Introduction

Stripping
and distillation columns are widely used at industrial scale to separate
molecules with different boiling points. The trays traditionally used in
distillation columns are being gradually substituted by structured packings offering good gas-liquid mass transfer
performances with low pressure drop. Many structured packing solutions exist
for the industrial scale. The choices are more limited at the laboratory scale.
The Sulzer EX structured packing represents one
solution for improving gas-liquid contact and mass transfer performances within
a stripping column at laboratory scale. Although the EX structured packing is
widely used for separation processes at the lab scale, available information
about mass transfer correlations for this specific packing is reduced. One can
mention the work from Roesler, J. et Muhammad, A.,
2018 115%;font-family:" calibri mso-fareast-font-family:calibri minor-latin new roman mso-ansi-language:en-us>[1],
in which the authors present the mass transfer rate parameters for this
particular packing within a column of 20 mm diameter. The authors determined correlations
for both gas and liquid mass transfer coefficients and gas-liquid contact area
as a function of the relevant dimensionless numbers (Reynolds and Schmidt).
During the experiments, the superficial velocities of the fluids and the driving
forces  were extremely low. No studies
were found in literature concerning the gas-liquid mass transfer phenomenon for
EX structured packing at high driving force. In this particular case, both gas
and liquid hydrodynamics might be affected by the mass transfer phenomenon.
Thus, mass transfer rate as well as the dependence of mass transfer
coefficients on superficial velocities may increase by means of the local
instabilities generated within the fluids at high driving forces.

Objectives and methodology

The objective
of this work was to establish new mass transfer parameters correlations for the
Sulzer EX structured packing at high driving forces. Experiments
carried out in a 20 mm ID column containing 1430 mm packing height for
separating the 370 °C^- boiling point fraction from the 370 °C⁺
fraction present in a vacuum gas-oil were used for determining the different
mass transfer rate parameters correlations and their dependence on gas and
liquid velocities. The column was placed inside an oven constituted by six heating
zones allowing the implementation of specific linear temperature profiles to improve
separation. The laboratory tests were carried out at atmospheric pressure and
temperatures in between 230 °C (top of the column) and 345 °C (bottom of the
column). The bottom of the column was fed with hydrogen which was used for
stripping purposes.

A dynamic mathematical
model was developed to extract the mass transfer parameters from the
experiments. The model considers both convective and diffusive mass transfer mechanisms
as well as the gas-liquid equilibrium for all the pseudo-components (lumps) of
the feed.

Results

The impact
of the hydrogen flow on the 370 °C^- cut mass fraction at the bottom
of the laboratory column is illustrated in Figure 1. The gas flow improves the
separation performances by increasing the driving force between the liquid and
the gas phases. However, the model shown that the separation improvement is
mostly related to the enhancement of mass transfer between phases. In fact, the
mass transfer between phases is a strong function of the gas flow at these
conditions. This is not the case at low driving forces, for which the gas phase
mass transfer coefficient is substantially higher than the liquid phase mass
transfer coefficient. Some flow instabilities are likely to appear at high
driving forces. These instabilities increases the rate of surface renewal and
consequently, the liquid side mass transfer coefficient. Thus, the mass
transfer phenomenon can become dependent of the gas mass flow at these
conditions.

EN-US">Figure

style='mso-element:field-begin'> lang=EN-US style='mso-ansi-language:EN-US'> style='mso-spacerun:yes'> SEQ Figure \* ARABIC style='mso-element:field-separator'>1

style='mso-bookmark:_Ref5022775'> – Evolution of the 370 °C^-
cut mass fraction at the bottom of the laboratory column as a function of the H₂
flow. Model and experimental results are represented by curves and symbols,
respectively. Liquid flow – 340 g/h, T_top
= 230 °C, T_bottom = 345 °C, 50 % of 370 °C⁺
and 370 °C^- fraction in a mass basis.

The
evolution of the 370 °C^- cut mass fraction at the bottom of the
column as a function of the liquid flow to be fractionated is shown in Figure 2
along with the experimental point used for model validation purposes (gas flow
of 100 NL/h and liquid flow of 534.6 g/h). The separation performance decrease
is slight despite a considerable liquid flow increase, which is traduced by a
lower liquid residence time in the column. The low residence time is
compensated by the improvement on the gas-liquid mass transfer through the
local instabilities generated in the liquid phase at high driving forces.
Consequently, the liquid size mass transfer coefficient is substantially higher
than the one obtained at the same liquid velocity and low driving force. This
conclusion is in good agreement with the dependence of the mass transfer
phenomenon on gas velocity.

The model
was used afterwards in order to optimize the separation for a liquid flow of
534.6 g/h containing 50 % of 370 °C⁺ and 370 °C^-  fraction in a mass basis. Thus, the needed gas
flow to obtain less than 3 % (2.6 % precisely) of 370 °C^-  fraction in a mass basis at the bottom of the
column was 100 NL/h, which perfectly agrees with the experimental result.

Finally,
the model was also used to establish the influence of the temperature profile
on separation performances. Both the model and the experiments show that the
dependence of the separation on the temperature profile was low when the
temperature at the top of the column was varied between 230 and 260 °C, and
between 345 and 365 °C at the top of the column.

EN-US">Figure

style='mso-element:field-begin'> lang=EN-US style='mso-ansi-language:EN-US'> style='mso-spacerun:yes'> SEQ Figure \* ARABIC style='mso-element:field-separator'>2

style='mso-bookmark:_Ref5024477'> – Evolution of the 370 °C^-
cut mass fraction at the bottom of the laboratory column as a function of liquid
flow. Model and experimental results are represented by curves and symbols,
respectively. Gas flow – 75 NL/h, T_top =
230 °C, T_bottom = 345 °C, 50 % of 370 °C⁺
and 370 °C^- in a mass basis. Model validation at a gas flow of 100
NL/h and a liquid flow of 534.6 g/h.

Conclusion

The
objective of this work was to understand the mass transfer phenomenon within a
laboratory scale stripping column of 20 mm ID containing 1430 mm of Sulzer EX structured packing at high driving forces.

Both the
gas and the liquid velocities influence gas-liquid mass transfer phenomenon.
The increase of the gas phase velocity allows the driving force to be
increased, as well as the global mass transfer coefficient. The liquid velocity
increase is traduced in a slight decrease on separation performance, lower than
expected. This is probably due to the occurrence of some liquid flow
instabilities at high driving forces that increases the surface renewal rate
and partially compensate the reduction on residence time. This conclusion is in
agreement with the high dependence of the mass transfer on gas velocity at high
driving forces, which is not the case at low driving forces.

A model
taking into account mass transfer by convection and interphase mass transfer,
as well as gas-liquid equilibrium was developed and validated within a wide
range of operating conditions. The model was used to optimize the performance
of the separation column after calibration, and the numerical results were in
good agreement with the experimental data.

Finally,
both the model and the experiments shown that the dependence of the separation
on the temperature profile was negligible when the temperature at the top of
the column was varied between 230 and 260 °C, and between 345 and 365 °C at the
top of the column.