CLEANER PRODUCTION:TOOLS FOR CLEANER PROCESSES IN TANZANIA

P.P.A.J. VAN SCHIJNDEL AND F.J.J.G. JANSSEN1

AbstractThis article focuses on tools for cleaner production processes with emphasis on two Tanzanian industries. First adescription of a process and ways to improve it in an economically and environmentally friendly manner are given.Secondly, tools to survey processes are discussed like exergy analysis, pinch analysis and environmental Life CycleAssessment. Furthermore, some practical results from an environmental audit and exergy analysis in the soap andcement manufacturing industry in Tanzania are summarised.

1 The authors are at the Centre for Environmental Technology (CMT), Faculty of Chemistry and ChemicalEngineering, Eindhoven University of Technology, STO 3.25, P.O.Box 513, 5600 MB, Eindhoven, TheNetherlands, Email: p.p.a.j.v.Schijndel@tue.nl, respectively f.j.j.g.Janssen@kema.nl

1. INTRODUCTION

The World is in a continuous state of change. There isan increase in global economy and world population,which may endanger earth as an ecosystem. Theanswer against this threat is called sustainabledevelopment.There are several definitions of sustainabledevelopment, which are abstract and can beinterpreted differently. Commonly used definitionsinclude the one given by the World Commission onEnvironment and Development (WCED, 1987) in theBrundtland report:

‘Humanity has the ability to make developmentsustainable - to ensure that it meets the needs of thepresent without compromising the ability of futuregenerations to meet their own needs. The concept ofsustainable development does imply limits - notabsolute limits but limitations imposed by the presentstate of technology and social organisation ofenvironmental resources and by the ability of thebiosphere to absorb the effects of human activities’.

In this concept of sustainable development by theWCED, there are three dimensions: environment,development and security.

2. CLEANER PRODUCTION

Cleaner production of materials, goods and services isone of the answers for sustainable development. Itmeans production in a way in which resources andenergy are used in an efficient way and only smallamounts of waste and emissions are produced. Otherimportant factors are the use of renewable resourcesand the increase in quality of the products. This

doesn’t mean that the cleaner production concept isdifferent from that of an economic approach,minimising costs and maximising profits. Becauseminimising the use of resources and in this way alsocutting back on emissions will decrease the costs of agiven process.

Some other important issues used in this context aresource (raw material) reduction, waste reduction andpollution prevention (Allen and Rosselot, 1997). FromTable 1, it becomes clear that there are differencesbetween these similar issues.

Table 1: Comparison of different issuesChange inreactor/process

Recycling,in process

Wastetreatment

Cleanerproduction

Yes Yes* -

Sourcereduction

Yes No No

Wastereduction #

Yes* / No Yes Yes

Pollutionprevention

Yes Yes No

-) Not applicable#) Reduction of solid and liquid wastes*) Not necessarilySource: Allen and Rosselot (1997)

Energy carriers belong to the most important resourcesfor mankind. The production of energy sources cancause many environmental problems like majoraccidents, water pollution, maritime pollution, landuse and siting impact, radiation and radioactivity,solid waste disposal, hazardous air pollution,deterioration of ambient air quality, acid deposition,stratospheric ozone depletion, and global climatechange. These problems may be decreased when the

energy efficiency of processes is increased. Thus thereis a need for cleaner production processes.

3. CLEANER INDUSTRIAL PROCESSES

An industrial process can be simply outlined, as inFigure 1. The process can be seen as a black box.Resources and energy (work) are the inputs andproducts, wastes, emissions (air, soil, water), excessheat etc. are the outputs of this process.

Figure 1. Schematical drawing of an industrialprocess

For the energy input, one may need to add a separateblack box, representing the fact that energy carriersare converted into electricity, power or heat beforebeing used in the actual process, as shown in Figure 2.

Figure 2. Schematical drawing of an energyproduction process

In both Figures 1 and 2 there are possibilities for in-process recycling. It means some waste products canbe separated from the waste stream and used again asresources. For instance waste heat can be used to makesteam. Cleaner production for the two Figures meansthat there should be a decrease of resources used andan increase of useful products.

The traditional way to improve processes towards amore sustainable production uses costly wastetreatment facilities added-on at the stack or dischargepipe of the manufacturing plant; so called end-of-pipetreatment. Sometimes end-of-pipe treatment isunavoidable. Looking closer at waste reduction in, andnot at the end of the process, it becomes clear this ismuch cheaper and more sustainable than the ordinaryseparation procedures using filters, scrubbers, settlersetc. (Allen and Rosselot, 1997).

Other disadvantages of end-of-pipe treatment are:• It takes resources to remove pollution;• Pollution removal generates residues;• It takes more resources to disperse residues;• Disposal of residues also produces pollution.

It thus becomes clear that there must be a focus on theprocess as a whole and not only at the pipes leavingthe process or the plant. Several ways to improveprocesses into cleaner production processes are (fromsimple to more difficult):• improve a process by optimisation procedures, like

process control;• Use of other resources, of higher quality, with less

impurities;• Improve some process units;• Process integration;• Total change towards more sustainable processes.

In the next sections, tools for checking the energyefficiency and environmental impact of a process andpossible solutions to increase efficiency and decreaseenvironmental impact are discussed.

4. TOOLS

There are several tools available to check, for a givenprocess, whether there are possibilities to decrease theenvironmental impact. For most chemical and physicalprocesses thermodynamics provide a powerful tool,because thermodynamics can forecast the amount ofresources and energy used and sometimes emissions

Process

Useful Energy

Resources

Product(s)

Wastes

Materials

and

Waste Energy

Emissions

Utility Process

Useful Energy

Resources

WastesMaterials

and

Waste Energy

Emissions

Energyconversion

produced within a certain process (Swaan Arons andKooi, 1993). Two thermodynamically related toolsdiscussed here are exergy analysis and pinch analysis.Another, non-thermodynamic, tool discussed here isenvironmental Life Cycle Assessment, shortly LCA.

Exergy analysisExergy analysis is much familiar with the enthalpy orenergy analysis. The difference is that in exergyanalysis entropy is applied, contrary to energyanalysis, which only includes enthalpy. An exergyanalysis can be performed for a whole plant or fordifferent unit operations. Information about exergyanalysis can be found in literature, Szargut et al.(1988) and Kotas (1995).

The following definition for exergy is used normally:‘Exergy is the maximum amount of work that can beobtained from a stream of matter, heat or work as itcomes to equilibrium with a reference environment,and is a measure of the potential of a stream to causechange, as a consequence of not being completelystable relative to the reference environment. Exergy isnot subject to a conservation law, but it is destroyeddue to irreversibility’s during any process.’

A basic example is the possibility of convertingmechanical work into heat with 100% efficiency. Heathas a lower exergy, compared with work - exergy isalso called quality of energy. Heat can not beconverted into work by 100% efficiency, since heat hasa lower quality compared with work. Some examplesof the difference between energy and exergy are shownin Table 2. From this table hot water and steam withthe same enthalpy have different exergy or qualityvalues. Steam has a higher quality than hot water.Fuels like natural gas and gasoline have exergeticvalues comparable to their net combustion value.Work and electricity have the same exergy asenthalpy. Exergy can be calculated by product ofenergy and quality.

Table 2: Examples of energy and exergy ofdifferent matterMaterial Energy

[J]Exergy [J]

Quality [-]

Water 80°C 100 16 0.16Steam 1 bar and 120°C 100 24 0.24Natural Gas 100 99 0.99Electricity / work 100 100 1.00

The advantage of exergy analysis over an ordinaryenergy analysis is the fact that an exergy analysis ismore accurate and scientifically correct, because:• Exergy analysis provides a better view on the

efficiency of a process;

• Exergy analysis is very useful to find the unitoperation where efficiency improvements are themost suitable or useful.

Each process designer or process engineer shouldperform an exergy analysis to make all exergy lossesvisible in the process under study. The method is verypowerful when comparing improvement solutions inan objective and quantitative manner. Of course theexergy analysis does not give direct answers on how toimprove the process but it gives the best clues where tostart, namely at the point where the largest exergylosses appear.Exergy analysis is also used in the design phase andduring optimisation of processes. It is a very usefultool when used for comparison of different productionroutes of a specific product.

Using the knowledge from exergy analysis it becomesclear that, for instance, a heat exchanger can beoptimised by increasing its heat-exchanging surface,because this decreases the temperature difference, ΔT,of the heat-exchanger at the same heat load conditions.At the same time costs will go up with increasing heatexchanging surface. Therefore there will be aneconomical/exergetical optimum as visualised inFigure 3.

Figure 3. Heat exchanger optimisation

Pinch AnalysisPinch analysis is designed for the optimisation of HeatExchanger Networks (HEN) by matching excess ofheat and cold streams in a system (Linnhoff andAlanis, 1991). An economical analysis is also includedin the optimisation phase. Experiences (Varwijk et al.,1998) show that using pinch analysis energy savingsfrom 20%-40% are possible with pay back periods lessthan three years.

exergy consumption(operational costs)

Heat exchanger surface(capital costs)

minimum ΔT

Larger ΔT

Optimum ΔT

The theory behind pinch technology is quite simple.For a process all hot and cold streams are inventoried;cold streams are the streams that need to be heated andhot streams need cooling. Necessary data aretemperatures, flows and heat/cooling duty. A diagramis made with the hot composite and the coldcomposite, with temperature at y-axis and the enthalpyon the x-axis (see Figure 4). The cold and hotcomposites may be moved horizontally until the twocurves are very close together at one point. This pointis called the pinch point; the temperature differenceΔT at this pinch is often chosen to be an offset value ofabout 10°C. When the optimal situation is calculatedfrom the diagram, an improved HEN can be designed.An important rule of thumb for HEN design is not toperform a heat exchange process from above to belowthe pinch point.

Figure 4. Composite curve with pinch point

The area between the hot and cold compositetemperature represents an exergy loss, which can ofcourse be lowered by decreasing ΔT to a minimalvalue. It appears that pinch analysis is a good tool toimprove a given HEN for certain conditions. But for agiven process, exergy analysis is a much morepowerful tool as shown in Wall and Gong (1996) andin two articles (Gaggioli et al., 1991 and Linhoff andAlanis, 1991). In the last two articles pinch and exergyanalysis were used to optimise a given process fornitric acid production. Pinch analysis led to a goodoptimisation, however the optimisation exergy analysisgave a two times higher efficiency than found by pinchanalysis.

Life Cycle Assessment or LCAThe Environmental Life Cycle Assessment is used toimprove the environmental impact of products andservices but can also be used to improve processes.

The LCA of a product studies the environmentalaspects and potential impacts throughout a product’s

life cycle, (i.e. from cradle-to-grave) from raw materialacquisition through production, use and disposal. Incontradiction to exergy and pinch analysis, LCA looksat all environmental impacts instead of only problemsrelated to energy use.

LCA’s of production processes can be of interestbecause they are useful for comparison with otherproduction processes in order to distinguish anenvironmental ranking. Four steps have to be carriedout to perform a LCA (Balkema, 1998; Guinée et al.,1993a and b):

1. Goal and scope definition to define the study focusand depth;

2. The inventory analysis; which consists of athorough mass balance over the productionprocesses;

3. An impact assessment; in which all the emissionsand resource uses are translated to environmentaleffects;

4. Interpretation of the results.

Environmental effects used in LCA include ozonelayer depletion, greenhouse effect, smog formation,eutrophication, toxic effects for humans and theecosystem and resource depletion. In this article it isnot possible to give an elaborate discussion of all theadvantages and disadvantages. Instead of this, a case ispresented (Kniel et al., 1996).

Case: The Nitric Acid Plant.This example describes the use of LCA for theimprovement of a nitric acid plant. In this process NH3is converted into NO2 which dissolves into waterforming HNO3. Because this process is not so efficient,a lot of NOX (NO and NO2) is emitted. There are twoeffects from this deficiency, high production cost forHNO3 and eutrophication and acidification throughthe NOX emission. Two solutions presented in thearticle (Kniel et al., 1996) are (1) end-of-pipetreatment of NOX with ammonia in which N2 and H2Oare formed or (2) cleaner production methodology byreaction at a higher pressure to increase the efficiencyin which the NO2 is absorbed to form HNO3.

Both methods have the same aim, although method (2)is cheaper because it is more efficient in point of viewof the product. In method (1) NH3 is used twice in theprocess, both as reactant for HNO3 and as the end-of-pipe treatment compound. It is also notable that duringthe production of NH3 one important emissioncompound is NOX. The amount of acidification andeutrophication for process (2) is lower than for process(1). The conclusion of this case, based on LCA:cleaner production in process is much better than end-of-pipe treatment.

T

H

Pinch

ΔTmin

Composite hotstream

Composite coldstream

Qout Qin

Although the usefulness of this tool has been proved,some scepticism on LCA’s carried out will alwaysexist. Without proper data or good set-up of the studiesthe outcome of the LCA will be of low quality.

Other toolsAnother important tool is the ‘Common Sense’ of theengineer. The process has to be seen as a total not justas a sum of separate processes.Combination of all the tools presented here canprovide an even more powerful tool.

It is possible to give a summary of some commonsense solutions:

• Alter the production technique: Use techniques thatproduce less waste and/or also use less resources.

• Alter the resources used; do not use resourceswhich contain material that will be wasted at theend of the process. For instance paints withoutsolvents (powder paints) will not emit solventsduring application and curing. Use resources withless waste and less toxic components.

• Alter the process management; better maintenancemethods, better training for operators.

5. OPTIONS FOR TANZANIAN INDUSTRIES

Like is the case for all industries World-wide, thereare a lot of opportunities for the Tanzanian industriesto improve the quality of production. The efficienciesin use of both resources and energy can increase.There is room as well to decrease effects on theenvironment, like air, water and noise pollution.

The advantages from cleaner production can be verygenerous; production costs can drop and make roomfor more possibilities for improving the processes. Atthe same time security and health conditions at andaround the plant will improve. And in almost all casesthe increase in process efficiency will decreaseemissions to air, water as well as the amount of solidwastes.

Ydhego (1993) stresses the fact that there should beput more emphasis on waste reduction and cleanerproduction rather than on end-of-pipe technologiesbecause of the high costs for these technologies. Inorder to stimulate cleaner production there must befacilities for training etc. The establishment of theCleaner Production Centre of Tanzania (CPCT),located at the offices of the Tanzania IndustrialResearch & Development Organisation, TIRDO, in

Dar Es Salaam is one of the first steps to introducecleaner production in Tanzania. In the past there hasalso been a first cleaner production programme calledCEPITA, cleaner production in Tanzania, the resultsof which are being audited at this time. Next steps areto educate the engineers and scientist to think in termsof environmental friendly processes, as is intended inthe courses at UCLAS (University College of Land &Architectural Studies), and the start of a M.Sc. coursein environmental engineering at UDSM (University ofDar es Salaam). A Centre for Environmental Scienceand Technology at UDSM is planned for beyond theyear of 2000. Some examples of cleaner production inTanzania are listed below.

Soap productionOne of the companies involved in the cleanerproduction programme of CPCT in 1997 was the‘Mshindi’ soap factory. Located in an area with a lowcapacity sewerage system they managed to modify theprocess into a ‘zero’ waste production plant. The onlywastewater they now have comes from sanitarysystems, like toilets, showers and kitchen. They areplanning to treat this water using a heliophyt filter onsite (Van Schijndel, 1997). Other options being carriedout are: better truck offloading system to decreasespillage, increased amount of steam pipes in the soaptank to decrease steam use, use of other raw materials,effluent recycling filter to decrease disposable wasteand the increase of overall power factor to decreaseelectricity consumption (CPCT, 1998).

Cement production:In 1997 an exergy analysis was performed at the WazoHill cement factory at kiln number three to check ifsuch an analysis was possible with scarce data. Theanalysis proved to be possible. An exergetic efficiencyof 37% was calculated for the existing process; therewere plenty of possibilities for further improvement to43% efficiency with pay back times less than 1.5 years(Van Schijndel et al., 1998). Improvement optionsinclude improvement of the pre-heater system andinstallation of a pre-calciner (20% less fuel),improvement of the dust filters (5% less fuel) andbetter kiln isolation (4% less fuel). Other options arethe installation of new burners, isolation of the heatexchangers and preheating of the fuel. Note that anincrease in exergetic efficiency decreases the demandfor heavy fuel oil and therefore puts less pressure onnon-renewable resources.

6. CONCLUSIONS

Three tools for cleaner production have beendiscussed. Pinch analysis can be used in a good way tooptimise heat exchanger networks, however exergyanalysis appears to be a more powerful tool than pinch

analysis. LCA on processes shows to be a suitable toolwhen focusing on environmental effects, howeverthere still exists some scepticism about the method. Acombination of the tools mentioned, includingcommon engineering sense, may prove to be a muchmore powerful tool for cleaner production.

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