**4. Downstream facilities**

The oil refining sector is vast and diverse with multiple processes that are tailored to the type of crude oil feed (light, medium, heavy, etc.). Everything starts with the CDU (crude distillation unit) which fractionates the desalted feed into various cuts—light-end gases, naphtha (gasoline), kerosene (jet fuel), diesel, Light Gas Oil (LGO), Heavy Gas Oil (HGO), and tower Bottoms. The bottoms stream may or may not to send to a vacuum distillation unit (VDU) for further fractionalization into more valuable cuts, depending upon the flow rate and composition. Other process units include typically include hydro-treaters for sulfur removal, catalytic reformers for raising the octane value, hydro-crackers, delayed cokers, visbreakers, alkylation units, hydrogen production, sulfur recovery units, etc. An oil refinery requires a multitude of supporting facilities and utilities, such as fuel storage and distribution, steam generation and distribution, power distribution, onsite power generation (whether as mechanical shaft work or electricity), compressed air, cooling water, refrigeration, freshwater supply, wastewater treatment and disposal, chemicals storage and distribution (e.g., caustic soda, nitrogen, etc.), firewater system, chillers, boilers, steam turbines, and gas turbines. Although all are important for the smooth operation of the processes, they are not equally important from an energy consumption perspective. The seven main ones—steam, fuel gas, fuel oil, electric power, air cooling, water cooling, and refrigeration are energyintensive and important from the energy conservation point of view. All of these systems could involve significant energy losses.

#### **4.1 Applications in the downstream**

To identify the gaps, it is important to consider the energy assessment of the entire unit instead of just equipment by equipment. It starts with drawing the

boundary around the processing unit, calculating energy losses, and comparing them by equipment component first, followed by comparing with the unit's "target" energy consumption to identify the efficiency gaps for the whole integrated unit and subsequently for the whole facility.

The CDU + VDU system is generally the largest single energy-consuming unit in a refinery, the majority of them utilize fuel for the fired heaters to provide heat for reboiler in the columns, followed by steam used in the feed preheat train, for side strippers, and for vacuum jets. The goal of the preheat trains is to deliver the feed streams to the fired heater at the highest achievable inlet temperature on a sustained basis. The efficiency of the preheat train (PHT) can be easily measured as the actual heat recovery divided by the target heat recovery.

PHT efficiency is governed by two principal factors: (a) design the heat exchanger network (HEN) so as to follow the temperature profile of the hot and cold streams (using Pinch Analysis), and (b) the rate of asphaltene fouling in critical heat exchangers, generally at crude temperatures above 200°C (390°F). Fouling, especially in the processing of heavier high-boiling crudes, is a major problem. It can shorten the run length between maintenance shutdowns by as much as 1 year, with a significant negative impact on profitability. So, fouling control is a critically important energy efficiency improvement measure. However, it is not easy to manage.

Ebert and Panchal (1995) first identified and modeled the fouling rate as a competition between deposition and removal mechanisms. Deposition rate depends mostly on the surface temperature while removal depends on the mechanical shear rate (flow velocity). Fouling will be negligible if the removal rate surpasses the formation rate. Significant progress has been made in the 25 years since 1995, and there is commercial software available that is able to predict fouling rates throughout the PHT, and therefrom the optimum HX cleaning strategy (**Figure 17**), while the refinery is running, which can extend run times back to near non-fouled conditions [10].

An important perspective is that there is no such thing as waste heat; there is only WASTED Heat, whether deliberate (due to reluctance to invest in recovery) or accidental wasted due to ignorance). Because of historical anti-investment bias in the industry, there will almost always be opportunities to recover such wasted heat

**Figure 17.** *Furnace coil inlet temp was maintained within target range via optimized HX cleaning strategy.*

at a payback that far exceeds what one can hope to get by other legally permitted investments.

One major area of opportunity is where systems are oversized/overdesigned. These are the consequence of engineers habitually putting extra safety margins into the original design. So, by clever modifications, the excess equipment capacity can be converted into better energy efficiency.

Traditional refinery energy efficiency "optimization" initiatives normally focus only on the obvious equipment performance improvements: furnace stack temperature, furnace excess air, adding extra HX surface to the PHT in an effort to raise column feed temperature, steam usage for stripping, and process heating, recovering obviously wasted heat, reducing flow rates recycle streams, cleaning of poorlyperforming heat exchanger, power recovery, rotating equipment and motors/ machinery efficiency improvements, adding insulation, reducing steam leaks and improving condensate return. While these all are good things to do, but the benefits of optimal system integration are far greater, yet they are seldom addressed.
