Size of Storage Facilities to Facilitate Refinery Revamp
- Amit Choudhary, Process Design Engineer, Flour Daniel India Pvt Ltd

An efficient utility and overall OSBL facility is vital to the refinery economics and its importance can no less be undermined in the overall refinery revamp job. This article focusses on the storage and dispatch facilities' configuration to meet the revamp requirements and talks about considerations required during conceptual study phase for arriving at adequate size of storage facilities to facilitate a major refinery revamp.

Revamp of existing refineries has its own challenges at each stage of the engineering cycle starting from conceptualisation to final erection or modification and commissioning of the revamped facilities. Once the licensor selection and ISBL configuration are frozen, the utilities - power and offsite infrastructure required to assist the production requirements- needs an overhaul. The configuration of the OSBL facilities requires a brainstorming so as to assist the refiner to limit the project spending while meeting the production needs.

The possibilities and means of conducting and carrying out the OSBL revamp are many and it is imperative that the right philosophy is adopted to yield the most cost effective yet most efficient OSBL design. A more efficient utility and overall OSBL facility is vital to the refinery economics and its importance can no less be undermined in the overall refinery revamp job.

The intention of this paper is to mainly highlight the challenges faced and the solutions finally rendered to such problems, during the configuration study for the storage and dispatch facilities for a major revamp for one of the recent refineries' revamp project. The group of refineries', for which the study was carried out, is located in Asia and has an existing crude processing capacity of approximately 550,000 BPSD. Post revamp this processing capacity shall almost double to 13,00,000 BPSD. The increase in the processing capacities for each of the refineries range from as low as 6 percent for one of the refineries to as high as 136 per cent for the other.

The paper shall concentrate on the operational philosophy of ISBL units, the product marketing requirements and the plot plan considerations that were the deciding factors for arriving at the most appropriate and suitable storage and dispatch facility configuration for each of the refineries undergoing revamp. The central idea is to provide a better perspective to the commonly occurring problems in projects of similar kind which will perhaps enable the project team to better execute the job at hand.

Some of the key highlights of the configuration study were, possible modifications to make existing tank suitable for new service, finalising the operational philosophy to optimise the storage requirement, unit wise slop segregation and minimising separate product blending tanks by providing inline blending.

Background
Refinery revamp is often necessitated owing to a lot of conditions, viz, ageing of the refinery units, rise in demand of distillates which may be either local or global, change in the refinery configuration so as to meet a particular product slate, modernisation of the refinery for improved GRM, or most importantly change in the environmental regulations of a country for sale of the distillate yields. While all or one of these conditions may require the refiner to consider undergoing a revamp, most commonly occurring rather dictating condition that pushes refineries to undergo revamp, off -late, is the local law and environmental regulations. Due to the rising awareness and concerns of the impact of fossil fuels' use on the environment around us, most developed countries and many developing ones are pushing towards a clean fuels approach. This means adding of mainly the hydrodesulphurisation units so as to reduce the sulphur content in the final distillate.

Revamp of existing refineries has its own challenges at each stage of the engineering cycle starting from conceptualisation to final erection or modification and commissioning of the revamped facilities.

Introduction
After the licensor selection and ISBL configuration are frozen the utilities - power and offsite infrastructure required to assist the production requirements - needs an overhaul. The configuration of the OSBL facilities requires a brainstorming so as to assist the refiner to limit the project spending while meeting the production needs. The possibilities and means of conducting and carrying out the OSBL revamp are many and it is imperative that the right philosophy is adopted to yield the most cost effective yet most efficient OSBL design. A more efficient utility and overall OSBL facility is vital to the refinery economics and its importance can no less be undermined in the overall refinery revamp job.

The revamp for the refinery, in discussion here, had been initiated with the following objectives:
  • Increasing margin for existing refinery units
  • Increasing flexibility of current refineries to process high sulphur crude
  • Meet future product requirement of Euro IV diesel and gasoline
  • Meet security of supply by increasing production of diesel and gasoline
As part of the OSBL facility configuration, the focus area of this paper shall be the storage and dispatch facilities' configuration to meet the revamp requirements.

Configuration
As mentioned earlier in the background section, the ways and means of approaching a new configuration for the offsite facilities are more than one . There is also no one particular way of doing this activity. However the intention of this paper shall be to highlight the experience of conducting this job such that it provides an insight into one of the many ways in which this configuration can be addressed.

The starting point for the storage configuration study was the material balance of the process units. This can be in form of an HMB document or the LP model output of the Process units' licensor. The initial material balance for one of the refineries yielded the block flow diagram. The block flow diagram helped in deciding the flow of mainly the intermediate streams to and from the units in a more clear perspective.

Operational Philosophy
The unit operating philosophy in terms of the overall sequencing of process units' start-ups was finalised based on the client considerations. The operational philosophy with any such project, for the process units, is site specific. However, the refiner may ask the engineering contractor to suggest the sequence of secondary unitsí start-up so as to decide on the storage requirement. In this case, past process unit operating experience comes handy and the possible options can be suggested. For eg, many refiners prefer to store the sweet naphtha from NHT so as to assist CCR (Platformer unit) start-up which assists to generate the hydrogen required for other hydrotreating units. But this storage was not considered in this case since the Hydrogen plant of the refinery was accounted to provide and maintain the required H2 balance for operating the secondary units of the refinery. Likewise the requirement of storing the NHT feed and Isomerisation unit feed were not considered by adopting the philosophy of keeping all these naphtha and gasoline processing units shut along with CDU shutdown. This led to a substantial saving on plot space requirements combined with adopting the approach of reducing the number of days of storage for inter-unit feed tanks from a very conservative 10 days production requirement storage approach to a more practical 2-3 day of 100 percent production requirement and about 6 days of turndown production storage. Over the years, with improved automation and overall technology combined with the increase in the number of skilled operation personnel and increased emphasis safety matters(which includes all key operation aspects like commissioning, start-up, normal operation with SOPs and shutdown procedures) has improved the refinerís ability to maintain the units on-stream for a longer time.

Also, typically once the power requirements of the refinery are uniformly met and once the overall power block is stabilised in a new or revamped refinery, then the reasons for any unit and particularly CDU to stay shutdown for long may be very rare.

Accounting all these aspects of experience gained in operation of a refinery the recommendation of having a reduced number of days of storage for the inter-unit feed tanks seems a reasonably fair conclusion.

These suggestions having been accepted by the client helped in finalising the process units operating sequence and in turn it helped finalise the storage required for different services.

Reuse of the Existing Tanks for New Service
After firming the operational philosophy and the number of days of storage, the numbers of tanks required for each service were worked out.

The important aspect here was to allocate the existing tanks in newer services and thus reduce the requirement of addition of new tanks. Based on the data available for existing tanks, the tanks were allocated to new service as shown in the Table 1. The important considerations while allocating the tanks were the vapour pressure of stored substance, the corrosive nature of the stored substance and the location of the stored substance (ie, storage tank) on the plot plan.

The tanks allocated to new service would require some modification. The suggested modifications to the tank, as required, have been tabulated in Table 2.

It can be seen from the Table 1 that among the many stocks required to be stored, the requirement of new tanks is significantly less despite a considerable increase in the total refinery processing capacity. This is due to the excess number of days of storage available with most of the existing tanks. Also, the operational philosophy as discussed above helped in reduction of the number of days of storage as also the storage requirement of a few components.

The spheres for propylene with one of the refineries were in excess compared to the requirement. Now propylene spheres being of a higher design pressure were allocated for LPG storage purpose since the LPG vapour pressure is very less as compared to propylene.

This is one of the examples of how the allocation has avoided addition of new spheres/tanks and thus helped in keeping the cost in control.

Further, from Table 2 the modifications are suggested on a few existing tanks. For eg, instead of constructing a new IFR the existing Floating Roof (FR) tank has been recommended to be modified for storage of products like feed and product of the RFCC Naphtha HT unit.

IFR storage is required for this service so as to minimise the gum forming tendency of naphtha. The modification is possible since the vapour pressure of the tank shall be the same due to the tank being in similar service earlier. The only additional aspect is the addition of a fixed roof with N2 blanketing. Since the roof's mechanical design will be based on the design pressure due to this blanketing, the new roof shall be designed to meet the same. Hence the modification is an executable one.

Plot Plan Considerations
The overall plot is arranged considering process flow, safety, ease of maintenance, operation, and construction requirements along with client preferences. In this particular refinery which is being discussed here, the considerations were based on storing requirements of the feed and products at different locations. The crude was being received at a distant island from the refinery complex while the products and intermediates were to be stored at the refinery complex itself. The new crude storage tanks to be added as a result of the increase in refining capacity also were to be accommodated on the island.

On the other hand, the refinery orientation was such that the North side of the refinery complex housed all the units while the south had the product tanks and product jetties for dispatch. While allocating the product and intermediate storage tanks for the new service this philosophy was taken into account and accordingly the tanks were allocated for the storage of the same. These were the kinds of considerations that were taken in to account for the plot plan configuration of one of the refineries.

Minimising the Blending Tank Requirements
The blending tanks that were separately used in the refinery for blending the crude and/or products were replaced by the recommendation of inline blending of the streams. For eg, the various blend components of Gasoline like Isomerate, Reformate, Hydrotreated RFCC Naphtha, Alkylate and MTBE are blended through an automatic inline gasoline blending which is controlled by a feedback controller working on RON and Sulfur measurements being measured in the final product. It should be noted that few of the components shall be intermittently required to tune the final product quality requirements.

The Alkylate flow will be governed by the octane number requirement and will generally be an intermittent requirement.

The flow of components like Alkylate and MTBE are also very less compared to the continuous flow of other gasoline components. Hence the storage was only recommended for these intermittent components. As it turned out for all the refineries in this case, the flow of these components was very less and hence small existing tanks were allocated for its storage.

The remaining streams were designed for continuous flow in the Gasoline pool and hence the separate storage for these streams was not required. This method reduced the number of components to be stored for each of the product and feed pool and thereby was an important aspect in minimising the overall storage requirement.

Conclusion
Thus it can be concluded that the detailed study of existing facilities and the operational philosophy can yield various areas of improvement at the time of a refinery turnaround. When these aspects are studied in detail and are looked at with a fresh approach there can be various cost reducing factors which will surface up as a result. Hence by considering a few or all of the above discussed methods and approaches the storage facilities can be optimally sized and thus contribute to the goal of delivering a cost friendly and economically feasible project.