REFINING INDIA 2025

refining india 2025

Maximising alkylation unit throughput during the summer months

Rinav Shah and Lalit Mathpal reliance industries limited

the tube bundles in the last 10 exchangers were replaced with Curran-coated bundles. Although the reliability of the frequently fail- ing refrigerant condensers was improved due to restrictions on heat transfer by coat- ing, the exchangers’ condensing capabili- ties decreased and adversely impacted the refrigeration duty. The impact is more prominent in summer, with the higher cool- ing water temperature, while the effect is minimal during the winter. HTRI software was used to evaluate the duty impact of one condenser, and a con- tribution of 1% production loss was esti- mated per condenser. It was decided to replace the coated bundles in two out of 10 condensers with non-coated ones to restore heat transfer efficiency. The risk of fouling is addressed by enhanced cool- ing water flow and parameter monitoring. To ensure no reliability issues (fouling, leak- age) in the long term, the flow measure- ment frequency has been increased from once a quarter to twice a quarter, and the exchanger inspection and cleaning practice has been institutionalised every winter. Installing two additional Condensers A simulation study was used to validate the benefit of installing two additional con- densers. It shows that installing an addi- tional heat exchanger to each refrigeration condenser train can handle approximately 8% more refrigerant vapour flow and/or further subcool the refrigerant to a colder temperature. Cooler refrigerant/higher refrigerant flow reduces the temperature in the alkylation reactor, allowing a higher feed rate to be processed. A 3-4% pro- duction improvement is expected follow- ing the implementation of this scheme. These simulation findings were also con- firmed by actual operating data, which showed that under the same set of condi- tions and with colder refrigerant tempera- ture, higher plant throughput was achieved. Another challenge was to design a layout that considered space constraints and cost minimisation. The other important requirement for sus- taining the higher throughput is to maintain sulphuric acid regeneration (SAR) capac- ity at maximum. A SAR unit that undergoes corrosive service, has high reliability con- cerns due to frequent shutdown require- ments. Reliability upgrade drives for the SAR unit have also been initiated to ensure that SAR availability does not constrain achieving higher throughput. Conclusion This article discusses the optimisation of a refining alkylation unit using innovative, carefully planned modifications, designed to improve alkylate production by 8-10% during the summer months. Replacing non- coated tube bundles in two condensers has already yielded benefits this summer. The other two modifications are currently being implemented.

The alkylation process is an important cata- lytic refining process used to convert light olefins (such as butylene) produced in fluid- ised catalytic cracking units and cokers into a highly valued gasoline component called alkylate. Alkylate is one of the best gasoline blending components as it contains no ole- fins, no aromatics, very low sulphur, a low Reid Vapor Pressure, high octane, and good distillation properties. This makes alkylate an ideal blending component for meeting stringent emissions requirements. Jamnagar Refinery operates a sulphuric acid (H₂SO₄) catalysed C₄ alkylation unit to produce alkylate from liquefied petroleum gas (LPG) feeds. In alkylation, a light olefin feed (C₃ through C₅) reacts with isobutane in the presence of H₂SO₄ to produce alkylate, a branched paraffinic hydrocarbon with a high-octane value. Due to the high margin in alkylate, maximising alkylation unit pro- duction remains a key focus. A comprehen- sive study was undertaken to identify and resolve these limitations, aiming to maxim- ise production and profitability. While the unit operates efficiently in cooler months, summer throughput drops due to temper- ature-induced constraints in the refrigera- tion system. This article summarises the throughput constraint and improvement plan for maximising the throughput during the summer months. Alkylation reaction is favoured at lower temperatures. Higher temperature results in increased olefin polymerisation, lower octane, and higher acid make-up. Efficient heat management, including heat removal in the refrigeration section, is important for maintaining optimal reaction conditions and unit throughput. This exothermic reaction’s heat is managed through auto refrigera- tion, which involves vaporising some of the lighter hydrocarbons, primarily isobutane, from the reaction mixture. These vapours are then compressed and condensed in the plant’s refrigeration section. The alkylation unit is equipped with refrigeration compres- sors and multiple condensers. The refrig- eration section takes vapours from the alkylation reactors and the feed chillers, compresses and cools the stream, and then returns it to the chillers and reactors for fur- ther cooling ( Figure 1 ). Alkylation unit throughput is strongly dependent on refrigeration duty and very sensitive to chilled water (CW) supply tem- peratures. Refrigerant condensers play a significant role in condensing compres- sor discharge vapours for refrigerant liq- uid supply to reactors. During the summer, when heat losses are high and the cool- ing water temperature is high, the limits of the refrigerant condensers are reached. This causes a rise in compressor discharge pressure, leading to a reduction in com- pressor discharge condensation and thus refrigerant flow. Reactors operated at high reaction temperatures dramatically favour polymerisation reactions and higher acid consumption. The overall impact is a rise in

Refridgeration compressor

Electric motor

Economiser

K.O. drum

Vapours from reactor

Refrigerant receiver

Cooling tower

Depropaniser bottoms

Refrigerant purge to treating

Refrigerant to reactor

Chiller

Reactor feed

Coalescer

Reactor product

Water purge

Figure 1 Alkylation unit’s refrigeration system

CW inlet temp©

Alkyl t’put (MT/day)

30.50 31.00 31.50 32.50 32.00 33.00 33.50

Figure 2 Summer cooling water temperature and alkylation throughput

around 5°C, compared to the design approach of <4°C. During the summer months, the wet bulb temperature exceeds 28°C, and due to the higher approach, the CW supply temperature increases, impact- ing the refrigeration condenser capac- ity and resulting in throughput limitation. The maximum actual cooling water sup- ply temperature during the months of May and June reaches 33.5-34°C, compared to the design supply temperature of 32°C. The increase in temperature versus design resulted in heat duty loss across refrigerant exchangers. The reason for the underperformance of the cooling tower is attributed to the age- ing/deterioration of the internals. The CT performance issue was studied by the RIL team in collaboration with the original equipment manufacturer (OEM) to evalu- ate the cooling tower performance and for- ward path. Based on the study outcome, it was decided to replace ageing fills with high-efficiency ‘trickle fills’, which offer bet- ter efficiency. The expected improvement in cooling water temperature is 1.0°C, which would improve the refrigerant condenser duty substantially. The estimated improve- ment is verified with actual operating data and simulation results. The implementation of this scheme is being done in a phased manner, as cooling tower cells are only available during the winter months. Heat Transfer Improvement These condensers historically had issues with fouling and frequent leaks, leading to production losses on each occasion. To increase the reliability of these exchangers,

reactor temperature, resulting in through- put restriction. A study was undertaken to maxim- ise alkylation throughput in the summer months, sustainably and cost-effectively. The objective of the study was to identify and implement engineering solutions that would: ○ Overcome summer-specific refrigeration condenser limitations. ○ Enhance cooling efficiency. In this article, the impact of refrigerant condenser limitations during summer con- ditions, which can restrict production, is discussed along with a plan for remedial action. The following three options were identi- fied and evaluated to enhance refrigeration duty for alkylation throughput improvement:  Cooling water temperature reduction by cooling tower cell fills replacement.  Replacing existing Curran-coated tube bundles with non-coated in two condensers to improve heat transfer.  Installation of two additional condensers to provide an additional heat transfer area. Cooling Water Temperature Reduction During the summer, the cooling tower tem- perature exceeds the design value and remains at and above 33°C. As the cooling water supply temperature increases, the compressor discharge pressure increases, while heat transfer, compressor flow, and alkylation unit decrease ( Figure 2 ). A 10-cell cooling tower is supplying cool- ing water to the alkylation plant. The cooling tower’s approach temperature (cold water temp – wet bulb temp) consistently remains