**1. Introduction**

Industrial sector relies on fossil fuels as the primary source of energy that is in control of approximately one-third of world-wide energy usage. A major percentage of energy consumed is disbursed to provide utilities for oil & gas facilities in order that energy may well be engendered for other sectors. The adverse impact of fossilfuel combustion has instigated efforts to attenuate carbon emissions, whilst environmental regulations have a considerable effect on the energy's cost. Furthermore, it's a competitive world with ever-increasing energy prices, industrial communities are convened to brainstorm over conservation of energy resources and profitability. Energy efficiency should be a foundational strategy to support plans to fulfill the climate policy goals moreover as for being lucrative. Energy efficiency with its integral

role in energy intensive industrial processes is critically essential to realize desired energy-saving potential and reduction in Carbon Dioxide gas (CO2) emissions.

The economics of industrial production, environmental conservation realities and global energy supply limitations are persistent concerns for the whole industrial sector. Regardless of where on turns, there's an unswerving appeal to preserve energy, reduce carbon emissions, and safeguard environment for future generations. These enablers made energy efficiency as an entrenched thanks to conduct the business. In fact, energy efficiency for any industrial facility (i.e. several processes/plants tied with a central utilities-plant) is extremely dynamic and wish to be improved throughout its lifetime, nonetheless of how energy-efficient facility design was achieved during design-phase. On the opposite hand, in its entire lifetime, a plant undergoes various retrofit subjected to high-profile operational changes/improvements/profits. The changes that include process disturbances, uncertain feedstock conditions, and products demand are short-term, requiring no major retrofit. However, long-term transformations comprise of must process more raw feed, feedstock changes, improve performance etc. warrants the debottlenecking of the whole facility and comprises key retrofits. In summary, may be stated that, the retrofitting of a plant may well be needed numerous times, in its lifespan, to upgrade energy efficiency and satisfy the required increase in production rates.

The main challenge is to develop the proposed facility design into a Sustainable Energy-Efficient Design, which implies a design that's capable of improving its energy efficiency supported by the energy-capital trade-off dynamics. Conceptually the problem will be formulated as a bi-level programming problem with two objectives, the minimum disruption within the energy utility supply (zero deficiency) and minimum energy utility consumption. In such cases the energy utility is minimized subject to least energy deficiency in supply. The outer problem of optimization of minimum energy consumption should be solved at the best-case scenario of the inner problem which is that the least possible deficiency in energy utility supply to the process plant.

On the macro level, there are three energy system components i.e. generation, distribution and utilization. Throughout the process facility design, the main target is to boost all three components of energy i.e. energy demand (process/user) minimization, efficiently fulfill energy demand (efficiently site-wide utility/supplier) and efficiently transport energy from supplier(s) to user(s). Objectives are to minimize: waste in energy, fresh resources and capital (de-bottlenecking) in these three components. This may be done via the continual upgrade of the efficiency of energy system components in generation, distribution and utilization. However, the utilization component has a unique feature, where its boundaries do not seem to be completely dictated by the process/users. Therefore, the space of improvement within this component is way wider than the others. The interaction among all three components specified waste from one component to be utilized by others and also modifications of one component supported the other components waste availability will suffice in developing energy efficient process design. Moreover, as highlighted earlier that over the last forty plus years several design standards/criteria aside from safety; health and environment besides capital and operating costs are considered, like switch-ability; flexibility; reliability; maintainability; availability; controllability; operability; acceptability and then on. Although, almost every industrial facility has to be retrofitted several times throughout its lifetime to satisfy its objectives which was coined as "retrofit with retrofit in mind approach". "Retrofit-ability" as a design requirement has not been addressed or may be coined within the chemical engineering design
