4.5.2. Catalyst regeneration
The catalyst regeneration can be proceeded in-situ or ex-situ the process. In case of in-situ
regeneration or oxidation, effluents are a non negligible issue even if they do not occur often.
Indeed these gaseous effluents contain a lot of pollutants that have never really been qualified
and quantified and the treatment method currently used seems to not be efficient.
Origin of contaminants
a) Impurities in raw materials
Feed to selective hydrogenation units come from steam cracking or other olefins sources,
consequently many impurities are present, that come from upstream processes. In the same
way, hydrogen contains impurities. Even if these impurities are in very little quantity, they
can fix themselves on the catalyst. When the catalyst is regenerated or oxidised, all these
impurities, which are no longer in trace amount as they concentrated, leave the catalyst and go
out with the gaseous effluent.
These contaminants are various:
- Sulphur compounds (H
2
S, COS, disulphides and mercaptans)
- Methanol and oxygenated compounds
- Nitrogen compounds (HCN, NH
3
, amines …)
- Inorganic chlorides and other mineral salts
- Arsenic
- Mercury
- Chlorhydric acid
b) Parasite reactions
As described before, catalyst and operating conditions in selective hydrogenation aim at
privilege the hydrogenation of dienes or alkynes into olefins and disadvantage other chemical
reactions that could occur. However dimerization (chemical union of two identical molecules)
and then oligomerization can occur. It is assumed that 10% of dienes and alkynes are
dimerized. In case of C
4
hydrogenation, the product of dimerization is called green-oils. These
molecules (C
4
to C
20
) coat the catalyst and deactivate it but can be removed by regeneration.
Catalyst regeneration principle
During catalyst regeneration procedure, a combustion (or oxidation) is performed in order to
burn deposited coke (or green oils). Other impurities can also be stripped by this process.
Operations of catalyst regeneration are a bit different depending from the process.
Nevertheless, main steps are more or less the same:
- Heating the catalyst bed by circulating heated nitrogen through the reactor.
- Catalyst bed stripping by establishing a steam circulation through the reactor (this step is not
always performed).
- Catalyst pre-oxidation by slowly adding air to the steam.
- Catalyst impurities burning-off by raising temperature and injecting air again.
- Cooling and purge with steam first and then nitrogen.
51
The out coming gas is consequently contaminated with CO
2
and CO and also unburned coke
and other impurities. Here below is a list of compounds that can be found in the effluent
regeneration gas:
Compound Maximum
content
Green oils
10 % of the compound that have to be
hydrogenated.
Sulphur (H
2
S, SO
2
, SO
3
) (1)
Mercury (1)
Arsenic (AsH
3
) (1)
Chloride (HCl)
(1)
(1) The quantity of contaminants deposited on the catalyst can not be known. The only data is
the maximum content of contaminants in the raw materials.
Table B-4-2: Compounds possibly present in catalyst regeneration effluents
Decoking drum
The most usual way to treat in-situ regeneration gas in hydrogenation processes are currently
decoking drum. This decoking drum is basically a drum in which the gas is simply washed
with water before being released in the atmosphere. This washing appears to not be really
efficient and pollutants are consequently released both in the atmosphere and in the waste
water.
The following tables summarize effluents from each step of catalyst regeneration for each
type of process.
Heating
Stripping
Pre-oxidation
Burning
Cooling 1
Cooling 2
Gas
components
N
2
Steam
Steam + air
Steam + air
Steam
N
2
Flowrate (1) (1)
(1)
(1)
(1)
(1)
Notice
Presence
of HC
and H
2
O
Presence
of HC
Presence of
CO
2
and HC,
impurities (2)
Presence of
CO
2
and HC,
impurities (2)
Presence of
air, impurities
(2)
-
Destination Flare (3)
Flare (3)
Atmosphere
via decoking
drum (3)
Atmosphere
via decoking
drum (3)
Atmosphere
via decoking
drum (3)
Flare (3)
(1) Refer to operating instructions
(2) Impurities correspond to arsenic, mercury, and other compounds present in trace amount
in raw materials and which are trapped on the catalyst.
(3) These are the usual destination but some units can have a special collecting drum.
Table B-4-3: Gaseous effluents during catalyst regeneration in C
3
selective hydrogenation
52
Heating
Oxidation I
Oxidation II
Air purge
Gas
components
N
2
N
2
+ air
O
2
content:
0,3 % vol
N
2
+ air
O
2
content: 2
% vol
Air
Flowrate (1)
(1) (1)
(1)
Notice Presence
of
HC
Presence of
CO
2
and HC,
impurities (2)
Presence of
CO
2
and HC,
impurities (2)
-
Destination Flare
(3)
Decoking
drum (3)
Decoking
drum (3)
Decoking drum
(3)
(1) Refer to operating instructions
(2) Impurities correspond to arsenic, mercury, and other compounds present in trace amount
in raw materials and which are trapped on the catalyst.
(3) These are the usual destination but some units can have a special collecting drum.
Table B-4-4: Gaseous effluents during catalyst oxidation in C
4
selective hydrogenation
First stage reactor:
Heating
Stripping
Pre-
oxidation
Burning
Cooling 1
Cooling 2
Gas
components
N
2
Steam
Steam +
air
Steam + air
Steam N
2
Flowrate (1) (1) (1) (1) (1) (1)
Notice
Presence of
HC + H
2
O
Presence of
HC
Presence
of HC,
CO
2
Presence of
HC, CO
2
,
impurities (2)
- -
Destination
Decoking
drum or
flare
Decoking
drum
Decoking
drum
Decoking
drum
Decoking
drum
Decoking
drum or
flare
(1) Refer to operating instructions
(2) Impurities correspond to arsenic, mercury, and other compounds present in trace amount
in raw materials and which are trapped on the catalyst.
Table B-4-5: Gaseous effluents during catalyst regeneration in GHU first reactor
53
Second stage reactor:
Heating
Stripping
Pre-
oxidation
Burning
Cooling 1
Cooling 2
Gas
components
N
2
Steam
Steam +
air
Steam + air
Steam N
2
Flowrate (1) (1) (1) (1) (1) (1)
Notice
Presence of
HC + H
2
O
Presence of
HC, H
2
S
Presence
of HC,
H
2
S, CO
2
Presence of
HC, H
2
S,
CO
2
,
impurities (2)
- -
Destination
Decoking
drum or
flare
Decoking
drum
Decoking
drum
Decoking
drum
Decoking
drum
Decoking
drum or
flare
(1) Refer to operating instructions
(2) Impurities correspond to arsenic, mercury, and other compounds present in trace amount
in raw materials and which are trapped on the catalyst.
Table B-4-6: Gaseous effluents during catalyst regeneration in GHU first reactor
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