Retrofitting Crude Oil Refinery Heat Exchanger Networks to Minimize Fouling While Maximizing Heat Recovery

Abstract

The use of fouling factors in heat exchanger design and the lack of appreciation of fouling in traditional pinch approaches have often resulted in crude preheat networks that are subject to extensive fouling. The development of thermal and pressure drop models for crude oil fouling has allowed its effects to be quantified so that techno-economic analyses can be performed and design options compared. The application of these fouling models is described here on two levels: the assessment of increasing heat recovery in stream matches (e.g., by adding extra area to exchangers) and the design of a complete network using the Modified Temperature Field Plot. Application to a refinery case study showed that, at both the exchanger and network levels, designing for maximum heat recovery (e.g., using traditional pinch approaches) results in a less efficient system over time due to fouling effects.     

Heat Exchanger Fouling in Phosphoric Acid Evaporators

Multistage shell and tube evaporators are frequently used in phosphoric acid plants to increase the concentration of dilute phosphoric acid to 52–55 wt% P₂O_5. The concentrated phosphoric acid solution is supersaturated with respect to calcium sulfate. As a result, part of the calcium sulfate in the liquor deposits on the heat exchanger tube walls. Because the thermal conductivity of these scales is very low, thin deposits can create a significant resistance to heat transfer. Therefore, regular cleaning of heat exchangers is required, frequently at shorter than biweekly intervals. As the major costs in modern phosphoric acid plants are the cost of energy, a thorough understanding of the fouling kinetics and of the effects of various operational parameters on the behavior of calcium sulfate is required to improve operation and design of the shell and tube heat exchangers, which are extensively used. In this investigation, a large set of heat exchanger data was collected from shell and tube heat exchangers of the phosphoric acid plant of the Razi Petrochemical Complex in Iran, and the fouling deposits were analyzed with respect to appearance and composition. The overall heat transfer coefficients and fouling resistances were evaluated at different times, and a kinetic model for the crystallization fouling was developed. It is shown that the crystallization rate constant obeys an Arrhenius relationship with an activation energy of 57 kJ/mol. The average absolute error of 12.4% shows that the predictions of the suggested model are in good agreement with the original plant data.     

Real Time Fouling Diagnosis and Heat Exchanger Performance

The issue of fouling in preheat trains of crude oil distillation units in Petrobras's refineries is a major concern—especially now, as heavier Brazilian crudes with higher asphaltene content are being refined. As the efficiency of the preheat train plays an important role in the energy consumption of a distillation unit, its performance must be tracked as precisely as possible in order to identify operational problems. This work describes an online heat exchanger performance evaluation system based on rigorous simulation of the equipment in order to predict both the operational and clean overall heat transfer coefficient. A real-time comparison between these two values indicates the actual performance of the heat exchanger and of the preheat train. The use of a rigorous process simulator (Petrox from Petrobras) together with a rigorous calculation of the global heat transfer coefficient (using the program Xist from HTRI) allows one to consider aspects that are not usually taken into account in this kind of evaluation. These aspects include crude vaporization after the desalters and variations of crude and products composition with the distillation unit run. The system is being implemented at the biggest Petrobras refinery (360,000 bpd) in a 25 heat exchanger preheat train.     

Controllability of Heat Exchanger Networks

A heat exchanger network (HEN) is the heart of a heat-integrated plant, and classical synthesis methologies assume all design operating conditions are constant values and overlook controllability issues. The controllability measures presented in this paper are intended to assess the controllability of a HEN through a simple and comprehensive methodology consisting of five steps that allows an easy screening process to determine a more controllable HEN.     

Measuring Local Crystallization Fouling in a Double-Pipe Heat Exchanger

Abstract

Although fouling is a problem varying in space and time, sizing and assessment of a process apparatus is almost always based on one single integral fouling resistance value. Furthermore, the integral fluid dynamic behavior, e.g. the development of time-dependent pressure drop in a heat exchanger, can be influenced by local constrictions. While it is generally possible to determine the time dependency of the integral fouling behavior, local differences are not taken into consideration at present. Therefore, this paper introduces a metrological, an incremental and a segmental approach to study the local development of crystallization fouling by calcium sulfate in a countercurrent double-pipe heat exchanger. The consecutive approaches allow for thermal, volumetric, gravimetric, and optical fouling investigations, aiming to examine the axial distribution of deposit as well as local differences in the deposit morphology. All methods provided congruent results and local fouling could be described properly. An almost clean surface at the colder end of the heat exchanger and an exponential increase of deposit thickness were observed towards the hot end. Hence, the section near to the hot water inlet turned out to be a key area with regards to increasing fouling mass and structural changes of the layer.     

Experimental Study of Fouling Resistance in Twisted Tube Heat Exchanger

This paper discusses fouling of a twisted tube heat exchanger under different conditions of fluid velocity and heat input. The fluid velocity was varied from 0.5 to 2.0 m/s, whereas the heat input to the heat exchanger was varied from 200 to 800 W. The experimental results show that for low fluid velocity of 0.5 m/s, the fouling resistance showed noticeable variation with respect to heat input, whereas for high velocity ranges, that is, 1.0–2 m/s, the variation in fouling resistance is less. The fouling in twisted tube steadily increases with time for different values of heat input from 1000 min onward for fluid velocity in the range from 1.0 to 2.0 m/s. It is also observed that fouling resistance curves overlap for various values of heat input. During the initial 1000 min of the test duration, the maximum fouling in a twisted tube heat exchanger decreases with increase in fluid velocity from 1.0 to 2.0 m/s. This behavior of the fouling rate can be attributed to the fact that at higher fluid velocity, flow becomes turbulent, and this in turn flushes the fouling particles. The time-series correlations for the fouling resistance are found to be logarithmic in nature.     

Detection of Fouling in a Cross-Flow Heat Exchanger Using Wavelets

Detection of fouling in a heat exchanger experiencing perfect steady-state conditions is not very difficult. But the challenge is to detect fouling when all inputs (inlet temperature of the fluids and the mass flow rates) are simultaneously varying. In this paper it has been considered that the mass flow rates can vary in a ratio of 2, and that the inlet temperatures can vary by about ±20%. This first approach is dedicated to show the feasibility of using the wavelet transform. It has been considered that getting simulated data is the best way. In fact, it is then possible to introduce an arbitrary fouling factor. Thus, in the first part of the paper the model of the heat exchanger is presented. It is developed using Simulink. The validation is carried out on an electrical heater, for which it is possible to find an analytical solution for transient states. It is also shown that steady states are accurately computed over a large range of the number of transfer units and heat capacity rate ratios. Then a brief overview of the wavelet transform is given. Then basic examples show that the wavelet transform can help to find the trend of time series. It is then applied to the analysis of the “wavelet-transformed” effectiveness of the heat exchanger. This analysis is carried out on a sliding observation window (to be able to detect fouling on-line). It is shown that fouling is detected at a very early stage.     

Fouling of Heat Exchanger Surface: Measurement and Diagnosis

Fouling in heat exchange equipment results in a significant energy toss by increasing heat transfer resistance and fluid frictional resistance. This paper deals with an effective way to monitor fouling and describes a potentially useful diagnostic approach for discriminating different types of deposit in situ. Information regarding deposit type would be useful in selecting an appropriate treatment procedure. Some cautions regarding the indiscriminate use of fouling factor or fouling resistance are also discussed.     

烟气换热器堵灰原因及解决办法

介绍了数家电厂GGH堵灰情况，总结出GGH堵灰的主要原因，并且提出了对策。     

一种监测换热器污垢的新方法

在考虑污垢对换热器传热性能影响的基础上,提出换热器当量总污垢热阻和污垢函数的概念,并给出换热器当量总污垢热阻的监测方法,讨论了换热流型、传热有效度ε和冷热流体热容量率比R对换热器当量总污垢热阻的影响。     

非清洁水源热泵系统换热器除污方法研究

非清洁水源热泵系统中,污水换热器换热面的污染问题导致流动阻力增大,换热系数降低,已成为污水源热泵发展的主要技术障碍之一.本文分析了现存的几种在线物理清洗方法应用于污水换热器除垢的局限性,基于污水换热器污垢的特点,针对管壳式换热器提出了适合于污水换热器去污方法,即小水量强力轮替冲洗部分换热管工艺,介绍了强力自冲洗换热器结构和原理,分析了其经济可行性.     

Fouling Reduction Characteristics of a No-Distributor-Fluidized-Bed Heat Exchanger for Flue Gas Heat Recovery

Heat exchangers and heat exchanger networks are extensively used for the purpose of recovering energy. In conventional flue gas heat recovery systems, the fouling by fly ashes and the related problems such as corrosion and cleaning are known to be major drawbacks. To overcome these problems, a single-riser no-distributor-fluidized-bed heat exchanger is devised and studied. Fouling and cleaning tests are performed for a uniquely designed fluidized bed-type heat exchanger to demonstrate the effect of particles on the fouling reduction and heat transfer enhancement. The tested heat exchanger model (1 m high and 54 mm internal diameter) is a gas-to-water type and composed of a main vertical tube and four auxiliary tubes through which particles circulate and transfer heat. Through the present study, the fouling on the heat transfer surface could successfully be simulated by controlling air-to-fuel ratios rather than introducing particles through an external feeder, which produced soft deposit layers with 1 to 1.5 mm thickness on the inside pipe wall. Flue gas temperature at the inlet of heat exchanger was maintained at 450°C at the gas volume rate of 0.738 to 0.768 CMM (0.0123 to 0.0128 m³/sec). From the analyses of the measured data, heat transfer performances of the heat exchanger before and after fouling and with and without particles were evaluated. Results showed that soft deposits were easily removed by introducing glass bead particles, and also heat transfer performance increased two times by the particle circulation. In addition, it was found that this type of heat exchanger had high potential to recover heat of waste gases from furnaces, boilers, and incinerators effectively and to reduce fouling related problems.     

A Theory Describing Asphaltene Adhesion Fouling Inside Heat Exchanger Tubes

A theory for mathematically modeling asphaltene adhesion fouling in heat exchanger tubes was derived and its agreement with experiment suggested feasibility. The premise of this theory is that asphaltene adhesion fouling requires the formation of a chemical bond—modeled here as a sulfur–sulfur bond—which is strong enough to resist fluid dynamic forces. This theory suggests that once an asphaltene monolayer is adsorbed onto a heat transfer surface, shear stress alone may be insufficient for preventing further fouling unless the asphaltene flocculate size distribution can be manipulated, the chemically labile heteroatoms can be deactivated, or the asphaltenes can be kept in solution. A method for calculating fouling threshold shear stresses and flocculate diameters is expounded.     

Study of the Effect of Fin-and-Tube Heat Exchanger Fouling on its Structural Performance

This paper presents the thermal and structural analysis of high temperature fin-and-tube heat exchanger. Water flowing in tubular space and flue-gas flowing in the intertubular space, were considered as working fluids. The effect of limescale fouling on thermal and structural performance of heat exchanger was studied. The analysis considered an industrial heat exchanger, which failure occur from time to time. The expert inspection, after the failure indicated the existence of fouling layer within the heat exchanger tubes. In order to understand the reasons of heat exchanger failure, a detailed fluid flow analysis (both in the tubular and intertubular spaces) was performed. The analysis indicated that the silicate limescale fouling layer with thermal conductivity of 0.35 W/(mK) and thickness up to 1.5 mm existing in the tube, may increase the tube wall temperature even more than 150°C. The study also includes the impact of outer tube wall surface fouling with thickness of 0.2 mm and heat transfer coefficient of 2 W/(mK). As a result, the compressible stresses may increase over three times compared to the situation where the tube wall fouling does not exist.     

Fouling and Uncertainty Margins in Tubular Heat Exchanger Design: An Alternative

Fouling is a costly problem in heat exchanger design and operation. Over the past 20–30 years, design capabilities have advanced such that most fouling can be mitigated through effective design techniques. A design margin is added to the initial clean design in order to handle uncertainties in design and any deterioration in performance due to the reduced fouling that occurs in spite of good design practice. This article explains the advantages and disadvantages of three common methods for adding design margin to heat exchangers. It then introduces a new design margin method that combines the strengths of the previous three while avoiding their weaknesses.     

A Theory Describing Sedimentation Particulate Fouling Thresholds Inside Heat Exchanger Tubes

This paper expounds a simple, fundamental theory for predicting sedimentation particulate fouling thresholds for horizontal flows inside heat exchanger tubes. The velocities and shear stresses at the tube wall predicted by this theory for keeping particulate matter suspended compare favorably with industrial experience and proprietary Chevron data. This theory is also not inconsistent with the literature in that the shear stress required for sedimentation fouling mitigation is roughly 4–6 Pa. However, the situation where small particles become encapsulated in the viscous sublayer cannot be resolved with a simple sedimentation particulate fouling threshold theory at this time, necessitating future research.     

Experimental Investigation of Microbial Fouling and Heat Mass Transfer Characteristics on Ni-P Modified Surface of Heat Exchanger

Fouling deposition problem on heat transfer surface is widely distributed in the field of energy and chemical industry,and microbial fouling is a common fouling type in heat exchanger.In this article,the surface modification was used for inhibiting or mitigating the microbial fouling deposition on heat exchange surface.Firstly,the experimental system for real-time monitoring the fouling deposition process was built,and then the Ni-P modified surface was prepared.Further,the slime forming bacteria(SFB)microbial fouling characteristics and corresponding influencing factors on Ni-P modified surface were investigated experimentally.The results indicated that Ni-P modified surface had an excellent fouling inhibition property.Comparing with carbon steel,Ni-P modified surface reduced the fouling heat resistance by 80%.Accordingly,the influencing factors of microbial fouling deposition including temperature,flow rate and microbial concentrations were discussed.With cooling water temperature increasing given in the experiment arrangement,the microbial fouling resistance was increased first and then decreased,while with bacteria concentration and flow rate increasing,the fouling resistance was increased and decreased separately.The work can provide experimental reference for the fouling inhibition surface development and fouling inhibition mechanism study.     

Thermal Fouling of Heat Exchanger Tubes due to Heavy Hydrocarbon Droplets Impingement

ABSTRACT

This work discusses fouling in the vapor–steam mixture overheater in the convection section of an industrial steam cracker due to the thermal degradation of heavy hydrocarbon droplets deposited on the tube wall. A spray of heavy hydrocarbon multicomponent droplets is injected in a tube of the vapor–steam mixture overheater and the path of the droplets through the tube is followed by an Eulerian–Lagrangian computational fluid dynamics simulation. To study tube fouling, the droplet impingement behavior on the wall, the evaporation of the deposited liquid, and a coking model describing thermal coke formation due to degradation of heavy hydrocarbons are required. To describe the droplet impingement behavior, a regime map for single component millimeter-sized droplets is taken from the literature. Two simulations are performed to study fouling problems in a vapor-mixture overheater tube. Simulation results are found to be grid sensitive. By analyzing and comparing simulation results it is concluded that reliable fouling data require a regime map for the impingement of multicomponent heavy hydrocarbon micron-sized droplets.