VISBREAKING AND DELAYED COKING Content Visbreaking 1 Introduction

VISBREAKING AND DELAYED COKING

Content: Visbreaking 1. Introduction 2. Process Description 3. Purpose 4. Classification of Visbreaking 5. Coil visbreaking 6. Soaker visbreaking 7. Difference between Coil and Soaker Visbreaking 8. Problems encountered in visbreaking 9. Solutions of problems

Introduction: A visbreaking is a processing unit in oil refinery that converts atmospheric or vacuum residues via thermal cracking to gas, naphtha, distillates and visbroken residues. In other words, A visbreaker thermally cracks large hydrocarbon molecules in the oil by heating in a furnace to reduce its viscosity to produce small quantities of light hydrocarbons(LPG and Gasoline).

Purpose: 1. To reduce the quantity of residual oil produced in the distillation of crude oil. 2. To increase the yield of more valuable middle distillates(heating oil and diesel). 3. To reduce the pour point and viscosity of waxy residues used for blending with lighter fuel oil. 4. Viscosity is reduce by : a. Vaccum distillation b. Hydro skimming 5. Increase the proportion of middle distillates in the refinery output. Middle distillate is used as diluent with residual oils to bring their viscosity down to their marketable level.

Visbreaker Unit A schematic diagram of a Visbreaker unit ( Reference Visbreaking Chemical Engineering Process)

Ø Process Description: • The process name of visbreaker refers to the fact that the process reduces i. e. breaks the viscosity of the residual oil. • The process is mild form of thermal cracking that significantly lowers the viscosity of heavy crude oil without affecting the boiling point. • The process is non catalytic.

Vacuum residue fed to a visbreaker can be considered to be composed of the following: • Asphaltenes: Large polycyclic molecules that are suspended in the oil in a colloidal form. • Resins: Also polycyclic but of a lower molecular weight than asphaltenes • Aromatic hydrocarbons: Derivatives of benzene, toluene and xylenes • Paraffinic hydrocarbons: Alkanes Aromatic hydrocarbons Resins Vacuum residue fed Paraffinic hydrocarbons Asphaltenes

Classification of Visbreaking Coil Visbreaking Soaker Visbreaking

q Coil visbreaking: The term coil (or furnace) visbreaking is applied to units where the cracking process occurs in the furnace tubes (or "coils"). Charge is fed to the visbreaker heater where it is heated to a high temperature, causing partial vaporization and mild cracking. A schematic diagram of a Coil Visbreaker unit ( Reference Wikipedia)

The heater outlet stream is quenched with gas oil or fractionator bottoms to stop the cracking reaction. The vapor liquid mixture enters the fractionator to be separated into gas, naphtha, gas oil and visbroken tar. The visbroken bottoms are then blended with lighter materials (cutter stock) to meet fuel oil specifications. The fractionated visbreaker gas oil is often used as the cutter stock. A schematic diagram of a Coil Visbreaker unit (Reference Garry)

q Soaker visbreaking: Soaker visbreaking In soaker visbreaking, the bulk of the cracking reaction occurs not in the furnace but in a drum located after the furnace called the soaker. Here the oil is held at an elevated temperature for a pre determined period of time to allow cracking to occur before being quenched. The oil then passes to a fractionator. In soaker visbreaking, lower temperatures are used than in coil visbreaking. A schematic diagram of a Soaker Visbreaker unit (Reference Garry)

Coil Visbreaking vs Soaker Visbreaking Coil Visbreaking (Low Residence time Cracking) Soaker Visbreaking (high residence time cracking ) • Coil visbreaking is applied to the cracking • In this bulk cracking reaction occur not in the process units where cracking process occur in furnace but in a drum located after the a furnace tube. furnace. • The higher temperatures are used in coil visbreaking. • the lower temperatures are used in Soaker visbreaking. • . De coking: In this petroleum coke deposits in the tube of the furnace and leads to fouling or blocking of the tubes. • The same will occur in the drum of a soaker visbreaker, though the lower temperatures used in the soaker drum leads to fouling at a much slower rate.

Problems in visbreaking: 1. Fouling: It is the accumulation of unwanted material on solid surfaces to the detriment of function. . 2. Formation of coke: The formation of coke normally starts at temperatures >300°C, and is therefore a major problem. 3. Asphaltenes precipitation: Asphaltenes makes precipitation with i-octane. .

Solution of the problem: High pressure spiral heat exchanger(HPSHE): • Lower costs the unique self cleaning effect in an Alfa Laval high pressure spiral heat exchanger (HPSHE) can avoid fouling and ensure long operation times. • The HPSHE is designed for pressures up to 100 bar(g). • Lower internal volumes result in shorter residence times which helps avoid asphaltene precipitation. • There is no fouling, there is no extra heat required in the furnace. This leads to no extra fuel consumption required in the furnace and much less production downtime.

Content: Ø Delayed Coking 1. What is coking 2. Types of coking 3. Delayed coking 4. Process Description

What is coking? Ø Coking is a well known process in the refining industry, the objective of which is to upgrade heavy fractions and, more especially, the distillation residues by subjecting them to heat decomposition. Ø The residual products of the refinery form the feed for coking. Ø The liquid feed is converted into gas, naphtha, heating oil, gas oil and coke. Ø Coking is done to get either gas oil or coke as the main product. Ø The gas oil is used as the feed for catalytic cracking. Ø While the coke used as a fuel or in the production of carbon electrodes and chemicals.

Main types of coking processes. 1. Delayed coking – continuous thermal cracking process which generates coke and produces various distillates in fractionator from vacuum residue. Coke drum temperature remains around 415 °C – 450 °C. 2. Fluid coking continuous coking process where residue is sprayed on to fluidised bed of hot coke particles. Here cracking takes place at much higher temperature than delayed coking (temperature up to 565 °C). • Both are physical processes that occur at pressures slightly higher than atmospheric and at temperatures greater than 900 o. F that thermally crack the feedstock into products such as naphtha and distillate, leaving behind petroleum coke.

DELAYED COKING ØA delayed coking is a type of coking whose process consists of heating a residual oil feed to its thermal cracking temperature in a furnace with multiple parallel passes. ØDelayed coking is one of the unit processes used in many oil refineries. ØThis cracks the heavy, long chain hydrocarbon molecules of the residual oil into Coker gas oil and petroleum coke. ØDelayed Coking is the most commonly used carbon rejection process that upgrades residues to a wide range of lighter hydrocarbon gas and distillates through thermal cracking.

DELAYED COKING Ø The by product of delayed coking process is petroleum coke. Ø The goal for delayed coking operation is to maximise the yield of clean distillates and minimise the yield of coke. Ø Three operating parameters govern the yield pattern and product quality of Delayed Coking are – I) Temperature II) Pressure III) Recycle Ratio (RR)

Delayed coking

Process description Ø In delayed coking, two or more large reactors, called coke drums, are used to hold, or delay, the heated feedstock while the cracking takes place. Ø Residual oil from the vacuum distillation unit is pumped into the bottom of the distillation column called the main fractionator. Ø The residual oil by having it contact the hot vapours in the bottom of the fractionator. Ø From fractionator it is pumped along with the some injected steam into the fuel fired furnaces and heated to its thermal cracking temperature. Ø The injection steam helps to minimize the deposition of coke with in the furnace tubes. Ø The heated stream transferring to coking drum where the temperature and pressure are maintained at coking conditions so that stream decompose into coke and volatile components.

Ø The process is called delayed coking because in this process the coal is heated using a much more complex system , and it consists of multiple furnaces or coke drums. Ø The Coker fractionating system separates the volatile components generated in the coking drum into various lighter hydrocarbon streams and components. Ø After coking, the coke drum is full of coke and it is then removed using water jets. Ø Like other secondary processing units, coking play an important role in refinery economics depending on the type and cost of the crude oil run at a refinery. Ø Decoking Process for the removal of coke from a coking reactor of a series of coking reactors, comprising the projection of water under pressure at the coke via a water ejection device hanging from a flexible tube guided by a guide pulley movably mounted above the reactors for translation between each reactor of the series of reactors.

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