Analysis of sustainability of resource reproduction
When the production of these basic chemicals would be based on bioethanol, the limited amount of bioethanol has to be distributed proportionally for each chemical. This approach has been used during the assessment of SVrep in each country. The simplest representative of this approach is the SVrep calculation in Czech Republic. 1.3 thousand barrel/day bioethanol was produced in Czech Republic in 2008, i.e., 0.06 million tons were produced annually. This small amount of bioethanol has to be distributed between benzene and toluene, because only these two basic chemicals of the selected ones were produced in this country. Altogether, 0.71 million tons of ethanol would be needed to cover the production of toluene and benzene, which is 10 times more than the available bioethanol. Overviewing the available data between 2008 and 2012, it can be stated that the produced bioethanol is far below the amount would be needed resulting in SVrep values lower than 0.12 (Fig. 4a). Even if the total bioethanol would have been used to produce one of the chemicals, the SVrep value still remains far below the sustainable value (SVrep = 1).
Similar calculations were performed for Hungary, Poland, and Slovakia using the assumption of evenly distributed bioethanol as a resource. In case of Hungary (Fig. 4b), a significant difference can be seen between 2015 and 2016, which can be explained by the increase in the ERoE value: ERoE = 2.3 (Shapouri, et al. 2010) was used from 2008 to 2015 and ERoE = 4 (Gallagher et al. 2016) from 2016 onward. Although the bioethanol production was also grown up to 7.2 from 7.09 thousand barrels/day, this increase alone cannot result in a doubled SVrep value. The remarkable increase in ERoE was not conducted to significant growth of SVrep in Poland. Because the chemical industry in Poland produces six different basic chemicals, and the bioethanol has to be distributed proportionally, SVrep values show a moderate increase over the years (Fig. 4c). A significant increase in SVrep can be seen in year 2016 in Slovakia (Fig. 4d) due to both the significant increase in the chemical industry and the ERoE value. When enough bioethanol would be available for the chemical reaction, moreover, the energy requirements could be covered from bioethanol as well, SVrep value would reach one or increase beyond. As it can be seen in Fig. 4, SVrep values remain below 0.3 in all cases, which is far away from the sustainable 1 and proves the scarce volume of existing bioethanol. It should be noted that the SVrep value of ethylene, propylene, and toluene for the USA in 2014 did not exceed 0.2, which also proves the limited volume of bioethanol (Horváth et al. 2017).
Analysis of sustainability of the fate of the waste
SVwaste equals one when all the generated waste is treated during the chemical production and no wastes are released to the environment. Incineration, chemical and/or biological treatment, and disposal can be applied as treatment methods, and their time requirements have to be applied as treatment time. If no treatment is applied, then all wastes are released to the environment and must be considered as untreated waste, which possesses corresponding natural decomposition times. Because the generated waste during the production of the given chemical is either treated or untreated, every single atom is accounted for. When the EE of the waste is calculated, the situation is similar and mass balance can be expressed according to Eq. 4.
$${\text{EE}}_{\text{generated waste}} = {\text{EE}}_{\text{treated waste }} + {\text{EE}}_{\text{untreated waste }}$$
(4)
Consequently, sustainability of the fate of the waste depends on exclusively on the waste treatment and waste natural decomposition times, so Eq. 2 can be simplified to formula of Eq. 5.
$${\text{SV}}_{\text{waste}} = \frac{{t_{\text{waste generation}} }}{{t_{\text{waste treatement}} + t_{\text{waste natural decomposition}} }}$$
(5)
Production and consumption data represent annual volumes, and waste is generated during the whole year, thus waste generation time must equal 1 year. Waste can be divided into two parts: treated or untreated and treatment methods could include incineration, chemical and/or biological treatment, and disposal. In the case of the selected chemicals treatment is narrowed to combustion, which is a prompt reaction and takes place immediately after purge. Therefore, waste treatment time equals 1 year. Although combustion does not require additional time, natural decomposition of chemical if released to the environment does. So, all the wastes have to be identified first, then their half-lives in the different environment have to be determined. The detailed chemical reactions from ethanol to the named chemicals including conversion, selectivity, and identified waste compounds are summarized in our previous work (Horváth et al. 2017). Wastes formed during each reaction step with corresponding natural decomposition times for ethylene, propylene, benzene, toluene, xylenes, and styrene are summarized in Supporting Information Table S1. When waste is released to the environment, natural decomposition occurs and the time requirement must include the generation time i.e., 1 year in our case. Additionally, the time needed to reach the local governments’ regulation level in the environment based on its natural half-life. Applying the rule of thumb, concentration reduced to 0.1% of the initial concentration may have a negligible effect on the environment, so we calculated the time required to reach 0.1% of the annual volume of waste.
Multistep technologies are used during propylene, benzene, toluene, xylenes, and styrene production, and wastes are generated almost in each step. In these cases, the longest time of natural decomposition should be used for the overall process. To cover all occasions of contamination of air, water, or soil, the longest time of natural decomposition should be used (Table 2). When calculating SVwaste treated wastes have to be considered, which are incinerated immediately during the chemical production, therefore, 1 year of waste treatment is used in the calculations. Table 2 and Fig. 5 show that the chemical production technology governs the value of SVwaste, which is close to sustainable in the case of ethylene production (SVwaste = 0.957). Because the other basic chemicals can be produced via 3, 4, 7, 8, and 6 steps (propylene, benzene, toluene, xylenes, and styrene, respectively), and the conversion and or/selectivity of the reactions are lower than 100%, wastes form that are either treated or untreated but worsen the overall SVwaste value. The same chemical production technology is assumed in all countries, consequently, SVwaste values are the same thus environmental sustainability is governed by the reproduction. It was also revealed that the same industrial-scale chemical technology, chemical reactions (moreover equipment) can be used for the synthesis of each chemical in all countries on the one hand; and due to the simplification of the waste formula (see Eq. 5), waste generation, waste treatment, and waste natural decomposition times influence the value of SVwaste on the other hand; consequently, SVwaste values of a chemical are the same in each country. It should be noted that the SVwaste values do not differ from data calculated for the USA and depends exclusively on the decomposition of the chemicals.
Table 2 Representatives of wastes formed during the production of basic chemicals: untreated wastes having the longest natural decomposition times and incinerated wastes Analysis of environmental sustainability: sustainability indicator
According to the calculation methodology and Eq. 3, SUSind is governed by the smaller parameter, in our assessment always SVrep, because the same chemical production technologies having the same SVwaste values can be used in each country. Due to the limited amount of bioethanol and its proportional distribution between the chemicals the low SVrep values result in SUSind values lower than 0.09, 0.18, 0.05, and 0.25 in Czech Republic, Hungary, Poland, and Slovakia, respectively (Fig. 6). If SVrep is sustainable its value reaches or overcomes 1, similarly to SVwaste, consequently SUSind must be at least 0.5 or even higher to reach sustainability. Of the countries studied the ethylene production in 2017 in Slovakia shows the highest SUSind value (0.226), but it is far below 0.5.
In comparison with the data calculated for the USA in 2008 and 2014 (Horváth et al. 2017), the SUSind values shows same order of magnitude verifying the fact that the sustainable production of these basic chemicals is not feasible at the current stage and primarily depends on the SVrep values.