stream data extraction 3.pdf

8/9/2019 stream data extraction 3.pdf

http://slidepdf.com/reader/full/stream-data-extraction-3pdf 1/43

STREAM DATA EXTRACTION

By Siti Shawalliah Idris, AMIChemE

CPE 633 PROCESS ENGINEERING II



Data Extraction

1• Existing Process

Flowsheet

2• Stream Data - DT

min

3• Pinch Analysis

4• New Flowsheet



Data Extraction

Extracting the stream data from the process flow sheet is an unspectacular, unexciting but absolutely crucial part of pinch

analysis.

It is not always clear-cut and sometimes alternative approachespresent themselves.

If the wrong method is chosen, one may either end up withimpossible targets which cannot be achieved by realisticequipment, or conversely with a system which is so

constrained that the only possible network is the one youalready have!



The key information that needs to be extracted includes the temperature levels of the process streams, and the amount of heat

required to bring about desired temperature changes.

Heat capacity and flowrate are key pieces of information fordefining the enthalpy change for a given process stream.

In summary the data required for each process streaminclude: Mass flowrate (kg/s),

Specific heat capacity (kJ/kg oC)

Supply and target temperatures (oC), and

Heat of vaporization for streams with a phase change (kJ/kg).



A key objective of data extraction is : to recognize which parts of the flowsheet are subject to

change during the analysis (e.g. possibility of makingmodifications to the piping, or adding new heat exchangers,

possibility of making temperature changes in the process ormodifying the utility that heats a given piece of equipment(MP steam instead of HP steam for example), etc.).

If, during extraction, all features of the flowsheet areconsidered to be fixed, there will clearly be no scope

for improvement.



Definition of stream

A sensible criterion for a stream is that it should change in heat load but notincomposition.

Hence a flow of liquid through a heat exchanger, or a single component

liquid being evaporated, or a mixture which is being cooled without anyseparation of the components, can all be represented as streams.

Conversely, a flow of liquid through an absorption column or a scrubber, ora mixture which is reacting, or a flow through a distillation column whichemerges with a volatile component removed, should not be treated as a

single stream; better methods will be found in later lecture.



How to extract stream data?



Data Extraction

Given Process Flow Diagram



Strip the plant!



Strip the plant!



Strip the plant!



Strip the plant!



Basic Principles of Stream Data Extraction

1. Stream Identification

Figure a shows part of a flowsheet in which a feed stream isheated from 10 to 70◦C before being filtered. After the filter, itis heated from 70◦C to 135◦C and then from 135◦C to 200◦Cbefore it is fed to a distillation column.



When is a stream a stream?

Is the feed one stream, two streams or three streams?







Extract the feed as three streams, together with thethree hot streams matched against the feed.

Exact matches (just like the old design!)




Question: What exactly is essential as far as the streamdata are concerned?

Heating the outlet of the filter from 70◦C to 135◦C is part of a previouslysuggested solution. It is not a constraint that the process heating should stop at135◦C. The feed stream needs to be heated to 200◦C, but the midpoint at135◦C is not a constraint. The feed is heated from 10◦C to 70◦ C beforeentering a filter. It may be that the temperature at which the filtration cantake place has some flexibility and does not need to be rigidly at 70◦C. Thus,for this problem, it would seem to be appropriate to extract the feed stream

to the distillation as two streams, one from10◦C to 70

◦ C and a second from70◦C to 200◦C. The operation of the filter at 70◦C is kept flexible, as shown

in Figure b.




There are two streams with flexibility to choosetemperatures




2. Temperature-enthalpy profiles

How do we obtain T-H Profiles? –

Use physical property data

BUT usually more convenient to take heat loads from

flowsheet






Rough T-H data can beobtained from flowsheet

heat loads and

temperatures

T*

DH

150o

70o

25o

10o

DH1 DH

2 DH

3




The known temperatures and heatduties for the existing heatexchangers can be plotted on atemperature-enthalpy profile.

This shows a nonlineartemperature-enthalpy profiletaken directly from the flowsheetbut represented as three linearsegments. Such an approximationis good enough for many

purposes. It is also a articularlyconvenient approach to adoptwhen dealing with retrofit ofexisting flowsheets.




3. Mixing

Two cold streams leavingseparate units at different

supply temperatures, mixing

and then requiring heatingto

a common targettemperature. How to extract the

data?



Mixing act as heat transfer



Do you extract the stream as…



What if?...

Heat trasfer across pinch!.. Miss theopportunity for energy saving!



Therefore:



Generally:

To ensure the best energy performance atthe targeting stage, the mixing must beassumed isothermal

Hence in stream mixing, the data for unitstargeting is incompatible with the data forenergytargeting. However, this should notcause confusion at the design stage if theabove principles are thoroughly understood.It merely means that the designer mightrequire one more unit than minimum if non-

isothermal mixing cannot be allowed in anMER design.





4. Utilities

General: Do not extract utilities



If we have alternatives

Do not extract existing utilities design.(add them later using the grand composite curve)



Watch out for process steam

Steam is not a utility



For direct use:

Need to provide the steam somehow







6. Soft Constraints

The data is ‘soft’



Extracting ‘Soft’ Data:

Apply +/- principle to reduce targets



For example

Recover waste heat to hot pinch temperature

S



Summary

Guideline : ensure that all energy savingopportunities have a change. Therefore :

“Free” the system

Do not copy the existing solution if any

Identify and set aside existing utilities‟ supplies

Differentiate between “hard” and “soft” data

Prefer optimisation of the process prior to solving actual

concepts that might be “second best”



Avoid mixing of streams of different temperatures Respect practical temperature levels which streams are

available at (cfr. quenching)

Take precautions for streams that might give rise to

forbidden matches

Accept intermediates if imposed by process constraints

Partition streams with non-linear profiles and define

extracted sections “on the save side”







The crude feed is supplied at ambient temperature

and fed to a distillation column at atmosphericpressure, where it is split into three fractions: light oil,middle oil and residue.

Naturally, the feed has to be heated up to theoperating temperature of the column and in this case itmust also be partially vaporised, as there is noseparate reboiler; some flashing also occurs as the hotliquid enters the column.

The light oil comes off the top as vapour (overheads),is condensed and the majority is recycled to providethe top reflux; the remainder is cooled and a smallamount of water is removed in a gravity separator.

The various products are cooled to different levels,depending on their viscosity. The crude feed passesthrough two heat exchangers, and is heated by theoverheads and middle oil, before entering a furnace

which brings it up to its final feed temperature.

Documents

stream data extraction 3.pdf