Principal Tools for a Cleaner Chemical Technology

Patrick van Schijndel and Han van Kasteren∗

Faculty of Chemical Engineering and ChemistryKunststoffenhuis, Eindhoven University of TechnologyPO BOX 513, 5600 MB Eindhoven, The Netherlands

SummaryIn our developing world-economy the need for materials and products is growing very fast. This has an enormous effect on resource depletion and could exceed the ecological capacity of our planet. A major challenge lies in designing better (chemical) processes, and retrofitting existing plants. However it is a fact that economies grow much faster compared to efficiency improvements. As world-wide the attention towards better and cleaner processes grows, also the understanding of the principles of efficiency and environmental impact of processes becomes more important. This can be translated into a growing need for objective and certified tools to assess these processes. This paper focuses on the relationship between exergy, ecology and economy of chemical processes. The tools used, i.e. LCA, exergy analysis and economic assessment, are investigated for their usefulness in analysing, retrofitting and optimally designing (chemical) processes. Combination of the different tools can provide more information on the overall sustainability of processes. Therefore more attention to combination of tools in all engineering curricula besides or integrated in the general sustainability courses should be given.

IntroductionIn order to reach a sustainable society, large efforts are requested from scientists and engineers. These are necessary to improve production chains and processes in the (chemical) industry. However defining sustainable production is difficult. For an easier interpretation and decision making a single sustainability indicator is necessary. However, such an indicator does not exist at the moment. Instead of energy efficiency it is better to use exergy efficiency as it is linked with thermodynamics and gives more information on potential process improvements. Furthermore it has been standardised and it has been suggested that exergy efficiency would be a good candidate for a single sustainability indicator.

According to Brundtland most important criteria for a sustainable process are:1. More efficient use of resources, measured by thermodynamic perfection.2. Overall decrease of emissions, harmful for humans and eco-systems.3. Increased ratio of renewable resources against non-renewable resources.4. Closing of cycles, taking into account the other 3 aspects.

If these criteria are used in the design of better processes it means different tools have to be used of which the most important are exergy analysis and environmental Life Cycle Assessment. As in the case of the design and improvement of industrial processes also economical evaluations have to be performed. This paper will describe the use of these indicators and possible combinations to achieve a valid sustainability tool for industrial processes.

Sustainable Process ChainsThinking in terms of life cycles started since the energy crises in the seventies. The idea was that the environmental burden of a product is not only situated at the actual production step but also in the other stages of production and utility generation. For a product a simple life cycle is shown in Figure 1.

Tel. + 31 40 247 5435; Fax: + 31 40 2475430E-mail address: j.m.n.v.kasteren@tue.nl

An environmental Life Cycle Assessment of a product determines the environmental impact of all phases in the product life cycle calculated for a certain functional unit. Every process step has to be investigated by determining all inputs and outputs, like shown in Figure 2.

Figure 1. Process chain Figure 2. Black box model for a single process

Several authors have proposed to combine the LCA with exergy analysis of the whole product chain, because cumulative exergy efficiency gives an indication of total resource depletion. This is only the case if all exergy losses are connected to non-renewable exergy resources.

Exergy, Ecology and EconomyA Combination of exergy efficiency, environmental impact and an economic evaluation, the 3 E’s, would be a good indicator for sustainability according to its definition (Rivero 1997).

Exergy analysis toolIn process analyses it is important to use good defined exergy efficiency definitions (Van Schijndel 1998). The second law efficiency of a process is the ratio of all exergy streams out and in (Ex out / Ex in). The Fratzcher or real efficiency is defined as the ratio of utilisable product streams and exergy in, equation 1 (Sorin, 1998):

in

ext

in

ureal Ex

DI1

Ex

Ex +−==η (1)

If a larger part of the exergy input is not transformed into the useful products, it is better to use the utilisable exergy coefficient, ηu,tr:

utrin

utrextuu ExEx

ExDEx

,

,

−−−

=η (2)

In most cases it is advisable to use real or utilisable exergy efficiency instead of the second law efficiency because the main interest lies in the efficiency in which an utilisable product is produced. Since all production routes are linked and because thinking in cycles is getting large attention it is obvious that cycle efficiencies should be introduced in exergy efficiency calculations too. Szargut (1990) defined the cumulative degree of perfection as ratio of specific exergy and CExE:

CExE

ExuCExE =η

(3)

CExE is the consumption of exergy of natural resources in all links of a technological manufacturing network leading from raw materials to a product. The exergy efficiency calculated in this way also takes the upstream processes into account, which can also be extended into a full LCA.

Some other extensions to the exergy analysis method should be made to make it more useful for engineers working on process improvements. One important issue is to distinguish between renewable and non-renewable exergy streams. Renewable streams, for instance solar light, wind and biomass can be zero depletion resources. Although renewables can cause environmental problems, the exergy losses in using renewed resources are less problematic compared to exergy losses in non-renewable resources in terms of sustainability. A ‘renewed exergy index’ can be

raw materialextraction

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defined focusing on the fact that renewed exergy streams are ‘free’ because they are generated in a certain amount per annum:

( ) renewedin

u

Ex - Ex

Ex index exergy Renewed =

(4)

In this equation the Exrenewed should be the net cumulative renewed exergy in. This index can be larger than one as explained in Table 1.

Table 1. Renewed exergy index

Index

Explanation

<<1 very small efficiency, low use of renewable exergy

<1 small efficiency, low use of renewables

= 1 fossil exergy input equals exergy value of product (no net fossil loss)

>1 highly efficient process, high ratio of renewables

>>1 Reversible process without waste, large input of renewables

Biomass as sustainable resourcePlant biomass is used as both an energy carrier and a raw material (construction material, resource for chemicals, paper production). World wide about 15% of energy use is accounted for as biomass. Last 10 years research on biomass as feedstock has received renewed attention in an effort to decrease the use of fossil fuels (fossilised biomass). By using biomass instead of fossil fuel the net amount of CO2 emitted into the atmosphere will be less. Another reason to use biomass is its composition. Biomass contains less S and N compounds compared to oil and coal, which means their combustion, can be less environmentally unfriendly.

However, these advantages should not be generalised without a proper analysis. Biomass needs a large ground surface to be grown (250 GJ poplar wood per hectare⋅annum as stated by Schaidhauf, 1998). Still with increasing productivity of agricultural food production the available area for extra biomass production is growing every year in the western world. Also, it is known that in growing 10 MJ of biomass a fossil fuel input of about 3,5 MJ is needed through the process chain i.e. transport, tractors, fertilisers and pesticides production. Still the switch from fossil based to biomass resources will cause a net reduction in CO2 emissions, however not for the full 100%. Furthermore Haberl and Geissler (2000) state that an increased use of biomass can disturb biochemical flows (i.e. nitrogen and carbon) and decrease biodiversity regionally. Also extended use of natural biomass sources leads to over cutting and overgrazing causing soil erosion and related environmental problems. Because large amounts of bio waste, about 10% of commercial energy supply, are not being used at the moment, the use of these wastes could be a first answer to these disadvantages.

Economic evaluationNowadays economy is based upon ‘free’ trade and costs being determined by demand and supply. The free economy works very well in determining prices. However many scientists argue that certain costs are not being reflected in these prices. It is true that the price of using ecological services like trees cleaning the air, the capacity of soil to filter groundwater, photosynthesis, and different cycles are kept outside the classical economy. Parts of these ecological services are being damaged by economic activities through emissions to water, soil and air, use of surface etc. Therefore there is a need to combine both systems and internalise them. It is estimated that total global national product lacks about 50% of money due to ecological services not accounted for.

However these calculations are very difficult and only give some feeling on the fact that the economy would come to a total halt without ecological services.

The big question remains on how to internalise environmental costs in our economy. Costs to be specified are pollution via emissions and the use of resources, which is sometimes calculated as the entropy production or exergy loss. The costs of emissions on the environment are clear, like in health problems, corrosion of iron and concrete structures, and damage to eco systems. But the financial quantification of these costs is difficult. Another way to calculate costs caused by emissions is via cleaning up costs. This can be done for instance for oil spills. But these clean up costs are also hard to quantify. A compromise solution is to calculate prevention costs, which are made up by the different costs of cleaner technology and end-of-pipe solution needed to cut emissions to a more sustainable level. It is easy to understand that prevention costs are lower than clean up costs or costs of ecological damage. For use in The Netherlands Vogtländer (2000) has determined prevention costs for some important environmental impact categories as shown in Table 2. Table 2. Marginal prevention costs for important environmental impact categories

Environmental Impact Category Marginal prevention costs*

In € / kg equivalentsAcidification 6,40 € / kg SO2

Eutrophication 3,05 € / kg PO4

Summer smog 50,00 € / kg VOCWinter smog 12,30 € / kg fine dustHeavy metals 680 € / kg ZnCarcinogenics 12,30 € / kg PAHGlobal warming 114 € / 1000 kg CO2

* Data calculated for Dutch situation

Environmental prevention costs can be applied to compare environmental impact of different techniques. It is also possible to check the real economic feasibility of environmental solutions. The calculation of depletion costs is very difficult. A certain amount of exergy is produced on earth ‘for free’, which is called renewable. This should be left out of depletion cost calculations. A possible way to calculate the depletion costs is by coupling the exergy values of the resource with the price of fossil fuels. However the price of fossil fuels is not depending on the actual proven reserves but on a ratio between production capacity and anticipated or actual demand for oil. As oil reserves are proven to be well beyond 10 years from now there is no real effect on its price other than a sort of psychological effect and some extra costs due to increasing complexity of oil extraction techniques used. By combining the prevention costs, raw materials costs and depletion costs a sort of combined triple E model based on economics can be established, which is useful as a sustainability indicator.

Industrial casesWithin major industries there have been important developments world-wide like:- Increase in production (see Figure 3), especially in major developing countries like China.- Efficiency improvement and decrease of specific environmental impact mainly by instalment

of new technology.- Overall strong increase of electricity production- Overall strong increase in use of fossil fuels and raw materials

Figure 3. Total world production of some important metals and minerals

Exergy analysis of some chemical processes as carried out by Hinderink (1999) show that the process industry still has enough potential for improvements, as shown in the adapted Figure 4.

Figure 4. Exergy losses during production of important commodities

The conclusion is that most processes have become more efficient but due to the enormous growth (potential) the use of resources, fossil fuels and the environmental pollution has increased. Although new technology is available many older plants remain in function and are not being upgraded due to lack of funds or lack of government intentions and environmental law enforcement.

Case study 1: Simple economical and environmental decision model

Biomass as a sustainable energy source within The NetherlandsThis study was done to find the best technique of generating electricity using biomass in respect of economy and the environment (van Schijndel, 1999). The available land surface within The Netherlands is too small to make biomass farming a very large and attractive activity in the near future. However different biomass waste streams are available in high enough quantities to built small-scale power plants or to co-combust biomass in existing power plants. First a preliminary literature survey was carried out to find out possible techniques for using biomass waste to generate electricity and ranking them according to technology and market perspectives.

For the Dutch situation following top 5 ranking of techniques was found:1. Co-combustion of biomass in a pulverised coal power plant with steam cycle.2. Gasification of biomass in an atmospheric fluidised circulating bed and co-firing the gas in a

natural gas steam boiler system.3. As 3 but with co-combustion of the gas in a natural gas fired combined cycle system.

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4. As 3 but with stand-alone combustion of the gas in a combined cycle system.5. Atmospheric stand-alone fluidised bed combustion with steam cycle system.Three of the best-ranked techniques (1,4 and 5) were evaluated on their environmental impact using a functional unit based upon exergy. The functional unit was defined as: Thermal treatment of an amount of biomass equal to 966 TJ calorific value (LHV) and joined production of 424,80 TJ electricity in one year. An elaborate discussion can be found in the Entrée 1999 proceedings (Van Schijndel 1999).

The results of the LCA on the three alternatives in Figure 5 show that the process with highest exergy efficiency, namely co-combustion with coal, also scores best on environmental impact. In Figure 5 the LCA scores are calculated using eco-indicator ’95 method. High scores mean high environmental impact.

Figure 5. Results in Eco-Indicator 95 points for environmental comparison of techniques

It can be concluded that in the case of power production the exergy efficiency has a large impact on the outcome of the LCA. A consequent ranking of technical, economical and environmental issues is a good way to choose a sustainable option.

Case study 2: Eco costing method as a tool to internalise environmental burden

Production of container glassThe production of container glass involves basic chemicals like sand and sodium carbonate and a large input of fossil fuels. The high temperature glass melting process has been improved tremendously over the years by technical improvements of the kiln, re-use of exhaust gases and not least the recycling of glass, see Figure 6.

Figure 6. Historical development of specific fuel use in container glass production

Using exergy analysis and an AspenPlus simulation model a process improvement using fuel reforming through exhaust cooling, chemo-recuperator (TCR), has been investigated. LCA was combined with the emission prevention methodology (Table 2) in order to calculate the total costs for container glass production for a standard glass kiln and improved processes.

When environmental issues are internalised within the ‘normal’ production price of standard container glass, the eco costs of emissions are about 66%. Raw material and fuel costs per kg of glass equal about 0,09 €, environmental protection costs are much bigger since they are about 0,18 € per kg of glass. For the two types of kiln eco cost calculations are shown in Figure 7

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In this Figure, BAT stands for best available technique or best available glass kiln system. When hydrogen is used, for instance from biomass, the eco costs are much lower. The new TCR kiln natural gas fired system also has a better eco costs score almost equal to the hydrogen-fuelled system. The high eco costs compared to raw materials used are clearly shown in the figure. Use of this prevention costs model, the triple E model, can be a good one number indicator for process improvement studies or comparison studies. The figure also shows that the ecological impact is not solely determined by the exergy (fuel) efficiency.

Figure 7. Results for triple E costs of glass production (Van Schijndel 2000)

Besides emission also the depletion of resources should be quantified. It is hard however to quantify the costs for depletion. A possible pricing method can be pricing the exergy value of a resource and linking it to a specific price for instance the price of fossil fuels.

Case study 3: Relationship between exergy efficiency and environmental impact

Cement industry This industry is a great importance for world-wide economical development as it forms the basis for concrete, main material for buildings and other civil constructions. Figure 3 shows the tremendous volume growth in cement production. China and India mainly caused the growth in the nineties. The efficiency of cement making for new plants lies around 50% as shown in Figure 4, however many plants in operation are outdated and their efficiency lies around 40%. Main environmental effects in cement making are pollution by dust particles, SO2, NOx and CO2. Large amounts of CO2 are produced by fossil fuel combustion but also due to decomposition of the calcium carbonate. The relationship between the exergy efficiency (eq. 1) of an oil-fired cement kiln and the CO2 emissions is shown in Figure 8.

Figure 8. Relationship between CO2 emission and exergy efficiency of a cement kiln

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CO

2 em

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[kg

/kg

cli

nke

r]

A life cycle over the cement plant itself shows that the production phase has a much larger impact compared to construction and demolition phase (i.e. over 90%). As a large part of external exergy losses are caused by heat radiation most important developments have been the enlargements of the production facilities. In Japan the hot flue gases are used for electricity generation, in other countries these energy savings are economically non-feasible due to low electricity prices. Exergy analysis in combination with LCA has been used to compare three plants (two in Tanzania and one in South Africa) on their efficiency, environmental impact and low cost improvement options. Upgrading dust filters and decreasing driving forces are the most important improvement options. More attention also should be on the milling and grinding of the resources and final product as their efficiencies are very low.

Figure 9. Influence of electricity generation on environmental impact of cement production

Results for the influence of exergy efficiency and choice of fuel for electricity generation on the environmental impact are given in Figure 9. The influence of the type of electricity generation on the environmental impact is higher than the increase of exergetic efficiency of the cement kiln. This result clearly illustrates the importance of chain analyses when improving a process.

Results and discussionDifferent tools have been introduced and discussed in the case studies. Most tools are known although the combined use of them has not become general practice yet in industry and in the engineering companies. Therefore focus on combination of these tools should be an important issue in engineering education. The relationship between exergy efficiency on one hand and environmental impact on the other hand is not very clear. Therefore exergy analysis is not a good sustainability indicator. Another important aspect is the way renewable resources are used in exergy analysis. As renewable streams do not deplete resources they should be treated different from non-renewable exergy streams, like in a sort of renewed exergy index.

ConclusionsProcess improvements have been tremendous in the last century but production volume increase will overshadow these good results in terms of resource use and environmental impact. It will be important to use the right tools in order to achieve the necessary sustainable development within the industry. These tools should be combinations of exergy analysis, LCA and economic analysis. The focus should be on the development of these combinations and on the teaching of these combinations in engineering curricula.

NomenclatureDext external exergy loss (kJ)Ex exergy value (kJ)ExRenewed renewable exergy (kJ)Extr transiting exergy (kJ)Ex u total utilisable exergy output (kJ)

ReferencesHaberl, H. and Geissler S., 2000, “Cascade utilization of biomass: strategies for a more efficient use of a scarce resource”, Ecological Eng., Vol 16, pp. S111-S121.

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ct [ 

mic

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 ] PPCPretoria

TangaTPCCDar es Salaam

fuel mix

coal

electricity NL

hydro power

Hinderink, P., Swaan Arons J.d. and Kooi, H. v.d., 1999, “On the efficiency and sustainability of the process industry”, Green Chemistry, December 1999, pp. G176-G180.

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Van Schijndel, P., Janssen, F., Mrema, G. and Greeff, I., 2000, “Exergy analysis and environmental impact assessment of cement production: Tanzania and South African case studies”, Proceedings of ECOS 2000, Hirs, G., ed., University Twente, The Netherlands, pp. 1425-1436.

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PersonaliaThe author;Patrick van Schijndel studied chemical engineering at the Eindhoven University of Technology (TU/e) and graduated in 1994. He got his teaching degree in chemistry at TU/e in 1995. Since 1996 he is working on his PhD thesis in the field of cleaner production. He combined this work with setting up a MSc. course and several research projects in environmental technology at the University of Dar Es Salaam in Tanzania. He will receive his PhD degree in the beginning of 2002. In November 2001 he joined the Kunstoffenhuis group at the Dutch Polymer Institute (DPI) as process designer and environmental consultant.

The co-authors;Han van Kasteren studied chemical engineering at TU/e and in 1990 he received his PhD degree. In 1990 he worked at the Inter-University Environmental Institute Brabant (IMB). From 1991 he started working as assistant professor at the TU/e, in the field of environmental technology. From 1996-1999 he was director of the Polymer-Recycling Institute, PRI at the TU/e. At PRI economic and technical feasibility studies of the recycling of wastes are carried out. Since 1999 he is director of the Kunstoffenhuis at TU/e. The Kunstoffenhuis is the intermediate between the polymer industry and knowledge institutes. Several projects running are: recycling of toner waste, small-scale plastic waste gasification and design of environmental friendlier products in the electronic industry.

Address:Kunststoffenhuis, Faculty of Chemistry and Chemical EngineeringEindhoven University for Technology, P.O. Box 513, 5600 MB EindhovenThe Netherlands; Tel. + 31 40 247 5435; Fax: + 31 40 2475430E-mail address: j.m.n.v.kasteren@tue.nl

Photos of both authors are placed in a separate file!