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• Luis Enrique Leyva Ovalle

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Training Services

Vacuum Unit Also called “Vacuum Distillation Unit” (VDU)

EDS 2004/VDU-1

1

Outline „

Introduction

„

Design Topics

„

Design Examples

„

Operating Tips

„

Optimization, Revamps

EDS 2004/VDU-2

2

Introduction „

Vacuum Unit - Why Do We Need One?

„

What Is A Vacuum Unit?

„

How Does It Work?

„

How Is It Designed?

EDS 2004/VDU-3

3

Crude Oil Mixture Boiling Point F C < 30

30 years ago were generally not interested in maximum VGO yield. The economics of refinery operation at the time did not demand it, and operating the heater and flash zone of the vacuum unit at lower temperatures would minimize coking, leading to long run lengths with minimal operator attention. The current economic climate demands maximum VGO yield. This pushes design and operations to the limit of cracking. Greater care is needed in design and operation to still yield long run lengths.

9

Cracking Depends On:

EDS 2004/VDU-10 EDS-R01-3719

A key point about cracking and coke laydown. Cracking is a time AND temperature relationship.

10

Operating Conditions „

Flash Zone Pressure – 30 – 50 mmHg absolute

„

Flash Zone Temperature – Approximately 750ºF (400°C)

EDS 2004/VDU-11

2 of the key parameters in vacuum unit operation are the flash zone pressure and temperature. This two points have the greatest influence on VGO yield. They are limited by cracking and coke laydown.

11

Vacuum Unit Design

EDS 2004/VDU-12

12

Design Considerations „ „ „ „ „ „ „

Wet vs. Dry Design Level of Vacuum, Pressure Drop Number of Packed Beds Heater Transfer Line Flash Zone and Bottoms Section HVGO and Heat Removal Sections LVGO and Overhead Section

EDS 2004/VDU-13

13

Design Considerations „

Wet vs. Dry Design – Stripping Steam – Heater Coil Steam – With or without Precondenser

„

Level of Vacuum – 3 versus 4 Stage Ejectors – With or without Precondenser – Column pressure drop

„

Number of Packed Beds – Benefits of Intermediate Cut – Fractionation between HVGO and LVGO EDS 2004/VDU-14

14

Wet vs Dry „ „

Major Design Decision - once set, cannot be changed There really are 3 types - Dry, Wet with Precondenser, and Wet without Precondenser (which some call “Damp”)

EDS 2004/VDU-15

A key design consideration is whether the tower will be dry (no steam input at all) or wet. Steam can be injected in either/both of two locations - into the heater coils (to decrease heater coking) and in the bottom boot (to strip lighter materials from the VR). If steam is injected, another major decision is whether or not to include a precondenser in the design.

15

Typical Vacuum Distillation Unit - No Precondenser

EDS 2004/VDU-16 EDS-R04-3709

16

Typical Vacuum Distillation Unit - With Precondenser

EDS 2004/VDU-17 EDS-R04-3709

17

Wet vs. Dry „

„

Dry Advantages: Much lower utility consumption (no steam injected and less motive steam for the ejectors) Dry Disadvantages: – With no steam, distillate yield is limited by absolute

pressure – With no steam in the heater, coking is more of a concern – Steam is often needed in the stripping section to produce proper asphalt

EDS 2004/VDU-18

With no precondenser, the overhead vapor from the column is sent directly to the 1st stage ejector. Therefore, the load on this ejector is high.

18

Wet vs. Dry „

Wet Disadvantages: – Much higher utility consumption (steam injection

and more ejector motive steam) – Greater chance for water damage on startup „

Wet Advantages: – With steam, distillate yield is greater because steam

lowers hydrocarbon partial pressure, thereby enhancing vaporization and ultimate gas oil yield – With steam in the heater, coking is less of a concern – Steam is often needed in the stripping section to produce proper asphalt EDS 2004/VDU-19

19

Precondenser vs. No Precondenser „

„

„ „

As the sketch shows, a precondenser will condense most of the steam that was injected into the heater and/or stripping section. However - this comes at a price. The condensed water in the precondenser exerts its own vapor pressure (at condensing temperature) - this means the overhead pressure cannot go lower than this pressure. At 95°F (35°C) water exhibits 41 mmHg abs pressure to this must be added the inert gas pressure. Without a precondenser, overhead pressure can be any value (although for a practical matter it is usually 20-25 mmHg at least). EDS 2004/VDU-20

With a precondenser in the design, the great majority of the steam coming overhead from the column is condensed before the 1st stage ejector. This significantly reduces the load on the 1st stage ejector. However, there is a water phase vapor/liquid equilibrium that exists at the precondenser. This exerts a vapor phase water partial pressure at the temperature that it has been condensed at. This effectively means the overhead pressure must be at least this value (actually, to this water partial pressure must be added the hydrocarbon and air partial pressures). This means that a design with a precondenser will have a higher top pressure, and thus a higher flash zone pressure as well. The only way to make up for these higher pressures is to increase the amount of steam injected in order to lower the hydrocarbon partial pressure (which partially reduces the benefit of having a precondenser).

20

Precondenser vs. No Precondenser „ „

End Result - For maximum lift (deep cut) do not use a precondenser. Precondensers will be more of a problem in hot climates - if cold cooling water is available most of the year, then the effect (while still present) will be small.

EDS 2004/VDU-21

Thus, if cold cooling water is available, the problems of having a precondenser are significantly reduced compared to hot climates.

21

Pressure Drop, Level of Vacuum „

Pressure Drop – From the column top to the flash zone, typical pressure

drop for a packed column ~10 mmHg – Each accumulator tray ~1 mmHg – Each packed bed ~1-2 mmHg – Each tray 3-5 mmHg (that’s why new columns only have packing - BIG pressure drop for old columns „

Level of Vacuum – Pressures vary greatly – Wet/No Precondenser UOP designs 35 mmHg flash zone – Wet/Precondenser often 45-65 mmHg flash zone – Dry often 20-30 mmHg in flash zone EDS 2004/VDU-22

22

LVGO Section

HVGO Section

Wash Section

Typical Vacuum Column Dimensions How do we determine column diameter?

Flash Zone

Bottom Boot EDS 2004/VDU-23 EDS-R02-3708

Some typical column dimensions are shown. The height of sections is determined from spacing requirements for spray headers. For packed sections, the calculation methods will be shown. We will now show how to calculate the diameter.

23

Vacuum Column Sizing Glitsch Method

C = VS

VS = C

ρv

ρl −ρv

ρl −ρv ρv

EDS 2004/VDU-24

The “C” factor is central to the calculation of vapor loading in distillation. For vacuum service, we use a design value (0.35 ft/sec) in order set the column diameter. For an operating column, it is important to calculate the C factor to determine how heavily vapor loaded the column is. It is a very good method to determine if more vapor can be put through the column.

24

Vacuum Column Sizing Glitsch where: c VS ρv ρl

= = = =

0.35(1) ft/s superficial vapor velocity, ft/s vapor density, lb/ft3 liquid density, lb/ft3

Note: (1) For new designs using grid and limited by re-entrainment, not flooding.

EDS 2004/VDU-25

25

Vacuum Column Sizing Old UOP Method

PV = 2.9

760 P

where: PV = permissible vapor velocity, ft/s P = pressure at a given point in column, mmHga

EDS 2004/VDU-26

This is an older criteria that UOP has used in the past. With a constant of 2.9, this results in larger diameters than the 0.35 C factor. For the rest of this session, only the C factor method will be used (the PV method was shown to let you know that there is more than one sizing method).

26

„

Heater Transfer Line

„

Flash Zone

„

Bottoms

EDS 2004/VDU-27

27

EDS 2004/VDU-28 EDS-R05-3709a

28

Heater Transfer Line

EDS 2004/VDU-29

The heater transfer line is another critical piece of equipment. From a process perspective, the vacuum heater transfer line sets the pressure drop between the furnace outlet and the flash zone. This is important because the real limit for the unit is the furnace outlet vaporization. A high pressure drop between the furnace outlet and the flash zone will result in a lower level of VGO yield. UOP typically designs for approximately 125 mmHg pressure drop between the furnace outlet and the flash zone. However, in reality this is a detail design layout issue, and the ultimate pressure drop is a function of the furnace type and header layout, and the plot plan location determines the length and number of bends in the transfer line.

29

Heater Transfer Line „

To Size Line – Use 70% of sonic velocity at column inlet conditions – Maximum 350 ft/s

„

„

Transfer line pressure drop will impact flash zone temperature and distillate recovery (temperature will drop from heater outlet to flash zone inlet). We want to minimize the pressure drop. Recommend “telescope” expanding diameter design for deep cut designs.

EDS 2004/VDU-30

This line is typically a very large diameter because of the need to stay away from sonic velocity. This is because, for example, if the flash zone is at 35 mmHg, the furnace outlet may be 160 mmHg. This means the volume of the gas will increase by a factor of about 4! This is a huge increase in absolute pressure terms. This is also why a telescoping design is recommended, both to save on materials and maintain a constant velocity. .

30

Heater Transfer Line  Cp  T  Vs = 223     Cv  M  where: Vs

= sonic velocity, ft/s

Cp/Cv = ratio of specific heats (use 1.0) T

= vapor temperature, ºR

M

= vapor mol weight

EDS 2004/VDU-31

This is the equation for sonic velocity.

31

Feed Distribution „

„ „

The Vapor-Liquid Separation is not an Equilibrium-Flash separation in reality - with the vapor velocity at 50-100% of sonic There is a large amount of entrainment Many different designs have been used in order to minimize entrainment – Tangential – Box – Many others

EDS 2004/VDU-32

Once the feed gets to the flash zone, the feed must be introduced to the column. The ideal is to get a pure vapor liquid separation. However, because of the very high velocities, and the very heavy materials we are dealing with, there is a large amount of entrainment that occurs. Thus, there has been a lot of interest in the vapor/liquid separation devices that can provide improved process performance. The industry has moved toward tangential vapor horns of various styles (90° vs. 360° for example). UOP has traditionally designed a “box” distributor. There are other proprietary designs available.

32

Vacuum Column Tangential Feed Distributor

EDS 2004/VDU-33 EDS-R00-3730

This is an illustration of a tangential vapor horn. The theory is that the the centrifugal force will help to separate liquid and vapor. The top is closed as is the side. Most distributors have closed ends. This forces all of the vapor and liquid to exit the distributor from the bottom, and thus forces the vapor to make a 180 degree turn to go up the column.

33

Vacuum Column Box Feed Distributor

EDS 2004/VDU-34 EDS-R02-3715

In this distributor, the top is closed and there are some holes on the side to allow some vapor to escape, however, as in the vapor horn, most of the vapor and liquid exit the distributor from the bottom.

34

Wash Section „

Wash Section is to De-entrain resid material. Any fractionation is incidental.

„

Column Diameter set by Glitsch C-Factor limit for packing 0.3 to 0.35 ft/s

„

Overflash or slop wax rate to be controlled by minimum packing wetting rate.

EDS 2004/VDU-35

The wash section is, for most refiners, a critical operational section. This section effectively controls the level of contaminants in the HVGO product. If the destination of the HVGO is a hydrocracker or hydrotreater, then there will normally be strict controls on concarbon (coke precursors) and metals (catalyst poisons). Most concarbon and metals are entrained in small liquid droplets that exit with the vapor from the flash zone (in other words, the flash zone does not produce a true equilibrium flash as produced by most process simulators). (However, it should be noted that when maximizing VGO yield (“deep cut operation”), the distillate actually can contain significant amounts of concarbon and metals). The primary purpose of the wash zone is to de-entrain residue from the vapors rising up from the flash zone.

35

Grid Bed

EDS 2004/VDU-36

The traditional packing in the wash zone is known as “Glitsch” grid (other vendors sell similar material). As can be seen in the picture, it is a thick walled grid with large open area. This has two advantages. The wash zone is a service highly susceptible to coking. The high level of open space means that even if some coking occurs, there is a decent chance that the packing will not close off (which would cause the unit to shutdown). In addition, the thick walled material also can better handle small amounts of coking than typical thin walled structured packing. In recent years, there has been a trend, when going for “deep cut” operation (maximum VGO), to place a “Glitsch” grid packing in the lower portion of the wash zone, and a thin walled structured packing at the top of the packing. This tends to give some fractionation to the wash zone, in addition to the deentrainment that grid can give.

36

Grid De-entrainment Efficiencies

Grid Depth, ft-in

Entrainment Removal Efficiency, Percent

2–0

90.0

2–6

95.0

3–0

98.0

3–6

99.0

4–0

99.5

5–0

99.9

6–0

99.99

EDS 2004/VDU-37

In general, if using only grid, it is best to use 6 feet of grid depth to ensure maximum entrainment removal. Despite the apparent effectiveness of grid at de-entrainment, some entrainment may still occur due to poor liquid or vapor distribution, especially at low wash oil rates.

37

Accumulator Tray Details

EDS 2004/VDU-38 EDS-R02-3722

The slop wax (and HVGO and LVGO) is typically withdrawn from an accumulator tray, a total trap tray. The slop wax withdrawal tray must be especially designed to minimize liquid residence time since this is heavy material at high temperature and is prone to coking. An accumulator tray must also be designed so as to minimize the pressure drop across the tray. Using the appropriate open cross sectional area will accomplish this.

38

Wash Section „

Critical Performance parameter TRUE Wash Oil Rate in slop wax = minimum of 0.2 gpm/ft2

„

Valuable to determine % entrainment in slop wax (by performing a concarbon or metals balance)

EDS 2004/VDU-39

A key performance parameter for wash section performance is to ensure that, at the bottom of the wash section packing, the flux rate of true wash liquid (clean HVGO) is at least 0.2 gpm/ft2. This will ensure that the entire cross section of packing is kept adequately wet and ensure adequate deentrainment. In order to calculate the true wash rate, it is necessary to calculate the percentage of entrainment that is contained in the slop wax. This can be done by performing a component balance with either concarbon or metals analysis in the resid, HVGO and slop wax, and the flow rates of the wash oil and slop wax.

39

„

Heavy Vacuum Gas Oil Section

„

Heat removal Sections

EDS 2004/VDU-40

40

Heavy Gas Oil Section „

Primary function is a a condensing/heat removal section.

„

Note that all withdrawals from the column (slop wax, HVGO, LVGO) come from Total Trap Trays - once vapor comes up through the chimney tray, it can’t go back down the column (except for wash oil, which is pumped from the HVGO draw to the slop oil section).

EDS 2004/VDU-41

The primary function of the remaining packed sections of the vacuum tower are to condense the rising vapors. The packed beds above the HVGO and LVGO withdrawal trays are contact condensers. Any vapor that exits the top of the HVGO packed bed will be condensed in the LVGO bed and withdrawn with the LVGO product.

41

Heat Removal Sections

Q = U “A” (LMTD)

where:

“A” = volume of packing

EDS 2004/VDU-42

The heat removal beds are designed in using an equation analogous to heat exchange design, where instead of and area “A” a packed volume is substituted.

42

EDS 2004/VDU-43 EDS-R04-3728

43

Heat Removal Sections U = h o = 4 2 1 C 0 .8 ( g p m A t ) where:

0 .5 8

C

= capacity factor at bottom of bed, ft/s gpm = liquid leaving bed, gal/min At = column cross sectional area, ft2

EDS 2004/VDU-44

The heat transfer coefficient “U” is calculated as shown above. However, it should be noted that there is a maximum value for design of 400. An example is shown in the design example session.

44

GRC Bed - Grid/Ring Combination Bed

EDS 2004/VDU-45 EDS-R02-3726

The heat removal section has been traditionally designed using a “Grid-Ring” combination bed. The high vapor loaded bottom section uses the same grid as in the wash zone. Once the vapor load is reduced below a C factor of 0.20-0.23, then random packing (rings) are placed in the top bed. The rings have greater surface area compared to the grid, but would be overwhelmed if placed in the bottom of the bed. Some more recent designs have used structured packing in this service, as have revamps.

45

GRC Bed

EDS 2004/VDU-46 EDS-R01-3711

46

Grid Layer

EDS 2004/VDU-47

47

Grid Bed

EDS 2004/VDU-48

48

Liquid Distribution „

As with any packed bed, liquid distribution is critical to packed bed performance.

„

There are 2 main types - spray nozzles and gravity fed distributors.

„

Spray nozzles have been the primary device.

„

Gravity fed distributors are gaining more acceptance with new designs.

EDS 2004/VDU-49

As mentioned above, there are 2 main types of liquid distributors: pressurized (spray nozzles) or gravity fed (drip type) distributors. Spray nozzles have been the traditional choice. Gravity fed devices (drip trays, others) are gaining more acceptance in the industry. New designs have overcome many of the previous performance issues. The specific vendors (Koch-Glitsch, Sulzer, etc) should be contacted to obtain specific recommendations and experience lists.

49

Spray Nozzle Assembly Layout

EDS 2004/VDU-50 EDS-R01-3717

When laying out spray nozzles, the nozzle assembly will require a certain amount of column height. It is important to allow for this height, both for the nozzle performance and for assembly and maintenance access.

50

Vacuum Column Spray Nozzles HVGO

(37 Spray Nozzles) CU-R00-31 EDS 2004/VDU-51

Typically, there are a large number of spray nozzles in any particular layout.

51

Spray Nozzles Spray Distributor Nozzle

Packing Holddown Grid

CU-R00-32 EDS 2004/VDU-52

The spray pattern should be set up to have spray pattern overlap between each nozzle. This is because we need to ensure that ALL of the packing remains wetted.

52

Liquid Distribution „

Improper liquid distribution leads to all sorts of problems.

„

Plugged spray nozzles lead to dry sections of the packed bed, which (especially in the wash zone) leads to a coked bed and a shutdown.

„

Normally a filter is installed near the spray nozzle inlet, and the line after the filter is made out of stainless steel to prevent rust from clogging spray nozzles.

EDS 2004/VDU-53

53

„

Light Vacuum Gas Oil Section

„

Overhead System

EDS 2004/VDU-54

54

LVGO Process Flow Vacuum Column

Ejector s PR

FR C

LC T I FRC LVGO to Blending

CU-R00-30 EDS 2004/VDU-55

The LVGO section is similar to the HVGO section, as you can see. One major difference is that there is no reflux (called wash oil for the HVGO) that is sent to a bed below. Actually, some refiners have a fractionation bed below the LVGO draw, and in this case reflux would be sent below the LVGO draw to this additional fractionation bed. Normally, the LVGO is only cooled against a utility (cooling water or air). This is because the temperature of the LVGO draw is dependent on the amount of light material slumped in the crude column. If the amount of crude slump is higher than design, then the LVGO draw temperature will be lower than expected. If a LVGO exchanged with another process (such as cold crude) this would mean the LVGO would not be sufficiently cooled. This would lead to a large quantity of hydrocarbon in the overhead system.

55

Vacuum Column Overhead „

Consists of multi sets of ejectors, condensers, and ejector overhead receiver

„

Provides the vacuum atmosphere necessary for the proper operation of the vacuum column

„

Provides for separation of the non-condensible gases, liquid hydrocarbon, and water

EDS 2004/VDU-56

56

Typical Vacuum Distillation Unit

EDS 2004/VDU-57 EDS-R04-3709

57

Vacuum Producing Equipment

EDS 2004/VDU-58 EDS-R04-3709b

This is a sketch of the vacuum overhead system. Steam (typically MP steam is sent at a controlled pressure to the ejectors. The outlet of the ejectors is sent to an ejector condenser. There is then a large barometric leg between the condenser outlets and the receiver. The receiver is a gas/water/hydrocarbon separator. A major issue in many refineries is what to do with the gas from this receiver. Traditionally, this gas sent sent to a furnace to be burned. However, this gas contains a large amount of H2S. Some refineries compress this gas so that it can be put in the fuel gas system, where the H2S can be removed.

58

Diagram of a Jet Ejector D Steam

A

C B

E

Compressed Vapors

Suction

IRP-R01-67 EDS 2004/VDU-59

The principles behind ejector operation are covered in another session.

59

Ejector Condensate Drum Liquid from Condensers Non Cond. Vapor

Oil to Slops

Gas to Heater

Water Outlet

CU-R00-33 EDS 2004/VDU-60

The ejector condensate drum allows simultaneous separation of oil/water mixtures. The oil spills over the weir on the left, while water is forced under the 1st baffle on the right, and over the second.

60

Non-condensables „

Air leakage

„

Gases produced in the heater and vacuum column due to thermal cracking

„

Dissolved non-condensable in the feed

„

Water of saturation in the feed

EDS 2004/VDU-61

The non-condensable gases that the ejector system must remove consist of air leakage, light ends produced by cracking reactions in the furnace, and water vapor that is in equilibrium with a water phase.

61

Non-condensable in Vacuum Columns

EDS 2004/VDU-62 EDS-R01-3710

This chart is used for design purposes to estimate the load on the ejector system. The use of the chart should be self explanatory.

62

Ejector Design Figure 1: Ejector Performance Curve 1.2

1000 Correction Factor

Ejector Inlet Pressure (mmHg)

1.1

Single Stage

1 0.9 0.8 0.7

100

0.6 0

50

100

150

200

250

300

350

400

Steam Pressure (psig)

2 Stage, Condensing

10

3 Stage, Condensing

4 Stage, Condensing 1 1

Add 2 % to Steam Consumption for Every 1.0 PSI Increase over 0.5 psig discharge

10

Steam Ratio (# Steam / # DAE)

100 EDS 2004/VDU-63

The plot above illustrates the effect of having different ejector stages. For the same vapor rate and overhead pressure, less steam is needed with more ejectors. The charts are based on the ratio of motive steam(ejector steam) to the “dry air equivalent” (DAE). The dry air equivalent is a calculation of the overhead vapor rate in terms of an equivalent rate of air A key point in ejector design and operation is the steam pressure. Ejectors are sensitive to the motive steam pressure. Once an ejector is designed for a certain steam level, it should always be operated at that steam level. Ejector performance degrades very quickly when moving away from the design pressure. This is also the reason why there is a pressure let down controller on the motive steam line (in order to maintain a constant pressure).

63

Vacuum Unit Design Examples

EDS 2004/VDU-64

64

Vacuum Unit Design Examples „ „ „ „ „ „

Column Sizing at the Flash Zone Column Sizing at the HVGO Draw HVGO Circulation Rate Column Sizing at the LVGO Draw and LVGO Circulation Rate Bottom Boot Sizing - Stripping Steam Rate Ejector Sizing

EDS 2004/VDU-65

65

Material Balance Mol Weight

(106 scfd) bpsd

Lb/h

Lb -mol/h

Volume Percent

Weight Percent

12.0

474

39,370

532,602

1124

100.00

100.00

–

–

36

(0.381)

1,589

41.9

Vac Naph

51.7

11.9

131

13

151

1.1

LVGO

33.5

12.0

289

5,397

67,894

234.8

13.7%

12.8%

HVGO

24.9

12.1

436

20,158

267,087

612.4

51.2%

50.2%

Slop Wax

17.7

12.2

630

1,197

16,631

26.4

3.0%

3.1%

Resid

14.3

12.2

761

12,497

179,405

235.5

31.7%

33.7%

Reduced Crude

ºAPI Gravity

UOP K

22.4

Estimated Yields Gas

-

0.3%

0.03%

0.03%

EDS 2004/VDU-66

This is the material balance we will use for the examples that follow.

66

Design Temperatures

EDS 2004/VDU-67 EDS-R02-3703

These are the temperature estimates that will be used for the examples that follow. These temperatures also make a good check on simulation data.

67

Column Size at Flash Zone 750ºF at 40 mmHga (0.774 psia) Vapors Rising

Lb/h

Mol/h

Gas

465

15.5

Steam

667

37.0

LVGO

17355

61.3

HVGO

68575

149.1

4890

8.9

91952

271.8

Slop Wax Total

EDS 2004/VDU-68

This is the detailed data we will use to size the column diameter at the flash zone. In general, one can do this calculation, for either design or operation, without having data from a simulation.

68

Column Size at Flash Zone – Molecular weight (mol wt) of vapors rising:

h  91952 lb        = 338.3 h    271.8 mol 

EDS 2004/VDU-69

69

Column Size at Flash Zone – The equation for c-factor in this service is:

VS = C

where: c VS ρv ρl

= = = =

ρl −ρv ρv

0.35 ft/s superficial vapor velocity, ft/s vapor density, lb/ft3 liquid density, lb/ft3 EDS 2004/VDU-70

70

Column Size at Flash Zone – Vapor density (ρv):

ρv =

( mol wt)( psia ) ( 10.73) ( o R )

= lb ft 3

where:

10.73 = Gas Constant ( R ) =

( psia ) ( ft 3 )

( lb mol) ( o R ) EDS 2004/VDU-71

71

Column Size at Flash Zone – Therefore, ρv at this point in the column is: o  338.3 lb   0.774 psia   lb mol - R  ρv =    = 0.0202 lb ft 3  o 3  lb mol   ( 750 + 460) R   10.73 psia - ft 

– The cubic feet per second (CFS) of vapors rising is: 3   91952 lb   h   ft CFS =     3600 s  0.0202 lb  = 1267 h     

EDS 2004/VDU-72

72

Column Size at Flash Zone – Therefore, Vs at this point in the column is:

Vs = 0 .35

(46.8 - 0.0202) = 16.9 ft s 0.0202

EDS 2004/VDU-73

This is the key value is the above calculations. The C factor chosen for design (in our case 0.35 ft/sec) effectively determines the column diameter.

73

Column Size at Flash Zone – The required column cross sectional area (CSA) is:

1267 ft 3   s  CSA =  = 75.0 ft 2    s  16.9 ft  – This results in a column diameter (ID) of:

4 ID = 75.0 ft 2   = 9.80 ft (2990 mm ) π 

EDS 2004/VDU-74

We will see in the next calculation (size at HVGO draw) that this in not the maximum diameter in the column.

74

Size at HVGO Draw

EDS 2004/VDU-75 EDS-R02-3705

The next area to check for the column diameter is at the HVGO draw. The envelope show streams into and out of the envelope in question.

75

Column Size at HVGO Draw 665ºF at 37 mmHga (0.716 psia): ºF

lb/h

Feed to Column

750

138975

459

Stripping Steam

413

667

1174*

HVGO Reflux

545

W

271

Btu/lb

106 Btu/h

Heat In

139642 + W

63.74 0.78 271(W) 64.52 + (271W)

Heat Out Resid from Column

710

47690

364

17.35

Net Slop Wax

725

4890

377

1.84

Steam

665

667

1340*

0.89

Gas

665

465

511

0.24

Net LVGO

665

17355

448

7.78

Net HVGO

665

68575

438

30.04

HVGO Reflux

665

W

438

438(W)

Vapors Rising to HVGO Draw

139642 + W

58.14 + (438W)

*Corrected to liquid at 60ºF EDS 2004/VDU-76

The above table shows the heat into and out of the HVGO envelope.

76

Column Size at HVGO Draw IN

OUT

64.52 ⋅ 10 6 Btu  271 Btu   W lb  58.14 ⋅ 10 6 Btu  438 Btu   W lb  + +  =   h h  lb   h   lb   h 

W = 38204 lb h

EDS 2004/VDU-77

77

Column Size at HVGO Draw Vapors Rising

lb/h

Mol/h

Gas

465

15.5

Steam

667

37.0

Net LVGO

17355

61.3

Net HVGO

68575

149.1

HVGO Reflux

38204

83.1

125266

346.0

Total

– Molecular weight of vapors rising:

h  125266 lb        = 362.0 h    346.0 mol  EDS 2004/VDU-78

78

Column Size at HVGO Draw – Vs at this point in the column is:

Vs = 0.35

45.74 − 0.0215 = 16.43 ft s 0.0215

– ρv at this point in the column is: o  362.0 lb   0.716 psia   lb mol - R  ρv =    = 0.0215 lb ft 3  o 3 lb mol    ( 665 + 460) R   10.73 psia - ft 

EDS 2004/VDU-79

If you compare the Vs at this point to the Vs for the flash zone, you will see this is a smaller value. This will require a larger cross sectional area than at the flash zone.

79

Column Size at HVGO Draw – The CFS of vapors rising is: 3   125266 lb   h   ft CFS =     3600 s  0.0215 lb  = 1621 h     

– The required column cross sectional area (CSA) is:

1621 ft 3   s  CSA =  = 98.7 ft 2   s 16.43 ft    – This results in a column diameter (ID) of:

4 ID = 98.7 ft 2   = 11.2 ft (3410 mm ) π  EDS 2004/VDU-80

As can be seen when compared to the flash zone diameter, this is significantly larger. In most cases, the size of a vacuum tower will be set at the point just below the HVGO draw. The flow below the HVGO section is always the largest in the column because of vaporization of wash oil and the lower pressure (compared to the flash zone) at this point. Immediately in the HVGO bed, vapor is condensed using subcooled pumparound liquids, so the vapor rate begins to go down.

80

HVGO Circulation

EDS 2004/VDU-81

Another major design activity required is to calculate the quantity of HVGO needed to condense the net HVGO product. In modern designs, the required quantity and heat duty is calculated using a process simulator.

81

HVGO Heat Removal Section „

Using the previous calculated vacuum column diameter and other necessary data, calculate the amount and type of packing required for the HVGO heat removal section

EDS 2004/VDU-82

What must still be calculated by hand is the height of packing required to transfer the heat that was removed by the HVGO pumparound.

82

HVGO Heat Removal Section – 11 feet, 6 inch ID = 103.9 ft2 column cross sectional

area (CSA) – Calculate the capacity factor (C) below the packing

1621 ft 3   0.0215  C= = 0.339 ft s  2  s  103.87 ft  45.74 − 0.0215

EDS 2004/VDU-83

Since we rounded up the diameter from 11.2 to 11.6 ft, the C factor is actually slightly less than 0.35.

83

HVGO Heat Removal Section – Calculate the capacity factor (C) above the packing

 444 ft 3   0.01157  C = = 0.064 ft s  2 s 122.72 ft 51 . 23 − 0.01157    – Based on the above capacity factor calculations, a

Grid/Ring Combination (GRC) bed is required in the HVGO section of the column

EDS 2004/VDU-84

The C factor above the bed is also calculated. As can be seen, a tremendous amount of vapor is condensed in the HVGO heat removal bed (the majority of the vapor generated in the flash zone).

84

HVGO Heat Removal Section – Calculate the overall heat transfer coefficient (U) using

Glitsch’s equation:

U = ho = 421(0.286 )

0.8

 704  103.87   

0.58

= 469 Btu h −ο F − ft 3

– Use 400 Btu/h - ºF-ft3 as a maximum practical U for

design.

EDS 2004/VDU-85

The method that UOP uses to size the heat removal bed is the Grid Ring Combination bed method (GRC). The key equation is shown above. The result of the equation is a U value that is based on volume instead of area. This U value can then be used to calculate a bed volume.

85

HVGO Heat Removal Section – LMTD is:

665

⇒

375

545

⇐

325

120

50 80ºF

EDS 2004/VDU-86

Since the HVGO heat removal bed is involved in heat transfer, a temperature difference is needed as a driving force. A log mean temperature difference (as is used in heat transfer calculations) is used in the heat transfer calculation.

86

HVGO Heat Removal Section – Calculate heat removed (Q) in the HVGO section

of the column

 151151 lb   ( 271 − 132) Btu  6 Q=  = 21.01 ⋅ 10 Btu h  h lb   

EDS 2004/VDU-87

The amount of heat removal can be from the HVGO that is condensed.

87

HVGO Heat Removal Section – Calculate volume of packing:

 21.01 ⋅ 106 Btu     h −o F − ft 3    h    80o F   400 Btu

  = 656.6 ft 3 

  ft 656.6 ft 3   = 6.32 ft required bed depth  103.87 ft 3  – Use 6 feet, 6 inch actual bed depth.

EDS 2004/VDU-88

88

Bed Composition

C factor, ft/sec

Bed Composition 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

C factor, ft/sec

0

1

2

3

4

5

6

7

Bed Height, ft

3 feet, 6 inch grid plus 3 feet, 0 inch rings if operation at turndown is not a prime concern. 3 foot, 0 inch grid plus 3 feet, 6 inch rings if operation at turndown is a prime concern. EDS 2004/VDU-89

At the bottom of the bed, the C factor is near 0.35. This portion of the bed requires the use of grid. As the vapor velocity decreases, random packing rings can be put in the bed (hence the name Grid-Ring Combination bed). The breakpoint is between a C factor of 0.20-0.23.

89

HVGO Circulation Rate ºF

lb/h

Btu/lb

Feed to Column

750

138975

459

Stripping Steam

413

667

1174*

HVGO Circulation

325

W

132

106 Btu/h

Heat In

139642 + W

63.74 0.78 132(W) 64.52 + (132W)

Heat Out Resid from Column

710

47690

364

Net Slop Wax

725

4890

377

17.35 1.84

Net HVGO

545

68575

271

18.58

HVGO Circulation

545

W

271

271(W)

Vapors Rising to LVGO Draw Steam

375

667

1202*

Gas

375

465

315

0.15

Net LVGO

375

17355

276

4.79

139642 + W

0.80

43.51 + (271W)

*Corrected to liquid at 60ºF EDS 2004/VDU-90

90

HVGO Circulation Rate

IN

OUT

6

6

64.52 ⋅ 10 Btu  132 Btu   W lb  43.51 ⋅ 10 Btu  271 Btu   W lb  + +  =   h h  lb   h   lb   h 

W = 151151 lb h

EDS 2004/VDU-91

91

LVGO Circulation Size at LVGO Draw

EDS 2004/VDU-92

The vapor load in the LVGO section is much lower than in the HVGO section. The diameter of this section must be reduced compared to the HVGO section. Most column are reduced in external diameter. A few columns keep the same outside shell diameter, but use an internal column inside the shell to process the LVGO. This is why most columns have a swaged section at the top.

92

LVGO Circulation Rate 375ºF at 33 mmHga (0.638 psia): 106 Btu/h

ºF

lb/h

Btu/lb

Steam

375

667

1202*

0.80

Gas

375

465

315

0.15

Net LVGO

375

17355

276

4.79

LVGO Circulation

110

W

23

23(W)

Heat In Vapors Rising to LVGO Draw

18487 + W

5.74 + (23W)

Heat Out Net LVGO

200

17355

70

1.21

LVGO Circulation

200

W

70

70(W)

Steam

115

667

1084*

0.72

Gas

115

465

175

0.08

Vapors to Ejectors

18487 + W

2.01 + (70W)

*Corrected to liquid at 60ºF EDS 2004/VDU-93

93

LVGO Circulation Rate

IN

OUT

6

6

5.74 ⋅ 10 Btu  23 Btu   W lb  2.01 ⋅ 10 Btu  70 Btu   W lb  + +  =   h h  lb   h   lb   h 

W = 79360 lb h

EDS 2004/VDU-94

94

Column Size Above LVGO Draw

Vapors Rising

lb/h

Mol/h

Gas

465

15.5

Steam

667

37.0

Net LVGO

17355

61.3

Total

18487

113.8

– Molecular weight of vapors rising:

h  18487 lb        = 162.5 h    113.8 mol  EDS 2004/VDU-95

95

Column Size Above LVGO Draw – Vs at this point in the column is:

Vs = 0.35

51.2 − 0.0116 = 23.25 ft s 0.0116

– ρv at this point in the column is: ο  162.5 lb   0.638 psia   lb mol - R  ρv =    = 0.01157 lb ft 3  ο 3 lb mol    ( 375 + 460) R   10.73 psia - ft 

EDS 2004/VDU-96

96

Column Size Above LVGO Draw – The CFS of vapors rising is:

 ft 3  18487 lb   h   CFS =   = 444     h    3600 s   0.01157 lb  – The required column cross sectional area (CSA) is:

 444 ft 3   s  2 CSA =    = 19.1 ft  s   23.25 ft  – This results in a column diameter (ID) of:

4 ID = 19.1 ft 2   = 4.93 ft (1940 mm ) π  EDS 2004/VDU-97

97

Bottom Boot Size „

Diameter is selected to give a liquid velocity of approximately 2 feet/minute based on total flow (net bottom plus quench)

„

Typical DP cell length used is 60 inches

„

Gives 2.5-5 minutes residence time based on net product

EDS 2004/VDU-98

The bottom boot typically contains 4-6 trays to assist in stripping the liquid that flows from the flash zone. There then is also is a boot to contain the residue before it is pumped out of the column. The residence time is minimized in order to avoid coking. There is also a cooled residue flow (quench) that is recirculated into the boot in order to reduce the temperature and avoid coking. In general the temperature must be quenched below 690F.

98

Bottom Boot Size – For this example:

 139* gal   ft 3   min  = 9.29 ft 2 required      min   7.48 gal   2 ft  – This results in a boot diameter of:

ID =

4 9.29 ft 2   = 3.44 ft ( 1050 mm ) π

– The final boot diameter can be set at 3 feet 6 inches ID

(1070 mm). * Refer to Heat and Weight Balance sheets included in reference material. EDS 2004/VDU-99

99

Bottom Boot Size – Bottom Stripping Section utilizes large hole sieve decks – Boot Diameter based on requirements for sieve tray

design – Check the residence time across the 60 inch DP cell based on net resid (e.g.):

 9.62 ft 3   5 ft   min   7.48 gal      = 2.93 min    123* gal   ft 3   ft  

* Refer to Heat and Weight Balance sheets included in reference material. EDS 2004/VDU-100

Most recent designs utilize large diameter (25mm) hole sieve trays for the stripping section. Older designs have used shed decks.

100

Stripping Steam Rate  3,203 bbl Resid   5 lb Steam    bbl Resid  = 667 lb/h Steam = 37 mol/h 24 h    „

„ „

For a deep cut design, this level of steam stripping will result in oversized ejectors and large motive steam requirements Benefits of bottoms stripping will be reduced by coil steam injection For deep cut, recommend 2 lb Stm/bbl Resid  3203 bbl Resid   2 lb Steam     bbl Resid  = 267 lb/h Steam = 15 mol/h 24 h    EDS 2004/VDU-101

Another design value that must be set is the stripping steam rate. The values shown above are for a wet-no precondenser design. More steam would be required for a design with a precondenser.

101

Ejector Sizing Example Non-condensable „

Air Rate – At 37,000 BPD, Air Rate is 72 lb/hr – Note this calculation is independent of pressure

„

Cracked Gas Rate – At 37,000 BPD and 750oF, base rate is 720 lb/hr – Correction for 11.9 UOP K is 1.25 – Net Rate is 900 lb/hr – Cracked gas Molecular Weight is 36

EDS 2004/VDU-102

The methods described earlier are used to calculate the load of air and cracked gases going to the ejector.

102

Ejector Performance „

Ejector steam – Large portion of unit operating costs – A function of gas rate and desired pressure

corrected for the following: — Offgas MW — Offgas Temperature — Steam pressure — Offgas % Non-condensable „ „

Curves are based on Dry Air Equivalents, which must be calculated from charts Method presented is independent of condenser cooling water temperature EDS 2004/VDU-103

103

Ejector Design Figure 1: Ejector Performance Curve 1.2

1000 Correction Factor

Ejector Inlet Pressure (mmHg)

1.1

Single Stage

1 0.9 0.8 0.7

100

0.6 0

50

100

150

200

250

300

350

400

Steam Pressure (psig)

2 Stage, Condensing

10

3 Stage, Condensing

4 Stage, Condensing 1 1

Add 2 % to Steam Consumption for Every 1.0 PSI Increase over 0.5 psig discharge

10

Steam Ratio (# Steam / # DAE)

100 EDS 2004/VDU-104

The above chart allows the user to calculate steam requirements given the number of ejectors, the required overhead pressure and the mass rate of Dry Air Equivalent (DAE). The foot note points out that ratios are based on a discharge pressure of 0.5 psig. Adjustments to the ratio are required if the discharge pressure is higher or lower.

104

Ejector Design Figure 2: MW Entrainment Ratio

Figure 3: Temp Entrainment Ratio

1.8

1

1.6

0.95

Air

Entraiment Ratio

Entraiment Ratio

1.4 1.2 1 0.8 0.6 0.4

Weight of gas Entrainment Ratio = Weight of air

0.2

0.9 0.85

Steam

0.8 0.75 0.7

Entrainment Ratio =

Weight @ Temp Weight @ 70

0.65

0 0

20

40

60

80

100

Molecular Weight

120

140

0

100 200 300 400 500 600 700 800 900 1000

Gas Inlet Temperature (Deg F)

EDS 2004/VDU-105

Dry air equivalent is calculated based on the Molecular weight and temperature of the overhead gas. Generally the overhead is a mixture of condensable hydrocarbons, noncondensable gases, and steam. The user can calculate the dry air equivalents for each of these subcomponents and sum the total dry air equivalent rate.

105

Ejector Design Figure 4: % Condensable Correction 1.8 1.6

CORRECTION FACTOR

1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

20

40

60

80

100

% NON CONDSENSABLE EDS 2004/VDU-106

If a high portion of the gas is condensable, ejectors will function better as the gas rate will decrease as the materials is condensed, improving the motive steam to offgas ratio. To adjust for condensation, a correction factor can be determined based on the % noncondensables

106

Ejector Design - Example Calculate Motive Steam Requirements for the vacuum overhead system. Given: Non condensible Rate - 2652 lb/hr MW - 31.3 Condensible Rate - 18 lb/hr MW - 140 Steam Rate - 8336 MW - 18 Vacuum - 25 mmHg Discharge - 1.5 psig Temp - 120ºF Steam Pressure - 150 psig Equivalent Air Non-Cond + Cond MW - 32.8 MW ER (Fig 2) - 1.06 Temp ER (Fig 3) - 0.985 Steam MW ER (Fig 2) - 0.81 Temp ER (Fig 3) - 0.982 Eq. Air Mass Flow =

(2652 + 18) 8336 + (1.06)(0.985) (0.81)(0.982)

= 13037 lb/hr of Equivalent Air EDS 2004/VDU-107

As noted, the first step to calculating motive steam requirements is to determine the off gas rate and convert it into a dry air equivalent. The three components of offgas are condensable hydrocarbons (referred to as condensables), non condensable gas, and steam. The temperature and molecular rate and temperature factor for all these components can be determined from figures 2 and 3.

107

Ejector Design - Example Motive Steam Requirements Two Stage Base Ratio (Fig 1)

Three Stage

6.8

4.5

0.89

0.89

Discharge Modifier

1.02

1.02

Non Cond Corr - 24% (Fig 4)

0.65

0.72

Steam Pressure

Corr*

Two Stage Ratio = 6.8*0.89*1.02*0.65 = 4.01 lb steam/lb air Steam Rate = 4.01*13037 lb air/hr = 52,310 lb/hr of steam Three Stage Ratio = 4.5*0.89*1.02*0.72 = 2.94 lb steam/lb air Steam Rate = 2.94*13037 lb air/hr= 38,345 lb/hr of steam Installing a third stage will save 13,965 lb/hr of steam or $1039 / day * Based on 140 psig - 10 psi Control valve drop EDS 2004/VDU-108

Using figure one we can determine that to get to an overhead pressure of 25 mmHg we need 6.8 # motive steam/# DAE for two stages and 4.5 # motive steam/# DAE for three stages. Corrections for steam pressure, discharge pressure and non condensible amount also need to be factored. Doing so gives the benefit for installing a third compressor stage. There are other methods available for calculating ejector performance, and most are based on the concept of Dry Air Equivalent ratios. Do you see any weaknesses of this methodology, especially in calculating operating ejector performance? Will it adequately reflect changes in operation from summer to winter?

108

Typical Vacuum Column Dimensions

EDS 2004/VDU-109 EDS-R02-3720

109

Vacuum Unit Operating Tips

EDS 2004/VDU-110

110

Operating Tips „

Flash Zone Pressure

„

Flash Zone Temperature

„

Circulating HVGO and LVGO

„

Slop Wax Draw

„

Steam

EDS 2004/VDU-111

111

Pressure „

Must Maintain Steady Pressure

„

Effect on Gas Oil Yield

„

Effect on Capacity

EDS 2004/VDU-112

As has been stated earlier, most refiners desire to maximize VGO yield. A low pressure maximizes gas oil yield. However, just as important is that the pressure in the column remain steady. Without pressure control in the overhead ejector system, maintaining a steady pressure in the flash zone will be impossible. A varying pressure in the flash zone often leads to poor HVGO and VR quality, and a large amount of quality giveaway in order to compensate.

112

Temperature „

Increases Gas Oil Yield

„

Sign of Cracking if Temperature Increase Reduces Vacuum

„

Controlled at Heater Outlet

„

Bottoms to be Quenched to