Recovery of dinitrotoluenes and trinitrotoluene from spent acid of toluene nitration process by solvent extraction

Separation and Purification Technology 43 (2005) 95–101

Recovery of dinitrotoluenes and trinitrotoluene from spent acid of toluene nitration process by solvent extraction Wen-Shing Chena,∗ , Chien-Neng Juana , Kuo-Ming Weib a

Department of Chemical Engineering, National Yunlin University of Science and Technology, Yunlin 640, Taiwan, ROC b Refining and Manufacturing Research Center, Chinese Petroleum Corporation, Chia-Yi 600, Taiwan, ROC Received 1 May 2004; received in revised form 21 September 2004; accepted 10 October 2004

Abstract Extraction using hexane or heptane solvent was employed to recover dinitrotoluene (DNT) isomers and 2,4,6-trinitrotoluene (TNT) from spent acid after toluene nitration process. Multiple stages experiments were conducted to elucidate the influence of various operating variables on the performance of removal of organic compounds from spent acid, including extracting temperature, volume ratio of solvent versus acid, spent sulfuric acid concentration and organic compounds content of the feedstock. It was found that organic compounds extracted reached nearly 78%, of which solubility in spent acid depends strongly upon the sulfuric acid concentration. In both cases of hexane and heptane, the extracting priority of organic compounds is in the following order: 2,3-DNT > 2,6-DNT > 3,4-DNT > 2,4-DNT > 2,4,6-TNT. Furthermore, this method established is promising for regeneration of spent acid preliminarily. © 2004 Elsevier B.V. All rights reserved. Keywords: Dinitrotoluene; Trinitrotoluene; Extraction; Spent acid

1. Introduction Aromatic nitration processes using a mixture of sulfuric and nitric acid have been well developed. The former leads to formation of the nitrating agent (nitronium ion, NO2 + ) and inhibits the dissociation of nitric acid into an oxidizing ion (NO3 − ). Additionally, the mixed acid serves to enhance solubility between the aqueous and organic phases [1]. Due to some organic compounds dissolving in mixed acid and production of water byproduct, the spent mixed acid is regenerated industrially in two steps, including purification and concentration. The former aims to abate organic compounds and residual nitric acid, and the latter is responsible for elevation of concentration of sulfuric acid. Recently, Bodenbenner et al. [2] investigated the oxidative degradation of organic compounds by hydrogen peroxide or ozone. Subsequently, the dilute sulfuric acid was ∗

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concentrated to 60–70 wt.% in a vacuum evaporator firstly, and 97 wt.% sulfuric acid concentration was achieved in a packed column or plate column. In their technical reports, Plessen and Schiessler [3] and Plessen et al. [4] proposed the oxidative degradation of organic compounds by potassium permanganate. In contrast, Parks and Martin [5] converted organic compounds into high molecular weight ones by heating procedure. In the concentration unit, the organic compounds would be removed by distillation column. Several researchers modified the purification system wherein organic compounds were removed directly by superheated steam [6]. Sawicki [7] studied the recovery of dinitrotoluenes and organic byproducts from wastewater at a PH value of 8 by toluene extraction. In another publication, the authors focused their interest on the removal of organic compounds from spent acid, which had undergone oxidation of nitrous acid into nitric acid in advance, by toluene extraction [8]. Further, Witt and Beckhaus [9] investigated the recovery of sulfuric and nitric acid from dinitrotoluenes of organic phase by means of blending with deionized water. Another report

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has been issued that the nitric and sulfuric acid was extracted from crude dinitrotoluenes by a dilute aqueous solution of said acid [10]. According to the study of Chou and Chen [11] and Chou et al. [12], the organic compounds dissolving in spent acid were oxidized into carbon dioxide by hydroxy radical (• OH), generated from anodic oxidation of water. Therefore, regeneration of spent sulfuric acid could be carried out in a single unit. Until now, numerous investigations have dealt with the oxidative degradation of organic compounds contained in spent acid. In our previous paper [13], high purity of 2,4DNT could be recovered from spent acid by simply diluting or refrigerating method. This work explores the feasibility of solvent extraction of organic compounds, e.g. dinitrotoluene isomers and 2,4,6-trinitrotoluene from spent acid of toluene nitration process. Hexane and heptane were selected as the solvent due to their common usage industrially [14,15]. The effects of operating temperature, volume ratio of solvent versus acid, spent sulfuric acid concentration and organic compounds content on the removal of total organic compounds were elucidated. It is remarkable that not only organic compounds but also water contents of spent acid were diminished significantly. Thus, it reveals that the method established could be in use to regenerate spent acid.

2. Experimental 2.1. Extraction testing Extraction tests were performed in an autoclave (Autoclave Engineers, Erie PA 16512 Model) system (see Fig. 1) under atmospheric pressure at 300, 323 and 343 K, respectively. Prior to tests, a proportionate amount of spent acid

(rendered by military ammunition plant) was situated in the autoclave. The identical amount (volume basis) of hexane or heptane (≥ 99.5%, Fluka) was supplied by a liquid metering pump (LDC Analytical Consta Metric 3200 Model). The extractor was made of stainless steel 316, and equipped with both cooling circulation bath and heating jacket. One thermocouple was inserted into the extracting zone for reading and controlling the temperature. The flow rate of gas effluent was measured by a wettype gas meter (Ritter TG 1 Model). The typical spent acid was composed of H2 SO4 :HNO3 :H2 O:organic compounds = 74.3:2.7:21.97:1.03 on the weight basis. At the beginning of extraction tests, the spent sulfuric acid concentration and organic compounds content were adjusted in the range of 46.9–74.3 wt.% and 1.03–1.64 wt.%, respectively. After extraction experiment (ca. 8 min), the extract decanted from the extractor was analyzed by a gas chromatograph (Hewlett Packard 6890 SERIES) equipped with a flame ionization detector. A capillary column (DB-1, 100 m × 0.25 mm, film thickness 0.5 ␮m) operated from 373 to 493 K was used to identify the compositions. Furthermore, the raffinate was undergone both total organic compounds and Karl–Fischer analyses to evaluate the organic compounds and water contents. To find out the steady state concentration of organic compounds residued in spent acid, six stages of extracting treatment were carried out in series. The extracting percentage of dinitrotoluene isomers and trinitrotoluene in each stage respectively was defined as follows, e.g. 2,4-DNT was defined as shown: 2, 4-DNT(%) =

the amount of 2, 4-DNT in each extract sum of amounts of 2, 4-DNT in six extracts

Fig. 1. Schematic diagram of the solvent extraction system.

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2.2. Gas chromatograph/mass spectrometer analysis (GC/MS) The proportionate amount (0.3 ␮l) of extract was injected into a gas chromatograph/mass spectrometer (Hewlett Packard 59864B/HP 5973 MASS) equipped with a capillary column (Metal ULTRA ALLOY UA-5, 30 m × 0.25 mm, film thickness 0.25 ␮m). The mass spectra obtained were used to identify the ingredients involved in the extract by the reference database (Wiley 275.L). 2.3. Total organic compounds analysis (TOC) The spent acid after carrying out extracting tests was analyzed on Tekmar Dohrmann Phoenix 8000 equipped with both UV reactor and NDIR detector, wherein sodium persulfate served as an oxidizing agent. Prior to analysis, the concentrations of samples were diluted to fiftieth one to meet the requirement of measuring limits. The data obtained was corrected by the potassium hydrogen phthalate standard solution (20–1000 ppm). 2.4. Karl–Fischer analysis The water contents of spent acid of serial raffinates were measured by a Karl–Fischer Coulometric Moisture Titrator (MKC-510N Model), in which iodine serves as a titrant. The weight of sample was kept below 0.5 mg to prevent the interference of sulfuric acid.

3. Results and discussion 3.1. Effect of extracting temperature According to the study of Witt and Beckhaus [9], the nitrating spent acid consisted of 70–75 wt.% sulfuric acid, 0.3–0.7 wt.% nitric acid, 0.4–2.0 wt.% nitrous acid, 0.5–1.7 wt.% organic compounds and water balanced. Many researchers have paid attention to oxidative degradation of organic compounds in spent acid derived from nitration processes [2–4,16]. In this research, organic compounds involved in spent acid would be recovered by solvent extraction. The four curves in Fig. 2 illustrate the residual organic compounds content in spent acid at different extracting temperatures among six extraction stages. It clearly indicates that the extracted amounts of organic compounds in each stage exhibit a decreasing trend with increasing number of stage. As far as fifth and sixth extraction stages are concerned, the amount of organic compounds in extract was only 1.5–2.5 wt.% of the feedstock. It means that six stages operation could meet the steady state requirement of extraction testing. Obviously, the residual organic compounds content at 300 K was higher than that of 323 K at sixth stage using hexane solvent (3116 ppm versus 2748 ppm). In fact, an identical trend was observed for heptane solvent as compared the data

Fig. 2. Effect of the extracting temperature on the removal of organic compounds in spent acid among six stages under solvent/acid (1/1) and atmospheric pressure.

between 323 and 343 K. The phenomenon may be interpreted with enhancing solubility of organic compounds into aqueous spent acid with increasing temperature [17–20]. Nonetheless, the effect of enhancing solubility of organic compounds into hexane or heptane, resulted from temperature increase, is more apparent [20]. That leads to more organic compounds extracted by hexane at high temperature. As the extracting performance at the same temperature is considered, heptane seems better than hexane (2604 ppm versus 2748 ppm). The plausible explanation is longer-chain unbranched alkanes being more suitable for extracting polar solutes in comparison with shorter-chain ones, as described in the textbook [21]. In order to identify all components involved in the extract, the samples were chiefly analyzed by GC/MS spectrometer. Table 1 summarizes the results obtained, wherein the compositions consist of 2,3-DNT, 2,6-DNT, 3,4-DNT, 2,4-DNT and 2,4,6-TNT. The amount of each organic component among six extracts was summed up individually and overall components distribution was listed in Table 2. With regard to the weight ratios of dinitrotoluene isomers, e.g. 2,4-DNT:2,6DNT:2,3-DNT:3,4-DNT = 78.59:17.90:0.42:1.53 in the case of hexane, it appears that dinitrotoluene isomers dissolving in aqueous spent acid are in equilibrium with their isomers in organic product of toluene nitration process, of which compositions are as follows [9]: 2,4-DNT:2,6-DNT:2,3-DNT:3,4DNT = 77:18:1.3:2.8. Additionally, similar weight ratios of dinitrotoluene isomers of the extracts were also observed for the case of heptane. That provides another piece of evidence for above hypothesis, as consistent with our previous report [13]. Furthermore, the content of dinitrotoluene isomers in spent acid was extracted completely by hexane after 18 extraction stages. As expected, similar weight ratios of dinitrotoluene isomers were obtained, i.e. 2,4-DNT:2,6-DNT:2,3DNT:3,4-DNT = 78.51:17.80:0.44:1.60. To understand the extractable tendency of each organic component in spent acid, the extracting percentage, as mentioned in experimental section, was clearly defined. As shown in Fig. 3, it is remarkable that 2,3-DNT has been extracted

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Table 1 Components involved in extract identified by GC/MS analysis Mass/charge ratios of different relative abundance (%)

2,4-DNT 2,6-DNT 2,3-DNT 3,4-DNT 2,4,6-TNT

30 (12.5%), 39 (12.6%), 51 (13.0%), 63 (35.6%), 64 (13.0%), 77 (12.2%), 78 (16.3%), 89 (60.6%), 90 (25.9%), 119 (25.5%), 165 (100%), 166 (13.7%) 51 (16.0%), 63 (35.7%), 64 (15.6%), 77 (19.6%), 78 (16.2%), 89 (40.1%), 90 (27.0%), 91 (15.7%), 121 (17.4%), 135 (14%), 148 (20.8%), 165 (100%) 30 (13.2%), 39 (11.8%), 51 (12.2%), 62 (10.1%), 63 (32.6%), 64 (14.2%), 78 (16.4%), 89 (51.2%), 90 (17.6%), 91 (11.4%), 119 (25.3%), 166 (100%) 30 (64.3%), 39 (32.7%), 51 (22.4%), 52 (32.7%), 63 (47.1%), 65 (28.8%), 66 (32.9%), 77 (28.9%), 78 (46.2%), 89 (51.0%), 94 (32.2%), 182 (100%) 30 (14.7%), 39 (10.4%), 51 (10.9%), 62 (16.2%), 63 (31.9%), 76 (14.7%), 89 (43%), 134 (11.7%), 180 (13.4%), 193 (13.1%), 210 (100%), 211 (9.6%)

Table 2 Product distribution of organic compounds within six extracts

Component

2,4-DNT (wt.%) 2,6-DNT (wt.%) 2,3-DNT (wt.%) 3,4-DNT (wt.%) 2,4,6-TNT (wt.%)

Solvent (operating conditions) Hexane/acid (1/1), 300 K

Hexane/acid (1/1), 323 K

Hexane/acid (2/1), 323 K

Heptane/acid (1/1), 323 K

Heptane/acid (1/1), 343 K

Heptane/acid (2/1), 323 K

78.59 17.90 0.42 1.53 1.56

78.47 17.35 0.43 1.60 2.15

78.34 17.05 0.43 1.52 2.66

79.30 16.94 0.42 1.52 1.82

79.54 16.30 0.38 1.44 2.34

80.11 16.20 0.38 1.42 1.89

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extraction stages. It reaches a level as low as 17 wt.% water content at sixth stage. Due to the insolubility between water and hexane, the phenomenon may be explained by a little amount of water dispersing in the extract, in which the slight acidity had been detected by PH meter. The water content of nitrating acid was properly in the range of 5–20 wt.% in accordance with the literature [23]. Thus, our results suggested strongly the suitability of recycling of spent acid into toluene nitration processes after solvent extraction. 3.2. Effect of volume ratio of solvent versus spent acid

Fig. 3. Extraction percentage of dinitrotoluene isomers and trinitrotoluene in a series of extracting testing by heptane extraction under solvent/acid (1/1) at 323 K.

completely within four stages of extraction tests. That reveals 2,3-DNT was more easily extracted from spent acid by means of heptane. Therefore, the slopes of these curves represent the extracting priority of organic compounds. That is, organic component with higher absolute value of slope is more easily extracted than that of lower values. Consequently, in the case of heptane, the extracting priority of organic compounds is in the following order: 2,3-DNT > 2,6DNT > 3,4-DNT > 2,4-DNT > 2,4,6-TNT. An analogous extracting behavior was also observed in the case of hexane. It may be attributed to the influence of relative location of electron-withdrawing nitro groups on the solubility of dinitrotoluene isomers and 2,4,6-TNT in the aqueous spent acid. In other words, the solubility of organic compounds into spent acid was as follows: 2,3-DNT < 2,6-DNT < 3,4-DNT < 2,4DNT < 2,4,6-TNT, in agreement with the report by Spanggord et al. [22]. Fig. 4 presents the water contents of spent acid in a series of extracting treatments respectively. Apparently, there exists a decreasing trend of water contents of spent acid with

Fig. 4. The water content of spent acid in a series of extracting testing by hexane extraction under solvent/acid (1/1) at 323 K.

It has been recognized that the adjustment of solvent amounts used is an important operating variable industrially. Fig. 5 demonstrates the influence of volume ratio of solvent versus acid on the extracting performance. As far as hexane is concerned, the high volume ratio of solvent/acid (2/1) is more beneficial to extract organic compounds of spent acid as compared with that of low one during the course of extracting testing (2674 ppm versus 2748 ppm). A similar trend was observed in the case of heptane. Besides, the extracting efficiency of high volume ratio of solvent versus acid was significantly superior to that of low one prior three stages operation (3018 ppm versus 3867 ppm). This implies that high volume ratio of solvent versus acid would be preferred from economic consideration in industrial application. As expected, the extracting performance of heptane gets better than that of hexane, corresponding to results as described in previous paragraph. The amount of each organic component was also summed up within six extracts respectively and overall component distribution was presented in Table 2. It is worth noting that the weight ratios of dinitrotoluene isomers in hexane solvent were in the following: 2,4-DNT:2,6-DNT:2,3DNT:3,4-DNT = 78.34:17.05:0.43:1.52, which were analogous to those of dinitrotoluene isomers in organic product of toluene nitration process. The result was consistent with

Fig. 5. Effect of the volume ratio of solvent/acid on the removal of organic compounds in spent acid among six stages at 323 K.

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that mentioned in previous paragraph. That is, another evidence is given on the inference that dinitrotoluene isomers in aqueous spent acid are in equilibrium with their isomers in organic product of toluene nitration process. Moreover, an identical trend of extracting priority of organic compounds in spent acid was found for both hexane and heptane, i.e. 2,3-DNT > 2,6-DNT > 3,4-DNT > 2,4-DNT > 2,4,6-TNT. This fact supports strongly our previous hypothesis of extracting priority of organic compounds contained in spent acid. 3.3. Effect of spent sulfuric acid concentration For the sake of enhancing recovery of dinitrotoluenes and trinitrotoluene from spent acid, the concentration of spent acid was adjusted by diluting manner. Effect of spent sulfuric acid concentration on the removal of organic compounds is illustrated in Fig. 6. Apparently, the concentration of organic compounds was significantly decreasing at lower concentration of sulfuric acid in comparison with those of higher ones. In other words, the extracting efficiency of lower concentration of sulfuric acid was superior to those of higher ones. It may be ascribed to the solubility of organic compounds in aqueous spent acid, which depends strongly upon the concentration of spent sulfuric acid. Namely, high concentration of spent sulfuric acid promotes solubility of organic compounds in aqueous phase, corresponding to other publications [13,17]. As far as the removed content of organic compounds is concerned, it reaches a level as high as 68% of the feedstock at first stage under the 46.9 wt.% concentration of spent sulfuric acid. One may deduce that the recovered percentage of organic compounds from spent acid would increase under lower concentration of sulfuric acid. It reveals the solvent extraction method established is promising in recovery of dinitrotoluenes and trinitrotoluene from the wastewater stream of toluene nitration process.

Fig. 7. Effect of organic compounds content of the feedstock on the removal of organic compounds in spent acid among six stages by heptane extraction under solvent/acid (1/1) at 323 K.

3.4. Effect of organic compounds content Fig. 7 demonstrates the influence of organic compounds content on the extracting behavior by heptane among six extraction stages. The residual amount of organic compounds in the case of low organic compounds content of the feedstock (1.03 wt.%) was significantly less than that of high one (1.65 wt.%). Whereas, organic compounds extracted reaches nearly 78% for the former, which is almost equivalent to that of the latter. It means the extracting tests were kinetically controlled on account of short contact time (8 min). Nonetheless, high content of organic compounds in the feedstock has been reduced to the value of about 2600 ppm in further extracting stages. Therefore, the residual content of organic compounds in spent acid depends upon the spent sulfuric acid concentration, and is independent of organic compounds contents of he feedstock.

4. Conclusion On the basis of the above discussion, it seems that dinitrotoluene isomers dissolving in aqueous spent acid are in equilibrium with their isomers in organic product of toluene nitration process. High volume ratio of solvent/acid and extracting temperature are more beneficial to recover dinitrotoluenes and trinitrotoluene. Moreover, high concentration of spent sulfuric acid enhances solubility of organic compounds in aqueous phase. That leads to lower extracting efficiency. Further, in both cases of hexane and heptane, the extracting priority of organic compounds is as follows: 2,3-DNT > 2,6DNT > 3,4-DNT > 2,4-DNT > 2,4,6-TNT.

Acknowledgement Fig. 6. Effect of the spent sulfuric acid concentration on the removal of organic compounds in spent acid among six stages by heptane extraction under solvent/acid (1/1) at 323 K.

The financial support of the fund of National Yunlin University of Science & Technology is gratefully acknowledged.

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