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Retrofit Design of Heat Integrated Crude Oil Distillation Systems
Mamdouh Ayad Gadalla
[Thesis].University of Manchester;2003.
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Abstract
Heat-integrated crude oil distillation systems are energy and capital intensive, and havea very complex structure with strong interactions between the individual units. Retrofitof these systems is of major interest to petroleum refiners. Retrofit objectives arevarious and preferably achieved with minimum capital expenditure, while equipmentconstraints are met.Traditional approaches to retrofit design of crude oil distillation systems identifypromising modifications based on experience or pinch analysis. Later, sequentialapproaches to retrofit design were developed, in which distillation and heat recoveryunits are modified individually. Recent approaches considered simultaneously thedistillation column and heat integration targets, rather than the existing heat recoverysystem. That shortcut models for retrofit design of distillation columns are not availableis an additional limitation of established methodologies.In this thesis, a new approach is presented for retrofit design of heat-integrated crude oildistillation systems. Shortcut models are developed for distillation retrofit, includingreboiled and steam-stripped columns. These models are based on the Underwoodequation and are appropriate for retrofit design of simple columns and various complexcolumn arrangements. Models are also proposed for exchanger network retrofit,addition of new columns to the existing distillation unit, modifying column internals,enhancing heat transfer in exchanger tubes and for evaluating CO2 emissions in existingcrude oil distillation units.The retrofit design methodology is optimisation-based, and considers the existingdistillation process simultaneously with the details of the associated heat recoverysystem. Existing equipment limitations, such as the hydraulic capacity of the distillationcolumn, exchanger network pressure drop and bottlenecked exchangers, are accountedfor. The approach considers various structural modifications and design optionsresulting in significant benefits. Examples of these are the installation of preflash andprefractionator units to the existing column configuration, replacement of columninternals with packing, enhancement of exchanger heat transfer and integration of a gasturbine with an existing furnace.The optimisation framework comprises column and exchanger network retrofit models,cost models and suitable objective functions. The approach optimises all operatingconditions of the existing distillation process and any new columns to minimise ormaximise a specified objective function, while satisfying existing constraints. Theobjective function is flexible and varies according to retrofit objectives. Severalobjectives are taken into account, such as reducing energy consumption and overallcost, increasing capacity, improving profit and reducing CO2 emissions. The approachallows these objectives to be met by considering several design alternatives.The new retrofit approach is applied to different industrial cases of crude oil distillationunits, for energy and total cost savings, throughput enhancement, product yield changes,profit increase and emissions reduction. Typical results conclude that retrofit goals canbe achieved with substantial savings in energy and total cost, and improved profit with minimal capital investment.