Feb-2023

Reconfiguration of naphtha splitters using divided wall column technology

Integration of DWCs into existing refining configurations reduces energy consumption and GHG emissions in distillation systems while also lowering operating costs.

Ratheesh S
Bharat Petroleum Corporation Limited (BPCL)

Article Summary

Distillation systems are responsible for the highest amount of energy consumption in refineries. Use of fossil fuels is responsible for environmental problems such as global warming and air pollution, causing health problems and affecting the quality of life of populations. Technologies coupled with improved energy efficiency can support a more sustainable energy system with fewer carbon emissions.

The most effective way to reduce greenhouse gas (GHG) emissions is to reduce the consumption of fossil fuels. The BPCL Mumbai Refinery (MR) is committed to pioneering energy conservation stewardship and environmental protection in the oil and gas industry through innovative approaches. Reconfiguration of the Mumbai refinery naphtha splitters was such an attempt to reduce fuel consumption.

Naphtha management is crucial for improving the overall gross refining margin (GRM) of a refinery. Naphtha can be managed by effective separation and upgradation and by optimising the blending strategies, which minimises overall naphtha production. MR consists of two crude distillation units (CDUs), namely CDU-3 and CDU-4. The refinery had three naphtha splitters, namely the reformer feed preparation unit (RFU) Splitter I, RFU Splitter II, and the isomerisation unit (ISOM) naphtha splitter, to split 6000 tons/day of stabilised naphtha from the CDUs into light and heavy components.

RFU Splitters I and II were operating in a series configuration wherein Splitter I bottoms were fed to Splitter II. Along with its scheduled turnaround for maintenance and inspection activities, the RFU had also undergone a reconfiguration in Sept-Oct 2021 to enhance its feed processing capacity from 4300 t/d to 6000 t/d through modification of column internals. With the post-reconfiguration of its RFU naphtha splitters, MR can operate Splitters I and II in a parallel configuration, thereby shutting down the ISOM naphtha splitter to conserve energy and reduce emissions.

Background
Straight-run naphtha from the stabiliser bottoms of both CDU-3 and CDU-4 is received by the naphtha splitters. The quantity and quality of naphtha depends on the type of crude. Typically, naphtha generated from Bombay High (BH) crude contains a high amount of benzene, whereas naphtha generated from Middle East crude contains relatively lower benzene. For upgradation purposes, naphtha has to be separated into ISOM feed (C5 and C6 with low benzene content) and CCR feed (C7+).

In the pre-reconfiguration scenario, the RFU, with a series configuration of the two splitters, was processing 4300 t/d of stabilised naphtha, and the ISOM naphtha splitter was processing 1700 t/d of stabilised naphtha. Details of these columns (pre-reconfiguration) are provided in Table 1. Top cuts (C5-85ºC) from RFU Splitter I and the ISOM naphtha splitter were routed to the naphtha hydrotreating unit (NHT), where they were hydrotreated to yield ISOM feed. Bottom cut from Splitter I was fed to Splitter II. Mid-cut from Splitter I, top cut from Splitter II, and mid-cut from the ISOM naphtha splitter were routed to the naphtha product pool. Bottom cuts (95-135ºC) from Splitter II and the ISOM naphtha splitter were routed to the NHT, where they were hydrotreated to yield CCR feed. The RFU pre-reconfiguration is shown in Figure 1.

With the objective of improving the performance of these splitters, BPCL’s Corporate R&D Centre performed detailed analysis of naphtha operations at MR. Based on analysis of feed and products data, it was found that:
-    Existing configuration was outdated and required upgradation as per changes in the refinery configuration
-    Separation efficiency was low and overall energy consumption was high           
-    ISOM feed had low amounts of C6 components, which are active reactants in the ISOM unit (for RON improvement).

To address these issues, different schemes were configured and simulated using Aspen Plus. Stream properties (quality and quantity) and the energy consumption of each configuration were compared with the existing configuration. Based on economic and operational benefits, one of these configurations was finalised and proposed for implementation. The selected configuration was to convert RFU Splitter II into a divided wall column (DWC) so that RFU Splitters I and II can be operated in parallel and the ISOM naphtha splitter can be shut down.

Objectives
The main naphtha splitter objectives were as follows:
-    Increase RFU feed processing capacity from 4300 MTPD to 6000 MTPD by operating Splitters I and II in parallel configuration so that the ISOM naphtha splitter can be shut down. Feed distribution to both the splitters will be optimised suitably to minimise the quantum of modifications and maximise product quality and yield
-    Maximise the quantity of top cut while minimising the benzene, C6 naphthenes, and C₇+ hydrocarbons in it. This stream will be fed to the NHT-ISOM unit
-    Minimise the quantity of mid-cut and minimise overlap with the top cut and bottom cut. This stream will be diverted to naphtha storage
-    Maximise the quantity of bottom cut while maximising the toluene in it. This stream will be fed to the NHT-CCR unit.

Post-reconfiguration of RFU
Post-reconfiguration, RFU Splitter I continues to operate as a conventional side-cut column, whereas Splitter II has been converted into a middle DWC. The RFU post-reconfiguration is shown in Figure 2.
With this configuration in place, RFU Splitters I and II are sufficient to split the entire quantity of stabilised naphtha from the CDUs.

Major modifications during reconfiguration
No modifications were envisaged in the Splitter I and II reboilers, which are fired heaters. Four new heat exchanger shells were added by replacing three existing ones to meet the revised cooling requirement of various process streams. Further, seven new pumps with LT motors replaced five existing pumps. Major modifications during the reconfiguration are listed in Table 2. Post-reconfiguration product specifications are provided in Table 3.

Divided wall column technology
The reconfiguration of naphtha splitters is based on optimised DWC, which maximises product yields. The added benefit of lower capital investment combined with lower operational cost has helped DWCs gain popularity over conventional columns. When applied to sequential multi-component separation, DWCs can separate the feed into two or more purified streams within a single tower, thus eliminating the need for a second tower. DWCs offer the following benefits:
-    Suitable for separating multi-component mixture into three or more high-purity product streams in a single column
-    Ideal alternative for revamp of side-cut columns when high purity is required from the three product streams
-    Lower footprint as equipment count is reduced by half
-    Equipment turnaround time and other miscellaneous expenditure are reduced
-    Operational and capital expenditure are reduced by approximately 20-50%.
The following are the salient features of reconfiguring the naphtha splitters:
-    Integration between Splitters I and II: the side-cut from Splitter I substantially overlaps C5 components. The side-cut from Splitter I is fed to Splitter II on the feed side of the dividing wall. This leads to a considerable reduction in the overall side-cut rate from the two columns. The new packings in both columns are high-efficiency packings, which are capable of handling high vapour liquid traffic
-    Flexibility has also been provided to operate Splitter I and II columns in a series configuration.