Hydrocarbon Processing - December 2020 - 35

Plant Design, Engineering and Construction
The SCOT process occurs in essentially
four stages:
1. Heating and reduction
2. Cooling and quenching
3. H2S absorption, stripping and
recycle to the SRU
4. Incineration of offgas from
the amine absorber.
The SCOT process is the best treating unit for use with the conventional
SRU; the number of commercialized sulfur plants proves this assertion. However,
what if the SCOT process is combined
with the modified SRU? Some inefficiencies can be anticipated. Exothermic reaction on the reduction catalyst will be minor; subsequently, the temperature rise
will be negligible. Utility consumption to
treat a small amount of sulfur species will
be excessive with regard to amine regeneration by steam and amine circulation,
while no steam production occurs through
heat recovery. Also, equipment cost will be
extreme for the reaction and amine systems, and operating cost will be higher.
Due to these disadvantages, a caustic
scrubbing system is considered instead.
The process first incinerates SRU tail
gas, generating a stream containing SO2.
This gas, containing 36 mol% of water, is
cooled and the water is removed in the
quench tower to avoid caustic dilution
in the scrubber. The gas is then sent to
a caustic scrubber, where the SO2 is absorbed in an aqueous caustic solution, as
shown in Eq. 6:
2NaOH + SO2 t Na2SO3 + H2O (6)
The purge water is transferred to a
wastewater treatment facility, and the flue
gas is released to atmosphere. The process
uses stainless-steel material or alloy cladding in the caustic scrubber to prevent
corrosion. Daily caustic (NaOH) consumption is estimated at 2.8 t-3 t, based
on a 200-t sulfur production facility. SO2
emissions limits can be designed with a
wide range of 50 ppm-250 ppm. Several
qualified vendors for scrubber design and
manufacturing offer their own technologies, such as venturi types, sprays, water
curtains, etc. A schematic flow diagram of
the process is presented in FIG. 8.
The comparison of the two units is
summarized in TABLE 3. The parameters
of the caustic scrubbing system have been
estimated, while those for the SCOT unit
are taken from actual cases. The SCOT
unit has higher overall sulfur recovery

efficiency than the caustic scrubbing system due to H2S recycle to the SRU and
high efficiency of amine absorption for
H2S. On the other hand, however, only
chemical conversion occurs in the caustic
scrubber, not recovery.
Also, further wastewater treatment is
required for the scrubbing system, e.g.,
oxidizing solution by air. Byproduct sodium sulfate (Na2SO4 ) is generally regarded as nontoxic. The equipment cost
for a caustic system is much lower than
that for the SCOT unit, since the scrub-

bing system consists of a simple configuration without reaction and amine facilities. Its operation cost is more economical
than that of the SCOT unit, mainly due
to steam consumption in the amine unit.
The caustic system is not sensitive to tail
gas composition because there is no catalytic reactor, and all H2S is converted to
SO2 in the upstream incinerator.
Furthermore, the caustic system can
handle a wide range of tail gas composition changes without process upsets like
SO2 breakthrough or amine foaming/carFlue gas to atmosphere

SRU tail gas
Quench
tower
Caustic
scrubber

HP steam
LP steam
Incinerator
WHB/steam
superheater

Fuel

Makeup water

Purge to
wastewater
treatment

CW

Incinerator
Air

Caustic
tank

BFW
Blowdown

FIG. 8. Caustic scrubbing system.

TABLE 3. Comparison summary between two TGTU processes
Item

SCOT process (combined with
the conventional SRU)

Caustic scrubbing system
(combined with the modified SRU)

Total capacity

200 tpd of sulfur production

200 tpd of sulfur production

Configuration

2 trains, catalytic reactors, quench 1 train, incinerator, quench system,
system, amine system, incinerator caustic scrubber

Total investment cost

$35.8 MM

$16.2 MM

LP steam consumption

11.3 tph (amine regeneration
reboiler)

0 tph

Chemical consumption

Amine makeup for loss or
degradation

Caustic makeup for
consumption (2.8 tpd-3 tpd)

Overall sulfur recovery
efficiency (includes
SRU performance)

Min. 99.9%

Min. 99.5%

SO2 emissions*

Max. 250 ppmv

50-250 ppmv

Advantages

*
*
*
*

Disadvantages

* Higher investment cost
* Required additional wastewater
* Sensitive to tail gas composition
treatment
* Required amine quality control
* Not applicable to conventional
SRU

Higher sulfur recovery efficiency * Lower investment cost
Easy, flexible operation
* Economical operation and
Familiar process and equipment
utility cost
No water pollution
* Wide range of operating
conditions
* Minimized SO2 and CO2 emissions

* The concentrations of SO2 emissions to atmosphere are calculated based on 3% excess oxygen and dry conditions
at the incinerator and the caustic scrubber stack, respectively.

Hydrocarbon Processing | DECEMBER 2020

35



Hydrocarbon Processing - December 2020

Table of Contents for the Digital Edition of Hydrocarbon Processing - December 2020

Contents
Hydrocarbon Processing - December 2020 - Intro
Hydrocarbon Processing - December 2020 - Cover1
Hydrocarbon Processing - December 2020 - Cover2
Hydrocarbon Processing - December 2020 - Contents
Hydrocarbon Processing - December 2020 - 4
Hydrocarbon Processing - December 2020 - 5
Hydrocarbon Processing - December 2020 - 6
Hydrocarbon Processing - December 2020 - 7
Hydrocarbon Processing - December 2020 - 8
Hydrocarbon Processing - December 2020 - 9
Hydrocarbon Processing - December 2020 - 10
Hydrocarbon Processing - December 2020 - 11
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Hydrocarbon Processing - December 2020 - 15
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Hydrocarbon Processing - December 2020 - 18
Hydrocarbon Processing - December 2020 - 19
Hydrocarbon Processing - December 2020 - 20
Hydrocarbon Processing - December 2020 - 21
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Hydrocarbon Processing - December 2020 - 26
Hydrocarbon Processing - December 2020 - 27
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Hydrocarbon Processing - December 2020 - 31
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Hydrocarbon Processing - December 2020 - 40
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Hydrocarbon Processing - December 2020 - 42
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Hydrocarbon Processing - December 2020 - 50
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Hydrocarbon Processing - December 2020 - 79
Hydrocarbon Processing - December 2020 - 80
Hydrocarbon Processing - December 2020 - 80A
Hydrocarbon Processing - December 2020 - 80B
Hydrocarbon Processing - December 2020 - 81
Hydrocarbon Processing - December 2020 - 82
Hydrocarbon Processing - December 2020 - Cover3
Hydrocarbon Processing - December 2020 - Cover4
Hydrocarbon Processing - December 2020 - GP-1
Hydrocarbon Processing - December 2020 - GP-2
Hydrocarbon Processing - December 2020 - GP-3
Hydrocarbon Processing - December 2020 - GP-4
Hydrocarbon Processing - December 2020 - GP-5
Hydrocarbon Processing - December 2020 - GP-6
Hydrocarbon Processing - December 2020 - GP-7
Hydrocarbon Processing - December 2020 - GP-8
Hydrocarbon Processing - December 2020 - GP-9
Hydrocarbon Processing - December 2020 - GP-10
Hydrocarbon Processing - December 2020 - GP-11
Hydrocarbon Processing - December 2020 - GP-12
Hydrocarbon Processing - December 2020 - GP-13
Hydrocarbon Processing - December 2020 - GP-14
Hydrocarbon Processing - December 2020 - GP-15
Hydrocarbon Processing - December 2020 - GP-16
Hydrocarbon Processing - December 2020 - GP-17
Hydrocarbon Processing - December 2020 - GP-18
Hydrocarbon Processing - December 2020 - GP-19
Hydrocarbon Processing - December 2020 - GP-20
Hydrocarbon Processing - December 2020 - GP-21
Hydrocarbon Processing - December 2020 - GP-22
Hydrocarbon Processing - December 2020 - GP-23
Hydrocarbon Processing - December 2020 - GP-24
Hydrocarbon Processing - December 2020 - GP-25
Hydrocarbon Processing - December 2020 - GP-26
Hydrocarbon Processing - December 2020 - GP-27
Hydrocarbon Processing - December 2020 - GP-28
Hydrocarbon Processing - December 2020 - GP-29
Hydrocarbon Processing - December 2020 - GP-30
Hydrocarbon Processing - December 2020 - GP-31
Hydrocarbon Processing - December 2020 - GP-32
Hydrocarbon Processing - December 2020 - GP-33
Hydrocarbon Processing - December 2020 - GP-34
Hydrocarbon Processing - December 2020 - GP-35
Hydrocarbon Processing - December 2020 - GP-36
Hydrocarbon Processing - December 2020 - GP-37
Hydrocarbon Processing - December 2020 - GP-38
Hydrocarbon Processing - December 2020 - GP-39
Hydrocarbon Processing - December 2020 - GP-40
Hydrocarbon Processing - December 2020 - GP-41
Hydrocarbon Processing - December 2020 - GP-42
Hydrocarbon Processing - December 2020 - GP-43
Hydrocarbon Processing - December 2020 - GP-44
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