Carrying out exothermic reactions between a gas and a liquid

Process for carrying out exothermic reactions between a gas and a liquid in the presence of a solid catalyst by passing the gas and the liquid cocurrently through a packed reaction vessel, preferably of elongated shape, wherein the gas and liquid pass through the packed reaction vessel in transition flow.

It is known that exothermic reactions between gases and liquids with 
formaldehyde to form butynediol (of. Ullmanns Encyclopadie der technischen 
Chemie, volume 3 (1953), pages 109 to 119), can be carried out in such a 
way that the liquid trickles over the catalyst used as a packing in a 
packed column while at the same time the gas is passed through 
cocurrently. In this method, however, only a poor space-time yield is 
achieved. Byproducts are formed very readily and/or the catalyst is easily 
damaged because of the occurrence of local over-heating due to the 
difficulty of removing heat. 
It is known from A.I.Ch.E. Journal, volume 10 (1964), pages 951 to 957, 
that when air and a water are passed cocurrently through a packed column 
(i.e. a column containing glass beads) the following types of flow occur 
depending on the loading of the column by air and water: 
(1) In the region of gas continuous flow, the liquid trickles over the 
tower packing and the gas phase flows continuously through the voids in 
the packing. The liquid flows as a laminar film over the individual 
packing bodies. 
(2) In the region of transition flow, the liquid moves through the packing 
in a type of turbulent flow. 
(3) In pulsing flow, pulses in the form of waves of higher density pass 
through the packed column at a specific frequency. 
We have now found that exothermic reactions between a gas and a liquid in 
the presence or a solid catalyst in which the gas and the liquid are 
passed cocurrently through a packed reaction vessel, preferably of 
elongated shape, can be carried out advantageously by passing the gas and 
liquid through the packed reaction vessel in transition flow, which is 
defined below. 
In the new process an intimate mixture of gas and liquid flows through the 
bed of packing. The reaction products from the gas and liquid are 
therefore obtained in a considerably higher space-time yield than in 
production by the conventional method, which is carried out in the region 
of gas continuous flow. The heat of reaction may be removed very easily 
without the occurrence of hot spots. The formation of byproducts is thus 
substantially prevented so that the reaction products are obtained in 
higher purity than according to prior art processes. 
The gas used as starting material, such as carbon dioxide, carbon monoxide, 
ethylene, hydrogen, acetylene and oxygen may be used as such or diluted 
with inert gas such as nitrogen. The liquid starting material may also be 
used alone or in admixture with liquids which are inert under the reaction 
conditions, for example organic solvents or the reaction product itself. 
The liquid may also be a solution of a solid or gaseous starting material 
in an inert solvent. 
The exothermic reaction between gas and liquid is carried out in the 
presence of a solid catalyst. The catalyst may be used as such or, after 
application to an inert carrier material, as a supported catalyst. The 
catalyst generally serves at the same time as the tower packing. It is 
possible however to use inert packing in addition to the catalytically 
active tower packing. The tower packing may be for example in the form of 
spheres, rings, cylinders or tablets. When spheres are used, they 
generally have a diameter of from 2 to 8 mm. Cylindrical tower packing is 
usually from 2 to 15 mm in length and from 2 to 6 mm in diameter. Tower 
packing which is not spherical or cylindrical generally has a volume 
roughly equivalent to that of the spherical tower packing. 
The process is particularly suitable for exothermic reactions between a gas 
and a liquid in the presence of a solid catalyst where a narrow 
temperature range has to be maintained, i.e. for reactions in which the 
occurrence of fluctuations in temperature during the reaction or the 
occurrence of hot spots have proved to be unfavorable. A narrow 
temperature range is taken to mean a range of fluctuation of 
.+-.20.degree. C., preferably of .+-.5.degree. C. The process may 
therefore be used with particular advantage for example for catalytic 
hydrogenation and for the ethynylation reaction. Other suitable reactions 
include the oxidation of hydrocarbons, such as cyclohexane or p-xylene, 
with molecular oxygen and the halogenation of hydrocarbons. When the new 
process is used for individual reactions, the general reaction conditions, 
such as the use of solid catalyst or the temperature are generally not 
affected. The more rapid and more intense mixing of gas and liquid brought 
about by the new process can however be of influence on the speed of 
reaction and it may be advantageous, on the basis of the new higher 
reaction speed, to re-optimize the process parameters, such as mean 
residence time, temperature and amount of catalyst, which have proved to 
be optimal in an industrial process. 
It is an essential feature of the new process that the gas and liquid 
should be passed through the packed reaction vessel in transition flow. 
Contrary to the results described in A.I.Ch.E. Journal, volume 10 (1964), 
pages 952 to 953, according to which there is no sharp variation in the 
pressure difference .DELTA.p between the point of supply of water to the 
packed column and the point of withdrawal of the water from the column in 
the transition from gas continuous flow to transition flow and from 
transition flow to pulsing flow, we have found that the transition from 
gas continuous flow to transition flow is characterized by a sudden 
increase in the pressure difference .DELTA.p (as shown in FIG. 1). 
The occurence of transition flow may for example be done by visual 
inspection and/or by measurement of the pressure difference .DELTA.p. 
Transition flow may be observed visually for example by occurence of 
turbulence of liquid flow in the column, e.g., as in A.I.Ch.E. Journal, 
volume 10 (1964), pages 952 to 953. The start of transition flow by 
measuring the pressure difference .DELTA.p may be carried out for example 
by adjusting the gas loading required for the reaction (measured in parts 
by volume (STP) per unit time) and, beginning at a loading L of the 
reaction vessel with liquid (measured in parts by volume per unit time) of 
about zero, passing increasing amounts of liquid through the reaction 
vessel. The region of gas continuous flow is first traversed, and this 
region is characterized by an almost linear slow rise in the pressure 
difference .DELTA.p as load L increases. Upon further increasing the 
liquid loading L, the beginning of the region of transition flow is 
indicated by a sudden increasingly steeper rise in the pressure difference 
.DELTA.p. Generally the region of transition flow is reached when the rise 
in the pressure difference .DELTA.p with increasing load L, expressed as 
(.DELTA.p)/L, is at least twice, preferably three times, the average rise 
in the region of gas continuous flow. When the liquid loading L is 
increased further, the rise becomes linear again, but is now considerably 
steeper than in the region of gas continuous flow (cf. FIG. 1). When the 
liquid loading L is further increased, the region of transition flow is 
left and the region of pulsing flow is entered, which is characterized by 
fluctuations in the pressure difference .DELTA.p caused by the pulses. The 
fluctuations have about the same frequency as the pulses. 
It is advantageous to use reaction vessels of elongated shape for the 
process according to this invention. The vessels may have any cross 
section, e.g. a square or elliptical cross section. In general, 
cylindrical reaction vessels are used. The ratio of diameter to length of 
the vessel is as a rule from 1:2 to 1:60, preferably from 1:10 to 1:40. 
The vessels may be arranged vertically or horizontally or inclined. 
Vertical vessels are preferred. 
The process according to the invention may be carried out batchwise or 
continuously. When using columns conventionally used in industry, complete 
conversion is generally not achieved in a single passage of the liquid. In 
this case the liquid is advantageously recycled more than once, for 
example from three to thirty times, through the packed column. It is also 
possible however to achieve complete conversion in a single passage of the 
liquid by using very long narrow packed columns, for example packed 
columns whose ratio of diameter to length is from 1:50 to 1:100. 
Continuous operation of the process may be carried out for example by 
recycling the reaction mixture through the packed vessel, the starting 
materials being fed into the recycled reaction mixture prior to entry into 
the reaction vessel and the reaction product being removed from the 
recycled reaction mixture after it has left the reaction zone. Continuous 
operation may also be carried out by allowing the reaction mixture to flow 
through several, for example three to five, successive recycle apparatus. 
When using the new process for the ethynylation reaction, i.e. the 
production of alkynols and/or alkynediols by reaction of acetylene with 
aldehydes in the presence of a heavy metal acetylide (a heavy metal being 
defined as a metal having a specific gravity of more than 5) and in the 
presence or absence of basic reagents, acetylides of heavy metals of the 
first or second group of the Periodic System are usually used as solid 
catalysts. Heavy metal acetylides may be used as such for the reaction. It 
is possible however to use the heavy metals themselves or their salts 
which are then converted into the corresponding acetylides at the 
beginning of the reaction. Examples of suitable heavy metals are silver, 
gold, mercury and particularly copper. When using heavy metal salts, the 
nature of the anion is not critical. Examples of heavy metal salts which 
may be used are copper phosphate, copper acetate, copper(I) chloride, 
copper(II) chloride, copper acetate, copper formate, silver nitrate and 
mercury chloride. The heavy metal acetylides are preferably used after 
they have been applied to shaped carrier material which acts as the same 
time as as tower packing. Examples of suitable carrier materials are 
aluminum oxide, animal charcoal, diatomaceous earth and particularly 
silica gel. 
Ethynylation is advantageously carried out in the presence of an inert 
solvent or diluent such as an alcohol, ether, ester, carbonamide, aromatic 
or aliphatic hydrocarbon or water. Specific examples are ethanol, 
isobutanol, n-butanol, ethyl glycol, dioxane, tetrahydrofuran, 
dimethylformamide and N-methylpyrrolidone. The end product itself or 
excess liquid starting material may also serve as diluent. 
Alkylacetylenes, preferably those having three to six carbon atoms, 
arylacetylenes, preferably those having up to twelve carbon atoms, and 
alkenyl or alkynyl acetylenes preferably having four to six carbon atoms 
and particularly acetylene itself are used for the ethynylation. Examples 
are methylacetylene, ethylacetylene, phenylacetylene, vinylacetylene and 
diacetylene. 
Aromatic aldehydes preferably having up to eleven carbon atoms and 
particularly aliphatic aldehydes are used for the ethynylation. The 
aliphatic aldehydes generally have one to twelve, preferably one to six, 
carbon atoms. Examples of suitable aldehydes are acetaldehyde, 
butyraldehyde, n-caproaldehyde, benzaldehyde and preferably formaldehyde. 
Formaldehyde may be used in monomeric form, for example as 
commercial-grade aqueous formaldehyde solution, for example as a 20 to 50 
wt%. solution, or in polymerized form, for example as trioxane and 
particularly paraformaldehyde. It is preferred to use commercial-grade 
aqueous formaldehyde solutions. 
The reaction is generally carried out without the addition of basic 
reagents. It is also possible however to carry out the ethynylation in the 
presence of basic reagents. Examples of suitable basic reagents are salts 
of carboxylic acids, carbonates, hydroxides of the alkaline earth metals 
or of the alkali metals. Specific examples are potassium formate, sodium 
acetate, sodium carbonate, potassium carbonate, magnesium carbonate, and 
calcium hydroxide. The basic reagent may be used for example in dissolved 
form in the reaction mixture. 
The starting material having the lower boiling point is used in gaseous 
form and the starting material with the higher boiling point is supplied 
in liquid form. Ethynylations are carried out generally at temperatures of 
from -10.degree. to 120.degree. C., particularly of from -10.degree. C. to 
100.degree. C. 
Generally the starting materials are reacted in a molar ratio of about 1:1. 
It is also possible however to use one of the starting materials in 
excess, and it is advantageous to maintain a molar ratio of the starting 
materials of from 1:1 to 1:10, particularly from 1:1 to 1:3. 
When the new process is used for catalytic hydrogenation, the conventional 
hydrogenation catalysts may be used, e.g. metallic platinum, palladium, 
rhodium, ruthenium, nickel or cobalt, advantageously applied to carriers 
such as animal charcoal, barium sulfate, calcium carbonate, silica gel or 
aluminum oxide. The new method may be used for carrying out conventional 
catalytic hydrogenations, for example the hydrogenation of carbon-carbon 
triple bonds to corresponding double or saturated bonds, the hydrogenation 
of double bonds, the hydrogenation of aromatic hydrocarbons to 
cycloaliphatic hydrocarbon, of carbonyl groups to hydroxyl groups, of 
nitro groups to amino groups, of nitrile compounds to amines, of amine 
oxide groups to amines, the hydrogenolysis of protected groups such as 
benzyl ester or benzyl ether groups, and the hydrogenolysis of acid 
chlorides to aldehydes. 
Hydrogenation may be carried out in the absence of solvents. It is also 
possible however to carry it out in the presence of liquids conventionally 
used for catalytic hydrogenations, such as ethers, esters, lower aliphatic 
carboxylic acids, alcohols or water. Temperatures of for example from 
10.degree. to 300.degree. C. and pressure to 325 atmospheres may be used 
for the catalytic hydrogenation according to the invention. It is also 
possible however to use subatmospheric pressure, for example 600 mm Hg.

The following Examples illustrate the invention. 
EXAMPLE 1 
This Example is given with reference to FIG. 2. A pressure-resistant packed 
column 1 of stainless steel having a length of 6 meters and a diameter of 
45 mm is used for the reaction. The packed column is filled with a 
supported catalyst shaped in the form of pellets 3 mm in diameter 2.5 to 3 
mm in length. The analysis of the catalyst is as follows: 85% by weight of 
silica gel, 12% by weight of CuO and 3% by weight of bismuth. 1 liter per 
hour of 37% by weight aqueous formaldehyde solution 11 is fed in through 
line 11a and 170 liters (STP) per hour of acetylene 9 is fed in through 
line 9a. Recycled reaction liquid is supplied through line 12 and recycled 
acetylene gas is supplied through line 13 to the packed column. After they 
have passed through the packed column, separation of the acetylene phase 
from the liquid reaction mixture is carried out in a separator 2. 114 
liters per hour of the liquid reaction mixture is withdrawn from the 
separator 2 through outlet 17 by way of circulation pump 3 and flow meter 
6 and recycled through a circulation cooler 5. Some of the liquid reaction 
mixture is withdrawn from the separator 2 through line 14 as reaction 
product. Unreacted acetylene gas is recycled in an amount of 750 liters 
(STP) per hour from the separator 2 through line 16, circulation pump 4 
and flow meter 7. Some of the recycled gas is withdrawn through line 10a 
as offgas. The difference in pressure between supply line and discharge 
line of the column is measured by means of a differential pressure meter 8 
and is 1.2 atmospheres gauge. At an acetylene pressure of 5 atmospheres 
gauge the reaction temperature in the packed column is 105.degree. C. 50 
liters (STP) of offgas 10 per hour is withdrawn at 10a. 1.2 liters per 
hour of reaction product 15 is removed from the separator; it has a 
formaldehyde content of 10% by weight. The yield of butynediol is 97% of 
the theory, based on formaldehyde reacted. The conversion of formaldehyde 
is 73%. During the reaction under the said conditions, the whole packed 
column is traversed by an intimate mixture of acetylene gas and reaction 
liquid in transition flow. No decline in activity of the catalyst is 
observed in the said apparatus using the same catalyst even after sixty 
days. 
If however pump 3 for circulating the liquid is switched off so that the 
flow regime in the column is that of gas continuous flow, the catalyst 
becomes encrusted within a short time and is completely inactive after 
only twenty-four hours. 
EXAMPLE 2 
The apparatus described in Example 1 in which an additional separating 
vessel arranged downstream of the circulation cooler 15 is used for the 
reaction. The packed column 1 described in Example 1 is filled with a 
supported catalyst in the form of pellets 4 mm in diameter and 3 to 8 in 
length, analysis of which gives a content of 25% by weight of nickel and 
75% by weight of silica gel. 1 liter per hour of nitrobenzene is supplied 
through line 11a and 650 liters (STP) per hour of hydrogen through line 
9a. 1.1 m.sup.3 (STP) of reacted hydrogen gas and 130 liters of liquid 
reaction mixture per hour are recycled through the column analogously to 
Example 1. The hydrogen pressure is 100 atmospheres gauge and the reaction 
temperature in the packed column is 100.degree. C. 50 liters (STP) of 
offgas is withdrawn per hour. The pressure difference between the upper 
and lower ends of the column is 1 atmosphere. Entrained water of reaction 
is deposited in a separating vessel arranged downstream of the circulation 
cooler 5. 1.3 liters per hour of reaction product (water and aniline) is 
withdrawn from the separator. This has a nitrobenzene content of 0.1%. The 
yield of aniline is 99.9% based on reacted nitrobenzene. The conversion of 
nitrobenzene is 99.9%. There is no decline in hydrogenation efficiency 
even after fourteen days in the said apparatus using the same catalyst. 
When the pump 6 for recycling the liquid is switched off, however, and the 
flow regime in the column is that of gas continuous flow, the temperature 
in the column rises immediately to more than 200.degree. C. and a mixture 
of completely hydrogenated and cracked products is obtained. 
EXAMPLE 3 
The apparatus described in Example 1 is used but the packed column 
described is replaced by a glass tube having an internal diameter of 40 mm 
and a length of 2 meters. This is filled with 2.55 liters of a catalyst 
consisting of pellets having a diameter of 2 mm and a length of 2 to 6 mm 
and consisting of 0.5% of palladium and 99.5% of silica gel. 1 liter per 
hour of 25% by weight solution of trimethyl-p-benzoquinone in isobutyl 
alcohol is pumped in through line 11a 50 liters (STP) of hydrogen is 
supplied per hour through line 9a. 120 liters per hour is passed through 
the liquid circulation and 125 liters (STP) per hour through the gas 
circulation. The pressure drop is 220 mm Hg. Hydrogenation is carried out 
at atmospheric pressure and a temperature of 90.degree. C. 20 liters (STP) 
of offgas is withdrawn per hour. 1.0 liter per hour of reaction solution 
is withdrawn from the separator; it contains 25% of trimethylhydroquinone. 
The conversion and yield are practically 100% of the theory. 
If the circulation pump is switched off so that the flow regime in the 
column is that of gas continuous flow, the originally water-clear solution 
of trimethylhydroquinone flowing out becomes brown to black within three 
hours by reason of the formation of quinhydrone. 
Dependence of the Pressure Difference .DELTA.p on the Liquid Loading of the 
Packed Column (L) 
The following apparatus (cf. FIG. 3) is used to measure the dependence of 
the pressure difference .DELTA.p on the liquid loading L of the packed 
column. A glass tube 1 having a length of 130 cm and an internal width of 
45 mm is filled over a length of 120 cm with glass spheres having a 
diameter of 3 mm. The glass spheres are supported at the lower end of the 
column by a sieve having a mesh width of 1.5 mm. The outlet has a 
cross-section which is larger than the free cross-section available 
between the spheres so that no additional pressure can be produced by the 
build-up of gas and liquid in the outlet. Measurement of the amount of gas 
and liquid supplied is carried out by means of flowmeters (Rotameters) 7 
and 6. The pressure difference .DELTA.p is measured in a manometer 8. To 
measure the pressure difference .DELTA.p, in case the amount of hydrogen 
(9) is kept constant and the liquid loading L is slowly increased, water 
being used as the liquid. Dependence of the pressure difference .DELTA.p 
on the amount of liquid L at different amounts of gas (which are however 
kept constant in each case) is shown in FIG. 1.