Investigation into the effectiveness of feed spacer configurations for reverse osmosis membrane modules using Computational Fluid Dynamics

© 2016 Elsevier B.V.Reverse osmosis operations for water treatment are usually energy intensive and responsible for most of the product price. Several studies used flow characteristics to compare different geometries of feed spacers, but these cannot completely explain the effectiveness of feed spacers for promoting mass transfer near membranes. A few recent studies introduced a concept (Spacer Configuration Efficacy, SCE) combining mass transfer and energy consumption, but SCE has been applied only to a limited extent. The present study uses 3-dimensional steady state Computational Fluid Dynamics with mass transfer to compare four channels with feed spacer configurations (Ladder-type, Triple, Wavy and Submerged) and an empty plain channel using SCE and other performance measures. In contrast to previous studies, a saturated concentration boundary condition is employed at the membrane surface and optimised meshing of the domain is discussed. Power law correlations for SCE and other performance measures developed from the simulation results enable quick evaluation of the spacers. Results indicated that the assumed saturated solute concentration at the membrane strongly affects the mass transfer coefficient. Based on SCE, the Wavy spacer configuration showed the highest performance for Re>120 among the obstructed geometries considered, while Ladder-type was better for Re<120

Prediction of bulk modulus and volumetric expansion coefficient of water for leak tightness test of pipelines

To determine whether any pressure variation in pipeline hydrostatic test is a result of temperature changes or the presence of leaks, the calculation of pressure/temperature changes is required for test sections. In these calculations, bulk modulus and volumetric expansion coefficient of fresh or sea water must be taken into account. In this study, a simple-to-use correlation is developed to predict the bulk modulus and volumetric expansion coefficient of both fresh and sea water as a function of temperature and pressure. The proposed correlation helps to cover the bulk modulus and volumetric expansion coefficient of both fresh and sea water for temperatures less than 50 °C (40 °C for sea water) as well as pressures up to 55,000 kPa (550 bar). The results can be used in follow-up calculations to determine whether any pressure variation in pipeline hydrostatic test is a result of temperature changes or the presence of leaks. The proposed correlation showed promising results with average absolute deviations for volumetric expansion coefficient and bulk modulus of water being around 0.58% and 0.08% respectively. The novel correlation is easy to use and will prove to be of immense value for project engineers to test the critical limits accurately

Simple methodology for sizing of absorbers for TEG (triethylene glycol) gas dehydration systems

Natural gas is an important source of primary energy and it is saturated with water vapor under normal production conditions. In the design of natural gas dehydration systems, correct estimation of absorption column size is crucial. Once the lean TEG (Triethylene glycol) concentration has been established, the circulation rate of TEG and number of trays (height of packing) must be determined. The current methods to correlate the TEG circulation rate, TEG purity, water removal efficiency, number of equilibrium stages (or height of packing) and the diameter of contactor employs rigorous calculation techniques involving more complicated and longer computations. The aim of this study is therefore to develop a simple-to-use method, by employing basic algebraic equations to correlate water removal efficiency as a function of TEG circulation rate and TEG purity for appropriate sizing of the absorber at wide range of operating conditions of TEG dehydration systems. Estimates from simplified approach were found to be quite reliable and accurate, as evidenced by the comparisons with literature data where the average absolute deviation percent from reported data in the literature shown to be around 0.05%

Novel predictive tools for design of radiant and convective sections of direct fired heaters

Direct fired heaters are used considerably in the energy related industries and petroleum industries for heating crude oil in the petroleum refining and petrochemical sectors. The aim of the current study is to formulate simple-to-use correlations to design the radiant and convective sections of direct fired heaters. The developed tools are easier than currently available models and involves a fewer number of parameters, requiring less complicated and shorter computations. Firstly, a simple correlation is developed to provide an accurate and rapid prediction of the absorbed heat in the radiant section of a fired heater, expressed as a fraction of the total net heat liberation, in terms of the average heat flux to the tubes, the arrangement of the tubes (circumferential), and the air to fuel mass ratio. Secondly, another simple correlation is developed to approximate external heat transfer coefficients for 75, 100, and 150 mm nominal pipe size (NPS) steel pipes arranged in staggered rows and surrounded by combustion gases. Finally, a simple correlation is presented to predict the gross thermal efficiency as a function of percent excess air and stack gas temperature. This study shows that the proposed method has a good agreement with the available reliable data in the literature. The average absolute deviations between reported data and the proposed correlations are found to be around 1.5% demonstrating the excellent performance of proposed predictive tool. The proposed simple-to-use method can be of significant practical value for the engineers and scientists to have a quick check on the design of radiant and convective sections of direct fired heater. In particular, mechanical and process engineers would find the proposed approach to be user-friendly involving no complex expressions with transparent and easy to understand calculations

Estimation of performance of steam turbines using a simple predictive tool

Mechanical drive steam turbines are major prime movers for compressor, blower, and pump applications. Steam turbines are available for a wide range of steam conditions, power ratings and speeds. In this work, a simple predictive tool, which is easier than existing approaches, less complicated with fewer computations, is presented for rapid prediction of steam rate, turbine efficiency, and the inlet and exhaust nozzle diameters to determine the actual steam rate (ASR) and total steam requirements for both multi-stage and single-stage turbines. The proposed method predicts the above mentioned parameters for inlet steam pressures up to 12,000 kPa, turbine ratings up to 10,000 kW as well as the exhaust air over inlet air ratios of up to 0.55. The predictions from the proposed predictive tool have been compared with reported data and found good agreement with average absolute deviation hovering around 1.4%

Prediction of temperature drop accompanying a given pressure drop for natural gas wellstreams

Za učinkovitu konstrukciju postrojenja za proizvodnju prirodnog plina neophodno je sigurno predviđanje pada temperature koji prati određeni pad tlaka. Sadašnji rigorozni kompozicijski modeli ovise o više varijabli i iziskuju informacije o sastavu fluida. U ovom je radu prikazana metoda, jednostavna za korištenje i lakša od danas dostupnih modela, za sigurno predviđanje odgovarajućeg pada temperature praćenog odgovarajućim padom tlaka u sustavima za proizvodnju prirodnog plina temeljena na modelu "black oil", a u svrhu dobivanja brzog aproksimativnog rješenja za pad temperature struje prirodnog plina u sustavima za proizvodnju plina. S obzirom na rezultate, novorazvijena korelacija preporuča se za brzu procjenu pada temperature u sustavima proizvodnje plina za tlak do 45 MPa i pada tlaka do 25 MPa. Dobiveni rezultati pokazuju visoku podudarnost između podataka iz izvještaja i izračunatih vrijednosti dobivenih novom metodom. Srednja vrijednost odstupanja, između podataka iz izvještaja i predložene korelacije je oko 4,6 posto. Čini se da je predložena metoda superiorna zbog svoje preciznosti i pregledne numeričke podloge u čemu se relevantni koeficijenti za razne podatke mogu dobiti brzo.Accurate prediction of the temperature drop accompanying a given pressure drop for the natural gas production systems is necessary in the effective design of natural gas production facilities. Current rigorous compositional models depend on many variables and require information about fluid composition. In this paper, a simple-to-use method which is easier than current available models, is presented to predict accurately the appropriate temperature drop accompanying a given pressure drop in natural gas production systems based on the black-oil model to get a quick approximate solution for the temperature drop of a natural gas streams in gas production systems. Considering the results, the new developed correlation is recommended for rapid estimation of temperature drops in gas production systems for pressures up to 45 MPa and pressure drops up to 25 MPa. The obtained results illustrate that good agreement is observed between the reported data and the values calculated using the new developed method. The average absolute deviation between reported data and the proposed correlation is around 4.6%. The proposed method appears to be superior owing to its accuracy and clear numerical background, wherein the relevant coefficients can be retuned quickly for various data

Simple equations to correlate theoretical stages and operating reflux in fractionators

Natural gas is an important source of primary energy. Virtually all gas processing plants producing natural gas liquids require at least one fractionator to produce a liquid product which can meet sales specifications. Fractionation is one of the pivotal unit operations in refineries, gas processing and other industries utilized to separate mixtures into individual products. However, it is capital and energy intensive and, with decreasing relative volatility, the size and energy requirements of a column tend to increase. The primary parameters involved in the design of fractionators are the number of stages and the reflux ratio. The aim of this study is to develop easy-to-use equations, which are simpler than current available models involving a large number of parameters and requiring more complicated and longer computations, for an appropriate prediction the operating reflux ratio for a given number of stages. Alternatively, for a given reflux ratio, number of stages can be determined. The accuracy of the proposed equations was tested and found to be in excellent agreement with the reported data for the wide range of conditions, wherein the average absolute deviation percent of proposed equations being 1.5%. These simple-to-use equations can be of immense practical value for the engineers. In particular, process engineers would find the proposed approach to be user friendly involving no complex expressions with transparent calculations