Jun-2021

Dividing wall revamp boosts octane and throughput

Dividing wall column technology reduces energy and capex, but increasingly contributes to higher revenues from better products, as in this application to light naphtha isomerisation.

MANISH BHARGAVA and ANJU PATIL
DWC Innovations

Article Summary

Economic growth and environmental awareness require refineries to produce clean, high-octane gasoline products. The octane number or RON is primarily the knock resistance measure of gasoline. It has a numerical value from 0 to 100 and primarily describes the behaviour of the fuel in the engine during combustion at lower temperatures and speeds. To take RON values to higher levels, the reforming and light naphtha isomerisation process became an integral part of refineries. Light naphtha isomerisation not only produces high octane isomerate products but it also takes care of the latest stringent gasoline specifications. Isomerisation units can handle the benzene content of the gasoline pool and most benzene and its precursors are diverted to the light naphtha fraction as the feed to this unit. The isomerisation unit saturates benzene to cyclohexane. The configuration of an isomerisation unit depends on the required RON value of the gasoline pool.

Overview of a light naphtha isomerisation unit
Isomerisation and reforming are processes which help to improve the octane barrel of the end product by either converting straight chain paraffins to their branched isomers or by changing linear hydrocarbons into branched alkanes and cyclic naphthenes which are then partially dehydrogenated to produce high-octane aromatic hydrocarbons.

Isomerisation reactions are reversible and mildly exothermic. Conversion to iso-paraffins does not reach completion since the reaction is equilibrium governed. The presence of other components in the feed such as benzene and naphthenes tends to raise the reaction temperature as benzene saturation and naphthene ring opening are highly exothermic, while low temperatures favour the conversion of n-paraffins to iso-paraffins. However, operating at low temperatures will decrease the reaction rate, so to overcome this a very active catalyst is usually employed.

Light naphtha isomerisation is evaluated on the basis of the product yields and the RON of the isomerate product. The liquid product yield is determined principally by the extent of hydrocracking which takes in the isomerisation unit. Hydrocracking is an undesirable side reaction which converts light naphtha into light hydrocarbon gas molecules which are low RON components. There is an inbuilt tendency in molecules with higher molecular number such as the heptanes and above to crack and produce undesirable components. C7+ molecules are diverted to the CCR unit instead of the isomerisation unit, and benzene and benzene precursors help to manage this well. Figure 1 shows the primary light naphtha isomerisation reactions.

The isomerisation reaction enhances the octane values of straight chain alkanes by isomerising the n-pentane (nC5, RON value 62) to iso-pentane (iC5, RON 93.5). Other low RON components like nC6 (RON 31) are isomerised to 2-methylpentane (2MP, RON 74) and 3-methylpentane (3MP, RON 76). These 2MP and 3MP molecules are then isomerised to 2,2-dimethylbutane (22DMB) with a RON of 94 and 2,3-dimethylbutane (23DMB) with a RON of 104. Table 1 shows the comparative RON values of individual components in an isomerisation unit.

Unit configurations to meet RON requirements
As isomerisation reactions are in equilibrium, various methods are used to push the reactions in the forward direction. The target RON desired for the combined isomerate product depends on two criteria, the first being removal of products from the isomerate stream and the other recycling low octane molecules from the product back to the isomerisation reactors. For this purpose, the depentaniser and deisopentaniser are installed. It can be concluded that, since the isomerisation reactions are in equilibrium, the product octane number is defined by the number of separation units in the process. The sequence of columns used for separation of isopentanes or isohexanes clubbed with the isomerisation unit with recycle give benefits on account of managing the equilibrium of the reactions taking place in the isomerisation unit to maximise RON. As the benchmark for RON is increased, there is an increase in reboiler duties which can be attributed to the following factors:
- Increase in RON requires a sharp separation between low and high-octane molecules due to which the reboiler load of the column increases.
- RON can also be increased by recycling low RON components back to the reactor, which increases the reboiler duties of the downstream columns.

Apart from increased opex, the capex involved in deploying various columns in the process of boosting octane makes the technocrats in the facility rethink other options. Also, with facilities that have already invested in these columns, further enhancement of RON is always on the table. In this article, we will be discussing the various configurations of an isomerisation unit. Figure 2 shows the typical location of an isomerisation unit in a refinery. The top product from the naphtha splitter with C7 less than 3% is sent to the isomerisation unit.

Typical configurations of isomerisation units in a refinery
Once-through vs hexane recycle

The simplest isomerisation units are once-through units. Fresh feed is sent to a feed pretreatment section and then passes through a series of isomerisation reactors after mixing with hydrogen gas where it comes into contact with the catalyst. The reactor effluent is sent to a stabiliser column where hydrogen and light hydrocarbons produced due to hydrocracking are removed from the top as off-gas and the isomerate is removed from the stabiliser bottoms. The RON achievable through these once-through processes is about 85.

Refineries that want to achieve RON beyond a value of 85 deploy ways to recycle low octane molecules back to the isomerisation reactor. Facilities which have an isomerisation reactor followed by a deisohexaniser (DIH) column can achieve a product RON of up to 88 by recycling a high percentage of the n-hexane, 2-methylpentane, and 3-methylpentane, which are low in RON, back to the reactor.
This is achieved by drawing the mid-cut from the DIH column. The DIH produces a light isomerate distillate product consisting of C5 and branched C6 molecules (rich in DMBs), a C6 recycle side draw, and a C7+ bottoms product. For a recycle stream at 60% of the fresh feed, an octane increase of several points is achieved compared to the once-through operation. Typically, one can expect a RON increase from 83-84 to 87-88 with DIH column hexane recycle to the isomerisation reactor. Figure 3 shows a typical configuration of a light naphtha isomerisation unit with hexane recycle.