Simulation of a visbreaking unit

T he visbreaking unit utilises vacuum residue as a feed and converts it

into fuel oil. In this study, the visbreaking unit of Tehran refin-ery 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 vali-dated simulator, the sensitivity of the yield of fuel oil, gasoline and fuel oil viscosity to varia-tions in furnace temperature (reaction temperature) was investigated. The validated simulator can be used to opti-mise 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 visbro-ken residue. Atmospheric and vacuum residues are typically charged to a visbreaker to

Simulation of a commercial visbreaking unit supports optimisation of the unit’s performance

S Reza Seif Mohaddecy, SepehR Sadighi, oMid ghabuli and Mahdi RaShidzadehResearch Institute of Petroleum Industry

www.digitalrefining.com/article/1000396 PTQ Q2 2011 1

Furnace

Feed480ºC

Quench

Fractionator

Gas oil stripper

Overheaddrum

Gas

Gasoline

Gas oil

Visbroken residue

CW

figure 1 Coil visbreaker

Furnace

Soakerdrum

Feed450ºC

Quench

Fractionator

Gas oil stripper

Overheaddrum

Gas

Gasoline

Gas oil

Visbroken residue

CW

430ºC

figure 2 Soaker visbreaker

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 charac-teristics. 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 proc-ess can be attractive when it comes to producing feedstock for catalytic cracking plants.1 The process severity is control-led by the interchangeable operational variables (being essentially a first-order reaction) such as temperature and resi-

2 PTQ Q2 2011 www.digitalrefining.com/article/1000396

dence 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 visbreak-ing 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 prop-erties 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 clean-ing the soaking drum.5

To effectively design and perfect the control of any proc-ess, a simulation of the process is needed to predict product yields and qualities against vari-ables such as space velocity and temperature. The aim of this research was to develop a simple yield predictor model, according to a process simula-tion, 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 influ-ence of operating conditions on the yield of products such as LPG and gasoline. The soaker visbreaking unit of the Tehran

Variable ValueNumber of tubes 128Number of convection tubes 76Number of radiation tubes 52Tube length, m 18.745Outside diameter, m 0.114

Specifications of the coil of the visbreaking unit

Table 1

Variable ValueOutside diameter, m 2.405Length, m 16.5

Specifications of the soaker of the visbreaking unit

Table 2

Variable ValueFeed rate, kg/hr 13 2500Feed density, kg/m3 1006Feet temperature, °C 93Feed pressure, bar 11.89distillation analysis (aSTM d1160)IBP, °C 2035 vol%, °C 40910 vol%, °C 45720 vol%, °C 50330 vol%, °C 54350 vol%, °C 585Nitrogen content, wt% 0.4Sulphur content, wt% 3.19Asphaltic content, wt% 5.1Kinematic viscosity (100°C), cSt 430Nickel content, ppm 53Vanadium content, ppm 135

Specifications of the feed

Table 3

Furnace Soaker

Str

ipp

er

Sta

bili

ser

Frac

tio

nato

r

Feed

Steam

Lightgas

Light gas

LPG

Tar

Gasoline

figure 3 Block flow diagram of visbreaking process

2 PTQ Q2 2011 www.digitalrefining.com/article/1000396

refinery has been simulated, and the effects of operating variables on the yield and qual-ity of products have been studied.

process descriptionThe 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.

After the coil furnace, the two hot streams converge in a trans-fer line, then the mixed product

Table 6

Variable ValueInlet temperature, °C 345.8Outlet temperature, °C 440.5Inlet pressure, bar 7Outlet pressure, bar 31Number of tubes 128Number of tubes (convection zone) 76Number of tubes (radiation zone) 52

Specifications of the furnace

Table 4

Variable ValueRate, kg/hr 150Temperature, °C 316Pressure, bar 44.82

Specifications of the injected steam

Table 5

Variable ValueFlow rate, barrel/day 901Density 0.001compositionMethane, vol% 36.9Ethane, vol% 24.38Propane, vol% 20.56Isobutene, vol% 4.94n-butane, vol% 5.03Isopentane, vol% 0.77n-pentane, vol% 0.52Hydrogen sulphide, vol% 6.91

Specifications of gas production

Variable ValueFlow rate, barrel/day 1222Density 0.744Sulphur, wt% 3.4distillation analysis (aSTM d86)IBP, °C 485 vol%, °C 6710 vol%, °C 7630 vol%, °C 11050 vol%, °C 14170 vol%, °C 16390 vol%, °C 18495 vol%, °C 190FBP, °C 201

Specifications of gasoline production

Table 7

VBFeed

VBsteam 1

FractionatorV-302

Furnace 301A

VBsteam 2

Fueloil

Water To Visbreaker heaterFurnace

301B

E301 E302

E306

Steam

C1 C2

Off gas

LPG

Gasoline

StripperV-303

StabiliserV-306

R

R

figure 4 Simulation of visbreaking unit at Tehran refinery

is fed into the soaker drum. A quench stream of cooled prod-uct is added on flow control, and the combined stream enters the flash section of the flash fractionators. In the flash section, operating at 80 psig pressure, much of the gas, gaso-line and distillate formed during the cracking process flashes off. To split some of the light gas content in the fuel oil and gaso-line products, stripper and stabiliser columns are used. A simplified process flow diagram

www.digitalrefining.com/article/1000396 PTQ Q2 2011 3

shown in Tables 3–8.As Figure 4 shows, off-gases

including C1 and C2, as well as LPG, gasoline and tar are the output streams from the visbreaking plant. It is possible to take the gas oil product from the stripper tower, but it is usually blocked so that the gas oil can be mixed as a cutter blend with the fuel oil.

A comparison of operating data from the Tehran refinery and from the simulation runs was made to evaluate the simu-lation of the visbreaking unit (see Tables 9 and 10). These results confirmed the ability of a simulation to predict the desired outputs.

influence of furnace outlettemperature on product flowsThe effect of increasing the

furnace outlet temperature on the flow rates of products at a constant inlet feed rate (132 500 kg/hr) and operating conditions was investigated. According to the results of this exercise (see Figures 5 and 6), increasing the furnace outlet temperature leads to a decrease in the rate of production of fuel oil and an increase in the rate of gasoline production.

The effect of increasing the furnace outlet temperature on the viscosity of fuel oil was also investigated and the results are shown in Figure 7.

conclusionOperating data from the Tehran refinery’s visbreaking unit was gathered to calibrate a simula-tion of the unit in Petro-Sim. Following confirmation of the results of the simulation, the effect of increasing the furnace outlet temperature on the rate of production of fuel oil and gasoline, and on fuel oil viscos-ity, was investigated. A sensitivity analysis for these values showed that increasing the furnace temperature leads to an increase in the gasoline production rate and a decrease in the fuel oil’s production rate and viscosity. These results and other constraints, such as prod-uct quality and furnace operating temperature, can be used to optimise the unit.

4 PTQ Q2 2011 www.digitalrefining.com/article/1000396

of this configuration is shown in Figure 3.

The specifications of the coil and the soaker drum at the Tehran refinery are shown in Tables 1 and 2. The output product from the soaker drum is quenched by the cooled prod-uct to prevent more cracking reactions after the soaker and so inhibit coke formation. The combined stream is transferred to the fractionation tower and side strippers to separate the visbreaking products.

process simulation andvalidationPetro-Sim can simulate catalytic and non-catalytic processes on an industrial scale.6 It can simu-late a visbreaking unit with or without a soaker drum and, in this study, it was used for the simulation and sensitivity anal-ysis of the Tehran refinery’s visbreaking unit.

The soaker-visbreaker unit was simulated as a case study (see Figure 4). This unit was designed to visbreak 20 000 b/d of a mixture of vacuum resid-uum and slop vacuum gas oil, which are both taken from the vacuum tower. The composition of the fresh feed can vary slightly with time from start of run to end of run.

To prepare a simulation of the visbreaking unit, data were gathered during a test run of the Tehran unit. The data are

Table 9 Table 10 Table 11

Variable ValueFlow rate, barrel/day 18180Density 0.9995distillation analysis (aSTM d1160)IBP, °C 4525 vol%, °C 50210 vol%, °C 52820 vol%, °C 55930 vol%, °C 584Sulphur content, wt% 3.4Asphaltic content, wt% 8.3Kinematic viscosity (100°C), cSt 80Nickel content, wt% 0.004Vanadium content, wt% 0.0153

Specifications of fuel oil production

Table 8

Variable Simulation actual Rate, barrel/day 18 190 18 180Hydrogen sulphide, vol% 3.1 3.4Kinetic viscosity (100°C), cSt 80.23 79

comparison of fuel oil product between actual data and

simulation results

Variable Simulation actual Rate, barrel/day 1230 1222Hydrogen sulphide, vol% 3.322 3.4

comparison of gasoline product between actual data and simulation

results

Variable Simulation actual Rate, barrel/day 887.8 901Hydrogen sulphide, vol% 6.57 6.91

comparison of gas product between actual data and

simulation results

4 PTQ Q2 2011 www.digitalrefining.com/article/1000396

Since the simulation showed high accuracy when compared with real operating data, the results of an optimisation based on variations in operating conditions and feed have proved to be practical and acceptable.

References 1 Benito A M, Martinez M T, Fernandez I, Miranda J L, Visbreaking of an asphaltenic coal residue, Fuel, 74, 1995.2 Kataria K L, Kulkarni R P, Pandit A B, Joshi J B, Kumar M, Kinetic studies of low severity visbreaking, Ind. Eng. Chem. Res., 43, 2004.3 Wiehe I A, Process Chemistry of Petroleum Macromolecules, CRC Press, 2008.4 Joshi J B, Pandit A B, Kataria K L, Kulkarni R P, Sawarkar A N, Petroleum residue upgrading via visbreaking: a review, Ind. Eng. Chem. Res., 47, 2008.5 Upgrading Process of Heavy Oil, JCCP Technical Training Course, Jun 2005.6 Petro-Sim User Guide, KBC Advanced Technologies, KBC Profimatic.

S Reza Seif Mohaddecy is a Senior Researcher in the Catalytic Reaction Engineering Department at the Catalyst Research Centre, Research Institute of Petroleum Industry (RIPI), Tehran, Iran. Email: Seifsr @ ripi.irSepehr Sadighi works in the Faculty of Chemical and Natural Resources Engineering, University of Technology, Johor Bahru, Malaysia.omid ghabuli is a Senior Researcher in the Catalyst Synthesis Department, Catalyst Research Centre, RIPI.Mahdi Rashidzadeh is Head of the Catalyst Research Center, RIPI.

www.digitalrefining.com/article/1000396 PTQ Q2 2011 5

dp

b,

etarw

olF

Temp, ºF

figure 5 Sensitivity of produced fuel oil vs the furnace outlet temperature

1200

1300

1100

814 816 818 820 822 824 826

Flo

w r

ate

, b

pd

Temp, ºF

1000

figure 6 Sensitivity of produced gasoline vs furnace outlet temperature

79.85

80.05

79.95

79.75

79.65

79.80

80.00

79.90

79.70

79.60

814 816 818 820 822 824 826

Vis

cosi

ty,

cS

t

Temp, ºF

79.55

figure 7 Sensitivity of fuel oil viscosity vs furnace outlet temperature

linkS

More articles from the following categories: process Modelling & SimulationVisbreaking