PTQ Q3 2022 Issue

Fired heater

Purge gas

To amine wash

Heater exchanger

High pressure separator

Light naphtha

Reactor

Stripper column

Full range naphtha

Reux drum

Naphtha feed

Euent cooler

Reboiler

Reux pump

Hydrogen

Naphtha product

NS bottoms to gasoline pool

Hydrogen compressor

Naphtha cooler

Figure 1 CS 2 removal by hydrotreating

a nickel-based catalyst have also observed that the catalyst is poisoned in the presence of CS 2. A specific catalyst treat - ment is required to recover activity. It is observed that the nickel-based catalyst can tolerate CS 2 contamination in the range of 10-20 ppm with mild temperature elevation as compared to palladium (Pd) cata- lyst. But they tend to be poisoned with further increase in the CS 2 level. However, Pd-based catalyst is more active than a nickel (Ni) catalyst and recovers its activity more easily without any specific treatment once feed conditions come back to their initial level (with CS 2 in the specified range). Moreover, Ni catalyst’s residual activity is minimised with CS 2 poisoning, and it is impossible to recover the initial activity when operating with feed containing a high amount of CS 2 . Thus, while keeping up with growing petrochemical demand, technocrats face greater technological challenges because of the stricter specifications of petrochemical derivatives in comparison with transportation fuels. Processes for producing petrochemical naphtha When facilities were finding ways to diversify in view of the declining fuel market, petrochemical naphtha (PCN)

production emerged as a lucrative option. PCN is light naphtha without CS 2 . Considering that it tends to concen- trate in the light fraction of naphtha, it must be treated for CS 2 removal. Mainly seen in the world’s refineries, the naphtha splitter separates full-range naphtha into heavy naphtha which is routed to the CCR unit, while the light naphtha, primarily used as fuel, is mainly upgraded to a higher RON through the process of isomerisation. Chemically, light naphtha is the fraction which boils between 30°C and 90°C and con- sists of molecules with 5-6 carbon atoms, while the heavy naphtha boils between 90°C and 200°C and consists of molecules with 6-12 carbon atoms. Petrochemical naphtha typically has an IBP of 40°C and an FBP of 130°C. With light naphtha being converted to PCN by processes that remove CS 2 , it caters to the making of ethylene. Steam cracking PCN converts to ethylene, the raw material for most plastics. Other ethylene feedstocks include ethane (C 2 H 6 ) and propane (C 3 H 8 ). Table 1 shows typical petrochemical grade naphtha specifications where the CS 2 specification is as low as 1-2 ppm. CS 2 removal by naphtha hydrotreating Naphtha hydrotreating is an important unit in the refinery, performing hydro-desulphurisation (HDS) of hydrocarbon feed. It is used in petroleum refining to remove sulphur and nitrogen compounds from natural gas and other refined petroleum products like diesel, gasoline, and jet fuel. HDS is the first choice of facilities when it comes to the removal of CS 2 from naphtha (subjected to the availabil- ity of processing). The process flow diagram for a typical naphtha hydrotreating unit is shown in Figure 1 . The unit operates in the presence of hydrogen, wherein CS 2 is con- verted into H 2 S according to the following reaction. The effluent is routed to a stripper where the produced H2 S is recovered from the top product and sent to the sulphur recovery unit (SRU):

Specifications for petrochemical naphtha

S.No. Parameter

Specification

Std.

Typical analysis

method

1 2 3 4 5 6 7 8

Colour, Saybolt

Plus 30 ASTM D 156 Plus 30

Density @ 15ºC, kg/m³, max Allowable CS ² in ppm Total paraffins, vol%, max

700 ASTM D 4052

690

1-2

60-65* ASTM D 5134 1.0 ASTM D 5134

Olefins, vol%, max Aromatics, vol%

<1.0

10-12 ASTM D 5134 10-12

Naphthenes, vol%, max

By Balance

Balance

ASTM distillation

ASTM D 86

IBP, °C

Report

FBP, °C, Max

130

CS 2 + 4H 2  CH 4 + 2H 2 S

Table 1

PTQ Q3 2022