Chemical Engineering - Industrial Organic Chemistry

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Oral questions 1)Ethylene oxide : processes, catalysis, mechanisms of reaction, reactor, process plant. General thermodynamics and kinetics questions about random processes. Reazione bilanciata con valore del deltaH, effetti della temperatura sulla selettività, ragionamento sulla struttura del catalizzatore e sulle limitazioni diffusive (perché alfa allumina e perché uso uno strato sottile), schema dell'impianto, temperature e pressioni usate. Ethylene oxide is a very reactive gas , due to its strong ly tensionated ring . Its main uses are to produce Et -glycol and Amines or polymers. In the past, it was produced via HidroClorination of Etilene : this is a 2 step process which involves first the reaction of etilene and gaseus cloro (in water solution) to give OH -Et-Cl and wasted HCl, and then the saponification reaction with Ca(OH)2 to give the desired product, water and the wasted salt CaCl2. This reaction was very exothermic, corrosive and had many sideproducts in stechiometric am ounts. For this reasons, nowadays, it is produced by selective catalytic oxidation of Et by Ag. As a partial oxidation, is very exothermic and led to total combustion as side reaction, both of reagent and product. In order to increase the selectivity (aro und 70%) the operating condition are managed: low T (250°C), high p (30bar), to favour the TD. Moreover, inibitors and inerts are added; and the Ag catalyst is supported on a alfa -Al2O3 support, with large pores, in order to reduce diffusion problems and c ontrol thereaction time (100°C . Each company uses a different catalyst: 1) BASF - Co, at 300bar and 250°C: start ing with cat activation from CoI2 +H2O+CO → (2step) → Co(CO)4 - H+ + HI+CO2 and RDS which is MeI nucleofic addition to the catalyst . The CO is then inserted for coordination with high p between Co and Me and I - helps MeCO to detached and hydrolize then to acid, regenerating HI and catalyst. 2) Monsanto -Rh at 30bar and 150°C: this catalyst is much more active and we can use less of it, si nce it’s also more expensive. It s insertion of MeI is indeed faster thanks to electronic addition (redox) and its RDS is still the same. The CO inserts after a structural reconfiguration that creates an empty bond on Rh. 3) CADIVA - Ir is 100 times faster its insertion but the RDS is the contemporaneous substitution of CO lwith I , making then the reaction rate depending on the CO conc as well. The process starts with a reactor and is then followed by a catalyst separation and recirculation and a rectification part . It is a TD limited reaction and I need to operate at high p if I need high T to avoid catalyst decomposition . _______________________________________________________________________________________ _______________________________________________________________________________________ 3)Which reactions are limited by TD and which by KIN? MeOH production is limited by TD because its DG0R=0 @ 135°C, T at which any catalyst is active. Is thus necessar y to work at higher T and increase the pressure to increase the equilibrium yield . Moreover, special reactors are needed, in order to follow the “ optimal thermal profile” and act always at maximum R rate. The same happens in the steam cracking, for which we operate at T of equilibrium, but at low p to favour the yield in low HC. Same also for the production of styrene from ethylbenzene. As regards kinetics limitations, they are present every time we need to control the reaction time , thus mainly in oxidation , to avoid complete combustions. _______________________________________________________________________________________ _______________________________ ________________________________________________________ 4)Pthalic anhydride : naphthalene's oxidation and possible reactors. It’s a solid compound, used mainly as a plasticizer for PVC . In the past it wa s produced from naphthalene, obtained from coal distillation, but this was not sufficient for the request of PA. Then, it was produced from o-xylene , giving not CO2 but 3 water. This reaction is also less exothermic , but the temperature was un both cases around 350°C and the residence time was controlled below 1/10s. The catalyst is Ti/VO in the shape of alumina rings to increase the available surface , and activated by SO2. The possible reactors were both fixed and fluidized , with switch condenser , due to the fact that they need to be periodically cleaned from a solid and this allows to keep the process in continuous . Since it’s not possible to remove the flammability limits, it’s preferred to work inside but to use safe equipment like rupture disks and with a catalyst as a third body, in order to avoid excess of air and high energy costs . _______________________________________________________________________________________ _______________________________________________________________________________________ 5)Maleic anhydride sythesis : reactants origins, reactions, catalysts in details, plant schemes It’s a solid compound, used mainly for copolimers . In the past it was produced from benzene, coming from BTX , oxidated in excess of oxygen in order to stay above LFL=1,4% and to diluite the highly exothermic reaction. In the process was used a multitubolar reactor in order to control the reaction time below 1/10s and keep T in the small range 300 -400 where the catalyst of V/MoO was active. Nowadays, it is produced from C4, obtained from steam cracking, which are much more safe and cheap than benzene. They are selectively oxidized with VPO catalysts, able to activate the C4 de -H2 both the etilic H, and catalyze the in termediates reactions. The mechanism is redox, with many unk now intermedi ates, mainly olefins, and 14 electrons. The side product here i t’s only 4 water, whereas in the previous process were also produced 2CO2 which was C lost. The process starts with a reactor, which can be fixed or fluidized or even transport bed. The fixed bed allows mainly a good time control but has risks of hot spot at the entrance, whereas the fluidized bed allows a double concentration of C4 at the inlet, and thus bigger productivity, even if it needs stronger catalyst for the higher mechanica l stress. The transported bed is similar to the FCC and it’s safer since the catalyst only enters in contact with air for regeneration, while the reaction happens in the riser which take fresh catalyst and reactants to the disengagement space of the reacto r. This has an higher selectivity too. _______________________________________________________________________________________ _______________________________________________________________________________________ 6)BTX The mixture coming from reforming a nd cracking is distilled in order to separate first aliphatic and aromatic compounds and then the families of aromatics.The first separation can’t be done with a simple distillation because of the extremely similar Teb of this compounds. It is industruial ly held with a SOLVENT EXTRACTION and regeneration (stripping) in rotative extractors with the process: -UDEX with Et -glycol -SULFINOL -Lurgi with N -Methil -pirrolidone that allows separe in liquid phase All this solvents have in common the fact to be inert , cheap, low Teb and affine to aromatics compound for polarity, with high capacity too, to minimize the amount of solvent (on the contrary of water, which is only added sometimes to increase the selectivity). The hard part is then to separate the similar i somers between each other: for the first benezene separation is possible to use a simple distillation column at 80°C, with 60 plates. The same for the following separation of etilbennzene, at 100°C. In the C8 -C9 compounds column there’s also the isomeriza tion of etilbenzene in xylene. In the following isomers separations we can separate directly o -xylene with 150 plates at 144°C and the other isomers by crystallization at 13°C, exploiting their different solidification temperatures or their structural diff erences for disposition of Me group around the ring. _______________________________________________________________________________________ _______________________________________________________________________________________ 5)steam cracking: production of olefins in general , TD aspects (Francis diagram), reactor, operative conditions , ethilene process . Separation of HCmixture , alternative pathways to obtain olefins (FTS, metathesis, and especially oxidative coupling of CH4) . Olefines can be pr oduced in refinery via thermal treatments (cracking) or cc, but these are mainly long chain olefins. Shorter ones like ethylene and propylene are produced via steam reforming. This is an endothermic, TD limited, reaction: according to the Francis diagram, olefins become more stable than corresponding paraffines above 800°C, T at which C and H2 are always favoured. For this reason, it’s really important to control the reaction time and temperature and avoid disomogeneity, in order to block C formation and ot her side reactions (polymerization, condensation, pyrolysis,..), due also to the fact that the mechanism is radical. To increase the light products is desired low pressure; since this can held to explosions risk, it is diluited with steam, which also helps as a thermal diluent and C cleaning. The operative conditions are in general dependent on the feed: it can be pure ethane, mainly in US, or nafta, preferred in Europe. It’s possible to define a parameter “severity” as a function of T and time: for nafta it’s asked to have high severity for higher ethane conversion, whereas for ethane it’s better to keep it low. With nafta we need 0,4 diluition of steam, but the feed is cheaper and leads to higher conversion (thus lower selectivity) and wider distribution of products: H2, CH4, unconverted ethane and propane, propene, pyrgas are the main side products. It will be fundamental a good products separation. The reaction is held in a special furnace, with a convective preheating section, and a radiative tubular section to reach 800°C. The tubes design and disposition is fundamental: they are long (80m) and small (80mm) in order to have high flow rates and smal l residence time. They are made of Cr -Ni but poisoned with 20ppm of S in order to block C gasification and polymerization reactions. C is always forming, there will be a deckoing stage necessarly. It is also important to guarantee both fast preheating at the inlet and fast cooling at the outlet, to avoid condensation. After the furnace there’s quenching with oils a heat transfer heat exchanger, which has low residence time. For nafta there’s then a primary fractionation and the cleaning section, to remove first H2S with NaOH and then H2O and CO2 to avoi their precipitation in the following cold part of the plant. Indeed, the fractionation part needs to work at T below the Teb of HC, which are very similar and low. ___________________________________________ _________________________________________ The “cold part” of t he plant starts with a first column of separation of light H2 and CH4 from heavier HC: it works with a liquid ethylene reflux at -95°C and 42atm, maximum allowed by Tc of 50°C. Before the C2 col umn, it’s necessary to hydrogenate acetylene to ethylene to avoid explosives components and to not loose C2. After separation of C2 in a distillation column, theC2 alchane and alchene can be splitted (15°C difference) with 100 trays, working around -100°C , with ethylene reflux at 5atm. The C3 split starts to be difficult because it would need 200 trays for only 5°C difference . We can use 1 column at 20 atm and -50°C, or 2 in series at 12atm , with propylene as cooling fluid, or solvent extraction affine to propylene. C4 splitting includes: butano, buthene, isobutano, butadiene, with all similar Teb. It is not possible to use a 300 trays distillation column, so we separe first butadiene , us ing : 1) Physical adsorption: Exxon Cu -NH4 salts , affine for structural characteristics 2) solvent extraction: affine for polarity • Philips → FURFUROLO • Union Carbide → DMAC • Shell → CAN They all involve first an extractive column and a splitting regeneration one. Then it’s separated isobutene with H2SO4 or with addition of MeOH or H2O, giving ethers . These are known as “tank reactions” because they allow to convert them back to highly reactive isobuthene when needed. It is done mainly with the Hulls process. _________________________________________ __________________________________________ Due to the low yield (16%) and selectivity of Steam Cracking, there are other possible routes to obtain olefins in organic chemical industry: • Selective de -H2 of alkanes, held with metal catalysts at high T (600°C) in different reactors, according to the company. This increase the yield to 20% and selectivity to 40%. • Methatesis: consists in double exchange of alchil groups around double bond to produce mainly propylene. It uses W or Re catalysts at low T and FCC -like reactors+ fractionation. • CH4/O2 coupling : 2CH4 react with ½ O2, without catalists, giving first the corresponding alcane and then the alchene, and water. The catalyst are still don’t known. • MeOH/EtOH condensati on , first to ether and then to alchenes, with high conversion. Needs acid catalysts • FT , gives mainly long chain olefines 7)formaldeide: processi di produzione, condizioni operative, disegnare gli schemi d'impianto e di confrontare i due processi dal punto di vista delle emissioni e dell'impatto ambientale 8)styrene production : process, scheme justifying why the second distillation column is bigger than the first two , recycle, temperatures limits choices for catalysts , pressure drops Styrene is largely produced (25Mt) , together with ethylene and VCM (13Mt) to produce plastics. It is a 2 step reaction: first benezene alkylation with ethylene , via a Friedel -Craft reaction, and then a selective de -H2 of ethylbenzene : 1. The first was carried with acid catalists : - Monsanto: AlCl3, hav s the advantage of being active at lower T (160°C) but they need purification from water (which is present because it forms an azeotrope with benzene) and they can cause corrosion. The pressure is 5atm to stay in liquid phase. T he process starts with water azeotropic distillation, then biphase reactor where ethylene is bubbled in excess of benzene to reduce side reactions of polyalchilation . The y happen anyway, thus the product is then fed to a second reactor of transalkilation . The catalyst is then separated and recycled , while the product is neutralized with NaOH to avoid corrosion. - Mobil -Badger:zeolites, are less active, then works at higher T ▪ GAS phase: 400°C/20atm , with ethilene quench multistage adiabatic reactor ▪ LIQUID: 298°C/40atm , with benzene excess slurry reactor The advantage of this process is that it doesen’t need to be separate from water and it’s not corrosive, moreover zeolites are more selective for monoalchilates and we can reduce the polialchilate s, but with a second reactor anyway. The process starts with the reactor, followed by a benzene column to recycle the esxcess, and a purification section, where ethylbenzene is separated from the top, before recycling the liquid res idues to transalkilation reactor and back to the benzene column. 2. The catalytic de -H2 of ethylbenzene is ENDOTHERMIC and limited by TD: its DG0R=0 @ 700°C, but this T is too high to avoid decomposition of catalists and radical dealkilation of ethilbenzene ! We prefer then to decrease the T at 600°C and decrease the p with steam diluition, to favour the TD. The selected catalyst is a mixture of Me oxide s: Fe (active) -Cr(support) -K(copromoter) . The reactor (Monsanto process) is a radial multistage adiabatic reactor , heated in steps by splitted steam feed . Both feed are preheated in a furnace and quenched after for heat recover. Water is used to strip the light products, whereas the heavy HC is taken to purification , which happens in vacuum distillation, in order to decrease the T, with TC and adding inibitors to avoid polymerization. Moreover high purity is required for followin g polymerization. _______________________________________________________________________________________ _______________________________________________________________________________________