High sulphur recovery with ease 
of operation

A process performing sulphur recovery and tail gas treatment in two catalytic reactor units exceeded guaranteed recovery limits from start-up.

MICHAEL HEISEL, ITS Reaktortechnik

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Article Summary

There are thousands of Claus plants worldwide to convert poisonous H2S into elemental sulphur. A typical two-stage Claus plant reaches 95% to 97% sulphur recovery efficiency. In most countries this is not sufficient to meet environmental protection requirements. Almost always a tail gas treatment has to follow downstream.

In the past, authorities mostly asked for a sulphur recovery efficiency as per local legislation, equivalent to a certain emission level for SO2. Other emissions, notably CO2, were of much less or even no concern. However, since the climate change debate focuses on limiting all emissions, CO2 has become important.

Smartsulf is a relatively new process, but already well proven to minimise sulphur emissions. The only products are bright yellow premium quality sulphur and steam. It does not make by-products. Fuel consumption is limited to the obligatory incineration of the tail gas. In all these aspects it compares very favourably to other, more conventional sulphur recovery and tail gas treatment processes. In addition, the process is easy and reliable to operate. Normal operation is fully on automatic control. Only conditions out of the normal steady state, as in starting up and shutting down, need to be operated on manual control. As a co-licensor with ITS Reaktortechnik GmbH of this sub-dewpoint technology at the time the project was sanctioned, WorleyParsons developed the Basic Engineering Design Package for the SRU at the Pemex Salamanca Refinery. WorleyParsons also provided commissioning, start-up and performance  test assistance to Pemex for the new Salamanca SRU under separate contract

Process principles
Smartsulf is a sulphur recovery process with an improvement to the catalytic part in contrast to the conventional Claus process. A typical process flow diagram is shown in Figure 1.

The fundamental idea of Smartsulf is to remove the heat of the Claus reaction directly in the catalyst bed rather than in a downstream heat exchanger. This controls the temperature throughout the catalyst bed very close to the optimum for chemical equilibrium which results in a substantial increase of sulphur recovery efficiencies. The heat exchanger, applied to absorb the heat of reaction, consists of thermoplates with large clearances. The space in between the plates is filled with catalyst in order to control efficiently its temperature. This is advantageous for a number of reasons.

In the catalytic converter, several reactions take place at the same time. Maximum COS and CS2 hydrolysis require a high temperature, while the Claus reaction needs a low temperature to achieve a favourable chemical equilibrium and thus maximum conversion and recovery of sulphur. The internally cooled Smartsulf reactor can solve this issue: the top layer of the catalyst is not equipped with thermoplates. The feed temperature to this adiabatic section can be adjusted to reach safely the required temperature for COS and CS2 hydrolysis. The second section downstream in the same reactor is cooled and a fixed outlet temperature is set by an external heat sink, for instance by evaporating boiler feed water. This combination of adiabatic and cooled reaction optimises conversion efficiencies.

A second identical reactor is added downstream to take advantage of the thermoplate exchanger and operate at even lower temperature, and thus optimise conditions for the Claus reaction even further. At the outlet of the second reactor, the temperature is below the sulphur dew point or even the sulphur solidification point. The elemental sulphur formed is adsorbed by the catalyst. The evenly low temperature throughout the bed causes a substantial increase in sulphur recovery efficiency up to 99.9%. For such high sulphur recovery efficiency in other processes, much more complex plant configurations are required.

Detailed process description
With reference to Figure 1, as in any conventional Claus plant, the feed acid gas is burned substoichiometrically in a Claus furnace and the resulting heat recovered in a waste heat boiler (1), followed by the 
No. 1 condenser and liquid sulphur separation. Then the process gas is sent via the No. 1 preheater (2) and a first four-way valve (3A) to the first Smartsulf reactor (4A) for continuation of the Claus reaction:

2 H2S + SO2 ⇔ 3/x Sx + 2 H2O                      (1)

x = 2,4,6,7,8 indicates the different number of sulphur atoms per molecule

In addition, COS and CS2 are converted in this reactor.

The outlet gas from the first reactor passes via the second four-way valve (3B) to the No. 2 sulphur condenser (5) which operates at temperatures between 135°C and 150°C and produces low pressure steam. The process gas passes through the sulphur separator. Before entering the second reactor the gas is again reheated (6). 
In the 2nd reactor (4B) the Claus reaction proceeds towards even more favourable equilibrium at lower temperature. The purified gas flows via the second four-way valve (3B) to an incinerator (8). From there, it is released to the atmosphere.

Switch-over procedure
During operation in the sub-dewpoint mode, elemental sulphur is accumulated on the catalyst in the lower bed of the second reactor. Once the pores of the catalyst are saturated with sulphur, it has to be regenerated. Regeneration is accomplished by simply reversing the sequence of the two reactors. What was the first reactor (4A) is free of sulphur and shifts to the adsorber position. The sulphur saturated second reactor (4B) then becomes the first reactor. The sulphur in this reactor will be desorbed in the hot operation condition of the first reactor. A reactor sequence control device calculates and initiates the cyclic switch-over procedure. The sequence runs fully automatic without requiring any operator action. The cycle time is typically 24 hours between switch-overs.

Start-up experience at Pemex Salamanca refinery - The Smartsulf plant
Figure 2 shows the Smartsulf unit installed at the Pemex refinery Ing. Antonio M. Amor in Salamanca, Guanajuato, Mexico.

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