Process and device for preparing pure formaldehyde

Details Process and device for preparing pure formaldehyde Process and device for preparing pure formaldehyde

2000-05-25

2002-07-02

Padmanabhan, Sreeni (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Oxygen containing

C568S449000

Reexamination Certificate

active

06414196

ABSTRACT:

The present invention relates to a process and an apparatus for the post-purification in particular of pyrolysis gases for preparing pure formaldehyde.
To prepare engineering plastics (polyacetals, polyoxymethylene), high-purity formaldehyde is required. The quality of the plastic produced, which should be of as high a molecular weight as possible, primarily depends on the purity of the gaseous formaldehyde, in addition to the polymerization conditions selected. Various processes for preparing formaldehyde from methanol are known (e.g. Oxidation in the formox process, oxidative dehydrogenation, direct dehydrogenation of methanol) which deliver formaldehyde in liquid (aqueous) or gaseous phase. Regardless of the phase state (gaseous or liquid), formaldehyde never arises as a pure substance, but in a mixture with water and other byproducts. However, both the anionic and cationic polymerization require gaseous formaldehyde which must contain only minimal amounts of the minor components which are harmful to or disrupt the respective polymerization.
What are termed the hemiformal processes are known for preparing high-purity gaseous formaldehyde. In these processes, aqueous formaldehyde (approximately 40-70% by weight water) reacts with an alcohol, forming hemiformals: e.g. with cyclohexanol, forming cyclohexyl hemiformals and possibly polyformals. The reaction appears as follows:
n
CH
2
O+R—OH→R(CH
2
O)
n
OH
The alcohols used should not contain reactive groups such as carbonyl, nitro or amino groups, which can participate in unwanted side reactions. Monohydric or polyhydric alcohols can be used whose boiling point is advantageously markedly above that of water. Cyclohexanol is advantageous.
The hemiformal mixture can be separated from water by distillation, the overhead vapors, after condensation, forming a two-phase mixture of water and cyclohexanol. The bottom product is the desired hemiformal, containing small amounts of water, methanol, methyl formate and other byproducts.
After removal of byproducts, the hemiformal is thermally cleaved at 1 to 1.5 bar and 120 to 180° C. (pyrolysis, thermolysis). The resulting formaldehyde vapors, if they are not yet sufficiently low in byproducts, are post-purified in a further process stage. Here, both scrubbing processes (gas flows in the opposite direction to a falling film) and prepolymerization processes are known for separating off methanol and water. In the scrubbing processes, the absorption capacity of certain alcohols is exploited (accepting significant losses of formaldehyde), and in the prepolymerization processes the fact that, in the polymerization of formaldehyde on cold surfaces, as in RU 262 45 42 or DD 124 92 48, or cold liquids, as described in DE 115 94 20, an increased separation of minor components can be observed is exploited (accepting the accompanying solids problem). In the prepolymerization, owing to the spontaneous formation of low-molecular-weight polymer, water and other components are removed from the reaction medium by the resulting end groups.
Customarily, the pyrolysis is carried out in an evaporator, e.g. a falling-film evaporator. The formaldehyde vapors are taken off at the top of the evaporator. Thereafter, the vapors are forced in cocurrent or countercurrent through tubes, down the inner wall of which a trickling film of laden cyclohexyl hemiformal runs. The trickling film is usually cold (0 to 50° C.) and absorbs the interfering byproducts (e.g. methanol, water and methyl formate), but also formaldehyde, which leads to quantitative losses at the level of up to 30% of the formaldehyde used. Processes of this type are described, for example, in U.S. Pat. No. 2,848,500 or U.S. Pat. No. 2,943,701. Usually, only smooth tubes are used, because other geometries (e.g. packed columns or plate columns) cannot be cleaned in the event of unwanted polymerization. In addition, other absorption processes are known, e.g. using poly(ethylene glycol ester)s from GB 221 8089.
Since polymerization can also be observed in the pyrolysis, considerable efforts have been made to overcome the solids problem. Thus, pyrolysis apparatuses have been developed in the form of plate columns which comprise individually heated plates, occasionally having special heatable geometries, and are described, for example, in DD 254 847, DE 521 33 or DD 124 98 46. For post-purification, processes have been developed in which a heated rotor is used in the falling-film evaporator, disclosed in DD 333 11 91. In order to exploit the prepolymerization effect specifically, low-temperature apparatuses (−20° C.) have been developed in which two heatable screws engage with one another and discharge resulting solids, disclosed by DD 124 92 48.
In order to separate off the byproducts, which are present in the cyclohexyl hemiformal solution, from the liquid, in addition to the above-described distillation processes, adsorption processes have also been developed, e.g. using ion exchangers for adsorbing formic acid and methyl formate, disclosed in DD 118 64 53, IT 119 61 75 or IT 311 87 47.
In order to decrease the byproduct formation in the pyrolysis, which is frequently carried out at high temperatures of 150 to 180° C., processes are known in which salts are added to the cyclohexyl hemiformal solution. Adding salt can suppress the formation of high-boiling substances, disclosed by U.S. Pat. No. 6,705,098 or U.S. Pat. No. 6,705,100; on the other hand, problems with salt encrustation on heat exchangers etc. occur.
For the post-purification of the gaseous formaldehyde, a process is also disclosed, for example, by JP 413 70 54, DE 305 17 55 or IT 318 49 00, in which water is separated off from the gas mixture by adsorption using dry beads or other adsorbents (silica gel, zeolites). In this process as well, the formaldehyde loss is very high, since formaldehyde is adsorbed on adsorbents of this type.
Temperature and pressure are fundamentally critical for an optimum pyrolysis. The boiling temperature of the alcohol provided for the hemiformal formation should therefore be at a sufficient distance from the pyrolysis temperature.
However, all these known processes for the post-purification of gaseous formaldehyde have the disadvantage of an increased equipment requirement, owing to a sometimes highly complicated structure using different coupled apparatuses, and high formaldehyde losses, and in some cases also exhibit unsatisfactorily high residual contamination.
Against this background, the object of the present invention is therefore to provide a process for the post-purification in particular of pyrolysis gases for preparing pure formaldehyde, which process overcomes the described disadvantages, and an apparatus for carrying out the process.
This object is achieved according to the invention by a process for the post-purification in particular of pyrolysis gases for preparing pure formaldehyde, having the process steps of:
pyrolyzing of hemiformal in an evaporator,
passing the resulting formaldehyde vapor through the condensed liquid component collected in the bottom area of the evaporator,
passing out the condensed liquid component through a cooled outlet tube disposed in the bottom area of the evaporator, so that the condensed liquid component flows down as a falling film on the inner wall of the cooled outlet tube,
passing the formaldehyde vapor in cocurrent through the cooled outlet tube,
taking off the formaldehyde vapor at the outlet tube end remote from the evaporator, and
taking off the condensed liquid component at the outlet tube end remote from the evaporator.
Although this process can be carried out with any desired evaporator, pyrolysis is advantageously carried out in a thin-film evaporator. The resulting vapors are according to the invention no longer taken off at the top end of the evaporator, that is overhead, but are ejected in what is termed the bottom liquid-phase area of the evaporator together with the condensed liquid component. From this liquid phase disposed in the bottom area of the evaporator there exits a cooled outl

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