【3D Process Flow】Summary Post--The Core of TDI Process: Two-Stage Nitration of Toluene to Produce DNT

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【3D Process Flow】Summary Post – Useful for learning about the process flow: https://bbs.hcbbs.com/thread-5711616-1-1.html ---------------------------------------------------------- The nitration reaction is the key step in converting toluene into DNT. Moreover, it determines the ratio of the key isomers (2,4-TDI and 2,6-TDI) in the final TDI product, directly affecting the load on the downstream hydrogenation process, the lifespan of the catalysts, as well as the properties of the final polyurethane products. Paragraph 1: Mononitration – precise control of isomers is the key task: Toluene + mixed acid → Mononitrotoluene (MNT). The core equipment is the R-101 Toluene Mononitration Reactor, which features an external jacket and efficient stirring. Toluene reacts with mixed acid here; isomer ratio control (80/20 & 65/35 Isomer Ratio Control): This is the core of the R-101 design. Here, it is not only necessary to control the reaction yield; it is also essential to precisely adjust the ratio of the mixed acids, as well as the reaction time and temperature, in order to keep the ratio of 2-nitrotoluene to 4-nitrotoluene in the resulting MNT strictly at the desired level (such as 80/20 or 65/35). This ratio directly determines the specifications of the final TDI product. Reaction conditions: T: 30-50°C, P: atmospheric pressure. This is a typical exothermic reaction at moderate intensity, and external jacket cooling along with stirring are essential to keep the reaction under control and minimize by-products. The mixture after the reaction is fed into the S-101 Mononitration Phase Separator for static settling and layer separation. The organic phase (MNT) is sent to the V-101 MNT storage tank to prepare for the second reaction stage ； The aqueous phase at the bottom (spent acid) is sent to the V-102 spent acid collection tank. https://mmbiz.qpic.cn/mmbiz_png/5N1wxpcBmPdn9yvHTpV52N1Jv1IjQEaAH0AbhrlsuHI4p2qr38RNPFLaboFib1XbhQibQho7pLVlnhjFt8h0aUnBpTgbk2KexFchEEEkuAwc4/640?wx_fmt=png#imgIndex=1Second paragraph: Dinitration – the key task involving high temperature and efficiency: MNT + additional nitric acid + mixed acid—to produce dinitrotoluene (DNT). After single-nitration is completed, the organic phase moves on to the green-colored second stage of dinitration. The core equipment is the R-102 Dinitration Reactor. Unlike R-101, R-102 is a large CSTR with efficient stirring, and an external heat exchanger is installed to enhance heat transfer. MNT, supplemental nitric acid, and mixed acid undergo further reactions here. Reaction efficiency and conditions: Unlike mononitration, introducing further nitro groups into the benzene ring is a more difficult process (nitro groups are deactivating groups). Therefore, the operating temperature of R-102 is higher (T: 60-80°C). To maintain reaction efficiency at higher temperatures and remove a large amount of reaction heat, the design of the external heat exchanger must be highly optimized, and temperature control must be extremely precise. Advantages of the two-stage design: This two-stage design eliminates the need for extremely strong nitrating agents and harsh conditions in one of the reaction stages, effectively reducing the formation of high-boiling by-products and yielding a purer DNT precursor. The reaction mixture is separated using an S-102 Dinitration Phase Separator. The crude DNT (Crude DNT, dark red stream) at the top is sent to the refining section ； The aqueous waste acid is also collected in V-102. Third paragraph: DNT refinement – the goal is to achieve exceptional purity. The main task is to remove acids and impurities from crude DNT in order to obtain “Purified DNT”. Crude DNT coming out of S-102 cannot be sent directly to the hydrogenation unit, as it contains large amounts of residual nitric acid, sulfuric acid, and acidic by-products. These substances not only deactivate the expensive hydrogenation catalysts but are also highly corrosive. In the purple DNT purification area, crude DNT undergoes multiple stages of treatment: T-101 Water Wash Tower: Process water is used to remove the majority of the acids present in crude DNT. T-102 Alkali Wash Tower: A critical step. An alkali is added to neutralize the residual trace acids and convert them into water-soluble salts, with a focus on removing acidic by-products such as phenols. T-103 Final Water Wash Tower: Uses fresh process water for the final washing to ensure that all residual salts and alkalis are completely removed. D-101 DNT Dryer: Finally, and most importantly, this is the last step. Drying is carried out under high temperature and vacuum to ensure that the moisture content of the DNT sent downstream is at an extremely low level. Final product: Purified DNT destined for the hydrogenation unit. The refining section generates acidic wastewater (from T-101) and alkaline wastewater (from T-102 and T-103), both of which are sent to the wastewater treatment unit. Paragraph 4: The core task of waste acid treatment and concentration (WAC): converting ~70% dilute sulfuric acid into ~93-98% concentrated sulfuric acid, thereby achieving acid recycling. Finally, there is the WAC area, which represents a major advantage of the TDI process from both environmental and economic perspectives. In the nitration reaction, at least 1 mole of dilute sulfuric acid (~70%) is produced for every 1 mole of DNT generated. WAC-101 Multi-stage Waste Acid Concentration Unit: The core equipment consists of a three-stage series-connected vacuum evaporation system (T-201, T-202, T-203), with each evaporator equipped with an external heat exchanger. High-temperature vacuum evaporation: The evaporator must operate under conditions of P: Vacuum, T: High Temp. Through multi-stage vacuum pressure reduction, thermal energy is utilized to the fullest extent, and water is evaporated at high temperatures. The thermometers and vacuum level indicators in the infographic clearly highlight this point. Rapid increase in acid concentration: Dilute H2SO4 (~70%) collected from V-102 enters the system, and ultimately Concentrated H2SO4 (~93-98%) is produced via the concentrated sulfuric acid storage tank (V-103). The concentrated sulfuric acid is reused in the mixed acid systems of the first and second stages, thereby achieving a green cycle for the entire life cycle of sulfuric acid. It also treats the acidic and alkaline wastewater generated.