Figure 1 shows the hydrotreating unit in operation, which typically receives feed from the top (overhead) of the naphtha splitter. This is premixed with hydrogen before routing it to the hydrotreater reactor, which converts the sulphur present in the feed into H 2 S, yielding product free from sulphur. Though hydrotreating is the process of choice when it comes to attaining the desired specification with respect to the CS 2 content in the petrochemical naphtha, this only applies to cases when the facility has the available capacity in the hydrotreating unit, along with surplus hydrogen. In refineries where the existing hydrotreating units are already bottlenecked, treating light naphtha by this route becomes very capital intensive. Also, a big reason for facilities not considering the hydrotreating of petrochemi- cal grade naphtha is because this stream is otherwise low in contaminants. For this reason, hydrotreating results in substantial increases in operating costs. CS 2 removal by physical adsorption Another processing route to control CS 2 content in pet- rochemical grade naphtha is physical adsorption. For the adsorption process to be effective, the contaminant con- centration and temperature become important parameters. As these processes are fixed-bed type, continuous regen - eration is needed. This makes the process OPEX intensive and tedious, and for reasons mentioned, technocrats look forward to processes that are less OPEX and CAPEX inten- sive and help attain the desired PCN purity. CS 2 removal by distillation As CS 2 has a boiling point higher than C 5’ s and lower than C 6 hydrocarbons, CS 2 can be effectively removed along with C 5’ s via distillation. Though an accurate thermody- namic model is required to predict the behaviour of CS 2 in PCN, distillation is nonetheless another promising and interesting method for CS 2 removal from PCN. As distillation takes advantage of the CS 2 boiling point lying between isopentane and hexane, it is possible to use distillation and obtain three cuts from the column. By this route, isopentanes and normal pentane carry most of the CS 2, while PCN, which is withdrawn as the bottom cut, will have minimal CS 2 content. Table 2 provides the boiling points of components in a typical PCN mix. Figure 2 shows PCN production using the distillation method. Though distillation is an effective way to reduce CS 2 content, employing conventional distillation methods has its own set of limitations: • High energy consumption to get desired PCN specifications • Overhead product is predominantly a mix of iC 5 /NC 5 • Low RON, blending into the gasoline pool will lower the overall RON. This bottleneck with conventional distillation schemes was addressed by technocrats, who developed new mod- els, including hybrid schemes based on divided wall col- umn (DWC) technology. DWC technology was successfully implemented in a Southeast Asia facility, to be discussed further.
Boiling point of components in PCN mix
Typical components in petrochemical
Boiling point,ºF
naphtha Isobutane N-Butane
10.9 30.2 82.0 97.0
Iso-Pentane N-Pentane
Carbon-Disulphide
115.3 120.6 121.5 136.4 140.0 147.2 156.2 161.2 176.2 177.4 174.2 176.9 177.8 186.8
Cyclopentane
2,2-Dimethyl-Butane 2,3-Dimethyl-Butane 2-Methyl-Pentane 3-Methyl-Pentane
N-Hexane
Methylcyclopentane
Benzene
Cyclohexane
2,2-Dimethylpentane 2,4-Dimethylpentane 2,2,3-Trimethylbutane 3,3-Dimethylpentane
Table 2
Why DWCs? Process optimisation techniques of late, especially DWC technology, are for improving overall profitability through process intensification and optimisation. This is done for better product specifications, decreased energy consump - tion, or capacity augmentation through robust simulation models and engineering. DWCs have provided an effective way to reinvent age- old distillation methods and offer the benefits of lower capital investment and lower operational costs compared to their conventional counterparts. It is a highly adaptable technology in which either a single wall or multiple walls can be installed inside the shell according to the process requirement. The benefits of this technology can be sum - marised as follows: •It can separate the feed into three or more high-purity streams from a single column in a sequential distillation •Ideal alternative for revamp of side-cut columns when high purity is required from the three product streams • Lower footprint as equipment count is reduced by half • Operational and capital expenditure are reduced by approximately 20-50%.
C rich to gasoline pool
Light naphtha
Full range naphtha
Naphtha splitter
Depentaniser
NS bottoms to gasoline pool
Petrochemical naphtha
Figure 2 Conventional distillation scheme for obtaining PCN
71
PTQ Q3 2022
www.digitalrefining.com
Powered by FlippingBook