Edward M. Ishiyama

The Effect of Fouling on Heat Transfer, Pressure Drop, and Throughput in Refinery Preheat Trains: Optimization of Cleaning Schedules

Optimizing cleaning schedules for refinery preheat trains requires a robust and reliable simulator, reliable fouling models, and the ability to handle the thermal and hydraulic impacts of fouling. The interaction between thermal and hydraulic effects is explored using engineering analyses and fouling rate laws based on the “threshold fouling” concept; the potential occurrence of a new phenomenon, “thermo-hydraulic channeling” in parallel heat exchangers, is identified. The importance of the foulant thermal conductivity is highlighted. We also report the development of a highly flexible preheat train simulator constructed in MATLAB/Excel. It is able to accommodate variable throughput, control valve operation, and different cost scenarios. The simulator is demonstrated on a network of 14 heat exchangers, where the importance of optimizing the flow split between parallel streams is illustrated.

Ageing is important: closing the fouling-cleaning loop

Process units subject to fouling often require regular cleaning, giving rise to repeated cycles of fouling and cleaning. The initial stages of fouling are strongly influenced by the effectiveness of the most recent cleaning step and, similarly, the effectiveness and rate of cleaning are determined by the extent and nature of the deposit layer present on the surface. The optimal operating cycle will therefore be determined by fouling–cleaning interactions. Deposit aging is an important factor in this, as an aged deposit is usually more difficult to clean. Aging therefore introduces an element of choice into fouling–cleaning operating cycles, between in situ “chemical” methods and ex situ “mechanical” methods, with associated differences in effectiveness, time, and cost. This paper reports a reformulation of the cleaning scheduling problem to consider the choice of cleaning method, as well as the timing of cleaning. A case study based on a shell-and-tube heat exchanger processing crude oil is used to illustrate the concepts and scope of application of this approach. A novel and more general formulation of the problem, linking design, fouling, and cost aspects via dimensionless groups, is then presented and illustrated with a second case study based on a simpler exchanger model.

Management of Crude Preheat Trains Subject to Fouling

Crude oil refinery preheat trains are designed to reduce energy consumption, but their operation can be hampered by fouling. Fouling behaviors vary from one refinery to the next. Effective management of preheat train operation requires inspection of historical plant performance data to determine fouling behaviors, and the exploitation of that knowledge in turn to predict future performance. Scenarios of interest can include performance based on current operating conditions, modifications such as heat exchanger retrofits, flow split control, and scheduling of cleaning actions. Historical plant monitoring data are frequently inconsistent and usually need to be subject to data reconciliation. Inadequate data reconciliation results in misleading information on fouling behavior. This article describes an approach to crude preheat train management from data reconciliation to analysis and scenario planning based around a preheat train simulator, smartPM, developed at Cambridge and IHS. The proposed methodology is illustrated through a case study that could be used as a management guideline for preheat train operations.

Considering In-Tube Crude Oil Boiling in Assessing Performance of Preheat Trains Subject to Fouling

Oil refinery preheat trains can exhibit unwanted two-phase flow behavior. An example is boiling of crude oil inside heat exchangers, when the local pressure is not high enough to keep crude in a liquid state. This often arises when the pump is undersized. Understanding the two-phase behavior and assessing the boiling heat transfer coefficients would result in a better prediction and estimation of exchanger fouling. Where single-phase modeling is used under boiling conditions, the anomalous behavior leads to unrealistic estimates of fouling resistance, and can severely underpredict the increased pressure drop and consequent loss of crude throughput. There is little public information on fouling in two-phase flows as laboratory experiments are very costly, despite the importance of this in refinery heat exchangers and furnaces. Indeed, the importance of crude boiling is likely to increase as lighter crudes such as shale oils are processed. These lighter crudes are often blended with heavier crudes to maintain an appropriate refining average density. This paper consists of two sections. The first section uses industrial monitoring data to illustrate fouling behavior for a heat exchanger that undergo both boiling and fouling. The second section discusses simulations to evaluate thermohydraulic behavior when the crude undergoes boiling. The analysis requires coupled heat transfer, and hydraulic and surface fouling aspects; a commercial preheat train network simulator, SmartPM, was used for this study.

Effect of flow distribution in parallel heat exchanger networks: use of thermo-hydraulic channeling model in refinery operation

Abstract Parallel branches are commonly observed in industrial heat exchanger networks (HENs). Despite the important relationship between flow distribution and network efficiency, not all parallel branches comprise of flow controllers or not least, flow measurements. When the network is subject to fouling, uncontrolled flow branches can introduce undesired phenomenon such as thermo-hydraulic channeling (THC) [presented at the 2007 HEFC conference; Ishiyama et al., Effect of fouling on heat transfer, pressure drop and throughput in refinery preheat trains]. Recent analysis of crude preheat train heat exchangers has shown the need to use THC models, in particular, for situations where there is insufficient flow measurement data, especially in nonsymmetric branches. This paper revisits the THC model and highlight practical importance of the THC phenomenon through analysis of plant data. The hydraulic aspect of the analysis is strongly linked to the knowledge of deposit thermal conductivity. A case study of a section of a crude refinery HEN is used to illustrate the use of thermo-hydraulic models in data reconciliation to understand flow imbalances caused due to differences in operating conditions and fouling of heat exchangers in each branch of a parallel network.AbstractParallel branches are commonly observed in industrial heat exchanger networks (HENs). Despite the important relationship between flow distribution and network efficiency, not all parallel b...

Quantifying implications of deposit aging from crude refinery preheat train data

Abstract Heat exchanger fouling has been studied for some time in the petroleum industry. As understanding of fouling dynamics and mitigation methods improves, refinery fouling mitigation strategies are changing. The implications of deposit aging in refinery units have not been addressed in detail: aging refers to where the deposit undergoes physical and chemical conversion over time. In the 2009 Heat Exchanger Fouling and Cleaning conference, Wilson et al. [Ageing: Looking back and looking forward] presented a simple framework illustrating how deposit aging impacts heat exchanger thermal and hydraulic performance. This paper presents insights into deposit aging gained from analysis of refinery monitoring data. Two case studies are presented: (i) one from the Preem refinery in Sweden where stream temperature, flow and gauge pressure measurements indicated a higher deposit thermal conductivity in exchangers located in the hotter section of the preheat train. (ii) US refinery stream temperature, flow and plant cleaning log data, showing an increased resistance to cleaning when deposits are exposed to high temperature for a prolonged period. The use of deposit aging analysis to improve exchanger operation is discussed.

Fouling Management of Thermal Cracking Units

ABSTRACT Visbreakers and other thermal cracking units are thermal process units in crude oil refineries that upgrade heavy petroleum, usually residual oils produced from atmospheric or vacuum distillation of crude oil. The associated process streams of these units consist of heavy hydrocarbons with very high viscosities and impurities, resulting in fouling of the heat exchangers used to cool or heat these streams. This paper presents a practical fouling analysis for thermal cracking units in a refinery in Germany. Fouling management at this refinery was initiated as part of the refinery energy-saving program. Following similar analysis of the refinerys crude preheat trains, heat exchanger networks associated in the thermal cracking units were modeled by entering the plant monitoring data, network topology, and heat exchanger geometries into a commercial heat exchanger network simulator, SmartPM. Fouling behaviors of vacuum residue streams and thermal cracker residue streams were identified and quantified. Both chemical reaction fouling and particulate fouling mechanisms were identified to be responsible for the fouling in these streams. Dynamic fouling models were fitted and used to predict fouling of these heavy petroleum streams, which fouled on both the shell and tube sides of the shell-and-tube heat exchangers.