Improvement in Transient Operation of Ethane Cracker

Introduction

A1.1.       
This
report outlines the task of improvement in transient operation of Ethane Cracker
for ethylene production at Saudi Petrochemical Co. (SADAF), Jubail Industrial
City, Kingdom of Saudi Arabia.

Background

A1.2.       
SADAF
is a joint venture between Saudi Basic Industries Corporation (SABIC) and Shell
Chemicals Arabia LLC. It is a petrochemical complex containing six world scale
petrochemical plants with a total average output of more than 4 million metric
tons per year of various chemicals. The plants include Ethylene, Methyl
Tertiary Butyl Ether (MTBE), Ethylene Dichloride (EDC), Styrene-I&II, Crude
Industrial Ethanol (CIE) supported by Utilities distribution department (UDD)
with production of 12,600 Metric tons per day steam and 250 MW power.

A1.3.       
Ethylene
plant was commissioned in 1986 and had gone through expansion in 1991 and 1997
to an increased production capacity of 1.1 million metric tons per year. The
plant receives ethane feed from Saudi Aramco and delivers produced ethylene to
other SADAF plants and KEMYA (SABIC affiliate) to manufacture different
derivatives. Ethane feed is cracked to ethylene and byproducts in furnaces
(crackers). Ethane is fed along with steam to lower the hydrocarbon partial
pressure in the radiant coils and to prevent rapid coke formation so as to
increase furnace run length. Furnace is followed by primary and secondary
transfer line exchangers (PTLE & STLE) where hot furnace effluent is cooled
by steam and boiler feed water respectively to stop reaction and avoid unwanted
products. The water quench system provides final quench to the furnace
effluents before the cooled cracked gas is sent forward to compression and
treatment. In addition, a small amount of the heavier components, mainly
gasoline, is condensed out in the water quench tower.

A1.4.       
There
are total 12 furnaces, out of which 9 or 10 furnaces are kept in cracking mode.
For each furnace, cracking mode is followed by ‘hot steam standby’ (HSSB) and
de-coke modes, to remove coke from inside cracker tubes and subsequently to improve
efficiency. This transient operation is repeated for each cracker almost every
180 days depending upon the furnace performance, feedstock availability and
production targets. Recently, a fatal incident happened during transient
operation of a naphtha cracker. Sadaf management, therefore, decided to carry
out assessment and to review the Ethylene furnaces transient operation
philosophy with an objective to examine the applicability of that incident at
Sadaf site. A multi-disciplinary team was chartered to complete this critical
task, with members from process operations, process engineering, process safety
and environment, health & safety (EHS).

A1.5.       
The
team selected the most suited methodology to complete the activity from
conception of problem to producing the solution. It included development of
activities flow chart, task of data collection, review of past process safety
studies, interviews of process operations team, qualitative and
semi-quantitative analyses, and proposing possible solutions with actions.

A1.6.       
First
of all, critical incident details were collected. There had been an outside
fire from a naphtha cracking furnace which resulted in a fatality. Available
information suggests that transient operation (moving from cracking mode to
decoke) may have played major role.

A1.7.       
Team’s
objective was to analyze cracking furnace transient operation in relation to
the above incident. It is to be noted that, owing to number of furnaces,
transient operation is almost fortnightly activity which makes it very critical
for business continuity.

A1.8.       
Next
step was to collect process information. It included process flow diagrams (PFD),
piping & instrumentation diagrams (P&ID), equipment and instrumentation
specifications, operation philosophy, standard operating procedures (SOP) – 
startup, shut down, normal and transient, process safety studies like process
hazard analysis (PHA) and incidents history.

A1.9.       
Following
critical steps and information were noted during the review of SOP’s and data:

a.    Furnace effluent line
is divided into two lines of different sizes downstream secondary transfer line
exchanger (STLE). Larger dia pipe is to line up furnace effluent to quench
tower through a motor operated valve (MOV) during cracking and HSSB modes.
While smaller dia line is used for aligning the flow to decoke pot and cyclone.

b.    Larger MOV gate is of
double wedge design. Sealing steam at a pressure higher than that of quench
tower side, is supplied in cavity between the two wedges to avoid any valve
passing.

c.    SOP asks for Double
block & bleed  to cut off ethane feed to furnace prior to switch over from
cracking mode to HSSB mode

d.    SOP includes insertion
of blind at downstream of 1st block valve of ethane feed line before switching
furnace effluent (steam) from quench tower to decoke header (by MOV’s).

e.    Verification in area
and panel is included in SOP for closing larger MOV, opening sealing steam to
prevent cracked gas backflow to de-coke header and opening smaller MOV to
de-coke header.

f.     SOP ensures reading
is Zero at hydrocarbon analyzer installed on decoke header, otherwise larger MOV
tightness and cavity pressure is checked

g.    Each step is signed
by panel & field operators on separate copies. After completing the
activity, the field operator transfers his signatures on panel copy and wastes
field copy (soiled usually).

h.    Furnace Feed In /
Feed Out is part of job qualification program (JQP) for field and panel operators

A1.10.     Important
feedback extracted from interviews of field and panel operators included:

a.    Field operators,
sometimes, face difficulty in closing MOV’s. One of either MOV’s stick during
switchover from HSSB to decoke mode approximately once every ten times. This
situation needs to be resolved very fast within 2-3 minutes using hand-jack and
motor; or otherwise revert to HSSB.

b.    There is a pressure
gauge provided to ensure “No Backflow” during switch over, i.e., closing and
opening MOV’s in increments one by one.

c.    One operator is
present in field for normal switchover. If there is problem, more operators are
arranged.

d.    MOV position
indicator to show percent opening is not provided at panel. Only ‘open’, ‘close’
& ‘moving’ indications are present.

e.    All communication
between field & panel operators is done via radio which is satisfactory

f.     Sealing steam
pressure is noted in the field once per shift when applied.

g.    There are few
opportunities of improvement regarding STLE outlet PG.

A1.11.     While
reviewing incident history, team came across one relevant incident at site. In
addition, root cause analysis and corrective actions were also reviewed for the
same incident. Actions found complete and effective.

A1.12.     After review of all
information, risk assessment was conducted. First part of analysis was qualitative,
based on hazard & operability (HazOp) technique following 5x5 risk
assessment matrix for the threat of hydrocarbon breakthrough to decoke header,

Risk (R) =
Consequence (C) x Likelihood (L)

As released hydrocarbon may catch fire
resulting into a fatality (C2) while likelihood was low or very low (L4 / L5).

A1.13.     Being
fatality scenario, semi-quantitative risk assessment was carried out using
layer of protection analysis (LOPA) technique.

RR = IEF x PFD_CB1
x PFD_CB2 x PFD_CB3 x PFD_RM1 x PFD_RM2

Where,

RR = Residual Risk; IEF = Initiating event
frequency

PFD = Probability of failure on demand

CB = Control barrier; RM = Recovery measure

A1.14.      LOPA
results concluded that cracking furnaces transient operation carries certain
residual risk which needs to be mitigated through various actions.

A1.15.     Keeping
in view the fortnightly cycle, it was recommended to revise furnace diversion
SOP immediately to ensure maintaining MOV cavity pressure once MOV is closed
and sealing steam open; adding warning box to sensitize the operator about
consequences in case of backflow from cracked gas header to decoke header; and elaborate
few steps to keep decoke header side pressure higher than that of quench tower
side while opening or closing MOV’s. It was also advised to immediately improve
JQP resource material by adding detailed discussion of furnace transient
operation and consequences of backflow from cracked gas header to decoke header.

A1.16.     Team
also furnished following short term actions:

a.    Complete previous PHA
revalidation action related to sealing steam to MOV.

b.    Improve hardware and
operation regarding STLE outlet PG for a better control as a best practice.

c.    As per OSHA standard
29 CFR 1910.119 Appendix A, add hydrocarbon analyzer on de-coke header to safety
critical instruments list and define its preventive maintenance accordingly. 

A1.17.     A
long term action was recommended to automate switchover from quench tower to
decoke header based on pressure differential transmitter (PDT) across main
cracking mode MOV in light of newer designs; as the critical step of furnace
switchover is managed manually based on SOP, operator knowledge and field
indications only. 

A1.18.     After
taking immediate and short term actions, the company can improve furnace
transient operation saving down time as well as reducing a significant risk in
a business critical activity to tolerable. Automation requires few long lead
items, therefore, can be taken as a separate project, completion of which will save
even more downtime and will bring the risk down to as low as reasonably
practicable (ALARP).

A1.19.     A
revalidation / review will be carried out after detailed RCA report of the
accident is received from affiliate site.

*By 2017
CCPS MEPSC, improvement results would be available.