tool for better process control and

process understanding. We see a number of oppor-

tunities to optimize the recovery process as focus can

be put on the recovery boiler and its performance. As

we get the reduction degree values much more fre-

quent, we can make sure that the recovery boiler runs

stable. It is early days to say exactly how the recovery

boiler performance will be optimized but with proven

accuracy of the analysis result we will step by step

optimise it further.”

“We noticed in the beginning that the increased

frequency of analysis results encouraged operators

to make quicker changes based on the latest result,”

Sandström recalls. “This is however dangerous as

if you start to change process parameters too often

the result might be that the process gets out of your

hands. We have therefore learnt not to make changes

based on one single result but on an accumulated

trend, which we get from the automated system.

Earlier we measured the reduction degree every four

hours and if the value wasn’t as expected we got a bit

nervous knowing that we had to wait another four

hours for the next analysis value. Sometimes we start-

ed to make changes believing that something could

be wrong and sometimes we did nothing hoping that

the value itself, and not the process, was wrong. Now,

however, as we get frequent reliable data we can wait

recoveryoperations

35Pulp & Paper International (PPI) F e b r u a r y 2 0 1 5

Metra measurement points at the smelt dissolving tank

In this chart, typical reduction degree variation due to dry solids firing rate is shown to operators. Data are from a Finnish kraft pulp mill.

recoveryoperations

36 F e b r u a r y 2 0 1 5 Pulp & Paper International (PPI)

To read more articles on Automation & IT, visit our Automation & IT Technology Channel at www.risi.com/technologychannels/automation

for a couple of reduction degree values until we know

if a process change has to be done or not.”

“An important reason for the investment was

work safety,” Jonsson stresses. “Risks connected to

manual sampling are now eliminated so the working

environment is in this respect safer, which is very

important to us. Changes in the recovery process are

slow, which brings stability but it also means that it

takes a long time to correct a downward trend in e.g.

reduction degree before it is back to normal. During

such a down period changes to counteract the effect

of a lower reduction degree have to be done in the di-

gesting plant, which increases the production costs as

well as causes quality problems. As frequent analysis

data now are available, downward trends are spotted

early and corrections can be done much quicker

making the recovery process more stable and hence

the white liquor quality more stable.”

EXCEEDED GOALSHas the mill achieved their goals with this invest-

ment? Sandström responds positively: “We have not

only achieved the goals, but also exceeded them. In all

fairness, this is not only due to the automatic system

but it is a combined effect of the new causticizing

plant and the automated analyzing system. The

whole white liquor production is more automated

nowadays and its progress is frequently followed by

the Metra analysis system. The process is followed in

real time and hence we have a better control of what

is happening.

“In the future we might add one or more sam-

pling points in the chemical recovery plant, or even

close the loop by letting the results from the automat-

ed measurement control the process. However, this is

no priority at this point as we are focusing on learning

more about the recovery boiler performance and how

the whole recovery process can be optimized further,”

he adds.

APPROACHES TO CONTROLThe way to manage the recovery boiler variables to

achieve the target reduction degree is now becoming

apparent. Operators at one mill in Finland first try to

set the char bed in a stable state and then by adjust-

ing the air feeds they try to improve reduction. The

reduction degree at this mill is 95-97% when on stable

operation. The recovery boiler is being controlled by

Metso optimization control.

While these clients are looking at ways to utilize

data from the analyzers to optimize reduction degree

in their individual mills Metso is putting together on-

line process response data that would help to formu-

late a general approach to control reduction degree.

Metso representatives have recently presented these

findings at the 2014 PAPTAC “PacWest” Conference in

Canada and at the 2014 APPITA Conference in New

Zealand. These papers look at the process responses

associated with char bed building, liquor firing tem-

perature and combustion air distribution. The results

are published in the conference proceedings.

To date, four analyzers with reduction degree

measurement are in the process of commissioning.

Sören Back is the owner of SB Kommunikation in Örnsköldsvik, Sweden. Mark Williamson is a journal-ist engineer based in Thornhill, ON,Canada PPI