02+Sizing+and+Rating+of+Trays+Advanced+Distillation+With+Aspen+Plus

Advanced Distillation with Aspen Plus

Sizing and Rating of Trays and Packing

Sizing and Rating of Trays and Packing Advanced Distillation with Aspen Plus

© 2002 AspenTech. All Rights Reserved.

Lesson Objectives • Learn about: - Tray Efficiencies - Sizing (Design) of Trays and Packing - Rating of Trays and Packing



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Aspen Technology, Inc.



RadFrac Efficiencies • Efficiency type - Overall efficiency - Vaporization efficiencies - Murphree efficiencies

• Can be specified - Overall - By stage - For column sections - For individual components


Overall Efficiency • Direct conversion of actual trays to theoretical plates. • Not directly input to model; instead Overall Efficiency sets the number of stages in the RadFrac block. • Example: - Column with condenser, reboiler and 30 trays - At 70% efficiency, 30 trays = 21 theoretical stages - Add stages for condenser and reboiler => 23 total

• Most common, but least flexible method.



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Vaporization Efficiency • At equilibrium: • With efficiency:

y

i

y

i

Ki ix i K i ix

• Efficiency should be different for different components. • As decreases, the vapor pressure must increase which means that the temperature must increase. • Using vaporization efficiency alters the T profile and this may alter relative volatility. • In general it is better to use Murphree efficiencies.


Murphree Efficiency

Eff

M i, j

y

y i, j i, j 1

y K i, j x i, j i, j 1

• Strictly, this is a Murphree vapor efficiency. • Subscripts are i for components and j for stages. • Many correlations exist for Murphree efficiencies



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Specifying Efficiencies in the Column • RadFrac Efficiencies Options sheet • Do not specify efficiencies in both the column and in the tray rating forms


The Problem with Efficiency • Efficiencies vary enormously - they can be under 10% or over 100%. • Efficiencies vary with composition, with purity, and with location in the column. • Efficiencies are different for different components.

» Next to physical properties, this is the most important thing to know to successfully model a column.



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Determining Column Efficiency • Previous experience. - Be sure experience is comparable

• Generalized correlations. • Fit to plant data. - Use Data-fit to regress Murphree efficiency to data

• RateFrac can be used to calculate efficiencies. - RateFrac calculates actual separation - Efficiency backed out from calculated compositions

• Vendor packages. © 2002 AspenTech. All Rights Reserved.

HETP • Comments similar to those for tray efficiency - It is important - It is hard to know well

• RateFrac can be used to calculate HETP. - RateFrac calculates actual separation for specified height of

packing - Separation is converted to NTU, and NTU to HTU

• Use of vendor data is common.



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Sizing and Rating for Trays and Packing (1) • Aspen Plus has extensive capabilities to size, rate, and perform pressure drop calculations for trayed and packed columns. • The calculations are based on vendor-recommended procedures when available. When vendor procedures are not available, well-established literature methods are used. • You can use the computed pressure drop to update the column pressure profile.


Sizing and Rating for Trays and Packing (2) • Rating Mode - Specify column section diameter and other tray or packing

details. - Model calculates performance and hydraulic information such

as flooding approach, downcomer backup, and pressure drop.

• Sizing Mode - Model determines column diameter to satisfy the flooding

approach or pressure drop* specified for each stage. The largest diameter is selected.

*pressure drop specification valid for packed columns only



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Sizing and Rating for Trays and Packing (3) • For sizing and rating calculations, column is divided into sections - Each section can have a different tray type, packing type, and

diameter. - The tray details can vary from section to section. - A column can have an unlimited number of sections. - You can size and rate the same section with different types of

trays and packing.


Sizing and Rating for Trays and Packing (4) • Aspen Plus calculates sizing and performance parameters such as: - Column diameter - Flooding approach or approach to maximum capacity - Downcomer backup - Pressure drop

• These results are based on: - Column loadings (vapor and liquid traffic) - Tray geometry or Packing characteristics - Transport properties



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Sizing and Rating for Trays and Packing (5) • Use the following Tray/Packing forms to enter specifications: - Tray Sizing - Tray Rating - Pack Sizing - Pack Rating

• These capabilities are available in the following column unit operation models: - RadFrac - MultiFrac - PetroFrac


Example: RadFrac Tray Rating Using the MEG column example, add tray rating calculations for the following configured trays: All Bubble Cap Trays One pass tray Tray Spacing: 2 ft Diameter: 5 ft Cap diameter: 3 in

FILENAME: EX1B-MEG-COL-TRAYRATE.BKP © 2002 AspenTech. All Rights Reserved.


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Tray Types • You can choose from the following five commonly-used tray types: - Bubble caps - Sieve - Glitsch Ballast

® ®

- Koch Flexitray

- Nutter Float Valve


Packing Types • Aspen Plus can model a variety of random packings. • You can also use any of the following types of structured packings: - Goodloe

® ®

- Glitsch Grid

- Norton Intalox Structured Packing - Sulzer BX, CY, Mellapak, and Kerapak - Super-Pak and Ralu-Pak - Koch Flexipac, Flexeramic, Flexigrid



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Trays (1) • When specifying Weir heights, cap positioning, and number of valves: For Specify One-pass tray A single value Two-pass tray

Up to two values, one for each panel A and B

Three-pass tray

Up to three values, one for each panel (A, B and C)

Four-pass tray

Up to four values, one for each panel (A, B, C and D)


Trays (2) • The values for the number of caps and number of valves applies for each panel. For example, two -pass trays have two A panels for tray AA, and two B panels for tray BB. Therefore, the number of caps per panel is the number of caps per tray divided by two. Similar consideration is necessary for three- and four -pass trays. • If you specify only one value for multi-pass trays, that value applies to all panels.



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Trays (3) • When specifying downcomer clearance and width: For Specify One-pass tray A single value for the side downcomer Two-pass tray

Up to two values, one for the side downcomer, one for the center downcomer

Three-pass tray

Up to two values, one for the side downcomer, one for the off-center downcomer

Four-pass tray

Up to three values: one for the side downcomer, one for the center downcomer, and one for the off-center downcomer


One Pass Tray

Outlet Weir Length

Column Diameter

DC-WTOP WEIR-HT DCWBOT

DC-HT

DC-CLEAR



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Two Pass Tray

Outlet Weir Length

Column Diameter

CTR. DC

DC-WTOP

CTR. DC Below

~

~ DCWBOT

Panel A

WEIR-HT DC-HT DCWTOP

DC-CLEAR

Tray AA Side Downcomer

Panel B Tray BB DC-HT

DCWBOT

Center Downcomer

DC-CLEAR

~~

~ ~ © 2002 AspenTech. All Rights Reserved.

Three Pass Tray

Outlet Weir Length

Column Diameter OFF-CTR.DC

OFF-CTR.DC DC-WTOP

DC-WTOP

WEIR-HT DC-HT

Panel A. B. C. DCOF DC-WBOT

DC-

DC-WTOP

CLEAR

B

A

B

A

C

Panel C. B. A.

Panel A. B. C.



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Four Pass Tray

Outlet Weir Length

Column Diameter OFF-CTR.DC OFF-CTR.DC

SIDE DC

CTR.DC DC-WTOP

DC-WTOP

WEIR-HT DC-HT

Panel A. B. DC-WBOT

DC-

CLEAR

A

D

D

B

B

DC-WBOT

C DCOF

A

Panel C. D.

Panel A. B.


Specifying Efficiencies in Tray Rating (2) • If specifying Murphree or Vaporization efficiency, each stage in RadFrac corresponds to one actual tray. Calculated pressure drops will be correct. • If using Overall efficiency, each stage in RadFrac will correspond to more than one physical tray. To calculate the column pressure drop, the assumed overall efficiency must be specified on the Tray Rating Design/Pdrop sheet. » Specify efficiency on either the Efficiencies form or on the Tray Rating form but not both.



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Specifying Efficiencies in Tray Rating (2) • Specified on Tray Rating Design/Pdrop sheet when column simulation is based on overall efficiency.


Example: RadFrac Pack Sizing (1) Working with EB/Styrene example, add Packed Column Sizing information: Packing Type

Norton IMTP 2 in.

HETP

1.5 ft.

Approach to Flooding

80%

Maximum Pressure Drop

55 torr

FILENAME: EX1A-EBSTRYRENE-PACKSIZE.BKP



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Example: RadFrac Pack Sizing (2) Packed Column Sizing Input:





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Example: RadFrac Pack Sizing (5) Packed Column Sizing Results:



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Example: RadFrac Pack Sizing (6) Packed Column Sizing Results:


Workshop 2A: HCL Column Rating (1) Use Workshop 1, perform tray rating. Tray details: Tray type Nutter float Valve BDH Number of passes 2 Column diameter 7 ft. Actual number of trays 66 Tray spacing 1.5 ft. Tray efficiency 0.5 Weir height 2 in. Valve density 12/sq.ft. Downcomer clearance 1.5 in. Side downcomer width 8.7 in. Center downcomer width 7.1 in. FILENAME:WS2A-HCLCOL-RATE.BKP



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Workshop 2A: HCL Column Rating Results (2) Condenser Duty Reboiler Duty Maximum flooding factor Maximum DCBA Predicted P drop

-9.837 MMBtu/hr 10.927 MMBtu/hr 0.329 (Stage 2) 0.384 (Stage 32) 5.65 psi


Workshop 2A: HCL Column Rating Results (3)



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Workshop 2B: Increased Throughput (1) Sensitivity Analysis Study the effect of an increase in flow rate on the flooding approach and downcomer/ backup tray spacing. Consider increases of 5 to 50%. Use a Multiplier block to handle the feed variation.

HCL-PROD

HCL-COL

FEEDA

FEEDB MULT VCM-EDC

FILENAME:WS2B-HCLCOL-FLOW.BKP © 2002 AspenTech. All Rights Reserved.

Workshop 2B: Increased Throughput (2) Results:



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02+Sizing+and+Rating+of+Trays+Advanced+Distillation+With+Aspen+Plus

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