A.G.J. van der Ham

A small-scale regularly packed circulating fluidized bed: Part I: Hydrodynamics

The present investigation is based on the idea of intensifying the gas?solids contact in a circulating fluidized bed by introducing obstacles into it. Such obstacles may effectively suppress radial inhomogeneities in the solids flux and concentration, increase the dynamic solids hold-up, and break up solids clusters. This article (Part I) deals with the hydrodynamics (pressure drop and solids hold-up) investigated at ambient conditions, for cocurrent upward flow of air and microsize solid particles (FCC, 70 µm diameter) over a regularly structured inert packing introduced into the riser part of a circulating fluidized bed unit. The packed section has a height of 0.48 m, a cross-sectional area of 0.06 × 0.06 m2 and contains regularly-stacked 0.01 m diameter Perspex bars as the obstacles meant to enhance the gas?solids contact. Slide-valves mounted above and below the packed section can be used to trap the solids inventory and determine the (dynamic) solids hold-up. Gas and solids mass fluxes have been varied in the range of 0.7 < Gg < 4.4 and O < Gs < 15 kg m-2s-2, respectively. Part II will report on the results of gas?solids mass transfer measurements, which have been carried out in the same set-up at comparable experimental conditions. Results of this work show that: (i) the pressure gradient over the packed section increases linearly with increasing solids mass flux, but faster than linearly with increasing applied gas mass flux, (ii) the dynamic solids volume fraction can be described quite well by the correlation s dyn = 0.0084 GsGg-1.22 for almost the entire range of applied gas and solids mass fluxes, (iii) the value for the solids friction factor derived for the gas flux range 0.7 < Gg < 3.7 kg (m-2s-1) varies from 1.4 to 2.5 and is linear with the solids volume fraction. These fs values are about 2 to 3 decades higher than those obtained from fs correlations derived for dilute-phase pneumatic conveying lines operated under the same experimental conditions.

Techno-economic evaluation of high temperature pyrolysis processes for mixed plastic waste.

Three pyrolysis processes for Mixed Plastic Waste (MPW) with different reactors (Bubbling Fluidized Bed, Circulating Fluidized Bed and Rotating Cone Reactor, respectively BFB, CFB and RCR) were designed and evaluated. The estimated fixed capital investment for a 50 kton/year MPW pyrolysis plant build in 1996 in the Netherlands varies from 14 million Nlg for the RCR to 20 million Nlg for the CFB pyrolysis plant. The Return On Investment (ROI) of the RCR plant (29.5%) is significantly higher than the ROI of the BFB and CFB pyrolysis plants (respectively 21.7 and 14.2%). The lower temperature levels (less energy required) and the fact that the RCR requires no or little additional utilities are important operational advantages of the RCR. The chlorine from PVC was removed in this process using low temperature pyrolysis converting 98% of the chlorine into HCl gas. The remaining chlorine is subsequently converted to CaCl2 by reaction with CaCO3 , but this is unattractive from an economical and environmental point of view as a large amount of sand has to be disposed of to prevent agglomeration of CaCl2 and sand at high CaCl2 concentrations. Removal of the last 2w% of chlorine using NH3 or NaHCO3 is probably preferable. A dedicated product separation section for a MPW pyrolysis plant of this size is economically not feasible as only 50 to 60% of the polymer is converted to product gas, resulting in an economically unattractive scale for the product separation section. Integration of the pyrolysis plant with an existing chemical complex (naphtha cracker) should be preferred. The use of a dedicated product separation section should only be considered, if this is required for non technical/economic or geographical reasons

A small scale regularly packed circulating fluidized bed. Part II: Mass transfer

The underlying objective of the present study is to increase gas?solids contact in a circulating fluidized bed by the introduction of obstacles in the riser portion. The presence of such obstacles leads to suppression of radial inhomogeneities in the solids mass flux and concentration, and break-up of solids clusters. At ambient conditions, gas?solids mass transfer was investigated for cocurrent upward flow of air and microsize solid particles (FCC, 70 ?m diameter) over a regularly structured inert packing introduced into the riser part of a circulating fluidized bed unit. The packed section has a height of 0.48 m, a cross-sectional area of 0.06 × O.06 m2, and contains regularly stacked 0.01 m diameter Perspex bars as the obstacles meant to enhance the gas?solids contact. Gas mass fluxes used were 1.4 and 2.7 kg m?2 s?1. Solids mass fluxes were varied in the range 0Gs 12 kg m?2 s?1. Experimental mass transfer data were obtained by applying the method of adsorption of naphthalene vapor on FCC particles. A conservative estimate of the apparent gas?solids mass transfer coefficient kg* could be derived from the naphthalene vapor concentration profile along the packed section on the basis of a plug-flow-model interpretation, while assuming single-particle behaviour and neglecting intraparticle diffusion effects. Such kg* values appear to increase with increasing gas mass flux, but decrease with increasing solids mass flux (and consequently increasing solids volume fraction) probably due to the corresponding increase in particle shielding. Comparison of the present results with available literature data for similar solid materials suggests that the effect of the packing inserted into the CFB is significant: the Sherwood numbers derived from the present study are relatively high.

Hydrogenation of Carbon Dioxide for Methanol Production

A process for the hydrogenation of CO2 to methanol with a capacity of 10 kt/y methanol is designed in a systematic way. The challenge will be to obtain a process with a high net CO2 conversion. From initially four conceptual designs the most feasible is selected and designed in more detail. The feeds are purified, heated to 250 °C and fed to a fluidized bed membrane reactor equipped with a Cu/ZnO/Al2O3 catalyst. Zeolite membranes mainly remove the methanol and shift the equilibrium reaction towards methanol. A yield of 25 % per pass is obtained. The permeate and the water-methanol mixture from the phase separator is finally separated in a distillation column. In the final design 15.4 kt/y of carbon dioxide is needed in order to produce 10 kt/y methanol. The net CO2 reduction is about 2/3, which is significant. The process is technical but currently not economically feasible.

Axial dispersion in gases flowing through a packed bed at elevated pressures

Axial dispersion in upward gas flow is investigated by pulse and displacement experiments in a vertical, packed column with different concentrations of the tracer and at pressures up to 1.5 MPa. The responses to the introduced pulse and step changes are measured at two locations and the extent of axial dispersion, respresented by the Bodenstein number, is determined by curve fitting in the time domain. The performed experiments demonstrate that the residence time distribution is considerably affected by density differences between the tracer and carrier gas, particularly at elevated pressures. Obtained Bodenstein numbers for step changes from nitrogen to a helium/nitrogen mixture and vice versa differ by as much as a factor ten, depending on the helium concentration and column pressure. The difference in axial dispersion may be ascribed to gravitation-driven instabilities as due to vertical density gradients in the case of a heavy gas displaced by a light gas; density gradients in the step changes from a light to heavy gas evidently inhibit axial dispersion. The presented observations are of major importance for the description of flow behaviour of gases in packed bed reactors where density gradients exist due to temperature and concentration gradients, particularly because many processes operate at elevated pressures.

Techno-economic evaluation of the direct conversion of CO2 to dimethyl carbonate using catalytic membrane reactors

The production of dimethyl carbonate (DMC) caught more interest in the past decades due to its versatile use (e.g. as fuel additive), low toxicity and fast biodegradability. Different ‘green’ production routes are being developed to replace the conventional and rather toxic production of DMC via phosgene. The direct conversion of CO2 and methanol toward DMC is an environmental and economically interesting production route for the chemical industry. This work describes the process design of the direct conversion of CO2 to dimethyl carbonate, providing a valuable insight and a better understanding of the process limitations. In this design, membrane reactors are used for continuous removal of water by-product, in order to overcome the equilibrium limitations. The rigorous Aspen Plus simulations show that even when using an excess of methanol, the attainable conversion is low and the DMC concentration in the reactor effluent is less than 1.5 mol%. Purifying this diluted stream to the desired concentrations demands large size equipment and a substantial amount of energy (13.61 kWh/kg DMC) resulting in high investment and utility costs, thus making the process not profitable. The focus for new membrane reactors could be on the selective removal of DMC (instead of water) from the reaction area to allow for a more concentrated DMC stream.

Survey of Desulfurization Processes for Coal Gas

An overview is given of recent developments in the field of regenerative, high temperature, coal gas desulfurization. The results of a wide variety of sorbents and reactor types either tested on lab-, bench- as well as on pilot-plant scale are presented. The sorbents discussed are mainly oxides of transition metals either pure, mixed or deposited on an inert carrier. Regeneration performance of the sorbent and the regeneration off-gas composition are also taken into account.

Evolution patterns and family relations in G-S reactors

Reactor selection strategies for gas–solid (G–S) heterogeneously catalysed processes can be based on the requirements of the desired process and the properties of the reactions and catalysts involved. Ultimately a reactor selection will nearly always be grounded on existing or emerging reactor types slightly modified for adaptation to the specific chemical process. This procedure results in radiation of different reactor modifications from the archetypes towards niche applications. It is shown that this process has a lot of resemblance with the evolution process of animal species. The G–S heterogeneous catalytic reactors can be classified into three or four families. They are presented as adaptations from only three archetypes: packed bed, fluid bed and barrier wall. The properties of these reactors and their family members are discussed. Examples are given of a few relatively new variants and of competition of very different reactors for the same application niche. The classification system can be used as a means for the creation of new reactors and if extended with a database or knowledge system it can facilitate reactor selection. Similar classifications can be set-up for other types of chemical reactors like G–L and G–L–S reactors.