65263851 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 © 2002 AspenTech. All Rights Reserved.

© 2002 AspenTech. All Rights Reserved. 3 - 1 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 © 2002 AspenTech. All Rights Reserved.

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. © 2002 AspenTech. All Rights Reserved.



Vaporization Efficiency

y • At equilibrium:

y • With efficiency:

i Ki ix

i 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. © 2002 AspenTech. All Rights Reserved.

Murphree Efficiency

M Eff

y y i, j i, j 1

i, j K i, j

x y 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 © 2002 AspenTech. All Rights Reserved.



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. © 2002 AspenTech. All Rights Reserved.



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. © 2002 AspenTech. All Rights Reserved.



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. © 2002 AspenTech. All Rights Reserved.


• 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 © 2002 AspenTech. All Rights Reserved.




• 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.



• 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 © 2002 AspenTech. All Rights Reserved.




• 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 © 2002 AspenTech. All Rights Reserved.

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




Tray Types

• You can choose from the following five commonly-used tray

types:

- Bubble caps

- Sieve

- Glitsch Ballast®

- Koch Flexitray®

- Nutter Float Valve © 2002 AspenTech. All Rights Reserved.

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 © 2002 AspenTech. All Rights Reserved.



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) © 2002 AspenTech. All Rights Reserved.

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. © 2002 AspenTech. All Rights Reserved.


Ou

tlet

Wei

r L

en

gth


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 Column Diameter

DC-WTOP

WEIR-HT

DC- DC-HT WBOT

DC-CLEAR



Outl

et W

eir

Len

gth

Outl

et W

eir

Len

gth


Two Pass Tray

Column Diameter

CTR. DC

DC-WTOP CTR. DC Below

~ DC- WEIR-HT WBOT DC-HT

DC- DC-CLEAR WTOP

DC- WBOT

~ Panel A Tray AA

Side Downcomer

Panel B Tray BB

DC-HT Center Downcomer

DC-CLEAR

~ ~ ~~ © 2002 AspenTech. All Rights Reserved.

Three Pass Tray Column Diameter

OFF-CTR.DC

OFF-CTR.DC

DC-WTOP DC-WTOP

WEIR-HT Panel A. B. C.

DC-HT DCOF

DC-WBOT

DC- DC-WTOP

CLEAR B A Panel C. B. A.

A B C Panel A. B. C.



Outl

et W

eir

Len

gth


Four Pass Tray Column Diameter

OFF-CTR.DC OFF-CTR.DC

SIDE DC

CTR.DC

DC-WTOP DC-WTOP

WEIR-HT Panel A. B.

DC-HT

DC-WBOT DC-WBOT

DC-CLEAR D D C Panel C. D.

DCOF

A B B A 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. © 2002 AspenTech. All Rights Reserved.



Specifying Efficiencies in Tray Rating (2) • Specified on Tray Rating Design/Pdrop sheet when column

simulation is based on overall efficiency. © 2002 AspenTech. All Rights Reserved.

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





Packed Column Sizing Results: © 2002 AspenTech. All Rights Reserved.

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




Workshop 2A: HCL Column Rating Results (2)

Condenser Duty -9.837 MMBtu/hr

Reboiler Duty 10.927 MMBtu/hr

Maximum flooding factor 0.329 (Stage 2)

Maximum DCBA 0.384 (Stage 32)

Predicted P drop 5.65 psi © 2002 AspenTech. All Rights Reserved.

Workshop 2A: HCL Column Rating Results (3)




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-COL

FEEDA FEEDB

MULT © 2002 AspenTech. All Rights Reserved.

HCL-PROD

VCM-EDC

FILENAME:WS2B-HCLCOL-FLOW.BKP

Workshop 2B: Increased Throughput (2)

Results: © 2002 AspenTech. All Rights Reserved.


65263851 02 sizing and rating of trays advanced distillation with aspen plus

Documents