Untitled 00sep55pdf

7/29/2019 Untitled 00sep55pdf

1/18

2000 TAPPI JOURNAL PEER REVIEWED PAPER SEPTEMBER 20001

2000 TAPPI JOURNAL PEER REVIEWED PAPER

ENERGY-EFFICIENT PRE-EVAPORATION OF BLEACH PLANT FILTRATES

ECONOMIC EVALUATION OF VARIOUS OPTIONS

Jessica Algehed*, Jan Strmberg, Thore BerntssonDepartment of Heat and Power Technology, Chalmers University of Technology,

S-412 96 Gteborg, Sweden

Abstract: Increased pulp mill closure will in many cases lead to evaporation of bleach plantfiltrates and a new energy situation in the mill. Due to the new conditions, various pre-evaporation plants are evaluated and compared from an energy and economic perspective in

order to find the best way of managing a certain evaporation demand. Waste-heat drivenplants are compared to utility-driven ones, such as pre-evaporation with steam of 12 or 4 bar

or mechanical vapor recompression (MVR).

Pre-evaporation with excess heat is found to be one of the best alternatives from energyperspective and economically reasonable under most conditions.

Application: Cost estimates and understanding of the energy consequences of new

evaporation demands play an important role in the energy-strategic planning of future mills.

Keywords: mill closure, evaporation, bleach plant filtrates, pinch technology, processintegration, optimization, heat pump and energy

INTRODUCTION

When striving towards the minimum-impact pulp mill, an important measure is to reduce

the water consumption by closing the water loop. Increased water recycling will stronglyinfluence the chemical, as well as, the energy balance of the mill. Two major consequencesare:

new demands for internal cleaning of different process streams (often referred to askidneys) will arise;

the demand for raw water heating will decrease or vanish, which will generate newwaste heat sources at higher temperatures than today.

Among several techniques for water cleaning we focus on evaporation as a means forconcentrating a dry substance content. This technique seems to be the most likely choice in

the near future and, furthermore, it is the most heat-demanding unit operation in a pulp mill.

In Figure 1, a future mill with an extremely closed cycle is shown. The bleach plant filtratenot being counter-currently used in the bleach plant is pre-evaporated and the condensates areused as process water again (after cleaning, if necessary). Many mills world-wide have


2/18


experiences of closing the water loop [1-5], and even though they rarely have this closedwater-loops, several have experiences of pre-evaporating bleach plant filtrates [6-7].

Figure 1. Location of a pre-evaporation plant in a future, minimum impact pulp mill.

In this paper, pre-evaporation plants with various heat supply sources (waste heat included)

are evaluated and compared from an energy and economic perspective. The aim is to show

energy-efficient and economic ways of managing a certain evaporation demand under the newconditions that mill closure leads to.

PRE-EVAPORATION PLANT CASE STUDIES

Future pre-evaporation of bleach plant filtrates will in many ways differ from traditional

black liquor evaporation:

The bleach plant filtrates will mainly be evaporated at fairly low dry substance contentand the boiling point rise will therefor be low. This will make it possible to use more

effects in a certain temperature range than for black liquor, and the use of vaporrecompression more suitable due to a low temperature lift.

The low dry substance content will keep the viscosity low, even at low temperatures, andwith new technology it is already today possible to evaporate bleach plant filtrates downto 40C [20].

New technology under development, as plastic evaporators, will make it economicallyfeasible to use even bigger heat transfer surfaces than today. Smaller waste heat sourcesthan today can then become economically feasible.

Cooking O2 Bleachplant

Drying

Bleach planteffluent

Rest products

Black liquorevaporation

Recoveryarea

Condensate

cleaning

Pre-evaporation

Dirtycondensate

CondensateClean condensate

Whiteliquor

Blackliquor

Clean condensateRecipient

PulpWood

Concentrate

Make-up water


3/18


New kidneys aiming at purging non-process elements may change the content in the futurebleach plant filtrate, and the scaling and foaming problems of today, limiting the

temperatures at which evaporation is performed, may not even remain. Pre-evaporation athigher temperatures than today might then be possible.

Due to the differences from traditional black liquor evaporation and the new energysituation in the minimum impact pulp mill discussed above, many new ways of running afuture pre-evaporation plant are interesting: compressor driven plants, waste heat driven ones

and plants driven at high temperatures. In this work, four different types of pre-evaporationplants have been studied. All have falling-film evaporators in series with countercurrent feedflow, but the heating and cooling demands, electricity consumption and working temperatures

differ. The plants are divided into two main groups: utility-driven ones, using live steam orelectricity, and waste-heat driven ones. The utility-driven plants are shown schematically in

Figure 2 and the waste-heat driven in Figure 3. The LP-driven plant uses 4 bar steam from a

back-pressure turbine and rejects steam at 60C. The MP-driven plant uses 12 bar steam froman extraction turbine, and rejects 4 bar steam that is fed to the LP steam network or directly toa consumer. The mechanical vapor recompression (MVR)-driven plant uses its own reject

steam, upgraded by using a compressor or fan(s). The waste-heat-driven plants, lastly, use

various amounts of excess heat at different temperatures and reject vapor at 40C.

MP steam line

Compressor

or electrical fans

LP steam line

Steam out

60C

LP steam line

Figure 2. Utility-driven alternatives. From left to right: LP, MP and heat pumpalternatives.


4/18


Steam from waste-heat source

Steam out40-60C

Figure 3. Waste-heat supplied pre-evaporation plant.

THE IMPACT OF PRE-EVAPORATION ON THE TOTAL ENERGY BALANCE OF

THE MILL

When heat-integrating different sections in a mill, heat rejected at a high temperature inone section is used as a heat source at a lower temperature in another section. An example ofcommon heat integration in existing mills is pre-heating of bleach plant wash water with

evaporation plant off-steam and flash steam from the digester. The more integrated alldifferent parts in a mill are, the smaller the total heat and cooling demands may be. A pre-evaporation plant can either be heatintegratedwith the existing pulp mill, i.e. the plant uses

heat recovered from elsewhere in the process and/or rejects heat to be used elsewhere, or

non- integrated, i.e. run separately from the rest of the process. An integrated pre-evaporationplant does not increase the total heating and cooling demands, while a non-integrated does

(The MVR-driven plant excepted).

To find opportunities for direct heat integration between parts of the mill and true waste

heat sources, the grand composite curve (GCC) is often used [19]. This curve shows theminimum net heating and cooling demands for a process at each temperature level, when, for

a defined minimal temperature difference in each heat exchanger, maximum heat recovery

within the process has been assumed (Figure 4). The pinch point (100C in Figure 4) dividesthe process into two parts: one above the pinch point which has a net demand for heat, andone below the pinch point which has a net surplus of heat. Only heat sources below the pinch

temperature should be considered as true waste heat sources. The GCC can be constructed forthe entire mill or individual departments, depending on the aim of the analysis. The GCC inFigure 4 represents the net curve for the example mill (for details see Base case data and

assumptions), including all departments except the pre-evaporation plant. In the same type ofT-Q diagram a single evaporator can be represented as a rectangle, where the lengthcorresponds to the heat demand and the height to the temperature difference between the


5/18


entering and leaving steam, and a multi-effect evaporator train can consequently berepresented as a pile of rectangles.

Figure 4. GCC for the model mill, pre-evaporation plant not included.

By including the different pre-evaporation options in Figure 4, it can easily be seen how muchthe total heating and cooling demands increase with the different pre-evaporation plants. Thewaste-heat and MP driven plants can all be integrated with the rest of the process, with

virtually no impact on the total heating and cooling demands (Figure 5 and Figure 6).

Figure 5. Two examples of excess-heat integrated pre-evaporation of bleach plant

filtrate.

0

50

100

150

200

0 20 40 60 80 100

Q (kW)

T

(C)

minimum steam demand

minimum cooling demand

pinch point

Waste heat from black liquor evaporation

Waste heat from bleach plant

0

50

100

150

200

0 20 40 60 80 100

Q (kW)

T

(C)



Pre-evaporation integrated with existing

black liquor evaporation

Pre-evaporation

integrated with

digester


6/18


Figure 6. Pre-evaporation with MP-steam.

Their maximum sizes (heat demands) are given by the shape of the GCC. The LP drivenplant on the other hand, cannot be integrated at all, and increases both the heating and cooling

demands of the mill (Figure 7). The MVR-driven plant is not integrated with the rest of the

process, but as it uses its own reject steam and not live steam, it does not increase the totalheat demand. The consequences for the electricity production caused by the different plantscannot be seen in the GCC. When using waste heat for pre-evaporation, the electricity

production is not affected. The MP and the MVR driven plants decrease the net production ofelectricity, while the LP-driven plant increases the possibilities for power generation.

From an energy perspective, pre-evaporation with excess heat will in most cases be thebest choice, due to its low impact on the total energy system. Using excess heat from theblack-liquor evaporation plant or the digester will neither increase the total heating or cooling

demands, nor affect the electricity production. The cooling temperature will, however,become lower than before, causing the size of the cooling equipment to increase even though

the cooling load is not influenced.

Compared to the LP-driven plant, both the MP and the MVR-driven plants are favorable

from an energy point of view, as increased evaporation is possible without increasing theheating or cooling demands for the total system.

BASE CASE DATA AND ASSUMPTIONS

Example mill

The pulp mill discussed in this paper is an assumed modern Scandinavian market kraftpulp mill, producing 335,000 ADt/year. The mill has 4 m3/ADt of bleach plant filtrate at

60C, to be pre-evaporated. The filtrate is pre-evaporated from 1.25% DS to 17% DS andthen, for example, fed to the existing black-liquor evaporation plant or other treatment. Whenevaporating the filtrate to 17% DS, the remaining 0.3 m3/ADt of concentrated bleach plant

0

50

100

150

200

0 20 40 60 80 100

Q (kW)

T

(C)



Pre-evaporation

with MP steam


7/18


filtrate normally can be evaporated with the black liquor, without increasing the evaporatorarea in the black liquor evaporation plant.

Figure 7. Non-integrated pre-evaporation with LP live-steam.

Evaporator design data

We have assumed that the pressure drop is constant and corresponds to a temperature loss

of 1C in each effect for the utility-driven plants and 2C per effect for the waste-heat drivenones (due to the low temperatures of the waste-heat sources) [18]. The filtrate is assumed tohave the same properties as diluted black liquor [9-11]. The heat transfer coefficients are in

the range of 1200-2700 W/m2,C. They are calculated using well-known expressions from

literature [12-13] and temperature and dry substance content dependence have been combinedto reflect the new conditions included in this study.

Heat recovery within the evaporation plants

In order to recover as much heat as possible within the pre-evaporation plant, the enteringbleach plant filtrate is heat-exchanged with the leaving condensates. As a minimaltemperature difference of 5C is used, all utility-driven plants will generate condensates of

65C and the waste-heat-driven ones condensates of 45C. We have found that if the heat-exchanger network is designed according to pinch technology rules, virtually all heat neededfor pre-heating the filtrate, both before entering the first effect and between the effects, can be

supplied within the pre-evaporation plant. This minimises the use of live steam for preheatingthe filtrate. The investment cost for the heat exchangers needed is taken into account.

Economic data

The investment cost for the evaporators and the size exponent used are based on price

estimations from a major Scandinavian supplier of process equipment, and include transport

and start-up costs [14]. Investment costs for four different cases (listed in Table 1) have beenused for calculating the investment costs for the different plants. To make the cost estimations

0

50

100

150

200

0 20 40 60 80 100

Q (kW)

T

(C)



Pre-evaporation

with LP steam


8/18


dependent on the number of effects, the cases with two evaporators have been used forcalculating the investment cost when up to three effects are needed, and the cases with fiveand six evaporators when more than three effects are needed. The equation for the investment

cost for the compressor is taken from a North American comparison of different compressorsuppliers [15].

All technical and economic data used for the calculations of the base case are listed inTable 1.

Economic and technical conditions Comments

Yearly production of pulp 335 000 ADt 1000 ADt/day, 8000h/Year

Filtrate flow 4 m3/ADtFiltrate temperature 60C

Dry solid contents in entering filtrate 1.25% 50kg DS/ADt

Dry solid contents in leaving filtrate 17%

Heat transfer coefficients 1800-3200 W/m

2

K

Electricity price 300 SEK/MWh

Internal fuel price 80 SEK/MWh boiler efficiency: 0.7

Investment cost for evaporators:

6x700 m2 50 MSEK 6.3 million US $1


2x4500 m2

47 MSEK 5.6 million US $1


Size exponent 0.4

Compressor investment cost 2[38500+3630[volume flow (m3/s) ]-0.39

+3720[power output (kW) ]0.74

+304[power output (kW) ]]Valid for 1996, 1SEK=6 US $

Annuity factor 0.2

11 US$=8 SEK

Table 1. Conditions for the base case calculations.

THE ECONOMIC COMPARISON: METHOD

The economic comparison of the different pre-evaporation plants has been performed in

three steps:


9/18


a) The utility-driven pre-evaporation plants are economically optimized regarding bothcapital and operational costs, using the computer program Optivap, developedespecially for this purpose [8]. The optimal pre-evaporation design for each case is thus

determined and the lowest total coststated.b) Using Optivap, capital costs for the waste-heat evaporator units are calculated as a

function of amount of heat available and temperature of the heat. Capital cost versus

amount of heat available and temperature is plotted.c) The capital costs calculated under b) do not include the cost for equipment that may be

needed to recover the excess heat from the process, such as heat exchangers, piping, steamreformers etc. By comparing the capital cost for the waste-heat plants (calculated underb), with the total cost for the utility-driven plants (calculated under (a)), the maximumallowable cost for recovering the excess heat from the process can be obtained. It is then

possible to assess under what conditions the waste-heat-driven plants are economicallyfeasible.

Finally, a sensitivity analysis is performed to determine how the results are influenced bychanged steam and electricity prices, as well as changed annuity factor and different cost

models for the evaporator investment cost.

ECONOMIC COMPARISON: TECHNICAL AND ECONOMIC RESULTS

General

The optimal total cost for the LP and MP driven plants is a trade-off between investment costs

and running costs. The investment costs generally depend on number of effects and total heattransfer area required, while the running costs depend on how large the heat and electricitydemand for the plant is. The more effects in series, the smaller the heat demand will be. Plants

with many effects will thus have the lowest running costs, but large investment costs. Large

temperature differences over the plant, as well as high temperatures of the input steam,decrease the heat transfer area required. Hence, plants working with large temperature

differences at high temperatures will have smaller investment costs for the evaporators thanplants working with small temperature differences at low temperatures.

The MVR-driven plants are more complex to optimize than the LP and MP-driven. Thetotal cost is a balance between the investment cost for the evaporators and compressor, and

the running cost for the compressor. However, many parameters tend to counteract, giving anextremely flat optimum. The allocation of the different costs is constant, independent ofnumber of effects. One can either have a few large effects, with a large steam flow to

compress and small temperature lift, or have many small effects with a small steam flow tocompress and large temperature lift.

Technical description of optimal utility-driven plants and waste-heat-driven plants

Technical specifications like number of effects, area requirement, working temperatures,

and temperature difference for the optimal design for all plants are listed in Table 2. From

this table one can see that for the MVR-driven plant it is optimal to have a rather small totaltemperature difference, which also has been found for smaller plants [16]. For the LP and MP


10/18


driven plants it is optimal to have as large total temperature difference as possible, since this

decreases the total area required. In Figure 8 total area for the waste-heat-driven and theoptimal utility-driven plants versus temperature and heat demand are shown.

Alternative Number ofeffects

Workingtemperature

(C)

Area pereffect

(m2

)

Heatdemand

(MW)

Electricity

(MW)

Total cost

(SEK/ADt)

Totalevaporator

investment

(MSEK)

LP 7 144-60 520 16 3.52 55 52MP 5 188-144 820 19 2.4 3 53 52Compressor 2 140-130 5000 - 1.64 46 49Fans 2 140-130 5000 - 1.64 44 49

Waste heatintegrated1

3 60-40 4000 35 - 34 56

1Integrated with black liquor evaporator condenser

2Increased power production 3 Lost power production 4 Power consumption

Table 2. Summary of important simulation and optimisation results for the optimal

utility driven pre-evaporation plants and waste heat supplied plants at basecase conditions.

0

5000

10000

15000

20000

25000

0 10 20 30 40 50 60

Heat demand (MW)

Totala

rea(m2)

MVR

MPLP

100C85C

60C

2

3

45

6

7

Figure 8. Total area requirements for waste-heat alternatives and optimal utility-

driven alternatives. All points vertically of each other correspond to the

same number of evaporators. The number of evaporators increases withdecreasing heat demand.

Economic results for utility-driven plants

The running and investment costs for the optimal utility-driven plants at the base case

conditions are shown in Figure 9. From this figure it can be seen that the total cost will differ


11/18


between the plants and be lowest for the MVR-driven. The investment costs are almost equalfor the different plants, while the running costs vary.

Figure 9. Optimal total cost for the utility-driven pre-evaporation plants under given

base case conditions.

Depending on the mills possibilities to produce electrical power, the cost for the LP steamvaries. If the mill is assumed to be able to increase the electrical power production withoutany extra investments when increasing the use of LP steam, the steam cost will be reduced by

the income for the produced electricity. The total cost for the optimal LP-driven plant willthen be 55 SEK/ADt, as shown in Figure 9. If the mill cannot increase the production of

electrical power without making new investments, and no income for electrical powerproduction is therefore assumed, the total cost will raise to 77 SEK/ADt. An additional costfor the LP-driven plant will be the cost of increasing the mills cooling capacity. This cost canbecome substantial in a mill with limited cooling capacity.

The total cost for the optimal MP-driven plant will be 53 SEK/ADt. An extra cost for thisplant might be for reforming the LP steam from the last effect before passing it on to a

consumer or the LP steam network. If the mill is throttling HP steam to LP steam due tolimited turbine capacity (which is unfortunate from an energy perspective as energy at a hightemperature is degraded to a lower temperature), this case is especially favored. Fewer effects

will become optimal, as the energy cost for MP essentially is negligible, and the total cost will

be reduced.

The total cost for the MVR-driven plant will be 46 SEK/ADt. Reports indicate that fansmay be used for compressing the steam instead of a compressor [17]. The temperature lift

over a fan is of course limited, but with two fans in series it will be possible to achieve thenecessary temperature lift. According to manufacturers the investment cost for the two fansneeded will be approximately 50% of the investment cost for a compressor. By using fans, the


12/18


total investment cost then decreases by approximately 5%, giving a total cost of 44 SEK/ADtfor the fan-driven plant.

Economic results for waste-heat driven plants

The estimated investment cost of the evaporators needed for pre-evaporation with waste

heat versus temperature and amount of waste heat available is shown in Figure 10. The

investment cost decreases with increasing amount and temperature of the waste heat. Theinvestment cost for the waste-heat supplied plants is not the total cost for these plants. Eventhough the waste heat is for free, there may arise additional costs to make the waste heat

available for pre-evaporation. Examples are costs for steam reformers, heat exchangers andadditional piping. These costs vary between different mills and applications. In general, theseextra costs increase with temperature and amount of waste heat; i.e. it will be more expensive

to extract a large amount of waste heat at a high temperature, than a small amount at a lowertemperature. In Figure 10, the total costs for the optimal utility driven plants are shown aswell, and the maximum allowable cost for making the waste heat available at different

amounts and temperature of waste heat can thus be seen. In Figure 10 it can be seen that themargin between the waste heat driven plants and the utility driven ones is highest at largeamounts of high temperature waste heat and lowest at small amounts of low temperature

waste heat. As stated earlier, large amounts of high temperature waste heat generally willrequire large investments to become available for pre-evaporation, and it is therefore notalways evident whether it will be economical to use a certain waste-heat source or not.

Figure 10. Investment cost for waste-heat-driven plants versus temperature and

amount of waste heat available compared to the optimal cost for the utility-

driven plants (at base case conditions). The total costs for the utility-driven

alternatives are represented as horizontal lines, as they only refer to the

economically optimal design, which is independent of amount of waste heat

available.


13/18


An example of pre-evaporation with excess heat in the closed mill

In future closed mills the heat from the black liquor condenser will often be considered aswaste heat, as the demand for warm water production decreases when the intake of fresh

water is decreased or eliminated.

The example mill used in this study will have about 35 MW excess heat at 60C from the

black-liquor evaporation condenser when closing the water loop. To evaporate 4 m3/ADt withthis amount of heat, 3 effects of approximately 4000 m2 each will be needed. It will be

possible to have this number of effects due to the low or non-existing boiling-point rise of thefiltrate and the low pressure drop assumed (2C in each effect). According to Figure 9, thecapital cost for a waste-heat-driven evaporation plant of this size would be 38 SEK/ADt and

thus be lower than the total cost for the best utility-driven plant. The cost to make the wasteheat available for pre-evaporation would, in this case, be small. The extra cost would mainlybe for the enlarged condenser, due to the lower off-steam temperature. According to Figure 10

this cost can be allowed to be up to 6 SEK/ADt, which corresponds to an investment of 10million SEK, without making the waste-heat-driven plant more expensive than the otherplants.

The waste heat from the black liquor evaporation is just one of many possible waste-heatsources in the future mill, but often the largest and easiest to use. Other smaller waste-heat

sources might be from, for example, the bleach plant, the exhaust gas scrubber or the digester.

SENSITIVITY ANALYSIS

Sensitivity of the optimal design of the utility driven plants

In general, optimal designs change towards fewer effects with increased investment costsand decreased running costs, and vice versa. The optimum curve (total cost versus number of

effects for each case) is rather flat around the optimal design, and even though the optimal

design for the MP-driven plant, for example, changes from 5 to 4 effects when the electricityprice is lowered, the total cost for 5 effects is virtually the same as for 4.

Due to the uncertainties in the estimations of the investment cost, we have studied how theshape of the optimum curves for the different utility-driven plants is influenced by the cost

estimations made. We found that the shape of the optimum curves and the optimal number ofeffects are rather independent of the cost model chosen. Using a simple linear model for theevaporator investment cost gives optimum curves very similar to the ones we get using an

exponential model, or a model based on a fixed cost per effect plus a part depending on thearea of each effect.

Sensitivity to changes in energy prices

The total cost for the waste-heat driven plants is not affected by changed energy prices, as

the waste heat is for free. The total cost for all utility-driven plants will on the other hand beinfluenced by changed energy prices, which affects the margin between the waste-heat-drivenplants and the utility-driven ones.


14/18


The total cost for the utility-driven plants at 30% lower and higher electricity price is

shown in Figure 11 and Figure 12 respectively, and in Figure 13 the results are shown withthe electricity lowered and steam price raised 30%.

The LP-driven plant is influenced by both the fuel price and the electricity price, and isfavoured by a low steam price and a high electricity price. Such price relations could occur ina mill with an over-capacity of steam, even though no external fuel like oil or bark is used,

and with no other potential steam consumers near by. The MP-driven plant is influenced

mainly by the electricity price but also by the fuel price, and is favoured by both a low fuelprice and a low electricity price. The MVR-driven plants are the least sensitive to changedenergy prices. They are only influenced by the electricity price, favoured by a low electricityprice. If the electricity price is assumed to be 30% lower and the steam price at the same time

30% higher than for the base case, the relationship between the utility-driven plants changesto the benefit of the MVR-driven plants; The MVR-plant will cost almost 50% less than theLP-plant.

Figure 11. Optimal total cost for the utility-driven plants at 30% lower electricity price

than for the base case.

The margin between the best utility-driven plant under different conditions and the waste-

heat driven is shown in Figure 14.

Sensitivity to changed investment costs

By varying the annuity factor, the effects of changed investment costs have been studied.The plants with the larger portion of the total cost as investment costs are naturally the mostinfluenced. For this reason, the relationship between the utility-driven plants is not

dramatically influenced by a changed annuity factor, but the margin between the best utility-driven plant and the waste-heat driven ones will change depending on which option that has

the larger investment cost.


15/18


Figure 12. Optimal total cost for the utility driven plants at 30% higher electricity pricethan the base case.

Figure 13. Optimal total cost for the utility-driven plants at 30% lower electricity price

and 30% higher steam price than the base case.

Investment cost for evaporation at low temperatures

The investment costs for traditional falling-film evaporators may become higher at lowtemperatures than at normal temperatures, but as evaporation in the pulp and paper industry

today rarely is done below 50C it is difficult to predict to what extent low temperatures willaffect the cost models. At low temperatures, larger areas will be required due to the decreased


16/18


heat transfer coefficients, and larger equipment (pipes, vessels, steam rooms etc.) will beneeded due to the large specific volume of the vapor and the large temperature losses alreadyat moderate pressure drops. The enlarged areas have been accounted for, but not other

additional costs.

Figure 14. Investment costs for waste-heat driven plants compared to best utility drivenplants at 30% lower and 30% higher electricity price than the base case.

By making heat transfer surfaces of plastic instead of steel, the cost for evaporation at lowtemperatures may be lowered. Plastic evaporators have been found to work well forevaporation of bleach plant filtrates, according to pilot runs at Stora Gruvn pulp mill in

Sweden [7], but have an upper temperature limitation of around 70C. Low-cost heat transfersurfaces will make it possible to use even smaller, or cooler, waste-heat sources thanexemplified in this work with reasonable economy, as the margin between the waste-heat

plants and the utility-driven ones will increase. Cheaper heat transfer surfaces will also lower

the total cost for the MVR-driven plant. The plastic heat transfer surfaces will make it optimalto build larger evaporators and thereby decrease the temperature lift and the electricity

consumption for the compressor or the fans.

SUMMARY AND CONCLUSIONS

There are several attractive ways of handling an increased evaporation demand in the

future pulp mill. The optimal solution will differ between mills and the local, technical,environmental and economic conditions for a specific case can cause the results to change, inboth ranking and absolute terms. In general we have found that:

The best choice from an energy perspective would be either the waste heat drivenplants or the MP or MVR driven ones. None of these alternatives increase the totalheating or cooling demands. For the waste-heat-driven plants the cooling will occur at

a lower temperature than before though, and the MP and MVR-driven ones bothdecrease the net production of electricity.


17/18


The results indicate that waste-heat-driven plants are economically reasonable undermost conditions, and should always be considered. Compared to the other pre-

evaporation plants discussed, waste-heat integrated options will be favourable in millswith large amounts of high-temperature excess heat available, low investment costs forthe evaporators and high investment costs for increased cooling demand. In mills with

smaller waste-heat sources, integrating parts of the pre-evaporation demand can be aninteresting solution.

The results also indicate that using a heat pump, as is done in the MVR alternatives,can be a highly interesting alternative. The MVR-driven plants are economicallyattractive under several economic conditions and flexible from a technical point of

view. It is also a stand-alone solution. By choosing a MVR-driven plant instead of awaste heat driven plant, the waste heat can be used for other purposes, like districtheating.

The MP-driven plant will have costs similar to the MVR-plant and the benefit of nothaving a compressor with moving parts, but will on the other hand have more effects

than the MVR-driven plant.

Environmental limitations, insufficient cooling capacity, and other departments of the mill

competing for the use of waste heat, are issues not addressed in this work, but may play animportant role when comparing the different pre-evaporation plants. For a deeper and total

understanding of the energy system in a future mill, energy-strategic planning through pinchanalysis including all parts of the mill therefore will be motivated.

Finally, the results presented are based on a model mill with several assumptions made,and show only the technical and economic consequences for the energy system. In reality, pre-evaporation of bleach plant filtrates, and the recycling of non-process elements connected

with it, is still a relatively undocumented technology, with several important technical andenvironmental issues left to be resolved.

ACNOWLEDGEMENTS

The work presented in this paper is part of a project on Energy Potential within the Eco

Cyclic Pulp Mill Programme, financed by MISTRA, the Swedish Foundation for StrategicEnvironmental Research.

The vision in this programme is an eco cyclic pulp mill producing high-quality products,using as much as possible of the energy and biomass potential in the raw material entering the

mill. The main aim in the project onEnergy Potential is to identify efficient energy systems inthe minimum-impact mill that are economically and technically attractive.


18/18

REFERENCES

1. Gleadow P. L., Vice K., Johnson A. P., Sorenson D. R., and Hastings C. R., Mill

applications of closed-cycle technology, International Non-Chlorine Bleaching

Conference, Orlando, FL (1996).2. Annergren G., Boman M., and Sandstrm P., Towards the Closed Bleach Plant,

International Conference on Available Techniques, SPCI, Stockholm (1996).3. Kinell P., Strm K. E., and Swan B., Sustainable Development in Stora Pulp Mills,

International Conference on Available Techniques, SPCI, Stockholm (1996).4. Lindeberg O., Lidn J., Ahlenius L., and Svensson G., Concurrent Bleaching and Metal

Management by Addition of EDTA to a Chlorine Dioxide Stage, InternationalConference on Available Techniques, SPCI, Stockholm (1996).

5. Larsson P., Malmstrm J., and Igerud L., TCF at Vr Pulp Mill - Development andExperiences, International Conference on Available Techniques, SPCI, Stockholm

(1996).6. Vlttil O., Tappi Pulping Conference, Chicago, October 1995.7. Personal communication with Sandstrm E., STORA, Sweden.

8. Algehed J., Pre-evaporation of bleach plant filtrates, Institutionen fr Vrmeteknikoch maskinlra, Chalmers University of Technology, 1996 (In Swedish).

9. Zaman A., Wight M. O., Fricke A. L., TAPPI Journal 77(8):175(1994).

10. Kocurek M. J., Grace T. M., Malcolm E., Alkaline Pulping vol 5, TAPPI PRESS, p.481.

11. Hultin S. O., 1968 Symposium on Recovery of Pulping Chemicals, IUPAC-EUCEPA,

p177, 179.12. VDI Vrmeatlas vol 4 1984, p. Ja 9.13. Persson L., Kemisk Tidskrift 9, (1987).

14. Personal communication with Lindberg T., Kvaerner Pulping, Sweden.15. Bliem C. J., Mills J. I., Chappell R. N.,DOE-report no. EGG-2408.16. Wimmerstet R., STU-report81-3018, 1982.

17. Koistinen P. R., TAPPI Minimum Effluent Mills Symposium, Atlanta, Ga., 1996.18. Personal communication with Wimby M., Kvaerner Pulping, Sweden.19. Linhoff, B. et al, Users guide on process integration for the efficient use of energy,

IChemE, Rugby, U. K. 1982.20. Wising U., Full scale pre-evaporation of bleach plant filtrates at AssiDomn Frvi,

Institutionen fr Vrmeteknik och maskinlra, Chalmers University of Technology,

1999 (In Swedish).

Received: May 26, 1998Accepted: May 18, 1999

This paper was accepted for abstracting and publication in the September issue of TAPPIJOURNAL.

Date post:	03-Apr-2018
Category:	Documents
Upload:	boehmit
View:	217 times
Download:	0 times

Untitled 00sep55pdf

Documents