Unit 4 Assessment Research

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Modular Process Design:Chemical and Thermal Recycling of Acids Date: Apr. 20, 2020 From: Chemical Engineering Publisher: Access Intelligence, LLC Document Type: Article Length: 2,687 words

Full Text: In order to recycle spent acids from different applications, a design approach based on different process modules gives operators the flexibility needed to handle different contaminants

Spent or waste acids (SA) containing sulfuric or nitric acid are a byproduct of various processes involving nitration or sulfonation reactions or of processes, in which these acids are used for leaching or as a catalyst. These reactions and processes are used in the industrial production of polymers, drugs, pigments, other specialty chemicals, explosives, battery chemicals or metallurgical products. The SA from such industries is diluted by reaction water and contaminated with organic and inorganic compounds.

Stricter environmental regulations and rising raw-material prices in recent years have led to considerable cost and organizational effort for the procurement of fresh acid and the disposal of SA. An effective acid-recycling system can contribute to a more cost- efficient, independent and environmentally friendly production process. A challenge for recycling systems, however, is the wide variety of production processes and the resulting SA with partly unknown contaminants.

Chemical and thermal recycling of SA consists of various steps and the respective unit operations. The applicability of these unit operations has to be determined individually, considering factors such as volatility, solubility and reactivity of the known and unknown contaminants, the required quality and concentration of the recycled acid and the available materials of construction for the treatment of the highly corrosive acids. For an efficient process design, pre-defined process modules can be combined, comprising unit operations for specific cases. With such a modular approach, the thermal SA recycling is divided into three steps: (1) removal of contaminants; (2) separation of acid mixtures; and (3) acid concentration increase and absorption. The used modules include oxidation reactors, precipitation, filtration, distillation and rectification, as well as absorption and stripping equipment.

Composition of spent acids

Spent acids result from a variety of processes in which nitric or sulfuric acids are used as reactants, leaching solutions or as a catalyst. Depending on the production process, the composition of the SA varies considerably in terms of acid concentration and impurities. Figure 1 illustrates the classification of SA resulting from nitration or sulfonation reactions (Figure 2) and acid leaching based on the containing acid (nitric or sulfuric acid or both) and the type of contaminants (organic or inorganic). While hydrochloric acid is also used in various industrial processes, chemical or thermal recycling of spent hydrochloric acid is applied less often as compared to spent nitric and sulfuric acids and therefore will not be discussed further here.

In industrial production processes involving nitration and sulfonation reactions (for example, in the production of polymers, drugs, explosives or pigments), the SA is contaminated with organic compounds. These contaminants include at least traces of the reactants and product (for instance, nitrocellulose, nitrobenzenes, benzenesulfonic acid, glycolnitrates, picric acid, nitrotoluenes) but also byproducts. For recycling of these SA, in some cases, a complete removal of certain contaminants is required for safety reasons (such as for explosive compounds) while some contaminants can be recycled to the originating process. Depending on the acid used for the nitration, the SA is either a nitric acid or a mixture of nitric and sulfuric acid with organic impurities.

Spent acids from inorganic industries like the production of inorganic pigments or battery chemicals (for example, titanium dioxide via the sulfate route or lithium refining) or metallurgical industries (such as copper electrolysis, or demister acid from smelter sulfuric acid plants) contain inorganic impurities that could precipitate in the production process and/or affect the product quality. These inorganic impurities must be depleted or removed entirely.

The concentration levels of the acid in the SA range from diluted solutions (below 10 wt.% acid) to high concentrations with partly anhydrous acids.

Process modules

The treatment of SA applying process modules consists of unit operations for mechanical separation (filtration), thermophysical separation (evaporation, distillation/rectification) as well as chemical (oxidation) and physicochemical operations (precipitation, stripping and absorption). Together with the auxiliary machines (pumps, vacuum pumps and compressors) and static equipment (heat exchangers, evaporators, tray columns, packed-bed columns, reaction vessels) made of corrosion- and temperature-resistant materials these unit operations form process modules that can be combined to treat various SA. In general, each process module serves one of the three above-described steps in the SA recycling process as is further described below. Table 1 shows a summary of the process modules.

Step 1: Removal of contaminants. The contaminants or impurities found in the SA can be classified as volatile or non-volatile. Modules for the removal of these contaminants are shown in Figure 3. Volatile compounds can be removed from a high-boiling SA by stripping with air or steam (module A). In general, the vapor pressure of many organic compounds (for example, nitrobenzene) and some inorganic compounds (such as hydrogen fluoride or oxides of nitrogen) increases with temperature. Therefore steam stripping promotes the separation. For an optimal gas-liquid contact, stripping is performed in packed bed columns. These columns are made of either glass-/polymer-lined steel, fiber-reinforced polymer or stainless steel. By stripping these impurities from the SA, it is possible to recover them in a condensation or absorption step downstream of the stripping unit. However, in some cases, recovery is not possible due to a severe safety hazard being posed by some organic nitrates.

Non-volatile or hazardous volatile organic compounds can be destroyed by thermal oxidation (module B). The oxidation must destroy all organics non-selectively to achieve a purified acid and protect downstream equipment from explosive compounds. In order to achieve a complete oxidation, the SA must be heated up to temperatures between 120 and 200[degrees]C. For diluted acids whose boiling point is lower than the required decomposition temperature, sulfuric acid can be added to achieve an oxidation in the liquid phase, resulting in smaller equipment sizes. Nitric or mixed acids already contain nitric acid as oxidizing agent, while for SA without nitric acid, an oxidizing agent consisting, of hydrogen peroxide or nitric acid, for example, must be added. An example of a reaction of an organic impurity (benzene) with hydrogen peroxide or nitric acid is shown in Equations (1) and (2). Figure 2 depicts a sulfonation SA before and after the thermal oxidation step.

C 6 H 6 + 15H 2 O 2 a 6CO 2 + 18H 2 O

(1)

2C 6 H 6 + 30HNO 3 a

12CO 2 + 21H 2 O + 15NO + 15NO 2

(2)

Precipitation and subsequent filtration removes non-volatile inorganic compounds (module C). The precipitation / crystallization is facilitated by increasing the concentration of the inorganic impurities beyond the solubility, which depends on the temperature and acid concentration. This can be achieved by evaporation of excess water and/or cooling of a hot SA to decrease the solubility. A higher acid concentration usually also decreases the solubility of inorganic compounds and therefore has a positive influence on remaining impurities in the system.

Step 2: Separation of acid mixtures. The separation of acid mixtures is done by rectification, as shown in module D (Figure 4a). The higher boiling sulfuric acid is discharged at the column bottom and the nitric acid as distillate from the column overhead. Due to the hygroscopic nature of sulfuric acid, the water from the feed SA is discharged with the sulfuric acid whereby high concentrations of nitric acid can be achieved. The highly corrosive nature of the boiling sulfuric acid-nitric acid-mixture requires glass or glass-lined packed bed columns for the rectification.

Step 3: Acid concentration increase and absorption. An increase of the acid concentration is achieved by evaporation (Figure 4b). Sulfuric acid concentrations near the azeotropic point at approximately 98.5 wt.% are achievable by evaporation of excess water. Depending on the concentration of the feed SA and the targeted concentration of the product acid, evaporators operated under atmospheric pressure (module E) or vacuum (module F) can be used. By combining evaporators at two different pressure levels, it is possible to reuse the evaporated water by vapor compression for the heating of vacuum falling-film evaporators. In vacuum evaporators with an absolute pressure as low as 40 mbar the sulfuric acid concentrations close to the azeotropic point can be achieved at significantly lower temperatures (approximately 225[degrees]C) compared to the atmospheric boiling point (approximately 335[degrees]C).

Precursors of nitric and sulfuric acid (NOx and SO 3 ) that are formed during oxidation (as described in Step 1), rectification and evaporation can be absorbed to produce moderate or high concentrated nitric or sulfuric acid (Figure 4c). Absorption of NOx produces nitric acid with concentrations close to 70 wt.%. Absorption of NOx to produce nitric acid involves a gas-phase oxidation of nitrogen monoxide and absorption of nitrogen dioxide in water [see Equations (3) and (4)]. The gas-phase oxidation as well as the dissolution of nitrogen dioxide in aqueous solution is favored by increased gas pressure. The absorption is performed in stainless- steel tray columns.

2NO + O 2 a 2NO 2 (3)

2NO 2 + H 2 O a HNO 3 + NO (4)

Evaporation of sulfuric acid above 90 wt.% produces a SO 3 -rich gas. The absorption of this SO 3 increases the concentration of sulfuric acid. The absorption does not necessarily require an increased gas pressure and can be performed in the same pressure system as the initial evaporation step.

Design criteria

The selection of the process modules is determined by the composition of the SA and the required concentration and quality of the product acids. For the removal of contaminants, modules A to C are selected depending on the type of contaminant. To determine the adequate design parameters for these process modules, depending on the SA to be treated, laboratory tests or even pilot plant tests may be required prior to a commercial design. If a separation of a SA consisting of nitric and sulfuric acid is necessary, the process module D is used. Depending on the target concentration of the sulfuric acid product, the evaporation modules E or F are applied or combined. With the aim of achieving high energy efficiency within the system, vacuum evaporation (module F) is used when very low (<15%) concentrated acid must be recycled or high sulfuric acid concentrations shall be achieved. The absorption modules are used for internal recovery of NOx (module G) or SO 3 (module H) whenever large quantities of these gases are produced. An efficient energy recovery is achieved by using the energy provided in the previous process step (for example, thermal oxidation) in the subsequent steps (for example, evaporation). The sequential design of process steps also protects sensitive downstream equipment by initially removing contaminants.

Example: Recycling of nitration acid

As an example, a recycling process for the treatment of nitration SA is shown in the following. It combines various process modules to produce concentrated nitric acid of 98.5 wt.% and sulfuric acid of 96 wt.% from a SA. The SA originates from nitroglycerine production with the composition shown in Table 2. The process design shown in Figure 5 considers a mass flowrate of 1,000 kg/h SA. All product acids are exported with 40[degrees]C.

The illustrated exemplary process comprises the decomposition of organic contaminants by thermal oxidation and a subsequent rectification to separate the acid mixture. The nitric acid is separated as distillate and cleaned from volatile NOx by stripping to produce a clear acid. The NOx are recovered as nitric acid in a pressure absorption. The sulfuric acid from the rectification column bottom is concentrated to 96% by vacuum evaporation while emerging SO 3 is recovered as sulfuric acid.

Heating, evaporation, condensation and cooling in the thermal recycling process requires heating and cooling energy. This exemplary process requires approximately 460 kW heat and 480 kW cooling energy to produce 125 kg/h of concentrated nitric acid and 675 kg/h of sulfuric acid. Besides the positive environmental effect of recycling, the feasibility of recycling SA rather than disposing it and buying fresh nitric and sulfuric acids from the market has to be evaluated by plant operators who are considering a financial investment into a SA recycling plant. Due to the high costs for SA disposal and fresh acid procurement, recycling can significantly reduce the operating expenses (opex) for any production process where great quantities of SA arise.

In order to determine the financial viability of such a capital investment project, the plant operators will have to perform a comprehensive financial assessment study that considers the initial capital expenses (capex) and location-specific opex including energy cost, disposal cost for hazardous waste, product acid pricing.

Final remarks

In order to recycle spent acids from various applications, a flexible design approach, applying process modules, enables operators to deal with spent acids containing different contaminants. The overall process design is combining independent process modules to an overall recycling process that can recycle different SA with varying acid concentrations and containing organic or inorganic contaminants to achieve different acid concentrations and purities depending on the specific requirements. An efficient energy recovery within the process can be achieved, thus enabling significantly reduced opex as compared to the disposal of SA as a hazardous waste and buying of fresh acid from the market. With increasing prices for fresh acids and more stringent environmental regulations applying to the disposal of SA, chemical and thermal recycling of SA becomes increasingly beneficial for plant operators. n

Authors

Kevin Schnabel is process/project engineer for Plinke GmbH (Kaiser-Friedrich-Promenade 24, 61348 Bad Homburg, Germany; Phone: +49-6172-126-156; Email: schnabel@plinke.de), where he works on the basic and detailed engineering and commissioning of treatment and concentration plants for sulfuric, nitric and mixed acids. He is also responsible for Plinke's CO 2 -related R&D projects. Schnabel earned two masters degrees in environmental technology and energy systems from the University of Applied Sciences Mittelhessen (THM) and is currently also a lecturer at THM for waste treatment and CO 2 -abatement technologies.

Sebastian Bialek is head of Laboratory for Plinke GmbH (same address as above; Phone: +49-6172-126-137; Email: bialek@plinke.de). He has more than 10 years of experience in industrial and research laboratories. Initially he started his career as a laboratory technician for Evonik Stockhausen GmbH, Germany. After that he earned a bachelor and master of science degree in business chemistry from the Heinrich-Heine-University in DA1/4sseldorf, with main focus on crystallization of arene complexes of main group metals. Since then, he has also worked as a laboratory engineer for GEA Messo GmbH, Germany. He joined Plinke GmbH in 2018 in his current position.

Peter Pataky is director of Technology for Plinke GmbH (same address as above; Phone: +49-6172-126-315: Email: pataky@plinke.de). In his current position, he is responsible for R&D, process design and technical proposals. He has more than 34 years of international experience with technologies for treatment and concentration of sulfuric, nitric, hydrochloric and mixed waste acids, as well as adiabatic nitration of benzene while working for Plinke GmbH in basic and detailed engineering, project management, procurement, erection supervision and commissioning. From 2005 to 2019, he was director of Engineering and Project Execution and took over his current position in 2019. He has an engineering diploma (masters degree) in process engineering from the University of Applied Sciences, Frankfurt am Main, and is inventor/co-inventor in various international patents and patent applications.

Max Heinritz-Adrian is managing director for Plinke GmbH (same address as above; Phone: +49-6172-126-134; Email: adrian@plinke.de) and for KBR Ecoplanning Oy (Pori, Finland). He has more than 20 years of international experience in petroleum refining, petrochemical and chemical technologies. He started his professional career with Uhde GmbH, Germany (now thyssenkrupp Industrial Solutions AG), where he held various leadership positions, including head of the Process Department, head of the Gas Technology Division and Member of the Board of Directors for KEPCO-Uhde Inc., a joint venture of thyssenkrupp Industrial Solutions and Korea Electric Power Generation. Heinritz-Adrian joined KBR as director Technology, Olefins in 2016 and took over his current position in 2018. He has an engineering diploma (masters degree) in process engineering from the Technical University in Clausthal- Zellerfeld, Germany, and is inventor/co-inventor in various international patents and patent applications.

Note: For more on acid recovery, see part 2, "Acid Recovery and Recycle Technologies," on pp. 46-51, as well as the Newsfront, "Acid Recovery Becomes the Norm," Chem. Eng., October 2018, pp. 14-19).

Copyright: COPYRIGHT 2020 Access Intelligence, LLC http://www.accessintel.com/ Source Citation (MLA 8th Edition) "Modular Process Design:Chemical and Thermal Recycling of Acids." Chemical Engineering, 20 Apr. 2020. Gale Academic OneFile,

link.gale.com/apps/doc/A625737399/AONE?u=lirn50909&sid=AONE&xid=70df1bad. Accessed 6 Feb. 2021. Gale Document Number: GALE|A625737399