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An energy efficient evaporation process for treating bleach plant effluents
~
Pergamon
Applied Thermal Engineering Vol. 16, No. 1, pp. 33~12, t996
1359-4311(95)00015-1
Copyright © 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 1359-4311/96$9.50+ .00
AN ENERGY EFFICIENT EVAPORATION PROCESS FOR TREATING BLEACH PLANT EFFLUENTS A. G i d n e r , ~ . Jernqvist* a n d G. Aly Department of Chemical Engineering I, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden (Received 7 February 1995)
Abstract--Simulation results of an energy efficientevaporation process are reported for the treatment of bleach effluents in the pulp and paper industry. Due to the low concentration of the effluent stream, the evaporation process must have a high degree of energy efficiencyin order to compete with other treatment alternatives, such as ultrafiltration, adsorption, ion exchange and biological treatment. For a pulp and paper mill with an annual capacity of 335,000 ton of bleached kraft pulp, the capacity of the spent black liquor evaporation plant is about 7.5 ton water/ton pulp if its concentration is increased from 16 to about 65 wt% dry substance. An evaporation plant for the bleach effluent was simulated for a flow rate corresponding to 7.2 m3/ton pulp and a concentration of 1.4 wt% dry substance. This stream is to be concentrated to 16 wt% resulting in an evaporation capacity of 245 ton water/h. The total evaporation capacity for both evaporation plants would be increased by 91%. Optimal energy conservation strategies were investigated where an absorption heat transformer unit (AHT) is integrated with both the existing black liquor evaporation plant and the proposed bleach effluent evaporation process. Different process configurations were simulated using a flow sheeting program, developed for simulation of general multiple-effectevaporation processes and absorption heat pump systems. Using real operating data from a major Swedish pulp and paper mill, simulation results are reported for the optimum location of the AHT unit. Consequently, the energy requirements for both evaporation plants would increase by only 6.6%. One of the main practical features of this process is that integrating the AHT unit would require minimum changes in the existing plant. An economic analysis resulted in a cost of 23 SEK/ton of pulp for the first alternative where the pre-concentrated 16 wt% effluent stream is concentrated further to 65 wt%, within the existing black liquor evaporation plant, and then recycled to the recovery boiler. The corresponding cost of 49 SEK/ton of pulp would result for the second alternative, where the 16 wt% effluent stream is sprayed on the waste from bark peeling and co-burned in the bark boiler. Keywords--Effluent treatment,
evaporation, absorption heat transformer, paper industry.
INTRODUCTION I n response to stringent e n v i r o n m e n t a l regulations a n d a strong c o n s u m e r d e m a n d for e n v i r o n m e n tally friendly products, the p u l p a n d p a p e r i n d u s t r y is intensifying its efforts to m a k e significant changes to the milling process, as well as the effluent m a n a g e m e n t systems, to virtually eliminate the g e n e r a t i o n a n d discharge o f toxic c o n t a m i n a n t s at a n affordable cost. Nearly all mills have already closed up the digesting cycle, while further research is still needed to close u p the bleaching cycle. U s i n g c o u n t e r - c u r r e n t w a s h i n g schemes, the v o l u m e o f the bleaching effluents have been c o n s i d e r a b l y decreased from 2 0 - 3 0 to less t h a n 10m3/ton pulp. F u r t h e r efforts are targeting possible flows o f less t h a n 5.5 m3/ton pulp for the near future [1]. Different m e t h o d s to treat the kraft bleaching effluents, i n t e r n a l as well as external, have been investigated; for example, ultrafiltration [2-4], ion exchange [5], a d s o r p t i o n [5, 6], a n d biological t r e a t m e n t [6-8]. I n a n experimental study where seven different m e m b r a n e s were investigated, J 6 n s s o n [3] c o n c l u d e d that some o f the tested m e m b r a n e s could p r o b a b l y c o m b i n e a satisfactory flux a n d a n acceptable retention. A r o u g h cost estimate revealed that t r e a t m e n t of the caustic E-stage effluent with ultrafiltration could be p e r f o r m e d within a total cost of a b o u t 25 S E K / t o n pulp. Effective c o l o u r r e m o v a l by a d s o r p t i o n o n cross-linked polymers i n c l u d i n g a n i o n exchange resins was studied experimentally [5]. This m e t h o d requires a n acidification of the caustic extraction stage (E-stage) effluent and, to avoid the precipitation o f extractives, the effluent was passed *Author to whom correspondence should be addressed. 33
34
A. Gidner et al.
through a column packed with a non-polar resin before acidification. The resins used had to be regenerated for re-use. Adsorption was studied as one of some physiochemical methods to apply on biologically pre-treated bleaching effluents [6], where about 90% COD (chemical oxygen demand) and DOC (dissolved organic carbons) elimination rates could be achieved. Spent bleaching effluents from both chlorination and E-stages were subjected to bench-scale biological treatment followed by one of some physiochemical methods such as ozonization, ozonization/irradiation, and adsorption [6]. It was shown that, for the biologically pre-treated effluent, which was further treated by ozonization or ozonization/irradiation, removal rates of 61, 81, 98 and 92% could be achieved for DOC, COD, colour (436 nm) and AOX (adsorbable organic halides), respectively. Conversely, adsorption could achieve removal rates exceeding 90% for DOC, COD and colour (436 nm). In another recent study, a combined ozonization-biological method was applied on the alkaline E-stage effluent [7]. It was concluded that both ozone and biological treatment effectively destroyed effluent chromophores, but the fungal process resulted in greater degradation as expressed by COD removal. The combining of a brief ozone treatment with a subsequent fungal treatment revealed a synergism between the two decolorization mechanisms on the complete alkaline E-stage effluent but not on the high-molecular-weight fraction alone. In another recent study using biological treatment, AOX removals averaged 46% for two activated sludge systems and 34% for five aerated stabilization basins. Both schemes averaged removal of over half of the influent low molecular weight AOX while the average removal of high molecular weight AOX varied from 1 to 47%. It should be observed that the above-mentioned methods were applied on chlorine bleaching effluents. The Swedish pulp and paper industry applies, since 1992, either ECF (elemental chlorine free) or T C F (totally chlorine free) bleaching sequences. Due to the rapid development in the bleaching technology, the resulting effluents have different compositions and volumetric flow rates. It should therefore be recognized that very little hard data are available on the characteristics and toxicity from the ECF and T C F bleaching processes. An evaluation of the economic feasibility of concentrating the kraft bleaching effluent using an energy-efficient evaporation scheme is presented in this paper.
ABSORPTION HEAT CYCLES Absorption heat pumps (AHP) are based on the principle of utilization of the enthalpies of evaporation-condensation of solutes in appropriate working fluid mixtures. These systems consume very little electric energy and mainly use a high-temperature primary heat source and a low-temperature waste heat source to recover useful heat at an intermediate temperature, as illustrated in Fig. 1. Some electric energy is used only for the purpose of recirculating the working fluid. An absorption heat transformer (AHT) is a reversed absorption heat pump, where heat is supplied at an intermediate level and useful heat is delivered at the highest temperature, provided there is a heat sink with a lower temperature level than that of the waste heat supplied. The heat released in the heat sink is normally not used, since it is at a relatively low temperature. The heat transformer, which consists of four main components, is normally operated at three temperature and two pressure levels, as can be seen in Fig. 2. Besides those four components, the system usually includes one heat exchanger and two pumps. Both absorber and evaporator operate at a higher pressure level than the generator and condenser. The waste heat input is supplied to the generator and evaporator, at a middle temperature level, while about half of the heat input is recovered at a higher temperature in the absorber and the other half is dissipated at a lower temperature in the condenser. The available useful energy output of an A H T is, accordingly, about 50% of the waste heat input. However, an A H T does not consume high-grade thermal energy and only a small amount of electric energy is used to recirculate the liquid streams. The circulation pumps can be totally eliminated if the heat transformer is operated in the self-circulation mode [9-1 1]. A working fluid pair is circulated between the absorber and generator to provide the heat pump loop. Absorption occurs in the absorber and desorption occurs in the generator. Working fluids
Treating bleach plant effluents
35
~T
/
useful heat
QGI--
%
I--
%
QE
Absorption
Absorption
heat pump
heat transformer
Fig. I. Schematic definition of absorption heat cycles (AHP and AHT).
consist of a refrigerant and an absorbent. When heat is added to the working fluid mixture, the refrigerant evaporates, leaving behind a weak working fluid. As the refrigerant is condensed, the latent heat is given off to the cooling water. When the refrigerant is vaporized in the evaporator and rejoined with the weak working fluid, heat is liberated to the process. A portion of the waste heat is theoretically recoverable to produce steam or to supply other process heating requirements. The heat recovered would reduce the primary energy input into the process. Incorporating various types of heat pumps within different units in a process burdened with waste heat streams having appropriate temperature levels is an efficient method for energy recovery. A detailed literature survey concerning industrial applications of absorption heat cycles was recently conducted [10, 12, 13]. It was revealed, however, that relatively few applications have so far been implemented in the process industry. In contrast, based on technical feasibility and preliminary economic competitiveness, it was concluded that the commercial development of industrial heat pumps in 27 selected processes in the US is projected to be 1110 units by the year 2010 [13]. Based on the types of processes evaluated, most of the heat pump installations will be in the chemical industry (684 units). The food processing, pulp and paper, and petroleum refining industries are projected to install 184, 168 and 71 units, respectively. E V A P O R A T I O N S Y S T E M S IN T H E P U L P AND P A P E R I N D U S T R Y Due to the fact that evaporation is one of the most energy-intensive unit operations in the pulp and paper industry, together with the availability of many waste heat streams at moderate temperatures, and since evaporation and absorption heat cycle processes are interrelated with respect to operation and dynamics, it is quite possible to achieve energy savings by process integration of an AHT.
Evaporaito
sorber
I ~
enser I Tc
Heatexchanger
j era,or I I TG TE
.... ........ I TA
Fig. 2. Schematic flowsheet for the AHT system.
Condensate Vapour Weaksolution Strongsolution
>
36
A. Gidner et al.
Fig. 3. Schematic flowsheet of the black liquor evaporation plant.
Figure 3 shows a schematic flow-sheet of an existing black liquor evaporation plant consisting of six effects. The thin liquor, with a concentration of 16 wt% dry solid contents, is fed to the fourth effect and the thick liquor leaves the second effect at a concentration of 65 wt%. The layout of the unit is quite conventional and comparable to most Scandinavian black liquor evaporation plants. SIMULATION RESULTS The flow sheeting program EVAPSYS, developed for both design and evaluation calculations of any type of evaporation plant was used for the simulation of the multiple-effect evaporation processes investigated in this work [14]. This program enables the evaporation plant to be simulated either separately or integrated with an energy conservation scheme, such as a mechanical compressor, a steam ejector, or an absorption heat cycle. Different component modules are incorporated to provide the user with maximum flexibility in constructing any evaporation plant. These modules include an evaporator, heat exchanger, expansion vessel, mixer, splitter, mechanical compressor and a steam ejector and would thus enable the user to easily assemble evaporation flowsheets of arbitrary complexity. Furthermore, other component modules are incorporated to simulate single-stage absorption cycles. These modules include an absorber, generator, condenser and heat exchanger. This arrangement allows for executing the program in three different modes: evaporation plants, absorption heat cycles, and heat-pump-driven evaporation systems. One of the difficulties in applying energy conservation techniques to existing multiple-effect evaporators is the problem of accurately predicting the change in heat transfer coefficients. For this reason, three subfunctions for computing the overall heat transfer coefficients are incorporated. The first subfunction is used for black liquor in vertical tube evaporators, the second for forced circulation evaporators and heat exchangers and the third is used for falling-film evaporators. Physical properties of different liquors for evaporation applications (such as sea water, sugar solutions, skim milk, black liquor, Mg-based sulfite liquor and Ca-based sulfite liquor) and different working fluid pairs for absorption heat cycle applications (such as H20-LiBr, H zO-NaOH, H 20-CAC12, CH30H-LiBr, H 20-Glycerol, H 20-Glyco1, NH3-H 20 and H20-LiNO3/KNO3/NaNO3) are available in the database of the program. Using a minimum of input data, EVAPSYS computes pressure, temperature, concentration, enthalpy and flow rate profiles for the simulated system. The program is designed to provide maximum flexibility, as well as maximum simplicity of the flowsheet input data procedure. The program is considered as a powerful tool for the simulation of different process configurations for evaluation, design and optimization of energy efficient evaporation systems [10-12, 14]. A large number of computer runs were performed to establish both local and global optimal energy conservation strategies for the evaporation plants investigated in this work. The black liquor evaporation plant
Considering a pulp and paper mill with an annual capacity of 335,000 ton pulp, the flow rate of the spent black liquor would be reduced in the evaporation plant from 100 to 25 kg/s. This corresponds to an evaporation capacity of 270 ton water/h or about 7.5 ton water/ton pulp. Some boundary conditions must be fulfilled during the search for optimal energy conservation strategies. The most important boundary condition is to avoid major changes in the existing black liquor evaporation plant when the AHT system is incorporated.
Treating bleach plant effluents
37
The black liquor evaporation plant was first simulated using actual operating data, provided by a major Swedish pulp and paper mill, to calculate complete temperature, concentration and overall heat transfer coefficient (U-values) profiles. The U-values and apparent temperature differences were in the range 0.35-1.82 kW/m2K and 3.1-14.4°C, respectively. Further, the simulations resulted in a total heat transfer area of 30,500 m 2 and a total live steam requirement of 15.2 kg/s, which is equivalent to 32.6 M W of thermal energy.
The proposed bleach effluent evaporation plant The proposed flowsheet of an evaporation plant for the bleach effluent was simulated for an estimated flow rate of 75 kg/s, corresponding to 7.5 ton efffluent/ton pulp, and a concentration of 1.4 wt% dry substance. This stream is to be concentrated to 16 wt%, resulting in a flow rate of 6.6 kg/s. This corresponds to an evaporation plant capacity of 246 ton water/h. The concentrated effluent stream can either be fed to the black liquor evaporation plant for further concentration, or sprayed on the waste from bark peeling and co-burned in the bark boiler. The proposed evaporation plant consists of six falling-film effects with equal heat transfer areas. Due to small boiling point elevations of the bleach effluent within the concentration range considered in this work and the relatively smaller temperature differences which can be attained in falling-film evaporators, the evaporation plant can preferably be operated at pressures above atmospheric. Consequently, the plant will be driven by live steam at 140°C (i.e. the same live steam source as used for the black liquor evaporation plant) and the condenser temperature will be maintained at 100°C. Consequently, the U-values and apparent temperature differences were in the range 2.50-3.40 kW/m2K and 5.7-6.9°C, respectively. The simulations also resulted in a total heat transfer area of 8, 100 m 2 and a total live steam requirement of 12.9 kg/s, which is equivalent to 27.7 MW of thermal energy. It may be pointed out that, compared to the black liquor evaporation plant, considerably higher U-values were obtained in the effluent plant due to the concentration and temperature ranges, as well as the falling-film configuration of the heat transfer area. Consequently, the heat transfer area required for the bleach effluent evaporation plant is only 26.6% of that required for the black liquor evaporation plant, although both plants have nearly the same evaporation capacity expressed in kg evaporated water/s.
The hybrid AHT-driven evaporation plant Different system configurations were simulated to determine the optimum number of effects to be used in the bleach effluent evaporation plant, and the optimum location and size of the heat transformer to be incorporated between both evaporation plants. It was concluded from the simulation results that five effects would be required in the bleach effluent evaporation plant. This can be explained by the fact that it is necessary to achieve an optimum relationship between the flow rates of the vapour streams leaving the black liquor and bleach effluent evaporation plants
10.7 k~/s 9~C 11.6 kg/s
vapourfromthe black liquorevaporationplant 12.7 kg/s 6.6 k~s
97 'C 125 'C 3.4 kg/s 15.0 kg/s 60 "C 0.014 kg/kg Fig. 4. The hybrid AHT-driven evaporation plant (closed cycle).
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A . G i d n e r et al.
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Table 1. Basic design parameters for the bleach effluent evaporation plant Variable
Unit
Effect 1
Effect 2
Effect 3
Effect 4
Effect 5
Area U-value AT
m2 kW/(m2"C) C
1960 3.2 5.0
1960 3.0 5.1
1960 2.8 5.4
1960 2.6 5.8
1960 2.5 6.0
and which will be utilized as input thermal energy in the generator and evaporator components of the AHT, respectively. Further, it would be necessary for the bleach effluent evaporation plant to be driven by live steam having a saturation temperature of 125°C, instead of 140°C, in order to minimize the heat transfer area required in the solution heat exchanger. To find a configuration that achieves considerable energy conservation, the A H T has to be designed to deliver steam to the first effect of the bleach effluent evaporation plant. This would give the plant a total temperature difference which is given by the optimum design of the AHT. This arrangement can be achieved by utilizing 10.7 kg/s or 80.5% of the vapour leaving the last effect of the black liquor evaporation plant, at 69°C and 30kPa, as heat input to the generator component of the AHT, while all vapour leaving the bleach effluent evaporation plant, 12.7 kg/s at 95°C and 85 kPa, will be utilized in its evaporator component, as illustrated in Fig. 4. For this configuration, the simulation results revealed that the bleach effluent evaporation plant would require a total heat transfer area of 9800 m 2 and 3.4 kg/s live steam. This means that although the total evaporation capacity, for both black liquor and bleach effluent plants, has been increased by 91%, the energy requirements for both plants would increase by only 22.5%. Tables 1 and 2 display basic design parameters for the bleach effluent evaporation plant and for the different components of the A H T unit, respectively. The heat transformer would deliver an output of 25.4 MW and was simulated using the HEO-NaOH as the working pair. This results in operating pressures of 4 and 73.2 kPa and concentrations of 54.4 and 52.4 wt% for the strong and weak solutions, respectively. This configuration represents the closed cycle mode of operation for the A H T unit. If we choose to operate the heat transformer in the open cycle mode, then it is possible to deliver the vapour leaving the bleach effluent evaporation plant directly into the absorber, without using the evaporator component of the AHT. This modification would result in a reduced equipment cost for the A H T and even lower steam consumption. A drawback of this alternative configuration, however, is that the working pair solution may be contaminated by the vapour from the bleach effluent evaporation plant. A small bleeding stream of the working pair solution would therefore be required and the corresponding amount must be fed to the system as a make-up stream. However, when using H z O - N a O H as the working pair, it is possible to use the bleeding stream from the A H T as make-up chemical for the black liquor. The simulation results of this alternative configuration showed that 12.8 kg/s or 96.2% of the vapour leaving the last effect of the black liquor evaporation plant, at 69°C and 30 kPa, will be utilized as heat input to the generator component of the AHT, while the balance can be used for preheating duties. Conversely, all vapour leaving the bleach effluent evaporation plant, 12.7 kg/s at 95°C and 85 kPa, will be utilized in its evaporator component, as illustrated in Fig. 5. With this modification of the AHT, the flow rate of live steam needed in the bleach effluent evaporation plant would only be 1.0 kg/s, while the total energy requirement for both evaporation plants would be increased by only 6.6%. Table 2. Basic design parameters for the heat transformer unit (closed cycle mode) Variable Effect Flow rate Temp LMTD Pressure Conc.
U.A.
Unit
Evap.
Generator
Absorber
Cond.
Heat Ex.
MW kg/s ~'C ~C kPa wt% n20
28.9 12.7" 95 4 73.2 100 7.2
25.0 10.7" 69 4 4 52.4 6.2
25.4 11.6" 125 4 73.2 54.4 6.4
28.4 1360]" 20-25 6.2 4 100 4.6
51.2 --5.5 --9.3
MW/C
*Steam. "~Cooling water.
T r e a t i n g b l e a c h p l a n t effluents
12.8kg/s 9~C 14.0 kg/s
39
Vapourfromthe black
liquor evaporation plant
12.7 kg/s 95 'C 6.6 kg/s 97 "C
125 'C 1.0 kg/s
lSO " - W
" ¢XJ
"W
h,
"W
60 "C
0.014 kg/kg F i g . 5. T h e h y b r i d A H T - d r i v e n e v a p o r a t i o n p l a n t ( o p e n cycle).
Table 3 displays the basic design parameters for the different components of the open-cycle heat transformer which would deliver an output of 10.8 MW. Based on the same working pair, the operating pressure range would be 4-85 kPa, while the concentration range would be 57.2 and 52.4 wt% for the strong and weak solutions, respectively. It should be mentioned that the preheating duties currently performed by the evaporation plant, to preheat external streams of mainly process and boiler water, have been taken into consideration in both transformer-integrated process configurations. It may also be observed that installing a heat transformer according to either or both configurations would require minimal changes in the black liquor evaporation plant. Furthermore, sodium hydroxide solutions are more propitious to the pulp and paper industry compared to other working pairs. ECONOMIC EVALUATION An economic evaluation was performed to determine the economic feasibility of concentrating the bleach effluent using an energy efficient evaporation scheme. The values displayed in Table 4 were calculated for the specific conditions considered in this work. The exact costs are very site-specific. It is therefore necessary to study each mill individually to determine the impact of any new process modification. The cost estimate is based on a flow rate of 7.2 m 3 effluent per ton of pulp. The 16 wt% effluent stream leaving the evaporation unit can either be concentrated further to about 65 wt%, in the existing black liquor evaporation plant, and then recycled to the recovery boiler where it undergoes combustion (the recovery boiler alternative), or sprayed on the waste from bark peeling and co-burned in the bark boiler (the bark boiler alternative). Both alternatives have been considered in the economic analysis. Capital cost The capital cost of the evaporator vessels and the plate heat exchangers for preheating the incoming bleach effluent stream was assumed to be 1100 SEK/m 2 of heat transfer area (1 US$ = 7.50 SEK). This results in an investment cost of 11.1 million SEK and an installed cost of 22.2 million SEK, assuming a Lang multiplication factor of 2.0. This factor is used quite frequently Table 3. Basic design parameters for the heat transformer unit (open cycle mode) Variable Effect Flow rate Temp. LMTD Pressure Conc. U.A.
Unit
Evap.
Generator
Absorber
Cond.
Heat Ex.
MW kg/s °C °C kPa wt% H : O MW/"C
-12.7" 95 -85 100 --
30.0 12.8" 69 4 4 52.4 7.5
30.1 14.0" 125 4 85 57.2 7.6
31.8 1530t 20-25 6.2 4 100 5.2
22.3 --7.2 --3.1
*Steam. tCooling water. ATE
16/I--D
A. G i d n e r et al.
40
Table 4. Estimated cost (SEK/ton pulp) for treating bleach effluents by evaporation Item Capital cost Operating costs: Thermal energy Electricity Credit: Heat value Total cost
Recovery boiler alternative
Bark boiler alternative
57
57
17 1
9 1
52 23
18 49
to obtain order-of-magnitude cost estimates which usually result in a probable accuracy estimate of about _+30% [15]. It recognizes that the cost of a process plant may be obtained by multiplying the basic equipment cost by some factor to approximate the capital investment. These factors vary depending upon the type of process being considered. The installed equipment cost for the A H T unit was calculated as 2400 SEK/kW absorber output [10], corresponding to 72 million SEK. It should be pointed out that, owing to the lack of a sufficient number of commercial full-scale A H T plants, some empirically determined cost functions were used to estimate the investment cost of the A H T unit simulated in this work [10, 16]. The annual plant production rate and operation time were taken as 335,000 tons of pulp and 8000 h, respectively. Returns on investment and pay-back time requirements may differ between companies, but a common value in the Swedish pulp and paper industry is 15% and 10yr, respectively. This corresponds to an annuity of 20% of the installed cost. The investment cost would then be 101 million SEK, corresponding to 57 SEK/ton of pulp. Operating costs The main operating costs are live steam and electric power to drive all pumps. Based on a pump head of 10m, a pump efficiency of 60% and a flow rate of 150 kg/s for the circulated stream in all evaporator vessels, the electric power needed to drive the five circulation pumps would be 123 kW. The feed pump would require 12kW and the total power needed becomes 135 kW corresponding to 3.2 kWh/ton of pulp. Based on an average price for electric power in the Swedish pulp and paper industry of 0.30 SEK/kWh, the total cost for the electric power needed in both alternatives would be 1 SEK/ton of pulp. The steam required in the bleach effluent evaporation unit was earlier computed to 1.0 kg/s, corresponding to 86 kg steam/ton of pulp. Based on an average thermal energy cost of 100 SEK/ton of generated steam, the steam cost would be 9 SEK/ton of pulp. The simulations revealed that, for the recovery boiler alternative, an equal amount of live steam is required, 1.0 kg/s corresponding to 0.20 kg steam/kg evaporated water, to further concentrate the effluent stream from 16 to 65 wt%. The additional steam cost would then be 9 SEK/ton of pulp. Credit The concentrated effluent may be used as fuel in the recovery boiler, where its heat value can be utilized and its sodium content can also be integrated in the spent liquor recycle loop. The heat value of the organic matter is approximately 19 MJ/kg of total dry solids (TDS) [3]. Based on an energy price of 0.032 SEK/MJ, corresponding to an oil price of 1500 SEK/m 3, and 92.8 kg TDS/ton of pulp, a net income of 52 SEK/ton of pulp should be credited to this alternative. For the bark boiler alternative, the flow rate of the 16wt% effluent stream is 6.75 kg/s, corresponding to 580 kg/ton of pulp and its heat value is only 0.94 MJ/kg due to its high water content. This results in an income of 18 SEK/ton of pulp. Sodium sulfate is commonly added as a make-up chemical to compensate for the chemical losses in the spent liquor cycle. Sodium hydroxide is sometimes used instead of Na2 SO4 in order to limit the emission of sulfur to the air. Due to the comprehensive research efforts currently made to develop the bleaching technology, the economic impact of the sodium content of the effluent stream has been neglected in this analysis.
Treating bleach plant effluents
41
PRACTICAL ASPECTS A simulation study [12] revealed that an A H T installed on-line with one of the evaporation plants of a major Swedish pulp and paper mill would reduce the amount of live steam requirement by 18.5%, corresponding to 3.09 MW. As a direct response of this result, an A H T has been built and incorporated with one of the evaporation plants in that mill. To avoid any operational disturbances to the evaporation plant itself, the size of the A H T was limited to approximately 200 kW of heat input, corresponding to approximately 100 kW of heat output. The reference A H T was built based on the experience gained from a pilot-plant A H T unit [10]. The A H T was designed to operate both with self-circulation alone and also with self-circulation together with forced circulation of the working solution around the absorber. It can be operated by utilizing vapour supply at a temperature range of 87 100°C and to produce steam at a temperature range of 110-135°C using the working pair H 2 0 - N a O H . Currently, the A H T unit is still under the first test period. Consequently, the operating data available are not sufficient to determine accurately the efficiency of the AHT. The first tests were performed with the A H T operated in the forced circulation mode of the working solution around the absorber. A set of measured temperature profiles and other measured data are reported elsewhere [11] and reveal that the total temperature lift for the A H T is about the same, irrespective of the unavoidable transients in the operation behaviour of the evaporation plant. No electronic control systems are currently installed in the AHT, since the reference plant was deliberately designed for manual operation in the first hand. Together with another research group working in our department with knowledge-based risk management (KRM) [17], a database will be developed containing the operational know-how of the A H T on-line with the evaporation plant. This database will be valuable to process engineers not familiar with this type of energy conservation system. Based on careful analysis of the operational behaviour during the test period of both the evaporation plant and the A H T unit, some changes in the A H T might be suggested. For instance, an appropriate control system will be designed and installed at a later stage of the project. CONCLUSIONS This simulation study revealed the feasibility of an energy-efficient evaporation process for the treatment of bleach plant effluents. For the plant capacity considered, the total evaporation capacity for both the existing black liquor evaporation plant and the proposed bleach effluent evaporation process would be increased by 91%. However, the energy requirements for both evaporation plants would increase by only 6.6%. This can be achieved by integrating an absorption heat transformer unit with both plants. It should be noticed that as the concentration of the bleach effluent increases, as a result of developing the bleaching process, the proposed evaporation process becomes more feasible. Furthermore, one of the main practical features of this process is that integrating the heat transformer unit would require minimum changes in the existing plant. A reference heat transformer unit, delivering 200 kW and directly incorporated with one of the evaporation plants of a major Swedish pulp and paper mill, has demonstrated the technical feasibility of this energy-saving technique under real industrial conditions. The economic analysis resulted in a cost of 23 SEK/ton of pulp for the first alternative, where the 16wt% effluent stream is concentrated further to 6 5 w t % , in the existing black liquor evaporation plant, and then recycled to the recovery boiler. The corresponding cost of 49 SEK/ton of pulp would result for the second alternative, where the 16 wt% effluent stream is sprayed on the waste from bark peeling and co-burned in the bark boiler. Acknowledgements--The authors are grateful to Dr Ann-SofieJ6nsson and Klas Abrahamsson for stimulating discussions. The financial support of the SwedishNational Board for Industrial and TechnicalDevelopmentis gratefullyacknowledged.
REFERENCES 1. P. L. Gleadow and C. Hastings, Steps towards kraft mill closed cycle.Part I: Designs for reduced water usage, Pacific Paper Expo, Vancouver, Canada (1991).
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A. Gidner et al.
2. J. Dorica, Ultrafiltration of bleach plant effluents--a pilot plant study. J. Pulp Paper Sci. 12(6), 172-177 (1986). 3. A.-S. J6nsson, Ultrafiltration of bleach plant effluents. Nordic Pulp & Paper Res. J. 2, 23-29 (1987). 4. A. Zaidi, H. Buisson, S. Sourirajan and H. Wood, Ultra- and nano-filtration in advanced effluent treatment schemes for pollution control in the pulp and paper industry. Water Sci. Technol. 25(10), 263-276 (1992). 5. O. Samuelson and B. Wennergren, Purification of bleach plant effluents by adsorption on cross-linked polymers. Svensk Papperstidn. 15, 477-479 (1977). 6. F. Cecen, W. Urban and R. Haberl, Biological and advanced treatment of sulfate pulp bleaching effluents. Water Sci. Technol. 26(1-2), 435-444 (1992). 7. L. Roy-Arcand, F. S. Archibald and F. Briere, Comparison and combination of ozone and fungal treatments of a kraft bleachery effluent. Tappi 74(9), 211 218 (1991). 8. C. W. Bryant, J. J. Avenell, W. A. Barkley and R. N. Thut, The removal of chlorinated organics from conventional pulp and paper waste-water treatment systems. Water Sci. Technol. 26(1-2), 417-425 (1992). 9. K. Eriksson and/~. Jernqvist, Heat transformer with self-circulation: design and preliminary operational data. Int. J. Refrig. 12, 15-20 (1989). 10. K. Abrahamsson, Absorption heat cycles--an experimental and theoretical study, Ph.D. thesis, Dept of Chem. Eng. I, University of Lund, Sweden (1993). 11. K. Abrahamsson, A. Gidner and ~. Jernqvist, Design and experimental performance evaluation of an absorption heat transformer with self-circulation. Heat Recovery Systems & CHP 15(3), 257-272 (1995). 12. K. Abrahamsson, G. Aly and/~. Jernqvist, Heat transformer systems for evaporation applications in the pulp and paper industry. Nordic Pulp & Paper Res. J. 7, 9-16 (1992). 13. RCG/Hagler-Bailly, Inc., Opportunities for industrial chemical heat pumps in process industries. Final Report prepared for the US DOE Office of Industrial Technologies (1990). 14. A. Gidner and/~. Jernqvist, Modelling and simulation of integrated systems of evaporation plants and different energy conservation techniques. Proc. lASTED Int. Conf. on Applied Modelling and Simulation, Lugano, Switzerland, 1994, pp. 1-6. 15. M. S. Peters and K. D. Timmerhaus, Plant Design and Economics for Chemical Engineers, 4th Edn. McGraw-Hill, New York. (1991). 16. J. Berghmans, High temperature industrial heat pumps, Final report prepared for the IEA Advanced Heat Pumps Annex IX (1990). 17. R. Wennersten and A. Gr/infors, K R M - - a n integrated system for process analysis and operator support. Plant~Operation Prog. (submitted).
Pergamon
Applied Thermal Engineering Vol. 16, No. 1, pp. 33~12, t996
1359-4311(95)00015-1
Copyright © 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 1359-4311/96$9.50+ .00
AN ENERGY EFFICIENT EVAPORATION PROCESS FOR TREATING BLEACH PLANT EFFLUENTS A. G i d n e r , ~ . Jernqvist* a n d G. Aly Department of Chemical Engineering I, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden (Received 7 February 1995)
Abstract--Simulation results of an energy efficientevaporation process are reported for the treatment of bleach effluents in the pulp and paper industry. Due to the low concentration of the effluent stream, the evaporation process must have a high degree of energy efficiencyin order to compete with other treatment alternatives, such as ultrafiltration, adsorption, ion exchange and biological treatment. For a pulp and paper mill with an annual capacity of 335,000 ton of bleached kraft pulp, the capacity of the spent black liquor evaporation plant is about 7.5 ton water/ton pulp if its concentration is increased from 16 to about 65 wt% dry substance. An evaporation plant for the bleach effluent was simulated for a flow rate corresponding to 7.2 m3/ton pulp and a concentration of 1.4 wt% dry substance. This stream is to be concentrated to 16 wt% resulting in an evaporation capacity of 245 ton water/h. The total evaporation capacity for both evaporation plants would be increased by 91%. Optimal energy conservation strategies were investigated where an absorption heat transformer unit (AHT) is integrated with both the existing black liquor evaporation plant and the proposed bleach effluent evaporation process. Different process configurations were simulated using a flow sheeting program, developed for simulation of general multiple-effectevaporation processes and absorption heat pump systems. Using real operating data from a major Swedish pulp and paper mill, simulation results are reported for the optimum location of the AHT unit. Consequently, the energy requirements for both evaporation plants would increase by only 6.6%. One of the main practical features of this process is that integrating the AHT unit would require minimum changes in the existing plant. An economic analysis resulted in a cost of 23 SEK/ton of pulp for the first alternative where the pre-concentrated 16 wt% effluent stream is concentrated further to 65 wt%, within the existing black liquor evaporation plant, and then recycled to the recovery boiler. The corresponding cost of 49 SEK/ton of pulp would result for the second alternative, where the 16 wt% effluent stream is sprayed on the waste from bark peeling and co-burned in the bark boiler. Keywords--Effluent treatment,
evaporation, absorption heat transformer, paper industry.
INTRODUCTION I n response to stringent e n v i r o n m e n t a l regulations a n d a strong c o n s u m e r d e m a n d for e n v i r o n m e n tally friendly products, the p u l p a n d p a p e r i n d u s t r y is intensifying its efforts to m a k e significant changes to the milling process, as well as the effluent m a n a g e m e n t systems, to virtually eliminate the g e n e r a t i o n a n d discharge o f toxic c o n t a m i n a n t s at a n affordable cost. Nearly all mills have already closed up the digesting cycle, while further research is still needed to close u p the bleaching cycle. U s i n g c o u n t e r - c u r r e n t w a s h i n g schemes, the v o l u m e o f the bleaching effluents have been c o n s i d e r a b l y decreased from 2 0 - 3 0 to less t h a n 10m3/ton pulp. F u r t h e r efforts are targeting possible flows o f less t h a n 5.5 m3/ton pulp for the near future [1]. Different m e t h o d s to treat the kraft bleaching effluents, i n t e r n a l as well as external, have been investigated; for example, ultrafiltration [2-4], ion exchange [5], a d s o r p t i o n [5, 6], a n d biological t r e a t m e n t [6-8]. I n a n experimental study where seven different m e m b r a n e s were investigated, J 6 n s s o n [3] c o n c l u d e d that some o f the tested m e m b r a n e s could p r o b a b l y c o m b i n e a satisfactory flux a n d a n acceptable retention. A r o u g h cost estimate revealed that t r e a t m e n t of the caustic E-stage effluent with ultrafiltration could be p e r f o r m e d within a total cost of a b o u t 25 S E K / t o n pulp. Effective c o l o u r r e m o v a l by a d s o r p t i o n o n cross-linked polymers i n c l u d i n g a n i o n exchange resins was studied experimentally [5]. This m e t h o d requires a n acidification of the caustic extraction stage (E-stage) effluent and, to avoid the precipitation o f extractives, the effluent was passed *Author to whom correspondence should be addressed. 33
34
A. Gidner et al.
through a column packed with a non-polar resin before acidification. The resins used had to be regenerated for re-use. Adsorption was studied as one of some physiochemical methods to apply on biologically pre-treated bleaching effluents [6], where about 90% COD (chemical oxygen demand) and DOC (dissolved organic carbons) elimination rates could be achieved. Spent bleaching effluents from both chlorination and E-stages were subjected to bench-scale biological treatment followed by one of some physiochemical methods such as ozonization, ozonization/irradiation, and adsorption [6]. It was shown that, for the biologically pre-treated effluent, which was further treated by ozonization or ozonization/irradiation, removal rates of 61, 81, 98 and 92% could be achieved for DOC, COD, colour (436 nm) and AOX (adsorbable organic halides), respectively. Conversely, adsorption could achieve removal rates exceeding 90% for DOC, COD and colour (436 nm). In another recent study, a combined ozonization-biological method was applied on the alkaline E-stage effluent [7]. It was concluded that both ozone and biological treatment effectively destroyed effluent chromophores, but the fungal process resulted in greater degradation as expressed by COD removal. The combining of a brief ozone treatment with a subsequent fungal treatment revealed a synergism between the two decolorization mechanisms on the complete alkaline E-stage effluent but not on the high-molecular-weight fraction alone. In another recent study using biological treatment, AOX removals averaged 46% for two activated sludge systems and 34% for five aerated stabilization basins. Both schemes averaged removal of over half of the influent low molecular weight AOX while the average removal of high molecular weight AOX varied from 1 to 47%. It should be observed that the above-mentioned methods were applied on chlorine bleaching effluents. The Swedish pulp and paper industry applies, since 1992, either ECF (elemental chlorine free) or T C F (totally chlorine free) bleaching sequences. Due to the rapid development in the bleaching technology, the resulting effluents have different compositions and volumetric flow rates. It should therefore be recognized that very little hard data are available on the characteristics and toxicity from the ECF and T C F bleaching processes. An evaluation of the economic feasibility of concentrating the kraft bleaching effluent using an energy-efficient evaporation scheme is presented in this paper.
ABSORPTION HEAT CYCLES Absorption heat pumps (AHP) are based on the principle of utilization of the enthalpies of evaporation-condensation of solutes in appropriate working fluid mixtures. These systems consume very little electric energy and mainly use a high-temperature primary heat source and a low-temperature waste heat source to recover useful heat at an intermediate temperature, as illustrated in Fig. 1. Some electric energy is used only for the purpose of recirculating the working fluid. An absorption heat transformer (AHT) is a reversed absorption heat pump, where heat is supplied at an intermediate level and useful heat is delivered at the highest temperature, provided there is a heat sink with a lower temperature level than that of the waste heat supplied. The heat released in the heat sink is normally not used, since it is at a relatively low temperature. The heat transformer, which consists of four main components, is normally operated at three temperature and two pressure levels, as can be seen in Fig. 2. Besides those four components, the system usually includes one heat exchanger and two pumps. Both absorber and evaporator operate at a higher pressure level than the generator and condenser. The waste heat input is supplied to the generator and evaporator, at a middle temperature level, while about half of the heat input is recovered at a higher temperature in the absorber and the other half is dissipated at a lower temperature in the condenser. The available useful energy output of an A H T is, accordingly, about 50% of the waste heat input. However, an A H T does not consume high-grade thermal energy and only a small amount of electric energy is used to recirculate the liquid streams. The circulation pumps can be totally eliminated if the heat transformer is operated in the self-circulation mode [9-1 1]. A working fluid pair is circulated between the absorber and generator to provide the heat pump loop. Absorption occurs in the absorber and desorption occurs in the generator. Working fluids
Treating bleach plant effluents
35
~T
/
useful heat
QGI--
%
I--
%
QE
Absorption
Absorption
heat pump
heat transformer
Fig. I. Schematic definition of absorption heat cycles (AHP and AHT).
consist of a refrigerant and an absorbent. When heat is added to the working fluid mixture, the refrigerant evaporates, leaving behind a weak working fluid. As the refrigerant is condensed, the latent heat is given off to the cooling water. When the refrigerant is vaporized in the evaporator and rejoined with the weak working fluid, heat is liberated to the process. A portion of the waste heat is theoretically recoverable to produce steam or to supply other process heating requirements. The heat recovered would reduce the primary energy input into the process. Incorporating various types of heat pumps within different units in a process burdened with waste heat streams having appropriate temperature levels is an efficient method for energy recovery. A detailed literature survey concerning industrial applications of absorption heat cycles was recently conducted [10, 12, 13]. It was revealed, however, that relatively few applications have so far been implemented in the process industry. In contrast, based on technical feasibility and preliminary economic competitiveness, it was concluded that the commercial development of industrial heat pumps in 27 selected processes in the US is projected to be 1110 units by the year 2010 [13]. Based on the types of processes evaluated, most of the heat pump installations will be in the chemical industry (684 units). The food processing, pulp and paper, and petroleum refining industries are projected to install 184, 168 and 71 units, respectively. E V A P O R A T I O N S Y S T E M S IN T H E P U L P AND P A P E R I N D U S T R Y Due to the fact that evaporation is one of the most energy-intensive unit operations in the pulp and paper industry, together with the availability of many waste heat streams at moderate temperatures, and since evaporation and absorption heat cycle processes are interrelated with respect to operation and dynamics, it is quite possible to achieve energy savings by process integration of an AHT.
Evaporaito
sorber
I ~
enser I Tc
Heatexchanger
j era,or I I TG TE
.... ........ I TA
Fig. 2. Schematic flowsheet for the AHT system.
Condensate Vapour Weaksolution Strongsolution
>
36
A. Gidner et al.
Fig. 3. Schematic flowsheet of the black liquor evaporation plant.
Figure 3 shows a schematic flow-sheet of an existing black liquor evaporation plant consisting of six effects. The thin liquor, with a concentration of 16 wt% dry solid contents, is fed to the fourth effect and the thick liquor leaves the second effect at a concentration of 65 wt%. The layout of the unit is quite conventional and comparable to most Scandinavian black liquor evaporation plants. SIMULATION RESULTS The flow sheeting program EVAPSYS, developed for both design and evaluation calculations of any type of evaporation plant was used for the simulation of the multiple-effect evaporation processes investigated in this work [14]. This program enables the evaporation plant to be simulated either separately or integrated with an energy conservation scheme, such as a mechanical compressor, a steam ejector, or an absorption heat cycle. Different component modules are incorporated to provide the user with maximum flexibility in constructing any evaporation plant. These modules include an evaporator, heat exchanger, expansion vessel, mixer, splitter, mechanical compressor and a steam ejector and would thus enable the user to easily assemble evaporation flowsheets of arbitrary complexity. Furthermore, other component modules are incorporated to simulate single-stage absorption cycles. These modules include an absorber, generator, condenser and heat exchanger. This arrangement allows for executing the program in three different modes: evaporation plants, absorption heat cycles, and heat-pump-driven evaporation systems. One of the difficulties in applying energy conservation techniques to existing multiple-effect evaporators is the problem of accurately predicting the change in heat transfer coefficients. For this reason, three subfunctions for computing the overall heat transfer coefficients are incorporated. The first subfunction is used for black liquor in vertical tube evaporators, the second for forced circulation evaporators and heat exchangers and the third is used for falling-film evaporators. Physical properties of different liquors for evaporation applications (such as sea water, sugar solutions, skim milk, black liquor, Mg-based sulfite liquor and Ca-based sulfite liquor) and different working fluid pairs for absorption heat cycle applications (such as H20-LiBr, H zO-NaOH, H 20-CAC12, CH30H-LiBr, H 20-Glycerol, H 20-Glyco1, NH3-H 20 and H20-LiNO3/KNO3/NaNO3) are available in the database of the program. Using a minimum of input data, EVAPSYS computes pressure, temperature, concentration, enthalpy and flow rate profiles for the simulated system. The program is designed to provide maximum flexibility, as well as maximum simplicity of the flowsheet input data procedure. The program is considered as a powerful tool for the simulation of different process configurations for evaluation, design and optimization of energy efficient evaporation systems [10-12, 14]. A large number of computer runs were performed to establish both local and global optimal energy conservation strategies for the evaporation plants investigated in this work. The black liquor evaporation plant
Considering a pulp and paper mill with an annual capacity of 335,000 ton pulp, the flow rate of the spent black liquor would be reduced in the evaporation plant from 100 to 25 kg/s. This corresponds to an evaporation capacity of 270 ton water/h or about 7.5 ton water/ton pulp. Some boundary conditions must be fulfilled during the search for optimal energy conservation strategies. The most important boundary condition is to avoid major changes in the existing black liquor evaporation plant when the AHT system is incorporated.
Treating bleach plant effluents
37
The black liquor evaporation plant was first simulated using actual operating data, provided by a major Swedish pulp and paper mill, to calculate complete temperature, concentration and overall heat transfer coefficient (U-values) profiles. The U-values and apparent temperature differences were in the range 0.35-1.82 kW/m2K and 3.1-14.4°C, respectively. Further, the simulations resulted in a total heat transfer area of 30,500 m 2 and a total live steam requirement of 15.2 kg/s, which is equivalent to 32.6 M W of thermal energy.
The proposed bleach effluent evaporation plant The proposed flowsheet of an evaporation plant for the bleach effluent was simulated for an estimated flow rate of 75 kg/s, corresponding to 7.5 ton efffluent/ton pulp, and a concentration of 1.4 wt% dry substance. This stream is to be concentrated to 16 wt%, resulting in a flow rate of 6.6 kg/s. This corresponds to an evaporation plant capacity of 246 ton water/h. The concentrated effluent stream can either be fed to the black liquor evaporation plant for further concentration, or sprayed on the waste from bark peeling and co-burned in the bark boiler. The proposed evaporation plant consists of six falling-film effects with equal heat transfer areas. Due to small boiling point elevations of the bleach effluent within the concentration range considered in this work and the relatively smaller temperature differences which can be attained in falling-film evaporators, the evaporation plant can preferably be operated at pressures above atmospheric. Consequently, the plant will be driven by live steam at 140°C (i.e. the same live steam source as used for the black liquor evaporation plant) and the condenser temperature will be maintained at 100°C. Consequently, the U-values and apparent temperature differences were in the range 2.50-3.40 kW/m2K and 5.7-6.9°C, respectively. The simulations also resulted in a total heat transfer area of 8, 100 m 2 and a total live steam requirement of 12.9 kg/s, which is equivalent to 27.7 MW of thermal energy. It may be pointed out that, compared to the black liquor evaporation plant, considerably higher U-values were obtained in the effluent plant due to the concentration and temperature ranges, as well as the falling-film configuration of the heat transfer area. Consequently, the heat transfer area required for the bleach effluent evaporation plant is only 26.6% of that required for the black liquor evaporation plant, although both plants have nearly the same evaporation capacity expressed in kg evaporated water/s.
The hybrid AHT-driven evaporation plant Different system configurations were simulated to determine the optimum number of effects to be used in the bleach effluent evaporation plant, and the optimum location and size of the heat transformer to be incorporated between both evaporation plants. It was concluded from the simulation results that five effects would be required in the bleach effluent evaporation plant. This can be explained by the fact that it is necessary to achieve an optimum relationship between the flow rates of the vapour streams leaving the black liquor and bleach effluent evaporation plants
10.7 k~/s 9~C 11.6 kg/s
vapourfromthe black liquorevaporationplant 12.7 kg/s 6.6 k~s
97 'C 125 'C 3.4 kg/s 15.0 kg/s 60 "C 0.014 kg/kg Fig. 4. The hybrid AHT-driven evaporation plant (closed cycle).
,~
A . G i d n e r et al.
38
Table 1. Basic design parameters for the bleach effluent evaporation plant Variable
Unit
Effect 1
Effect 2
Effect 3
Effect 4
Effect 5
Area U-value AT
m2 kW/(m2"C) C
1960 3.2 5.0
1960 3.0 5.1
1960 2.8 5.4
1960 2.6 5.8
1960 2.5 6.0
and which will be utilized as input thermal energy in the generator and evaporator components of the AHT, respectively. Further, it would be necessary for the bleach effluent evaporation plant to be driven by live steam having a saturation temperature of 125°C, instead of 140°C, in order to minimize the heat transfer area required in the solution heat exchanger. To find a configuration that achieves considerable energy conservation, the A H T has to be designed to deliver steam to the first effect of the bleach effluent evaporation plant. This would give the plant a total temperature difference which is given by the optimum design of the AHT. This arrangement can be achieved by utilizing 10.7 kg/s or 80.5% of the vapour leaving the last effect of the black liquor evaporation plant, at 69°C and 30kPa, as heat input to the generator component of the AHT, while all vapour leaving the bleach effluent evaporation plant, 12.7 kg/s at 95°C and 85 kPa, will be utilized in its evaporator component, as illustrated in Fig. 4. For this configuration, the simulation results revealed that the bleach effluent evaporation plant would require a total heat transfer area of 9800 m 2 and 3.4 kg/s live steam. This means that although the total evaporation capacity, for both black liquor and bleach effluent plants, has been increased by 91%, the energy requirements for both plants would increase by only 22.5%. Tables 1 and 2 display basic design parameters for the bleach effluent evaporation plant and for the different components of the A H T unit, respectively. The heat transformer would deliver an output of 25.4 MW and was simulated using the HEO-NaOH as the working pair. This results in operating pressures of 4 and 73.2 kPa and concentrations of 54.4 and 52.4 wt% for the strong and weak solutions, respectively. This configuration represents the closed cycle mode of operation for the A H T unit. If we choose to operate the heat transformer in the open cycle mode, then it is possible to deliver the vapour leaving the bleach effluent evaporation plant directly into the absorber, without using the evaporator component of the AHT. This modification would result in a reduced equipment cost for the A H T and even lower steam consumption. A drawback of this alternative configuration, however, is that the working pair solution may be contaminated by the vapour from the bleach effluent evaporation plant. A small bleeding stream of the working pair solution would therefore be required and the corresponding amount must be fed to the system as a make-up stream. However, when using H z O - N a O H as the working pair, it is possible to use the bleeding stream from the A H T as make-up chemical for the black liquor. The simulation results of this alternative configuration showed that 12.8 kg/s or 96.2% of the vapour leaving the last effect of the black liquor evaporation plant, at 69°C and 30 kPa, will be utilized as heat input to the generator component of the AHT, while the balance can be used for preheating duties. Conversely, all vapour leaving the bleach effluent evaporation plant, 12.7 kg/s at 95°C and 85 kPa, will be utilized in its evaporator component, as illustrated in Fig. 5. With this modification of the AHT, the flow rate of live steam needed in the bleach effluent evaporation plant would only be 1.0 kg/s, while the total energy requirement for both evaporation plants would be increased by only 6.6%. Table 2. Basic design parameters for the heat transformer unit (closed cycle mode) Variable Effect Flow rate Temp LMTD Pressure Conc.
U.A.
Unit
Evap.
Generator
Absorber
Cond.
Heat Ex.
MW kg/s ~'C ~C kPa wt% n20
28.9 12.7" 95 4 73.2 100 7.2
25.0 10.7" 69 4 4 52.4 6.2
25.4 11.6" 125 4 73.2 54.4 6.4
28.4 1360]" 20-25 6.2 4 100 4.6
51.2 --5.5 --9.3
MW/C
*Steam. "~Cooling water.
T r e a t i n g b l e a c h p l a n t effluents
12.8kg/s 9~C 14.0 kg/s
39
Vapourfromthe black
liquor evaporation plant
12.7 kg/s 95 'C 6.6 kg/s 97 "C
125 'C 1.0 kg/s
lSO " - W
" ¢XJ
"W
h,
"W
60 "C
0.014 kg/kg F i g . 5. T h e h y b r i d A H T - d r i v e n e v a p o r a t i o n p l a n t ( o p e n cycle).
Table 3 displays the basic design parameters for the different components of the open-cycle heat transformer which would deliver an output of 10.8 MW. Based on the same working pair, the operating pressure range would be 4-85 kPa, while the concentration range would be 57.2 and 52.4 wt% for the strong and weak solutions, respectively. It should be mentioned that the preheating duties currently performed by the evaporation plant, to preheat external streams of mainly process and boiler water, have been taken into consideration in both transformer-integrated process configurations. It may also be observed that installing a heat transformer according to either or both configurations would require minimal changes in the black liquor evaporation plant. Furthermore, sodium hydroxide solutions are more propitious to the pulp and paper industry compared to other working pairs. ECONOMIC EVALUATION An economic evaluation was performed to determine the economic feasibility of concentrating the bleach effluent using an energy efficient evaporation scheme. The values displayed in Table 4 were calculated for the specific conditions considered in this work. The exact costs are very site-specific. It is therefore necessary to study each mill individually to determine the impact of any new process modification. The cost estimate is based on a flow rate of 7.2 m 3 effluent per ton of pulp. The 16 wt% effluent stream leaving the evaporation unit can either be concentrated further to about 65 wt%, in the existing black liquor evaporation plant, and then recycled to the recovery boiler where it undergoes combustion (the recovery boiler alternative), or sprayed on the waste from bark peeling and co-burned in the bark boiler (the bark boiler alternative). Both alternatives have been considered in the economic analysis. Capital cost The capital cost of the evaporator vessels and the plate heat exchangers for preheating the incoming bleach effluent stream was assumed to be 1100 SEK/m 2 of heat transfer area (1 US$ = 7.50 SEK). This results in an investment cost of 11.1 million SEK and an installed cost of 22.2 million SEK, assuming a Lang multiplication factor of 2.0. This factor is used quite frequently Table 3. Basic design parameters for the heat transformer unit (open cycle mode) Variable Effect Flow rate Temp. LMTD Pressure Conc. U.A.
Unit
Evap.
Generator
Absorber
Cond.
Heat Ex.
MW kg/s °C °C kPa wt% H : O MW/"C
-12.7" 95 -85 100 --
30.0 12.8" 69 4 4 52.4 7.5
30.1 14.0" 125 4 85 57.2 7.6
31.8 1530t 20-25 6.2 4 100 5.2
22.3 --7.2 --3.1
*Steam. tCooling water. ATE
16/I--D
A. G i d n e r et al.
40
Table 4. Estimated cost (SEK/ton pulp) for treating bleach effluents by evaporation Item Capital cost Operating costs: Thermal energy Electricity Credit: Heat value Total cost
Recovery boiler alternative
Bark boiler alternative
57
57
17 1
9 1
52 23
18 49
to obtain order-of-magnitude cost estimates which usually result in a probable accuracy estimate of about _+30% [15]. It recognizes that the cost of a process plant may be obtained by multiplying the basic equipment cost by some factor to approximate the capital investment. These factors vary depending upon the type of process being considered. The installed equipment cost for the A H T unit was calculated as 2400 SEK/kW absorber output [10], corresponding to 72 million SEK. It should be pointed out that, owing to the lack of a sufficient number of commercial full-scale A H T plants, some empirically determined cost functions were used to estimate the investment cost of the A H T unit simulated in this work [10, 16]. The annual plant production rate and operation time were taken as 335,000 tons of pulp and 8000 h, respectively. Returns on investment and pay-back time requirements may differ between companies, but a common value in the Swedish pulp and paper industry is 15% and 10yr, respectively. This corresponds to an annuity of 20% of the installed cost. The investment cost would then be 101 million SEK, corresponding to 57 SEK/ton of pulp. Operating costs The main operating costs are live steam and electric power to drive all pumps. Based on a pump head of 10m, a pump efficiency of 60% and a flow rate of 150 kg/s for the circulated stream in all evaporator vessels, the electric power needed to drive the five circulation pumps would be 123 kW. The feed pump would require 12kW and the total power needed becomes 135 kW corresponding to 3.2 kWh/ton of pulp. Based on an average price for electric power in the Swedish pulp and paper industry of 0.30 SEK/kWh, the total cost for the electric power needed in both alternatives would be 1 SEK/ton of pulp. The steam required in the bleach effluent evaporation unit was earlier computed to 1.0 kg/s, corresponding to 86 kg steam/ton of pulp. Based on an average thermal energy cost of 100 SEK/ton of generated steam, the steam cost would be 9 SEK/ton of pulp. The simulations revealed that, for the recovery boiler alternative, an equal amount of live steam is required, 1.0 kg/s corresponding to 0.20 kg steam/kg evaporated water, to further concentrate the effluent stream from 16 to 65 wt%. The additional steam cost would then be 9 SEK/ton of pulp. Credit The concentrated effluent may be used as fuel in the recovery boiler, where its heat value can be utilized and its sodium content can also be integrated in the spent liquor recycle loop. The heat value of the organic matter is approximately 19 MJ/kg of total dry solids (TDS) [3]. Based on an energy price of 0.032 SEK/MJ, corresponding to an oil price of 1500 SEK/m 3, and 92.8 kg TDS/ton of pulp, a net income of 52 SEK/ton of pulp should be credited to this alternative. For the bark boiler alternative, the flow rate of the 16wt% effluent stream is 6.75 kg/s, corresponding to 580 kg/ton of pulp and its heat value is only 0.94 MJ/kg due to its high water content. This results in an income of 18 SEK/ton of pulp. Sodium sulfate is commonly added as a make-up chemical to compensate for the chemical losses in the spent liquor cycle. Sodium hydroxide is sometimes used instead of Na2 SO4 in order to limit the emission of sulfur to the air. Due to the comprehensive research efforts currently made to develop the bleaching technology, the economic impact of the sodium content of the effluent stream has been neglected in this analysis.
Treating bleach plant effluents
41
PRACTICAL ASPECTS A simulation study [12] revealed that an A H T installed on-line with one of the evaporation plants of a major Swedish pulp and paper mill would reduce the amount of live steam requirement by 18.5%, corresponding to 3.09 MW. As a direct response of this result, an A H T has been built and incorporated with one of the evaporation plants in that mill. To avoid any operational disturbances to the evaporation plant itself, the size of the A H T was limited to approximately 200 kW of heat input, corresponding to approximately 100 kW of heat output. The reference A H T was built based on the experience gained from a pilot-plant A H T unit [10]. The A H T was designed to operate both with self-circulation alone and also with self-circulation together with forced circulation of the working solution around the absorber. It can be operated by utilizing vapour supply at a temperature range of 87 100°C and to produce steam at a temperature range of 110-135°C using the working pair H 2 0 - N a O H . Currently, the A H T unit is still under the first test period. Consequently, the operating data available are not sufficient to determine accurately the efficiency of the AHT. The first tests were performed with the A H T operated in the forced circulation mode of the working solution around the absorber. A set of measured temperature profiles and other measured data are reported elsewhere [11] and reveal that the total temperature lift for the A H T is about the same, irrespective of the unavoidable transients in the operation behaviour of the evaporation plant. No electronic control systems are currently installed in the AHT, since the reference plant was deliberately designed for manual operation in the first hand. Together with another research group working in our department with knowledge-based risk management (KRM) [17], a database will be developed containing the operational know-how of the A H T on-line with the evaporation plant. This database will be valuable to process engineers not familiar with this type of energy conservation system. Based on careful analysis of the operational behaviour during the test period of both the evaporation plant and the A H T unit, some changes in the A H T might be suggested. For instance, an appropriate control system will be designed and installed at a later stage of the project. CONCLUSIONS This simulation study revealed the feasibility of an energy-efficient evaporation process for the treatment of bleach plant effluents. For the plant capacity considered, the total evaporation capacity for both the existing black liquor evaporation plant and the proposed bleach effluent evaporation process would be increased by 91%. However, the energy requirements for both evaporation plants would increase by only 6.6%. This can be achieved by integrating an absorption heat transformer unit with both plants. It should be noticed that as the concentration of the bleach effluent increases, as a result of developing the bleaching process, the proposed evaporation process becomes more feasible. Furthermore, one of the main practical features of this process is that integrating the heat transformer unit would require minimum changes in the existing plant. A reference heat transformer unit, delivering 200 kW and directly incorporated with one of the evaporation plants of a major Swedish pulp and paper mill, has demonstrated the technical feasibility of this energy-saving technique under real industrial conditions. The economic analysis resulted in a cost of 23 SEK/ton of pulp for the first alternative, where the 16wt% effluent stream is concentrated further to 6 5 w t % , in the existing black liquor evaporation plant, and then recycled to the recovery boiler. The corresponding cost of 49 SEK/ton of pulp would result for the second alternative, where the 16 wt% effluent stream is sprayed on the waste from bark peeling and co-burned in the bark boiler. Acknowledgements--The authors are grateful to Dr Ann-SofieJ6nsson and Klas Abrahamsson for stimulating discussions. The financial support of the SwedishNational Board for Industrial and TechnicalDevelopmentis gratefullyacknowledged.
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