Barry Crittenden

Preparation and characterisation of chemisorbents based on heteropolyacids supported on synthetic mesoporous carbons and silica

Abstract The preparation of chemisorbents based on tungsto- and molybdophosphoric acids supported on two types of synthetic mesoporous carbons and two types of mesoporous silica is described. Strong solid acids with good accessibility to acid sites may potentially be effective adsorbents for the removal of basic molecular impurities, such as amines, from ultrapure manufacturing environments. Prepared materials were characterised by scanning electron microscopy, nitrogen adsorption, Fourier-transform infrared spectroscopy, powder X-ray diffraction, and equilibrium ammonia uptake. Composites of SBA-15 with heteropolyacids were synthesised. It was shown that the inclusion of HPAs into SBA-15 results in the loss of long range order. Adsorbents based on the HPAs impregnated into the supports with the open-pore morphology (Novacarb and SBA-15) were found to be promising materials. A composite of tungstophosphoric acid with sol–gel SiO2 was found to have the highest ammonia uptake.

Use of the linear driving force approximation to guide the design of monolithic adsorbents

The objective of this paper is to provide the basic tools necessary to guide the optimal design of monolithic adsorbents. Previous work has concentrated on optimizing monolith manufacturing processes and experimental studies have suggested that the mass transfer performance of the monolithic form might be inferior to that of the more traditional packed bed form. In this paper, the classical linear driving force approximation, along with the parabolic concentration gradient assumption, is applied to a number of simple geometries. Transformation of square, rectangular, triangular and hexagonal geometries to an equivalent hollow cylinder on the basis of equal volume and equal internal surface area, facilitates use of the linear driving force analytical solution for a cylinder in order to guide the design of the more complex monolith geometries. Taking channel mass transfer performance and pressure gradients into consideration as well, the analyses indicate that regular hexagonal channels offer the best compromise on overall performance, with minimization of the wall thickness being the key design objective. Use of the algebraic design equations for the circular channel provides an excellent approximation for the regular hexagon and thus design work can be carried out with the former and simpler geometry. The engineering challenge now becomes one of manufacturing monoliths with appropriately thin walls. A challenge for the future is to obtain the full numerical solutions for the square, rectangular, triangular and hexagonal geometries.

The effect of the binder on the manufacture of a 5A zeolite monolith

Abstract The effect of the binder on the manufacture, by a paste extrusion process, of a 2 mm square lattice channel zeolite monolith with a 0.98 mm wall thickness and an overall diameter of 20 mm has been studied using a variety of visual and analytical techniques. Crucial factors for manufacturing defect-free 5A zeolite monoliths have been found to be the use of a binder with good plasticity properties, such as Na–bentonite, extrusion conditions, and a well-controlled drying process. Pastes containing different amounts of water and binder were characterised from the relationship between pressure drop and extrudate velocity during flow from a circular barrel into a circular die-land. From the relationship between the extrusion pressure and the extrudate velocity, six extrusion parameters were derived for each paste. A higher extrusion pressure is required when there is either a decreased water content or an increased binder content.

Fouling in Crude Oil Preheat Trains: A Systematic Solution to an Old Problem

A major cause of refinery energy inefficiency is fouling in preheat trains. This has been a most challenging problem for decades, due to limited fundamental understanding of its causes, deposition mechanisms, deposit composition, and impacts on design/operations. Current heat exchanger design methodologies mostly just allow for fouling, rather than fundamentally preventing it. To address this problem in a systematic way, a large-scale interdisciplinary research project, CROF (crude oil fouling), brought together leading experts from the University of Bath, University of Cambridge, and Imperial College London and, through IHS ESDU, industry. The research, coordinated in eight subprojects blending theory, experiments, and modeling work, tackles fouling issues across all scales, from molecular to the process unit to the overall heat exchanger network, in an integrated way. To make the outcomes of the project relevant and transferable to industry, the research team is working closely with experts from many world leading oil companies. The systematic approach of the CROF project is presented. Individual subprojects are outlined, together with how they work together. Initial results are presented, indicating that a quantum progress can be achieved from such a fundamental, integrated approach. Some preliminary indications with respect to impact on industrial practice are discussed.

Predicting ozone levels : A statistical model for predicting ozone levels in the Shuaiba Industrial Area, Kuwait.

This paper presents a statistical model that is capable of predicting ozone levels from precursor concentrations and meteorological conditions during daylight hours in the Shuaiba Industrial Area (SIA) of Kuwait. The model has been developed from ambient air quality data that was recorded for one year starting from December 1994 using an air pollution mobile monitoring station. The functional relationship between ozone level and the various independent variables has been determined by using a stepwise multiple regression modelling procedure. The model contains two terms that describe the dependence of ozone on nitrogen oxides (NOx) and nonmethane hydrocarbon precursor concentrations, and other terms that relate to wind direction, wind speed, sulphur dioxide (SO2) and solar energy. In the model, the levels of the precursors are inversely related to ozone concentration, whereas SO2 concentration, wind speed and solar radiation are positively correlated. Typically, 63 % of the variation in ozone levels can be explained by the levels of NOx. The model is shown to be statistically significant and model predictions and experimental observations are shown to be consistent. A detailed analysis of the ozone-temperature relationship is also presented; at temperatures less than 27 °C there is a positive correlation between temperature and ozone concentration whereas at temperatures greater than 27 °C a negative correlation is seen. This is the first time a non-monotonic relationship between ozone levels and temperature has been reported and discussed.

Zeolite Monoliths for Air Separation: Part 1: Manufacture and Characterization

A 2 mm square lattice channel monolith with a 0.98 mm wall thickness and an overall diameter of 20 mm has been prepared from 5A zeolite powder and Na-bentonite (as a binder) by the unit operations of paste preparation, extrusion, drying and firing. The manufactured monolith, which shows good strength, retains the crystal structure and micropore size of the source 5A zeolite powder, and adsorption measurements made by the constant volume method confirm that the monolith exhibits an equilibrium performance equal to that of commercial 5A zeolite pellets. In addition, the monolith possesses a higher macroporosity than the commercial pellets, a feature which is expected to be of benefit in pressure swing adsorption air separation processes.

Zeolite Monoliths for Air Separation: Part 2: Oxygen Enrichment, Pressure Drop and Pressurization

Experiments have been carried out on two individual idealized steps in a pressure swing cycle in order to compare the performance of a novel adsorbent monolith with that of a packed bed of commercial pellets containing the same weight of adsorbent. The application is the production of oxygen-enriched air using 5A zeolite. For feed pressures up to 3.8 bar, maximum oxygen compositions in the constant pressure production step up to 100% and 52%were achieved for the monolith with the previous step being carried out (1) by purging the column with pure oxygen at 1 bar, and (2) by evacuating the column, respectively. For virtually all experimental conditions studied, the separative performance of the monolith was found to be somewhat inferior to that of the bed of pellets, this being due largely to the former’ s poorer film mass transfer coefficient together with the virtual independence of this coefficient on velocity. To its advantage, however, the pressure drop through the monolith was found to be 3-5 times lower than that through the equivalent packed bed and consequently the time to pressurize the monolith was found to be 3-5 times faster than for the bed of pellets. Thisexperimental feasibility study has demonstrated that the novel zeolite monolith configuration shows good potential for the production of oxygen-enriched air of low-to-medium purity, in a low energy, short cycle time, pressure swing process. The challenges associated with both improving and modelling the performance of the monolith are described.

Characterization of Crude Oils and Their Fouling Deposits Using a Batch Stirred Cell System

A small (1 L) batch stirred cell system has been developed to study crude oil fouling at surface temperatures up to 400°C and pressures up to 30 bar. Fouling resistance–time data are obtained from experiments in which the principal operating variables are surface shear stress, surface temperature, heat flux, and crude oil type. The oils and deposits are characterized and correlated with the experimental heat transfer fouling data to understand better the effects of process conditions such as surface temperature and surface shear stress on the fouling process. Deposits are subjected to a range of qualitative and quantitative analyses in order to gain a better insight into the crude oil fouling phenomenon. Thermal data that can be obtained relatively quickly from the batch cell provide fouling rates, Arrhenius plots, and apparent activation energies as a function of process variables. The experimental system, supported by computational fluid dynamics (CFD) studies, allows fouling threshold conditions of surface temperature and shear stress to be identified relatively quickly in the laboratory. The data also contribute to existing knowledge about the compensation plot.

Enhancement of liquid phase adsorption column performance by means of oscillatory flow: an experimental study

Abstract Oscillations have been applied to the bulk fluid flowing upwards through a column packed with spherical beads of 3A zeolite that is used to dry an ethanol solution containing about 3.2 wt.% water. For a given bulk flow rate, concentration and temperature of the feed, the application of oscillations leads to a delay in the breakthrough time and a sharper breakthrough curve than for the case when there is no oscillation. Use of oscillatory flow, therefore, leads to an increased bed capacity up to the point of breakthrough, a lower length of unused bed and a reduced mass transfer zone length (MTZL). Improvements in all these parameters of up to 20% have been found for oscillations with frequencies up to 2.4 Hz and amplitudes up to 0.001 m. The best improvements were found for the highest frequencies and amplitudes that could be achieved with the apparatus. In the development of the process it is believed that a further 20–30% improvement could realistically be achieved. However, practical limitations to the upper bounds of these two oscillatory parameters arise from the strength of the adsorbent particles and the risk of movement of particles within the bed. The main reason for the improvement in the process, when oscillations are applied, is the increase in interparticle transport. The increase in interparticle heat transfer is potentially as relevant as that in interparticle mass transfer . The results obtained for this model chemical system show that process improvements through the use of oscillatory flow can be obtained with well-rounded particles and it is speculated that the method could be applicable more generally to all liquid phase adsorption processes.

Manufacture and Characterisation of Silicalite Monoliths

Multichannel monoliths containing up to 90% silicalite by weight and with cell densities up to 28 cells/cm2, wall thicknesses down to 0.6 mm and an overall diameter of 40 mm have been prepared from silicalite powder and sodium bentonite (as a binder) by the unit operations of paste preparation, extrusion, drying and firing. The manufactured monoliths, which show good strength, retain the crystal structure and micropore size of the source silicalite powder, and adsorption measurements made by using a dynamic flow system confirm that the monoliths exhibit an equilibrium performance broadly similar to that of commercial silicalite pellets. In addition, the manufactured monoliths possess a higher macroporosity than the commercial pellets. Regeneration of the monoliths was found to be possible at mildly increased temperature. These features augur well for the recovery and/or separation of organic compounds in simple pressure swing and thermal swing processes.