Jan-2012

Integrated monitoring for optimising crude distillation

On-line process analysers in crude distillation units prevent lost throughput caused by non-conforming process conditions and crude oil switching

Gregory Shahnovsky, Tal Cohen and Ronny McMurray
Modcon-Systems Ltd

Article Summary

Refineries are among the most complex of processing sites. They include many, entirely different, physical and chemical processes. These include atmospheric and vacuum distillation, and chemical reactions such as cracking, isomerisation, hydrogenation, desulphurisation, aromatisation and blending. Many processes are interlinked. The product of one unit serves as the feed for other, consecutive processes. Any failure, shutdown or lack of control of one of these processes will immediately affect another process in the production chain and will have an impact on the entire economy of the refinery, its revenue, profit or loss.

The majority of chemical industries process raw materials with defined specifications. Refineries must cope with a dependency on non-specific, fluctuating crude oil compositions. Trends and differences in crude oil prices, the size of tankers that can make harbour to supply a refinery, political instabilities in oil exporting countries, changing product specifications and the equipment available in the refinery dictate that crude switching and/or crude blending are inevitable. Crude blending is also one of the practices applied by refineries to increase the margin between the cost of the crude feed and the revenue from selling the final products.

Differences in crude oil compositions from various locations impact the production capacity of the refinery to deliver the volume of required distillates that the refinery is committed to bring to market.

The economics of refining are even more complicated because each refinery is unique. Many refineries are designed differently to fulfill their initial target to produce a certain range of petroleum products from a defined quality of crude oil. At present, refineries must be flexible enough to respond immediately to crude oil changes and deviations in product demands as a result of the changing global economy.

The required flexibility in the management of a refinery and the complexity of the different processes, crude oils and distillates can only be achieved by stringent monitoring of the quality of the incoming material and the outgoing product streams in each refinery unit. None of the product streams are standalone. The root of each stream is found in the crude oil that has been delivered to the distillation towers.

Challenge of crude distillation unit optimisation
The efficiency of a refinery to produce petroleum distillates is directly linked to:
• The crude oil that is delivered to the refinery
• The equipment of the refinery
• The maximum throughput of crude oil and petroleum products
• The ability to produce the distillates with the highest value at maximum yield.

Optimising the process conditions of the crude distillation unit is a main challenge for each refinery. It increases profit by producing the required range of distillates at maximum yield and at minimum cost. To achieve this goal, full and real-time monitoring and control of each incoming stream of crude oil and outgoing distillate stream is an inevitable requirement to ensure:
• Minimum influence on production capacity for each required distillate due to crude oil changes
• Minimum influence on distillate quality upon crude oil switching
• Maximum production 
of high-value distillates. Overlapping characteristic boiling ranges exist between two neighbouring refinery fractions. Maximum distillation profit is achieved by shifting the cut points towards the highest value products
• Maximum stability of the quality of each distillate throughout the entire distillation process
• Minimised production of off-spec or borderline materials and, as a result, the need for re-reprocessing or blending.

The quality and the cost of crude oil depends on its origin. Blending various types of crude oil is required to reduce the cost of the crude oil feed to be distilled and to adapt the crude oil feed so that it can be processed properly by the equipment available in the refinery.

Crude oil differences result in a variation in the distillate distribution produced by the crude distillation unit under the same process conditions. To achieve maximum production efficiency and product yield, continuous re-adjustment and fine-tuning of process conditions is inevitable.

Ongoing laboratory analyses are expensive and time consuming. The time lapse from sampling to analytical reporting increases the likelihood of a refinery to produce off-spec or borderline material. Delayed  awareness of the deficient qualities of distillates will delay the implementation of process adjustments. The crude distillation unit could operate in an inadequate and non-profitable mode for a long period of time. In addition, any other failure or malfunction of the crude distillation unit could result in reduced production capacity or even a total shutdown of the plant if not handled immediately.

To optimise process parameters to produce the required range of distillates at the highest yields, while taking into account the characteristics of the crude oil to be processed, strict and adequate monitoring of all streams is crucial to ensure maximum efficiency of the crude distillation unit.

Full control of product quality can only be achieved by an integrated system of on-line crude and distillate analysers. The analyser must provide continuous, instantaneous information on the quality and physical properties of the incoming crude oil and the outgoing streams of distillate streams of naphtha, kerosene, LGO and HGO, as well as the vacuum distilled products LVGO and HVGO. Real-time corrective actions need to be taken to guarantee optimised operation of the crude distillation unit.