Chemical Enyineerin0 Science, Vol. 51, No. 17, pp. 4065 4073, 1996 Copyright ~c3 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved PII: S0009-2509(96)00264-3 ooo9 2509/96 $15.00 + 0.00
MODELLING THE HYDRODYNAMICS A N D THE LIQUID-MIXING BEHAVIOUR OF A BIOGAS TOWER REACTOR*
G E O R G R E I N H O L D , t S T E F A N M E R R A T H , +"F R I E D R I C H L E N N EM A N N t a n d H E R B E R T M)i, R K L *'~ *Technische Universit~it Hamburg-Harburg, Bioprozel3 und Bioverfahrenstechnik, Denickestr. 15, D-2107 l Hamburg, Germany *+Preussag Noell Wassertechnik GmbH, Robert-Hooke-Str. 5, D-28359 Bremen, Germany
(First received 5 April 1995; revised manuscript received and accepted 20 November 1995)
Abstraet--A new concept of a biogas reactor for anaerobicwaste water treatment is presented. The characteristic features of this type of reactor are the tower shape, its modular structure and the internal installations. Gas-collecting devices are installed at different levels along the height of the reactor to withdraw the gas produced and to avoid gas accumulation in the upper zones of the reactor. The remaining gas causes a fluid circulation alongbaffles similar to airlift-loop reactors. A settler is integrated at the top of the reactor. As the reactor is built in a tower shape, the mixing behaviour is strongly linked to two questions: (1) how to supplement the microorganisms in the upper zones of the reactor with substrate and (2) whether there is a toxic concentration due to insufficient mixing near the inlet of the substrate. The scale-upof the biogas tower reactor, as far as the liquid mixing is concerned, was based on the knowledge of the mixing within a module and the intermixing between two modules. Two mathematical models are proposed to describe liquid mixing within the reactor. Model A describes the intramixing within one module as well as the intermixing between neighbouring modules. Model B describes only the globalintermixing between different modules. Therefore, model A can be used for investigations on the internal concentration profile of a module, e.g. over concentration in the feeding zone of the reactor, while model B is able to calculate the global mixing of a reactor. The experimental studies were performed in the laboratory- and in pilot-scale plants. An excellent agreement between the computersimulation and the experimental results was obtained with both models. The characteristic parameters of the system which include the axial dispersion coefficient Dax, the mean circulation velocity w,, and the exchange flow rate l~cxeha,g were calculated by means of the mathematical model from the experimental data according to the e least-squares method. The mean circulation velocity w,, increases withthe enlargement in diameter since the drag coefficient for the circulating flow decreases. The exchange flow rate between two neighbouring modules Vexchangerelated to the connecting area remains constant during the scale-up. Copyright ((?~1996 Elsevier Science Ltd
Keywords: biogas-reactor design, liquid mixing, hydrodynamics modelling, airlift, scale up.
INTRODUCTION In reactors for a n a e ro b i c waste water t r e a t m e n t the gas loading increases with the height of the reactor due to the microbiological p r o d u c t i o n of fermentation gas. This has adverse effects on the overall perform a n c e of the reactor when the rising gas causes a loss of biomass t h r o u g h flotation a n d turbulences. In the present work, a biogas tower reactor (BTR) with a height of a b o u t20-30 m will be discussed, where the p r o d u c e d biogas can be w i t h d r a w n from the reactor already in the lower zones of the reactor. W i t h these constructions, the gas h o l d - u p can be controlled below a critical value even at high gas p r o d u c t i o n rates a n d by this the loss of biomass can be avoided. The mixing of the B T R is caused by internal airliftloop units. The...