Lecture notes on Petrochemicals

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NPTEL – Chemical – Chemical Technology II Lecture 13: Petrochemicals: Overview 13.1 Introduction - In this lecture, we present a brief overview of petrochemical technologies and discuss upon the general topology of the petrochemical process technologies. - Petrochemicals refers to all those compounds that can be derived from the petroleum refinery products - Typical feedstocks to petrochemical processes include o C1 Compounds: Methane & Synthesis gas o C2 Compounds: Ethylene and Acetylene o C3 Compounds: Propylene o C4 Compounds: Butanes and Butenes o Aromatic Compounds: Benzene - It can be seen that petrochemicals are produced from simple compounds such as methane, ethylene and acetylene but not multicomponent products such as naphtha, gas oil etc. - 13.1.1 Definition : These are the chemicals that are made from petroleum and natural gas. Petroleum and natural gas are made up of hydrocarbon molecules, which comprises of one or more carbon atoms, to which hydrogen atoms are attached. - About 5 % of the oil and gas consumed each year is needed to make all the petrochemical products. Petrochemicals play an important role on our food, clothing, shelter and leisure. Because of low cost and easy availability, oil and natural gas are considered to be the main sources of raw materials for most petrochemicals. 13.1.2 Classification: Petrochemicals can be broadly classified into three categories- a. Light Petrochemicals: These are mainly used as bottled fuel and raw materials for other organic chemicals. The lightest of these methane, ethane and ethylene are gaseous at room temperature.The next lightest fractions comprise petroleum ether and light naphtha with boiling points between 80 and 190 degrees Fahrenheit. b. Medium Petrochemicals: Hydrocarbons with 6 – 12 carbon atoms are called "gasoline", which are mainly used as automobile fuels. Octane, with eight carbons, is a particularly good automobile fuel, and is considered to be of high quality. Kerosene contains 12 to 15 carbons and is used in aviation fuels, and also as solvents for heating and lighting. c. Heavy Petrochemicals: These can be generally categorized as diesel oil, heating oil and lubricating oil for engines and machinery. They contain around 15 and 18 carbon atoms with boiling points between 570 and 750 degrees Fahrenheit. The heaviest fractions of all are called "bitumens" and are used to surface roads or for waterproofing. Joint initiative of IITs and IISc – Funded by MHRD Page 1 of 83 NPTEL – Chemical – Chemical Technology II Bitumens can also be broken down into lighter hydrocarbons using a process called "cracking." 13.2 Process Topology - Reactors: Reactors are the most important units in petrochemical processes. Petrochemicals are manufactured by following simple reactions using relatively purer feedstocks. Therefore, reaction chemistry for petrochemicals manufacture is very well established from significant amount of research in this field. Essentially all petrochemical processes need to heavily depend upon chemical transformation to first product the purification. - Separation: With distillation being the most important unit operation to separate the unreacted feed and generated petrochemical product, the separation processes also play a major role in the process flow sheet. Where multiple series parallel reactions are involved, the separation process assumes a distillation sequence to separate all products from the feed. A characteristic feed recycle will be also existent in the process topology. Apart from this, other separation technologies used in petrochemical processing units include phase separators, gravity settling units and absorption columns. Therefore, the underlying physical principle behind all these separation technologies is well exploited to achieve the desired separation. - Dependence on Reaction pathway: A petrochemical can be produced in several ways from the same feedstock. This is based on the research conducted in the process chemistry. For instance, phenol can be produced using the following pathways o Peroxidation of Cumene followed by hydrolysis of the peroxide o Two stage oxidation of Toluene o Chlorination of Benzene and hydrolysis of chloro-benzene o Direct oxidation of Benzene - We can observe that in the above reaction schemes, there are two reaction pathways for phenol from benzene i.e., either chlorination of benzene or oxidation of benzene. Therefore, choosing the most appropriate technology for production is a trivial task. - Complexity in pathway: In the above Cumene example case, it is interesting to note that toluene hydrodealkylation produces benzene which can be used to produce phenol. Therefore, fundamentally toluene is required for the generation of various petrochemicals such as benzene and phenol. In other words, there is no hard and fast rule to say that a petrochemical is manufactured using a suggested route or a suggested intermediate petrochemical. Intermediate petrochemicals play a greater role in consolidating the manufacture of other downstream petrochemicals. Joint initiative of IITs and IISc – Funded by MHRD Page 2 of 83 NPTEL – Chemical – Chemical Technology II 13.3 Summary of petrochemical processes presented in the course We next present a summary of the petrochemical processes that would be presented in the course - Lecture 13 o Methanol from Synthesis gas route - Lecture 14 o Formaldehyde from Methanol o Chloromethanes from methane - Lecture 15 o Ethylene and acetylene production via steam cracking of hydrocarbons - Lecture 16 o Vinyl chloride from ethylene using two step process - Lecture 17 o Ethanolamine from ethylene - Lecture 18 o Isopropanol from Propylene o Cumene from propylene - Lecture 19 o Acrylonitrile from propylene o Oxo process for converting olefins and synthesis gas to aldehydes and alcohols - Lecture 20 o Butadiene from Butane o Hydrodealkylation of Toluene - Lecture 21 o Phenol from Cumene o Phenol from Toluene Oxidation - Lecture 22 o Styrene from Benzene o Pthalic anhydride from o-xylene - Lecture 23 o Maleic anhydride from Benzene o DDT manufacture from Benzene Joint initiative of IITs and IISc – Funded by MHRD Page 3 of 83 NPTEL – Chemical – Chemical Technology II 13.4 Manufacture of Methanol from Synthesis Gas 13.4.1 Introduction - Synthesis gas is H + CO 2 - When synthesis gas is subjected to high pressure and moderate temperature conditions, it converts to methanol. - Followed by this, the methanol is separated using a series of phase separators and distillation columns. - The process technology is relatively simple 13.4.2 Reactions - Desired: CO + 2H  CH OH 2 3 - Side reactions: CO + 3H  CH + H O 2 4 2 2CO + 2H  CH + CO 2 4 2 - All above reactions are exothermic - Undesired reaction: zCO + aH  alchohols + hydrocarbons 2 - Catalyst: Mixed catalyst made of oxides of Zn, Cr, Mn, Al. 13.4.3 Process Technology (Figure 13.1) Figure 13.1 Flow sheet of manufacture of Methanol from Synthesis Gas - H and CO adjusted to molar ratio of 2.25 2 - The mixture is compressed to 200 – 350 atms - Recycle gas (Unreacted feed) is also mixed and sent to the compressor - Then eventually the mixture is fed to a reactor. Steam is circulated in the o heating tubes to maintain a temperature of 300 – 375 C Joint initiative of IITs and IISc – Funded by MHRD Page 4 of 83 NPTEL – Chemical – Chemical Technology II - After reaction, the exit gases are cooled - After cooling, phase separation is allowed. In this phase separation operation methanol and other high molecular weight compounds enter the liquid phase and unreacted feed is produced as the gas phase. - The gas phase stream is purged to remove inert components and most of the gas stream is sent as a recycle to the reactor. - The liquid stream is further depressurized to about 14 atms to enter a second phase separator that produces fuel gas as the gaseous product and the liquid stream bereft of the fuel gas components is rich of the methanol component. - The liquid stream then enters a mixer fed with KMNO so as to remove traces 4 of impurities such as ketones, aldehydes etc. - Eventually, the liquid stream enters a distillation column that separates dimethyl ether as a top product. - The bottom product from the first distillation column enters a fractionator that produces methanol, other high molecular weight alcohols and water as three different products. 13.4.4 Technical questions 1. Why pressure is not reduced for the first phase separator? Ans: Methanol is separated out in the liquid stream by just cooling the reactor product stream. Therefore, since the separation is achieved physically, there is no need to reduce the pressure of the stream. Also, if pressure is reduced, then again so much pressure needs to be provided using the compressor. 2. Why the pressure is reduced to 14 atms for the phase separator? Ans: The second phase separator is required to remove dissolved fuel gas components in the liquid stream at higher pressure. If this is not done, then methane will remain in the liquid stream and fractionators will produce methane rich ethers which don’t have value. Fuel gas on the other hand has value or it can be used as a fuel to generate steam in a boiler or furnace. 3. Why two compressors are used in the process flowsheet but not one? Ans: The main compressor is the feed compressor where feed is compressed to 3000 – 5000 psi. The second compressor is for the recycle stream which is brought to the reactor inlet pressure conditions by taking into account the pressure losses in the reactor, cooler and phase separator. Joint initiative of IITs and IISc – Funded by MHRD Page 5 of 83 NPTEL – Chemical – Chemical Technology II 4. How multiple products are obtained from a single distillation column? Ans: This is an important question. Any distillation column consists of liquid reflux stream. A careful simulation of a distillation column using process simulators such as ASPEN or HYSYS or PRO II will give the liquid compositions at each tray. Using this information, one can exploit whether the intermediate liquid stream is having composition of any specific product. In such case, the liquid stream from the column can be taken out (as a pump around stream in the crude distillation column) and the balance could be cooled and sent back to a section above the distillation unit. Alternatively, without pump around also we can operate the column, but the basis of keeping pump around or not is based on the desired liquid reflux flow rates on the particular tray. 5. Can heat integration be carried out in the flowsheet? Ans: Yes, the reactor product is at higher temperature and can be energy integrated with the feed stream after compression. This is also due to the fact that compression usually increases the temperature and feed stream can be subjected to further heating after compression. 6. From engineering perspective, what is the most difficult part in the process flow sheet Ans: The design and operation of the high pressure reactor is the most difficult. To withstand such high pressure, thick walled reactor needs to be designed. Other materials of construction need to be as well looked into for safeguarding the long term shelf life of the reactor. References: Dryden C. E., Outlines of Chemical Technology, East-West Press, 2008 Shreve R. N., Austin G. T., Shreve's Chemical process industries, McGraw – Hill, 1984 Joint initiative of IITs and IISc – Funded by MHRD Page 6 of 83 NPTEL – Chemical – Chemical Technology II Lecture 14: Formaldehyde and Chloromethanes 14.1 Introduction - In this lecture, we present the production technology for formaldehyde and chloromethanes. - Formaldehyde is produced from methanol - Chloromethanes are produced from methane by chlorination route. 14.2 Formaldehyde production 14.2.1 Reactions a) Oxidation: CH OH + 0.5 O HCHO + H O 3 2 2 b) Pyrolysis: CH OH  HCHO + H 3 2 c) Undesired reaction: CH OH + 1.5 O 2H O + CO 3 2 2 2 In the above reactions, the first and third are exothermic reactions but the second reaction is endothermic. The reactions are carried out in vapour phase. Catalyst: Silver or zinc oxide catalysts on wire gauge are used. Operating temperature and pressure: Near about atmospheric pressure and o 500 – 600 C 14.2.2Process Technology ( Figure 14.1 ): - Air is sent for pre-heating using reactor outlet product and heat integration concept. - Eventually heated air and methanol are fed to a methanol evaporator unit which enables the evaporation of methanol as well as mixing with air. The o reactor inlet temperature is 54 C. - The feed ratio is about 30 – 50 % for CH OH: O 3 2 o - After reaction, the product is a vapour mixture with temperature 450 – 900 C - After reaction, the product gas is cooled with the heat integration concept and then eventually fed to the absorption tower. - The absorbent in the absorption tower is water as well as formaldehyde rich water. - Since formaldehyde rich water is produced in the absorption, a portion of the rich water absorbent solution from the absorber is partially recycled at a specific section of the absorber. - From the absorber, HCHO + methanol rich water stream is obtained as the bottom product. - The stream is sent to a light end stripper eventually to remove any light end compounds that got absorbed in the stream. The vapors from the light end unit consisting of light end compounds can be fed at the absorption unit at Joint initiative of IITs and IISc – Funded by MHRD Page 7 of 83 NPTEL – Chemical – Chemical Technology II specific location that matches with the composition of the vapors in the absorption column. - Eventually, the light end stripper bottom product is fed to a distillation tower that produces methanol vapour as the top product and the bottom formaldehyde + water product (37 % formaldehyde concentration). Figure 14.1 Flow sheet of Formaldehyde production 14.3Technical questions 1. Why water + HCHO + methanol stream is sent to a specific section of the absorber but not the top section of the absorber? Ans: This is to maximize the removal efficiency of both water and formaldehyde rich solution. If both are sent from the top, then formaldehyde rich solution will be dilute and not effective in extracting more HCHO + methanol from the gas phase stream. 2. Explain Why light end stripper is used after absorber? Ans: Water + HCHO + Formaldehyde solution may absorb other light end compounds which are not desired for absorption. This is due to the basic feature of multicomponent absorption where absorption factors for various absorbing components is not biased sharply and other undesired components also get absorbed. Therefore, the light end stripper would take care of removing these unwanted components by gently heating the same. Joint initiative of IITs and IISc – Funded by MHRD Page 8 of 83 NPTEL – Chemical – Chemical Technology II 3. Suggest why pure formaldehyde is not produced in the process? Ans: Pure formaldehyde is not stable and tends to produce a trimer or polymer. Formaldehyde is stable in only water and therefore, 37% formaldehyde solution with 3 – 15 % methanol (stabilizer) is produced as formalin and sold. 4. What type of process design is expected for the air preheater? Ans: Since we have a problem of vapour and air, we should use extended surface area heat exchanger or finned heat exchanger. 14.4Chloromethanes Chloromethanes namely methyl chloride (CH Cl), methylene chloride 3 (CH Cl ), Chloroform (CHCl ) and Carbon Tetrachloride (CCl ) are 3 2 3 4 produced by direct chlorination of Cl in a gas phase reaction without any 2 catalyst. 14.4.1 Reactions CH + Cl CH Cl + H 4 2 3 2 CH Cl + Cl CH Cl + H 3 2 2 2 2 CH Cl + Cl CHCl + H 2 2 2 3 2 CHCl + H CCl + H 3 2 4 2 - The reactions are very exothermic. The feed molar ratio affects the product distribution. When CH /Cl is about - 4 2 1.8, then more CH Cl is produced. On the other hand, when CH is chosen as 3 4 a limiting reactant, more of CCl is produced. Therefore, depending upon the 4 product demand, the feed ratio is adjusted. 14.4.2Process Technology - Methane and Cl are mixed and sent to a furnace 2 - The furnace has a jacket or shell and tube system to accommodate feed pre- o heating to desired furnace inlet temperature (about 280 – 300 C). - To control temperature, N is used as a diluent at times. 2 - Depending on the product distribution desired, the CH /Cl ratio is chosen. 4 2 - The product gases enter an integrated heat exchanger that receives separated CH (or a mixture of CH + N ) and gets cooled from the furnace exit 4 4 2 o temperature (about 400 C). - Eventually, the mixture enters an absorber where water is used as an absorbent and water absorbs the HCl to produce 32 % HCl. - The trace amounts of HCl in the vapour phase are removed in a neutralizer fed with NaOH Joint initiative of IITs and IISc – Funded by MHRD Page 9 of 83 NPTEL – Chemical – Chemical Technology II - The gas eventually is compressed and sent to a partial condenser followed with a phase separator. The phase separator produces two streams namely a liquid stream consisting of the chlorides and the unreacted CH /N . 4 2 - The gaseous product enters a dryer to remove H O from the vapour stream 2 using 98% H SO as the absorbent for water from the vapour. 2 4 - The chloromethanes enter a distillation sequence. The distillation sequence consists of columns that sequentially separate CH Cl, CH Cl , CHCl and 3 2 2 3 CCl . 4 Figure 14.2 Flowsheet of Chloromethane production Joint initiative of IITs and IISc – Funded by MHRD Page 10 of 83 NPTEL – Chemical – Chemical Technology II 14.4.3Technical questions 1. Why compressor is used before partial condenser? Ans: The compressor increases the pressure of the system which is beneficial to increase the boiling points of the mixtures. Note that the o boiling points of chloromethanes are -97.7, -97.6, -63.5 and -22.6 C for CH Cl, CH Cl , CHCl and CCl respectively. On the other hand, the 3 2 2 3 4 o boiling point is -161.6 C. For these boiling point mixtures, when the system pressure is increased substantially, the boiling points of the compounds increase and could reach close to those of the cooling water (20 o – 30 C). Cooling water is required in the partial condenser and if it is not used, a refrigerant needs to be used which requires an additional refrigeration plant. Therefore, the system pressure is increased. 2. Why water is removed using the dryer? Ans: Water enters the vapour system due in the absorption column where solvent loss to the vapour will be a common feature. Water molecule can react with the highly active intermediate chloromethanes to form oxychlorides, which are highly undesired. 3. Will there be any difficulty in separation by increasing boiling points of the chloromethanes in the distillation sequences? Ans: Definitely yes. This is because the relative volatility of compounds atleast slightly increases with reducing pressure and viceversa. But due to cooling water criteria in the distillation sequences also, there is no other way economical than doing distillation at higher pressure. 4. Since the boiling point of CH Cl and CH Cl are very close, what do you 3 2 2 expect for the production of CH Cl from the first column? 3 Ans: It is indeed difficult to separate CH Cl and CH Cl and therefore, good 3 2 2 number of separation trays be used. Or structured packing be used to reduce the height of the first column. 5. When the reactions are highly exothermic, why is the feed pre-heated? Ans: Irrespective of the reactions being exothermic or endothermic, the reaction rate always increases with temperature for non-equilibrium reactions. Therefore, feed is pre-heated to the desired temperature so as to fastly convert the reactants to products. References: Dryden C. E., Outlines of Chemical Technology, East-West Press, 2008 Shreve R. N., Austin G. T., Shreve's Chemical process industries, McGraw – Hill, 1984 Joint initiative of IITs and IISc – Funded by MHRD Page 11 of 83 NPTEL – Chemical – Chemical Technology II Lecture 15: Hydrocarbon Steam Cracking for Petrochemicals 15.1 Introduction In industrial processes, hydrocarbons are contacted with H O, depending 2 upon the desired effect. When hydrocarbon vapors at very high pressures are contacted with water, water which has a very high latent heat of vaporization quenches the hydrocarbon vapors and transforms into steam. In such an operation, chemical transformations would not be dominant and energy lost from the hydrocarbons would be gained by water to generate steam. The quenching process refers to direct contact heat transfer operations and therefore has maximum energy transfer effeiciency. This is due to the fact that no heat transfer medium is used that would accompany heat losses. The steam cracking of hydrocarbons is an anti-quenching operation, and will involve the participation of water molecule in reactions in addition to teh cracking of the bnydriocarbond on their own. Since steam and the hydrocarbons react in the vapour phase the reaction products can be formed very fast. Therefore cracking of the hydrocarbons on their own as well as by steam in principle is very effective. When steam cracking is carried out, in addition to the energy supplied by the direct contact of steam with the hydrocarbons, steam also takes part in the reaction to produce wider choices of hydrocarbon distribution along with the generation of H and CO. 2 - Hydrocarbons such as Naphtha and LPG have lighter compounds. - When they are subjected to steam pyrolysis, then good number of petrochemicals can be produced. - These include primarily ethyelene and acetylene along with other compounds such as propylene, butadiene, aromatics (benzene, toluene and xylene) and heavy oil residues. - The reaction is of paramount importance to India as India petrochemical market is dominated by this single process. 15.2 Reaction C H + H O + O C H + C H + C H + H + CO + CO + CH + C H + x y 2 2 2 4 2 6 2 2 2 2 4 3 6 C H + C H + C H + C H + C+ Heavy oils 3 8 4 10 4 8 6 6 - The reaction is pretty complex as we produce about 10 to 12 compounds in one go - The flowsheet will be reaction-separation-recycle system only in its topology. But the separation system will be pretty complex. - Almost all basic principles of separation appears to be accommodated from a preliminary look. - Important separation tasks: Elimination of CO and CO , Purification of all 2 products such as ethylene, acetylene etc. Joint initiative of IITs and IISc – Funded by MHRD Page 12 of 83 NPTEL – Chemical – Chemical Technology II - The process can be easily understood if we follow the basic fundamental principles of process technology - Typical feed stocks are Naphtha & LPG o - Reaction temperature is about 700 – 800 C (Vapor phase reaction). 15.3 Process technology (Figure 15.1) Figure 15.1 Flow sheet of Hydrocarbon Steam Cracking for Petrochemicals - Naphtha/LPG saturates is mixed with superheated steam and fed to a furnace fuel gas + fuel oil as fuels to generate heat. The superheated steam is generated from the furnace itself using heat recovery boiler concept. - The C -C saturates are fed to a separate furnace fed with fuel gas + fuel oil as 2 4 fuels to generate heat. - In the furnace, apart from the steam cracking, steam is also generated. This is by using waste heat recovery concept where the combustion gases in the furnace. - After pyrolysis reaction, the products from the furnace are sent to another heat o recovery steam boiler to cool the product streams (from about 700 – 800 C) and generate steam from water. - After this operation, the product vapours enter a scrubber that is fed with gas oil as absorbent. The gas oil removes solids and heavy hydrocarbons. - Separate set of waste heat recovery boiler and scrubbers are used for the LPG furnace and Naphtha steam cracking furnaces - After scrubbing, both product gases from the scrubbers are mixed and fed to a compressor. The compressor increases the system pressure to 35 atms. Joint initiative of IITs and IISc – Funded by MHRD Page 13 of 83 NPTEL – Chemical – Chemical Technology II - The compressed vapour is fed to a phase separation that separates the feed into two stream namely the vapour phase stream and liquid phase stream. The vapour phase stream consists of H , CO, CO C -C + components in excess. 2 2 1 3 The liquid phase stream consists of C and C compounds in excess. 3 4 - Subsequently, the vapour phase and liquid phase streams are subjected to separate processing. Gas stream processing: o CO in the vapour phase stream is removed using NaOH scrubber. 2 Subsequently gas is dried to consist of only H , CO, C -C components 2 1 3 only. This stream is then sent to a demethanizer which separates tail gas (CO + H + CH ) from a mixture of C -C components. The C - 2 4 1 3 2 C + components enter a dethanizerwhich separates C from C 3 2 3 components. o Here C components refer to all kinds of C s namely ethylene, 2 2 acetylene etc. Similarly, C the excess of propylene, and propane. 3 o The C2 components then enter a C2 splitter which separates ethane from ethylene and acetylene. o The ethylene and acetylene gas mixture is fed to absorption unit which is fed with an extracting solvent (such as N-methylpyrrolidinone) to extract Acetylene from a mixture of acetylene and ethylene. o The extractant then goes to a stripper that generates acetylene by stripping. The regenerated solvent is fed back to the absorber. o The ethylene stream is fed to a topping and tailing still to obtain high purity ethylene and a mixture of ethylene and acetylene as the top and bottom products. The mixture of ethylene and acetylene is sent back to the C2 splitter unit as its composition matches to that of the C2 splitter feed. - Liquid stream processing o The liqiuid stream consists of C3,C4, aromatics and other heavy oil components is fed to a NaOH scrubber to remove CO 2 o Eventually it is fed to a pre-fractionator. The pre-fractionator separates lighter components from the heavy components. The lighter components are mixed with the vapour phase stream and sent to the NaOH vapour phase scrubber unit. o The pre-fractionator bottom product is mixed with the deethanizer bottom product. o Eventually the liquid mixture enters a debutanizer that separates C3, C4 components from aromatics and fuel oil mixture. The bottom product eventually enters a distillation tower that separates aromatics and fuel oil as top and bottom products respectively. o The top product then enters a depropanizer that separates C3s from C4 components. o The C4 components then enter an extractive distillation unit that separates butane + butylenes from butadiene. The extractive Joint initiative of IITs and IISc – Funded by MHRD Page 14 of 83 NPTEL – Chemical – Chemical Technology II distillation unit consists of a distillation column coupled to a solvent stripper. The solvent stripper produces butadiene and pure solvent which is sent to the distillation column. o The C3 components enter a C3 splitter that separates propylene from propane + butane mixture. Thesaturates mixture is recycled to the saturates cracking furnace as a feed stream. 15.4 Technical questions 1. Why two separate furnaces are used for C2-C4 saturates and Naphtha feed stocks? Ans: The purpose of steam cracking is to maximize ethylene and acetylene production. For this purpose if we mix C2-C4 saturates and naphtha and feed them to the same furnace, then we cannot maximize ethylene and acetylene production. The napntha steam cracker has its own operating conditions for maximizing ethylene and acetylene and so is the case for C2- C4 saturates. 2. Why the product gases from naphtha and C2-C4 saturates steam cracker processed separately before mixing them and sending them to the compressor? Ans: Both crackers produce products with diverse compositions. Both cannot be fed to a single scrubber and remove the heavy hydrocarbons and oil components. While the scrubber associated to naphtha steam cracking needs to be remove significantly the oil and heavy hydrocarbons, this is not the case for steam cracker product vapour processing. An alternate way of designing a single scrubber is to design a complex scrubber that has multiple feed entry points correspond to both product gases entering from various units. This refers to process intensification and would be encouraging. 3. Why specifically the gases are compressed to 35 atm? Ans: The distribution of light and heavy components in vapour and liquid streams is critically dependent on the pressure. Therefore, the pressure of the system plays a critical role in the distribution of these key components. 4. Why is it not possible to sharply split C3 components in the phase separator? Ans: This is the basic problem of the phase equilibrium factors associated to the intermediate components. Usually, phase equilibrium factors are highest for lighter components and lowest for the heavier components. But intermediate components such as C3s have phase equilibrium factors in between. Therefore, C3s get distributed between both vapour and liquid equally. This will be the case even with higher pressure and going for higher pressure is not economical as the pressurizing costs will be significantly. Joint initiative of IITs and IISc – Funded by MHRD Page 15 of 83 NPTEL – Chemical – Chemical Technology II 5. Why a tailing and topping still is required for ethylene production? Ans: The distillation column for separating ethylene from ethylene from C2 components needs to carry out a difficult separation. This is also due to the fact that the boiling points of C2 components is very close. Therefore, there needs to be two columns (indicating good number of trays). 6. Explain how extractive distillation enables the separation of butadiene? Ans: Dimethyl formamide (solvent) is fed to the distillation column fed with butadiene, butane and butylenes. The solvent interacts differently with the components and therefore adjusts the relative volatility of the mixture which was close to 1 previously. Thereby, the solvent forms a high boiling mixture at the bottom with butadiene and thereby enables the difficult separation of butadiene from the C4 compounds. Thereby, the solvent + butadiene is fed to a stripper which removes butadiene from the DMF. One important issue here is that the solvent does not form an azeotrope with the butadiene and is therefore, easy to separate. 7. When acetylene is not required, what process modifications will exist to the technology? Ans: When acetylene is not required, then the top product from C2 splitter (which is a mixture of acetylene and ethylene) is fed to a packed bed column and H to convert the acetylene to ethylene. Eventually, one does 2 not require the absorber-stripper technology for acetylene purification. References: Dryden C. E., Outlines of Chemical Technology, East-West Press, 2008 Shreve R. N., Austin G. T., Shreve's Chemical process industries, McGraw – Hill, 1984 Joint initiative of IITs and IISc – Funded by MHRD Page 16 of 83 NPTEL – Chemical – Chemical Technology II Lecture 16: Vinyl Chloride from Ethylene Introduction - In this lecture we study the process technology involved in the production of Vinyl Chloride from Ethylene - Vinyl chloride is produced in a two step process from ethylene o Ethylene first reacts with Chlorine to produce Ethylene dichloride o The purified Ethylene dichloride undergoes selective cracking to form vinyl chloride - We first present the process technology associated to Ethylene Chloride 16.1 Ethylene dichloride 16.1.1 Reactions - C H + Cl C H Cl 2 4 2 2 4 2 - Undesired products: Propylene dichloride and Polychloroethanes - Reaction occurs in a liquid phase reactor with ethylene dichloride serving as the liquid medium and reactants reacting the liquid phase - Catalyst is FeCl or Ethylene dibromide 3 16.1.2 Process Technology (Figure 16.1) Figure 16.1 Flow sheet of production of ethylene dichloride Joint initiative of IITs and IISc – Funded by MHRD Page 17 of 83 NPTEL – Chemical – Chemical Technology II - C H and Cl are mixed and sent to the liquid phase reactor. 2 4 2 - Here, the feed mixture bubbles through the ethylene dichloride product medium o - Reactor operating conditions are 50 C and 1.5 – 2 atms. - The reaction is exothermic. Therefore, energy is removed using either cooling jacket or external heat exchanger - To facilitate better conversion, circulating reactor designs are used. - FeCl traces are also added to serve as catalyst 3 - The vapour products are cooled to produce two products namely a vapour product and a liquid product. The liquid product is partially recycled back to the reactor to maintain the liquid medium concentration. - The vapour product is sent to a refrigeration unit for further cooling which will further extract ethylene dichloride to liquid phase and makes the vapour phase bereft of the product. - The liquid product is crude ethylene dichloride with traces of HCl. Therefore, acid wash is carried out first with dilute NaOH to obtain crude ethylene dichloride. A settling tank is allowed to separate the spent NaOH solution and crude C H Cl (as well liquid). 2 4 2 - The crude ethylene dichloride eventually enters a distillation column that separates the ethylene dichloride from the other heavy end products. - The vapour phase stream is sent to a dilute NaOH solution to remove HCl and produce the spent NaOH solution. The off gases consist of H , CH , C H and 2 4 2 4 C H . 2 6 16.1.3Technical questions 1. Provide an insight into the liquid phase guided gas phase reaction? Ans: The liquid phase acts as a resistance phase for the movement of various gases. The recirculator enables greater turbulence of the liquid phase stream. Thereby, using these mechanisms, the gases are allowed to react with one another and produce ethylene dichloride which gets dissolved in the liquid. 2. Why a water condenser followed by refrigeration is used when the single refrigeration can serve the purpose of cooling? Ans: This is an important question. Allowing only refrigeration enhances process costs drastically. Therefore, water is used to carry out partial condensation and then refrigeration, even though in principle, water condensation can be bypassed and reactor operation and stream contacting can be further optimized. Joint initiative of IITs and IISc – Funded by MHRD Page 18 of 83 NPTEL – Chemical – Chemical Technology II 3. Why do we need a settling tank after the acid wash unit associated to the crude ethylene dichloride? Ans: Typically, we observe HCL removal from vapour streams. In such case, the unit used is a scrubber or absorber. The gas/vapour is fed to the absorption column and is obtained as a gas. When a liquid is allowed for scrubbing, it is possible to obtain emulsions of the organic phase in the aqueous phase. Therefore, provide gravity settling mechanism should exist so as to separate the crude ethylene dichloride from the mixture emanating from the acid wash tank. 16.2 Vinyl chloride production 16.2.1 Reaction - C H Cl CH CHCl + HCl 2 4 2 2 - Charcoal is used as the catalyst - The reaction is a reversible gas phase reaction 16.2.2Process Technology (Figure 16.2) Figure 16.2 Flow sheet of production of vinyl chloride - Ethylene dichloride is initially vaporized using a heat exchanger fed with process steam - Ethylene vapors then enter a dryer that removes traces of water molecules - After drying, the vapors enter a pyrolysis furnace operated at 4 atm and 500 o C. The furnace is similar to a shell and tube arrangement with the gases entering the tube side and hot flue gas goes past the tubes in the shell side. - The product vapors eventually enter a quenching tower in which cold ethylene dichloride is used to quench the product gases and cool them. Joint initiative of IITs and IISc – Funded by MHRD Page 19 of 83 NPTEL – Chemical – Chemical Technology II - The gases from the quench tower then enter a partial condenser which produces HCl as a gas and the liquid stream consisting of vinyl chloride, unreacted ethylene dichloride and polychlorides. - The liquid stream from the quench tower as well as the condenser is fed to the vinyl still which produces the vinyl chloride product. The product is stabilized using a stabilizer as vinyl chloride is highly reactive without stabilizer. - The bottom product from the vinyl still is fed to a distillation column which separates the ethylene dichloride from the polychlorides. The ethylene dichloride vapors are recycled back to the cracking furnace and the ethylene dichloride liquid is sent to the quenching tower to serve as the quenching liquid. 16.2.3 Technical questions 1. Why ethylene dichloride is dried before entering the cracking furnace? Ans: To avoid the formation of other compounds during cracking. Vinyl chloride cracking is a very selective cracking that we wish to happen. The selective cracking needs very clean feed stock. 2. Why quenching is carried out? Ans: The selective cracking reaction is a reversible reaction. Therefore, by doing cold ethylene dichloride quenching, we are suppressing the backward reaction and ensuring that only vinyl chloride gets formed in good quantities. 3. Can heat integration be carried out in the process? Ans: IN principle it can be done but in reality no. The reason is that if quenching is not done immediately, then vinyl chloride can get converted back to the ethylene dichloride. Therefore, though there is a hot stream available, heat integration cannot be done due to prevalent process conditions. 4. Can a partial condenser be used in the last distillation column to serve for both quenching, distillation reflux and produce vapour for the ethylene dichloride? Ans: Yes, this arrangement will be excellent as all requirements in the process will be met by going for a partial condenser. But it all depends on the quenching tower requirements and hence if ethylene dichloride needs to be cooled more than its boiling point, then partial condenser will not serve the purpose. Joint initiative of IITs and IISc – Funded by MHRD Page 20 of 83

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