Shortstopping and jet mixers in preventing runaway reactions

Runaway reactions are continuing to be a major problem in the chemical industry (26% of major accidents). One of the main reasons for runaways is power failure. Runaway reactions could be inhibited in two ways: by the addition of cold diluents and by the addition of an inhibitor (chemical reaction stopper). This technology is called shortstopping. After a power failure, the process of adding an inhibiting agent and mixing it with the reactor contents becomes a major problem in the shortstopping process. Jets or impellers, driven by a small generator, however, can be used for mixing the inhibitor with the reactor contents.Dakshinamoorthy et al. [2006. CFD simulations of shortstopping runaway reactions in vessels agitated with impellers and jets. Journal of Loss Prevention in the Process Industries 19, 570-581] compared the efficiency of using jet mixers versus impeller stirred vessels in shortstopping runaway reactions. On the basis of equal power consumption, this comparative study showed that jet mixers are ineffective when used for shortstopping. One needs to identify additional factors, to effectively shortstop when using jet mixers.Due to the hazardous nature of runaway reactions, these factors cannot be determined with lab scale or pilot plant scale experiments. Recent developments with CFD make it possible to carry out virtual experiments. The computational model is solved using FLUENT. Shortstopping studies via the addition of a reaction inhibitor and cold diluent are discussed in detail. The results reported in this study identify the major and minor factors, which contribute to effective shortstopping; i.e., power requirements, locations for adding the inhibitor, the quantity of inhibitor added, rate of the inhibition, the use of cold diluent and the use of multiple nozzles. These results especially demonstrate the value of using CFD simulations in situations that are experimentally prohibitive.     

Distribution of technetium in PUREX process streams

Extraction of technetium has been carried out from the aqueous medium containing nitric acid under different experimental conditions to investigate its extraction behaviour in 30% tri-n-butyl phosphate in n-dodecane. In order to study the distribution behaviour of technetium in different streams of PUREX process, experiments were carried out under process conditions. The distribution of technetium was also studied using anion-exchange resin. Based on these results, the path of technetium in the PUREX process streams has been established which will be useful in the development of an advanced PUREX flow-sheet for containment and isolation of technetium in an environmental friendly fuel cycle.     

Titanium complexes based on aminodiol ligands for the ring opening polymerization of l- and d,l-lactide

A series of new titanium isopropoxide complexes (1-4-Ti(OⁱPr)₂ based on enantiopure (1-H₂), racemic (2-H₂), meso (3-H₂) and diastereomeric (4-H₂) aminodiol ligands have been prepared and tested as initiators for the ring opening polymerization (ROP) of l/rac-lactide in solution and in bulk conditions. All complexes were shown to have significant activity in solution at 70 °C and higher activity in bulk at 130 °C with a good control over the molar mass distribution and molecular weights. The complex derived from the racemic-aminodiol ligand gave partially heterotactic polylactide in ROP of rac-lactide and afforded atactic polylactide in the bulk, whereas all other complexes yielded atactic polylactides both in solution and in bulk. Ligand variation (chirality) in the complexes has little effect on either the activity or selectivity of the initiators. The polymerization kinetics using (1-Ti(OⁱPr)₂) as an initiator indicated a first order reaction with respect to the monomer concentration.     

Separation of palladium from high level liquid waste of PUREX origin by solvent extraction and precipitation methods using oximes

Separation of palladium from high level liquid waste (HLLW) solution originated from the reprocessing of spent fuel by PUREX process is carried out by solvent extraction and precipitation methods using oximes. In solvent extraction, palladium is extracted from the waste using 1.1×10⁻²M α-benzoin oxime in Solvesso 100 as extractant. The loaded organic phase is scrubbed with water prior to the stripping of palladium using the strippants 0.05 M thiourea in 0.1 M HNO₃ and 4 M NH₄OH separately. The recovery of the method is found to be >99.5%. The method has been used to recover Pd in sub mg quantities from actual waste solution originated from the reprocessing of spent fuel from pressurised heavy water reactor (PHWR-HLLW). Studies are also carried out for the separation of palladium using an aqueous solution of sodium salt of dimethyl glyoxime (0.1% w/v) as precipitant. Various parameters which influence the precipitation of palladium such as concentration of the precipitant, concentration of nitric acid in the feed and the behaviour of uranium, plutonium and other fission products at the concentration level encountered in the waste are studied in detail. Under the optimized precipitation conditions separation of Pd is carried out from a synthetic PHWR-HLLW and the recovery is found to be >99.9%. The palladium product obtained from both the methods show high decontamination factors with respect to other fission products and constituents of the waste. Based on these results a scheme has been proposed for the recovery of palladium from HLLW.     

Radiolytic Degradation Studies on N,N‐dihexyloctanamide (DHOA) under PUREX Process Conditions

The radiolytic stability of DHOA, a high molecular weight N,N‐dialkyl amide has been investigated to evaluate its performance under PUREX process conditions vis–a–vis TBP. Gas chromatographic studies revealed the presence of caprylic acid, dihexylamine and dihexylketone in irradiated DHOA. Batch distribution studies of Pu, U, and fission products (¹⁴⁴Ce, ^103,106Ru, and ¹³⁷Cs) were carried out using the irradiated samples of 1.1 M DHOA and TBP in n‐dodecane, which showed significant retention of Pu, U, and fission products in the irradiated TBP as compared to that of DHOA even after successive contacts with the stripping solutions. Typically at 60 M Rad dose, the Pu content for DHOA was 1.4 mg/L after three contacts with 0.5 M HNO₃, and that for TBP was ～24 mg/L. White precipitate was observed at the interface during the stripping of Pu (with 0.5 M HNO₃) from the loaded irradiated TBP phase. The DHOA system, on the other hand showed no such problem during the stripping cycle but there was an increase in the density and viscosity for the irradiated DHOA.     

Effects of graphite filler loading and heat treatment temperature on the properties of phenolic resin based carbon–carbon composites

Carbon/carbon composites (C/Cs) were prepared through polymer pyrolysis using PAN based carbon fabric (Panex^(R) 35) and resol type phenolic resin having 0, 10, 20, 30, and 40 wt% of graphite fillers. These precursor composites were heat treated at 600, 900, and 1200°C. The effects of filler loading on the precursor composites and their C/Cs were investigated through density, microstructure, and mechanical properties. Since, the precursor composites were prepared under similar processing conditions and technique, at any particular filler loading when the heat treatment temperature increases, the bulk density of the samples decreases. The filler addition accelerates the formation of the carbon basal planes in the matrix supported by X‐ray diffraction studies. The properties such as tensile strength and strain decrease continuously mainly due to change in the matrix structure and decrease of density, whereas, the interlaminar shear strength (ILSS) and interlaminar fracture toughness (ILFT) increase mainly because of improvement in the modulus of matrix. At any particular heat treatment temperature, depending on the filler content and matrix type, the density, tensile properties, ILSS, and ILFT of the samples show different trends. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Studies on the recovery of uranium from phosphoric acid medium using synergistic mixture of (2-Ethyl hexyl) Phosphonic acid,mono (2-ethyl hexyl) ester (PC88A) and Tri-n-butyl phosphate (TBP)

This paper describes the extraction of uranium from aqueous phosphoric acid medium using (2-Ethyl hexyl) Phosphonic acid, mono (2-ethyl hexyl) ester (PC88A) and tri-n-butyl phosphate (TBP) individually as well as their synergistic mixture in different diluents. The various experimental parameters are investigated to optimize optimise the suitable extraction conditions. Results indicate that a synergistic mixture of 0.90 M PC88A + 0.15 M TBP in xylene, can be used for the extraction of uranium from low phosphoric acid medium. Back extraction studies reveals that among all the common strippants used, 0.50 M solution of (NH₄)₂CO₃ was most suitable. The synergistic mixture of 0.90 M PC88A + 0.15 M TBP as extractant system and 0.5 M (NH₄)₂CO₃ as strippant is used to recover uranium from a conditioned wet process phosphoric acid and from actual radioanalytical waste generated during uranium analysis by modified Davies–Gray method. The recovery is found to be around 80% from conditioned WPA whereas better than 90% from modified Davies–Gray waste.     

Highly active novel Ni-diimine pre-catalyst containing bis-ketimine ligand for the vinyl polymerization of norbornene

Novel Nickel-diimine pre-catalyst (1NiBr₂) containing N,N′-bis(diphenylmethylene)-ethylenediamine ligand was able to quantitatively polymerize norbornene (NBE) into polynorbornene (PNBE) at 50 °C upon activation with methylaluminoxane (MAO). The monomer concentration governs the rate of reaction and the polymer characteristics. Thermal and spectral analysis revealed the polymerization pathway to occur via vinyl addition and not ring opening. The homopolymerization of 1-hexene or ethylene could not be achieved using 1NiBr₂/MAO catalyst system, but the presence of 1-hexene did not prevent the quantitative polymerization of NBE without 1-hexene incorporation. The characteristics of the polymers formed were found to be different in terms of solubility and molecular weight compared to those of the product generated by the homopolymerization of NBE in the absence of 1-hexene. Here 1-hexene acted as a chain transferring agent and control the molecular weight of PNBEs produced.     

HOTSPOT DISTRIBUTION WHILE SHORTSTOPPING RUNAWAY REACTIONS DEMONSTRATE THE NEED FOR CFD MODELS

Runaway reactions continuing to be a problem in the chemical industry. A recent study showed that 26.5% of major chemical plant accidents are due to runaways. Runaways are caused by (a) mischarges of the reactants, catalysts, or contaminants or (b) loss of temperature control. Our studies cover the concept of shortstopping the runaway reactions to prevent accident scenarios. Experiments are conducted with CFD (Fluent) models. Shortstopping runaway reactions can be carried out by (a) adding an inhibitor to neutralize the reaction and/or (b) adding a cold diluent to lower the rate of reaction. In this present work we study the characteristics of runaway reactions and inhibition techniques with a full 3-D CFD simulation to explore nonsymmetric addition points for inhibition. Our 3-D simulations are performed using the multiple reference frame method, and reactions are enabled using user defined functions in Fluent. These CFD results show the distribution of hotspots, that characterizes the shortstopping performance. They also clearly demonstrate the value of using CFD simulations in situations that are experimentally prohibitive.