switching between cracking and decoking

1

Switching between

cracking and decoking

Menno VAN DER BIJ - Deputy Manager Instrumentation Department32th Annual European AIChE / Delta Process Academy Seminar

‘Safe start-up of chemical plants’ October 11th, 2016, Domein Martinus / Halle - Zoersel

AMERICAN INSTITUTE OF CHEMICAL ENGINEERS Netherlands / Belgium Section

Pyrolysis Furnaces

2

Switching between cracking and decokingIntroduction

CRAC

KED

GAS

HEA

DER

Deco

ke E

fflue

nt V

alve

Cracking furnace in CRACKING mode of operation

3

Switching between cracking and decokingPossible Hazards

Cracked gas leaking through decoke valve(s) to atmosphere Decoke air routed to furnace, mixing with HC High temperature downstream QOF in case of quench oil

failure Backflow of cracked gas from header to cracking furnace in

case of multiple radiant coil failure Low temperature downstream QOF due to failure of the

emergency water back-up

While in CRACKING mode:

4


CRAC

KED

GAS

HEA

DER

Deco

ke E

fflue

nt V

alve

Cracking furnace in DECOKING mode of operation

5


Cracked gas leaking through cracked gas valve(s) to atmosphere

Hydrocarbon feed routed to furnace, mixing with decoke air DMDS connected to atmosphere Quench oil connected to atmosphere Decoke effluent routed to cracked gas analyzer

While in DECOKING mode (open connection to atmosphere):

6


FEEDFEED

PRE-HEATER

DILUTION STEAM

SUPERHEATER

STEAM

HIGH TEMP. COIL

TLE

QUENCHOIL

FITTING

DECOKE CYCLONE

To Atmosphere

CRAC

KED

GAS

HEA

DER

Oil

AIR

To Stack

Deco

ke E

fflue

nt V

alve

Cracked Gas Valve

To Fire Box

DMDS

Cracking furnace in HOT STANDBY mode of operation

7

Switching between cracking and decokingPre-conditions for change-over

Hydrocarbon feed lines isolated

Decoke air line isolated

DMDS line isolated

Quench oil line isolated

Decoke air and HC feed lines purged with steam

8

Switching between cracking and decokingHistory

Deco

ke E

fflue

nt V

alve

Cracked Gas Valve

Individual operated valves

Deco

ke E

fflue

nt V

alve

Cracked Gas Valve

M

Mechanically linked valvesWith 1 electric motor

M

M

Deco

ke E

fflue

nt V

alve

Cracked Gas Valve

Electrically linked valves

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Overpressure due to simultaneously (partial) closed decoke and cracked gas valve(s)

Backflow of cracked gas to atmosphere while both valves are open

During TRANSFER from cracking mode to decoking modeDe

coke

Effl

uent

Val

ve

Cracked Gas Valve

Deco

ke E

fflue

nt V

alve

Cracked Gas Valve

M

M

M

Deco

ke E

fflue

nt V

alve

Cracked Gas Valve

Overpressure is possible in all 3 configurations

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Switching between cracking and decokingSafety Instrumented System requirements

Overpressure due to simultaneously (partial) closed decoke and cracked gas valve(s)

Backflow of cracked gas to atmosphere

During TRANSFER from cracking mode to decoking mode

PREVENT

M

Cracked Gas Valve

M

Cracked Gas Valve

11

Switching between cracking and decokingSafety valve for overpressure protection

Deco

ke E

fflue

nt V

alve

Cracked Gas Valve

SP SPLC LC

Deco

ke E

fflue

nt V

alve

Cracked Gas Valve

SP SPLC LC

Deco

ke E

fflue

nt V

alve

Cracked Gas Valve

SP SPLC LC

To Atmosphere

LC

Cracked Gas Valve

SP SPLC LC

Cracked Gas Valve

SP SPLC LC

Cracked Gas Valve

SP SPLC LC

To Atmosphere

LC

SP = Steam PurgeLC = Locked Closed

12

Switching between cracking and decokingConceptual design

Decoke cyclone

CRAC

KED

GAS

HEA

DER

Deco

ke E

fflue

nt V

alve

Cracked Gas Valve

AIR

FEED

STEAMFSLL

FI

Oil

L

DMDS

PSHH PDSLLPI

PAHPDI

PDALPressure transmitter

Differential pressure transmitter

Note: Switch-over can be manually from local panel or DCS or fully automatic through the safeguarding system

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CRAC

KED

GAS

HEA

DER

Deco

ke E

fflue

nt V

alve

14

Switching between cracking and decokingPressure profile

PAHH

PAH

PDAL

PDALL

dP = 0 Bar

Closed

Open DEV

CGVDEV

CGV

Decoke Effluent Valve DEV Cracked Gas Valve CGV

Differential pressure

T1 T2 T3 T4 T5 T6 T7 T8

CRACKING TO DECOKING: STEP 1De

coke

Effl

uent

Val

ve

PAHH

PAH

PDAL

PDALL

dP = 0 Bar

Closed

Open DEV

CGVDEV

CGV



T1 T2 T3 T4 T5 T6 T7 T8


coke

Effl

uent

Val

ve

PAHH

PAH

PDAL

PDALL

dP = 0 Bar

Closed

Open DEV

CGVDEV

CGV



T1 T2 T3 T4 T5 T6 T7 T8


coke

Effl

uent

Val

ve

PAHH

PAH

PDAL

PDALL

dP = 0 Bar

Closed

Open DEV

CGVDEV

CGV



T1 T2 T3 T4 T5 T6 T7 T8


coke

Effl

uent

Val

ve

PAHH

PAH

PDAL

PDALL

dP = 0 Bar

Closed

Open DEV

CGVDEV

CGV



T1 T2 T3 T4 T5 T6 T7 T8


coke

Effl

uent

Val

ve

Cracked Gas Valve

15


CRAC

KED

GAS

HEA

DER

Deco

ke E

fflue

nt V

alve

M

16

Switching between cracking and decokingDifferential pressure transmitters replaced

Decoke cyclone

Cracked Gas Valve

PI

PDI

PDSLL

PDAL

PAH PSHH

Pressure transmitter

17

Switching between cracking and decokingImproved reliability Safety Integrity Level

CRAC

KED

GAS

HEA

DER

Deco

ke E

fflue

nt V

alve

18

Switching between cracking and decokingEmergency isolation

Closing the cracked gas valve after a multiple coil rupture

Initiate furnace total shutdown:

> Close feed and DMDS valves

> Close decoke air valve

> Close fuel gas valves

Close quench oil valve (if applicable)

Overpressure protection remains active

Steam purge of HC and decoke air lines bypassed

19

Switching between cracking and decokingEmergency isolation

Maintaining overpressure protection and preventing backflow of cracked gas to decoke drum only possible with electrically

linked valves

Deco

ke E

fflue

nt V

alve

Cracked Gas Valve

Deco

ke E

fflue

nt V

alve

Cracked Gas Valve

M

M

MDe

coke

Effl

uent

Val

ve

Cracked Gas Valve

20

Switching between cracking and decokingConclusionsAdvantages of electrically linked valves compared to

mechanically linked valves

Easier to install

Easier to maintain pressures within safe limits

Less dependent of dilution steam flow fluctuations

Possibility for EMERGENCY ISOLATION after coil rupture

while maintain overpressure and back flow protection

21

Switching between cracking and decokingConclusions

Advantages of 4 gauge pressure transmitters compared to 2 dP-transmitters + 2 gauge transmitters

Continuous comparison of transmitter availability

Higher availability and reliability

Less process connections = less steam purges

No potential backflow over instrument equalization valve

22

Switching between cracking and decokingConclusions

Advantages of overpressure protection with pressure transmitters compared to safety valve

Overprotection always available

Higher reliability – no blocked PSV inlet

No key-interlocks required

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Switching between cracking and decoking

QUESTIONS ?

Thank you

switching between cracking and decoking

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