Energy Aspects in Drying

The energy performance of a dryer and a drying process is characterized by various indices such as volumetric evaporation rate, steam consumption, unit heat consumption and energy (thermal) efficiency. Of all indices, energy efficiency is the most frequently quoted, in technical specifications. A thorough analysis of available information, including the Handbook of Industrial Drying, points to the inconsistency of terminology, definition and data interpretation. Thus, reported data on energy efficiency vary significantly and frequently contradict both drying theory and industrial practice. To establish a common platform to deal with energy issues, this article provides a concise overview of the most common definitions of energy efficiency, along with a critical review of the published data. A need for energy audit and benchmarking is pointed out. To eliminate shortcomings of the energy efficiency as a lumped parameter, and to allow analysis of energy consumption over time (batch drying) or distance (continuous drying), instantaneous and cumulative indices are proposed. Using these indices, the energy performance of selected dryers is examined, and possible modifications to dryer design and operating parameters are indicated in order to reduce the overall energy consumption.     

Aromatic gas oil cracking under realistic FCC conditions in a microriser reactor

A paraffinic hydrowax feed spiked with naphthalene, anthracene, and phenanthrene was cracked in a once-through microriser reactor at 575 °C and with a catalyst-to-oil (CTO) ratio of 4.8 g_cat g_oil⁻¹. The conversion by cracking reactions is limited to the paraffinic fraction of the feed and the alkyl groups associated with the benzene ring in aromatic compounds; the aromatic probes did not crack under the applied conditions, and in fact an additional amount of naphthalene was formed by complex dealkylation and hydrogen transfer reactions. The ‘uncrackabilty’ of aromatics was directly demonstrated by processing an aromatic gas oil, containing 33.3 wt% aromatics. Experiments were performed with residence times between 0.05 and 8.2 s, keeping the temperature (525 °C) and CTO ratio (5.5 g_cat g_oil⁻¹) constant. The data was interpreted with a simplified first-order five-lump kinetic model, where approximately 19 wt% of the feed was found to be uncrackable. HCO (feedstock) conversion took mainly place during the first two seconds and coke was only formed during the first 50 ms of catalyst-oil contact. Gasoline was not overcracked to gas. Approximately 50 wt% of the LCO fraction was formed during this 50 ms and did not change thereafter.     

INTEGRATING ALBEMARLE RESOLVE DESULFURIZATION TECHNOLOGY WITH NOVEL PETRO-CANADA PROCESS CONCEPTS IN COMMERCIAL FCCU OPERATIONS

The gasoline and distillate sulfur regulations promulgated throughout the world to reduce tail-pipe emissions are now strongly impacting refinery operations and investments. FCC gasoline is recognized as the principal contributor of sulfur to the gasoline pool and has become the focus for meeting the new specifications. The difficulty in removing sterically hindered sulfur species in the fluid catalytic cracking unit (FCCU) cycle oil drives up the hydrogen and investment costs for treating the distillates. Although installation of pre- and post-treatment facilities is planned by many refiners, other non-capital approaches such as undercutting are being evaluated to meet interim and future sulfur levels. Even when expensive treatment facilities are installed, operating costs can be lowered and the flexibility of the facility increased with improvements in the ability to remove sulfur in the FCCU. In this article, we detail Petro-Canada's experience in integrating Albemarle's RESOLVE sulfur reduction technology with a combination of innovative process ideas. These concepts include heavy naphtha recycle, coprocessing of hydrogen donor feeds, and recycle of light cycle oil (LCO) to a specially designed stripper reactor. Special attention is paid to the interaction of deep FCC feed hydrodesulfurization with the FCC performance. The results demonstrate that very low FCC gasoline sulfur levels can be achieved without significant capital investment through novel approaches to recycle, creative integration of cat feed hydrotreating unit (CFHTU)-FCCU designs and operations, and application of state-of-the-art sulfur reduction additive technology. An added benefit of the RESOLVE 950 sulfur reduction technology is the substantial elimination of sulfur oxides in the FCC flue gas. This has been observed in Petro-Canada operations and numerous other RESOLVE 950 applications around the world.     

A PROCEDURE FOR EVALUATION OF COMMERCIAL FCC CATALYSTS

Based on the correlations between laboratory units and commercial plants, a procedure has been developed to guide commercial FCC (Fluid Catalytic Cracking) catalysts selection. Examining the operability of catalyst mixtures during catalyst transition period is emphasized. The testing procedure is simple and reliable. A commercially available catalyst and the catalyst currently in use have been compared to demonstrate the applicability of this procedure. The commercial testing has confirmed the reliability of laboratory results. The use of the catalyst evaluated led to a 30% saving in catalyst cost.     

Three-dimensional modelling of Non-Newtonian fluid flow in a coat-hanger die

The three-dimensional model of isothermal flow of power-law fluid in a coat-hanger die has been developed using finite element method. The shape of coat-hanger die used in the present model was determined according to the previous analytical design equation which is based on one-dimensional flow model in the manifold and the slot. Because uniform flow rate across the die outlet is most important to achieve uniform thickness of extruded polymer sheet or film, flow rate distribution is mainly examined to determine the valid process condition for the design equation as the design parameters are changed. The effects of fluid property in terms of power-law index and process parameters not considered in one-dimensional design equation such as die inlet size and the presence of land were analyzed. Results show that the manifold angle is the most influencing design parameter on flow rate distribution. When the material of different power-law index from design value is processed, the change of power-law index affects the uniformity of flow rate appreciably.     

Shear rates investigation in a scraped surface heat exchanger

The electrodiffusion technique was performed in order to investigate the shear rate on a scraped surface heat exchanger. Microelectrodes were placed inside: the walls of the outer cylinder; the inlet and outlet bowls; the rotor and the blades. Highly viscous Newtonian fluid (Emkarox HV45 solutions) and non-Newtonian model fluid (aqueous solutions of CMC) were used. The electrodiffusion method allowed us to measure wall shear rates. Maximum shear rate was observed at the scraping surface and caused by blades scraping, high shear rate was also measured on the leading edge of the blades. In the other parts of the exchanger, shear rate remained low but the development of Taylor vortices completely modified the scraped surface heat exchangers behaviour inside the surface of the bowls. A dimensionless representation of the friction factor was established for the inner and outer wall surface of the exchanger.     

DAMPED OSCILLATIONS IN A BINGHAM PLASTIC FLUID

Stokes' second problem, the propagation and damping of waves into a semi-infinite fluid generated by harmonic oscillations of a flat pate on the surface, is solved for the simple Bingham Theological constitutive model. The solution reveals the existence of “windows” in the distance-time-stress space in which shearing is possible whereas outside these restricted regions no shearing can occur. Within these restricted regions the wave forms developed are exponentially damped, traveling waves which propagate from the excitation plane into the fluid and disappear along definitely prescribed boundaries determined by the yield condition. The most significant consequence of the existence of these “windows” of shear is that even very small yield stresses will radically modify the induced velocity wave patterns from that which would be expected based upon the classical Newtonian fluid solution of Stokes' second problem. At least in this physical setting, it is not necessary for shear to occur globally for motion to occur anywhere, as has been postulated in some recent studies of complex motions. Thus, the motion is consistent with a simple Bingham model which does possess a yield stress.     

Combustion characteristics and flame stability at the microscale: a CFD study of premixed methane/air mixtures

A two-dimensional elliptic, computational fluid dynamics (CFD) model of a microburner is solved to study the effects of microburner dimensions, conductivity and thickness of wall materials, external heat losses, and operating conditions on combustion characteristics and flame stability. We have found that the wall conductivity and thickness are very important as they determine the upstream heat transfer, which is necessary for flame ignition and stability, and the material's integrity by controlling the existence of hot spots. Two modes of flame extinction occur: a spatially global type for large wall thermal conductivities and/or low flow velocities and blowout. It is shown that there exists a narrow range of flow velocities that permit sustained combustion within a microburner. Large transverse and axial gradients are observed even at these small scales under certain conditions. Periodic oscillations are observed near extinction in cases of high heat loss. Engineering maps that delineate flame stability, extinction, and blowout are constructed. Design recommendations are finally made.     

Transition of the flow in a symmetric channel with periodically expanded grooves

Transition of the flow in a periodically grooved channel is numerically investigated for periodicity indices m=1 up to 6 by assuming the two-dimensional and fully developed flow field, where m is defined as a number of grooves in which the flow repeats periodically. Critical Reynolds numbers for the onset of a self-sustained oscillatory flow from a steady-state flow are evaluated by numerical simulations. It is found that the bifurcations occur at the critical Reynolds numbers as a result of Hopf bifurcation, and a period in the streamwise direction of the oscillatory flow is twice as long as the groove pitch of the channel. In addition, flow visualization with the aluminum dust method is carried out to confirm the results obtained from the numerical simulations. The experimental results are in good agreement with the numerical ones.