UOP‘S PENEX PROCESS:

The Penex process has served as the primary isomerization technology for upgrading C5/C6 light straight-run naphtha feeds since UOP introduced it in 1958. This process has a wide range of operating configurations for optimum design flexibility and feedstock processing capabilities.

The Penex process is a fixed-bed procedure that uses high activity chloride-promoted catalysts to isomerize C5/C6 paraffins to higher octane branched components. The reaction is conducted in the presence of a minor amount of hydrogen. Even though the chloride is converted to hydrogen chloride, carbon steel construction is used successfully because of the dry environment. For typical C5/C6 feeds, equilibrium will limit the product to 83 to 86 RON (Research Octane Number) on a single hydrocarbon pass basis.

The operating conditions are such that promote isomerization and minimize hydrocracking. Operating conditions are not severe, as reflected by moderate operating pressure, low temperature, and low hydrogen partial pressure requirements.

Ideally, this isomerization catalyst would convert all the feed paraffins to the high octane-number branched structures: normal pentane (nC₅) to isopentane (iC₅) and normal hexane (nC₆) to 2,2- and 2,3-dimethylbutane. The reaction is controlled by a thermodynamic equilibrium that is more favorable at low temperature.

Equipments Used in Penex Process:

 Methanator feed effluent exchanger

 Methanator feed steam exchanger

 Methanator

 Methanator knockout drum

 Make-up gas dryers (2 in number)

 Liquid feed dryer (2 in number)

 Regenerant super heater

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 Regenerant evaporator

 Liquid feed surge drum

 Charge pump (2 in number)

 Chloride drum

 Chloride injection pump (2 in number)

 Combine feed exchanger (3 in number)

 Reactors (2 in number)

 Stabilizer column

 Stabilizer re-boiler

 Stabilizer overhead air cooler

 Stabilizer overhead trim cooler

 Stabilizer overhead separator

 Stabilizer reflux pump (2 in number)

 Net gas scrubber

 Caustic circulation pump (2 in number)

 Caustic tank

 Water circulation pump (2 in number)

 Water Tank

 Water injection pump (2 in number)

Operation and Operating Conditions of some Penex Process Equipment:

2.1.1 Liquid Feed Driers Operation:

Hydro treated SR light naphtha at temperature 45 ⁰C & pressure 4.5 kg/cm² is passed through driers to control moisture at 1.0 ppmw in the feed. Drying medium is the molecular sieves. There are two drier, one remain in operation while the other is on regeneration. Isomerate is used as regenerant. Dry liquid feed is collected in feed surge drum. Molecular sieves are regenerated by isomerate & there replacement depends on the efficiency or after period of four year.

2.1.2 Make Up Gas Driers Operation:

Make up gas is dry by passing into dryers. Molecular sieves used as drying agents.

Dry gas is control at moisture < 1.0 ppmv. Before drying of gas CO & CO₂ is

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removed from the makeup gas. It is accomplished by passing the gas through Methanator. CO & CO2 are converted into methane in presence of Nickel oxide catalyst. Nickel catalyst cannot be regenerated. It is replaced totally; its life is 4-5 years. Temperature & pressure of Methanator is maintained 220 °C & 27 kg/cm². 2.1.3 Reactor Operations:

Combine liquid feed & make up gas is heated in pre-heat exchangers & chloride is injected before entering the reactors. The Reactor System is typically designed to operate at a minimum pressure of 31.6 Kg/cm² (g). Lead reactor inlet temperatures range from 131°C to 200°C and lag reactor inlet temperatures range from 142°C to 186°C. H₂/HCBN mole ratio is maintained as 0.20 at reactors inlet & 0.05 at reactors outlet.

2.1.4 Stabilizer Operation:

Reactor effluents passed through stabilizer where lighter gases & propane is separated from the isomerate. Stabilizer column is operated at temperature 145 °C & pressure 18.0 kg/cm².

2.1.5 Stabilizer Net Gas Scrubber Operation:

The purpose of net gas scrubbers is that to neutralize the net gas prior sending to fuel gas header with caustic (strength is 10%wt). Operating parameters of net scrubbers is that pressure is 6.5 kg/cm² and temperature is 45 °C.

The most common Penex process is Hydrocarbon Once-Through Penex process.

2.1.6 Hydrocarbon Once-Through Penex Process:

Figure 2.1

16 2.1.6.1 Process Description:

Hydrogen Once-Through Penex process flow scheme results in a substantial saving in capital equipment and utility costs by eliminating product separator and recycle gas compressor.

Light naphtha feed is charged to one of the two dryer vessels. These vessels are filled with molecular sieves, which remove water and protect the catalyst. After mixing with makeup hydrogen, the feed is heat-exchanged against reactor effluent. It then enters a charge heater before entering the reactors.

Typically, two reactors in series are used to achieve high on-stream efficiency. The catalyst can be replaced in one reactor while operation continues in the other. One characteristic of the process is that catalyst deactivation begins at the inlet of the first Reactor and proceeds slowly as a rather sharp front downward through the bed. The adverse effect that such deactivation can have on unit on-stream efficiency is avoided by installing two reactors in series. Each reactor contains 50% of the total required catalyst. Piping and valving are arranged to permit isolation of the reactor containing the spent catalyst while the second reactor remains in operation. After the spent catalyst has been replaced, the relative processing positions of the two reactors are reversed. During the short time when one reactor is off-line for catalyst replacement, the second reactor is fully capable of maintaining continuous operation at design throughput, yield, and conversion.

The reactor effluent flows to stabilizer after passing through the heat exchanger. The stabilizer overhead vapors are caustic scrubbed for removal of the HCl formed from organic chloride added to the reactor feed to maintain catalyst activity. After scrubbing, the overhead gas then flows to fuel. The stabilized, isomerized liquid product from the bottom of the column then passes to gasoline blending.

The Penex process (see below) uses the most active chlorided-alumina catalyst and operates in the range 120-180°C.

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LHSV is set during the design phase of any isomerization project and reflects the compromise between residence time and overall catalyst cost. At lower LHSVs, more catalyst is loaded resulting in a longer residence time. As a result lower temperature Operation is possible, resulting in a higher octane product.

System pressure is another variable and is considered in conjunction with the hydrogen flow rate to the reactor. Chlorided-alumina is more active at higher pressures. It requires only a slight excess over stoichiometric hydrogen, since the catalyst does not produce coke. A Penex unit operates at about 30 to 32 bar with once-through hydrogen.

Figure 2.2 (Block flow diagram of ―one through‖ process)

To achieve higher octane, UOP offers several schemes in which lower octane components are separated and recycled back to the reactors. These recycle modes of operation can lead to product octane as high as 93 RON.

2.1.7 Penex Process/DIH (De-isoHexanizer):

This flow scheme is same as Penex Process with an addition of deisohexanizer column to recycle the methylpentanes, n-hexane, and some C6 cyclics. It is the lowest cost option of the recycle flow schemes & provide high octane isomerate product, especially on C6 rich feed.

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Figure 2.3 (Block flow diagram of process with DIP)

2.1.8 Penex Process With Recycle And Fractionation (DIP/Penex Process/DIH):

Separation and recycle of unconverted normal C5 and C6 paraffins and low octane C6 isoparaffins back to the reactor, produce a higher octane product. The most common flow scheme uses a deisohexanizer (DIH) column to recycle methylpentanes, n-hexane, and some C6 cyclics. It is the lowest capital cost option of the recycle flow schemes and provides a higher octane isomerate product, especially on C6 rich feeds.

In the Penex/DIH process the stabilized isomerate is charged to a DIH column producing an overhead product containing all the C5 and dimethylbutanes. Normal hexane and some of the methylpentanes are taken as a side-cut and recycled back to the reactors. The small amount of bottoms (C7+ and some C6 cyclics) can be sent to gasoline blending or to a reformer

.

The addition of a deisopentanizer (DIP) or a super DIH will achieve the highest octane from a fractionation hydrocarbon recycle flow scheme. In this scheme, both low octane C5 and normal and isoparaffin C6 are recycled to the Penex reactors.

Figure 2.4 (DIP-Penex-DIH)

19 2.1.9 Penex/Molex Process:

Figure 2.5 (Penex/Molex Process flow scheme)