Megan Jobson

Integrated Design of a Conventional Crude Oil Distillation Tower Using Pinch Analysis

The substantial energy requirement of crude oil distillation columns is met partly by costly utilities, such as steam and fuel for fired heaters, and partly by heat recovered from the process, using process-to-process heat exchange. Energy savings, therefore, demand not only a distillation column that is energy-efficient, but also a heat exchanger network (HEN) which minimizes utility costs by maximizing heat recovery. A new crude oil distillation design procedure is presented which considers the column, the HEN and their interactions simultaneously, to minimize utility costs. Pinch analysis is used to determine minimum utility costs prior to the design of the HEN. In this method, the column is decomposed into a sequence of simple columns, which enables appropriate distribution of stages and simplifies analysis. Modifications, which further increase the efficiency of the process, are proposed: these are the installation of reboilers, rather than stripping steam, and the thermal coupling of column sections. The detrimental effects of these modifications on the heat recovery opportunities of the process are analysed for a distillation tower with side-strippers. A new step-by-step design procedure is derived from this analysis, and is applied to a case study. In the case study, the resulting design offers nearly 20% savings in utility costs over the base case design. The vapour flow in the column is reduced by a similar amount, offering capital savings, additional flexibility or the opportunity to increase throughput. The new integrated design procedure considers the column and its associated HEN simultaneously, aiming to minimize operating costs by obtaining the best fit between the process and the available utilities.

Shortcut Models for Retrofit Design of Distillation Columns

Shortcut models are well established for grassroots design of distillation columns and have been widely applied. However, no shortcut models are available that address retrofit. Shortcut models are quicker to solve, do not have significant convergence problems and are more robust than rigorous models for column optimization. In particular, shortcut models for retrofit would be valuable for evaluating retrofit design options, for improving the performance of existing distillation systems (columns and heat recovery systems) and can be combined with detailed heat-integration models for optimizing existing heat-integrated distillation systems. This paper presents retrofit shortcut models for design of both reboiled and steam-stripped distillation columns. These models are primarily based on a modified Underwood method, the Gilliland and Kirkbride correlations, the Fenske equation and the material balances. The retrofit models are applicable for simple distillation columns, sequences of simple distillation columns, and complex distillation configurations, including columns with side-strippers and side-rectifiers. The models fix both the column configurations and the operating conditions, including steam flow rates, and calculate the product flow rates, temperatures and compositions, and the various heat duties. A comparison of the model results with those of rigorous models of existing distillation columns is presented to validate the model.

OPTIMIZATION OF EXISTING HEAT-INTEGRATED REFINERY DISTILLATION SYSTEMS

Existing refinery distillation systems are highly energy-intensive, and have complex column configurations that interact strongly with the associated heat exchanger network. An optimization approach is developed for existing refinery distillation processes. The optimization framework includes shortcut models developed for the simulation of the existing distillation column, and a retrofit shortcut model for the heat exchanger network. The existing distillation process is optimized by changing key operating parameters, while simultaneously accounting for hydraulic limitations and the design and the performance of the existing heat exchanger network. A case study shows that a reduction in energy consumption and operating costs of over 25% can be achieved.

Multicomponent homogeneous azeotropic distillation 1. Assessing product feasibility

A new method for estimating the feasibility of a pair of product compositions from a single-feed, two-product distillation column is proposed. This method is particularly useful for columns that separate azeotropic mixtures of more than four components, where existing feasibility tests are limited in their applicability. Feasibility is determined using geometric principles, which are expressed numerically. The method therefore does not require visual aids and is applicable to mixtures containing more than four components, where graphical representation is not possible. The new feasibility test is extended to test the feasibility of a proposed class of split, which is defined by the specification of product regions, or sets of product compositions. Classes of splits are used extensively in the development of a column sequence synthesis procedure, which will be presented in a further work.

Retrofit Design for Increasing the Processing Capacity of Distillation Columns: 1. A Hydraulic Performance Indicator

Increasing the throughput of an existing distillation process is an important objective of retrofit design of distillation processes. When the throughput is increased, entrainment flooding may create a bottleneck in the column. Important goals of retrofit design are to identify and remove such a bottleneck. This work develops an analysis tool for retrofit design of distillation columns. A new measure, fractional utilization of area (FUA), is proposed to identify the extent to which the area available for vapour flow on an existing stage of the column is utilized. The area utilization is assessed relative to an acceptable approach to flooding conditions in the column section. FUA is defined as the ratio of the area required for vapour flow on a stage to the maximum area available for vapour flow in the column section. An FUA curve, indicating area utilization on every stage in the column, can be generated from converged simulation results and existing sizing correlations. The FUA curve is a useful analysis tool, as it allows the bottleneck in the column to be identified and allows the effectiveness of proposed modifications to be evaluated.

Multicomponent homogeneous azeotropic distillation 3. Column sequence synthesis

Abstract An algorithmic distillation column sequence synthesis procedure for multicomponent azeotropic mixtures is proposed. The synthesis procedure is divided into several stages, and makes use of the feasibility test for classes of splits, as defined by Thong and Jobson (Chem. Eng. Sci. (2001a) submitted), to systematically generate a range of feasible and potentially feasible column sequences for a specified separation requirement. A set of rules is applied to generate a recycle superstructure for every identified sequence. Stream compositions and recycles are then finalised. Column design methods presented in Thong and Jobson (Chem. Eng. Sci. (2001b) submitted) are then applied to every column in the sequence.

Conceptual design of reactive distillation columns using stage composition lines

Abstract A conceptual design methodology for the synthesis of reactive distillation columns is presented. The method assesses feasibility of a proposed reactive distillation column, designs the column and allows evaluation of the design for both fully reactive and ‘hybrid’ column configurations. Stage composition lines are used to represent all possible liquid compositions in a column section, for specified product compositions and for all reflux or reboil ratios. Reaction equilibrium is assumed on each reactive stage, and vapour–liquid equilibrium is assumed on all stages. The graphical column design method allows fast and relatively simple screening of different reactive distillation column configurations. The methodology is illustrated by application to an ideal reactive system and for MTBE production.

Multicomponent homogeneous azeotropic distillation. 2. Column design

Abstract A new design tool for single-feed, two-product columns separating multicomponent azeotropic mixtures is proposed. Column parameters are determined numerically, based on geometric principles. Many feasible combinations of design parameters can be determined rapidly, and can be represented conveniently in a feasibility matrix. This matrix can be used to identify favourable combinations of design parameters. The new column design method is developed separately for two cases—columns producing pure components, and columns performing general splits. In the case of columns producing pure components, the accuracy of the design method can be improved by the application of an iterative convergence procedure.

Retrofit Design for Increasing the Processing Capacity of Distillation Columns

A hydraulic performance indicator, fractional utilization of area (FUA) can be used to identify the bottleneck of an existing distillation column when the throughput is to be increased. This analysis tool allows identification of modifications to increase the throughput to an existing distillation column. The work focuses on design options that increase the effectiveness with which the existing shell and internals are utilized, rather than simply replacing the internals. Some effective modifications can be quantified directly (i.e. without using trial and error), while qualitative guidance for others can be obtained. The design tool allows relatively inexpensive design options to be identified and evaluated, reducing simulation and design effort and providing improved solutions, compared to conventional techniques.

Synthesis of distillation sequences for separating multicomponent azeotropic mixtures

Abstract An automatic procedure for the synthesis of sequences of distillation columns for separating homogeneous multicomponent azeotropic mixtures is developed. Key to the approach is the use of sets of compositions, or ‘product regions’, to specify product compositions. The synthesis procedure is implemented in two stages. The first stage consists of identifying classes of splits. Each class of split may be characterised as either feasible or potentially feasible. The synthesis procedure systematically combines these classes of splits to generate column sequences that satisfy a pre-determined separation requirement. The second stage of the synthesis procedure assesses the feasibility of a proposed flowsheet once the recycle options have been defined. Preliminary design of each column in the sequence can be performed. The feasibility assessment procedure will form the basis for systematic flowsheet generation and optimisation.