Hydrocarbon Processing - December 2020 - 32

Plant Design, Engineering and Construction
inline burners. In the case of fuel gas cofiring with acid gas, the analysis of fuel
gas components may be additionally required. The sulfur recovery efficiency is
calculated according to Eq. 4:
R (%) = [1 - (C/S)inlet ÷
(C/S)outlet ] × 100

the battery limit defined by the flue gas
stream leaving the top of the incinerator
stack, expressed as kmol/hr.
For calculation accuracy, a flowmeter
is required in the incinerator stack.

(4)

Case study: Selection of SRU (conventional SRU vs. modified SRU). A

where:
(C/S)inlet = Carbon-to-sulfur ratio upstream of the reaction furnace at Claus
thermal stage.
(C/S)outlet = Carbon-to-sulfur ratio
downstream of the catalytic reactors in tail
gas streams.
Vapor sulfur concentration in the outlet
stream should be calculated with an equation including pressure, temperature and
dewpoint depression created by the catalyst. Each carbon and sulfur component
can be measured with gas chromatography.
For the recovery calculation from the
SRU to the incinerator stack through the
TGTU, Eq. 5 can be used:

conventional SRU typically consists of the
equipment and piping shown in FIG. 4. Catalytic conversion is maximized in three (or
two) successive, fixed catalytic beds [with
activated alumina and titanium oxide
(TiO2 ) catalyst] at low temperature. The
reactor inlet temperatures are maintained
by reheat exchangers at approximately
240°C, 205°C and 190°C, respectively.
The first catalytic reactor is maintained at
sufficiently higher temperature to ensure
carbonyl sulfide (COS) and carbon disulfide (CS2 ) conversion on titanium catalyst,
which are formed in the thermal reactor.
The Claus catalytic reaction is exothermic
and is favored at low temperature. However, each reactor's inlet temperature should
be maintained above the sulfur dewpoint
to prevent sulfur condensation on the
catalyst. Produced sulfur at each step is removed in the condenser to improve desirable reaction in the following catalyst beds.
The sulfur is drained via seal legs into the
sulfur pit, degassed and sent for export.

R (%) = 100 × [1 - (Stail ÷ Sin)]

(5)

where:
Sin = The quantity of sulfur entering
the plant battery limits with acid gases and
any other streams, expressed as kmol/hr.
Stail = The quantity of sulfur in the gas
streams leaving the plant and crossing
Steam
Acid gas

Air

Waste heat
boiler
Claus burner and
thermal reactor

Catalytic
reactor

Reheater

Tail gas
Steam

Steam

Condenser
Sulfur

Sulfur

FIG. 4. Conventional SRU (typical Claus process).

Four-way valve
Steam
Acid gas

Waste heat
boiler
Claus burner and
thermal reactor

Air

Reheater

Steam

Catalytic
reactor
Steam
BFW

Steam

Reheater

Catalytic
reactor
Steam
BFW

Condenser
Sulfur

Sulfur
Tail gas

FIG. 5. Modified SRU: Switching reactors with internal cooling.

DECEMBER 2020 | HydrocarbonProcessing.com

The catalytic reaction system for the
second technology is unique. Other sections are almost the same as those of the
conventional SRU. The typical process
schematic is shown in FIG. 5. The process
is based on the Claus reaction carried out
in two steps, a first thermal stage and then
two catalytic stages, with the last operating in sulfur sub-dewpoint mode. The
fundamental idea of this process is removing the heat produced by the exothermic
Claus reaction directly in the catalyst
bed, rather than in a downstream heat
exchanger. This controls the temperature
throughout the catalyst bed within a narrow range. The heat exchanger applied is
a thermoplate type with a large clearance.
The space between the plates is filled with
catalyst so that the cooling heat exchanger
is embedded in catalyst.
The first catalytic reactor of a conventional Claus plant is always a compromise
between two targets: the reaction temperature must be high enough for maximum
COS and CS2 hydrolysis and as low as
possible for favorable equilibrium of the
Claus reaction and maximum conversion.
The internally cooled reactor can solve
this conflict. The top layer of the catalyst
is not equipped with thermoplates. The
feed temperature to this adiabatic section
can be adapted to safely reach the required
temperature for COS and CS2 hydrolysis
(usually 300°C-310°C). The second section downstream in the same reactor is
cooled, and a fixed outlet temperature
slightly above the sulfur dewpoint can
be set by an external heat sink (evaporation of boiler feedwater circulating in the
thermoplates). As the heat of reaction is
removed, more sulfur is produced. This
combination of adiabatic and cooled
(pseudo-isothermal) reaction reaches
conversion rates comparable to a twostage, conventional SRU catalytic reactor
at the outlet of the first stage of a modified SRU catalytic reactor. The adiabatic
section in the reactors is filled with TiO2 ,
and the subsequent isothermal section is
loaded with activated alumina.
Downstream follows a second identical reactor that is operated at lower
temperature, shifting the chemical equilibrium toward further sulfur formation.
Sulfur is adsorbed in the catalyst pores in
liquid form. The outlet temperature can
be chosen in a range of 100°C-128°C-
i.e., possibly even below the sulfur solidification point (113°C-119°C).

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Hydrocarbon Processing - December 2020

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