顺流式填充床减阻英文文献和中文翻译
INTRODUCTION
Three-phase packed bed reactors with co-current flow of gas and
liquid are extensively utilized in chemical, petrochemical, biochem-
ical and waste water treatment processes for many contacting pur-
poses, such as gas absorption, gas absorption accompanied by a
chemical reaction, and heterogeneous catalytic reactions. The co-
current mode of operation is usually preferred over the countercur-
rent mode when there is no significant difference in the mean con-
centration driving force offered by the two phases. The reason for
this lies in the additional advantages of the co-current mode, such as
no limitation on gas and liquid throughputs (no flooding), offering
a wide range of operable flow rates and a low pressure drop as com-
pared with countercurrent operation. Also, the liquid back mixing
is a more prominent disadvantage in the up-flow configuration than
in the down-flow configuration in packed columns. The main advan-
tage of the columns operating in the co-current down flow of the
two phases is the possibility of using high flow rates without causing
the onset of flooding in the column.
A phenomenon that is always an undesirable factor in packed
bed co-current down flow contactors is the high axial pressure gradi-
ent, which results in substantial energy consumption per unit vol-
ume of liquid throughput. Turbulent flows occur in the boundary
layer near solid surfaces where the associated friction increases as
the flow velocity increases. The energy losses due to turbulence fric-
tion can be of very high magnitude, which requires further atten-
tion for minimization. Drag reduction is a phenomenon by which a
small amount of additives, e.g. a few parts per million, can greatly
reduce the turbulent friction factor of a fluid. The aim for the drag reduction study is to improve the fluid-mechanical efficiency using
active agents, known as drag reducing agents (DRA) named as the
“Toms effect” [1].
Drag can be reduced by using diluted polymers, surfactants, micro
bubble and compliant coating over a solid surface. Though surfac-
tants and polymer additives offer almost 80% drag reduction, the
former needs some higher degree of concentration than polymer
solution [2]. During late fifties and early sixties, the effect of dilute
polymer solutions on drag reduction was actively investigated. Lum-
ley [3,4] defined drag reduction as the reduction of skin friction in
turbulent flow below that of pure liquid. Form the literature it was
concluded that the polymer solution can produce drag reduction
only when flow remains turbulent. After Toms discovered turbulent
drag reduction by polymer additives, many theoretical, experimental
and numerical studies were conducted to explain the mechanism
of polymer drag reduction. Two principal concepts have been sug-
gested to explain the phenomenon of drag reduction by polymers:
one is a mechanism based on the extensional viscosity and the other
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