Chemical Processing Plant Research Articles

Off-Line and On-Line Approach for Optimal Maintenance Management of Continuous Parallel Processes with Decreasing Performance

The efficiency of equipment units in chemical process plants decreases with time because of fouling, byproduct formation, catalyst deactivation, etc. Therefore, periodic equipment maintenance and/or cleaning are required to restore original operating conditions that allow the desired high productivity rates to be maintained. Maintenance imposes a tradeoff decision between the cost associated with equipment shutdown and the resulting benefit of improvements in productivity. When parallel processing lines are considered in a production facility, the problem becomes more complex because of the presence of material interrelationships. In this work, the solution to this complex maintenance problem is addressed by introducing a simple formulation (NLP) for the planning problem and a procedure for the subsequent implementation of the discrete decisions involved (clean or not, feed assignments) in a real-time environment. This methodology can be considered an extension of the real-time evolution (RTE) approach for continuous processes (Sequeira, S. E.; Graells, M.; Puigjaner, L. Ind. Eng. Chem. Res. 2002, 41 (7), 1815). The main advantage of the proposed methodology is the robustness resulting from the use of on-line information, which reduces the model dependency and, thus, the undesirable effects of variability and plant−model mismatch. The proposed off-line and on-line approaches are tested with benchmark case studies, their performance is evaluated using dynamic simulations, and their long-term behavior is monitored via Monte Carlo simulations.

Vibration and Stability of 3000‐hp, Titanium Chemical Process Blower

This 74‐in‐diameter blower had an overhung rotor design of titanium construction, operating at 50 pounds per square inch gauge in a critical chemical plant process. The shaft was supported by oil‐film bearings and was directdriven by a 3000‐hp electric motor through a metal disk type of coupling. The operating speed was 1780 rpm. The blower shaft and motor shaft motion was monitored by Bently Nevada proximity probes and a Model 3100 monitoring system.Although the blowers showed very satisfactory vibration levels during test runs at the manufacturer′s plant, the vibration levels in situ had always been higher than was desirable. After several months of monitoring showed ever increasing vibration levels, one of the blowers was shut down in order to diagnose and resolve the problem.Several steps were taken to diagnose the problem: (1) The rotor was removed and the shop balance was checked and corrected. (2) The bearing support movement due to thermal expansion was measured. Then the shafts were misaligned in the cold condition in order to achieve near‐perfect shaft alignment during normal operation. (3) The expected shaft vibration at the bearings was determined using lateral rotor dynamics analysis, including critical speed mapping. (4) A heavy sleeve was added to the blower shaft to increase the radial load on the drive‐end bearing. (5) The metal disk type of coupling was replaced by a gear coupling. (6) The finite element and impact of the bearing support pedestal were tested to determine the stiffness of the bearing support. (7) The shaft movement was measured during a coast‐down. (8)Tilting‐pad bearings were evaluated as a possible replacement for the original standard sleeve type of hydrodynamic oil‐film bearings.The final solution showed the importance of coupling angular stiffness (often rarely considered in machine design), rotor dynamic analysis, and field alignment.

A SIMULATION OF THE TURBULENCE RESPONSE OF HEAT EXCHANGER TUBES IN LATTICE-BAR SUPPORTS

Tube failures due to excessive flow-induced vibrations are a major concern with regards to the operation of heat exchangers in nuclear power and chemical process plants. Among the possible excitation mechanisms, turbulence-induced forces have a persistent effect and thus are an important factor in determining the long-term reliability of heat exchangers. Impact forces which occur when tube/support collisions take place play a vital role in determining tube wear. To address this issue, a tube/support interaction model was implemented in an in-house finite element program and validated against three published examples. Pseudo-forces in conjunction with modal superposition were utilized in solving the nonlinear equations of motion of loose tubes in lattice-bar supports. Time-domain simulations of the nonlinear response of the tube are presented to determine the effect of various tube and support parameters on the vibratory characteristics of the systems. Special attention was paid to the effect of increased clearance on the response of tubes in lattice-bar supports. The tube response, impact force and contact ratio were analysed and represented in dimensionless form. The dimensionless parameters developed proved effective in collapsing the data pertaining to different flow velocities over single curves. These are useful in identifying the roles of several key variables in altering tube dynamics. Moreover, these parameters may also be used to scale the results to account for differences in geometrical and material properties.

Controlling reverse-flow reactors via multiscale transient thermal dispersion

Since the chemical reactor is the heart of any chemical processing plant, there exist significant opportunities to increase industrial profits and minimize ecological impacts. One such opportunity is improving thermal stability in packed bed reverse-flow reactors. A fundamental study of the effect of multiscale, transient, thermal dispersion within packed bed reactors is undertaken here to evaluate its utility to improve thermal stability and satisfy economic and environmental goals. A new reactor configuration is investigated, where a regular pattern of cylindrical rods is inserted into the packed bed using a multiscale approach. These rods can increase the effective thermal dispersivity in the bed over thermal conduction by up to 2-orders of magnitude. It is found that the magnitude of the dispersion can be tuned by adjusting the key operating parameter, the reversal time, such that the reactor can stay ignited without the need for external fuel supply during extended lean conditions while remaining thermally stable during extended rich conditions. The ensuing multiscale thermal dispersion, in concert with a simple control strategy, allows for indefinite reactor operation without runaway or extinction, as predicted by simple, generalized analytical models for any chemical reaction and illustrated with numerical simulations. Some added benefits of this study are that the tight controllability of these reactors may allow for increased reaction selectivity, energy efficiency, and reduced pollution levels.

Theoretical and Experimental Methods for the Evaluation of Reactive Chemical Hazards

Evaluation of reactive chemical hazards is critical for the design and operation of safer chemical plant processes. Much effort is needed for experimental techniques to measure thermal reactivity of chemical systems. Studying all the various reaction pathways experimentally, however, is very expensive. Therefore, it is essential to employ simplified screening tools to reduce the number of experiments and to identify the most hazardous pathways. A systematic approach is presented for the evaluation of reactive chemical hazards. This approach is based on a combination of numerical computational methods and experimental thermal analysis techniques. Numerical computational methods are used to predict reaction stoichiometries, thermodynamics and kinetics, which will help to exclude thermodynamically non-feasible and non-hazardous reaction pathways. The experimental techniques are used to evaluate potentially hazardous systems for more accurate thermodynamic and kinetics parameters or to replace inadequate numerical methods. This approach employing theoretical and experimental analyses was used to evaluate the decomposition reaction of di-tert-butyl peroxide (DTBP) in toluene.

Multivariate process monitoring and fault diagnosis by multi-scale PCA

Chemical process plant safety, production specifications, environmental regulations, operational constraints, and plant economics are some of the main reasons driving an upward interest in research and development of more robust methods for process monitoring and control. Principal component analysis (PCA) has long been used in fault detection by extracting relevant information from multivariate chemical data. The recent success of wavelets and multi-scale methods in chemical process monitoring and control has catalyzed an interest in the investigation of wavelets based methods for fault detection. In the present work, multi-scale principal component analysis (MSPCA) is used for fault detection and diagnosis. MSPCA simultaneously extracts both, cross correlation across the sensors (PCA approach) and auto-correlation within a sensor (wavelet approach). Using wavelets, the individual sensor signals are decomposed into approximations and details at different scales. Contributions from each scale are collected in separate matrices, and a PCA model is then constructed to extract correlation at each scale. The multi-scale nature of MSPCA formulation makes it suitable to work with process data that are typically non-stationary and represent the cumulative effect of many underlying process phenomena, each operating at a different scale. The proposed MSPCA approach is able to outperform the conventional PCA based approach in detecting and identifying real process faults in an industrial process, and yields minimum false alarms. Additionally, the advantage of MSPCA, over the traditional PCA approach for sensor validation, is also demonstrated on an industrial boiler data set.

Hazard Analysis for the Evaluation of the Hazard Potential of Chemical Process Units

“Hazard analysis” based on many years of practical experience accrued by safety engineers at the former Hoechst corporation is an excellent tool for assessing potential dangers emanating in particular from chemical processing plants. The assessment of plant failures by this procedure permits a relating weighting of their safety engineering relevance. Existing resources for implementing measures which may prove necessary can thus be efficiently applied to risk reduction.

Hazard Analysis for the Evaluation of the Hazard Potential of Chemical Process Units

Hazard analysis based on many years of practical experience accrued by safety engineers at the former Hoechst corporation is an excellent tool for assessing potential dangers emanating in particular from chemical processing plants. The assessment of plant failures by this procedure permits a relating weighting of their safety engineering relevance. Existing resources for implementing measures which may prove necessary can thus be efficiently applied to risk reduction.

Horizontal block-modular air-cooling units

Types AVG, AVZ, AVZ—D, and other mass-produced air—cooling units are used in the process units of chemical plants, petroleum refineries and gas processing plants, and in pipeline compressor stations and underground gas storage stations for cooling and condensation of different media. Manufacturers deliver them in individual units and parts (heat—exchange sections, blades and bosses, fans, electric motors, fan commutator and diffuser units, beams, crossbeams, diagonal braces, crossbars, and cables for bearing metal structures), which requires a large amount of assembly and welding work at the assembly site.

Systematic Waste Minimization in Chemical Processes. 1. Methodology

Increasing public pressure, more stringent regulations, and escalating waste treatment and disposal costs have motivated the chemical industries to implement waste minimization at the source rather than to rely on end-of-pipe treatment. Waste minimization analysis is time-consuming, expensive, laborious, and knowledge-intensive. The objectives of this two-part paper are (1) to develop a systematic methodology to guide a nonexpert with the technical aspects of waste minimization and (2) to implement an intelligent system that can automatically perform a waste minimization analysis of a chemical process plant. In part 1, a systematic methodology for waste minimization analysis is presented. An intelligent decision support system that implements this methodology is presented in part 2. The proposed methodology comprises three fundamental elements: process graph (P graph) and cause-and-effect and functional knowledge. The P graph is a directed bipartite graph capable of abstracting the flow of materials in a process. An analysis based on the P graph provides a framework for diagnosing the origins of waste in the process and for deriving top-level waste minimization alternatives. These top-level alternatives can then be distilled further by using cause-and-effect and functional knowledge to obtain detailed alternatives. The application of the methodology is illustrated using an industrial case study.

Recovery From Failures in the Chemical Process Industry

The prevention of errors and other failures has always been a central theme in safety and reliability management. However, additional benefits could be gained from focusing on what can be done after a failure has occurred, before it leads to negative consequences. This article examines the processes followed to recover from failures. Incident and near-miss data collected in an exploratory case study at a chemical process plant indicate, in agreement with literature survey findings, that 3 main types of recovery actions exist. These action types correspond with the phases recovery processes go through: actions aimed at failure or problem detection, actions aimed at explanation of the causes, and corrective actions. The data also show that the explanation and correction phases can be repeated and do not necessarily occur in that order. Finally, the results clearly demonstrate the essential role of the human operator for each of the recovery process phases.

Service Temperature Characterization of Polytetrafluoroethylene-Based Gaskets

Abstract Gaskets based on polytetrafluoroethylene (PTFE) are used extensively in bolted flanged connections, especially in difficult chemical process plant applications where blowouts due to excessive bolt load loss are of major concem. The determination of their service temperatures requires the characterization of their short- and long-term creep relaxation resistance. An experiment-alanalytical procedure has been developed to determine a recommended service temperature for PTFE-based gaskets. Based on an improved version of the Hot Blowout Test (HOBT) in which thermal cycling has been incorporated, the procedure quantifies the short-term hot relaxation resistance, and the margin of safety against an in-service blowout. The improved HOBT test provides an excellent tool for the selection of blowout-resistant PTFE gaskets for difficult service. Currently, the procedure estimates the cooldown load loss due to the thermal contraction difference between the gasket and the flange, based on a rough estimate of the gasket thickness and the coefficient of thermal expansion and their vanation with temperature. The proposed procedure provides not only a better estimation of the gasket thickness but also accounts for thermal ratcheting and the temperature lag between flange and bolts.

Power of Process and Sensor Fault Detection Methods for Stationary Mineral Processing Plants

The power of a statistical test for fault detection in chemical or mineral processing plants is an indicator of the efficiency of the detection method. The paper proposes a model-based method which is able to detect fault in the process model as well as in the measurements. The model consists of stationary mass conservation equations, allowing the calculations of the covariance matrix of the model uncertainties. The test is a X2 test, the power of which can be analytically calculated. The method is illustrated for a flotation circuit using either instantaneous data or measurements over an observation window of varying width.

Planning plant operating procedures for chemical plant

All industrial plants require an extensive set of operating procedures. This paper discusses the use of hierarchical nonlinear least-commitment AI planning technology to generate plant operating procedures for chemical process plant. It considers the handling of flow through the interfacing of a valve sequencing subplanner, the handling of safety through the mechanism of goals of prevention, and the use of pairs as a way of mutually constraining planning variables and increasing planning efficiency. It concludes with some results and discussion of the advantages of the approach.

Sole Mates

This article focuses on one of the harshest environments confronting industrial apparel that takes place where boots hit the floor of chemical and food processing plants. LaCrosse Rainfair of Racine, WI, a manufacturer of footwear and clothing, developed a dual-density outer sole that combines toughness and elasticity with resistance to chemicals, fats, and oils. The company fuses the outer sole directly to the soft upper sole of its Flex3 boot in a two-stage process. The Flexalloy compound has very good oil resistance compared with other materials used in other injection molded outer soles. This property is doubly valuable in the Flex3 boot since, by overmolding the outsole directly on the upper, the need for adhesives, which could degrade under aggressive attack and cause total product failure, are eliminated. It was observed that Flexalloy OR-based samples retain 74% of their original elongation, a property that correlates with elasticity, versus just 1% for flexible vinyl. When it is immersed in lard, the Flexalloy OR-based sample shrank 5%, compared with 6% and 84% swelling of the SEBS styrenic and thermoplastic olefin, respectively, and 13% shrinkage of the flexible vinyl.

Sequence-dependent batch chemical scheduling with earliness and tardiness penalties

Production volume in the specialized agricultural chemical industry is typically too small to justify the capital expenditure required for continuous processing. As such, there is a trend towards building chemical processing plants for this market segment that are batch plants. Scheduling of this type of chemical plant under just-in-time operations, where both earliness and lateness penalties are included, is critical to the efficient operation of these plants. In this paper, a heuristic scheduling procedure is developed for the problem of minimizing the total weighted earliness and tardiness costs as well as the total set-up cost for the single-stage batch chemical manufacturing environment.

Quality costs and robustness criteria in chemical process design optimization

The identification and incorporation of quality costs and robustness criteria is becoming a critical issue while addressing chemical process design problems under uncertainty. This article presents a systematic design framework that includes Taguchi loss functions and other robustness criteria within a single-level stochastic optimization formulation, with expected values in the presence of uncertainty being estimated by an efficient cubature technique. The solution obtained defines an optimal design, together with a robust operating policy that maximizes average process performance. Two process engineering examples (synthesis and design of a separation system and design of a reactor and heat exchanger plant) illustrate the potential of the proposed design framework. Different quality cost models and robustness criteria are considered, and their influence in the nature and location of best designs systematically studied. This analysis reinforces the need for carefully considering/addressing process quality and robustness related criteria while performing chemical process plant design.

Imaging laser radar for 3‐D modelling of real world environments

The paper presents design details and applications of the recently developed 3‐D laser radar from Z+F. It presents models which have been constructed using the data from “inspection of tunnel tubes”, modelling of a “car body welding cell” and a “car body gripper” in the automotive industry as well as a “chemical process plant”. The laser radar was developed for use in industrial environments. Its twin design aims are measurement performance and robustness. The laser radar can be used with a range of mechanical beam deflection units to meet the needs of specific applications.

Crossflow filtration testing of INEEL radioactive and non-radioactive waste slurries

Development of waste treatment processes for the remediation of radioactive wastes is currently under way at the Idaho Nuclear Technology and Engineering Center (INTEC), located at the Idaho National Engineering and Environmental Laboratory (INEEL). INTEC, formerly known as the Idaho Chemical Processing Plant, previously reprocessed nuclear fuel to retrieve fissionable uranium. Liquid waste raffinates resulting from reprocessing were solidified into a calcine material. Waste treatment processes currently being considered include the dissolution of the solidified calcine material and separation of residual undissolved solids (UDS). UDS in solution must be removed prior to downstream processes such as solvent extraction and ion exchange. Filtration experiments were conducted at the INEEL using a crossflow filter apparatus on radioactive and non-radioactive waste slurries [N.R. Mann, T.A. Todd, Evaluation and Testing of the Cells Units Crossflow Filter on INEEL Dissolved Calcine Slurries, INEEL/EXT-98-00749, Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID, 1998]. The purpose of this testing was to evaluate the removal and operational efficiency of crossflow filtration on slurries of various solids loadings. The solids loadings tested were 0.19, 2.44 and 7.94 wt.%, respectively. A matrix of test patterns was used to determine the effects of transmembrane pressure and axial velocity on filtrate flux. Filtrate flux rates for each solids loading displayed a high dependence on transmembrane pressure, indicating that pressure filtration resistance limits filtrate flux. Filtrate flux rates for all solids loading displayed a negative dependency on axial velocity. This would suggest axial velocities tested were efficient at removing filter cake. Prior to testing of actual waste slurries, baseline water runs were performed. Filtrate flowrates observed during baseline water runs exhibited substantial decreases despite numerous backpulses and rinses, suggesting particles that were deeply embedded within the filter membrane as the result of shear-induced deagglomeration

Intelligent systems for HAZOP analysis of complex process plants

Process safety, occupational health and environmental issues are ever increasing in importance in response to heightening public concerns and the resultant tightening of regulations. The process industries are addressing these concerns with a systematic and thorough process hazards analysis (PHA) of their new, as well as existing facilities. Given the enormous amounts of time, effort and money involved in performing the PHA reviews, there exists considerable incentive for automating the process hazards analysis of chemical process plants. In this paper, we review the progress in this area over the past few years. We also discuss the progress that has been made in our laboratory on the industrial application of intelligent systems for operating procedure synthesis and HAZOP analysis. Recent advances in this area have promising implications for process hazards analysis, inherently safer design, operator training and real-time fault diagnosis.