Hydrocarbon Processing - December 2020 - 34

Plant Design, Engineering and Construction
SRU is 50% of that from the conventional SRU, since required heat duty in the
sulfur condensers in the modified SRU
is reduced due to internal cooling in the
catalytic beds. For other utilities, the two
units are almost identical.
Before the initial startup of the modified SRU, some personnel expressed
concerns about potential troubles for the
untried process:
* What if the sulfur conversion
efficiency drops during the
intermediate stage of switching
reactors?
* What if switching valves do not
work properly due to sticking
or a mechanical problem?
* What if adsorption or desorption
(regeneration) are affected by
plugging in catalyst pores, etc.?
To allay the first concern, tail gas
sampling over the switching period and
subsequent lab analysis were performed.

During the transient switch, as detailed in
FIG. 6, no loss in performance was discovered because the sequence forces the two
beds in sub-dewpoint operation before
the switch. This results in even better conversion efficiency during transient time.
To address the second issue, fully
steam-jacketed four-way valves were designed, and smooth operation has been
maintained. If separate on-off valves had
been applied instead of the four-way
valves, there might have been mechanical
trouble in those valves, especially under
corrosive and potential plugging (by sulfur carryover) conditions. Lastly, reactor
sequencing has performed well, with no
desorption problems.
The modified SRU is a proven and
commercialized SRU technology with
15 references worldwide. Sub-freezing
mode, instead of sub-dewpoint, is also
available. Through the sub-freezing operation mode, it is possible to increase the

Switch

280

Switch

260
240

Temperature, °C

220
200
180
160
Reactor A temperature, °C
Reactor B temperature, °C

140
120
100
0

4

8

12

16

20

24
28
Time, hr

32

36

40

44

48

52

FIG. 6. Trend of reactor outlet temperature and sequence step in modified SRU.
Treated gas to incinerator
SRU tail gas

Bypass
H2S recycle to SRU
Quench
tower
SCOT
reactor

Fuel

CoMo
catalyst

Air

Steam
CW

Inline burner
(RGG)
WHB

CW

Amine regenerator
(stripper)
Steam
Reboiler

Blowdown

FIG. 7. Shell Claus Offgas Treating (SCOT) process.

34

Amine
Rich
absorber amine
Lean
amine

DECEMBER 2020 | HydrocarbonProcessing.com

sulfur recovery efficiency to 99.8%. SO2
emissions will drop accordingly.
Case study: Selection of tail gas
treating processes. More than 16

kinds of tail gas treating units have been
commercialized. The available TGTU
processes can be divided into two broad
categories: dry bed processes and wet
scrubbing processes. However, considering SO2 emissions at a maximum of 250
ppmv, only four options are applicable:
* Shell Claus Offgas Treating
(SCOT) process
* Wellman-Lord (W-L) process
* Ammonium Thiosulfate (ATS)
process
* Parsons' BSRP (Beavon-Stretford)
process.
All four processes are in the wet scrubbing category. The SCOT process (FIG. 7)
is used more than 60% of the time. The
modified SRU has a very low concentration of sulfur species, so a combination
of the aforementioned TGTUs does not
look cost-effective nor operationally efficient. As a result, the conventional SRU
with the SCOT process, and a combination of the modified SRU with alternatives, will be compared to each other in an
effort to optimize their combination.
The SCOT appears reliable as long as
the parent conventional SRU is operated
reasonably close to design condition. The
SCOT process is sensitive to tail gas composition changes. If the H2S/SO2 ratio in
the tail gas from the SRU is grossly off,
then the SCOT unit may experience operating difficulties. For example, if excessive
temperature rise occurs across the reactor
bed, it indicates too much SO2 in the SRU
tail gas. Or, if all the SO2 in the feed to the
reactor is not converted to H2S, then this
excess SO2 (called SO2 breakthrough) fed
to the quench system is absorbed by the
circulating water, resulting in corrosion.
The trouble related to amine quality, such
as amine foaming, is another major upset
with SO2 breakthrough.
The SCOT process is reported to have
several benefits:
* Easy adaption to an existing SRU
* Use of familiar process technology
and equipment
* Easy and flexible operation
* Elimination of secondary air
and water pollution
* High degree of sulfur removal over a
wide range of operating conditions.


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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
Hydrocarbon Processing - December 2020 - 12
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Hydrocarbon Processing - December 2020 - 14
Hydrocarbon Processing - December 2020 - 15
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Hydrocarbon Processing - December 2020 - 17
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
Hydrocarbon Processing - December 2020 - 43
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Hydrocarbon Processing - December 2020 - 50
Hydrocarbon Processing - December 2020 - 51
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Hydrocarbon Processing - December 2020 - 55
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Hydrocarbon Processing - December 2020 - 60
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Hydrocarbon Processing - December 2020 - 63
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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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