Control factors of hydrogenation reaction

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The hydrogenation reaction is a process in which nitration products, namely ***toluene (DNT), react with hydrogen to produce toluenediamine (MTD). The main chemical equation for this reaction is as follows: Among the reactants, 2,4-toluenediamine accounts for about 80%, while 2,6-toluenediamine accounts for about 20%. The ***toluene molecules at the 2,3 and 3,4 positions of the reactants give rise to the corresponding toluenediamines. In addition, there are many other side reaction products as well. Therefore, the reaction conditions must be strictly controlled to reduce side reaction products and achieve the highest yield. In the reaction, there are many factors that have an impact, such as the activity of the catalyst, reaction temperature and pressure, the mixing of hydrogen and the reactants, the dispersion of the feed, the feed rate, stirring conditions, and heat removal. This article mainly discusses the following aspects. 1. Catalyst feed rate and activity: In hydrogenation reactions, metal palladium is used as the catalyst while activated carbon serves as the carrier; the system requires intermittent addition of new catalyst to maintain the activity of the reaction system. The system requires that the solid content of the palladium catalyst as a reactant be maintained between 1.5% and 3.0%. Analysis shows that the low solid content and insufficient catalyst directly lead to adverse reactions, resulting in the formation of nitroamines and azo compounds. Reaction equation: 2.4‑CH3(C6H3)(NO2) + 3H2 → CH3(C6H4)NH2 + NH3; 2CH3(C6H3)(NO2)2 + 9H2 → CH3(C6H3)NH2N=NONH2(C6H3)CH3 + 7H2O. When the solid content is high, the activity increases, and the reactants in the system become pseudoplastic fluids, resulting in increased viscosity that severely affects mass and heat transfer, thereby hindering the reaction process. Volatile by-products such as p-toluidine and methyldiaminocyclohexane are formed during this reaction. Reaction equation: CH3(C6H3)(NH2)2 + H2 → CH3(C6H4)NH2 + NH3; CH3(C6H3)(NH2)2 + 3H2 → CH3(C6H9)(NH2)2. In actual production, adjustments are made based on 25#DNT levels; as a result, there is some discrepancy between the actual amount of catalyst used and what is required. In April 2003, the solid content in the 25# system was kept within the range of 2.0–2.5%, while the DNT content remained stable at below 200 ppm with no fluctuations. However, due to errors in sampling for analysis, the solid content in the 25# system fluctuated twice, once at the end of April and once at the beginning of October. This led to an increased amount of catalyst being added, which affected the system’s production capacity and product quality. In actual production, there are certain variations in the total amount of catalyst after each batch is prepared. Therefore, at the end of October, it was required that after the catalyst preparation was completed, T2105 be filled with deionized water, in order to increase and stabilize the total amount of catalyst per batch, thereby facilitating control during use. In actual production, the catalyst usage per shift is controlled within the range of 7–10% (T2016). DNT feed rate, Catalyst feed per batch (Kg), Catalyst lifetime Kg/h, M3/h: 50000, 60000, 70000, 80000, 90000, 100000, 150000, 200000; 1708, 1.3, 139, 116, 99, 87, 77, 70, 46, 35; 1971, 1.5, 161, 134, 115, 100, 89, 80, 54, 40; 2234, 1.7, 182, 152, 130, 114, 101, 91, 61, 46; 2497, 1.9, 204, 170, 145, 127, 113, 102, 68, 51; 2628, 2.0, 214, 179, 153, 134, 119, 107, 71, 54; 2891, 2.2, 236, 196, 168, 147, 131, 118, 79, 59; 3154, 2.4, 257, 214, 184, 161, 143, 129, 86, 64; 3285, 2.5, 268, 223, 191, 167, 149, 134, 89, 67; 3416, 2.6, 279, 232, 199, 174, 155, 139, 93, 70; 3548, 2.7, 289, 241, 207, 181, 161, 145, 96, 72; 3778, 2.9, 308, 257, 220, 193, 171, 154, 103, 77; 3958, 3.0, 323, 269, 231, 202, 179, 161, 108, 81. 2. Reaction rate: There are three control steps that affect the rate of hydrogenation reactions: ① The mass transfer rate of hydrogen from the gas phase to the liquid phase ； ②Hydrogenation mass transfer rate from the liquid phase to the catalyst surface ； ③The rate of chemical reactions on the catalyst surface. Due to the slow diffusion rate of hydrogen, it becomes the controlling step of the reaction; the reaction rate is zero order with respect to the concentration of nitro compounds, and it depends directly on the pressure of hydrogen and the power of stirring. Therefore, the control of the hydrogenation reaction is governed by physical diffusion rather than chemical kinetics. In actual production, the DNT in the oil phase comes into full contact with hydrogen in the gas phase in the presence of a solid catalyst, and reacts completely instantaneously upon the feed of DNT. During the reaction of DNT with hydrogen, a large amount of heat is released; the reaction temperature is maintained at 100–115°C, and an increase in temperature accelerates the reaction rate. When the temperature is below 100°C, the reaction rate decreases significantly. 3. Control of hydrogen pressure: As mentioned above, the diffusion rate of hydrogen in the liquid phase is one of the key factors determining the reaction rate; therefore, controlling the hydrogen pressure is an important requirement. An increase in pressure is equivalent to an increase in gas density, which increases the gas-liquid interfacial area and thereby enhances gas-liquid mass transfer per unit volume. The hydrogen must have sufficient pressure in order to enter the reaction system smoothly; the gauge pressure of the system is 840 kPa. The hydrogen pressure must be maintained at least at 1000 kPa, so as to ensure that hydrogen can come into contact with DNT rapidly and thus facilitate the reaction. In the hydrogenation reaction, the chemical equation is DNT + 6H2 = MTD + 4H2O. The molecular weights are 182 for DNT, 2 for H2, 122 for MTD, and 18 for H2O. With a feed rate of DNT set at full capacity of 2.8 M3/h, taking into account that the density of DNT is 1.32 and its purity is 95%, while the purity of H2 is 99.9%, the theoretical amount of H2 with a purity of 99.9% required is 2537 NM3/h. The designed feed rate for H2 with a purity of 99.9% is 3000 NM3/h. To maintain the partial pressure of hydrogen, the vent line of the hydrogenation reactor is opened based on the DNT value and the hydrogen feed rate, in order to remove impurities such as carbon monoxide, oxygen, and sulfur present in the feed gas, thus keeping the hydrogen content on a dry basis above 85%.