Refining-Petrochemicals-Chemicals-Engineering ———
PDVSA
Process Engineering Applied To Petroleum Refining Module 8: REFINING PROCESSES (2)
VISBREAKING UNIT PERFORMANCES CHARACTERIZATION
I - UNIT DESCRIPTION................................................................................................................. 1 II - MAIN PROPERTIES OF FEED AND PRODUCTS ................................................................... 2 1 2 -
Feed characteristics .....................................................................................................................2 Products characteristics ...............................................................................................................2
III - UNIT PERFORMANCES CHARACTERIZATION ..................................................................... 4 1 2 3 4 5 6 7 8
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Material balance and yields.......................................................................................................... 4 Sulphur balance ...........................................................................................................................4 Energy consumption.....................................................................................................................5 Conversion ...................................................................................................................................5 Viscosity reduction .......................................................................................................................6 Diluant or cutter saving.................................................................................................................7 Heavy fuel oil reduction ................................................................................................................ 8 Net distillate gain .......................................................................................................................... 8
IV - REAL EFFECT OF VISBREAKING UNIT IN HEAVY CUTS UPGRADING .............................. 9 V - VISBREAKING CONDITIONS................................................................................................. 10 1 2 -
Residence time...........................................................................................................................10 Temperature profile .................................................................................................................... 11
RA VIS - 00103_A_A - Rev. 1
17/01/2005
„ 2005 ENSPM Formation Industrie - IFP Training
1
I-
UNIT DESCRIPTION The simplified scheme of the unit is given in appendix 1, including main control loops and operating conditions. After preheating by the atm distillate and the visbroken vacuum resid in six exchangers, the feed is sent to the feed drum B 101. The heater F 101 has two radiation cells and one common convection cell. The feed is divided into four passes and goes across: – the convection section – the radiation heating section – the radiation cracking section The firing of each radiation section is regulated separately under control of the outlet temperature of each section. A condensate injection is made at the inlet of each pass of the radiation cracking section. At the outlet of the heater, the feed is directed to the soaker drum B 102. The soaker effluent is expanded and cooled by a distillate injection before entering the atmospheric fractionator C 101. The temperature of the flash zone of the atmospheric fractionator is regulated by an injection of quench. The cracked products are fractionated into five cuts: gas, naphtha, gas oil, distillate and atm visbroken residue. The naphtha of the overhead drum B 103 is routed most of the time to the naphtha hydrotreater. On tray 9, the gas oil is withdrawn and steam stripped. The light vacuum gas oil is added to the atm gas oil. A part of the gas oil blend is used as flushing oil on the equipments and instrumentation of heavy products lines. The non stripped atm distillate is routed with the vacuum gas oil to the catalytic cracking FCC. It feed also the atm fractionator pump around and the quench. The atmospheric resid, is sent under level control, to the vacuum distillation tower C 102. The noncondensables are incinerated in the heater F 101. The top side stream: light vacuum gas oil is used as reflux to condense the overhead vapors of the vacuum unit. The blend of light vacuum gas oil and atm gas oil constitutes the visbroken gas oil. The second side stream: vacuum gas oil is blended with the atm distillate and fed to the catalytic cracking FCC. It feeds also the second pumparound of the vacuum distillation tower. The vacuum resid preheats the feed and generates steam. A stream of cooled vacuum resid is recycled to the vac tower bottom to control its temperature. Before being sent to storage, the viscosity of the visbroken vacuum residue is adjusted with a cutter.
00103_A_A
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II -
MAIN PROPERTIES OF FEED AND PRODUCTS 1-
FEED CHARACTERISTICS Specific gravity
1,017
Viscosity (cSt) at 100°C
1850
ASTM distillation (°C) IP
490
5%
530
10%
545
30%
cracking
Conradson Carbon Residue (% wt)
11,9
Asphaltenes (%wt) C 5 insolubles
5,9
C 7 insolubles
4,6
Sulphur (%wt)
2-
4,3
PRODUCTS CHARACTERISTICS • Gas analysis
% wt
00103_A_A
H2S
15,0
H2
0,3
C1
15,8
C2
19,3
C3
25,2
C4
16,4
C5+
6,7
CO 2 + CO + N2
2,3
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• Other products main properties
Specific gravity ASTM distillation (°C)
Gasoline
Atmospheric gasoil
Light vacuum gasoil
Heavy atmospheric gasoil
Vacuum distillate
Vacuum residue
0,696
0,807
0,849
0,899
0,920
1,045
24
141
103
193
240
400
157
148
294
295
485
IP 5%
40
162
166
306
310
515
10%
71
172
214
335
365
570
30%
92
192
258
360
400
cracking
50%
108
220
297
395
430
70%
128
252
352
470
460
90%
130
262
> 370
505
475
95% FP
132
280
550
515
Sulphur (%wt)
0,92
1,4
2,1
2,6
2,9
at 20°C
1,7
3,4
at 50°C
1,1
2,0
9,5
14,7
3,2
4,1
4,7
Viscosity (cSt)
at 100°C at 150°C
00103_A_A
1110 91
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III - UNIT PERFORMANCES CHARACTERIZATION 1-
MATERIAL BALANCE AND YIELDS Material balance and yields are given on the following table.
PRODUCTS
FLOWRATE (t/h)
Uncondensables
0,45
Gas
2,55
Overall gas
3,0
2
Gasoline
4,65
3,1
Atmospheric gasoil
8,4
Light vacuum gasoil
2,4
Total gasoil
10,8
Heavy atmospheric gasoil
9,75
Vacuum distillate
15,3
Total distillate
25,05
16,9
Vacuum residue
105
70,7
TOTAL
148,5
100,0
FEED
148,5
2-
YiELD (%wt)
7,3
SULPHUR BALANCE The sulphur flowrate in feed is 4,3% of the feed flowrate (148,5 t/h): 6,4 t/h The sulphur flowrate in vacuum residue is 4,7% of the vacuum residue flowrate (105 t/h): 4,94 t/h So, about 75% of the feed sulphur remains in vacuum residue. Only 25% of the sulphur intake can be removed by visbroken products desulphurisation.
00103_A_A
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ENERGY CONSUMPTION In the operating conditions in appendix 1, consumption or production of fuel, steam and electricity are the followings: - fuel consumption in the furnace: 2,5 t/h - steam production: 11,5 t/h This production is equivalent to a fuel saving of 0,9 t/h - electricity consumption: 1,6 kWh This consumption is equivalent to 0,25 t/h of fuel. As a result, fuel consumption is 2,5 – 0,9 + 0,25 = 1,85 t/h for a 148,5 t/h intake. Fuel consumption is 1,25% of the feed, in the same order as for an atmospheric distillation unit.
4-
CONVERSION Conversion can be defined either as: - Gas + Gasoline yield Gasoil and distillates are supposed to be incorporated in heavy fuel oil pool Gas + Gasoline yield = 5,1 % - Gas + Gasoline + Gasoil yield This yield is the yield of 350+ products: Gas + Gasoline + Gasoil yield = 12,4 % - Gas + Gasoline + Gasoil + Distillates yield This yield is the yield of products which can be used as 350+ products or feedstock to upgrading units: Gas + Gasoline + Gasoil + Distillates yield = 29,3 %
00103_A_A
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5-
VISCOSITY REDUCTION • Viscosity blending rule Viscosity of a blend cannot be directly calculated by weighting the viscosities of the bases. It is necessary to use blending indices (BI) given by formulae or tables as a function of the viscosity of the bases and can be weighted to obtain the blending index of the blend. A table of blend indices can be found in appendix 2. Example of use: blending of 50%wt of a 10 cSt viscosity base with 50% wt of a 90 cSt base: Blending index corresponding to a 10 cSt viscosity base: 23,58 Blending index corresponding to a 90 cSt viscosity base: 32,87 Blending index of the blend: 50% x 23,58 + 50% x 32,87 = 28,22 To this blending index corresponds a 25,5 cSt value of the viscosity. • Viscosity of the 165+ products Calculation of viscosity of the 165+ products is given in the following table
Flowrate (t/h)
Viscosity (cSt) at 100°C
Blending Index
Flowrate x BI
Atmospheric gasoil
8,4
0,8
0
0
Light vacuum gasoil
2,4
0,8
0
0
Heavy atmospheric gasoil
9,75
3,2
15,73
153,4
Vacuum distillate
15,3
4,1
17,71
271
Vacuum residue
105
1110
39,28
4124,4
140,85
75
32,3
4548,8
Product
165+ products blend
• Viscosity reduction Viscosity reduction can be expressed by the ratio between feed viscosity and products blend viscosity: Viscosity ratio =
1850 = 24,7 75
or by the difference between the blending indices of feed and products blend: BI difference = 40,31 – 32,3 = 8
00103_A_A
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6-
DILUANT OR CUTTER SAVING Diluant saving is the difference between the quantities of diluant necessary to cut to 40 cSt the + viscosities of 100 t of feed on one hand and of the 165 products on the other hand. The cutter used is supposed to have a viscosity of 2 cSt at 50°C and of 1,05 cSt at 100°C. The corresponding value of blending index is: 3,91. The blending index corresponding to 40 cSt is 30,03. • Diluant D1 necessary to cut the viscosity of 100 t feed
Weight
Viscosity at 100°C
Blending Index
Feed
100
1850
40,31
Diluant
D1
1,05
3,91
Blend
100 + D1
40
30,03
The viscosity blending rule gives: 100 x 40,31 + D1 x 3,91 = (100 + D1 ) x 30,03 D 1 = 39,4 t • Diluant D2 necessary to cut the viscosity of the 165+ products +
With 100 t feed, the 165 products are 94,9 t since the Gas + Gasoline yield is 5,1 % and have a 75 cSt viscosity.
Weight
Viscosity at 100°C
Blending Index
165 products
94,9
75
32,3
Diluant
D2
1,05
3,91
Blend
94,9 + D2
40
30,03
+
The viscosity blending rule gives: 94,9 x 32,3 + D2 x 3,91 = (94,9 + D2 ) x 30,03 D 2 = 8,25 t
00103_A_A
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• Diluant saving Diluant saving is the difference between D1 and D2. D 1 - D 2 = 39,4 – 8,25 = 31,15 t for 100 t feed
7-
HEAVY FUEL OIL REDUCTION Heavy fuel oil reduction is the difference between the heavy fuel oil production from 100 t of feed and from the 165 + products. • Heavy fuel oil production from 100 t feed As shown above, 39,4 t of diluant are necessary to cut the viscosity of 100 t feed. Due to this blending, the heavy fuel oil production from 100 t feed is: Heavy fuel oil production from 100 t feed = 100 + 39,4 = 139,4 t +
• Heavy fuel oil production from the165 products +
From 100 t feed, the 165 products are 94,9 t and the diluant is 8,25 t. The heavy fuel oil production is: +
Heavy fuel oil production from the165 products = 94,9 + 8,25 = 103,15 t • Heavy fuel oil reduction The heavy fuel oil production from 100 t feed is reduced of: Heavy fuel oil reduction = 139,4 – 103,15 = 36,25 t for 100 t feed The heavy fuel oil reduction is the sum of the 165 + conversion and of the diluant saving.
8-
NET DISTILLATE GAIN The Net Distillate Gain is the sum of valuable distillates produced by the visbreaking unit. • Liquefied Petroleum Gas LPG are about 40% of the produced gas. For 100 t feed, LPG are 0,8 t. • Gasoline Gasoline yield is 3,1 %. • Diluant saving Diluant saving is 31,15 t for 100 t feed. • Net Distillate Gain Net Distillate Gain = 0,8 + 3,1 + 31,15 = 35,05 t for 100 t feed
00103_A_A
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IV - REAL EFFECT OF VISBREAKING UNIT IN HEAVY CUTS UPGRADING A simplified refining scheme of the visbroken products is given in appendix 3. The real effect of the visbreaking unit in refinery appears as a result of the material balance shown on the following table. For 100 t feed
Without visbreaker
With visbreaker
Gain
Gas
0
6
6
Gasoline
0
10,1
10,1
Light fuel oil
- 39,4
10,7 – 25 = - 14,3
25,1
Heavy fuel oil
139,4
97,4
– 42
100
99,2 ( 0,8 t coke )
Total
These results show: - gas and gasoline production much higher than visbreaker conversion - light fuel oil production due to diluant saving (major effect in value) - heavy fuel oil reduction (major effect in volume)
00103_A_A
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V-
VISBREAKING CONDITIONS 1-
RESIDENCE TIME • Definition Residence time definition is: Residence time (hour) =
Equipment volume (m3 ) Feed flowrate (m3 /h)
The volume flowrate changes along the equipment parts due to volume expansion by heating and to light cracked products in vapor phase. By convention residence time is calculated assuming the flowrate for feed at 15°C. • Residence time on cold feed With a specific gravity equal to 1,017 and a148,5 t/h mass flowrate, the volume flowrate is 146 m3/ h. According to the volumes of equipment parts given in the following table, the residence time in each part is calculated.
Volume (m3)
Residence time (h)
Residence time (mn)
7
0,05
3
Heating cell
10,8
0,075
4,5
Cracking cell
10,8
0,075
4,5
Soaker
56
0,383
23
C 101 bottom
6
0,1
3
Heater convection zone
The major part of the residence time is obviously in the soaker. • Effect of steam or condensate injection In fact, residence time is reduced by steam or water condensate injection at the entrance of the cracking cell coil. The water condensate flowrate in the visbreaking unit operating conditions is 400 kg/h. The steam specific gravity in the average temperature and pressure conditions can be read on the diagram given in appendix 4. Average operating conditions in cracking coil and soaker: Pressure: 9 bar abs. Temperature: 450°C Steam specific gravity: 2,6 kg/m3
00103_A_A
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The steam volume flowrate is 400 / 2,6 = 154 m3/ h. The steam volume flowrate being slightly equal to the feed volume flowrate, the steam injection leads to a division of residence time by a factor 2.
2-
TEMPERATURE PROFILE The temperature profile is given in appendix 5. On this diagram, the vertical scale graduated in temperature is in fact proportional to chemical reaction velocity. As a consequence, the area under the temperature profile is proportional to chemical reaction rate. Obviously, chemical reactions begins in the cracking coil of the heater but the major part occurs in the soaker where temperature decreases due to endothermic effect.
00103_A_A
© 2005 ENSPM Formation Industrie - IFP Training
BI
0.00 0.92 1.76 2.54 3.25 3.91 4.53 5.11 5.65 6.16 6.64 7.09 7.52 7.93 8.32 8.69 9.04 9.38 9.71 10.02 10.32 10.60 10.88 11.15 11.40 11.89 12.34 12.77 13.17 13.55 13.91 14.25 14.58 14.88 15.18 15.46 15.73 15.98 16.23 16.46
V (cSt)
0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5
0.92 0.84 0.78 0.71 0.66 0.62 0.58 0.54 0.51 0.48 0.45 0.43 0.41 0.39 0.37 0.35 0.34 0.33 0.31 0.30 0.28 0.28 0.27 0.25 0.49 0.45 0.43 0.40 0.38 0.36 0.34 0.33 0.30 0.30 0.28 0.27 0.25 0.25 0.23
D
3.5 3.6 3.7 3.8 3.9 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10 11 12 13 14 15 16 17 18 19 20
V (cSt)
16.46 16.69 16.91 17.12 17.33 17.52 17.90 18.25 18.57 18.88 19.18 19.46 19.72 19.97 20.21 20.44 20.65 20.86 21.06 21.26 21.44 21.87 22.27 22.64 22.97 23.29 23.58 23.58 24.11 24.58 25.00 25.39 25.73 26.05 26.35 26.62 26.88 27.11
BI 0.23 0.22 0.21 0.21 0.19 0.38 0.35 0.32 0.31 0.30 0.28 0.26 0.25 0.24 0.23 0.21 0.21 0.20 0.20 0.18 0.43 0.40 0.37 0.33 0.32 0.29 0.00 0.53 0.47 0.42 0.39 0.34 0.32 0.30 0.27 0.26 0.23
D 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 55 60 65 70 75 80 85 90 95 100 110 120 130 140 150 160 170 180 190 200
V (cSt) 27.11 27.55 27.93 28.28 28.59 28.88 29.15 29.39 29.62 29.83 30.03 30.21 30.39 30.56 30.71 30.86 31.21 31.51 31.79 32.04 32.27 32.49 32.68 32.87 33.04 33.20 33.49 33.76 34.00 34.22 34.42 34.60 34.77 34.93 35.08 35.22
BI 0.44 0.38 0.35 0.31 0.29 0.27 0.24 0.23 0.21 0.20 0.18 0.18 0.17 0.15 0.15 0.35 0.30 0.28 0.25 0.23 0.22 0.19 0.19 0.17 0.16 0.29 0.27 0.24 0.22 0.20 0.18 0.17 0.16 0.15 0.14
D 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 600 700 800 900 1 000 1 200 1 400 1 600 1 800 2 000 2 500 3 000 3 500 4 000 4 500 5 000 6 000 7 000 8 000 9 000 10 000
V (cSt) 35.22 35.48 35.71 35.92 36.11 36.29 36.45 36.61 36.75 36.88 37.00 37.12 37.23 37.34 37.44 37.53 37.95 38.30 38.59 38.85 39.07 39.45 39.76 40.03 40.26 40.46 40.88 41.21 41.49 41.73 41.93 42.11 42.42 42.67 42.89 43.08 43.25
BI 0.26 0.23 0.21 0.19 0.18 0.16 0.16 0.14 0.13 0.12 0.12 0.11 0.11 0.10 0.09 0.42 0.35 0.29 0.26 0.22 0.38 0.31 0.27 0.23 0.20 0.42 0.33 0.28 0.24 0.20 0.18 0.31 0.25 0.22 0.19 0.17
D 10 000 12 000 14 000 16 000 18 000 20 000 25 000 30 000 35 000 40 000 50 000 60 000 70 000 80 000 90 000 100 000 150 000 200 000 250 000 300 000 400 000 500 000 600 000 700 000 800 000 900 000 1 000 000 1 500 000 2 000 000 3 000 000 4 000 000 5 000 000 6 000 000 7 000 000 8 000 000 9 000 000 10 000 000 15 000 000 20 000 000
V (cSt) 43.25 43.53 43.77 43.97 44.15 44.30 44.63 44.89 45.10 45.29 45.59 45.83 46.03 46.21 46.36 46.49 46.99 47.34 47.60 47.82 48.14 48.39 48.59 48.76 48.90 49.03 49.14 49.56 49.85 50.25 50.53 50.74 50.91 51.06 51.18 51.29 51.38 51.74 51.99
BI 0.28 0.24 0.20 0.18 0.15 0.33 0.26 0.21 0.19 0.30 0.24 0.20 0.18 0.15 0.13 0.50 0.35 0.26 0.22 0.32 0.25 0.20 0.17 0.14 0.13 0.11 0.42 0.29 0.40 0.28 0.21 0.17 0.15 0.12 0.11 0.09 0.36 0.25
D
— Appendix 2 —
VISCOSITY BLENDING INDEX (weighting by mass fraction)
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