THERMAL PROCESSES » When a hydrocarbon is heated to a sufficiently high temperature thermal Cracking occurs. sometimes referred to as pyrolysis (especially when coalis the feedstock). When steam is used it is called steam cracking. Q We will examine two thermal processes used in refineries. 1. Coking 2. VisbreakingVISBREAKING. cont... > Visbreaking is essentially a mild thermal cracking operation at mild conditions where in long chain molecules in heavy feed stocks are broken into short molecules thereby leading to a viscosity reduction of feedstock. > Now all the new visbreaker units are of the soaker type. > Soaker drum utilizes a soaker drum in conjunction with a fired heater to.achieve conversion . > Visbreakingis a non-catalytic thermal process. > It reduces the viscosity & pour point of heavy petroleum fractions so that product can be sold as fuel oil. » It gives 80 - 85% yield of fuel oil & balance recovered as light & middle distillates. The unit produces gas, naphtha, heavy naphtha, visbreaker gas oil, visbreaker fuel oil (a mixture of visbreaker gas @ and vsibreaker tar).VISBREAKING cont... A given conversion in visbreaker can be achieved by two ways: * High temp., low residence time cracking: Coil Visbreaking. * Low temp., high residence time cracking: Soaker visbreaking. Reaction in visbreaking CH,-CH,-CH,-CH,-CH,-CH,-CH, —- CH,-CH,-CH=CH, + CH,-CH,-CH, Process Process conditions Visbreaking Mild thermal cracking (low severity) Mild (470-5000C) heating at 50-200 psig Improve the viscosity of fuel oil Low conversion (10%) to 4300F Residence time1-3 min Heated coil or drumVISBREAKING TYPES Fractionation [——* Gases |—+ Gasoline Vacuum _ oy 9 485°C sidue ‘Sbar [+ Gas oil Proheater Q Visbreaking is either coil breaking when the resid is |___. Residue A broken in the furnace coil & Fraconaten [——* Gases 2 4 — soaker breaking when soaked in reactor have the reactor for few minutes. ober| [——*Gasoline |—+Gas Oi Vacuum oy" cy 4ssc Residue Preheater |___. Residue 8 ° ype visbreaker. (B) Soaker type visbreakerCOIL VISBREAKING Q Q Feed residue feedstock is heated & then mildly cracked in the visbreaker furnace. Reaction temperature range from 850 to 900 °F & operating pressures vary from as low as 3 bar to as high as 10 bar. Coil furnace visbreaking is used & the visbroken products are immediately quenched to stop the cracking reaction. Quenching step is essential to prevent coking in the fractionation tower. Gas oil & the visbreaker residue are the most commonly used as quenching streams.SOAKER VISBREAKING PROCESS > Furnace operators at a lower outlet temperature & a soaker drum is provided at the outlet of the furnace to give adequate residence time to obtain the desired conversion while producing a stable residue product, thereby increasing the heater run & reducing the frequency of unit shut down for heater decoking. > Products from soaker drum are quenched & distilled in the downstream fractionators. Objective: To lower the viscosity of heavy residues under relatively milder cracking condition than the conventional cracking processes. v Feed Atmospheric residues — To get gasoline and diesel oil v Vacuum residues — To reduce viscosity Reaction: ¥ Splitting of C-C bond. Oligomerization & cyclisation to naphthenes of olefinic compounds. v ¥ Condensation of the cyclic molecules to polyaromatics. ¥ Side reactions: Foramation of H,S, thiophenes, mercaptans, phenolSOAKER VISBREAKING PROCESS cont.. Products: Y Cracked product contains gas, naphtha, gas oil and furnace oil, the composition of which will depend upon the type of feedstock processed. Y Typical yield pattern may be gas 1-2%, naphtha 2-3%, gas oil 5-7%, furnace oil 90-92%. Visbreaking Conditions: ¥ Inlet Temperature: 305-325°C (15-40 bar) v Exit: 480-500°C (2-10 bar) ¥ With soaking 440-460°C (5-15 bar) v Feed: 90°C pre-treated with VB tar to 335°C Visbreaking Furnace: Y¥ Convection zone — top — to thermal efficiency. v Radiation zone — bottom tubes. Y Avg. heat flow — 22-30 kw/m? (SOAKER VISBREAKING PROCESS cont... Soaking Drum » Soaking drum is used to lengthen the feed residence time so that the furnace can operate at lower temperature. v Soaker results in saving of energy because of the lower temperature with less coke tendency, larger gas oil yield Advantages: ¥ 15% reduction in fuel oil v Larger running time between two decoking operations. coke deposit rate 3-4 times slower than in conventional units. v Better selectivity towards gas and gasoline productivity.VISBREAKING WITH SOAKER ARCOMPARISON OF COIL AND SOAKER VISBREAKINGS » Coil cracking is described as a high temperature, short residence time route whereas soaker cracking is a low temperature, long residence time route. Vv The yields achieved by both options are in principle the same, as are also the properties of the products. v Both process configurations have their advantages & applications. > Coil cracking yields a slightly more stable visbreaker products, which are important for some feed stocks & applications. Vv It is generally more flexible and allows the production of heavy cuts, boiling in the vacuum gas oil range. » Soaker cracking usually requires less capital investment, consumes less fuel & has longer on-stream times.