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Nitration and Sulfuric Recovery: Effective Reaction Control and Efficient Acid Recycling for Economic Production

itration of aromatic organics involves the handling of corrosive is one of the key processes of as well as potentially hazardous, toxic modern . It and organics. Downstream of N produces various precursors of the unit a product washing and chemicals and products (i.e. , separation is required (see Figure 1). nitrotoluene or chloronitrobenzene). The 79 The design and operation of nitration nitration is performed with a mixture of plants requires a balancing of various sulfuric and nitric acid which is brought design aspects. Those aspects can be into contact with the organic compounds expressed by a tension triangle (Figure by intensive mixing. Therefore, the process

Fig 1: Overview of the nitration process with acid recovery and product purification. www.jasubhaimedia.com December 2020 Chemical Engineering World FEATURES

2) where safety, CAPEX and OPEX are nitration acid). The is used located on opposite sides. The main as dehydration agent and promotes the driver for the investment cost of nitration formation of reactive nitronium (Eq. 1). plants is the requirement to use The nitration as addition of the nitronium resistant materials like glass-lined steel, to the occurs fluoropolymer-lining or for in the aqueous acid phase. Thus, the the major part of the nitration process. reaction rate is determined by the mass Instrumentation required for a safe and transfer rate between the organic and reliable operation further increases the the acid phase (Figure 3). An increase of CAPEX. An efficient recovery of acids the surface area and therefore dispersion (sulfuric & nitric acid) and an efficient of the organic phase in the acid phase is reaction control to achieve high product required for an efficient reaction. This can yield allows to optimize OPEX. By applying be achieved by intensified mixing. The advanced processes with optimized overall nitration reaction (Eq. 2 as example (tubular) reactors as well as optimized for benzene) is exothermal, consumes downstream processing of the product nitric acid and dilutes the sulfuric acid with mixture all design aspects can be the produced . 80 considered. Eq. 1 � � � � � � � � 𝐻𝐻 𝑆𝑆𝑆𝑆 + 𝐻𝐻𝐻𝐻𝐻𝐻 → 𝐻𝐻𝐻𝐻𝐻𝐻 + 𝐻𝐻 𝑂𝑂 + 𝑁𝑁𝑁𝑁 Eq. 2 �����

𝐶𝐶�𝐻𝐻� + 𝐻𝐻𝐻𝐻𝐻𝐻� �⎯⎯� 𝐶𝐶�𝐻𝐻�𝑁𝑁𝑁𝑁� + 𝐻𝐻�𝑂𝑂 + ℎ𝑒𝑒𝑒𝑒𝑒𝑒 Nitration reactions can be realized in isothermal and adiabatic reactors. Isothermal reactors provide the opportunity to effectively control the reactor conditions in terms of temperature, acid concentrations and stirring speed. However, relatively large heat exchange areas are required for the nitration reactors to maintain the isothermal conditions. Fig 2: Tension triangle for the plant design and Furthermore, the recovery of the sulfuric operation acid requires a subsequent heating and Nitration Reaction evaporation of the produced mixture. Adiabatic reactors, as developed and Nitration describes the addition of one successfully commercially applied by KBR, or more nitration groups to an aromatic allow for an efficient use of the reaction compound involving a reactive mixture heat released by the nitration (Eq. 2). of sulfuric and nitric acid (so called

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With this type of systems, heat is utilized to determine the necessary operation to flash the excess water and recover the conditions of the tubular reactor (acid sulfuric acid under vacuum conditions. concentrations, temperature, flow rates). Furthermore, adiabatic tubular reactors Such studies can be performed in small provide the possibility to efficiently tubular reactors as done for benzene, disperse the organic phase by applying chlorobenzene and toluene [2]. shear forces on the organic droplets within Although the control of adiabatic reactors the acid phase without use of stirrers. is more challenging, they integrate all An improved dispersion reduces the design aspects by combining an efficient formation of byproducts and increases the use of the reaction heat (reduced OPEX), yield. For that reason, tubular reactors are the integration of two plant units in one commonly used for the adiabatic nitration (reduced CAPEX) and reliable transfer of of benzene [1]. the reaction heat (increased safety). By A further transfer of that reactor concept achieving a high yield and a complete to other aromatic compounds is possible conversion of the nitric acid, the costs for and still ongoing. In many cases product purification and acid recovery are intensive screening studies are required reduced. 81

Acid Recovery

Traditionally the sulfuric acid recovery is an addon to the isothermal nitration (e.g. in the case of mono- and dinitrotoluene production). For those plants several waste streams consisting of spent sulfuric acid and washing liquid (yellow water) must Fig 3: Scheme of the nitration reaction as sequence of mass transfer between immiscible phases and the reaction (left side). The mass transfer be treated which is promoted with the application of tubular reactors which increase the contain residual nitric dispersion of the organic phase in the acid phase (right side) acid, sulfuric acid www.jasubhaimedia.com December 2020 Chemical Engineering World FEATURES

and organics. An efficient separation in the organic products, is discharged to the sulfuric acid recovery is required to glass-lined evaporators operating under return valuable compounds like acids and vacuum conditions to flash the water organics to the nitration process. Various and recover the sulfuric acid. To achieve processes involving stream stripping higher sulfuric acid concentrations two (denitration) and acid concentration are or more evaporation steps equipped with available to recover the acids and organics tantalum heat exchangers can be used. from those waste streams. The concentration of the sulfuric acid is adjusted by a combination of temperature For adiabatic nitration plants the sulfuric and vacuum. acid recovery is directly integrated into the process and the same building Although nitration processes target a (see Figure 4). These systems allow complete conversion of the nitric acid an efficient use of the reaction heat to and a high yield, some evaporate water in the acid recovery are formed in all nitration reactions. step. Sulfuric acid, that is separated from Effluent gases can be treated by catalytic reduction or alkaline washing to remove 82 nitrogen oxides. However, such off-gas treatment techniques destroy a valuable source of nitric acid. Pressure absorption is an established solution to recover nitric acid from nitration off-gases. Similar to nitric acid production, nitrogen monoxide is oxidized and absorbed into water. For nitration plants the use of liquid ring compressors allows an efficient absorption within the compressor and the absorption column equipped with cooled trays.

Fig 4: 3D model of a typical adiabatic benzene nitration (70’000 tons per year) with integrated sulfuric acid recovery. Equipment is indicated in pink and process pipes in orange.

Fig 4: 3D model of a typical adiabatic benzene nitration (70’000 tons per year) with integrated Product Purification sulfuric acid recovery. Equipment is indicated in pink and process pipes in orange. The product separated from the sulfuric

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acid after the nitration reactor contains and operation require detailed knowledge residual sulfuric acid, nitric acid, nitrogen of corrosion resistant materials, advanced oxides and other byproducts like reaction technology and the integration nitrophenols in case of benzene nitration. of the various processing steps. Adiabatic With optimized reaction conditions, nitration systems, as offered by KBR, the formation of byproducts can be provide the most efficient design in terms reduced but not eliminated. Therefore, of optimized CAPEX, OPEX and plant acidic substances as well as unwanted safety.  byproducts must be removed before a further use of the product. Additionally, References: many nitration processes operate with 1. Duehr, J. (2013). Nitration Technology for Aromatics an excess of the educts, which must be As Described in the Patent Literature. , separated. Process Design, and Safety for the Nitration Industry. ACS Symposium Series, 71-82. The purification of the crude nitration 2. Henke, L., & Winterbauer, H. (2005). A Modular Micro Reactor for Mixed Acid Nitration. Chemical product is a sequence of two or more Engineering & Technology, 28(7), 749–752. steps like washing to remove acids 3. H. Winterbauer, (2004). New Process to Minimize and phenolic byproducts, rectification Effluent Water in the Production of DNT. or stripping to remove the unreacted Engineering in Life Sciences 4(1), 97-100. 83 organics, and the separation of the product’s isomers. The washing process includes various steps of acidic, alkaline and neutral washing [3]. Removing the acid compounds, transfer the weakly acidic phenols into the aqueous phase Authors and lowering the temperature, allows Kevin Schnabel a change of the construction material. Process Engineer, Plinke GmbH, as material of construction KBR Technology, Germany instead of corrosion resistant glass- or fluoropolymer-lined steel reduces the Joachim Hetzel investment costs for the purification step, Director Engineering, Plinke GmbH, significantly. Accordingly, the various KBR Technology, Germany washing steps allow for a purification in a rectification or stripping column without Bhavesh Shah corrosion problems. Director Business Development, Plinke GmbH, KBR Technology, Germany Conclusion Max Heinritz-Adrian Nitration plants combine various Managing Director, Plinke GmbH, KBR Technology Germany challenging process steps. Their design www.jasubhaimedia.com December 2020