Simulation of a visbreaking unit

Simulation of a commercial visbreaking unit supports optimisation of the unit’s performance. The visbreaking unit utilises vacuum residue as a feed and converts it into fuel oil.

S Reza Seif Mohaddecy, Sepehr Sadighi, Omid Ghabuli and Mahdi Rashidzadeh
Research Institute of Petroleum Industry

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Article Summary

In this study, the visbreaking unit of Tehran refinery was simulated and then a parametric sensitivity analysis was carried out. KBC’s Petro-Sim simulator was used in this study. Initially, the simulator was validated using actual plant test runs and, after tuning, the simulations provided errors of less than 3%. Using the validated simulator, the sensitivity of the yield of fuel oil, gasoline and fuel oil viscosity to variations in furnace temperature (reaction temperature) was investigated. The validated simulator can be used to optimise the unit’s operating conditions, to obtain the required product specifications or to study possible changes in the feed conditions, such as the use of diluents.

Visbreaking is a non-catalytic 
thermal process that converts atmospheric or vacuum residues via thermal cracking to gas, naphtha, distillates and visbroken residue. Atmospheric and vacuum residues are typically charged to a visbreaker to reduce fuel oil viscosity and increase the distillate yield in the refinery. The process will typically achieve conversion to gas, gasoline and distillates of 10–50%, depending on the severity and feedstock characteristics. Visbreaking reduces the quantity of cutter stock required to meet the fuel oil specifications and, depending upon the sulphur specifications, can decrease fuel oil production by 20%. Additionally, this process can be attractive when it comes to producing feedstock for catalytic cracking plants.1 The process severity is controlled by the interchangeable operational variables (being essentially a first-order reaction) such as temperature and residence time.2

There are two types of commercial visbreaking units: the coil or furnace type3 and the soaker process. The coil visbreaker is operated at high temperatures (885–930°F, 473–500°C) and low residence times (one to three minutes), while in a soaker unit, by adding an adiabatic drum after the coil furnace, the product is held for a longer time so that the coil is kept at a relatively lower temperature (800–830°F, 427–443°C). Therefore, the heater duty and, in turn, the fuel consumption is only 70% of that for the coil visbreaking process.4 Worldwide, about 200 visbreaking units are in operation, and Europe alone accounts for about 55% of the total visbreaking capacity.4 Process flows of coil and soaker units are shown in Figures 1 and 2.

The product yields and properties are similar, but the soaker operation, with its lower furnace outlet temperatures, has the advantages of lower energy consumption and longer run times before having to shut down to remove coke from the furnace tubes. Run times of 3–6 months are common for furnace visbreakers, and 6–18 months is usual for soaker visbreakers. This apparent advantage for soaker visbreakers is at least partially balanced by the greater difficulty encountered in cleaning the soaking drum.5

To effectively design and perfect the control of any process, a simulation of the process is needed to predict product yields and qualities against variables such as space velocity and temperature. The aim of this research was to develop a simple yield predictor model, according to a process simulation, to predict the products with the highest added value — gas, LPG, gasoline, diesel and visbroken fuel oil — in a commercial soaker unit. The main advantage of this work is the investigation of the influence of operating conditions on the yield of products such as LPG and gasoline. The soaker visbreaking unit of the Tehran refinery has been simulated, and the effects of operating variables on the yield and quality of products have been studied.

Process description
The vacuum residuum, which is stored in two tanks at 93°C, is charged to the unit. It picks up heat from the partly cooled product in the cold charge heat exchanger and accumulates in the charge surge drum. The charge from the surge drum splits and goes through two parallel coils of the heater. The flow through each coil is on flow control. In the hip section of each coil is a steam injection point. The visbreaking furnace is constructed in two sections, which are fired independently.

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