7/25/2019 Ammonium Bi-sulphide Corrosion in Hydrocrackers

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REFINING

.

t i f I t j j J G : t 4 e : ; : 4 : 4

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.

mmon um bi sulphide ;

corrosion in hydrocrackers

Factors in the

corrosion

of effluent

condensers

in

hydrocrackers are

the

subject of this

artiele, which

ineludes

a case

study of the problems experienced

at

Riyadh refinery, Saudi Arabia,

and the corrective actions

that were

taken

Walid A Al-Naim

Saudi Arameo

W

orldwide hydrocracker opera

tional experience has drawn

attention to

unique corros ion

problem

that

is . developing in reactor's

effluent condensers. This corros ion prob

. . . .m is defined

as ammonium

bi-sulphide

. H

4

(H5))

corrosion. Ammonium bi-sul

phide corrosion is

considered very risky

and may lead to a catastrophe because

hydrocrackers operate at high pressure,

high t e m p e r a ~ e , and in a hydrogen

atmosphere. Numerous incidents of efflu

ent condenser failures and consequent

fires have been experienced in a number

of refineries around the world due to this

form of corrosion.

Ammonium bi-sulphide's inherent

corrosivity

is

a function of the organic

nitrogen (pyridines, quinolines, and

acridines)

and sulphur

(thiophene,

benzo and di-benzo-thiophene) content

in

the

hydrocracker fresh feed that is

finally corrcrted intc Nll

3

al1d H

2

S

in

the hydrotreating reactors. As organic

sulphur and nitro gen levels increase in

the

fresh feed to the hydrocracker, more

ammonium bi-sulphide is

produced,

ading to corrosion attack in the efflu

~ o n d e n s e r . ..

The ammonium bi-sulphide corrosion

is further w o r s ~ n e d

when

hydrocracker

feed

is

heavier than the conventional vac

uum gasoil (VGO) feed. An example of

heavier feed stock

is

VGO with demet

allised oil (DMO >

1050F)

feed which

contains high levels of organic nitrogen,

and asphaltene which

is

a precursor to

carbon lay

down

on

the

catalyst.

NH

4

(HS) formation

Ainmonium bi-sulphide corros ion is con

sidered a main source of potential hazard

to hydrocrackers in all parts of the world.

T ~ e recipitation of

the ammonium

bi

sulphide on t e effluent con enser tu es

causes under-deposit corrosin and tube

pitting. Thereafter, condenser failure is

bouna

to occur, with a potential disaster.

AS a result, remedial measures

and

correc

ive

actions at design stages

and

unit oper-

N

lO

o;

e.

c:

lf::

..:

Z

: ; ; ~

~

O e

0

00

ZN

OX

_ e.

1

e.

~ j j

o c.

ol f : :

VI

VI

i5

VI

X

J:

Z

100000

_0f.=:R"l'

: : ~ ~ f : : .

10000

~

~

IN THIS AREA, DEPOSITION OF SOUD

:

NH4HS

WlLL

OCCUR

1000

100

'-

/

-

10

-_

,

'

i , ~

1

)-+Hl

>-1

u, cf3

UR-M

NE

,,,

5

-(1

73

r -

10

30

50

70

Temperature, Deg.

e

90 110 1 3 ~

Figure

1

Impact

of p

factor on ammonium bi sulphide formation

ation re being take!1

and

implementecl in

an attempt to mitigate this phenomenon

as

much

as possible

and

thereby ensuring

the plant safety

and

integrity. Hydroc

racker operational experience has led to

the identification of several parameters

that are considered as driving forces for

ammonium

bi-sulphide corrosion attack,

as

listed in Table 1.

The table shows

the

optimum parame

ters to operate the hydrocracker reactor

effluent condenser free from

ammonium

bi-sulphide attack. These values were

brought into practice after extensive sur-

Optimum parameters

Operational

Optimum

parameters

value

1. Piehi Kp Factor 0.15

2.

NH.(HS) wt % in wash water 4.8

3.

Wash

water rate,

vol%

leed 5.2

4. Oxygen (ppm) in wash water 0.05

5_Condenser tube velocity,

Ips

15

Table 1

veys and e x p e r i ~ n d u c t e d by NACE

and analysed

~

Effect of Kp factor

The levels of nitrogen

and

sulphur con

tent in feedstocks mainly dictate

the

extent of ammonium bi-sulphide forma

tion in the reactor effluent condenser. In

hydrc-processing

plants

that process

heavy feed stocks such as VGO

and

DMO,

NH

3

and

HzS

are produced in fairly high

concentrations, leading to a

highKp

fac-

tor (mol% NH

3

x mol% H

2

S).

The ammonium bi-sulphide fouling

rate at the effluent condenser is classified

as severe, modera te,

and

low based on

the

Kp

factor levels. The severity of corrosion

is indicated as follows:

Severe

(5) Kp

>

0.5

M o d e r a ~ e

M) Kp