With the continuous depletion in world oil reserves and increasing demand of petroleum products, the refiners are forced to process more and heavier crude .
The cost advantage of heavy crudes over light crudes has incentivized many refineries to process heavier crude, therefore increasing the heavy residue produced at a time when fuel oil demand is declining. Technological upgradation have been carried out at refineries which takes care of processing heavy crudes as well as maximizing value added products and stringent product quality requirements.
Some of the residue upgradation technologies is given in Table below.
Cracking Process:
Cracking of heavy residue is most commonly used method for upgradation of residues. This involves of decomposition of heavy residues by exposure to extreme temperatures in the presence or absence of catalysts.
A- Thermal Cracking: Cracking at elevated temperatures in the absence of catalyst.
Examples: Visbreaking, delayed coking, Fluid coking etc.
B- Catalytic Cracking: Cracking in presence of catalyst.
Examples: FCC , Hydrocracking, DCC.
A- Thermal Cracking Process: Thermal cracking process for upgradation of heavy residue has been used since long and still it is playing an important role in the modern refinery through:
1- Upgradation of heavy residue, and
2- Improving the economics of the refinery through the production of lighter distillate and other valuable product like low value fuel gas and petroleum coke.
Heavy residues are a mixture molecules consisting of an oil phase and an asphaltene phase in physical equilibrium with each other in colloidal form. Asphaltenes are high molecular weight, relatively high atomicity molecules containing high levels of metals.
Under condition of thermal cracking, hydrocarbons, when heated, decompose into smaller hydrocarbon molecules.
Table below shows the various thermal cracking process and process conditions
Visbreaking Process:
Visbreaking is 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.
• It is a non-catalytic thermal process.
•It reduces the viscosity and pour point of heavy petroleum fractions so that product can be sold as fuel oil.
• It gives 80 - 85% yield of fuel oil and balance recovered as light and middle distillates.
•The unit produces gas, naphtha, heavy naphtha, visbreaker gas oil, visbreaker fuel oil (a mixture of visbreaker gas oil and vsibreaker tar).
•A given conversion in visbreaker can be achieved by two ways:
1- Low temp., high residence time cracking: Soaker visbreaking.
2- High temp., low residence time cracking: Coil Visbreaking.
• Reaction in visbreaking:
1- Soaker Visbreaking Process:
The furnace operators at a lower outlet temperature and 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 and reducing the frequency of unit shut down for heater decoking. The products from soaker drum are quenched and distilled in the downstream fractionator. Process diagram for visbreaking with soaker is shown in Figure below:
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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