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Improved Modeling of Rotary Momentum Mass Transport Equipment
Abstract and Figures
Pumps, compressors and turbines are vital parts of any process plant. Thus, accurate modeling of these machines is crucial for a dynamic process simulator, especially if the simulator is used for operator training. The operation of the machines is usually described by operating curves provided by the manufacturer, but these are strictly valid only for the set of process conditions they were created for. Accurately predicting the performance of the machines outside of the design conditions can be a challenging endeavor.
ProsDS is a software used by NAPCON to construct operator training simulators. In order to improve the performance of the compressors in these simulators, a new dynamic model for a centrifugal compressor was developed in ProsDS during the course of this thesis. The developed model was based on dimensionless operating curves defined by the head coefficient and the exit flow coefficient. Simple calculation of the outlet temperature and power consumption was also included.
The stability of the developed model was verified by dynamic ramp tests of the operating conditions. Furthermore, the accuracy of the developed model was determined by performing process data tests. The results were promising and, in most cases, an improvement from the performance of the currently implemented model, although some further development of the new model is needed to reach a fully acceptable accuracy of the performance.
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... Changes in the rotating speed affect the head of the pump quadratically. (Nyholm, 2020) =̇√ ∆ ...
... For compressors in general, the curve type should be steeper at higher loads than what the CV-based quadratic curve is. (Nyholm, 2020) The power requirement of the machines is also modeled in a simple manner. It is based on the flow and pressure difference over the machine, with a corresponding efficiency. ...
... The outlet temperature is also modeled in a simple way, based on either an empirically approximated constant or on the inlet and outlet pressures. (Nyholm, 2020) ...
Operator training simulators (OTS) are a valuable tool
for improving the competence of both process operators
and engineers. As pumps, compressors and turbines are
vital parts of any process plant, accurate modeling of
these machines is crucial for a dynamic process
simulator. The behavior of the machines is usually
described by operating curves provided by the
manufacturer. However, these curves are strictly valid
only for the specific process conditions they were
created for. Accurate prediction of the operation outside
of these specific conditions may present a formidable
challenge. To further improve the accuracy the
NAPCON Simulator, a new dynamic modeling method
was developed for the centrifugal compressors. The
stability of the developed model was verified by
dynamic ramp tests of the operating conditions.
Furthermore, the accuracy of the developed model was
determined by performing process data tests. The results
were promising and, in most cases, an improvement
from the performance of the currently implemented
model.
Shell Global Solutions uses an in-house developed system for remote condition monitoring of centrifugal compressors. It requires field process data collected during operation to calculate and assess the machine's performance. Performance is assessed by comparing live results of polytropic head and efficiency versus design compressor curves provided by the Manufacturer. Typically, these design curves are given for specific suction conditions. The further these conditions on site deviate from those prescribed at design, the less accurate the health assessment of the compressor becomes. To address this specified problem, a compressor map prediction tool is proposed. The original performance curves of polytropic head against volumetric flow for varying rotational speeds are used as an input to define a range of Mach numbers within which the non-dimensional invariant performance curve of head and volume flow coefficient is generated. The new performance curves of polytropic head vs. flow for desired set of inlet conditions are then back calculated using the invariant non-dimensional curve. Within the range of Mach numbers calculated from design data, the proposed methodology can predict polytropic head curves at a new set of inlet conditions within an estimated 3% accuracy. The presented methodology does not require knowledge of detailed impeller geometry such as throat areas, blade number, blade angles, thicknesses nor other aspects of the aerodynamic design - diffusion levels, flow angles, etc. The only required mechanical design feature is the first impeller tip diameter. Described method makes centrifugal compressor surveillance activities more accurate, enabling precise problem isolation affecting machine's performance.
This paper introduces a new iterative method to predict the equivalent centrifugal compressor performance at various operating conditions. The presented theoretical analysis and empirical correlations provide a novel approach to derive the entire compressor map corresponding to various suction conditions without a prior knowledge of the detailed geometry. The efficiency model was derived to reflect the impact of physical gas properties, Mach number, and flow and work coefficients. One of the main features of the developed technique is the fact that it considers the variation in the gas properties and stage efficiency which makes it appropriate with hydrocarbons. This method has been tested to predict the performance of two multistage centrifugal compressors and the estimated characteristics are compared with the measured data. The carried comparison revealed a good matching with the actual values, including the stable operation region limits. Furthermore, an optimization study was conducted to investigate the influences of suction conditions on the stage efficiency and surge margin. Moreover, a new sort of presentation has been generated to obtain the equivalent performance characteristics for a constant discharge pressure operation at variable suction pressure and temperature working conditions. A further validation is included in part two of this study in order to evaluate the prediction capability of the derived model at various gas compositions.
The performance of centrifugal compressors is analyzed on the basis of nondimensional pressure and impeller-exit flow coefficients ψ and ϕ2. Use of these coefficients for impellers of high solidity permits presentation of compressor performance in a single curve for all conditions up to a critical Mach number. To show compressor data in a characteristic family of curves, performance is represented by pressure and flow coefficients KP and KQ. These coefficients take into account the coefficient of compression and the molecular weight simultaneously with the induction temperature. Test data for two compressors are presented over a range of Mach numbers from 0.36 to 2.0, and molecular weights in the range of that of air to heavy refrigerants. The test data show experimental verification of the fact that for centrifugal turbomachines with blades of high solidity the performance is a function of the Mach number only.
Simulation of pumps in an Ammonia synthesis unit (Production of ammonia) process is gaining importance and so the process needs to be studied and better ideas suggested such that the production cost is reduced. With the advent of computers and simulating software's like "ASPEN PLUS" it is possible to design and optimi ze a particular process. Proper design can significantly reduce production cost as well as provide to make the process safe and reduce environment hazards. The material, unit operation and processes involved are been identified. Steady state simulation is done. Each unit is taken into consideration and the variables are been optimized. The units are sequentially optimized. Use of newer equipment's in the process is been going to be suggested. The distillation column will be optimized and number of trays will be seen for smooth production and to attain high efficiency in product.
Coulson and Richardson's Chemical Engineering has been fully revised and updated to provide practitioners with an overview of chemical engineering. Each reference book provides clear explanations of theory and thorough coverage of practical applications, supported by case studies. A worldwide team of editors and contributors have pooled their experience in adding new content and revising the old. The authoritative style of the original volumes 1 to 3 has been retained, but the content has been brought up to date and altered to be more useful to practicing engineers. This complete reference to chemical engineering will support you throughout your career, as it covers every key chemical engineering topic. Coulson and Richardson's Chemical Engineering: Volume 1B: Heat and Mass Transfer: Fundamentals and Applications, Seventh Edition, covers two of the main transport processes of interest to chemical engineers: heat transfer and mass transfer, and the relationships among them.
A multi-stage centrifugal compressor model is presented with emphasis on analyzing use of an exit flow coefficient vs. an inlet flow coefficient performance parameter to predict off-design conditions in the critical region of a supercritical carbon dioxide (CO2) power cycle. A description of the performance parameters is given along with their implementation in a design model (number of stages, basic sizing, etc.) and a dynamic model (for use in transient studies). A design case is shown for two compressors, a bypass compressor and a main compressor, as defined in a process simulation of a 10 megawatt (MW) supercritical CO2 recompression Brayton cycle. Simulation results are presented for a simple open cycle and closed cycle process with changes to the inlet temperature of the main compressor which operates near the CO2 critical point. Results showed some difference in results using the exit vs. inlet flow coefficient correction, however, it was not significant for the range of conditions examined. This paper also serves as a reference for future works, including a full process simulation of the 10 MW recompression Brayton cycle.
This paper presents a comprehensive model of a post-combustion CO2 compression system that uses a multistage centrifugal compressor train to take the CO2-rich gas from the regenerator of a CO2 capture system and compress it to the supercritical conditions required for pipeline transport. The compression system model includes inter-stage coolers, flash vessels, glycol tower for water removal, and the CO2 inventory of the associated system. Both inline and integral gear compressors are designed using dimensionless numbers and appropriate design constraints. The steady-state integral gear compressor model is validated against industrial compressor performance data provided by a commercial CO2 compressor manufacturer. Its performance is also compared to the performance of an inline compressor model using a typical feed stream from a post-combustion CO2 capture process. Performance curves using dimensionless exit flow coefficient and isentropic head coefficient are used for simulating off-design performance of the integral gear compressor. To avoid surge during operation, a controller has been designed. Transients of key process variables in response to various disturbances have been studied.
The steam turbines play a significant role in global power generation. Especially, research on low-pressure (LP) steam turbine stages is of special importance for steam turbine manufacturers, vendors, power plant owners and the scientific community due to their lower efficiency than the high-pressure steam turbine stages. Because of condensation, the last stages of LP turbine experience irreversible thermodynamic losses, aerodynamic losses and erosion in turbine blades. Additionally, an LP steam turbine requires maintenance due to moisture generation, and therefore, it is also affecting on the turbine reliability. Therefore, the design of energy efficient LP steam turbines requires a comprehensive analysis of condensation phenomena and corresponding losses occurring in the steam turbine either by experiments or with numerical simulations. The aim of the present work is to apply computational fluid dynamics (CFD) to enhance the existing knowledge and understanding of condensing steam flows and loss mechanisms that occur due to the irreversible heat and mass transfer during the condensation process in an LP steam turbine.
Throughout this work, two commercial CFD codes were used to model non-equilibrium condensing steam flows. The Eulerian-Eulerian approach was utilised in which the mixture of vapour and liquid phases was solved by Reynolds-averaged Navier-Stokes equations. The nucleation process was modelled with the classical nucleation theory, and two different droplet growth models were used to predict the droplet growth rate. The flow turbulence was solved by employing the standard k-ε and the shear stress transport k-ω turbulence models. Further, both models were modified and implemented in the CFD codes. The thermodynamic properties of vapour and liquid phases were evaluated with real gas models. In this thesis, various topics, namely the influence of real gas properties, turbulence modelling, unsteadiness and the blade trailing edge shape on wet-steam flows, are studied with different convergent-divergent nozzles, turbine stator cascade and 3D turbine stator-rotor stage. The simulated results of this study were evaluated and discussed together with the available experimental data in the literature. The grid independence study revealed that an adequate grid size is required to capture correct trends of condensation phenomena in LP turbine flows. The study shows that accurate real gas properties are important for the precise modelling of non-equilibrium condensing steam flows. The turbulence modelling revealed that the flow expansion and subsequently the rate of formation of liquid droplet nuclei and its growth process were affected by the turbulence modelling. The losses were rather sensitive to turbulence modelling as well. Based on the presented results, it could be observed that the correct computational prediction of wet-steam flows in the LP turbine requires the turbulence to be modelled accurately. The trailing edge shape of the LP turbine blades influenced the liquid droplet formulation, distribution and sizes, and loss generation. The study shows that the semicircular trailing edge shape predicted the smallest droplet sizes. The square trailing edge shape estimated greater losses. The analysis of steady and unsteady calculations of wet-steam flow exhibited that in unsteady simulations, the interaction of wakes in the rotor blade row affected the flow field. The flow unsteadiness influenced the nucleation and droplet growth processes due to the fluctuation in the Wilson point.
The FCC wet gas compressor's major function is reactor pressure control. The machine must compress gas from the main column overhead receiver to gas plant operating pressure while maintaining stable regenerator-reactor differential pressure. The wet gas compressor and its control system play a vital role in maintaining steady reactor operating pressure. To be sure, optimum FCC operation requires balancing regenerator and reactor pressures to wet gas and air blower constraints. Reactor operating pressure is regulated by the main column overhead receiver. The wet gas machine needs to have sufficient capacity to compress receiver wet gas to the gas plant operating pressure. Reactor effluent composition, overhead receiver pressure and temperature, and gasoline endpoint all influence the amount of wet gas and its molecular weight. Centrifugal machines are used in many refinery units to compress gases. The principles of compression are the same irrespective of the process units.
The performance maps can be used in variable speed, fixed-inlet guide-vane centrifugal compressors for the dynamic simulation of a compressor system. A performance map describes how a compressor's polytropic head and power vary with volumetric suction flowrate and rotational speed for a specific set of suction conditions. Process simulators combine the performance maps with material and energy balances and thermodynamic relationships to predict the performance of the compressor under various operating scenario. Dimensional analysis is used to construct a coordinate system that is independent of suction conditions. A key aspect of this analysis is the identification of a set of dimensionless parameters in which the pertinent forces can be accurately represented. Functional relationships are set up in which polytropic head and shaft power are the dependent variables and volumetric flowrate and rotational speed are the independent variables. The function's parameters and the segment' end points are determined by fitting the curves to the reduced data.