Optimisation of energy consumption

The true values of fuel, power and steam costs are needed for reliable estimation of energy saving projects

FARBOD RIKHTEGAR, PPG Consultant Engineering

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Article Summary

Refineries add value to crude oil by converting feed into marketable products using energy. Figure 1 shows the net margin of a crude oil refinery.

In a typical refinery, the terms shown in Figure 1 can be described as follows:
• Product value is the value received from the sale of products. Because most refined products are commodity items, their values are related to their prices on the open market; thus, engineers can adjust the operation of the plant to maximise the most profitable stream. This is a good start point to develop process improvement projects
• Feedstock cost is the cost of the refinery feed stream, taking into account any transport costs
• Fixed costs are generally the costs of running the refinery, the infrastructure, taxes, people, and corporate costs
• Variable costs include fuels, catalysts, additives, purchased utilities, and maintenance costs.

Assume that a 100 000 b/d refinery consumes energy at a pacesetting level – roughly 5% of the feed input. Assuming the cost of fuel at about $100/t, the total energy bill is about $25 million/year. By contrast, an inefficient site consuming approximately 8% of purchased crude as energy receives an energy bill of $40 million/year, $15 million higher than the pacesetter site.

Figure 2 shows the change in crude oil cost, product slate value, and energy cost for the 100 000 b/d conversion refinery over a year. This figure uses data gathered from two refineries (one consuming 5% fuel and the other consuming 8% fuel on crude) at each end of the typical energy efficiency spectrum.

During this period, the efficient refinery showed a mostly positive net margin, whilst the inefficient one operated mostly at a net loss, indicating the critical role of energy consumption on refining profitability. Depending on the fuel cost, the annualised loss of profit for the inefficient refinery is $20 million/y (around $50/bbl).

Assuming average energy consumption of 6.3% on crude for a refinery with 100 000 b/d crude oil processing capacity, total energy usage is 6300 b/d FOE or 400 Gcal/h. A breakdown of this is shown in Table 1.

The energy balance of this typical refinery is further illustrated in Figure 3. The assumed energy consumption – that is, 400 Gcal/h  – includes all types of fuel which can be further broken down into three main categories (see Table 2).

Table 2 indicates the major area of interest. Burning fuels in furnaces incurs the highest energy cost in a refinery. Consequently, this was the driving force for extensive research and development projects which were the beginning of a number of new design concepts in the early 1980s.

The useful power consumption of this average refinery accounts for only about 5% of total energy (24 MW or 20 Gcal/h), but incurs around 25% of the total energy cost (100/400 Gcal/h).

Some energy expenditures, such as those resulting from fired heater inefficiency or heat losses through insulation, are independent of process operations, and so can be independently managed for saving energy, regardless of how the processes operate. Some of the most typical methods are:
• Optimising overflash in distillation: too much overflash wastes energy; too little reduces distillate yields
• Pumparound duties: increased pumparound duty improves feed preheat and saves energy, but impairs fractionation quality above pumparound trays
• Use of stripping steam improves separation and therefore improves yields
• Increasing reflux ratios increases energy consumption for reboiling, but improves separation and product quality.

It can be concluded that optimising refinery energy systems requires an integrated approach comprising energy balancing, rigorous energy economics, process analysis, steam/power system 
analysis, analysis of process/energy interactions, and use of optimisation tools. These basic steps form a systematic approach to achieving the best energy management within the refinery. It is obvious that energy efficiency has a great impact on refining margins, and by increasing the cost of marginal fuel, the importance of sustaining an efficient operation increases. But how is energy-efficient operation defined, and can refineries be compared in terms of efficiency? Since more complex refineries are expected to consume more fuel than simpler ones, the percentage of crude input is obviously not a valid parameter. Therefore, the fuel consumption expressed as a percentage of crude input is a function of both the energy efficiency and the complexity.
The basis of best technology
Developing a method encapsulated in the ‘best technology’ (BT)concept, enables us to compare energy efficiencies between refineries with different configurations, capacities and performances.

Through process simulation, an optimised, energy-efficient design can be developed for all refinery processes, and the energy consumption of each process can be calculated as a function of throughput, feed quality, severity of operation, or other parameters. Therefore, the best economically justifiable design can be simulated according to the following rules:
• Preheat trains designed for a minimum network approach temperature of 20°C (36°F)
• All fired heaters at 92% efficiency
• Yield-efficient operation
• Efficient utility systems
• All power generated internally at 80% marginal efficiency.

Next, correlation of energy consumption for BT processes is applied to rank existing refineries. Moreover, BT allowances for individual units are calculated, taking into account actual throughput, feed quality, yields, and so on. To rationalise the comparison, energy efficiency is expressed as a single number, tonnes of equivalent fuel oil per hour (foet/h). All energy streams – fuels, steam, and power –are converted to foet/h using a systematic method of rigorous energy evaluation and costing. Their sum is the total BT (or %BT), and it can be compared with the actual refinery energy consumption. For example, an index of 180% BT means that the target refinery consumes 80% more energy than the energy consumption of a BT refinery with the same configuration, feed quality, and yield pattern. Existing refineries rarely approach the BT target, and it is not economical to bring them down to 100% BT. Practically, energy-efficient design is achievable and economically justifiable only in grassroots plants.

During the last few years, a greater focus has been put on building efficient new plants. These refineries, as well as some of the older refineries, have helped bring the average BT figure down towards the 180 point. Using the data in Table 3, refineries can be categorised according to their BT indexes.

Figure 4 shows some of the initial BT indices and the achievable BT after implementing the recommended energy-saving projects. There is a wide range of opportunities for the enhancement of efficiency from 20 to about 80 points on the BT scale. However, the difference between the achievable improvements resulting from different energy costs and investment policies for each site limits the number of investment related energy saving projects.

The potential for improvement can then be carried forward to a gap analysis in order to identify where the refinery is not meeting the BT energy performance. Trying to identify the gap, four main groups of operations should be apportioned:
• Fired heaters
• Heat integration
• Process
• Steam and power.

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