Mercury Saturation Profile Across the Sulphur-Impregnated Activated Carbon Bed
415
from the water wash column and entering the adsorber vessel, evaporate on
activated carbon, depositing a considerable amount of mercury. It is assumed
that such a large quantity of mercury cannot react instantaneously even with
activated carbon, and even less so with impregnated sulphur contained in
activated carbon pores. Furthermore, formation of stable and insoluble mercury
sulphide is there by prevented. Mercury accumulates, therefore, and deposits on
activated carbon particle's surface. As a possible confirmation to this assumption
can be taken the fact that the samples of investigated saturated and replaced
activated carbon show a "decrease" in mercury saturation after a certain
utilization time. Evidently, this may be the mercury that has not been converted
into stable and insoluble mercury sulphide upon chemisorption with elemental
sulphur.
A reduced concentration of impregnated sulphur (8%) is evidenced at the
top of the bed layer, when compared to original carbon. In the deeper layers of
adsorber bed, sulphur content rises to values typical for the fresh carbon (15%).
Regarding the above assumption, we were able to find out and to establish nonuniformity of (total) sulphur content across the adsorber bed (Fig. lC). This
means that during a certain stage of activated carbon utilization, sulphur
becomes flushed and migrates (redistribution from upper towards lower
layers), indicating possible problems and disturbances in the process, and/or
even certain omissions in the general design. Considerable amounts of sulphur
may become irrecoverably lost from activated carbon, since they may be
dissolved and/or flushed by higher (liquid) hydrocarbons if they were not
extracted in an appropriate manner during previous stages (separation,
pretreatment). Therefore, after the first year of plant operation, when problems
with reduction in sulphur content in activated carbon became evident, a
process improvement was introduced, related to change of gas flow direction.
Natural gas was sent towards the heat exchanger, and then to the adsorber and
not, as originally designed, to the adsorber and then to the heat exchanger and
warming-up. The new arrangement prevented adverse impact of liquid
hydrocarbons on sulphur present in activated carbon. But, also elevated
temperatures (100 "c and above) contribute to reduction of sulphur impregnated activated carbon efficiency, due to escape of certain quantity of sulphur
from carbon granule caused by evaporation and/or formation of S02. Already
at a temperature of >65 "c sulphur sublimes, i.e. passes from the solid to the
gaseous state, and this aspect of behaviour should also be included into design
and selection procedure.
Further, during the adsorption-chemisorption process, phenomenon whereby
mercury adheres to impregnated activated carbon surface, the sulphide product
is contained - fixed - within the activated carbon pores. Due to the physical
properties, activated carbon is able to absorb also a whole series of organic
compounds as well as water, thus considerably reducing its adsorption capacity
for mercury. We also assumed, that some of the finest pores, which have greatest
adsorption capabilities, may have been totally filled with sulphur during
impregnation stage, whereas an amount of sulphur will be distributed in the
deepest pores of activated carbon, in the section where any contact and reaction
with mercury becomes impossible.
415
from the water wash column and entering the adsorber vessel, evaporate on
activated carbon, depositing a considerable amount of mercury. It is assumed
that such a large quantity of mercury cannot react instantaneously even with
activated carbon, and even less so with impregnated sulphur contained in
activated carbon pores. Furthermore, formation of stable and insoluble mercury
sulphide is there by prevented. Mercury accumulates, therefore, and deposits on
activated carbon particle's surface. As a possible confirmation to this assumption
can be taken the fact that the samples of investigated saturated and replaced
activated carbon show a "decrease" in mercury saturation after a certain
utilization time. Evidently, this may be the mercury that has not been converted
into stable and insoluble mercury sulphide upon chemisorption with elemental
sulphur.
A reduced concentration of impregnated sulphur (8%) is evidenced at the
top of the bed layer, when compared to original carbon. In the deeper layers of
adsorber bed, sulphur content rises to values typical for the fresh carbon (15%).
Regarding the above assumption, we were able to find out and to establish nonuniformity of (total) sulphur content across the adsorber bed (Fig. lC). This
means that during a certain stage of activated carbon utilization, sulphur
becomes flushed and migrates (redistribution from upper towards lower
layers), indicating possible problems and disturbances in the process, and/or
even certain omissions in the general design. Considerable amounts of sulphur
may become irrecoverably lost from activated carbon, since they may be
dissolved and/or flushed by higher (liquid) hydrocarbons if they were not
extracted in an appropriate manner during previous stages (separation,
pretreatment). Therefore, after the first year of plant operation, when problems
with reduction in sulphur content in activated carbon became evident, a
process improvement was introduced, related to change of gas flow direction.
Natural gas was sent towards the heat exchanger, and then to the adsorber and
not, as originally designed, to the adsorber and then to the heat exchanger and
warming-up. The new arrangement prevented adverse impact of liquid
hydrocarbons on sulphur present in activated carbon. But, also elevated
temperatures (100 "c and above) contribute to reduction of sulphur impregnated activated carbon efficiency, due to escape of certain quantity of sulphur
from carbon granule caused by evaporation and/or formation of S02. Already
at a temperature of >65 "c sulphur sublimes, i.e. passes from the solid to the
gaseous state, and this aspect of behaviour should also be included into design
and selection procedure.
Further, during the adsorption-chemisorption process, phenomenon whereby
mercury adheres to impregnated activated carbon surface, the sulphide product
is contained - fixed - within the activated carbon pores. Due to the physical
properties, activated carbon is able to absorb also a whole series of organic
compounds as well as water, thus considerably reducing its adsorption capacity
for mercury. We also assumed, that some of the finest pores, which have greatest
adsorption capabilities, may have been totally filled with sulphur during
impregnation stage, whereas an amount of sulphur will be distributed in the
deepest pores of activated carbon, in the section where any contact and reaction
with mercury becomes impossible.