Aah Bart Drinkenburg

Liquid-induced pulsing flow in trickle-bed reactors

This contribution describes the experiments on pulse induction by cycling the liquid feed in a column of 3.2 m height. Based on a square-wave cycled liquid feed, two feed strategies are developed that involve the artificial induction of natural pulses and a separation of the wetting efficiency in time. The feed strategies aim at increasing the mass transfer rate of the limiting reactant and to prevent flow maldistribution and hot spot formation. The feed strategies are categorized upon a relatively fast and slow cycling of the liquid feed. The potential consequences of the developed feed strategies on reactor performance are evaluated. Cycling the liquid feed results in the formation of continuity shock waves in the column. The shock waves decay by leaving liquid behind their tail. This decaying process limits the frequency of the cycled liquid feed to rather low values since at relatively high frequencies, total collapse of the shock waves occurs. By the induction of natural pulses inside the shock waves, the integral mass and heat transfer rates during the liquid flush will be improved. Shorter flushes can therefore be applied and the usual encountered periodic operation is optimized. This feed strategy is termed the slow mode of liquid-induced pulsing flow. The second feed strategy termed the fast mode of liquid-induced pulsing flow may be viewed as an extension of natural pulsing flow. Individual natural pulses are induced at an externally set pulse frequency less than 1 Hz. This feed strategy is the only fast mode of periodic operation possible since pulses are stable while shock waves decay. The characteristics of the induced pulses equal the pulse characteristics of natural pulsing flow at equivalent gas flow rates. A critical liquid holdup in between pulses is necessary for the induced pulses to remain stable.

Tar removal from biomass-derived fuel gas by pulsed corona discharges

Tar removal from fuel gas obtained from biomass gasification offers a significant challenge in its deployment for power generation as well as for other applications such as production of chemicals by processes such as Fischer–Tropsch. The present investigation focuses on pulsed corona discharges for the mentioned objective. The paper is meant to give an overview of our developments in the area of pulsed power development for large-scale plasma processing. In addition, lab-scale results as well as pilot-scale results for tar removal on an actual gasifier are presented.

Nature and characteristics of pulsing flow in trickle-bed reactors

Abstract Pulsing flow is well known for its advantages in terms of an increase in mass and heat transfer rates, complete catalyst wetting and a decrease in axial dispersion compared to trickle flow. The operation of a trickle-bed reactor in the pulsing flow regime is favorable in terms of a capacity increase and the elimination of hot spots. Extending the knowledge on the hydrodynamic nature and characteristics of pulsing flow stands at the basis of further exploitation of the effects of this flow regime on reactor performance. An analysis of the hydrodynamics of pulsing flow reveals that pulse properties as liquid holdup, velocity and duration, are invariant to the superficial liquid velocity at a constant gas flow rate. The pulse frequency, however, increases with increasing superficial liquid velocity. The relative contribution of the pulses and the parts of the bed in between pulses to an average measured property can thus be obtained. By applying this concept it is shown that the linear liquid velocity inside the pulses varies between 0.1 and 0.2 m s −1 . The linear liquid velocity in between pulses, however, is invariant to gas and liquid flow rates and packing properties and equal to about 0.05 m s −1 . This suggests that a linear liquid velocity of about 0.05 m s −1 is the maximum velocity possible in the bed to maintain the trickle flow regime. All liquid in excess is transported as pulses. The liquid holdup in the parts of the bed in between pulses equals the liquid holdup at the transition to pulsing flow at all gas flow rates. The same trend holds for the linear liquid velocity in between pulses. Pulsing flow then is a hybrid of two transition states. The pulses reside at the transition to bubble flow, while the parts of the bed in between pulses reside at the transition to trickle flow. The enhanced particle-liquid heat transfer coefficient inside the pulses is mainly the result of the high linear liquid velocity inside the pulses. Particle-liquid heat transfer rates in between pulses are constant due to the constant linear liquid velocity.

Enlargement of the pulsing flow regime by periodic operation of a trickle-bed reactor.

Potential advantages of pulsing flow in trickle-bed reactors include capacity increase and elimination of hot spots through the enhanced mass and heat transfer rates. A disadvantage of naturally occurring pulsing flow is the necessity of relatively high gas and liquid flow rates, especially at elevated pressures, resulting in rather short contact times between the phases. To maintain the advantages but to avoid the drawbacks of pulsing flow, a study has been set up to expand the pulsing flow regime. This is achieved by periodic operation of a trickle-bed, e.g. by cycling the liquid, respectively, the gas feed. It is observed that, due to the periodic operation of a trickle bed, it is possible to shift the transition boundary from trickling to pulsing flow towards lower average gas and liquid flow rates. An additional effect of induced pulsing flow is the possibility to predetermine the pulse frequency, and therefore the time constant of the pulses.

Chemical Processes in Tar Removal from Biomass Derived Fuel Gas by Pulsed Corona Discharges

Cleaning or conditioning of fuel gas from biomass gasification is perhaps one of the main obstacles for utilization of biomass as a source of power generation. Various methods exist, but, so far, none of them have been reported to be reliable for long-term operation. In our present research, we try to couple our advancements in pulsed power technology for industrial applications to the application mentioned. Here we focus on the chemical processes that occur during pulsed corona fuel gas cleaning. Experimental results at 200°C show that the main process for tar (heavy aromatic hydrocarbons) removal is mainly via oxidation.

Pulsed Corona Discharges for Tar Removal from Biomass Derived Fuel Gas

To supply combustion engines or gasturbines with fuel gas obtained from biomass gasification, it is necessary to clean the fuel gas. Also the production of chemicals by processes such as Fisher-Tropsch requires a high gas quality. Especially heavy aromatic hydrocarbons (“tars”) must be removed. In this work, we give an overview of our investigations on tar removal by pulsed corona discharges as an alternative approach to catalytic or thermal tar cracking. Experimental results (at a gas temperature of 200°C) are reported for the removal of various model tar components in synthetic fuel gas. In order to identify the major reaction pathways, experiments were also done on tars in individual fuel gas components. The results show that tar removal by pulsed corona processing is possible. The process for tar removal is mainly via oxidation. Also termination reactions by CO play an important role.

Production of copolymers with a predefined intermolecular chemical composition distribution by emulsion polymerisation in a continuously operated reactor

The influence of residence time distribution on the intermolecular chemical composition distribution (CCD) for the emulsion copolymerisation of styrene and methyl acrylate has been investigated. A special tubular reactor, the pulsed packed column (PPC), has been used. The PPC combines intensive radial mixing with limited axial mixing, thus providing good heat transfer to the reactor wall and proper emulsification for low net flow rates. A simple backmixing model was developed for feed stream mixing in the PPC. A combination of this backmixing model with a simple mechanistic model for emulsion copolymerisation was used to calculate the CCD of the PPC product. Experimental and calculated results are in good agreement. Production of copolymer having a bimodal CCD in the PPC is compared to the performance of a semi-batch process. The two processes are similar in terms of conversion; however, the CCD of the PPC product shows a characteristic difference. In the PPC some copolymer with an intermediate chemical composition is formed through backmixing of side feed streams. This work demonstrates that the PPC is a promising alternative for semi-batch processes, although some product characteristics will be slightly different.

A novel method to model emulsion polymerization kinetics: The explicit radical-particle size distribution approach

Abstract: An alternative approach to model emulsion polymerization is presented that is capable of rigorously solving both particle and radical kinetics for emulsion polymerization: the explicit radical-particle size distribution approach. The method is based on a direct solution of all population balances and fully covers the strong influence of compartmentalization on rates of reactions between macroradicals and, consequently, on chain length averages. An essential and new feature is the compartmentalization factor (Df), which accounts for compartmentalization in a transparent manner. The generic approach allows for studying the complete emulsion polymerization conversion range, including gel-effect, and the effect of various parameters on both chain length and particle size distribution. Well-known kinetic regimes for emulsion polymerization naturally arise as limiting cases from our model. The dynamic behavior of the model was studied by simulating several realistic seeded emulsion polymerization reactions for styrene. The model dealt with compartmentalization accurately and was able to correctly reproduce the dynamic behavior known to be typical for emulsion polymerization.

Non-steady state operation of trickle-bed reactors.

A review with 12 refs.; in this contribution we discuss the opportunities of pulsing flow and a cycled liq. feed strategy, both resulting in a non-steady state behavior of the trickle-bed reactor. The focus is on the hydrodynamics and its effect on catalytic reactions is evaluated.

Experimental validation of the Explicit Radical-Particle Size Distribution Approach for modeling emulsion polymerization: The seeded emulsion polymerization of styrene

Abstract: The objective of the present paper is to demonstrate that the explicit radical-particle size distribution approach correctly predicts the effect of compartmentalization on the overall reaction rates and therefore chain length averages. Modeling results for the seeded emulsion polymerization of styrene were compared with experimental results. Several experiments were carried out with systematically varied compartmentalization of radicals by varying seed latex particle numbers and the amount of initiator. The overall polymerization rate was measured using reaction calorimetry and the final particle size distribution was measured using Transmission Electron Microscopy. The results demonstrated that the model is able to predict successfully the rate of polymerization and particle size distributions as a function of time for all recipes. This proves that the model deals correctly with the effect of compartmentalization on overall reaction rates and thus on chain length averages. The work described in this paper demonstrates that the explicit radical particle size distribution approach is a powerful method for predicting emulsion polymerization kinetics and product properties, such as particle size distributions and chain length distributions.