Study on Hydrogen Production by Supercritical Water Gasification of Unsymmetrical Dimethylhydrazine under Multi-Parameters

Abstract

Unsymmetrical dimethylhydrazine (UDMH) is very toxic and hard to decompose in traditional ways. In this paper, the gasification of unsymmetrical dimethylhydrazine (UDMH) in supercritical water was studied in a batch reactor under different conditions. The hydrogen production process of supercritical water gasification of UDMH in metal containers is a multiphase reaction process. The effects of reaction temperature, alkaline catalysts, residence time, and oxidation on gasification were systematically studied. COD and ammonia nitrogen of the residual liquid were tested. Results showed that the maximum molar fraction and yield of hydrogen were 87.0% and 97.9 mol/kg, respectively, with KOH at 600 °C, 23 MPa. The COD removal efficiency in relation to alkaline catalysts was in the following order: NaOH > Na₂CO₃ > KOH > K₂CO₃. The highest COD removal efficiency (up to 95%) can be obtained at the temperature of 600 °C, 23 MPa with NaOH as the catalyst, and a residence time of 20 min. Ammonia nitrogen can be decreased by adding an oxidant. The COD and ammonia nitrogen of the residual liquid can meet the requirement of the Chinese emission standard of water pollution for space propellants. In addition, the organic compounds formed under different conditions were also identified.

1. Introduction

Unsymmetrical dimethylhydrazine (UDMH) combined with dinitrogen tetroxide (N₂O₄) serves as a liquid propellant that was employed for rocket fuel (CZ-2, CZ-3, and CZ-4) and missiles in China. Although it owns the advantages of high specific impulses, high thrust levels, better thrust control [1], and easy storage at reasonable temperatures and pressures, UDMH has gradually been substituted with other propellants due to its high toxicity. It has a high potential to contaminate the environment and do great harm to human beings [2,3,4]. Nowadays, there is still a large amount of UDMH wastewater that needs to be processed in an environmentally friendly way.

In our last article [5], we proposed that supercritical water gasification (SCWG) technology could be an alternative to degrade wastewater containing UDMH, with many advantages such as full degradation in a short time, no carcinogen and secondary pollution formed, etc. The properties of supercritical water are quite different from those of liquid water under ambient conditions. The number and persistence of hydrogen bonds are both diminished, and the dielectric constant is much lower, which is one of the reasons why organic wastes enjoy complete miscibility in supercritical water. On the other hand, the dissociation constant (Kw) for supercritical water is much higher than it is for ambient liquid water, which makes it an ideal solvent for organic compounds [6,7,8,9,10].

Both resource utilization and harmless treatment were realized in our last work. What is more, a reaction pathway and kinetics were also proposed based on the results obtained in the quartz reactor. However, in the process of industrialization, UDMH wastewater is processed in a metallic reactor. Hence, a study on the behavior of UDMH wastewater processed in the reactor with a metallic wall should be conducted.

As a matter of fact, research concerning the effect of the metallic wall on the chemical reaction in supercritical fluid has been studied by many researchers. In P. Christian’s group [11], they implemented an experiment on the radical dispersion polymerization of methyl methacrylate in supercritical carbon dioxide. They found that the exposed metal surface appears to terminate the growing polymer and inhibit polymer formation. This might be related to the sensitivity of the free radical polymerization of methyl methacrylate in supercritical CO₂ to the condition of the internal surface of the high-pressure autoclave. When they control the initiator concentration and deposit coating on the walls of the autoclave, the coating appears to prevent metal termination. Yu et al. [12] studied supercritical water gasification of glucose at 600 °C, 34.5 MPa, a reaction time of 30 s with different reactors (Hastelloy and Inconel reactor). They found that when the reactant concentration is 0.8 M, the gasification efficiency is 85% in the new Hastelloy reactor while 68% in the Inconel reactor, corresponding with different catalytic mechanisms of different metal reactors. Zhu et al. [13] investigated the supercritical gasification of glycerol and glucose in a quartz reactor made of SiO₂ and a tubular reactor made of Hastelloy C-276. They found that the gasification of the feedstocks could be promoted by the catalytic effects of the metallic reactor wall. All these experiments demonstrate that the metallic wall of the reactor has a certain effect on the gasification of the reactants in supercritical water.

In this paper, the gasification of UDMH in supercritical water was systematically studied in a batch reactor under different conditions. The effect of different parameters including reaction temperature, alkaline catalysts, residence time, and oxidation on gasification was discussed. We hope that our results could provide some references for industrialization.

2. Experiment

2.1. Apparatus and Methods

The simulated UDMH wastewater is a mixture of UDMH (98 wt%) and a certain amount of deionized water. The mass ratio of the UDMH to water is 1:999. The COD concentration of the simulated UDMH is 1213 mg/L in accordance with that of UDMH wastewater, which was obtained from the Sixth Research Institute of China Aerospace Science and Technology Corporation. The reagents KOH, K₂CO₃, NaOH, and Na₂CO₃ were AR grade purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

The experiment was carried out in a batch reactor which is designed for temperatures up to 750 °C and pressure up to 30 MPa. The reactor is made of Inconel 625 and the volume of the reactor chamber is 10 mL. More details about the batch reactor can be found in [14].

Before each experiment, UDMH (98 wt%) was diluted with deionized water to the desired concentration and 1.6 g of the simulated wastewater was loaded into the reactor. The reactor was placed into an electric furnace after it had been sealed. Before heating, the sealed chamber was swept out of the air with high purity argon three times and pressurized to about 5 MPa. The initial pressure could change depending on the final temperature. Then, the reactor was heated from ambient temperature to the desired temperature with a heating rate of about 70 K/min in the electric furnace [14]. When the temperature reached the desired value, the corresponding pressure was 23 MPa. After a certain reaction time, the reactor was firstly cooled down to 300 °C by the air and then by water to the ambient temperature. The gaseous product was collected with a gas bag and the gas composition was tested by the gas chromatograph. The gas yield was measured by a wet gas flow meter. Each experiment under the same condition was repeated at least three times and the average value was calculated.

2.2. Analysis Methods

The composition of the gaseous product was detected by Agilent 7890A gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) and capillary column C-2000 purchased from Lanzhou Institute of Chemical Physics in China. The carrier gas was argon (99.999%) with a flow rate of 5 mL/min. A standard gas mixture of H₂, CO, CO₂, and CH₄ was used for calibration. COD and ammonia nitrogen concentrations were quantitatively analyzed by a multi-parameter controller (Lov-ibond-ET99722). The liquid residual was extracted by dichloromethane and qualitatively detected by gas chromatography/mass spectrometry (GC/MS, Agilent 6890A-5973N MSD).

3. Results and Discussion

3.1. Effect of Temperature

The thermodynamic equilibrium model based on the minimum Gibbs free energy methodology (using the RGIBBS block) was calculated with Aspen software (all values were obtained at 450 °C, 23 MPa) to predict the yield of different gaseous products. This method also has been employed by other researchers [15,16,17]. The result calculated by the RGIBBS block of ASPEN is shown in Table 1. The state of equilibrium can only be achieved under ideal conditions. Actually, our goal is to find a way to approach the equilibrium state. A small amount of methane, carbon monoxide, and ammonia, which were at least two orders of magnitude lower than nitrogen, appeared in the calculated result and was omitted in Equation (1). The equation is as follows:

(1)${\mathrm{C}}_2{\mathrm{H}}_8{\mathrm{N}}_2+{4\mathrm{H}}_2\mathrm{O}\to 8{\mathrm{H}}_2+2{\mathrm{CO}}_2+{\mathrm{N}}_2 \Delta \mathrm{H}=172.1\mathrm{kJ}/\mathrm{mol}$

However, the gas obtained from our experiment contained not only H₂ and CO₂, but also CO and CH₄. The deviation indicated that the reaction under our conditions was far from chemical equilibrium. As a matter of fact, once the gaseous products were generated, reactions between different gases may occur [18]. Relative reactions are below:

(2)${\mathrm{CO}+\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{CO}}_2+{\mathrm{H}}_2 \Delta \mathrm{H}=-34.5\mathrm{kJ}/\mathrm{mol}$

(3)${\mathrm{CO}+3\mathrm{H}}_2\rightleftharpoons {\mathrm{CH}}_4{+\mathrm{H}}_2\mathrm{O} \Delta \mathrm{H}=-226.5\mathrm{kJ}/\mathrm{mol}$

The effect of temperature on UDMH gasification in supercritical water (23 MPa, residence time 20 min) is shown in Figure 1. As can be seen, the COD removal efficiency increased from 90.3% to 96.0% as the temperature increased from 450 °C to 600 °C. The gas yield increased from 32.2 mol/kg to 42.4 mol/kg as the temperature elevated from 450 °C to 600 °C, in which H₂ accounted for 56.5% to 65.8%. The YH₂ and the molar fraction of hydrogen increased from 19.2 mol/kg, 58.1 % to 28.6 mol/kg, 66.1%, respectively, indicating that high temperature favored the degradation of UDMH. From Equation (1), it could be seen that the reaction of UDMH with supercritical water is endothermic. Hence, increasing temperature could promote the degradation of UDMH and it is an important way to increase hydrogen production because every one-unit consumption of UDMH can obtain eight-unit hydrogen. A theory proposed by Bühler’s group [19] is that there are two competing reaction pathways in supercritical water: the preference for the free radical reaction at higher temperature and lower densities and the preference for the ionic reaction at lower temperature and higher densities. When the temperature is above the critical point, the decrease in water density causes the drop of ion product, indicating that the free-radical mechanism is preferable [20]. On the other hand, gas is formed by a free-radical reaction [21]. As a result, high temperatures can promote the degradation of UDMH and gas yield. However, from the point of view of industrialization, the energy loss will increase with temperature, but decrease with volume [22]. Therefore, catalyst deserves to be added to improve the gas yield and carbon efficiency.

3.2. Effect of Catalyst

In theory, catalysts can promote the reaction rate but cannot change the final equilibrium state [23,24]. In Equation (1), it could be seen that the main composition of the gaseous products is hydrogen. It is reported that the addition of alkali catalysts can drive the molar fraction of gaseous products closer to the equilibrium state [25] and promote hydrogen production [26]. So, the effect of KOH, K₂CO₃, NaOH, and Na₂CO₃ on the UDMH wastewater gasification was performed at 550 °C, residence a time of 20 min, 23 MPa. The mass of the catalysts added was equal to the UDMH. The results are shown in Figure 2. As can be seen, the total gas yield was increased to varying degrees. The maximal gas yield was obtained with KOH and the maximal CE with Na₂CO₃. According to Figure 2, the effect of gasification is promoted with different catalysts. With the presence of KOH, the molar fraction of H₂ and YH₂ were 87.0% and 97.9 mol/kg, increasing by 30.4% and 258.6%, respectively, and the total gas yield was 112.5 mol/kg, increasing by 174.4%. It was unpredicted that CE was enhanced with Na₂CO₃ and K₂CO₃, which seemed to be inconsistent with other literature [25,27]. They believed that alkaline catalysts had an insignificant influence on carbon gasification efficiency. This phenomenon could be explained from two aspects: firstly, the difference in waste concentration, which is at least 4 wt% in their works while 0.1 wt% in this work, may result in catalysts having a greater influence on the gaseous products. Secondly, the carbonate anion might be decomposed and be dedicated to the increase in CE. As a result, catalysts may have a greater influence on gaseous products. As we can see from Figure 2a that CE was promoted in the presence of carbonate. It is generally accepted that water–gas shift reaction can be accelerated by alkali metal [18,27,28,29], which can explain the decline in CO molar fraction and the increase in H₂ molar fraction. CH₄ molar fraction declined, indicating the reverse Equation (3) was promoted. To sum up, considering all the factors above, the alkaline catalyst can increase gas yield and has a better selectivity for hydrogen.

3.3. Effect of Residence Time

The effect of residence time was studied at 650 °C, 23 MPa. The result is shown in Figure 3. It could be seen in Figure 3a that the total gas yield is significantly enhanced with the prolonging of the reaction time. The gas yield obtained at 30 min is 2.6 times as much as that obtained at 0 min, illustrating that reaction time is an important factor affecting the gas yield. The amount of CO₂ decreases with the addition of KOH and NaOH because CO₂ is absorbed by basic catalysts [29]. The COD removal efficiency was increased by prolonging the residence time. It also should be noted that the COD removal efficiency had already been above 90% when the residence time was 0 min and it increased only by 1% as the residence time increased every 10 min. The reason is that the temperature was high enough to degrade the UDMH wastewater and it was difficult to go further. The hydrogen molar fraction increased from 61.1% to 71.1%, increasing by 16.4% and the YH₂ increased from 16.5 mol/kg to 51.0 mol/kg, increasing by 209.1% when the residence time increased from 0 min to 30 min.

Compared with the chemical equilibrium mentioned above, we concluded that the distribution of different gases was approaching the thermodynamic equilibrium with the increase in the residence time. However, the ammonia nitrogen concentration was still too high to be discharged into the environment.

3.4. Effect of Oxidant and ER

In order to decrease the concentration of ammonia nitrogen and obtain the optimum amount of oxidant, different amounts of oxidant were added. Oxidation of UDMH has been implemented by many researchers [30,31,32,33] and many types of oxidants [30,32,34] have been attempted to realize effective degradation and safe discharge.

The effect of oxidant and ER on UDMH gasification was studied at different temperatures, 23 MPa, with a residence time of 20 min. The result is shown in Figure 4.

When oxidant is added, the following reactions may occur:

(4)${2\mathrm{C}}_2{\mathrm{H}}_8{\mathrm{N}}_2+{3\mathrm{H}}_2{\mathrm{O}}_2\to 8{\mathrm{H}}_2+2{\mathrm{CO}}_2+2\mathrm{CO}+2{\mathrm{NH}}_3+{\mathrm{N}}_2$

(5)$2{\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O} \Delta \mathrm{H}=-506.3 \mathrm{kJ}/\mathrm{mol}$

(6)${\mathrm{CH}}_4+1/2{\mathrm{O}}_2\to \mathrm{CO}+2{\mathrm{H}}_2 \Delta \mathrm{H}=-23.4 \mathrm{kJ}/\mathrm{mol}$

(7)$\mathrm{CO}+1/2{\mathrm{O}}_2\to {\mathrm{CO}}_2 \Delta \mathrm{H}=-284.6 \mathrm{kJ}/\mathrm{mol}$

(8)$2{\mathrm{H}}_2{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2 \Delta \mathrm{H}=-168.9 \mathrm{kJ}/\mathrm{mol}$

(9)$4{\mathrm{NH}}_3+3{\mathrm{O}}_2\to 6{\mathrm{H}}_2\mathrm{O}+2{\mathrm{N}}_2 \Delta \mathrm{H}=-1284.1 \mathrm{kJ}/\mathrm{mol}$

In this paper, hydrogen peroxide was used as the oxidant. As can be seen, CE in the presence of oxidant is higher. The yield and molar fraction of H₂ and CH₄ tends to decrease when ER increases from 0.2 to 0.4. This result is predictable because H₂ and CH₄ yields at the chemical equilibrium state decrease with the increasing amounts of oxygen addition [35]. On the other hand, most of the nitrogen appears as N₂ in the reaction products [6]. It could be assumed that the declined hydrogen yield resulted from the oxidation of reduced hydrogen atoms. Meanwhile, more carbon dioxide could be produced by the oxidation of carbon monoxide and other intermediate products containing carbon. It should be noted that the ammonia nitrogen of the residual liquid was decreased in the presence of the oxidant. When ER is 0.4, ammonia nitrogen of the residual liquid can meet the requirement of the Chinese emission standard of water pollution for space propellant (GB14374-93), which is 25 mg/L.

3.5. Reaction Mechanism

To have a deeper understanding of the degradation mechanism of UDMH processed in supercritical water in the batch reactor, GC/MS analysis was implemented. Considering that the residual liquid with low organic concentration may not be detectable comprehensively, UDMH solution with a concentration of 50 wt% was loaded into the reactor. Relative percentage content at a reaction time of 1 min and a certain temperature was used to investigate the major organic intermediate products. Through the trend of the relative percentage content at different temperatures (400 °C, 450 °C, and 500 °C), we could deduce the reaction mechanism and determine which products are hard to be degraded. Table 2 shows the identified organic compounds along with their detected information. Figure 5 gives the photo comparison of residual liquid extracted by dichloromethane at different temperatures.

In general, the color becomes lighter as the temperature increases, indicating that the residual liquid contains fewer organic compounds. Trimethylamine (TMA) accounts for a major part of all three temperatures and has a downward trend with the increase in temperature, illustrating that high temperature can promote the degradation of TMA. N-containing heterocyclic compounds have an increasing proportion and seem hard to degrade even at high temperatures. These compounds including pyridine, indole, and carbazole are essentially unreactive under supercritical water conditions [36]. The addition of formic acid or sodium formate results in the degradation of the aromatic rings [6]. The reason why the result is different from our previous work may result from the difference in the concentration of reactants. From the micro perspective, high reactant concentration leads to a high collision probability of UDMH molecules, thus the probability of forming a heterocyclic molecule is higher.

4. Conclusions

Supercritical water gasification technology used for processing UDMH wastewater proved to be a harmless, resourceful utilization technology. In this paper, gasification of unsymmetrical dimethylhydrazine in supercritical water was studied in the range of 450 °C to 650 °C, 23 MPa for the first time. The main conclusions could be drawn as:

• (1). Gas efficiency can be promoted with the addition of an alkaline catalyst and KOH owned the highest gas efficiency. The maximum molar fraction and yield of hydrogen reached 87.0% and 97.9 mol/kg, respectively, with KOH at 550 °C, 23 MPa, and a residence time of 20 min.

• (2). The COD removal efficiency reached 95% at 600 °C, 23 MPa, no catalyst added, and a residence time of 30 min. When ER was 0.4, the ammonia nitrogen concentration of the residual liquid can meet the requirement of the Chinese emission standard of water pollution for space propellant, which is 25 mg/L.

• (3). When the concentration of UDMH solution is up to 50 wt%, the major degradation product under low temperature is trimethylamine (TMA) whose relative amount has a reduction tendency as the temperature increases.

Author Contributions

Conceptualization, L.Y. and J.C.; methodology, Z.L. (Zhigang Liu); validation, H.C. and D.L.; formal analysis, Z.L. (Zheng Liu); investigation, L.Y.; writing—original draft preparation, L.Y.; writing—review and editing, B.C.; supervision, B.C.; project administration, L.Y.; funding acquisition, L.Y. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Nomenclature

CE	the mass of carbon in gaseous product/the mass of carbon in UDMH ×100, %
YH2	the molar number of produced hydrogen/the mass of UDMH, mol/kg
Gas fraction	the molar number of a certain gas product/the summation of the molar number of all the gaseous product ×100, %
ER	the mass of oxidation added/the mass of oxidant for complete oxidation of UDMH by stoichiometry calculation

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Effect of temperature on gasification characteristic: (a) gas yield and CE (b) gas fraction (c) COD removal efficiency and ammonia nitrogen (23 MPa, residence time 20 min).

Figure 2. Effect of catalyst on gasification characteristic: (a) gas yield and CE (b) gas fraction (c) COD removal efficiency and ammonia nitrogen (550 °C, 23 MPa, residence time 20 min).

Figure 3. Effect of residence time on gasification characteristic: (a) gas yield and CE (b) gas fraction (c) COD removal efficiency and ammonia nitrogen (650 °C, 23 MPa).

Figure 4. Effect of oxidant and ER on gasification characteristic: (a) gas yield and CE (b) gas fraction (c) COD removal efficiency and ammonia nitrogen (23 MPa, residence time 20 min).

Figure 5. Residual liquid of 400 °C, 450 °C, and 500 °C (from left to right) extracted by dichloromethane.

Table 1

Product distribution, calculated by RGIBBS block of ASPEN.

Component	Feed	Product
UDMH	0.00029997 1	0
H2O	0.9997	0.9967211
H2	0	0.00237643
CO	0	2.98 × 10−7
CH4	0	1.59 × 10−6
CO2	0	0.000596985
C2H4	0	3.77 × 10−18
C2H6	0	6.62 × 10−15
N2	0	0.000295299
NH3	0	8.27 × 10−6
N2O	0	1.22 × 10−26
NO2	0	1.08 × 10−34

¹ The unit of the value in this table is mole fraction.

Table 2

Identities and relative amounts of compounds in residual liquid.

RT (min)	Name	Chemical Formula	Structural Formula	400 °C	450 °C	500 °C
				Area %
1.430	Trimethylamine	C3H9N	[Image omitted. Please see PDF.]	35.184	16.396	17.477
1.527	Ethanamine, N,N-dimethyl-	C4H11N	[Image omitted. Please see PDF.]	4.411	1.133	2.504
2.168	Aziridine, 1,2,3-trimethyl-, trans-	C5H11N	[Image omitted. Please see PDF.]	2.753	-	-
5.465	Propanenitrile	C3H5N	[Image omitted. Please see PDF.]	1.016	1.784	4.049
5.642	Propanal, 2-methyl-, dimethylhydrazone	C6H14N2	[Image omitted. Please see PDF.]	1.098	8.128	1.327
7.071	Butanenitrile, 2-methyl-	C5H9N	[Image omitted. Please see PDF.]	0.764	0.826	0.466
8.963	1H-Pyrrole, 1-methyl-	C5H7N	[Image omitted. Please see PDF.]	2.118	1.060	-
10.272	Pyridine	C5H5N	[Image omitted. Please see PDF.]	1.348	1.579	0.542
11.626	1H-Pyrrole, 2,5-dimethyl-	C6H9N	[Image omitted. Please see PDF.]	2.186	1.754	2.262
13.124	1H-Pyrazole, 1-methyl-	C4H6N2	[Image omitted. Please see PDF.]	8.778	8.242	5.871
14.627	Pyridine,3-methyl-	C6H7N	[Image omitted. Please see PDF.]	8.659	11.191	14.148
17.130	Pyrazole, 1,4-dimethyl-	C5H8N2	[Image omitted. Please see PDF.]	12.790	10.653	5.218
19.931	Pyridine, 3,5-dimethyl-	C7H9N	[Image omitted. Please see PDF.]	8.983	13.135	13.581
33.956	1H-Pyrrole, 2,3,4,5-tetramethyl-	C8H13N	[Image omitted. Please see PDF.]	9.913	24.117	32.555

“-”: not detect.

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