Chemical Engineering - Industrial Organic Chemistry

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Andrea Landella POLITECNICO DI MILANO 1 Industrial Organic Chemistry Tips 1. Combined Steam Reforming : compact steam reformer (SR) in series with a secondary autothermal reformer (ATR), the latter fed with pure oxygen. This configuration reduces capital expenses on the SR, due to smaller dimensions, but the supply of pure oxygen increases operative expenses. Moreover, the outlet H 2/CO ratio is around 2.05, that is near stoichiometric value for the methanol synthesis reaction. 2. In order to improve the purity specifications in a disti llation sequence (with single or multiple columns) for a generic product , the reflux ratio R = L/D of the product column is tuned. There is no thumb rule for an absolute value for R, as it is only expressed relative to the minimum reflux ratio R min . 3. A stan dard distillation column , thus with total condenser and no side -cuts , has 3 degrees of freedom : 3.1. Synthesis Problem: the ratio R /R min and the top and bottom purity specifications x D, x B 3.2. Analysis Problem: the number of stages N and the top and bottom heat duty requirements Q cnd, Q reb 4. The thumb rule for estimating the optimal R/R min ratio is based on economic considerations , as OPEX(R/R min ) is an increasing function, and CAPEX(R/R min ) is a n upward convex function, respectively: 4.1. If R/R min → 1, then column traffic is very low, thus low pumping costs and heat duties, thus low OPEX. The number of stages required to obtain purity specifications N → infinity, thus CAPEX → infinity . 4.2. If R/R min → infinity, then column is operating near full -reflux regime, thus column traffic is very high and thus high OPEX. The number of stages converges to a definite value N → N min , thus in order to operate with high traffic, the column diameter is very high, thus high CAPEX. 5. A vacuum column has low feed pressure, thus the pressure drop ΔP/H must be reduced to avoid very low top pressure, and impractically low gas velocities v to the condenser. From Poiseuille Law ΔP/H = 32 μv/D 2 the column diameter must be increased quadratically to reduce ΔP/H. 6. Shell & Tube heat exchanger s and reactors have high construction (welding), maintenance (fouling clean up ) and transportation (weight and dimensions require appropriate transports) costs. 7. Intermediate cooling of Adiabatic Staged Reactors done by heat exchangers is less prone to the formation of hotspot s (and runaway) than feed quenching. This is due to the latter configuration being intrinsically more sensitive to temperature perturbances , as for instance in hydrocracking , or hydrotreating , processes the temperature control is of paramount importance . 8. The Thiele modulus Φ = V/S*(k/D) 1/2 is evaluated for a single first -order reaction in a catalyst pellet of any shape. The value V/S is called the equivalent radius, equivalent diameter or characteristic length. It is said that Φ is non -ideal when the reaction occurs at very hig h pressure and very low temperature. 9. Noble metals are suitable for hydrogenation reactions, as well as Zn, Cr and Fe oxides. Noble metals have high cost, but require less regeneration. One example is catalytic reforming, as γ -Al2O3 is impregnated with platinum (Pt) and other metals such as (M = Re, Sn, Ir, Ge, Pb) to form the active H2MCl6. 10. Silver (Ag) is the active metal suitable for bland oxidation, and Ag/α -Al2O3 is one of the standard catalysts for oxidative dehydrogenation as s een in the Silver Process for formaldehyde production from methanol. Further oxidation is prevented by the support, that must have low S/V ratio (to prevent limiting counter - diffusion due to small channels, thus low overall surface S ) and overall a very lo w contact time is required. Andrea Landella POLITECNICO DI MILANO 2 11. Hydrogenation and dehydrogenation reactions are always reversible . 12. All observed reactions that form water are very thermodynamically favorite (very negative ΔRG), therefore there is no thermodynamic control. One example is ethylene oxychlorination. The equilibrium conversion ξeq is very close to unity for very wide range of temperature, pressure and other operating conditions (such as the molar ratio of the reac tants) . 13. It is very costly to burn diluted VOC streams. (VOC = Volatile Organic Compounds, such as benzene, toluene, carbon tetrachloride, ethyl acetate, acetone and others. Major VOC sources are coatings and paints). 14. In order to maintain an oxidized cataly st, excess oxygen is used. However, flammability limits must always be considered. The only example of operation within such limits is the phthalic anhydride synthesis, this is not a guarantee of absence of runaway and explosions. 15. Fe 2O3/MoO 3 is a typical s elective oxidation catalyst, one example is formaldehyde synthesis by direct oxi - dation of methanol. In the example, as the catalyst performs a redox reaction, it has high activity and with the form of Raschig rings for enhanced S/V ratio, higher void frac tion ε and thus low pressure drop ΔP/L . It is observed, from the Ergun Equation dP/ dx = – f*G*v/d p*(1 – ε)/ε3, that dP/dx → 0 if ε → 1. 16. From the Francis Diagram, at very high temperatures, decomposition and (if oxygen is present) complete combustion is always reached. This is because of the thermodynamic instability of hydrocarbons at higher temperatures. 17. For reaction scheme in series, it is preferable to use a PFR type reactor, while for a parallel reaction scheme a CSTR type reactor is pr eferred. This is because selectivity is maximized as follows: 17.1. Series: A → B → C , with constants k 1 and k 2, and assuming all steps are first order . Assuming to re - cover B, we have the instantaneous selectivity σ I defined as the ratio between the rate of the desired reaction over the undesired reaction: σ I = R D/R U = k 1*CA/(k 2*CB). This ratio is not constant once the temperature is defined, as the concentrations of A and B depend on the contact time τ = V/Q. The reactor, PFR type, is chosen by maximizing the global selectivity σ = (CB – CB0)/C A0, for the contact time τ, and the conv ersion of A is evaluated as ξ PFR = 1 – exp( –k1*τ), with the optimal τ expressed as τopt = ln(k 2/k1)/(k 2 – k1). 17.2. Parallel: A → C and A → B, with constants k 1 and k 2, and assuming all steps are first order. Assuming to recover B, we have the instantaneous selectivity σ I defined as the ratio between the rate of the desired reaction over the undesired reaction: σ I = R D/R U = k 1*CA/(k 2*CA) = k 1/k2. This ratio is constant once the temperature is defined, thus doe s not depend on the contact time τ = V/Q. The reactor, CSTR type, is chosen by maximizing the concentration of species B for the contact time τ, and the conversion of A is evaluated as ξ CSTR = (k 1+k 2)*τ/(1 + (k 1+k 2)*τ), with the optimal τ that is expressed as τ opt = 1/(k 1*k2)1/2. 18. The cooling fluid selection is governed by the design (or maximum) operative temperature in the reactor: 18.1. For T < 250°C, then common service water is used. Such water comes from cooling towers usually at 26°C and it may be pressurized, to increase its boiling point. However, as the temperature and pres - sure reach the critical point (374°C and 218 atm) the enthalpy of evaporation gets close to zero. This is because near the critical point the liquid -vapor phase transition does require less energy to occur spontaneously , reaching exactly zero enthalpy required at the critical point. Andrea Landella POLITECNICO DI MILANO 3 18.2. For 250°C < T < 350°C, then a DowTherm like hydrocarbon mixture is used. Also known as di athermal oil, its composition varies by vendor. The DowTherm is an eutectic mixture of biphenyl (26.5%) and biphenyl ether (73.5%) and has approximately 2.76 kJ/kg/K for specific heat. If temperature reaches above 360°C, thermal cracking reactions occur, a nd the specific heat drops. 18.3. For 350°C < T < 50 0°C , then an eutectic mixture of molten salt s is used (62% NaNO 3 and 38% KNO 3, and in other cases 53% KNO 3, 40% NaNO 3, 7% NaNO 2 with mass percentages), and has approximately 1.53 kJ/kg/K for specific heat. The molten salt cooling system is very tough to control, in both startup and shutdown sequences , due to difficulty in reaching a constant coolant temperature . 18.4. For T > 5 00°C, the n other reactor configurations must be chosen to increase the heat transfer area. 19. UDEX Process stands for UOP -Dow Ex traction Process. UOP (or Universal Oil Products LLC) was acquired by Honeywell, and it is now referred as Honeywell UOP, a multi -national company delivering technology to the petroleum refining, gas processing, petrochemical production and other manufacturing industries. The UOP (now Honeywell UOP) roots date to 1914 since the Dubbs’ process for thermal cracking, perfected in 1919, was the first UOP product. Other processes, like UDEX, SORBEX, OLEFLEX and others are UOP patented. 20. Welding is a critical and costly operation for multi -tubular reactors and heat exchangers, since the welded junctions are subject to thermal and mec hanical stresses over the equipment lifetime. 21. Catalyst loading is a critical operation, as the catalyst must be uniformly placed in (all) tubes, otherwise dif - ferent tubes experience different void fractions and subsequent different pressure drops and conc entration or temperature profiles. The reactor will show unbalanced behavior, or equivalently, non -uniform reactivity on the tubes, leading to potential hotspot formation and subsequent runaway. 22. For homogeneous catalytic reactions, the single reaction steps are identifiable. An example is the difference between the BASF, Monsanto and Cativa processes for acetic acid production by methanol carbonylation. 23. Non -catalytic partial oxidations are not used, due to impractically low selectivity . 24. The only two Fischer -Tropsch ( that use Syngas from Natural Gas) plants currently in operation, are: 24.1. Malaysia (Bintulu), from Shell , operating at 14500 BPD from 1993. It is based on a multi -tubular fixed bed reactor, with cobalt catalyst (low temperature proces s). 24.2. South Africa (Secunda) , from Statoil/PetroSA, operating at 22500 BPD from 1992. It is based on the circulating fluidized bed reactor, with iron catalyst (high temperature process). Another Fischer -Tropsch 300 BPD pilot plant, from British Petroleum and built in Alaska (Nikiski), is based on a multi -tubular fixed bed reactor, with cobalt catalyst. This plant employs a new reforming technology developed by British Petroleum and Davy, named “Compact Reformer”. 25. Chlorophyll and porphyrin vanadium complexes (especially from marine crude oil) interact heavily with re - finery processes, thus they must be removed beforehand. 26. The Bouduard Reaction 2CO → CO 2 + C, is the disproportionation of carbon monoxide into coke. It is a highly exothermic reaction, Δ RH0 = –41 kcal/mol and Δ RG0 = 0 at 700°C. It is the primary source of coke formation in steam reforming, and a secondary source of coke formation in catalytic reforming. Andrea Landella POLITECNICO DI MILANO 4 27. Vis breaking, or vis cosity breaking , is a bland thermal cracking process to reduce the vis cosity of atmospheric residue , vacuum residue and VGO ( Vacuum Gas Oil). Bland conditions, as 400°C < T < 500°C are used to limit tar and ultimately, coke formation. At higher temperatures around 600°C , coking, a thermal cracking process is used to obtain h igh purity coke and olefin -rich gasoline from the same visbreaking feedstocks . 28. Typical aci d-type catalyzed reactions are isomerization and cracking. 29. Hydrocarbon radicals and carbocations follow the same stability rule: tertiary > secondary > primary. This is because the electron vacancy manifests as a free p orbital (cation) or half -free p orbital (radical). As a moving electron vacancy is equivalent t o a moving positive charge , the free p orbital holds more positive charge than the half -free p orbital, thus making it more mobile. Highly mobile positive charge (electron vacancy) within carbocations allows for very fast rearrangement reactions, to form t ertiary from both primary or secondary carbocations if the initial structure allows it (for instance, CH 3CH 2+ does not rearrange). 30. Hydrothermal aging is the process for which water, at around 800°C, wears to pieces the catalyst substrate thus collapsing its crystal structure. One example is steam reforming with γ -Al2O3 as the substrate. 31. In the steam cracking process, the higher the severity, the better the products quality (higher ethylene and propylene quantity) but also the higher the number of coproducts (aromatics, ramified C4 -C5, methane, coke and others). In general, higher severity leads to higher conversions and low selectivities. Also the exit stream composition depends on feedstock type: VGO feed at medium severity leads to 24%wt of aroma tics, while naphtha feed at medium severity leads only to 7% wt of aromatics. 32. Isomerization are almost athermal reactions, since the same type of bonds is both broken and formed. While dehydrogenation reactions are very endothermic, since the produced olef ins are less stable than paraffins. 33. Foul odors are very often caused by unsaturated hydrocarbons. Esters leave a fruity smell, while HCN leaves a distinct almond smell, the latter being so toxic that almost 300 ppm can kill a person in minutes. 34. Hydrotreat ing, or hydro gen treating , is an ubiquitous catalytic processes in refineries, that is placed before other catalytic processes such as catalytic cracking or catalytic reforming. The objective is to remove poisons such as sulfur (HDS, hydro desulfurization), nitrogen (HDN, hydro denitrogenation), oxygen (HDO, hydro deoxi- genation), metals (HDM, hydro demetallization), and also unsaturated/aromatics (HYD, hyd rogenation) with hydrogen . Feedstocks range from vapors (gasoline) to liquids (gasoils), while the catalyst is Me/ γ-Al2O3 with Me=Co -Mo or Me=Mo -Ni, depending on the feed type. Reactor configurations can be adiabatic staged with inter -mediate quench or expanded bed reactors, to have low conversions per pass, thus low heat generated. 35. Ethylene interferes in polymerization processes, thus it must be separated from monomers . In the styrene production process from ethylbenzene, the side reaction styrene → benzene + ethylene is unwanted mai nly for this reason. The styrene polymerization temperature is around 120°C (upper reboiler temperature limit) , and polymerization inhibitors are mixed both in the ethylbenzene feed and in the purified styrene stream together with running the separation un der vacuum (low pressure drops, and high column diameters) . 36. The C4 olefin separation, specifically the 1,3 -butadiene/isobutene separation exploits the high reactivity of isobutene with methanol to form MTBE, methyl -tert butyl -ether, from etherification. The product acts as a reversible tank of isobutene, which is then recovered by hydrolysis. Another reaction of isobutene is hydra - tation to form TBA, tert -butyl-alcohol , while the oldest reaction being sulphoration, reaction with H 2SO 4. Andrea Landella POLITECNICO DI MILANO 5 37. Tra y efficiency (η) data obtained for a separation with almost unitary relative volatility (α) is meaningless if not accompanied by VLE data. This is because the relative error on η increases as α is closer to unity: 37.1. At α = 3 .0, +10% error on α leads to –10% error on η (uncertainty of –0.08 , or η = 0.80 -0.72 ); 37.2. At α = 1.1, +10% error on α leads to –50% error on η (uncertainty of –0.40 , or η = 0.80 -0.40 ); Intuitively if the error on α is positive (higher volatility) the error on η is negative (less efficient) since higher boiling compounds enrich the gas phase , thus lowering the outlet molar fraction of the distillate. 38. Produced methane and hydrogen, for ins tance from steam cracking, are often kept as fuels in the refinery. In most instances, produced methane is used as both fuel and reactant for the steam reforming process , and produced hydrogen is used as reactant of hydrotreating units . 39. Carbonylation is a class of reactions that produce organic carbonyls ( addition of a C=O group) to any organic compound in the reduced state. Methanol carbonylation to produce acetic acid (alcohol → carboxylic acid), and hydroformylation of propene to produce butanal (alkene → aldehyde), are both carbonylation reactions. 40. Cobalt or better, cobalt carbonyl complexes , are a classical hydroformylation (called oxo synthesis ) catalyst s, that is the formation aldehydes with an additional carbon atom, reacting a given olefin with syng as. The most important oxo products are C3 -C19 aldehydes, with butanal being 75% of the overall production. From its discovery in 1938 (Ruhrchemie) and up to 1975, cobalt was the ubiquitous catalyst of choice. From the 1980 ’s (Union Carbide and Celanese), rhodium -based carbonyl complexes replaced cobalt. However, o ther metals, such as nickel may be used, as an example is the production of acetaldehyde by methanol hydroformylation, however the process is not industrially feasib le due to the impractically low activity of nickel. 41. The Deacon process is the catalytic oxidation of hydrogen chloride to produce chlorine and water, employing the reaction 4HCl + O 2 → 2Cl 2 + 2H 2O, strongly exothermic , ΔRH0 = –14 kcal/mol and Δ RG0 = 0 at 6 00°C. It is a hydrogen chloride recovery process, as the latter is formed in the synthesis of chemical intermediates such as isocyanates for polyurethane production. In its origin, the CuC l2 catalyst deactivated quickly due to copper chlorination, or formation of volatile CuCl x species above 400°C . The reactor employs molten salt cooled fixed bed s, as fluidized beds deactivate quickly due to corrosion effects, and more active and chlorination resistant RuO 2/CeO 2-CuO catalyst is used. T he Deacon reactors have the highest thermal gradients, reaching 250°C/m in most cases, thus requir es highly optimized graded catalyst beds , with different activities per bed. 42. Chlorine production, as of 2010, is around 60 Mt/y, while total hydrogen chloride production is 20 Mt/y, but the market size of hydrogen chloride is only 5 Mt/y because it is directly used/recovered by the producers, one example is the employment of the Deacon Process. 43. An industrial process that produces large quantities of unw anted products is not sustainable. Upper limits for both concentration and fluxes of unwanted products are process design constraints. 44. If possible, a reactant or product is used as solvent , lowering further separation costs. 45. Copper sintering occurs at ar ound 300°C, while silver sintering occurs at around 700°C . At 450°C , the irrever - sible phase transition TiO 2(anatase) → TiO 2(rutile) takes place. 46. The general thumb rule of separation sequence s consists of the following: after purging incondensable gases the most toxic/corrosive compound is removed, then the most abundant compound is removed, then the Andrea Landella POLITECNICO DI MILANO 6 subsequent purification train (as designed to avoid small boiling gaps) should not allow remova l of the main product in the last column . This is to avoid possible degradation or purity/material losses over a sequence of multiple distillation/extraction columns. 47. While the LFL ( Low Flammability Limit) d oes not significantly change while changing from air to oxygen as an oxidizing agent, the HFL ( High Flammability Limit) changes significantly. One example is ethylene/air, with LFL = 2.4% vol and HFL = 32% vol, whereas ethylene/oxygen, with LFL = 2.9% vol and HFL = 80% vol. 48. The specific heat of a molecul e depends on its global degrees of freedom. If a generic molecule is comprised of N bonded atoms, then it has a total of 3N degrees of freedom, however: 48.1. If the molecule is linear, there are 3 translational (Cv trs = 3/2*R) and 2 rotational (Cv rot = R) degrees of freedom thus leaving 3N – 5 vibrational degrees of freedom , thus Cv = 5/2*R + Cv vib. The specific heat is obtained from Mayer ’s relation, thus Cp = Cv + R = 7/2*R + Cv vib. 48.2. If the molecule is no t linear, there are 3 translational (Cv trs = 3/2*R) and 3 rotational (Cv rot = 3/2*R) degrees of freedom thus leaving 3N – 6 vibrational degrees of freedom , thus Cv = 3*R + Cv vib. The spe - cific heat is obtained from Mayer ’s relation , thus Cp = Cv + R = 4*R + Cv vib. The vibrational contribution to specific heat , Cv vib, depends both on N and the mo lar mass of the molecule , howev er if the latter is similar between a linear and nonlinear molecule, then Cv vib,lin > Cv vib,non lin. 49. Azeotropes are mixtures that b oil as a single pseudo -component, while mixtures that do not form azeotropes are called zeotropes (one instance is methanol and water) . Minimum boiling azeotropes have smaller boiling temperatures than the constituents, thus are recovered at the top of the distillation column . The opposite happens to maximum boiling azeotropes, but they are less frequent than minimum boiling azeotropes. Some comm on azeotropes encountered in the course are: 49.1. Ethanol and water , minimum boiling azeotrope at 1.30 % mol of water is removed by pressure -swing distillation, if P < 11 kPa the azeotrope disappears; 49.2. Ethyl acetate and water, minimum boiling azeotrope at 30% mol of water; 49.3. Me thyl acetate and water, minimum boiling azeotrope at 90% mol of water; 49.4. Ethyl acetate and methanol , minimum boiling azeotrope at 70% mol of methanol; 49.5. Me thyl acetate and methanol , minimum boiling azeotrope at 35 % mol of methanol ; 49.6. Generic alcohols and ketones, minimum boiling azeotrope is removed by pressure -swing distillation; 49.7. Generic oxygenated hydrocarbons and water, minimum boiling azeotrope is removed by introducing an entrainer such as MIK (methyl -isobutylketone) and methyl -cyclohexane. 50. If two components have a boiling gap above 50°C, then no azeotrope with indistinguishable prope rties from the original components is formed. The converse, however, is not true: acetic acid and water have a boiling gap of 18°C, and they do not form any azeotrope, as they instead form a zeotropic mixture. In this example, the acetic acid and water sep aration by simple distillation is not practical : ethyl acetate is added as it forms an azeotrope with water, leaving the dry acetic acid as a residue. 51. The mean catalyst particle diameter for a fluidized bed reactor is 10 μm (as lower diameters leads to hig her minimum fluidization velocities). Once the gas velocity reaches the minimum fluidization, the pressure drop Andrea Landella POLITECNICO DI MILANO 7 remains constant up to a terminal velocity called entrainment velocity (catalyst particles are entrained and dragged by the gas flow). The mean catalyst particle diameter for a fixed bed reactor is 2 mm. 52. It is harder to improve selectivity (σ) rather than yield (η = σ*ξ) . One instance is steam cracking process, for which ξ → 1, but the selectivity towards light olefins is strongly affected by the severity. 53. In the Fischer -Tropsch synthesis, the product distribution can be estimated using different chain growth mo - dels. In the Anderson -Schulz -Flory distribution model, the chain growth probability α is considered constant and independent of the chai n length, and the desorption probability the complementary event to the chain growth, thus equal to 1 – α. The formation probability of a generic n-paraffin is the just the joint probability of n – 1 carbon additions ������������−1 with a single chain desorption 1− ������, and it is equal to its molar fraction : ������������= ������������−1(1− ������)= Number of n -paraffins formed Number of total paraffins formed = ������� The n -paraffin has molar mass MM ������= ������(MM C+ 2MM H), and it i s a component in the product mixture, its mass fraction is given from the well -known definition from molar fraction and molar mass : ������������= MM ������������� ∑ MM ������������� ������ Substituting the molar fraction and the molar mass of the n -paraffin, calling MM CH 2= MM C+ 2MM H: ������������= ������MM CH 2������������ ∑ MM ������������� ������ = ������������������ MM CH 2 ∑ MM ������������� ������ = ������������������������ = ������������������−1(1− ������)������ with the constant ������, independent of the chain length ������. In the product mixture, the sum of all possible pro - duct’s mass fraction must equal unity, therefore ������ is a normalization constant: 1= ∫ ������������ ������������ +∞ 0 = ∫ ������������������������ ������������ +∞ 0 = ∫ ������������������−1(1− ������)������ ������������ +∞ 0 After rearrangement, the expression of the normalization constant depends on an improper integral: ������ = 1 1− ������ 1 ∫ ������������������−1 ������������ +∞ 0 = ������ 1− ������ 1 ∫ ������������������ ������������ +∞ 0 thus, l etting �= ������������ then ������= ln �/ln ������, and ������������ = 1/(�ln ������) ������� , from this the upper limit is ������+∞ = 0, since ������< 1, and the lower limit becomes ������0= 1. The improper integral is transformed and solved as follows : ∫ ������������������ ������������ +∞ 0 = 1 ln2������∫ ln �������� 0 1 = 1 ln2������(|�ln �|10− ∫ ������� 0 1 )= 1 ln2������ Therefore the normalization constant, and the subsequent mass fraction of the n -paraffin is: ������ = ������ 1− ������ln2������ → ������������= ������������������ln2������ thus the n-th component in the product mixture , measured by mass fraction, is described by the Anderson - Schulz -Flory distribution model. The plot of ln(������������/������) vs ������ gives the classical linear plot. Andrea Landella POLITECNICO DI MILANO 8 54. Streams with high component concentrations lead to lower pumping costs, as less volume is transported for the same mass. Moreover, reactors , columns, vessels and tubes have lower diameters (since A 2 = A 1*ρ 1/ρ2 from the flux definition F = ρ*v*A, with same velocity v but ρ 2 > ρ 1, leads to A 2 < A 1). However, higher con - centrations are limited by safety and handling constraints, one example being hydrochloric acid that cann ot be stored as an aqueous solution over 3 8% m/m due to highly corrosion effects and evaporation. 55. Vanadium is a strongly oxidative catalyst, more than molybdenum. The only known direct catalytic oxidation process of a paraffin is the production of maleic a nhydride from n -butane. Many other direct non -catalytic oxidation processes of paraffins exist, for instance the Celanese process of acetaldehyde production from propane/butane. The latter process is not industrially relevant anymore , due to very poor sele ctivities. 56. Solidification processes are inherently discontinuous. Two instances are the Phillips and the Atlantic Richfield process es for m-xylene/p -xylene separation by fractional crystallization . 57. The two major production chains of the Organic Chemical Industry are ethylene and BTX ( benzene -toluene - xylenes) with around 150 Mt/y of produced ethylene goes to polyethylene synthesis (54%) direct chlorination (15%) and ethylene oxide (12%), the remaining is used in alcohol/aldehyde synthesis. 58. SBR stands for Styrene -Butadiene Rubber , or Resin, (23% styrene and remaining butadiene), is the classical tyre , tubing and gasket rubber. Higher styrene quantity gives thermoplastic properties. 59. ABS stands for Acrylonitrile -Butadiene -Styrene, a thermoplastic polymer (15% acrylonitrile, 50% styrene and remaining butadiene) used for producing small light object such as tubing, musical instruments and famously LEGOs. ABS is a terpolymer, being the result of polystyrene -acrylonitrile reacte d with polybutadiene. 60. The optimal catalyst lifetime is 2 y, the mean period within which mechanical maintenance is issued. 61. Radial flow reactors have smaller pressure drops, due to higher surface area to the catalyst bed (hollow cylin - der), leading to smaller velocities and thus smaller ΔP/H value. One example is the styrene process, for which ethylbenzene dehydrogenation must be conducted at low t emperature (with low pressures and ΔP/H value) to prevent styrene polymerization. 62. If the column pressure is low, the number of theoretical stages decreases, since the relative volatility α and the repartition constants K i = y i/xi are affected , as the vapour phase is enriched with the lower boiling point component. The key concept in pressure swing distillation is the shift of the pinch point (azeotrope) towards one of the two components by changing the top pressure of the second column. 63. Structu red packing columns have s maller pressure drops than tray columns. Tray pressure drops depend on the operative top column pressure, thus if P = 5 kPa → ΔP/tray = 400 Pa, if P = 100 kPa → ΔP/tray = 800 Pa, if P = 2000 kPa → ΔP/tray = 1000 Pa. Vacuum columns often reach a top pressure of 1 psia = 7 kPa. 64. The general thumb rule of vessel design is that, known the height/diameter ratio (optimal is around 2 -3) and the inlet flow rate F, the mean half -filling time T is 5 minutes. From the mass balance , dM/dt = F, and since M(t) = ρV(t) = ρV*α(t) with α being the filled fraction that goes from 0.5 to unity. The equation is directly solved obtaining the total volume V = 2*F*T/ρ, and if H = 3 *D this leads to V = πD2/4*H = 3 π/4*D 3 = 2*F*T/ρ or a single equation for the diameter. Andrea Landella POLITECNICO DI MILANO 9 65. In fluidized bed r eactors it is possible to operate within the LFL -HFL range, since the very high catalyst surface favors radicalic termination reactions of complete combustion.