Example Desulphurization Process

The procedure that has been discussed will be applied to a desulphurization process (Fig. 33.5). points will be discussed simultaneously. 

In Fig. 33.5 a simplified flow diagram of the process is shown, the reactor is the central part in this diagram. Since the chemical reactions have an exothermal character, intermediate cooling takes place through injection with cold feed. 

Process boundaries and external disturbances. Several process units, such as the desulphurization with an absorber-stripper combination and the separation section, consisting of multiple distillation colunms, have been omitted, since they operate more or less independently. 

The input conditions of the first catalyst bed are fixed as much as possible by control of the following variables  

The inlet conditions of the second and third bed depend strongly on the reactions in the first bed, it is therefore more difficult to keep them constant. It is, however, possible to keep the inlet temperatures constant through injection with cold feed. 

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As there now exists a large body of laboratory studies on each of the variable systems, for example the effect of die lime/silica ratio in the slag on the desulphurization of liquid iron, the most appropriate phase compositions can be foreseen to some extent from these laboratory studies when attempting to optimize the complex indusuial process. The factorial uials are not therefore a shot in the dark , but should be designed to take into account die laboratory information. Any qualitative difference between die results of a factorial uial, and the expectations predicted from physico-chemical analysis might suggest the presence of a variable which is important, but which was not included in the nials. 

From the point of view of clean technology hydrotreatment is gaining increasing interest. A recent example is diesel fuel. Sulphur-containing compounds in diesel fuel cause serious difficulties in catalytic cleaning of the exhaust gases. Undoubtedly, in the near future, novel processes will be needed for the deep desulphurization of diesel. 

A convenient synthesis of sulphenyl chlorides has been developed in which a thiol is reacted with chlorocarbonylsulphenyl chloride and the resulting adduct desulphurized by triphenylphosphine. Further examples - have been enumerated in which olefins are stereospecifically synthesized by a two-fold extrusion process, one of which is the removal of sulphur by a phosphine, e.g. in the synthesis of bicyclobutylidene (25). 

The process of SO2 removal on activated coke followed by a simultaneous reduction of NO with ammonia has been successfully applied in industry [176]. Mitsui Mining Process [177] (Table 12) and the Sumitomo Heavy Industry Process [178] are examples of the simultaneous desulphurization and NO removal with the application of moving beds of carbon adsorbents. Apart from the two above mentioned target gases, these processes also exhibit high removal efficiency for heavy metals and dioxins. Removal of NO on activated carbons can also be carried out using two separate processes. As NO2 can be easily removed from gas streams by water, low-temperature oxidation of NO to NO2 on the porous carbon surface is considered as feasible for the removal of NO without the ammonia addition [179]. 

Atypically, there are a few materials which occur in an elemental form. Perhaps the most notable example is sulphur, which occurs in underground deposits in areas such as Louisiana, Southern Italy and Poland. It can be brought to the surface using the Frasch process in which it is first melted by superheated steam and then forced to the surface by compressed air. This produces sulphur of high purity. Substantial quantities of sulphur are also removed and recovered from natural gas and crude oil (petroleum). This amounted to 24 million tonnes out of a total world sulphur production of 37 million tonnes in 1991, and clearly demonstrates the vast scale on which the oil and petrochemical industries operate since crude oil normally contains between 0.1 and 2.5% of sulphur, depending on its source. Desulphurization of flue gases from some U.K. power stations will be another source of sulphur in the future. Over 80% of all sulphur is converted into sulphuric acid, and approximately half of this is then used in fertilizer manufacture. 

The absorption by the sea of SO2 released into the atmosphere from chimneys can be viewed as a particular case of flue gas desulphurization (FGD). In what follows we consider two examples of FGD processes using seawater the inertion of engine exhaust on oil tankers and the reduction of SO2 emission on coastal industrial plants. Because of its alkalinity, seawater can absorb approximately three times as much SO2 as fresh water. 

Hydrocyclones fitted with replaceable elastomeric wear liners are widely used in a number of applications where the solid material in suspension is abrasive, including mineral processing, flue gas desulphurization and coal processing. Some examples of the wide range of use are shown in Table 7.1. 

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