Program for North American Mobility in Higher Education

Introducing Process Integration for Environmental Control in Engineering

Curricula

Module 3: Environmental Challenges – Pulp & Paper

IndustryCaroline Gaudreault

Created at:École Polytechnique de Montréal &

Texas A&M University, 2003

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Tier III:Open-Ended Problem

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Tier III: Statement of Intent

Tier III: Statement of Intent

The purpose of this is to provide students with an open-ended problem which assimilates the concepts of minimum impact manufacturing including process integration and LCA .

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Problem Statement

You are an environmental engineer in a pulp and paper mill. The head office wants to enhance its competiveness by putting together a technology roadmap with the ultimate goal to be a minimum impact manufacturing mill.

Some information about the mill is given at the following page.

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Mill Description

Conventional pulping technology, ECF bleaching, drying, activated sludge plant

Debarking: dry Lime kiln: normal Lime kiln fuel: heavy fuel oil Lime kiln flue gas: high eff. ESP Bark boiler (HW bark):

Total efficiency 0.87 Fluidized bed boiler Electric power generation from excess heat in mill

condensation turbine Since no information is available concerning the

effluent treatment plant, its efficiency will be consider constant. As a consequence of that, from a relative point of view, the effluent ion loads can be considered proportional to the ones before the effluent treatment.

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Question 1

A few months ago the company as ordered a partial LCA study in order to have an idea about its life cycle environmental impacts. As a first step, your boss ask you to look at this study as well as at the mill simulation and make him your recommendations for environmental improvement. To do this look at unit process contribution to each impacts and perform sensibility analysis. Do not use any normalization or weighting. Without doing calculations, you can also use cost arguments. Also determine, by mass balances by how much fresh water can be theoretically reduced (by recycle).

System boundaries are defined in the LCA study and the main hypothesis are presented next pages.

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Functional Unit

All LCA results are presented relative to the functional unit. The functional unit has been defined as follow: The production of 1 admt of pulp.

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Chemical Production

Chemical production as been included into the system boundaries. Chemical are considered to be transported on an average distance of 100 km using 40 ton diesel trucks and empty trucks return to the supplier. For calculation purpose a weight of 1/10 of the transported chemicals has been assumed for the return of the truck.

No data was available for talc manufacturing. Therefore it has been excluded from the system boundaries. However, its transportation has been considered.

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Birch Growth and Harvesting

Birch growth and harvesting as been included in the boundaries. The wood is transported on an average of 100 km. The same assumptions as for chemicals apply.

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Others

By product have been located. A credit has been considered for the

generated energy (but only on the energy).

Pulp is transported on an average distance of 200 km to the customer (same assumptions as chemicals).

Industrial landfill is located at 5 km of the mill. 16 ton diesel trucks are used to transport the solid wastes, the return of the trucks is considered negligible.

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Necessary documents

LCA Base Case Process Simulation

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Question 2

Your boss is convinced that most of the competitive advantages that can be gain with environmental improvement are related with fresh water rereduction.

In this case, recycling the effluent water is the most obvious way to reduce fresh water consumption, but this can result in the build-up of non-process elements and so reduce process performance.

For this reason, he has also mandated a consulting company to perform a water pinch study subject to process constraints.

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Question 2 (Cont’d)

The consultant has first evaluated possibility of direct recycle because it does not implicate major capital costs. Major results are presented in the following table.

Water Consumption 23% reduction

Liquid Effluent Reduction of ion content of 2.3%

Gas Effluent Cl, K: 0.2% increaseNa: 6.8% increase

Energy produced 5% reduction (need more energy to pump)

Dust 13.4% increase

Solid wastes Neglictible difference

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Question 2 (Cont’d)

Using the LCA model, discuss if this represent a real environmental improvement. To compare results, normalize against the base case.

A panel of experts has determined that the importance of each impact category can be described by the weights in the following tables. Resources and emissions are weighted separatly.

What is the influence of the weights on the final decision.

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Question 2 (Cont’d)

Resource depletion:

Emissions:

Impact Weigth

Raw water consumption

0.83

Energy consumption

0.08

Virgin Fiber consumption

0.01

Other resources

0.08

Impact Weigth

Carcinogenic substances

0.70

Heavy metals

0.07

Acidification 0.01

Eutrophication

0.01

Summer smog

0.07

Winter smog 0.07

Solid Wastes 0.005

Global Warming

0.065

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Solution – Q1

The process simulation does not give a lot of insights on the environmental impacts of the process. However it is obvious that the bleaching plant consumes a lot on fresh water and rejects a lot in the environment. The following is the solution for potential water reduction

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Solution – Q1 (Cont’d)

Water balances can be summarized by this picture.

The total fresh water consumption is 9.33+0.97+3.83+20.66=34.79 ton/ton of dry pulp.

Only liquid water can be “directly” recycle: 0.967+5.92+0.681+20.66= 28.23 ton/ton of dry pulp.

For mass conservation reasons, only the min of fresh water or liquid effluent can be recycle ie. 28.23 ton.

So the minimum water consumption is 34.79-28.23=6.56 ton (ie a reduction of 81%).

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Solution – Q1 (Cont’d)

The following graph show the contribution of each process unit to resource consumption.

Process Contributions - Input Impact Categories

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Raw WaterConsumption

Virgin FiberConsumption

Natural ResourcesConsumption

Energy Consumption

Landfill

Transport

Chemical Production

Pulp Manufacturing

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Solution – Q1 (Cont’d)

The last figure show that the manufacturing activities consumes a lot of resources: water, virgin fiber and other natural resources.

It also shows that chemical production is particularly energy-consuming.

From a first look, reducing chemical and water consumption will result in a significant environmental benefit.

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Solution – Q1 (Cont’d)

The following graph show the contribution of each process unit to emission-related environmental impacts.

Process Contributions - Output Impact Categories

0%

20%

40%

60%

80%

100%

Globa

l War

min

g

Acidific

atio

n

Eut

roph

icatio

n

Heavy

Met

als

Carcin

ogen

ic Sub

stanc

es

Summ

er S

mog

Win

ter S

mog

Solid

Was

te

Growth

Harvesting

Landfill

Transport

Chemical Production

Pulp Manufacturing

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Solution – Q1 (Cont’d)

From this graph it is possible to note that: Manufacturing activities are a large contributor

to acidification, eutrophication, winter smog and solid wastes;

Chemical production is a large contributor to all impact categories but more specifically eutrophication, heavy metals and summer smog.

Transportation seems also to be a large contributor to several impact categories: global warming, carcinogenic substances and summer smog.

Global warming is due to almost all unit processes.

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Solution – Q1 (Cont’d)

Even if it is impossible to talk about the relative importance of each impacts since no weighting has been performed, it is clear from the to last graphs that manufacturing activities, including chemical consumption must be targeted in order to reduce the overall environmental impacts. Transport is also a significant contributor.

The followings results show how much a 5% reduction in transportation and chemical consumption will affect the environmental impacts. Manufacturing is more difficult to assess but the impact of an increase of 5% of the yield (from 50% to 52.5%) is also presented. It as been assumed that an increased yield will only impact the quantity of wood required and not the chemical consumption in order to keep both effect separate.

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Solution – Q1 (Cont’d)

It is important to note that here only easily manipulable variable have been modified in order to determine which changes will influence the more the environmental impacts.

The most important results are the following: A 5% increase in the yield will result in a:

5.64% reduction in fresh water consumption; 4.70% reduction in virgin fiber consumption; 4.39% reduction in natural resources consumption.

A 5% reduction in transportation will result in a: 4.86% reduction in energy consumption; 4.26 reduction in carcinogenic substances.

A 5% reduction in chemical will not affect significantly the environmental impacts.

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Solution – Q1 (Cont’d)

As an environmental engineer, you will propose the followings: Increase the process performance, which

will also reduce costs. Since reducing transportation distance is

not easily realizable, you suggest to find a mode of transportation less pollutant.

Even if a reduction of chemical consumption will necessarily reduce the cost, it is not an environmental priority.

The mass balances have shown that there is a lot of potential for fresh water reduction.

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Solution - Q2

Question 2 - Normalized Results - Resources

0

0.2

0.4

0.6

0.8

1

1.2

Raw Water Consumption Virgin Fiber Consumption Natural ResourcesConsumption

Energy Consumption

Pulp Manufacturing

Chemical Production

Transport

Harvesting

Total

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Solution - Q2 (Cont’d)

The last graph shows the LCA results (resources) for the direct water recycle option. The results have been normalized against the reference case. From this graph, it is possible to say that: Raw water consumption from the

manufacturing process unit has been reduced to 70% of the reference case.

Energy consumption by the manufacturing has been increase by 5%.

Everything else is constant.

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Solution - Q2 (Cont’d)Question 2 - Normalized Results - Resources - Emissions

0

0.2

0.4

0.6

0.8

1

1.2

Global

War

ming

Acidific

ation

Eut

roph

icatio

n

Heavy

Met

als

Carcin

ogen

ic Sub

stanc

es

Summ

er S

mog

Wint

er S

mog

Solid

Was

te

Pulp Manufacturing

Chemical Production

Transport

Landfill

Harvesting

Growth

Total

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Solution - Q2 (Cont’d)

The preceding graph shows a reduction in the following impact categories: Acidification from the manufacturing

process unit. It also shows an increase in:

Winter smog from the manufacturing process unit.

All the remaining impact categories are almost constant.

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Solution - Q2 (Cont’d)

The aggregated indicators are: Resources: 0.76 Emissions: 1.00

From this it is possible to conclude that the direct water recycle solution has a positive impact on the resource impact categories (almost 25% improvement) and almost no impact on the emissions.

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Solution - Q2 (Cont’d)

A lot of importance has been given to the raw water consumption. A sensitivity analysis on the weights has been conducted. First, weight of raw water has been decreased while maintaining the other relative weights constant.

The results are presented in the table. It can be seen than even if the raw water importance passes from 83% to 10%. There is still an environmental benefit.

Weight of the raw water

consumption

Aggregated Indicator

0.83 0.76

0.50 0.85

0.30 0.91

0.10 0.97

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Solution - Q2 (Cont’d)

The impact category the most influenced by the direct recycle other than raw water is the energy.

By increasing the weight of energy while maintaining the other ratios constant we obtain the results presented in the table.

The conclusion of the 2 tables is that the environmental improvement is robust to the weights.

Weight of the Energy

Aggregated Indicator

0.08 0.76

0.16 0.78

0.32 0.82

0.64 0.90

0.83 0.96

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Solution - Q2 (Cont’d)

The same strategy has been applied to the emission impact categories. Sensitivity analysis have been conducted on the acidification and winter smog weights.

Acidification has been reduced so the sensitivity analysis try to determine if more weight on this impact category will reduce significantly the aggregated indicator.

The table shows that even if acidification weight passes from 1% to 80% this will results in only 2% improvement.

Weight of the

Acidification

Aggregated Indicator

0.01 1.00

0.10 1.00

0.20 1.00

0.40 0.99

0.80 0.98

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Solution - Q2 (Cont’d)

Winter smog has been increased so the sensitivity analysis try to determine if more weight on this impact category will increase significantly the aggregated indicator.

The table shows that even if winter smog weight passes from 7% to 80% this will results in only 1% degradation.

The 2 previous tables show that the emissions indicator is robust to the weights.

Weight of the Winter

Smog

Aggregated Indicator

0.07 1.00

0.14 1.00

0.28 1.00

0.56 1.00

0.80 1.01

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Solution - Q2 (Cont’d)

Overall conclusion: Direct water recycle results in a

positive resource saving (24%) without compromising the other impact categories.

Furthermore, it is a low cost solution. In consequence, its implementation is

highly recommended.