Method of chemically reacting substances in a reaction column

A process carried out in a reaction column for the chemical reaction of substances the reaction of which is affected by an unfavorable equilibrium position of the main reaction or a preceding equilibrium, wherein during the reaction one or more substances to be separated are continuously removed from the reaction mixture by one or more auxiliaries.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process carried out in a reaction column 
for the chemical reaction of substances the reaction of which is affected 
by an unfavorable equilibrium position of the main reaction or a preceding 
equilibrium, wherein during the reaction one or more substances to be 
separated are continuously removed from the reaction mixture by one or 
more auxiliaries. 
2. Description of the Prior Art 
In the past, reactive distillation proved successful in processes wherein 
conversion is impaired by the position of the chemical equilibrium forming 
the basis for reaction. Besides the production of ethers which are used 
for example as antiknock additives in fuels, esterification is another 
field of application. Prior art processes of esterification are reactive 
distillation processes allowing continuous separation of the water from 
reaction by distillation. See e.g. J. Krafczyk and J. Gmehling, Chem. Ing. 
Tech., vol. 66 (1994), p. 1372; and C. Breucker, V. Jordan, M. Nitsche, 
and B. Gutsche, Chem. Ing. Tech., vol. 67 (1995), p. 430. During 
conventional reactive distillation the temperatures in the column are such 
that all substances, except for the bottoms, are heated to boiling point. 
In the known processes, separation of the water from reaction is effected 
by a reactant which, in most cases, must be charged in high excess. It is 
therefore necessary that at the given pressure the temperature for 
separation be equal to or higher than the distillation temperatures of the 
reactants. The reactants employed in said processes are an auxiliary and 
an additional substance. In esterification processes the alcohol, if used 
in excess, is both the reactant for the esterification reaction and the 
auxiliary for removing the water from reaction. Furthermore, it is 
necessary to determine the ratio at which the reactants are used in order 
to achieve efficient separation. According to C. Breucker et al., 
separation of the water from reaction, if the reactants are present in low 
excess, is a challenging task for process engineers. Up to now, this 
problem has been tackled by using batch or cascade processes employing 
several reactors. 
One embodiment which is well known in the art is the one-stage 
esterification using an entraining agent and taking advantage of azeotrope 
formation. See e.g. Organikum, VEB Deutscher Verlag der Wissenschaften, 
Berlin (1977), p. 71-73. Processes for producing esters from carboxylic 
acids and alcohols with continuous separation of condensed water in a 
column using entraining agents, such as benzene, methanol, and butyl 
alcohol are described in DE 976 413 B. The entraining agent used according 
to U.S. Pat. No. 2,384,793 is butane. 
WO 94/19079 discloses a process for producing bisphenol A from acetone and 
phenol while splitting off water. The resultant water from the reaction is 
removed using an inert stripping gas. The aforesaid process employs a 
reactor containing a liquid catalyst or a particulate, suspended catalyst, 
while the reactor itself is equipped with trays, drains, and/or hold-back 
screens. 
BRIEF SUMMARY OF THE INVENTION 
It is the object of the present invention to provide a process allowing to 
enhance the conversion in equilibrium reactions and, in particular, to 
remove a substance which has formed and must be separated in a continuous, 
gentle, and most effective way in a column by using an auxiliary. 
It was surprisingly found that in columns with fixed-bed catalysts 
permanent gases are particularly suitable stripping gases serving this 
end. The advantageous effect of the stripping process should be utilized 
to support the reaction course and to prevent at the same time that a 
reactant is present in high stoichiometric excess. Once the auxiliary has 
fulfilled its task, it should be returnable to the process in a simple 
way. By using the present process it should be possible to avoid coupling 
of the process parameters, such as process 
temperature-pressure-distillation temperature-educts ratio, which is 
inherent to prior art processes. 
According to the present invention, the problem is solved by a process 
carried out in a reaction column for the chemical reaction of substances 
the reaction of which is affected by an unfavorable equilibrium position 
of the main reaction or a preceding equilibrium, wherein during the 
reaction one or more substances to be separated are continuously removed 
from the reaction mixture by one or more auxiliaries. This is achieved by 
passing a stripping gas, i.e. a permanent gas or a mixture of permanent 
gases, as an auxiliary through the reaction column in which a solid 
catalyst is arranged as a fixed bed and adjusting temperature and pressure 
such that all the educts are present as liquids or as solutions of solids 
and that the substance(s) in the reaction column is (are) predominantly 
present in gaseous/vaporous form. The stripping gas is preferably led 
countercurrently to the liquid stream. Stripping and reaction take place 
simultaneously in the reaction column. The catalyst may be arranged as a 
regular packing or an irregular bulk. 
DETAILED DESCRIPTION OF THE INVENTION 
The continuously charged auxiliary is a permanent gas. According to the 
present invention, permanent gases are gases which cannot be liquefied at 
temperatures of greater than -30.degree. C. Furthermore, permanent gases 
as defined in the present invention are gases consisting of 5 or fewer 
atomic molecules and mixtures thereof, including C.sub.1 hydrocarbons. 
Particularly suitable substances are those the molecules of which consist 
of only one element, such as nitrogen (N.sub.2) or hydrogen (H.sub.2), 
hydrogen being particularly preferred. Said permanent gases serve as 
stripping gas for the product to be separated. This stripping gas differs 
from a conventional entraining agent in that it is already present in 
gaseous form, whereas the entraining agent must first be converted to a 
gas. 
Considerable evaporation heat is required for this conversion. Moreover, it 
is essential that the product to be withdrawn from the equilibrium be also 
present in the gaseous phase. In the instant case it can partly be present 
as a liquid. 
The process according to this invention is of advantage whenever it is 
impossible to heat all the reactants or products to boiling point which 
would result in decomposition of the products or undesired side reactions. 
In said cases, prior art reactive distillation processes cannot be 
employed because the reactants would decompose. Therefore, the instant 
novel process is particularly suitable whenever natural or 
temperature-sensitive substances are to be converted. During this 
conversion the continuous removal of a low-molecular reaction product has 
a favorable effect on the course of reactions wherein the reactants must 
not be heated to boiling point. 
The catalyst is a fixed-bed catalyst. In a preferred embodiment the 
reaction column is operated as a trickle column of which about 30 to 60 
vol %, preferably 50 vol % are utilized by the stripping gas as free gas 
space, whereas 30 to 50 vol %, preferably 40 vol % of the column is 
occupied by solid substance, i.e. the fixed-bed catalyst. The remaining 
reaction space, preferably 10 vol % or less, is occupied by the trickling 
liquid. Irrespective of other process parameters, high catalyst 
concentrations can be chosen in a wide range. In contrast to prior art 
processes, the ratio of catalyst:liquid phase is surprisingly high. 
When using a fixed bed, the residence time of the liquid phase can be 
adjusted by the stripping gas velocity. The residence time of the liquid 
phase is high with higher velocities of the stripping gas volume. The 
stripping gas throughput can be adjusted in a wide range without having an 
adverse effect on the course of process. 
The effect of the stripping gas can be modified by an additional auxiliary 
which is a conventional entraining agent, such as benzene, toluene, or 
hexane, the latter one being particularly preferred. In this case the 
stripping gas removes the substance to be separated together with the 
entraining agent which is in gaseous form under the operating conditions 
in the stripping column. 
When using entraining agents as auxiliaries in addition to permanent gases, 
the amount of permanent gas in the reaction column is greater than 80 mol 
% of the auxiliaries charge, while the amount of entraining agent 
accordingly makes up to 100 mol %. The molecular portion of entraining 
agent is by at least the factor 4 smaller than the amount of permanent 
gases in the reaction column. 
Reactions for which this novel process can be employed generally proceed 
according to the following equation: 
##STR1## 
At least one of the products--the substance to be separated--can be 
vaporized under the reaction conditions of the reaction stripper or can be 
converted to vapor by the stripping gas. The reaction is an equilibrium 
reaction or is influenced in its course by a preceding equilibrium. The 
following types of reaction are given as examples: 
Aldol Condensation 
##STR2## 
Knoevenagel Condensation 
This condensation is a special case of aldol reaction followed by aldol 
condensation: methylene components having particularly high CH acidity, 
such as malonic acid, malonic semi-ester, malonic ester, cyanoacetic acid, 
cyanoacetic acid ester etc., are reacted with aldehydes and ketones. 
##STR3## 
Example of a Substance 
The reactants are sensitive to temperature and have higher boiling points 
than the auxiliary. The water to be separated (substance to be separated) 
is removed by the stripping gas, or the water to be separated plus an 
additional entraining agent, such as hexane, benzene, or toluene, hexane 
being the preferred entrainer, form an azeotrope in which case the 
permanent gas leaves the reaction stripper overhead together with 
entraining agent and water. The product, benzal malonic acid diethyl 
ester, is the highest boiling substance (186.degree. C. at 18 torr) 
leaving the column at the bottom. 
______________________________________ 
Reactants 
Boiling Point 
Catalyst Auxiliary 
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Benzaldehyde 
177.degree. C. 
Bases, e. g. basic ion ex- 
Stripping gas 
changers, carbonates of or stripping 
alkali metals and alkaline gas + hexane 
earth metals 
Malonic acid 94-96.degree. C. 
diethyl ester at 11 torr 
______________________________________ 
Preparation of Enamines 
Enamines are formed by reaction of aldehydes and ketones with secondary 
amines. This route is an important one for the formation of intermediates 
for organic syntheses. An important field of application is the formation 
of heterocycles employing appropriate reactants. 
##STR4## 
Example of a Substance 
Like in the example given hereinabove, the product .beta.-benzyl 
aminocrotonic acid ethyl ester is the highest boiling substance (b.p. 
140.degree. C. at 0.5 torr). The stripping gas or stripping gas plus 
entraining agent as an auxiliary transport the water (substance to be 
separated) to the top of the apparatus. The product leaves the reaction 
stripper at the bottom. 
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Reactants 
Boiling Point 
Catalyst Auxiliary 
______________________________________ 
Acetoacetic 
68-79.degree. C. 
Acids, e. g. acidic ion ex- 
Stripping gas 
acid ethyl at 11 torr changers or toluene sul- or stripping 
ester fonic acid gas + hexane 
Benzyl amine 70-71.degree. C. 
at 10 torr 
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Reactions of Carbonyl Compounds with Bases 
Reactions of carboxylic acids with amines to form carboxylic acid amides 
Formation of the acid amide is favored by the continuous removal of water 
(substance to be separated) in the reaction stripper. The water-enriched 
auxiliary leaves the apparatus overhead, whereas the product is withdrawn 
at the bottom. 
##STR5## 
Example of a Substance 
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Reactants 
Boiling Point 
Catalyst Auxiliary 
______________________________________ 
Capronic acid 
205.degree. C. 
Acids, e. g. acidic ion ex- 
Stripping gas 
changer or stripping 
gas + hexane 
Dibutyl amine 159-161.degree. C. 
______________________________________ 
Guerbet Reaction 
##STR6## 
Esterification of Carboxylic Acids 
##STR7## 
Example of a Substance 
______________________________________ 
Reactants Catalyst Auxiliary 
______________________________________ 
Isobutanoic acid or 
Acids, e. g. acidic 
Hydrogen as a stripping gas, 
pivalic acid with hy- ion exchanger optionally together with 
droxybenzaldehyde benzene, toluene, or, preferably, 
hexane 
______________________________________ 
Reactions of fatty acids with fatty alcohols to form fatty acid esters 
It is an advantage of the process according to this invention that when 
using permanent gases as an auxiliary, ordinary coolers can be mounted 
into the condensers because the physical data of the permanent gases 
effecting the separation of substance are often very different from those 
of the substances to be separated (boiling point, heat of condensation, 
heat of evaporation). Thus, less energy is required for preheating because 
heat is only required for raising the temperature of the gas to reaction 
temperature, but not for heating a liquid entraining agent to effect 
vaporization. 
##STR8## 
Example of a Substance 
______________________________________ 
Reactants Catalyst Auxiliary 
______________________________________ 
Fatty acid Acids, e. g. 
Hydrogen or nitrogen as a 
C.sub.6 --C.sub.22, preferably C.sub.8 --C.sub.22 acidic ion stripping 
gas, optionally 
Fatty alcohols exchanger together with hexane 
C.sub.1 --C.sub.22, preferably C.sub.8 --C.sub.22 (azeotrope former) 
______________________________________ 
Advantageous embodiment of this process 
(general, applicable to any reaction variant)

When separating the water from reaction by means of an auxiliary, the 
reaction can be influenced to the advantage of the desired product, e.g. 
fatty acid ester. As shown in FIG. 1, a mixture consisting e.g. of fatty 
acid and fatty alcohol is charged to the upper section of the reaction 
stripper. Alternatively, the educts can partially be reacted by a 
preceding reaction which has the advantage of achieving the same 
conversion in a shorter reaction stripper column. In this case the product 
from the preceding reaction is charged to the reaction stripper. 
Temperature and pressure of the reaction stripper are such that both the 
two reactants and the product, e.g. fatty acid ester, are liquid, while 
the component to be separated, e.g. water, evaporates and is mainly 
present in vaporous form. 
The stripping gas, e.g. preheated, dry hydrogen, charged to the bottom of 
the apparatus flows to the top of the reaction stripper. While passing 
through the apparatus, it entrains the component to be separated, e.g. 
water. The hydrogen enriched with the component to be separated, e.g. 
water, leaves the apparatus overhead and reaches a condenser wherein it is 
separated from the component to be separated, e.g. water. Optionally, the 
hydrogen can be returned to the reaction stripper or may be employed in a 
different way. It is also possible to charge one of the reactants, 
preferably the alcohol, through a lateral inlet into the reaction 
stripper. This variant raises a further possibility of influencing the 
reaction to the advantage of the products. Furthermore, this process 
variant allows to control the temperature profile in the reaction stripper 
by preheating the side streams. It is a particular feature of the process 
according to this invention that the educts, e.g. carboxylic acid or, in 
the case of transesterification, the carboxylic acid ester, and the 
alcohol can advantageously be employed in equimolar quantities. If, 
however, overstoichiometric conversion is desired, the alcohol is charged 
in excess quantity. The product, e.g. the fatty acid ester, is removed 
from the lower section of the column. 
The reaction stripper is provided with a solid catalyst. Thus, the reaction 
rate which is already accelerated by the stripping procedure can be 
further increased. When employing acidic catalysts, e.g. for an 
esterification process, it is expedient to use a solid acid as catalyst, 
e.g. an acidic ion exchanger. Ion exchangers which can also be employed at 
high reaction temperatures of up to 230.degree. C. are particularly 
suitable. Water-sensitive catalysts, e.g. catalysts which adsorb the water 
from reaction, can be reactivated by drying after several cycles in order 
to increase conversion. 
The process temperature is from 20 to 300.degree. C., preferably 100 to 
230.degree. C. The process pressure is below atmospheric down to 50 bar, 
preferably 1.5 to 15 bar. It is surprising that during esterification a 
cross-sectional load of the liquid phase of &lt;0.48 kg/m.sup.2 s has no 
perceptible effect on the fatty acid conversion, whereas at &gt;0.48 
kg/m.sup.2 s the conversion decreases, as is expected, as the 
cross-sectional load increases. 
It is understood that any other reaction meeting the requirements described 
hereinabove can advantageously be carried out according to the novel 
process described herein. This process can be employed whenever the educts 
or products are higher-boiling, temperature-sensitive substances, e.g. in 
the production of intermediates for detergents, pharmaceuticals, and 
cosmetics. The reactions mentioned herein are only few examples of a large 
number of syntheses which can be performed employing the novel process 
presented herein. 
Examples of Experiments 
Experiments were carried out in a batch reactor in order to examine the 
kinetics of fatty acid esterification under stripping conditions. 
Furthermore, experiments were carried out in a lab-scale reaction 
stripper. 
Batch Reactor Experiments 
Fatty acid and fatty alcohol were charged to a batch reactor. The 
esterification reaction was carried out in the presence of a suitable 
catalyst. The acidic catalyst (ion exchanger) having the form of Raschig 
rings was arranged as a fixed bed in the reactor. The reaction volume of 
about 400 ml was filled with about 85 ml of solid catalyst. The liquid 
phase volume was about 230 ml. The remaining volume was the stripping gas 
volume. Nitrogen was employed as a stripping gas. This batch reactor 
simulates a section in a reaction stripper column. 
The volume ratios of catalyst:liquid phase:gaseous phase chosen herein are 
not applicabe to a reaction stripper column. Despite the unfavorable 
parameter, i.e. the relatively low catalyst concentration in proportion to 
the liquid phase, the experiments proved that the process of this 
invention offers some advantages. Reaction temperature, feed ratio of the 
reactants, and volume of the stripping gas stream were varied in these 
experiments. 
______________________________________ 
Fatty Acid Conversion [-] at 
Equimolar Feed Ratios 
120 I N.sub.2 /h (Volume of Stripping 
Reaction Time [min] 0; 17; 60 I N.sub.2 /h Gas Stream) 
______________________________________ 
20 .apprxeq.0.2 
0.5 
40 .apprxeq.0.4 0.7 
75 .apprxeq.0.62 0.8 
______________________________________ 
It became apparent that the conversion of ester increases significantly 
with the reaction time if at the same time stripping gas is led through 
the batch reactor. This is achieved by the improved removal of the 
by-product water according to this invention by means of the stripping gas 
stream. The conversion increases as the stripping gas stream increases 
with time. The effect of simultaneous stripping in a batch reactor is not 
felt when employing very small stripping gas streams (&lt;60 liters of 
nitrogen per hour). The experiments carried out in a batch reactor 
revealed that at a reaction temperature of 110.degree. C. and atmospheric 
pressure the stripping gas stream (in this case nitrogen) should be 
greater than about 0.025 kg of nitrogen/(m.sup.2 s). The reference area is 
the cross-sectional area of the empty reaction stripper. The experiments 
were carried out in such a manner that the by-product to be eliminated, 
water, is obtained at a point above its boiling point. From process 
engineering aspects, this is the more advantageous variant of evaporation 
stripping. 
In the second series of experiments the reaction temperature was 80.degree. 
C. at atmospheric pressure. In these experiments water was obtained below 
its boiling point, i.e. as a liquid. In this case, too, it could be proved 
that the conversion increases by simultaneous stripping. However, the 
stripping efficiency was lower. 
______________________________________ 
Fatty Acid Conversion [-] 
at Equimolar Feed Ratios 129 I N.sub.2 /h 
Reaction Time [min] (Volume of Stripping Gas Stream) 
______________________________________ 
20 0.07 
40 0.13 
75 0.25 
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Experiments in a Lab-Scale Reaction Stripper 
A lab-scale reaction stripper was developed in order to prove the 
feasibility of the stripping process according to this invention in a 
column. The column had an internal diameter of 80 mm. For the first 
experiments a fixed catalyst bed consisting of acidic ion exchangers in 
the shape of Raschig rings was employed. The catalyst bed had a length of 
1 m. The volume of the catalytic Raschig rings packing was about 5 liters. 
Equimolar quantities of fatty acid and fatty alcohol were charged to the 
top of the apparatus, while the stripping gas was fed counter-currently 
from the bottom. In the first experiments nitrogen was employed as a 
stripping gas. 
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Cross-sectional load with liquid phase 
0.24 kg/(m.sup.2 s) 
(fatty acid + alcohol) 
1.sup.st cross-sectional load with nitrogen stripping 0.049 kg/(m.sup.2 
s) 
gas 
2.sup.nd cross-sectional load with nitrogen stripping 0.085 kg/(m.sup.2 
s) 
gas 
3.sup.rd cross-sectional load with stripping gas 0 kg/(m.sup.2 s) 
Reaction temperature 110.degree. C. 
Reaction pressure atmospheric 
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Conversion of the fatty acid increased as the stripping gas stream 
increased (at least with small-volume to medium-volume streams). 
Conversion [-] as a function of the volume of stripping gas stream after a 
reaction time of 90 minutes at V.sub.I,ges .apprxeq.5.1 I/h, T=110.degree. 
C., and atmospheric pressure 
______________________________________ 
0 NI N.sub.2 /h (3) 0.15 
1,500-2,000 NI N.sub.2 /h (2) 0.27 
______________________________________ 
It was also found that the water content in the products decreased as the 
stripping gas stream increased (at least with low-volume to medium-volume 
streams), i.e. the quality improved. 
Water content after a reaction time of 85 minutes at V.sub.I,ges 26 5.1 
I/h, T=110.degree. C., and atmospheric pressure 
______________________________________ 
0.25 wt. % H.sub.2 O 
at 1,500-2,000 NI N.sub.2 /h 
(2) 
0.39 wt. % H.sub.2 O at 0 NI N.sub.2 /h (3) 
______________________________________ 
Further experiments were carried out employing hydrogen as a stripping gas. 
The start-up behavior of the lab-scale reaction stripper was examined at 
two pressures. The pressures were chosen such that at a low pressure the 
water was obtained as a gas, while at a high pressure the water was 
obtained as a liquid. The other process parameters were: 
______________________________________ 
Cross-sectional load with liquid 
0.48-0.54 kg/(m.sup.2 s) 
phase (= fatty acid + alcohol) 
1.sup.st cross-sectional load with H.sub.2 3.15 .multidot. 10.sup.-3 
kg/(m.sup.2 s) 
stripping gas at 2 bar 
2.sup.nd cross-sectional load with H.sub.2 1.42 .multidot. 10.sup.-3 
kg/(m.sup.2 s) 
stripping gas at 0.35 bar 
Reaction temperature 110.degree. C. 
1.sup.st reaction pressure 2 bar above atmospheric 
2.sup.nd reaction pressure 0.35 bar above atmospheric 
______________________________________ 
Like in the batch reactor experiments it became apparent that the stripping 
efficiency achieved by this evaporation stripping variant is higher than 
in a process wherein the water is obtained as a liquid. 
Conversion [-] as a function of the volume of stripping gas stream after a 
reaction time of 90 minutes at V.sub.I,ges .apprxeq.10-11 I/h and 
T=110.degree. C. 
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300 NI H.sub.2 /h at .apprxeq.2 bar pressure above atmospheric 
(1) 0.082 
300 NI H.sub.2 /h at .apprxeq.0.35 bar pressure above atmospheric (2) 
0.118 
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Furthermore, the residence time of liquid phase and liquid phase hold-up as 
a function of the volume of liquid phase stream was examined. When the 
liquid load in the reaction stripper was low, the hold-up was found to 
decrease, while the residence time increased. With low liquid loads the 
residence time of the liquid phase could be influenced significantly by 
the volume of the stripping gas stream. The reaction stripper ran smoothly 
even with wide variations of the liquid load (about 1:300 in the 
experiments). The volume of the stripping gas stream, too, could be varied 
in a very wide range, i.e. from 0 to about 1,000 l/h, without resulting in 
process stand- still for hydrodynamic reasons. 
Further experiments for the esterification of fatty acids and fatty 
alcohols using an acidic ion exchanger (Amber-list) and hydrogen as a 
stripping gas were carried out in a pilot plant. The fatty alcohol was 
used in 10% excess. In order to determine the maximum conversion, several 
runs were performed at constant reaction parameters, the esterification 
product being returned to the process. 
TABLE 1 
______________________________________ 
Conversion per Run 
Conversion [%] 
Experiment No. 
(1) (2) (3) 
______________________________________ 
1 53.6 62.3 62.3 
2 76 87 81.9 
3 89.8 90.6 89.6 
4 94.4 90.9 92.8 
5 96.6 93.3 
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TABLE 2 
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Parameters (Volume of Stripping Gas Stream: 900 I H.sub.2 /h) 
Experiment No. 
Pressure (above atmospheric) 
Temperature 
______________________________________ 
1 0.35 bar 140.degree. C. 
2 0.5 bar 160.degree. C. 
3 3 bar 180.degree. C. 
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