apc and ethylene rto _160713

7/27/2019 APC and Ethylene RTO _160713

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The world leader in serving science

Thermo Scientif ic Prima PRO

Advanced Process Control



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Advanced Process Control (APC)

52%39%

9%

Yield

Capacity

Energy



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Polyethylene APC Example

If there is no branching (left), it is linear high density polyethylene - HDPE

Branching results in low-density polyethylene – LDPE (right)

The ratio of hydrogen and ethylene in the reactor feed is an importantvariable for control ling the density of the polymer

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

( )nHDPE LDPE

Ethylene

Polyethylene



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Manual “regulation” – comparison of results by GC and MS

Hydrogen is added according to GC readings and operator know-how

– A certain percentage of trials are out of specification

Variations are under-estimated by the GC



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Hydrogen is introduced in an automatically controlled fashion to control the H2/C2ratio, first using PGC data then control is switched to the process massspectrometer

The process becomes more stable (smoothed kinetic profile)

PMS versus PGC –Polyethylene Production

Value Value

Use PMS Speed to Replace Multiple PGCs

Use PMS Precision to Improve Control1% improvement in y ield of LLDPE worth $13K per day on a typical unit



6The world leader in serving science

Thermo Scientif ic Prima PRO

Ethylene Cracking Furnace:

Real-Time Optimization



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Olefin furnace

• Hydrocarbon feed pyrolized with steam at 750-900°C in

tubular furnace

• Gas feed (ethane, propane)

• C 2 H 6 → CH 2 =CH 2 + H 2

• C 3H 8 → CH 3CH=CH 2 + H 2

• Liquid feed (typically C10+, can be up to C40)

• C nH 2n+2 → CH 2 =CH 2 +C 3H 6 +C 4H 8 + C nH 2n+ H 2

• Residence time 0.08-0.25 seconds, pressure 175-240 kPa

• Effluent product stream quenched by heat exchanger • Various components (particularly ethylene) separated out

by further processes

• Need to avoid undesired reactions

• Dehydrogenation to acetylene, MAPD• Condensation to cyclo-diolefins & aromatics

• Coking

To gasanalyzer



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Olefins process: schematic

Product Slate depends on cracking severity in the pyrolysis furnaces



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Ethylene Furnace Control

Control Variables:

• Burner Firing Rate

• Flow Distribution• Steam to Carbon Ratio

• Furnace Draft (Damper Position)

• Steam Drum Level

These are used to control severity & selectivityso that most profitable gas composition is sentto the recovery section



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Example: Steady State Control Model

ε =

P (yH2)(yC2H4

)where ε = equilibrium approach

P = hydrocarbon partial pressure

Kp = equilibrium constant for ethane dehydrogenation

reaction at coil outlet temperature (COT)

y = mole fraction of constituent in coil effluent

K p (yC2H6)

Requires:

• COT measurement

• Hydrogen measurement

• Ethylene measurement

• Ethane measurement



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Plant profitability depends on the quality of the analyzers

Olefins plant automation systems



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COT correction (COT bias) =calculated COT – actual COT

Open-Loop COT correction: Process GC

According to the Technip SPYRO Team“GCs are too slow and unreliableFor closed-loop control”

Open-loop



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Fast, complete & reliableanalysis facilitates closedloop control for moreaccurate severity control

Closed-Loop COT correction: Process MS



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Other APC models

Tube Section

Thickness of coke layer determines energy transfer and residence timeBoth affect cracking severity and therefore product slate

Coking Rate for Light Hydrocarbon feedstocks Plehiers and Fromen

r C = AC2H4exp [-E a,C 2 H 4] + AC3H6

exp [-E a,C 3H 6 ]

RT RT

Coking Rate for Naphtha and Heavy Condensate feedstocks Reyniers et al

r C = CH2 CCH4

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∑i=1K i C i

Coking ratemodels require these

furnace effluentconcentrations



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Example model requiring extended analysis

Kinetic Severity Function (KSF)* = ln1

1 - σ

Where σ is the conversion factor of n-pentane

KSF = 1.7 maximize propylene

= 2.3 maximize butadiene

= 2.7 maximize combined olefins= 3.9 maximize ethylene

Real-Time Optimization (RTO) models change severity set-points after

taking into account feedstock analysis, feedstock costs, market conditions(or down-stream requirements) and energy prices

* Stone & Webster technology – naphtha feedstock



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Prima PRO v GC Analysis (severity control)

• MS analyzes 22components in 30seconds

• 3 minute sample interval

• 1 MS for 5 furnaces• GC analyzes 5

components in 3-6minutes

• 1 GC per furnace• MS precision ~ 0.1%

relative

• GC precision ~ 0.5%relative

• Additional extendedanalysis (slow) GCsneeded to provide modelinput data

Liquid feed ethylene cracker MS mean GC meanHydrogen 13.4136

Helium 0.0011

Methane 28.6200 28.9980

Acetylene 0.3768

Ethylene 27.5646 26.9372

Ethane 4.7831 4.5889

Hydrogen sulfide 0.0188

Methyl acetylene 0.2928

Propylene 12.7321 12.9908

Propane 0.3246 0.3879

1,3 Butadiene 2.9312Isobutene 3.5980

Isobutane 0.3046

1,3 Cyclopentadiene 0.9627

Isoprene 0.7922

Pentene-1 0.5557

Isopentane 0.1951

N-Pentane 0.8675

Benzene 0.7501

Hexadiene 0.2226

Hexene-1 0.4439

Hexane 0.2489

Coking rate

Selectivity

KineticSeverity



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Traditional process analyzer installation: GC

• 1 GC per furnace for cracking severity control

• (C 1 – C 3 only)

• 5 minute sampling interval

• 1 GC per two furnacesfor APC model input

• H 2 , C 1 – C 4, C 5 +• 30 minute sampling interval

• Primary controlrequirement is to

optimize product slateand minimize cokingdue to excessive carbonradicals



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Alternative analytical installation: MS

• 1 MS per 10 furnacesfor cracking severitycontrol + APC modelinputs (H2, C1 – C6)

• 5 minute sampleinterval

• Although one MS can

replace all 15 GCs, inpractice a 2nd unit istypically provided for redundant operation

• 2.5 minute sampleinterval (99% of thetime)



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6 Furnaces chosen for the

test: 3, 5, 6, 7,11 & 12GC Cycle Time = 9 minMS Cycle Time = 3 min

Braskem Evaluation Setup for GC versus MS



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Significant GC measurement error when rapid changes occur in cracking severitySignificant GC measurement error when rapid changes occur in cracking severity

MS data validation: Braskem Camaçari



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24 hour cracking severity (propylene/ethylene) followingsimultaneous calibration of MS (brown) and GC (blue)

MS data validation: Braskem Camaçari



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This test established a 5-day calibration interval for the GC.

17 days is not nearly enough to assess the stability of the mass spectrometer.

Stability Data (17 days): Braskem Camaçari



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MS v GC Severity Plots Braskem Camaçari



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MS v GC Severity Plots Braskem Camaçari



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Additional MS option: Monitoring low-level CO

• Continuous addition of ppm levels of dimethyl disulfide (DMDS)into the feedstock can result in a reduction of coke formation by >50% and CO production by >90%

• Monitoring CO clearly indicates the effectiveness of the coke-reduction strategy and can be used to improve the accuracy of thecoke-rate predictive models

• CO is not measurable in low concentrations by MS due tosignificant spectral overlap with ethylene and other high-concentration hydrocarbons

• This measurement is best performed by NDIR analyzers



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External analyzer modification

Allows divertedfast-loop

sample to bedirected toexternal COanalyzer



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External analyzer modification

• Allows for otherwise-impossible measurement to beincorporated into MS data by reading analog output from IRanalyzer

• Thermo Scientific GasWorks software presents CO data asthough the measurement were made by Prima PRO

• Alarms on the CO concentration can be configured toindicate excessive CO production (and by implication anunacceptable increase in the coking-rate)



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Ethylene production cost components

• Naphtha ≈ $120 per barrel (130% of crude price) or $900 per tonne

• Cracker operators in Asia typically need a spread of at least

$250/tonne between naphtha and ethylene prices just to breakeven

• Therefore they require an ethylene price of $1,150 per tonne of

ethylene

• Current price is around $1,000 per tonne.

• Clearly profitability requires product slate optimization to reduce

required spread to something significantly less than $250



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Typical GC Solution: Cost of Ownership

Total cost of ownership(10-year):

10‐Year Total Cost of Ownership

Cost Quantity Total

GC $90,000 9 $810,000 Service $7,000 90 $630,000 Gases $1,000 90 $90,000

$1,530,000

3-minute measurement intervalfor C1, C2 and C3 only



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Typical MS Solution: Cost of Ownership

Total cost of ownership(10-year):

10‐Year Total Cost of Ownership

Cost Quantity Total

MS $175,000 3 $525,000 Service* $7,000 7.0 $49,000 Gases** 0 0 0

$574,000 *First 3 years included then one service every 3 years per machine** No fuel or carrier gases required

All furnaces connected to all Mass Specs for 100% availability

90 second measurementinterval for extended analysis to

include hydrogen andspeciation to C6



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Opportunity Cost

Assumptions

• Olefins production ≈ 700,000 tonnes per year @ $1,000 pt ≈ $700M pa

• Each 1% improvement in yield = $7M pa

• The improvement in the yield of ethylene and other high-valuehydrocarbons is expected to be at least 1% when MS monitoring replacesGC due to the extended analysis, superior precision, stability andavailability

• If we add the 10-year opportunity cost to the TCO10 in the 9-furnace

example we get the following cost comparison:

GC ≈ $1.5M + $70M ≈ $72M

MS ≈ $0.6M

Note that a 3% improvement is more typical



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Summary

• Tight control of cracking severity requires online furnace effluentanalysis to maximize profit

• Furnace effluent analysis cycle time needs to be 3 minutes or lessto track process kinetics

• Prima PRO provides• Superior precision (x5)

• Extended analysis (speciation to C 6 )

• Higher availability (100% with redundant installation)

• More flexible configuration (configured with software rather thanhardware)

• Prima PRO provides rapid ROI and low TCO10 due to 3-year maintenance interval and 3-4 hour PM

• All Prima PROs are shipped with performance guarantee and 3-year parts & labour warranty with travel expenses also covered for the first 12 months, significantly reducing risk of switching to MS

apc and ethylene rto _160713

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