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Study on the stability of nitroglycerine spent acid
process safety and environmental protection 8 7 ( 2 0 0 9 ) 87–93
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Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Study on the stability of nitroglycerine spent acid Kai-Tai Lu ∗ , Peng-Chu Lin School of Defense Science, Chung Cheng Institute of Technology, National Defense University, Taoyuan, Taiwan, ROC
a b s t r a c t Nitroglycerine has been widely used as an ingredient of explosives and propellants for a long time. It is produced by the glycerine nitration reaction. Many explosions have occurred during handling or storing spent acid after separating nitroglycerine from the reactor in nitroglycerine factories. Safety charts have been constructed by various authors in order to cope with these hazards. In this investigation we construct enthalpy diagrams that correspond with the safety charts produced by earlier researchers. These can be used to evaluate the variation of heat and the safety composition during the handling of spent acid. Furthermore, this study uses practical operating conditions at nitroglycerine factories to evaluate the stability of spent acids in storage. © 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Nitroglycerine; Spent acid; Enthalpy diagrams; Safety; Stability
1.
Introduction
The manufacture of nitroglycerine is one of the most dangerous procedures in the explosives industry. Although manufacturing techniques have been developed for over 140 years, industrial procedures have changed little. These procedures comprise preparing mixed acid, glycerine nitration, separation of nitroglycerine and spent acid, washing nitroglycerine, filtration of spent acid, recycling nitroglycerine from spent acid and treating spent acid (Urbanski, 1965). Many accidents have occurred during these manufacturing processes. Forty-five major industrial accidents involving nitroglycerine occurred between 1860 and 1984 worldwide (Badger Army Ammunition Plant, 2002). Statistics for China show that 43 industrial accidents involving nitroglycerine occurred between 1954 and 1982 (Han et al., 1988). Biasutti (1985) analyzed 131 industrial accidents that involved nitroglycerine. Among these, 13 events, accounting for approximately 10% of the total events, correlated with spent acid decomposition. Thus, the potential risk during handling or storing spent acid cannot be ignored. Generally, industrial accidents involving spent acid at nitroglycerine factories are mainly caused by the following reasons (Rigas et al., 1998).
∗
(1) The decrease in temperature of spent acid results in an increased viscosity and thus affects the separation of nitroglycerine from spent acid. (2) A reduction of temperature during the handling of spent acid may cause separation of nitroglycerine. (3) Improper dilution of spent acid with water. (4) Unintentional dilution of spent acid can destroy the stability of spent acid. Oehman et al. (1960a,b, 1961) performed a thorough investigation of nitroglycerine and spent acid systems. They described that the cause of system instability is the hydrolysis reaction of nitrate esters in spent acid. The same viewpoint also presented by Camera et al. (1982), the hydrolysis and oxidation mechanism of nitroglycerine in acid solution was described particularly. Rigas et al. (1997, 1998) analyzed three safety charts in the literature (SAFEX, 1987; FEEM, 1988; Oehman et al., 1960a,b, 1961). They simulated and evaluated safety charts via a simple reaction kinetics model and a computer code to explain the boundary conditions of safe regions in safety charts. Rigas et al. also pointed out that both composition and temperature of spent acid are central factors determining the content of dissolved esters in the acid phase.
Corresponding author. Tel.: +886 3 3891716; fax: +886 3 3892494. E-mail address: [email protected] (K.-T. Lu). Received 21 August 2008; Accepted 31 August 2008 0957-5820/$ – see front matter © 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2008.08.004
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In this investigation mathematical models were used to analyze the relative enthalpies of spent acids that are composed of HNO3 , H2 SO4 and H2 O at various weight ratios and temperatures. Furthermore, enthalpy diagrams were constructed that correspond with safety charts to evaluate the variation of heat and the composition of safety during the handling of spent simultaneously based on practical operating conditions at nitroglycerine factories. Storage stability of spent acid after dilution was also took into consideration.
2.
Presentation of safety charts
Acidic nitroglycerine and spent acid form when manufacturing nitroglycerine. Acidic nitroglycerine is very unstable and requires washing as soon as possible. As spent acid contains a small amount of nitroglycerine, its composition and temperature are the important factors that determine whether spent acid is safe or dangerous. Thus, practical safety charts for spent acids have been developed for manufacturing nitroglycerine. Based on numerous experiments, Oehman et al. (1960a,b, 1961) developed a diagram (Fig. 1) indicating the compositions of spent acids that are safe or dangerous when in contact with nitroglycerine. It was assumed that the spent acid system only comprises the three components mentioned on the axis labels. They also observed that HNO2 content in the spent acid affects the system stability; that is, the stability of spent acid is decreased by increasing the HNO2 content. In Fig. 1 they call “High safety”, the areas where not more 0.2% HNO2 is formed after heating for 2 h at 70 ◦ C and “Normal safety” where 0.2–1% HNO2 is formed. This diagram is based on the HNO3 content and the H2 O content in the spent acid, and is only valid when operating with pure materials. Federation of European Explosives Manufacturers (1988) constructed a safety chart at 35 ◦ C and HNO2 content of 0.2%. Fig. 2 presents a new safety chart that is similar to Fig. 1. This diagram indicates that storage stability of spent acid can be maintained for a long time at 35 ◦ C, a temperature commonly reached in storage tanks in summertime. “Very High Stability” in Fig. 2 is the area where is assumed that the HNO2 content in the spent acid should not increased to more than 0.2% after storing the spent acid at 35 ◦ C for 1 month and “Very Low Stability” where over 1% HNO2 is formed under the same conditions.
Fig. 2 – Safety diagram for spent acids from a nitroglycerine factory for 35 ◦ C (FEEM, 1988).
Fig. 3 – Safety diagram for spent acids from a nitroglycerine factory for 20 ◦ C (SAFEX, 1987). SAFEX (1987) also has constructed a safety chart for 20 ◦ C as Fig. 3. It is obvious that this diagram is the most complete since it covers the entire region of spent acid composition. The top region of Fig. 3 is labeled “stable area”, but it is a high H2 O content area. According to Oehman et al. (1960a,b, 1961), however, high H2 O content is dangerous. Rigas et al. (1998) explained that the oxidation power of nitric acid in a dilute system is considerably reduced. The nitroglycerine solubility in spent acid may be also an important factor when determining the safety of spent acid. This study found that spent acid contains less nitroglycerine in the region of higher H2 O contents (over 50%) than other regions from Pascal’s triangular diagram (Pascal, 1925), which shows the solubility of nitroglycerine in spent acid at different compositions. Furthermore, this study found the same characteristics in the bottom region of Fig. 3, which is also a stable area, containing low H2 O content in the spent acid. In view of this phenomenon, we propose that the reduction of nitroglycerine content in spent acid also reduces HNO2 formation, indicating that the spent acid system is stable in regions with high (>50%) or low (<10%) H2 O content.
3. Simulation of enthalpy diagrams corresponding with safety charts
Fig. 1 – Safety diagram for spent acids from a nitroglycerine factory for 70 ◦ C (Oehman et al., 1960b, 1961).
In practice, the composition of spent acid after nitration in nitroglycerine factories usually contains low H2 O content and high H2 SO4 content. To reduce the danger of handling spent
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process safety and environmental protection 8 7 ( 2 0 0 9 ) 87–93
Fig. 4 – The relative enthalpy of nitric acid, sulfuric acid, and water mixtures at 35 ◦ C.
Fig. 5 – The relative enthalpy of nitric acid, sulfuric acid, and water mixtures at 20 ◦ C.
acid immediately after the separation of nitroglycerine, adding a certain amount of water was suggested by Nathan et al. (1901, 1903, 1905, 1906). This method greatly enhances safety when handling spent acid, and supplementary separation of nitroglycerine is not required. When diluted with water, the solubility of nitroglycerine in the spent acid increases and some nitroglycerine is lost as it no longer separates. Addition of water to detain nitroglycerine in the spent acid not only increases nitroglycerine solubility in the spent acid, it also alters the equilibrium of the reaction between dinitroglycerine, nitroglycerine and acid in such a way that dinitroglycerine undergoes further nitration and trinitrate does not form. The dilution heat formed when adding water to the spent acid must be removed. The relative enthalpy and specific heat of spent acid of various compositions are important when calculating dilution heat. Mckinley and Brown (1942) drew a relative enthalpy diagram at 0 ◦ C and a specific heat diagram for mixed acid containing various weight ratios of sulfuric acid, nitric acid, and water. Based on these diagrams, the relative enthalpies of nitrating mixtures and of heat generated when mixing acids at various temperatures can be easily calculated. Figs. 4 and 5 show the relative enthalpies of nitric acid, sulfuric acid, and water mixtures at 35 ◦ C and 20 ◦ C, respectively. Furthermore, the curves in Figs. 4 and 5 were simulated with least square regression analysis to polynomials in the form: H = a + bx + cx2 + dx3 + ex4
(1)
where H is the relative enthalpy of nitric acid, sulfuric acid or mixed acid. x is the weight percent of nitric acid in anhydrous mixed acid and is in the range between 0 and 100. Table 1 presents the polynomial coefficients and the correlation coefficients R2 . In simulating enthalpy diagrams corresponding with safety charts, RW is weight ratio of water in spent acid and RN is weight ratio of nitric acid in anhydrous spent acid as in the following equations. RW =
WH2 O WHNO3 + WH2 SO4 + WH2 O
(2)
RN =
WHNO3 WHNO3 + WH2 SO4
(3)
The relative enthalpy of spent acid at various compositions can be expressed as a function of RW and RN . H = f (RW , RN )
(4)
Table 1 is used to draw the enthalpy diagram corresponding with Fig. 2 in the ranges of 0.15 ≤ RW ≤ 0.2 and 0 ≤ RN ≤ 0.2 by calculating method (Fig. 6). The relative enthalpies of spent acid for various compositions at 35 ◦ C were simulated with least square regression analysis by the following polynomial
Table 1 – Coefficients of the polynomials used to simulate the curves of Figs. 4 and 5 for the spent acid system Temperature
Percent by weight of total acid a
35 ◦ C
Pure HNO3 80% HNO3 60% HNO3 40% HNO3 20% HNO3 Pure H2 SO4
20 ◦ C
Pure HNO3 80% HNO3 60% HNO3 40% HNO3 20% HNO3 Pure H2 SO4
R2
Coefficients
147.29 149.40 147.42 145.28 144.78 147.10 83.845 86.145 84.169 82.092 82.013 84.598
b
c
d
e −4
−6.9707 −7.5424 −7.1803 −7.0329 −7.6174 −9.6501
0.0193 0.0207 −0.0066 −0.0284 −0.0255 0.0819
4.2547 × 10 4.2056 × 10−4 6.4137 × 10−4 8.4355 × 10−4 8.6255 × 10−4 −1.2000 × 10−3
– – – – – 1.2778 × 10−5
0.9998 0.9988 0.9998 0.9998 0.9990 0.9986
−6.0977 −6.8462 −6.5699 −6.4480 −6.9518 −9.2756
0.0076 0.0135 −0.0110 −0.0318 −0.0300 0.0903
4.8846 × 10−4 4.5905 × 10−4 6.6254 × 10−4 8.5822 × 10−4 8.8062 × 10−4 −1.4000 × 10−3
– – – – – 1.3504 × 10−5
0.9999 0.9990 0.9998 0.9998 0.9990 0.9999
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process safety and environmental protection 8 7 ( 2 0 0 9 ) 87–93
Table 2 – Coefficients of the polynomials used to simulate the relative enthalpies of Figs. 6 and 7 for spent acid of various compositions Coefficients
Hat 35 ◦ C (kJ/kg) 4.1507 × 101 −1.9681 × 103 3.9998 × 103 −1.9179 × 103 −2.1652 × 102 2.6680 × 103 −5.5612 × 103 −4.3305 × 101 5.3360 × 102 −8.6609 0.9999
a b c d e f g h i j R2
Fig. 6 – The enthalpy diagram corresponding with Fig. 2 at 35 ◦ C.
Hat 20 ◦ C (kJ/kg) −7.7744 −1.6133 × 103 2.7443 × 103 −9.5667 × 102 −8.4740 × 101 8.0731 × 102 −9.0758 × 102 1.1285 × 102 −2.2313 × 102 1.4385 × 101 0.9999
Yeh et al. (2006). 2 3 RW = a + bRN + cRN + dRN
(6)
Tables 3 and 4 list the polynomial coefficients. The value of RN can be determined from the weight ratios of sulfuric acid and nitric acid in the spent acid system; then using Eq. (6) and coefficients in Tables 3 and 4 to calculate the values of RW on the top, medium and last lines in Figs. 6 and 7. By using the actual value of RW in the spent acid system, the stability of the spent acid systems at various temperatures can easily be obtained. When the values of RN and RW are given for a spent acid system before and after dilution with water, the heat generated by dilution can be calculated using Eq. (5) and coefficients in Table 2 at various temperatures.
4. Fig. 7 – The enthalpy diagram corresponding with Fig. 3 at 20 ◦ C.
of RW and RN : 2 3 2 2 H = a + bRW + cRW + dRW + eRN + fRW RN + gRW RN + hRN 2 3 + iRW RN + jRN
(5)
In the same manner, the enthalpy diagram corresponding with Fig. 3 was drawn as Fig. 7, which shows the relative enthalpy in the ranges of 0 ≤ RW ≤ 0.6 and 0 ≤ RN ≤ 1.0. This diagram was also simulated by the polynomial of Eq. (5). The coefficients of the polynomials used to simulate the relative enthalpy of Figs. 6 and 7 are listed in Table 2. Furthermore, the relationship between RW and RN on the boundary lines in Figs. 6 and 7 was simulated using the following polynomial. This analytic manner had been applied by
Comparison of enthalpy diagrams
Although the enthalpy diagram in Fig. 6 only covers the case of low H2 O content in spent acid, it is very useful in practice because the compositions of spent acid after nitration in nitroglycerine factories usually contain low H2 O content. In Fig. 6, the relative enthalpy of a spent acid system decreases as H2 O content increases; hence, heat is generated by adding water to the spent acid system. After separating nitroglycerine from the spent acid, a certain amount of water must be added to dissolve suspended nitroglycerine and reduce the danger of handling and storing the spent acid. Thus, removal of this heat must be taken into consideration. Furthermore, the composition of spent acid after dilution with water must be also confined to stable area for safe storage. Fig. 6 can be used to evaluate heat generated by dilution and the safe composition of spent acid simultaneously. Fig. 7 is the most complete enthalpy diagram since it covers the entire region of spent acid compositions. Notably, Fig. 7 has two stable areas in the high and low H2 O content regions. From perspective of nitroglycerine solubility in a spent acid sys-
Table 3 – Coefficients of the polynomials used to simulate the boundary lines of Fig. 6 Boundary lines
Top Medium 1 Medium 2 Bottom
Range of RN
0.0473 RN 0.1817 0.0478 RN 0.1829 0.0484 RN 0.1844 0.0491 RN 0.1856
R2
Coefficients a
b
c
d
0.1375 0.1436 0.1580 0.1712
0.4236 0.4863 0.3944 0.3412
−1.2218 −1.5712 −1.2940 −1.1843
0.0511 0.0013 −0.0202 −0.2911
0.9999 0.9999 0.9999 0.9999
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Table 4 – Coefficients of the polynomials used to simulate the boundary lines of Fig. 7 Boundary lines
Range of RN
R2
Coefficients a
b
c
d
Top
0.5438 RN 1.0000 0.2689 RN 0.5346 0.0914 RN 0.2710 0.0000 RN 0.0886
0.0214 0.3195 0.2022 0.1500
0.0848 −0.2624 0.0601 0.7255
−0.0062 −0.3977 −0.0045 −0.5476
– – – –
0.9999 0.9999 0.9999 0.9999
Medium
0.0000 RN 0.0972 0.1001 RN 0.2912 0.2895 RN 0.8148 0.8148 RN 1.0000
0.1502 0.2361 0.0571 −0.3231
1.5535 −0.0832 0.7147 1.3495
−2.2574 −0.0608 −0.2721 −0.4765
– – – –
0.9999 0.9999 0.9999 0.9999
Bottom
0.0000 RN 1.0000
0.5000
0.0552
−0.0053
–
0.9999
tem, this study has explained that the spent acid system can dissolve small amounts of nitroglycerine to reduce formation of HNO2 in these two stable areas in Fig. 7, and the stability of spent acid system increases by decreasing the HNO2 content. Furthermore, this study also found that the relative enthalpy of a spent acid system decreases when H2 O content increases within 0 ≤ RW ≤ 0.2 as well as Fig. 6, however, the relative enthalpy increases as H2 O content increases within RW ≥ 0.5. If the composition of spent acid system is in doubtful or unstable areas, stability can be increased by increasing H2 O content or increasing HNO3 content to decrease H2 O content for storage. Fig. 7 can be also used to evaluate the variation in heat and composition of safe regions during handling of spent acid systems. A comparison of Figs. 6 and 7 shows that Fig. 6 is more conservative than Fig. 7 in the safe region. This was expected, since Fig. 6 refers to 35 ◦ C, while Fig. 7 to 20 ◦ C. For relative enthalpy, the temperature of spent acid system is a principal
influential factor. Relative enthalpy increases as temperature increases for same composition condition in a spent acid system.
5.
Application of enthalpy diagrams
Two factories in Taiwan use the Biazzi continuous process to produce nitroglycerine. Their operating conditions were considered from the perspectives of both safety and economy (Table 5) (Biazzi NG Plant, 1973). The temperature of spent acid after separation of nitroglycerine was maintained within 18–20 ◦ C; about 2% of H2 O was added to the spent acid to prevent suspended nitroglycerine. According to the compositions of spent acid before and after water dilution, this study evaluated dilution heat and spent acid system stability (Fig. 8 and Table 6). In Fig. 8, all compositions of spent acid before and after dilution are located in the stable area, indicating that the operating conditions in nitroglycerine factories are suit-
Table 5 – The operating conditions of manufacture nitroglycerine in Biazzi continuous process (Biazzi NG Plant FO-13 Technical Manual, 1973) RF
RN (%)
Inflow compositions (kg/min) C3 H5 (OH)3
Resident time (min)
Mixed acid HNO3
H2 SO4
Outflow compositions (kg/min) C3 H5 (ONO2 )3
Fractional conversion of C3 H5 (OH)3
Spent acid
4.8
49 50 51 52
1.06 1.06 1.06 1.06
2.4931 2.5440 2.5949 2.6458
2.5949 2.5440 2.4931 2.4422
7.5650 7.5315 7.4982 7.4652
2.5957 2.6146 2.6405 2.6526
3.8251 3.6956 3.5580 3.4425
0.9885 0.9900 0.9912 0.9921
4.9
49 50 51 52
1.06 1.06 1.06 1.06
2.5451 2.5970 2.6489 2.7009
2.6489 2.5970 2.5451 2.4931
7.4383 7.4051 7.3723 7.3397
2.6224 2.6410 2.6524 2.6612
3.8952 3.7661 3.6489 3.5333
0.9897 0.9909 0.9918 0.9927
5.0
49 50 51 52
1.06 1.06 1.06 1.06
2.5970 2.6500 2.7030 2.7560
2.7030 2.6500 2.5970 2.5440
7.3157 7.2830 7.2506 7.2184
2.6420 2.6510 2.6618 2.6747
3.9778 3.8576 3.7391 3.6227
0.9906 0.9918 0.9928 0.9933
5.1
49 50 51 52
1.06 1.06 1.06 1.06
2.6490 2.7030 2.7571 2.8111
2.7571 2.7030 2.6489 2.5949
7.1971 7.1648 7.1328 7.1011
2.6530 2.6554 2.6706 2.6788
4.0758 3.9586 3.8362 3.7200
0.9915 0.9924 0.9930 0.9937
5.2
49 50 51 52
1.06 1.06 1.06 1.06
2.7009 2.7560 2.8111 2.8662
2.8111 2.7560 2.7009 2.6458
7.0823 7.0504 7.0188 6.9875
2.6552 2.6639 2.6751 2.6854
4.1743 4.0528 3.9340 3.8167
0.9921 0.9931 0.9937 0.9943
RF : The weight ratio of mixed acid to glycerine. RN : The weight percentage of nitric acid in anhydrous mixed acid.
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process safety and environmental protection 8 7 ( 2 0 0 9 ) 87–93
Table 6 – The dilution heat of spent acid system at 20 ◦ C for Biazzi continuous process RF
Compositions of spent acid before dilution RN
Compositions of spent acid after dilution with 2% water RW
RN
RW
Hd (kJ/kg)
4.8
0.1456 0.1324 0.1184 0.1035
0.1608 0.1667 0.1733 0.1793
0.1456 0.1324 0.1184 0.1035
0.1808 0.1867 0.1933 0.1993
−13.8411 −13.4558 −13.0123 −12.6332
4.9
0.1571 0.1447 0.1315 0.1173
0.1581 0.1637 0.1691 0.1748
0.1571 0.1447 0.1315 0.1173
0.1781 0.1837 0.1891 0.1948
−13.9709 −13.6023 −13.2571 −12.8964
5.0
0.1680 0.1561 0.1435 0.1303
0.1549 0.1600 0.1652 0.1706
0.1680 0.1561 0.1435 0.1303
0.1749 0.1800 0.1852 0.1906
−14.1428 −13.8205 −13.4859 −13.1426
5.1
0.1782 0.1670 0.1552 0.1424
0.1514 0.1560 0.1610 0.1662
0.1782 0.1670 0.1552 0.1424
0.1714 0.1760 0.1810 0.1862
−14.3586 −14.0640 −13.7365 −13.4104
5.2
0.1880 0.1772 0.1658 0.1537
0.1479 0.1525 0.1572 0.1621
0.1880 0.1772 0.1658 0.1537
0.1679 0.1725 0.1772 0.1821
−14.5687 −14.2741 −13.9742 −13.6616
able on the part of the safety. Since the average temperature during summer in Taiwan is about 32 ◦ C and the daytime temperature often exceeds 35 ◦ C, storage stability of spent acid after dilution must be addresses when considering the safety of spent acid. This study also evaluated storage safety of spent acid after dilution at 35 ◦ C (Fig. 9). The storage stability of spent acid decreased as the weight ratio of mixed acid to glycerine in operating conditions decreased. To improve storage stability of spent acid, adding a certain amount of nitric acid has been suggested. This method can reduce effectively H2 O content in spent acid and increases storage stability. Variation in heat can be also evaluated in the same manner for handling spent acid.
6. Fig. 8 – The evaluation of spent acid properties before and after dilution with water at 20 ◦ C.
Fig. 9 – The storage safety of spent acid after dilution at 35 ◦ C.
Conclusions
The temperature and composition of spent acid are important factors to consider when determining the safety of spent acid in nitroglycerine factories during handling and storing. This study constructed enthalpy diagrams that can be used to evaluate simultaneously the variation in heat and composition after dilution of spent acid. The evaluation of dilution heat contributes to the controlling of the temperature of spent acid during dilution. The evaluation of the composition of spent acid contributes to the determining of storage stability. These enthalpy diagrams are suitable for nitroglycerine factories and may be applied to other nitration processes. Under practical operating conditions at nitroglycerine factories in Taiwan, this study found that all compositions of spent acid before and after dilution are stable, meaning that operating conditions in nitroglycerine factories are safe. This study also evaluated the storage stability of spent acid after dilution in summertime. Few compositions of spent acid have very low stability and must be adjusted. The potential risks during the handling or storage of spent acid cannot be ignored in nitroglycerine factories. Consequently, it is important to know that the operating conditions during handling and storage of spent acid are safe or danger-
process safety and environmental protection 8 7 ( 2 0 0 9 ) 87–93
ous. This study presented an analytical model that is useful when assessing the safety in handling and storing spent acids at nitroglycerine factories.
Acknowledgement This research was financially supported by the National Science Council of R.O.C. under grant number of 94-2211-E014-001.
References Badger Army Ammunition Plant in Sauk County, Wisconsin, USA, 2002. Explosive Decontamination and Demolition Process Information. Submitted by: Plexus Scientific Corporation, September. Biasutti, G.S., (1985). History of Accidents in the Explosive Industry. (Vevey, Switzerland). Biazzi NG Plant, 1973. FO-13 Technical Manual for Chung-Shan Institute of Science and Technology in Taiwan, p. 50. Camera, E., Modena, G. and Zotti, B., 1982, Propell Explos Pyrot, 7: 66–69. Federation of European Explosives Manufacturers, 1988. Code of Good Practice: Nitroglycerine/nitroglycol Handling of Spent Acid Effluents, Publication No. 11.
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Han, G.L., Li, L.X., Zhao, Z.Y. and Qi, Z.D., (1988). The Technology and Management of Industrial Safety for Nitroglycerine. (National Defense Industry Press, Beijing), pp. 2–4 (in Chinese) Mckinley, C. and Brown, G.G., 1942, Chem Metall Eng, 49: 142–145. Nathan, F.L., Thomson, R.A., Rintoul, W., Brit. Pat. 15,983 (1901). Nathan, F.L., Thomson, R.A., Rintoul, W., Brit. Pat. 3020 (1903). Nathan, F.L., Thomson, R.A., Rintoul, W., Brit. Pat. 2776 (1905). Nathan, F.L., Thomson, R.A., Rintoul, W., Fr. Pat. 366,593 (1906). Oehman, V., Camera, E. and Cotti, L., 1960, Explosivestoffe, 6: 120–127. Oehman, V., Camera, E. and Cotti, L., 1960, Explosivestoffe, 7: 148–158. Oehman, V., Camera, E. and Cotti, L., 1961, Explosivestoffe, 5: 95–113. Pascal, P., 1925, Explosifs, poudres, gaz de combat. Hermann, Paris. Rigas, F., Sebos, I. and Doulia, D., 1997, Ind Eng Chem Res, 36: 5068–5073. Rigas, F., Sebos, I. and Doulia, D., 1998, J Loss Prevent Process Ind, 11: 161–168. SAFEX International, 1987. 9th Congress. Casablance, Morocco, June 3–6, pp. 131–137. Urbanski, T., (1965). Chemistry and Technology of Explosives (Pergamon Press, New York), pp. 62–124 Yeh, T.F., Lu, K.T. and Lin, P.C., 2006, J Explos Propell ROC, 22(2): 1–18.
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Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Study on the stability of nitroglycerine spent acid Kai-Tai Lu ∗ , Peng-Chu Lin School of Defense Science, Chung Cheng Institute of Technology, National Defense University, Taoyuan, Taiwan, ROC
a b s t r a c t Nitroglycerine has been widely used as an ingredient of explosives and propellants for a long time. It is produced by the glycerine nitration reaction. Many explosions have occurred during handling or storing spent acid after separating nitroglycerine from the reactor in nitroglycerine factories. Safety charts have been constructed by various authors in order to cope with these hazards. In this investigation we construct enthalpy diagrams that correspond with the safety charts produced by earlier researchers. These can be used to evaluate the variation of heat and the safety composition during the handling of spent acid. Furthermore, this study uses practical operating conditions at nitroglycerine factories to evaluate the stability of spent acids in storage. © 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Nitroglycerine; Spent acid; Enthalpy diagrams; Safety; Stability
1.
Introduction
The manufacture of nitroglycerine is one of the most dangerous procedures in the explosives industry. Although manufacturing techniques have been developed for over 140 years, industrial procedures have changed little. These procedures comprise preparing mixed acid, glycerine nitration, separation of nitroglycerine and spent acid, washing nitroglycerine, filtration of spent acid, recycling nitroglycerine from spent acid and treating spent acid (Urbanski, 1965). Many accidents have occurred during these manufacturing processes. Forty-five major industrial accidents involving nitroglycerine occurred between 1860 and 1984 worldwide (Badger Army Ammunition Plant, 2002). Statistics for China show that 43 industrial accidents involving nitroglycerine occurred between 1954 and 1982 (Han et al., 1988). Biasutti (1985) analyzed 131 industrial accidents that involved nitroglycerine. Among these, 13 events, accounting for approximately 10% of the total events, correlated with spent acid decomposition. Thus, the potential risk during handling or storing spent acid cannot be ignored. Generally, industrial accidents involving spent acid at nitroglycerine factories are mainly caused by the following reasons (Rigas et al., 1998).
∗
(1) The decrease in temperature of spent acid results in an increased viscosity and thus affects the separation of nitroglycerine from spent acid. (2) A reduction of temperature during the handling of spent acid may cause separation of nitroglycerine. (3) Improper dilution of spent acid with water. (4) Unintentional dilution of spent acid can destroy the stability of spent acid. Oehman et al. (1960a,b, 1961) performed a thorough investigation of nitroglycerine and spent acid systems. They described that the cause of system instability is the hydrolysis reaction of nitrate esters in spent acid. The same viewpoint also presented by Camera et al. (1982), the hydrolysis and oxidation mechanism of nitroglycerine in acid solution was described particularly. Rigas et al. (1997, 1998) analyzed three safety charts in the literature (SAFEX, 1987; FEEM, 1988; Oehman et al., 1960a,b, 1961). They simulated and evaluated safety charts via a simple reaction kinetics model and a computer code to explain the boundary conditions of safe regions in safety charts. Rigas et al. also pointed out that both composition and temperature of spent acid are central factors determining the content of dissolved esters in the acid phase.
Corresponding author. Tel.: +886 3 3891716; fax: +886 3 3892494. E-mail address: [email protected] (K.-T. Lu). Received 21 August 2008; Accepted 31 August 2008 0957-5820/$ – see front matter © 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2008.08.004
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In this investigation mathematical models were used to analyze the relative enthalpies of spent acids that are composed of HNO3 , H2 SO4 and H2 O at various weight ratios and temperatures. Furthermore, enthalpy diagrams were constructed that correspond with safety charts to evaluate the variation of heat and the composition of safety during the handling of spent simultaneously based on practical operating conditions at nitroglycerine factories. Storage stability of spent acid after dilution was also took into consideration.
2.
Presentation of safety charts
Acidic nitroglycerine and spent acid form when manufacturing nitroglycerine. Acidic nitroglycerine is very unstable and requires washing as soon as possible. As spent acid contains a small amount of nitroglycerine, its composition and temperature are the important factors that determine whether spent acid is safe or dangerous. Thus, practical safety charts for spent acids have been developed for manufacturing nitroglycerine. Based on numerous experiments, Oehman et al. (1960a,b, 1961) developed a diagram (Fig. 1) indicating the compositions of spent acids that are safe or dangerous when in contact with nitroglycerine. It was assumed that the spent acid system only comprises the three components mentioned on the axis labels. They also observed that HNO2 content in the spent acid affects the system stability; that is, the stability of spent acid is decreased by increasing the HNO2 content. In Fig. 1 they call “High safety”, the areas where not more 0.2% HNO2 is formed after heating for 2 h at 70 ◦ C and “Normal safety” where 0.2–1% HNO2 is formed. This diagram is based on the HNO3 content and the H2 O content in the spent acid, and is only valid when operating with pure materials. Federation of European Explosives Manufacturers (1988) constructed a safety chart at 35 ◦ C and HNO2 content of 0.2%. Fig. 2 presents a new safety chart that is similar to Fig. 1. This diagram indicates that storage stability of spent acid can be maintained for a long time at 35 ◦ C, a temperature commonly reached in storage tanks in summertime. “Very High Stability” in Fig. 2 is the area where is assumed that the HNO2 content in the spent acid should not increased to more than 0.2% after storing the spent acid at 35 ◦ C for 1 month and “Very Low Stability” where over 1% HNO2 is formed under the same conditions.
Fig. 2 – Safety diagram for spent acids from a nitroglycerine factory for 35 ◦ C (FEEM, 1988).
Fig. 3 – Safety diagram for spent acids from a nitroglycerine factory for 20 ◦ C (SAFEX, 1987). SAFEX (1987) also has constructed a safety chart for 20 ◦ C as Fig. 3. It is obvious that this diagram is the most complete since it covers the entire region of spent acid composition. The top region of Fig. 3 is labeled “stable area”, but it is a high H2 O content area. According to Oehman et al. (1960a,b, 1961), however, high H2 O content is dangerous. Rigas et al. (1998) explained that the oxidation power of nitric acid in a dilute system is considerably reduced. The nitroglycerine solubility in spent acid may be also an important factor when determining the safety of spent acid. This study found that spent acid contains less nitroglycerine in the region of higher H2 O contents (over 50%) than other regions from Pascal’s triangular diagram (Pascal, 1925), which shows the solubility of nitroglycerine in spent acid at different compositions. Furthermore, this study found the same characteristics in the bottom region of Fig. 3, which is also a stable area, containing low H2 O content in the spent acid. In view of this phenomenon, we propose that the reduction of nitroglycerine content in spent acid also reduces HNO2 formation, indicating that the spent acid system is stable in regions with high (>50%) or low (<10%) H2 O content.
3. Simulation of enthalpy diagrams corresponding with safety charts
Fig. 1 – Safety diagram for spent acids from a nitroglycerine factory for 70 ◦ C (Oehman et al., 1960b, 1961).
In practice, the composition of spent acid after nitration in nitroglycerine factories usually contains low H2 O content and high H2 SO4 content. To reduce the danger of handling spent
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Fig. 4 – The relative enthalpy of nitric acid, sulfuric acid, and water mixtures at 35 ◦ C.
Fig. 5 – The relative enthalpy of nitric acid, sulfuric acid, and water mixtures at 20 ◦ C.
acid immediately after the separation of nitroglycerine, adding a certain amount of water was suggested by Nathan et al. (1901, 1903, 1905, 1906). This method greatly enhances safety when handling spent acid, and supplementary separation of nitroglycerine is not required. When diluted with water, the solubility of nitroglycerine in the spent acid increases and some nitroglycerine is lost as it no longer separates. Addition of water to detain nitroglycerine in the spent acid not only increases nitroglycerine solubility in the spent acid, it also alters the equilibrium of the reaction between dinitroglycerine, nitroglycerine and acid in such a way that dinitroglycerine undergoes further nitration and trinitrate does not form. The dilution heat formed when adding water to the spent acid must be removed. The relative enthalpy and specific heat of spent acid of various compositions are important when calculating dilution heat. Mckinley and Brown (1942) drew a relative enthalpy diagram at 0 ◦ C and a specific heat diagram for mixed acid containing various weight ratios of sulfuric acid, nitric acid, and water. Based on these diagrams, the relative enthalpies of nitrating mixtures and of heat generated when mixing acids at various temperatures can be easily calculated. Figs. 4 and 5 show the relative enthalpies of nitric acid, sulfuric acid, and water mixtures at 35 ◦ C and 20 ◦ C, respectively. Furthermore, the curves in Figs. 4 and 5 were simulated with least square regression analysis to polynomials in the form: H = a + bx + cx2 + dx3 + ex4
(1)
where H is the relative enthalpy of nitric acid, sulfuric acid or mixed acid. x is the weight percent of nitric acid in anhydrous mixed acid and is in the range between 0 and 100. Table 1 presents the polynomial coefficients and the correlation coefficients R2 . In simulating enthalpy diagrams corresponding with safety charts, RW is weight ratio of water in spent acid and RN is weight ratio of nitric acid in anhydrous spent acid as in the following equations. RW =
WH2 O WHNO3 + WH2 SO4 + WH2 O
(2)
RN =
WHNO3 WHNO3 + WH2 SO4
(3)
The relative enthalpy of spent acid at various compositions can be expressed as a function of RW and RN . H = f (RW , RN )
(4)
Table 1 is used to draw the enthalpy diagram corresponding with Fig. 2 in the ranges of 0.15 ≤ RW ≤ 0.2 and 0 ≤ RN ≤ 0.2 by calculating method (Fig. 6). The relative enthalpies of spent acid for various compositions at 35 ◦ C were simulated with least square regression analysis by the following polynomial
Table 1 – Coefficients of the polynomials used to simulate the curves of Figs. 4 and 5 for the spent acid system Temperature
Percent by weight of total acid a
35 ◦ C
Pure HNO3 80% HNO3 60% HNO3 40% HNO3 20% HNO3 Pure H2 SO4
20 ◦ C
Pure HNO3 80% HNO3 60% HNO3 40% HNO3 20% HNO3 Pure H2 SO4
R2
Coefficients
147.29 149.40 147.42 145.28 144.78 147.10 83.845 86.145 84.169 82.092 82.013 84.598
b
c
d
e −4
−6.9707 −7.5424 −7.1803 −7.0329 −7.6174 −9.6501
0.0193 0.0207 −0.0066 −0.0284 −0.0255 0.0819
4.2547 × 10 4.2056 × 10−4 6.4137 × 10−4 8.4355 × 10−4 8.6255 × 10−4 −1.2000 × 10−3
– – – – – 1.2778 × 10−5
0.9998 0.9988 0.9998 0.9998 0.9990 0.9986
−6.0977 −6.8462 −6.5699 −6.4480 −6.9518 −9.2756
0.0076 0.0135 −0.0110 −0.0318 −0.0300 0.0903
4.8846 × 10−4 4.5905 × 10−4 6.6254 × 10−4 8.5822 × 10−4 8.8062 × 10−4 −1.4000 × 10−3
– – – – – 1.3504 × 10−5
0.9999 0.9990 0.9998 0.9998 0.9990 0.9999
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Table 2 – Coefficients of the polynomials used to simulate the relative enthalpies of Figs. 6 and 7 for spent acid of various compositions Coefficients
Hat 35 ◦ C (kJ/kg) 4.1507 × 101 −1.9681 × 103 3.9998 × 103 −1.9179 × 103 −2.1652 × 102 2.6680 × 103 −5.5612 × 103 −4.3305 × 101 5.3360 × 102 −8.6609 0.9999
a b c d e f g h i j R2
Fig. 6 – The enthalpy diagram corresponding with Fig. 2 at 35 ◦ C.
Hat 20 ◦ C (kJ/kg) −7.7744 −1.6133 × 103 2.7443 × 103 −9.5667 × 102 −8.4740 × 101 8.0731 × 102 −9.0758 × 102 1.1285 × 102 −2.2313 × 102 1.4385 × 101 0.9999
Yeh et al. (2006). 2 3 RW = a + bRN + cRN + dRN
(6)
Tables 3 and 4 list the polynomial coefficients. The value of RN can be determined from the weight ratios of sulfuric acid and nitric acid in the spent acid system; then using Eq. (6) and coefficients in Tables 3 and 4 to calculate the values of RW on the top, medium and last lines in Figs. 6 and 7. By using the actual value of RW in the spent acid system, the stability of the spent acid systems at various temperatures can easily be obtained. When the values of RN and RW are given for a spent acid system before and after dilution with water, the heat generated by dilution can be calculated using Eq. (5) and coefficients in Table 2 at various temperatures.
4. Fig. 7 – The enthalpy diagram corresponding with Fig. 3 at 20 ◦ C.
of RW and RN : 2 3 2 2 H = a + bRW + cRW + dRW + eRN + fRW RN + gRW RN + hRN 2 3 + iRW RN + jRN
(5)
In the same manner, the enthalpy diagram corresponding with Fig. 3 was drawn as Fig. 7, which shows the relative enthalpy in the ranges of 0 ≤ RW ≤ 0.6 and 0 ≤ RN ≤ 1.0. This diagram was also simulated by the polynomial of Eq. (5). The coefficients of the polynomials used to simulate the relative enthalpy of Figs. 6 and 7 are listed in Table 2. Furthermore, the relationship between RW and RN on the boundary lines in Figs. 6 and 7 was simulated using the following polynomial. This analytic manner had been applied by
Comparison of enthalpy diagrams
Although the enthalpy diagram in Fig. 6 only covers the case of low H2 O content in spent acid, it is very useful in practice because the compositions of spent acid after nitration in nitroglycerine factories usually contain low H2 O content. In Fig. 6, the relative enthalpy of a spent acid system decreases as H2 O content increases; hence, heat is generated by adding water to the spent acid system. After separating nitroglycerine from the spent acid, a certain amount of water must be added to dissolve suspended nitroglycerine and reduce the danger of handling and storing the spent acid. Thus, removal of this heat must be taken into consideration. Furthermore, the composition of spent acid after dilution with water must be also confined to stable area for safe storage. Fig. 6 can be used to evaluate heat generated by dilution and the safe composition of spent acid simultaneously. Fig. 7 is the most complete enthalpy diagram since it covers the entire region of spent acid compositions. Notably, Fig. 7 has two stable areas in the high and low H2 O content regions. From perspective of nitroglycerine solubility in a spent acid sys-
Table 3 – Coefficients of the polynomials used to simulate the boundary lines of Fig. 6 Boundary lines
Top Medium 1 Medium 2 Bottom
Range of RN
0.0473 RN 0.1817 0.0478 RN 0.1829 0.0484 RN 0.1844 0.0491 RN 0.1856
R2
Coefficients a
b
c
d
0.1375 0.1436 0.1580 0.1712
0.4236 0.4863 0.3944 0.3412
−1.2218 −1.5712 −1.2940 −1.1843
0.0511 0.0013 −0.0202 −0.2911
0.9999 0.9999 0.9999 0.9999
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Table 4 – Coefficients of the polynomials used to simulate the boundary lines of Fig. 7 Boundary lines
Range of RN
R2
Coefficients a
b
c
d
Top
0.5438 RN 1.0000 0.2689 RN 0.5346 0.0914 RN 0.2710 0.0000 RN 0.0886
0.0214 0.3195 0.2022 0.1500
0.0848 −0.2624 0.0601 0.7255
−0.0062 −0.3977 −0.0045 −0.5476
– – – –
0.9999 0.9999 0.9999 0.9999
Medium
0.0000 RN 0.0972 0.1001 RN 0.2912 0.2895 RN 0.8148 0.8148 RN 1.0000
0.1502 0.2361 0.0571 −0.3231
1.5535 −0.0832 0.7147 1.3495
−2.2574 −0.0608 −0.2721 −0.4765
– – – –
0.9999 0.9999 0.9999 0.9999
Bottom
0.0000 RN 1.0000
0.5000
0.0552
−0.0053
–
0.9999
tem, this study has explained that the spent acid system can dissolve small amounts of nitroglycerine to reduce formation of HNO2 in these two stable areas in Fig. 7, and the stability of spent acid system increases by decreasing the HNO2 content. Furthermore, this study also found that the relative enthalpy of a spent acid system decreases when H2 O content increases within 0 ≤ RW ≤ 0.2 as well as Fig. 6, however, the relative enthalpy increases as H2 O content increases within RW ≥ 0.5. If the composition of spent acid system is in doubtful or unstable areas, stability can be increased by increasing H2 O content or increasing HNO3 content to decrease H2 O content for storage. Fig. 7 can be also used to evaluate the variation in heat and composition of safe regions during handling of spent acid systems. A comparison of Figs. 6 and 7 shows that Fig. 6 is more conservative than Fig. 7 in the safe region. This was expected, since Fig. 6 refers to 35 ◦ C, while Fig. 7 to 20 ◦ C. For relative enthalpy, the temperature of spent acid system is a principal
influential factor. Relative enthalpy increases as temperature increases for same composition condition in a spent acid system.
5.
Application of enthalpy diagrams
Two factories in Taiwan use the Biazzi continuous process to produce nitroglycerine. Their operating conditions were considered from the perspectives of both safety and economy (Table 5) (Biazzi NG Plant, 1973). The temperature of spent acid after separation of nitroglycerine was maintained within 18–20 ◦ C; about 2% of H2 O was added to the spent acid to prevent suspended nitroglycerine. According to the compositions of spent acid before and after water dilution, this study evaluated dilution heat and spent acid system stability (Fig. 8 and Table 6). In Fig. 8, all compositions of spent acid before and after dilution are located in the stable area, indicating that the operating conditions in nitroglycerine factories are suit-
Table 5 – The operating conditions of manufacture nitroglycerine in Biazzi continuous process (Biazzi NG Plant FO-13 Technical Manual, 1973) RF
RN (%)
Inflow compositions (kg/min) C3 H5 (OH)3
Resident time (min)
Mixed acid HNO3
H2 SO4
Outflow compositions (kg/min) C3 H5 (ONO2 )3
Fractional conversion of C3 H5 (OH)3
Spent acid
4.8
49 50 51 52
1.06 1.06 1.06 1.06
2.4931 2.5440 2.5949 2.6458
2.5949 2.5440 2.4931 2.4422
7.5650 7.5315 7.4982 7.4652
2.5957 2.6146 2.6405 2.6526
3.8251 3.6956 3.5580 3.4425
0.9885 0.9900 0.9912 0.9921
4.9
49 50 51 52
1.06 1.06 1.06 1.06
2.5451 2.5970 2.6489 2.7009
2.6489 2.5970 2.5451 2.4931
7.4383 7.4051 7.3723 7.3397
2.6224 2.6410 2.6524 2.6612
3.8952 3.7661 3.6489 3.5333
0.9897 0.9909 0.9918 0.9927
5.0
49 50 51 52
1.06 1.06 1.06 1.06
2.5970 2.6500 2.7030 2.7560
2.7030 2.6500 2.5970 2.5440
7.3157 7.2830 7.2506 7.2184
2.6420 2.6510 2.6618 2.6747
3.9778 3.8576 3.7391 3.6227
0.9906 0.9918 0.9928 0.9933
5.1
49 50 51 52
1.06 1.06 1.06 1.06
2.6490 2.7030 2.7571 2.8111
2.7571 2.7030 2.6489 2.5949
7.1971 7.1648 7.1328 7.1011
2.6530 2.6554 2.6706 2.6788
4.0758 3.9586 3.8362 3.7200
0.9915 0.9924 0.9930 0.9937
5.2
49 50 51 52
1.06 1.06 1.06 1.06
2.7009 2.7560 2.8111 2.8662
2.8111 2.7560 2.7009 2.6458
7.0823 7.0504 7.0188 6.9875
2.6552 2.6639 2.6751 2.6854
4.1743 4.0528 3.9340 3.8167
0.9921 0.9931 0.9937 0.9943
RF : The weight ratio of mixed acid to glycerine. RN : The weight percentage of nitric acid in anhydrous mixed acid.
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Table 6 – The dilution heat of spent acid system at 20 ◦ C for Biazzi continuous process RF
Compositions of spent acid before dilution RN
Compositions of spent acid after dilution with 2% water RW
RN
RW
Hd (kJ/kg)
4.8
0.1456 0.1324 0.1184 0.1035
0.1608 0.1667 0.1733 0.1793
0.1456 0.1324 0.1184 0.1035
0.1808 0.1867 0.1933 0.1993
−13.8411 −13.4558 −13.0123 −12.6332
4.9
0.1571 0.1447 0.1315 0.1173
0.1581 0.1637 0.1691 0.1748
0.1571 0.1447 0.1315 0.1173
0.1781 0.1837 0.1891 0.1948
−13.9709 −13.6023 −13.2571 −12.8964
5.0
0.1680 0.1561 0.1435 0.1303
0.1549 0.1600 0.1652 0.1706
0.1680 0.1561 0.1435 0.1303
0.1749 0.1800 0.1852 0.1906
−14.1428 −13.8205 −13.4859 −13.1426
5.1
0.1782 0.1670 0.1552 0.1424
0.1514 0.1560 0.1610 0.1662
0.1782 0.1670 0.1552 0.1424
0.1714 0.1760 0.1810 0.1862
−14.3586 −14.0640 −13.7365 −13.4104
5.2
0.1880 0.1772 0.1658 0.1537
0.1479 0.1525 0.1572 0.1621
0.1880 0.1772 0.1658 0.1537
0.1679 0.1725 0.1772 0.1821
−14.5687 −14.2741 −13.9742 −13.6616
able on the part of the safety. Since the average temperature during summer in Taiwan is about 32 ◦ C and the daytime temperature often exceeds 35 ◦ C, storage stability of spent acid after dilution must be addresses when considering the safety of spent acid. This study also evaluated storage safety of spent acid after dilution at 35 ◦ C (Fig. 9). The storage stability of spent acid decreased as the weight ratio of mixed acid to glycerine in operating conditions decreased. To improve storage stability of spent acid, adding a certain amount of nitric acid has been suggested. This method can reduce effectively H2 O content in spent acid and increases storage stability. Variation in heat can be also evaluated in the same manner for handling spent acid.
6. Fig. 8 – The evaluation of spent acid properties before and after dilution with water at 20 ◦ C.
Fig. 9 – The storage safety of spent acid after dilution at 35 ◦ C.
Conclusions
The temperature and composition of spent acid are important factors to consider when determining the safety of spent acid in nitroglycerine factories during handling and storing. This study constructed enthalpy diagrams that can be used to evaluate simultaneously the variation in heat and composition after dilution of spent acid. The evaluation of dilution heat contributes to the controlling of the temperature of spent acid during dilution. The evaluation of the composition of spent acid contributes to the determining of storage stability. These enthalpy diagrams are suitable for nitroglycerine factories and may be applied to other nitration processes. Under practical operating conditions at nitroglycerine factories in Taiwan, this study found that all compositions of spent acid before and after dilution are stable, meaning that operating conditions in nitroglycerine factories are safe. This study also evaluated the storage stability of spent acid after dilution in summertime. Few compositions of spent acid have very low stability and must be adjusted. The potential risks during the handling or storage of spent acid cannot be ignored in nitroglycerine factories. Consequently, it is important to know that the operating conditions during handling and storage of spent acid are safe or danger-
process safety and environmental protection 8 7 ( 2 0 0 9 ) 87–93
ous. This study presented an analytical model that is useful when assessing the safety in handling and storing spent acids at nitroglycerine factories.
Acknowledgement This research was financially supported by the National Science Council of R.O.C. under grant number of 94-2211-E014-001.
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