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Program for North American Mobility in Higher EducationIntroducing Process Integration for Environmental Control in Engineering Curricula Module 3: Environmental Challenges – Pulp & Paper Industry Created at: École Polytechnique de Montréal & Texas A&M University, 2003
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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 .
Problem Statement for Q-1 & 2 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.
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.
Question 1 A few months ago the company ordered a partial LCA study in order to have an idea about its life cycle environmental impacts. As a first step, your boss had asked you to look at this study as well as at the mill simulation and give 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.
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.
Chemical Production • Chemical production as been included into the system boundaries. Chemicals are considered to be transported 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.
Birch Growth and Harvesting • Birch growth and harvesting as been included in the boundaries. The wood is transported an average of 100 km. The same assumptions as for chemicals apply.
Others • By product have been located. • A credit has been considered for the generated energy (but only on the energy). • Pulp is transported an average distance of 200 km to the customer (same assumptions as chemicals). • Industrial landfill is located 5 km from the mill. 16 ton diesel trucks are used to transport the solid wastes, the return of the trucks is considered negligible.
Necessary documents • LCA Base Case • Process Simulation
Question 2 • Your boss is convinced that most of the competitive advantages that can be gained with environmental improvements are related with fresh water reduction. • 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.
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.
Question 2 (Cont’d) • Using the LCA model, discuss if this represents 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.
Resource depletion: Emissions: Question 2 (Cont’d)
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
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%). Solution – Q1 (Cont’d)
Solution – Q1 (Cont’d) • The following graph show the contribution of each process unit to resource consumption.
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.
Solution – Q1 (Cont’d) • The following graph show the contribution of each process unit to emission-related environmental impacts.
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.
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 last two 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 following 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Problem Statement – Q3-7 • Consider the following Kraft pulp mill depicted below wash pulp water chips D E D E D screening Brown Stock Washing To papermaking Digester Flue Recovery Boiler Gas concentrator cond. cond. SBL weak ESP smelt black liquor salt Multiple Effect Evaporators cake dust recycle wash white liquor lime fluegas water mud weak white liquor dissolving wash tank mud lime kiln water dregs white liquor filter dregs mud clarifier washer washer & filter green liquor clarifier causticizer grits slaker
Problem Definition • Chips = 6000 tons (wet basis) • Moisture = 50% = 0.5*6000 t = 3000 t • Pulp Yield = 50 % of Dry = 0.5 * 3000 t • Consistency (CY) = 0.12 • Dilution Factor (DF)= 2 • Wash Water for Pulp = [(1-CY)/CY] +DF • Ion Content of Process Water: • Cl = 3.7; K = 1.1; Na = 3.6 (values in ppm)
Problem Definition • Given this Kraft pulping process, it is desired to develop cost-effective strategies for the reduction of water discharge from the mill. It should be noted that any water reduction objectives will entail the use of recycle; consequently, various species will build up in the process, leading to operation problems.
Problem Definition • To alleviate the detrimental effect of build-up, comprehensive mass integration strategies are required to provide answers to the following questions: • What are the rigorous targets for reduction in water usage and discharge? • Which streams need to be recycled? To which units? • Should these streams be mixed or segregated? • What interception devices should be added to the process? To remove what load? • What new research needs to be developed to attain the optimum solutions? Q3 – 7 will address some of these questions
Question 3 • What are the rigorous targets in water discharge and reduction?
Species Tracking Model • Before one can begin to tackle the water targeting problem, it is crucial to develop a species tracking model of the system with the right balance in details. • A too-simplified model will not adequately describe the process nor will it capture critical aspects of the process. • A too-detailed model cannot be readily incorporated into the process integration and optimization framework and will negatively impact the effectiveness of the optimization computations.
Species Tracking Model • In order to develop the species tracking module, we will make use of path diagram equations, perform degrees of freedom analysis, and use the mixer splitter models. These topics were covered in Module II, though they are included here as a quick reference • Path Diagram Equation • Degrees of Freedom • Mixer-Splitter Model
Mathematical Modeling • The modeling techniques covered in module II allow one to describe unit performance without requiring detailed models while still capitalizing on nominal plant data and knowledge about the process. With this information, one can begin to make choices for the selected model and streams/species. • Consider the following unit:
Pollutant/Water Load Balance Representation W1 (kg water/s) P1 (kg pollutant/s W2 (kg water/s) P2 (kg pollutant/s W3 (kg water/s) P3 (kg pollutant/s W4 (kg water/s) P4 (kg pollutant/s
Mathematical Modeling • W and P refer to the loads of water and a pollutant, respectively. • Suppose that the load of the water were to change as a result of process improvement (e.g. mass integration). The load of the pollutant will be affected as well; thus, it will be necessary to determine the new load of the pollutant. • Furthermore, suppose that there exists a proportional relationship between the pollutant loads in streams 2 and 3 (much more so than between streams 1 & 3, 1 & 4, etc).
Mathematical Modeling • With this knowledge, the ratio model can be used to relate the pollutant loads in streams 2 and 3: P3new = (P3old/ P2old) * P2new • The pollutant load in stream 4 can then be determined by a simple component material balance: P4new = P1new + P1new + P3new
Nominal Balance Model • By using these modeling techniques, path equations can be developed for tracking water and targeted NPE’s throughout the process, resulting in a mathematical model for the nominal case study. The nominal case study can then be revised to reflect the impact of mass integration on the process.
Nominal Balance Model • For this case study, the nominal balance model will be developed with the purpose of tracking water and three nonprocess elements, chloride, potassium, and sodium. These ions were selected because they are among the most important species that cause buildup problems and limit the extent of mass integration
Nominal Balance Model • Using process knowledge, nominal plant data, modeling techniques, initial assumptions, etc., one can begin to develop the nominal balance model unit by unit. • The overall result for the nominal balance model will be provided at this time. However, the full development of the nominal balance is provided at the end of this module for the reader’s understanding. Nominal Balance
Material Balance Stripper Stripper W2 = 13995 W6 = 1450 Chemicals W4= 10995 W33 = 30990 C2 = 0.052 C6 = 0.005 33 C4 = 0.492 W7 = 10995 2 6 K2 = 0.015 K6 = 0.002 K4 = 0.819 C7 = 0.394 N2 = 0.050 N6 = 0.005 Bleach Plant N4 = 4.347 K7 = 0.655 7 Screening N7 = 3.478 W35 = 10995 Washer Pulp Bleached pulp 8 W8 = 1450 4 C8 = 0.104 to papermaking Wood chips K8 = 0.165 N8 = 0.875 W37 = 30990 37 W1 = 3000 10 C37 = 15.495 W10 = 8901 C1 = 1.000 W12 = 1024 W15 = 1202 K37 = 0.155 C10 = 0.000 K1 = 2.500 C1 2= 0.000 C15 = 0.197 N37 = 15.495 K10 = 0.000 Digester N1 = 0.973 W16 = 0 K12 = 0.000 K15 = 0.327 12 N10 = 0.000 C16 = 4.230 N12 = 0.000 N15 = 0.386 1 15 K16 = 9.904 W5 = 11127 N16 = 73.033 C5 = 9.838 W9 = 2225 W14 = 0 ESP K5 = 39.230 14 C9 = 9.838 C14 = 0.472 16 N5 = 483.020 W3 = 5127 K9 = 40.927 K14 = 1.146 W11 = 1202 C3 = 9.278 N9 = 483.020 N14 = 0.966 5 MEE Concent . C11 = 9.838 W13 = 1202 K3 = 39.230 13 K11 = 40.927 C13 = 4.899 N3 = 486.344 3 9 N11 = 483.020 K13 = 11.378 N13 = 74.385 11 Recovery W32 = 1016 White C32 = 2.173 W18 = 0 W26 = 423 Furnace K32 = 0.631 18 C18 = 0.182 C26 = 0.042 Liq Clar N32 = 89.882 K18 = 0.026 K26 = 0.042 26 N18 = 18.225 N26 = 12.033 Na SO Lime 2 4 W17 = 0 smelt 32 27 17 C17 = 9.351 W31 = 6143 Kiln K17 = 39.479 31 C31 = 11.451 N17 = 499.893 K31 = 39.862 W27 = 0 N31 = 576.226 W25 = 423 C27 = 2.297 W19 = 6402 Dissol . C25 = 2.339 K27 = 0.604 C19 = 1.272 K25 = 0.647 N27 = 0.633 K19 = 5.369 25 Causticizer N25 = 12.666 Tank N19 = 95.323 19 Washers/ W29 = 32 W23 = 3.84 24 W30 = 6143 W20 = 6402 C29 = 0.000 C23 = 0.038 Filters 20 C30 = 11.451 C20 = 10.623 K29 = 0.000 K23 = 0.004 30 23 K30 = 39.862 K20 = 44.848 N29 = 0.000 N23 = 0.960 W24 = 5762 N30 = 576.226 N20 = 595.216 C24 = 0.021 W22 = 51 K24 = 0.006 29 C22 = 1.455 N24 = 0.021 W21= 6351 Green Liq K22 = 5.382 Slaker C21= 9.167 N22 = 19.047 K21 = 39.466 W28 = 8 Clarifier 21 N21 = 576.169 C28 = 0.014 28 K28 = 0.209 N28 = 0.576 22