Vol. 44 No. 1
SCIENCE IN CHINA (Series B)
February 2001
Analysis of the suspension polymerization flows of polyvinyl chloride with the exergy analysis involving resource utilization and environmental influence WANG Yanfeng (王彦峰) & FENG Xiao (冯
霄)
Environmental and Chemical Engineering School, Xi’an Jiaotong University, Xi’an 710049, China Correspondence should be addressed to Feng Xiao (email:
[email protected]) Received November 3, 2000
Abstract In order to comprehensively assess the resource utilization of a process system and its influence on the environment, the application range of the traditional exergy analysis was expanded in this paper to include the pollution degree of the discharged wastes to the environment. So technology indexes can be obtained to comprehensively assess the resource utilization and the environment impact of a process system. The harm coefficient and effect coefficient were introduced to concern the different harm to the environment of the different wastes and the pollution effect relative to resource waste by environment. The determination method of the harm coefficient and effect coefficient was discussed. Two suspension polymerization flows of polyvinyl chloride were calculated with this method. The results show that the method can comprehensively assess the resource utilization and the environment impact of the chemical process system by comparing and analyzing. Keywords: exergy analysis, resource utilization, environment impact, polyvinyl chloride.
Because people blindly damage the nature, the ecological crisis, water crisis, food crisis and energy crisis are threatening the basis condition of people’s living and developing [1]. While the environmental problem is becoming increasingly serious, people have to re-evaluate their social and economic behavior. Now they have realized that the traditional developing model of simply seeking economic benefit and the pollution control strategies mainly concerning end harnessing cannot meet the demand of the present social development. Clean production is necessary, that is, the effective use of the resources and preservation of the ecological environment must be taken into consideration during the whole producing process from the designing stage till the whole operation. Researchers in the world have done much work in the aspect. Douglas[2] extended his hierarchical theory to waste minimization in process generation. Based on Douglas’s approach, Flower et al.[3] introduced the idea of graphical mass balance to screen processes initially complying with the environmental regulations. EI-Halwagi et al.[4] used the concept of mass pinch as a tool to derive cost-optimal mass exchange networks with minimum waste emissions. Based on the same concept, Wang and Smith[5] developed a method to obtain design targets for minimum waste water generation in process plants. The methodology for environmental impact minimization (MEIM), proposed by Pistikopoulos et al.[6], involves the estimation of environmental damage using adverse effects of processing systems on the environment. Stefanis et al.[7] proposed a
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methodology for environmental impact minimization, with embedded life cycle analysis principles within a process optimization framework. Rivero[8] proposed a performance parameter called exergoecologic improvement potential Potee based on exergoecologic analysis, and the larger the value is, the more serious the pollution becomes. Stefanis et al.[9] extended the MEIM to include aspects of solvent selection and reaction path synthesis, and in this way, an optimum route can be found in which system’s cost and environmental impact are both minimum. Rosen and Dincer[10] analyzed and improved system performance with exergy analysis to increase energy efficiency and resource utilization so as to reduce waste exergy emissions. The researches are focusing on two aspects. One is to consider only the influence on the environment and how to reduce the discharge. The resource utilization and economy are not taken into account in this method. The other is to treat the problem using a multi-objective mathematical program to consider the resource utilization and the influence on the environment, but it is very hard for the method to resolve the complexity and nonlinearity of the model. In this paper, an evaluation index that combines the resource utilization and the influence of the environment is considered, and by this way, the multi-targets are changed into a target so as to simplify the setup and solution of the model. Rant presented the concept of exergy, a parameter in thermodynamics, for the first time in 1956. After several decades of development, the exergy analysis has become the basic theory and a useful tool in analyzing the energetic system and has aroused worldwide attention and made application all over the world. Taking the environmental state as its dead state, the parameter of exergy can be used to measure the difference between a system/a stream and the environment. Therefore, if an appropriate environmental state is chosen, it can measure not only the energy or resource utilization in a system but also pollution of the discharged waste to the environment. But the traditional exergy analysis can only assess the energy efficiency of a system and if it is used directly to assess the environment impact of the system, some problems will arise. Therefore, the concepts of the system environment negative effect, the system negative effect and the system negative effect factor were presented by authors to expand the traditional exergy analysis to include the pollution of the discharged waste to the environment. The technology indexes can be used to comprehensively assess the resource utilization and the influence on the environment of a system, and can also be used to optimize the system [11]. In this paper, the suspension polymerization flows of polyvinyl chloride were analyzed with this method. 1
The exergy analysis method involving resource utilization and environmental influence
First the exergy analysis was broadened to take the environment impacts of a system into consideration. The environment impacts of a system can be measured through the exergy discharge losses of the system. A system’s exergy discharge losses include two parts: one is from the emitted heat, the other is the physical exergy and chemical exergy of the waste itself. The emitted heat is ab-
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sorbed by the environment with no harm to the environment. However, the various components in the wastes, with different chemical natures (toxicity, greenhouse effect, photochemical effect, ozone sphere loss of stratosphere, acid rain, etc.), bring different harm to the environment. So, while dealing with the exergy loss of the system’s wastes, we cannot simply put the exergy loss of each component together. A harm coefficient can be used to show the different harm to the environment. The exergy loss calculated in this way is no longer the actual exergy loss since it cannot satisfy the exergy balance equation. The environment negative effect(ENE) is defined as follows: ENE =
∑B E i
x ,i ,
(1)
i
where ENE is the environment negative effect of a system (kW), Ex,i is the physical and chemical exergy of the component i in the system’s wastes (kW), and Bi is the harm coefficient of component i to the environment. Based on the ENE defined above, the exergy analysis method can be expanded to assess the comprehensive effect of a system on resource use and environmental influence. We define the comprehensive assessing index as system negative effect (SNE). Because any exergy loss will lead to resource waste, this negative effect on the resource can be measured by Exl tot, the total exergy loss of the system, i.e. the sum of the exergy dissipation and exergy discharge of the system Furthermore, as mentioned above, the exergy discharge loss of a system can also cause environment pollution. So, while dealing with the total effect of a system, the exergy discharge loss should be considered with waste of resource in (Exl tot), and impact on the environment in the ENE. However, the resource waste and the environment pollution cannot be considered equally, so a simple sum cannot be used to get the system negative effect. Here the effect coefficient is introduced to deal with the unequals. The system negative effect is defined as B
SNE = C1 E xl tot + C 2 ENE,
(2)
where SNE is the system negative effect (kW), Exl tot is the total exergy loss of the system (kW), and C1, C2 are the effect coefficients. If the resource effect coefficient C1 is taken as 1, eq. (2) will be SNE = E xl tot + C 2′ ENE,
(3)
where C 2′ is the converted environment effect coefficient, which is the ratio of C2, the environment effect coefficient, to C1, the resource effect coefficient. The determination of the harm coefficient Bi in ENE is very complicated, for there are various pollutants and as chemicals they have different characteristics (toxicity, greenhouse effect, ozonosphere loss in the stratosphere, acid rain, etc.). Ref. [12] gave the latent harm coefficients of many chemical substances by fully considering their different characteristics. So we can use them as the harm coefficient in this method. B
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It is even more difficult to determine the effect coefficients of the system’s negative effect, C1, C2, since resource waste cannot be compared with environmental pollution in a direct way. In order to overcome this, the economic losses caused are used to derive the effect coefficient. The calculation of the economic losses caused by the environment negative effect of the system is very complicated, since it concerns a lot of aspects. Ref. [1] offered some methods and examples which have great reference value. The economic loss caused by the waste of resource of the whole exergy losses is E cll =
∑E
xl, j Pin , j ,
(4)
where Ecl1 is the economic loss of resource (￥), Exl , j is the exergy loss of Unit j in a system (kW), Pin , j is the input exergy cost of Unit j of the system (￥/kW). For all of the unsuspected parameters have been set, we can calculate the assessing index ——SNE and use it to assess the resource utilization and environment impact of a system. But
SNE is an absolute figure, which can be used to evaluate the different models of the systems with the same type or the different designs of a system, but cannot be used to evaluate the systems of different types. So a relative figure——system negative effect factor can be defined as follows: SNEF =
SNE , E x in
(5)
where SNEF is the system negative effect factor, Ex in is the system’s input exergy (kW). 2
Calculations and analysis of suspension polymerization flows of polyvinyl chloride
In view of the resource utilization and the environment effect, the calculation and analysis of two suspension polymerization flows, A and B, are as follows. The flow chart of suspension polymerization flow A of polyvinyl chloride is given in fig.1. The VCM tank is filled intermittently with the fresh VCM coming from the VCM generator. The fresh VCM then is mixed with the recovered VCM, additives and pure water according to a certain proportion as the raw stuff to be delivered to the polymerizer. After polymerization, PVC serum, which is separated with VCM monomer, is sent to the serum tank. PVC serum is delivered from the serum tank to the centrifuge continuously to be separated with water and additives and then goes to the fluidized drier to further remove air, water and VCM. The product after these procedures is delivered to the packaging unit. At the same time, the unreacted VCM monomer is recovered directly by steam stripping in polymerizer and stored in the gas tank. Having been distilled in VCM monomer treating process, the VCM monomer finally is sent to VCM monomer refinement tank. It is noted that the difference between this flow and general flows is the absence of steam stripping column between the polymerizer and the serum tank. The steam stripping of unreacted VCM monomer is done not only in the special steam-stripping column, but directly in the kettle after polymerization. The advantages of this flow are: (ⅰ) the flow is simplified; (ⅱ) the feeding period is shortened; (ⅲ) replacing two containers with one reduces the outlets of
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VCM monomer. The disadvantage, however, is obvious. The reclaim of VCM monomer is not sufficient.
Fig. 1. Flow process chart of the suspension polymerization flow A of polyvinyl chloride.
The flow chart of suspension polymerization flow B of polyvinyl chloride, which is the traditional polymerization flow, is given in fig. 2. The processes before the polymerization are the same as those in the flow A. After polymerization, PVC serum is sent to the reclaiming tank, and then to the steam-stripping column in succession. Via the mixture-storing tank, the PVC serum after steam stripping is sent to the separation and desiccation processes to remove water, air, additives and VCM. The product is delivered to the packaging unit. At the same time, the unreacted VCM monomer, most of which is reclaimed by the reclaiming tank with the little remaining by the steam-stripping column, is distilled in primary and second stage condensers. The distilled VCM monomer is stored in VCM tank as the raw stuff of later polymerization and sent intermittently to
Fig. 2. Flow process chart of the suspension polymerization flow B of polyvinyl chloride.
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the polymerizer by VCM pump. Compared with flow A, this flow is more complicated because of using the special reclaiming tank and steam-stripping column to strip the unreacted VCM monomer. Though the flow B can reclaim unreacted VCM monomer more adequately than the flow A, it also causes some problems. Firstly, the outlets of VCM monomer are added. Secondly, both the investment and the operating cost increase. The calculation results for the input exergy, the output exergy and the whole system exergy in these two-process systems are listed in tables 1 and 2. The sums of the physical and chemical exergy of all wastes can be seen in tables 3 and 4. Table 1 Exergy balance of the suspension polymerization flow A of polyvinyl chloride Item Input exergy Raw material at point 1 Heat air Nitrogen at point 7 Total Output exergy Product (PVC) at point 6 Reclaiming VCM at point 2 Total Outside exergy loss 11 12 13 14 15 16 17+18 Total Inside exergy loss Total exergy loss Total
Exergy ×106/kJ·(batch)−1
%
352.9800 37.8000 0.0020 390.7820
90.3265 9.6730 0.0005 100.0000
321.0000 35.1200 356.1200
82.1430 8.9871 91.1301
0.0234 0.0025 0.0133 21.50 0.0034 0.0047 0.7140 22.2613 12.4007 34.6620 390.7820
0.0060 0.0006 0.0034 5.5018 0.0009 0.0012 0.1827 5.6966 3.1733 8.8699 100.0000
The damage coefficients in these case studies are listed in table 5, so the environmental negative effect in system A is ENE = 0.76×106×10.5+0.00006×106×4 = 5.17×1010 kJ/a and that in system B is ENE = 4.63×109 kJ/a. The economic loss of the resource caused by system can be obtained by the following equation : [13]
E resource = input exergy cost of a system × total exergy loss of the system.
(6)
In these two cases, the price of raw material is 75000￥/batch both in systems A and B, so the input exergy cost of the systems A and B is 2×10−4￥/kJ. In system A, the economic loss of resource waste is 6652￥/batch, and output is 108000 t/a, that is 6480 batch/a, so the economic loss of resource waste is 4310.5×104 ￥/a. In system B, the economic loss of resource waste is 313
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￥/h, output is 30000 t/a, and producing time is 7750 h/a, so the economic loss of resource waste
is 243.35×104￥/a. Table 2 Exergy balance of the suspension polymerization flow B of polyvinyl chloride Exergy
Item Input exergy Raw material at point 2 Steam at points 16, 18 and 19 Total Output exergy Product (PVC) at point 9 Reclaiming VCM at point15 Total Outside exergy loss 3 5 8 14 Total Inside exergy loss Total exergy loss Total
×106/kJ·h−1
%
81.196 6.069 87.265
93.0453 6.9547 100.0000
74.910 10.790 85.700
85.8420 12.3646 98.2066
0.790 0.350 0.380 0.023 1.543 0.022 1.565 87.265
0.9053 0.4011 0.4355 0.0264 1.7683 0.0251 1.7934 100.0000
Table 3 Sums of the physical and chemical exergy of all wastes in system A Waste Exergy loss ×106/kJ·(batch)−1
Water
Air
VCM
Nitrogen
CO2
0.12
21.37
0.76
0.0006
0.00006
Table 4 Sums of the physical and chemical exergy of all wastes in system B Waste Exergy loss/kJ·h−1
Water 55
PVC 1487863
VCM 56887
Table 5 Harm coefficient of all wastes in systems A and B Waste Bi
Water 0
Air 0
VCM 10.5
Nitrogen 0
PVC 0
CO2 4
The influences on the environment can be divided into two parts, one is the influence on the natural environment and the other is that on the social environment, so the economic loss of the pollution caused by the system can be defined as E environment = E nature environment + E social environment .
(7)
It is hard to quantify the economic loss of the natural environment caused by the system. To some extent, the pollutant penalties established by the environment protection departments and the treatment cost to handle the contamination in the factories can reflect the approximate costs, so we use the sum of these to denote the economic loss, which can be seen in eq.(8)[14]. E nature environment = E cost of polluter releasing + E cost of treating .
(8)
The economic loss of the social environmental pollution by the system can be calculated through the following equation[1]:
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E environment =
∑E
i , direct economic loss
+ E i , indirect economic loss ,
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(9)
i
where i is the ith pollutant releasing from a system. E direct economic loss = E medical cost + E income loss ,
(10)
E indirect economic loss = E early death + E work delay .
(11)
It can be seen from tables 3 and 4 that in these wastes, only VCM monomer and CO2 can contaminate the environment. Due to the few outgoings of CO2, we only consider the economic loss caused by VCM monomer pollution. VCM monomer is a kind of dangerous substances. Many diseases are related to it, such as liver cancer, hepatic hemangioma, and it deteriorates the working ambience. Three workers died of cancer due to the employment disease of VCM monomer in B.F. Goodrich Company in Jan. of 1974[15]. In this instant, we assume that one person is killed by the employment disease of VCM monomer. Firstly, estimating the economic loss caused by the VCM monomer to the environment. In system A, the annual cost to treat the contaminations is 30×104￥, and pollutant penalty is 590×104￥, so the total economic loss due to pollutants is 620×104￥. In system B, the annual cost to treat the contaminations is 5×104￥, and pollutant penalty is 16×104￥, so the economic loss due to pollutants is 21×104￥. These data are obtained from several chemical factories in China. Secondly, estimate the economic loss of the social environmental pollution caused by the system. The equations and experiential data are got from ref. [1]. E medical service = PAR ⋅ W ⋅ C ,
(12)
where PAR is percentage of liver cancer caused by VCM monomer, taken as 0.9; W is number of persons who died of liver cancer, taken as 1 person; C is medical cost because of liver cancer, taken as 20×104￥/(person·a). Thus, the direct medical cost is 18×104￥/a. E income loss = PAR ⋅ W ⋅ E B ,
(13)
where EB is direct average income loss for early death, taken as 6000￥/(person·a). B
Thus, the direct income loss for early death is 5400￥/a. E work delay = D ⋅ PAR ⋅ W ⋅ m,
(14)
where D is national average income per person, taken as 6000￥/(person·a); m is the total work delay time because of liver cancer, taken as 2 years. Thus, the economic loss of work delay because of liver cancer is 10800￥/a. In this paper, the indirect economic loss caused by early death is ignored. Therefore, the economic loss of the social environmental pollution by VCM is 196200￥/a.
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Therefore, the economic loss caused by system A is 640×104￥/a and that caused by system B is about 40.62×104￥/a. If the resource effect coefficient of the system is 1, the environmental effect coefficient of system A, C 2′ , is 640/4310.5 = 0.148. The environmental effect coefficient of system B, C 2′ , is 40.62/243.35 = 0.167. SNE of system A is: SNE = (34.6620×106+0.148×7980240)×6480 = 24519.3×106 kJ/a, SNE of system B is: SNE = 12167.5×106+0.167×4547.7×106 = 12927.0×106 kJ/a, SNEF of system A is: SNEF = 24519.3×106 /(390.782×106×6480) = 0.092, SNEF of system B is: SNEF = 12927.0×106/(87.265×106×7750) = 0.019. 3
Analysis
(ⅰ) The calculation process and results show that the method systematically considers the resource utilization and environmental influence, which are integrated by the effect coefficient. The system negative effect can finally evaluate the resource utilization and environmental influence in the chemical process and can be used as an objective to further optimize the system. (ⅱ) The results show a less effect coefficient of ENE. This is because that in the method effect coefficient is calculated by economic loss. In the present system of price and pollutant penalty, the resource waste plays a primary role in SNE. But with the increasing recognition of the environmental protection, much more restrict criteria of contamination discharge will be issued. Therefore, the proportion of the influence to the environment will rise as well. (ⅲ) It can be seen from the results that the main contamination released by the system is VCM monomer. So the system should be improved by reducing the outgoing of VCM monomer, for example, adopting high effective steam-stripping technique to increase the reclaiming ratio of VCM monomer; applying the waste steam burning technology to further reducing the outgoing of VCM monomer. (ⅳ) The negative effect coefficient of system B is much less than that of system A, meaning that system B is much better than system A in the aspects of the resource utilization and the influence on the environment. Through the comparison of the two flows, it can be easily seen that unreacted VCM monomer in system A is directly stripped in the polymerizer after the reaction. Though a fixed facility is saved, and the flow is simplified, the reclaiming VCM monomer is reduced, which not only wastes the raw materials, but also has serious hazard to the environment. On the contrary, the operation in system B is handled by the reclaiming tank and steam-stripping column, though two more facilities are adopted and VCM monomer can be maximally retrieved, which can save the raw materials as well as lessen the damage to the environment. 4
Conclusion
Based on the traditional exergy analysis, a new method is proposed which can systematically
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consider the resource utilization and the influence on the environment. Case studies for two-suspension polymerization flows process of polyvinyl chloride have been conducted. The calculation steps have been introduced and the method to determine the effect coefficient has been adopted. The results show that the method widens the application range of the traditional exergy analysis to evaluate the resource utilization and influence on the environment. The assessment index——system negative effect can be the target to optimize the system. Finally, the method can find out the weak link of the system and point out how to improve the system. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 29836140) and the Major State Basic Research Development Program of China (Grant No. G20000263).
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