Miscellaneous

CHEN 4470 – Process Design Practice

Dr. Mario Richard EdenDepartment of Chemical Engineering

Auburn University

Lecture No. 17 – Equipment Sizing, Economics and Pinch Analysis

March 7, 2013

• General Rule of Thumb– Based on discussions with industrial design

engineers

– Each major piece of equipment results in approximately $1,000,000 in capital investment (including installation, engineering time, instrumentation etc. etc.)

– Thus for a plant with a reactor and two distillation columns, the typical price will be around $3,000,000

– Remember, that this is “order of magnitude” estimation

Process Economics

• Distillation Columns– Required data is column diameter and height– Height is found from tray spacing specification

Equipment Sizing 1:4

Remember that the first and last stage are the condenser and

reboiler, respectively.

• Distillation Columns (Continued)– If identified diameter is beyond the curves found

in P&T, assume (for pricing purposes) that the separation is performed in NC columns in parallel all with the same specifications.

– The number of columns required should be based on equal distribution of the cross-sectional area NOT the diameter:

Equipment Sizing 2:4

maxAspen P&T

2 2 2Aspen

2 2 2P&T

2Aspen

2P&T

D 20ft , D 5ft Intuitive solution NC = 4 columns

A (20ft) 400ft 100ft4 4

A (5ft) 25ft 6.25ft4 4

A 100ftNC 16 columns

A 6.25ft

• Heat Exchangers– Required information is usually the heat transfer

area, apart from furnaces and fired heaters, which may be estimated based on the duty.

– Overall heat transfer coefficients can be found in P&T (p. 663 in 5th ed.). Assume an average value.

– Assume inlet and outlet temperatures for utility stream. Sample calculations given P&T p. 693.

Equipment Sizing 3:4

0 log meanQ U A T Q: Provided by Aspen

U: Found in P&T

ΔT: Calculated after assuming temps.

• Reactors and Vessels– Required information is usually the reactor

volume along with the amount of catalyst.– Both require detailed kinetic expressions

– Can also be determined from residence time

– When pricing in P&T remember the rule of thumb for major pieces of equipment, i.e. the reactor will not cost $50,000 if the distillation columns cost $1,000,000

Equipment Sizing 4:4

R flowV V Gas phase catalytic reactions = Fast

Order of magnitude would be: seconds range

ex

iR i0

i0

dXV F

( r )

• Units to Include– Heaters, coolers and reactors– Aspen will provide supply and target

temperatures as well as enthalpy change for each hot and cold stream

– Remember that a hot stream in pinch terminology will actually be TWO streams in Aspen

Pinch Analysis 1:5

Supply temp

Target temp

ΔH

Integration of streams should be

done using the HeatX model

B1

1

2

3

4

• Identifying the CP values (No phase change)– The heat capacity (CP) of a stream that does

NOT undergo any phase changes is readily calculated from the information provided by Aspen

Pinch Analysis 2:5

Ts Tt

Q

CP = Q/T

• Identifying the CP values (phase change)– Dashed line represents approximation if the duty

of the exchanger is averaged over the temperature range.

– Quality of approximation depends on the relative values of CPL, CPV and Hvap

Pinch Analysis 3:5

T

H

TT

TS

TB

HvapCPL * T CPV * T

• Approximation

Pinch Analysis 4:5

For mixtures with very different boiling points of the constituents the outlet temperatures in exchangers 1 and 2 will be quite different, but it will still give a better approximation than just an average over the entire temperature range.

Liquid @ Ts Vapor @ Tt

Q

Liquid @ Ts Liquid @ Tb

Q1Vapor @ Tb Vapor @ Tt

Q2 Q3

Can be specified in Aspen as

having an outlet vapor fraction of

0.001

Can be specified in Aspen as

having an outlet liquid fraction of

0.001

Specify outlet temperature at Tt

• Units NOT to Include– Distillation columns and flash drums

– Due to dynamic implications, distillation columns should not be directly integrated with the utility system

– Evaluate all columns together in tabular form with duties and temperature levels

– If there is a condenser with a temperature higher than a reboiler in another column, then those two are candidates for integration.

Pinch Analysis 5:5

• ASPEN Help File– References for equipment such as

scrubbers/absorbers

General Comment

• Next Lecture – March 19– Process risk assessment and inherently safe

design

Other Business