Special Report Process/Plant OptimizationN. TORAMAN and K. KAHRAMAN, TÜPRAŞ-Turkish Petroleum Refineries Corp., Kırıkkale Refinery, Kırıkkale/Turkey; P. BILLINGS and O. SABITOV, Honeywell UOP, Guildford, UK

Optimize isomerization reactor temperatures and component RON

One of the components of a refinery’s gasoline pool is isomerate, which is created mainly from isomers of pentane and hexane fraction by turning straight-chain alkanes into branched molecules (Eq.1).

(1)

n-Pentane→Isopentane

H—

H|

H|C—

H|

H|C—

H|

H|C—

H|

H|C—

H|

H|C—H→H—

H|

H|C—

H|

CH3|C —

H|

H|C—

H|

H|C—H

Isomerate is useful because it is paraffinic and contains no benzene, aromatics or sulfur. This pentane and hexane fraction of crude oil is known as light naphtha. The main objective of reactions is to increase the research octane number (RON) of the components. Decreasing fuel consumption without changing the product RON of isomerization units is discussed here.

Light straight-run naphtha (LSRN) is a fractionation cut of crude oil that contains C5 and C6 hydrocarbons. To formulate gasoline blend, some additives, reformate and LSRN are used to increase the RON value. Reformate, which is produced from heavy naphtha via continuous catalyst regeneration (CCR) or semi-regeneration reforming reactions, has high RON and aromatics values. However, because of the benzene content with carcinogenic properties, a limited amount of reformate can be used in the gasoline pool.

Achieving higher RON values. To obtain the desired RON value and aromatics content, other components are used, such as LSRN with a RON value of 65 to 70 and methyl tertiary butyl ether (MTBE). Prior to the restrictions on unleaded gasoline, LSRN itself was used in the gasoline pool and a lead additive was used to increase the gasoline pool’s RON value. However, environmental regulations and legal restrictions prohibit adding lead.

To achieve a higher RON value for LSRN, many catalytic processes have been developed. Since the isomerization reactions are thermodynamically in equilibrium, it is not possible to convert all the normal paraffin into their isomers by a “once-through” isomerization process. If normal paraffin were separated from the isomerized hydrocarbons and combined with the main feed, it would be possible to obtain a product that is totally composed of hydrocarbons that are converted to

their isomers. A total isomerization process (TIP) product has been developed and has a RON value of 85 to 88, with a lower amount of benzene and aromatics content. This means more isomerate could be used in the gasoline blend, lessening the need for other components, such as reformate and oxygenates.

Isomerization scheme. A typical and general isomerization process is shown in FIG. 1. The unit’s TIP section comprises two parts. The first is isomerization, where the isomerization reactions and adsorption of iso-hydrocarbons from n-hydrocarbons take place. A naphtha hydrotreater section eliminates the sulfur content of feed, as sulfur is a poison for isomerization reactions and catalyst. The second section is the stabilization section, where offgases, which contain lower hydrocarbons than C5 and C6, are separated from the final product of the unit, isomerate.

The isomerization unit in the TUPRAS Kırıkkale refinery is licensed by Honeywell UOP and was put into operation in 1998. FIG. 2 illustrates the basic flow scheme. Isomerization reactions take place in fixed-bed reactors in the gaseous phase. TIP units typically use two reactors in series. The system pressure is kept around 14 barg to 28 barg and at a temperature range of 230°C to 280°C in a hydrogenated medium.

In a TIP reactor section, reactors have nearly 10 m3 and 30 m3 of catalyst volume, respectively. The catalyst has platinum metals on a zeolite basis. The isomerization of n-paraffins into iso-paraffins is the main reaction type occurring on the catalyst. However, some other reactions may also occur on the catalyst, affecting the product RON value and yield in a negative manner. These reactions are benzene hydrogenation, ring-opening reactions on naphthenes, and paraffin hydrocracking.

In the first reactor, mainly normal paraffins are converted into iso-paraffins. The ring opening of naphthenes, aromatic

Feed C5-C6Isomerization

O�gases

Isomerate

H2

Stabilization

FIG. 1. An example of a typical isomerization process.

Originally appeared in:June 2016, pgs 31-35.Used with permission.

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Process/Plant Optimization

hydrogenation and the cracking of hydrocarbons heavier than C5/C6 generally take place in the first reactor. The remaining isomerization reactions and side reactions, such as hydrocracking, generally take place in the second reactor.

The feed quality of the isomerization reactors is closely related with the temperature change (∆-t) occurring across the reactor. Normally, it should be between 8°C and 17°C. When the benzene and heptane content of the feed increases, the ∆-t also increases. Therefore, the inlet temperature of the reactor should always be kept under control to avoid temperatures above 282°C, which causes hydrocracking reactions and, therefore, yield loss in the isomerization process because hydrocracking reactions result in undesired LPG production.

Adsorption and desorption steps. A TIP configuration has an adsorption section that is different from other isomerization unit configurations. Adsorption of normal hydrocarbons from their isomers is an unsteady-state operation. There are four adsorbers in the system, which is loaded with molecular sieves. In the adsorption process, there are two main steps: adsorption and desorption. During the adsorption step, cavities of the molecular sieves are filled with “normal hydrocarbons”; in the desorption step, the normal hydrocarbons that were adsorbed in the previous step are desorbed with the help of H2-rich gas and sent back (recycled) to the reaction section to again be converted into their isomers.

In a final step, a stabilization section separates lighter hydrocarbons from the main product. Here, offgases are separated from the isomerate, the Reid vapor pressure of the product is set and the product is sent to its tank to be used in the gasoline pool.

Experimental section. The RON value and the aromatic content of gasoline are obtained by mixing reformate, isomerate and a chemical compound (MTBE), as stated previously. Product specifications are determined by the planning department, and production operations are conducted according to those specifications. In an isomerization unit, the desired isomerate RON value is obtained mainly by changing the reactor temperatures. The purpose of the onsite study was to learn how the second reactor affects the isomerization reactions.

Prior to the study, the RON value of the isomerate was 85.30 when the unit was operated at full capacity. As shown in FIG. 3, the first reactor outlet stream flows to the second reactor inlet. The first reactor outlet stream is cooled by the help of a heat exchanger, indicated with the red circle, before entering the second reactor. After the purge gas is heated within that exchanger and in the purge gas heater, it is sent to the adsorber section to desorb the normal hydrocarbons from adsorbents.

After the sample result (RON of 85.30) was obtained, the second reactor inlet temperature was decreased by 2°C and

FIG. 4. Inlet and outlet temperatures of the first reactor.

FIG. 5. Inlet and outlet temperatures of the second reactor.

FreshfeedProduction to

fractionation

Furnace

Furnace

Reactor

Absorbers

Reactor

Furnace

FIG. 2. The basic flow scheme of a TIP unit process flow.

Split range

Purge gas heater

First reactor Second reactor

FIG. 3. The first isomerization reactor outlet stream is cooled by the help of a heat exchanger, indicated with the red circle, before entering the second reactor.

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operations were halted until conditions were stabilized at the new temperature. Before and after the temperature decrease, the feed amount to the unit was not changed and kept constant at 100% capacity. Temperature trends of the first and second reactors are provided in FIGS. 4 and 5. Besides the feed amount, best efforts were made to keep the first reactor inlet temperature at a constant 257°C. Consequently, the outlet temperature remained unchanged due to any change in feed properties and inlet temperature.

Isomerization reactions are exothermic, so outlet stream temperatures are higher than inlet temperatures. FIG. 5 illustrates the decrease in the second reactor inlet temperature from 264°C to 262°C. Due to the constant outlet temperature of the first reactor and the decreased inlet temperature of the second reactor, the heat is transferred to the purge gas heater via a purge gas reactor intercooler. Heat that is transferred to the purge gas stream in the exchanger between the reactors increased the inlet temperature of the purge gas heater by approximately 10°C (FIG. 6), which caused lower fuel consumption in the heater. When analyzed, it was observed that fuel gas used in the heater was decreased by approximately 9%. During this operation (FIG. 7), fuel gas properties were unchanged.

Results and discussion. Data was obtained from the flow indicators. When compensated with real-life conditions (temperature, pressure, specific gravity), it was concluded that fuel gas consumption was decreased by 10%. Fuel gas consumption was 572.21 Nm3/hr before the application and 514.30 Nm3/hr after the application with compensated flows.

Fuel gas properties were provided to calculate total savings:• Specific gravity of fuel g: 0.579• Fuel gas lower heating value (LHV): 11,906 kcal/kg• Specific gravity of natural gas: 0.572• Natural gas LHV: 11,733 kcal/kg.A fuel gas savings of approximately 43.35 kg/hr was

obtained. The natural gas equivalent of this savings was 43.99 kg/hr, which calculated as $0.52 Gcal/hr.

As a result, the RON value of isomerate obtained was 85.1. The reasons for the RON value decrease from 85.3 to 85.1 can be attributed to the decrease of reactor inlet temperature, the change of feed quality and the adsorber section operation.

NIHAN TORAMAN is the process chief for the CCR/NHT/ISOM units at the Tüpraş Kırıkkale refinery. She has been working in CCR reformer, benzene removal, naphtha hydrotreater and isomerization units for four years. Ms. Toraman holds an MS degree in industrial engineering/engineering management from Middle East Technical University in Turkey.

KORAY KAHRAMAN is the process superintendent of the CCR/NHT/ISOM units at the Tüpraş Kırıkkale refinery. His nine years of refinery experience include the process side of hydrocracker and hydrogen production plants, sulfur recovery, naphtha hydrotreater, isomerization, CCR reformer, catalyst regeneration, benzene removal and diesel hydroprocessing units. He holds a degree in chemical engineering from Middle East Technical

University in Turkey.

PAUL BILLINGS is a regional sales manager for Honeywell UOP’s process technology and equipment. Since joining Honeywell UOP in 2004, he has held a variety of positions in the field and regional service departments. He holds an MS degree in chemical engineering from Loughborough University in the UK.

OLEG SABITOV is a lead process specialist in Honeywell UOP’s technical service department. He has been working for Honeywell UOP since 2006 and specializes in aromatics, light naphtha isomerization and reforming process technologies. He holds a degree in chemical engineering from Ufa State Oil Technical University in Russia.

FIG. 6. Purge gas heater temperature trends.

FIG. 7. Fuel consumption trends, before and after the application.

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