Process Design and Operational Controls To Safeguard Strong Nitric

Chapter 13

Downloaded by NORTH CAROLINA STATE UNIV on May 8, 2015 | http://pubs.acs.org Publication Date (Web): November 22, 2013 | doi: 10.1021/bk-2013-1155.ch013

Process Design and Operational Controls To Safeguard Strong Nitric Acid Recovery Systems Thomas L. Guggenheim,* Roy R. Odle, and John Pace SABIC Innovative Plastics, 1 Lexan Lane, Mt. Vernon, Indiana 47620 *E-mail: [email protected]

A nitric acid concentrator-sulfuric acid concentrator (NAC/SAC) is a manufacturing process that produces strong nitric acid, strong nitric being defined as >98 weight percent (wt%). Operating procedures, compositional control, and design of the system are all necessary to ensure a safe process. Typically, 80 to 90 wt% sulfuric acid and 65 to 80 wt% nitric acid are fed to a packed column to provide condensed strong nitric acid overhead and 60 to 70 wt% sulfuric acid as a bottoms stream. The mass balance of constituents across the column during normal, startup, and shutdown conditions must be controlled to allow for the proper operation of the system. This paper will describe the operational and design changes made to a NAC/SAC system after an event in which the strong nitric acid condenser failed. The authors conclude that the condenser failure, which occurred after unusual operating conditions, was most likely caused by rapid gas formation, resulting from the fast decomposition of a thermally unstable condensed phase that had collected in low spots in the NAC overheads system, which led to over-pressurization of the condenser.

Introduction The Innovative Plastics business unit of SABIC operates a nitration facility in Mt. Vernon, Indiana. The facility nitrates an N-alkylphthalimide (1) derivative in 99 wt% nitric acid to afford primarily an isomeric mixture of nitro-N-alkylphthalimides (2), used in the manufacture of an engineering thermoplastic (equation 1) (1). A by-product of the nitration reaction is © 2013 American Chemical Society In Chemistry, Process Design, and Safety for the Nitration Industry; Guggenheim, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


tetranitromethane (3, abbreviated as TNM, bpt 126 °C). The majority of the strong nitric acid (>97 wt% HNO3) is removed from the product by evaporation under slightly reduced pressure, and condensed.

The recovered nitric acid contains the majority of the TNM produced in the process. The recovered strong acid is combined with weak nitric acid process streams and clean commercial grade 69 wt% nitric acid to afford ~75 wt% nitric acid (2). This combined weak nitric acid, stored in the ‘weak nitric acid tank’, must be concentrated back to >99 wt% HNO3 for the efficient nitration of the N-alkylphthalimide. Production of 99 wt% nitric acid is accomplished by continuously feeding the ~75 wt% HNO3 from the weak nitric acid tank to a packed column (called a nitric acid concentrator, NAC) along with ~85 wt% sulfuric acid (balance being water) under slight vacuum. Strong nitric acid (99 wt% HNO3) is taken overhead of the column and condensed in a water cooled condenser, while weak sulfuric acid (~65 wt%) is taken off the bottom of the NAC. The exact concentrations of the NAC feed can vary. The sulfuric acid can be 80 to 90 wt%, and the nitric acid can be 65 to 80 wt%. Approximate concentrations of acid streams in wt% are used throughout this paper, as the exact strengths are not critical aspects of the discussion and conclusions. While shutting down the nitration facility, the overheads condenser on the nitric acid concentrator over-pressurized and failed. Fortunately, there were no injuries resulting from the failure. The subject of this paper explains (i) the investigation of the of the condenser failure, (ii) how the most likely cause of the failure was determined, and (iii) the redesign of the condenser.

Brief Description of the NAC Equipment and Operation A simplified process flow diagram of the NAC is shown in Figure 1 (3). It is a glass column packed with structured glass packing. The ~85 wt% H2SO4 and ~70 wt% HNO3 are continuously fed to the NAC. The sulfuric acid’s strong affinity for water effectively ‘breaks’ the maximum boiling azeotrope of HNO3/water (~68 wt% nitric acid), allowing for the distillation of >99 wt% nitric acid overhead of the NAC and the collection of weak sulfuric acid (~65 wt%) off the bottom of the 186 In Chemistry, Process Design, and Safety for the Nitration Industry; Guggenheim, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


NAC. A steam-heated reboiler provides heat to the NAC, and the 99 wt% HNO3 is taken overhead at about 660 mm pressure (b.pt. 80 to 81 °C). The 99 wt% nitric acid is condensed in a horizontal condenser, and collected in a storage tank. The two other significant species present in the overheads product are TNM (~0.5 wt%) and dissolved oxides of nitrogen (NOx, 3-5 wt%). Some of the condensed 99 wt% HNO3 is returned to the top of the column, through a flow control (FC) valve, which controls the composition of the NAC overheads product and the overall production efficiency of the NAC. The material exiting the bottom of the NAC is primarily composed of ~65% sulfuric acid containing some oxides of nitrogen, nitric acid, and nitrosyl sulfuric acid. This bottom product of the NAC enters a glass denitrification tower (DEN), where it is contacted with live steam. The steam hydrolyzes the nitrosyl sulfuric acid to nitrous acid (HNO2), H2SO4, and NOx, some of which is oxidized to nitric acid by the air present in the process. The nitric acid vapor, along with the oxides of nitrogen present in the DEN, is ‘chased’ by air that is also fed to the DEN back into the body of the NAC, and eventually overhead of the NAC. Finally, the temperature of the vapor exiting the horizontal condenser to the vent system is controlled by the flow rate of cooling water (CWS) through the U-tube bundle inside the condenser. The ~65 wt% sulfuric acid from the bottom of the DEN is concentrated back to 80 to 90 wt% sulfuric acid by simple evaporation of water. The ~85 wt% sulfuric acid is eventually recirculated back to the NAC.

Figure 1. Simplified Schematic of the NAC. 187 In Chemistry, Process Design, and Safety for the Nitration Industry; Guggenheim, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


A simplified schematic of the NAC overhead condenser is shown in Figure 2. The overhead vapor line from the top of the NAC enters the top of the condenser. The condenser is constructed with an internal U-tube bundle which is supported by a plurality of evenly spaced Teflon® baffles. The metallurgy of construction of the condenser was compatible with the chemical compositions that were potentially present in the system.

Figure 2. Simplified Schematic of the NAC Condenser. (see color insert) One half of each Teflon® support is cut out to afford an open space. The open spaces are alternated to the other side for each successive support. This construction forces the vapor off the NAC to contact the tubes evenly throughout the length of the condenser, increasing the heat transfer efficiency. The bottom of each support is trimmed to allow the drainage of the condensate off the bottom of the condenser.

Failure of the NAC Condenser and Damage Assessment During a shutdown of the nitration facility, the NAC was running in normal mode. The 99 wt% nitric acid and N-alkylphthalimide that feeds to the nitrators had been ceased and the nitric acid was being recovered from the nitrated imide product and stored in the weak nitric acid tank. For brevity, only the salient events leading up to the condenser failure are detailed below. At 9:45 am the nitrators were empty and the NAC was being fed ~70 wt% HNO3 (from the weak nitric acid tank) and ~80 wt% H2SO4. 188 In Chemistry, Process Design, and Safety for the Nitration Industry; Guggenheim, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


At 11:09 am, the nitric acid feed to the NAC was intentionally shut off as was the steam to the NAC reboiler; the ~80 wt% sulfuric acid feed to the NAC and the live steam to the DEN were continued. At 11:10 am, the flow of condensate off of the NAC condenser to the nitric acid storage tank stopped, but the flow from the condenser to the top of the NAC continued. The NAC was said to be “on total reflux”. From 11:10 am to 5:41 pm, the ~80 wt% sulfuric acid feed to the NAC and the live steam to the DEN were continued. This condition produced ~65 wt% sulfuric acid as the bottoms product of the DEN. The ~65 wt% sulfuric acid was concentrated back to ~80 wt% sulfuric acid in the sulfuric acid concentrator. The ~80 wt% sulfuric acid was then returned to the NAC. The NAC was on total reflux during this entire timeframe. The temperature at the top of the NAC at 11:00 am was ~80 °C, 92 °C at 1200 pm, and ~114 °C at 1:00 pm. The top of the NAC remained at ~114 °C up to the time of the event. At 5:42 pm, the sulfuric acid feed to the NAC tripped off (ceased) due to a high pressure interlock at the top of the column. Some steam flow was automatically reestablished to the NAC reboiler. The live steam feed to the DEN had tripped off. The temperature of the material in the reboiler was 128 °C at 5:42 pm. The reflux return flow indication to the top of the column became erratic, most likely indicating no flow. Finally, the temperature at the top of the NAC was 114 °C, and the vapor off of the condenser was 52 °C. At 6:02 pm the NAC overheads condenser exploded. At that time, the temperature in the NAC reboiler had climbed to 153 °C because the steam to the reboiler had been automatically reestablished at 5:42 pm, and the top of the NAC was 114 °C. No personnel were in close proximity to the equipment damaged, and there were no injuries. The body of the condenser fragmented and the condenser end plates were found intact 10 feet from their original location. The condenser tubes were crushed and fragmented. The tubes near the tube sheet were fragmented. The tube sheet is the plate where the ends of the tubes are secured (rolled to prevent leakage of coolant into the process), and this plate is bolted to the CWS (cooling water supply) and CWR (cooling water return) side of the condenser (see Figure 2). A one foot diameter hole was found in the concrete floor directly below the tube sheet of the condenser. The top of the NAC was destroyed and the DEN was cracked. The NAC reflux return line was completely fragmented but the valve bodies in the line were found intact. Failure mode experts from a consulting firm (4) found that the shrapnel from the metals of construction failed in a ductile manner indicating a rapid deflagration or low order detonation, as opposed to a brittle failure that would be more consistent with a high-order detonation. The survey of the damage suggested that the event took place in the condenser and most probably in the reflux return line as well. The consulting firm determined that the extent of the damage resulted from a low-order detonation which was the equivalent of 2 to 4 pounds of trinitrotoluene (TNT) (5). The consulting firm also estimated that the condenser could withstand a detonation equivalent of

Process Design and Operational Controls To Safeguard Strong Nitric

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