Nov-2016

Turnarounds deliver process improvements

An extended unit shutdown is relatively rare and an opportunity to take every advantage to improve performance.

MARK PILLING
Sulzer USA

Article Summary

Depending upon the particular unit, refineries typically shut down for turnarounds every four years or so. From an economics standpoint, a refiner never really wants to shut down. When economics are good, a shutdown means missing out on potential operating profits. When economics are bad, there are financial pressures to avoid capital outlays. Consequently, as equipment and maintenance conditions allow, the time between scheduled refinery turnarounds has been increased over time from four, to five, to six years.

Many refineries were built years ago when common practice was to include substantial additional operating capabilities beyond the specified design conditions. Over the years, refiners have used up most, if not all, of this over-design and now are operating up against the limits of their equipment. When refining margins shrink, it becomes a top priority to squeeze out the most profitability from the operations in terms of product yield, value, and energy efficiency. Knowing where this additional profit lies and what operating and/or equipment changes are needed to realise these profits is critical.

Fewer turnarounds mean there are fewer windows of opportunity to make improvements. Rather than just thinking of turnarounds as a status quo maintenance exercise, it is important to plan ahead and work with a knowledgeable partner who can deliver significant benefits during the turnaround window. This is especially true for distillation columns since very few modifications can be done to them while the unit is running. Although controls can be modified while running, or hot taps can be made for emergency fixes, little else is possible beyond that.

Economics of distillation columns
Along with catalytic reaction, mass transfer (distillation, absorption, stripping, extraction) remains at the heart of refinery processing. During the refining process, components are upgraded and then separated into the most valuable product slate. Distillation in itself is interesting because of its financial implications. By definition, distillation uses considerable amounts of energy ‘boiling oil’. Of course efforts are made to recover as much energy as possible, but it is still a very energy intensive process. However, the value of this oil separation into the desired products is always substantial. If we draw a ‘cash balance’ box around the process, we see that there is a lot of money flowing in and even more money flowing out. This fact makes revamps of distillation columns quite lucrative for the plant. Because energy usage is so high, even moderate reductions in energy use can create substantial savings. Also, since the product upgrade value from distillation is so high, there is nearly always a large incentive to make a cleaner cut between products.

It should be noted that the value of distillation does not generally lie in making a more pure product. Once a desired product specification has been met, additional purity has no financial value. The true value of improved distillation is creating a more precise separation so that lower value components can be shifted into higher value streams and still meet the overall product specification. This is especially true with multi-product main fractionators, but can be equally true with simple columns like debutanisers and naphtha splitters as well. This capability, along with proper blending strategies, brings substantial financial value to the plant’s bottom line.

A somewhat simplistic example is shown in Table 1. These numbers are for American refiners during 2015.1 Overall, the refinery profits are quite nice. Looking more closely, the gasoline price is higher than diesel and kerosene so it would pay to route more of the kerosene into the gasoline pool. If all the kerosene could be upgraded to gasoline, then the additional profit would be just over $11 million/y. This is not possible, but if 5% could be upgraded to gasoline, that would increase profits by over $500000 annually. So this provides an order of magnitude feel for the economics. This increase in profits would justify making some small to moderate changes costing less than $1 million, but would not justify spending $5 million or $10 million dollars on a substantial revamp.

It is important to note that the data set in Table 1 is a fixed snapshot in time. The margins shown are quite high. Sometimes refinery margins can, and do, go negative. Regardless of margin, it always pays to optimise operations. As for the product make-up, sometimes distillates can be more valuable than gasoline. Plant operations need to have the flexibility to move quickly to the most profitable production mode. This optimum operation mode will vary from country to country, season to season, and year to year.

The example above dealt with product slate changes. In order to maximise revamp payouts, it often pays to look for a variety of benefits from one physical unit modification. Increases in throughput, energy savings, and increased reliability are typical benefits that can justify the project cost. Also, an equipment upgrade can often be combined with a process control change to further optimise the operation.

Proper selection and planning
Refineries abound with columns. The big columns (crude, vacuum, main fractionators) receive a lot of attention because they are clearly central to unit operations. Evaluation of these columns and their associated equipment is complex and challenging, but they have a large potential for improvement. The saturated and unsaturated gas plants function in groups and are generally best evaluated as such. The other columns mainly serve to clean streams and products to meet specifications (strippers, debutanisers, and so on).  These columns are smaller, but are often overlooked with respect to efficiency and proper operation. The advantage of reviewing smaller columns is that they are quite straightforward in their function and they are not nearly so inter-
dependent on other operations. Therefore, it is easier to find opportunities for improvement and to quantify the benefits.

The most valuable thing that can be done during operation periods is unit optimisation with the existing equipment and process requirements. By doing this, a thorough understanding of the unit operation can be gained. This provides a solid foundation for possible revamps during upcoming turnaround times. This effort starts with a good heat and mass balance around the column. Typically, this means a mass balance closure within 5% and a heat balance within 10%. These balances are the foundation for optimisation. If the heat and material balance numbers are poor, find out what is causing this and fix it prior to beginning a serious study.

Next, a good process simulation needs to be constructed based on the heat and mass balances and laboratory samples and then unit instrumentation readings. Often, there is a trial and error between the process simulation and the other data. Any significant anomalies must be rectified.

Finally, an accurate understanding of the economics surrounding that particular operation is mandatory. Without fully understanding the economics, a project can never be confidently justified.