Studies on various organic reactions in thin film agitated evaporators for better process efficiency.
In our endeavour to carry out Process Intensification of some of the organic processes, we have explored use of Thin-Film Evaporators as reactor and got very interesting results in terms of selectivity, conversion & throughput. Usefulness of Thin-Film Evaporator henceforth termed as Thin-Film Reactor (TFR) is explored in view of its very good mass transfer & heat transfer efficacy.
Agitated thin-film evaporators are mainly used for the concentration, distillation, stripping of liquids from molten mass in variety of chemical processes where the process streams are temperature sensitive, viscous, or tend to foul or foam. A vertical thin-film evaporator system comprised of two major components: a heated cylindrical body and a high speed rotor of very close clearance from the cylindrical body. Because of the very efficient mixing and heat transfer, this equipment can be exploited to carry out several endothermic as well as exothermic reaction and hence can function as Process Intensified Reactor. Reaction reaches equilibrium very fast due to good mixing and heat transfer on the surface. Due to ideal relation of product surface vs. effective reaction volume, the TFR provides very high surface, through which light reaction products can be separated from the process. The rate of reactive particle collisions per unit time and per unit of the reaction volume, increases substantially and hence reaction equilibrium reaches very fast.
Separation of low boilers takes place very quickly, because the product film is so small. Normally, the feed or mixture of reactants are fed continuously under reduced pressure and from bottom of TFR, the final material is removed. Thus the process is a continuous Plug Flow process and hence back-mixing is substantially reduced.
Because of the distinct heat transfer and mass transfer advantages in TFR system, our endeavour has been to delve various relatively fast organic processes in TFR which is often limited by mass transfer & heat transfer issues in normal batch type processes. Firstly these processes can be made efficient due to execution in continuous manner instead of batch manner. Secondly, intrinsic kinetic rate of these processes may be achieved due to overcome of heat & mass transfer limitation. Also processes where back-mixing is undesirable, TFR provides an efficient platform. Moreover, TFR can be very effective in organic reactions where equilibrium shifts to right hand side (to Products) due to fast removal of some low boilers viz. water removal in amidation or esterification reaction. Another distinct advantage of TFR processes is process safety, as very small volume of material is present in reactor at any given time and hence specially advantageous when hazardous or heat sensitive material is used or formed in process. Also a reaction can be done at the highest possible temperature (just below the decomposition temp. of any reactant or products or intermediates) to achieve maximum rate, as the mass is exposed to very short time in TFR. In view of these advantages, the following reactions were studied in both Lab (0.013 m2 area, MOC glass) & Pilot (0.1 m2 area, MOC SS-316) plant TFR system and evaluated process efficiencies in comparison to batch processes.
Apparatus: Both the Glass TFR (0.013m2 area) set up & Pilot Plant SS TFR (0.1m2 area) were fitted with suitable condenser, vacuum system, Peristaltic pumps of appropriate capacities, Temp. & Pressure indicators and suitable utilities like hot oil system for lab and Steam for Pilot plant with Pressure control & other utilities like cooling water, N2 etc.
Generally most condensation reactions occur with elimination of water or small molecules and rate of reaction is limited by system's ability to remove these small molecules due to various process and system limitations. In TFR, due to its inherent good heat transfer and mass transfer ability, these small molecules can be removed very efficiently. A few examples of few condensation reaction are given below which were tried in TFR system.
Formanilide is normally prepared in batch manner by mixing aniline with slight molar excess of Formic acid in a reactor in presence of aromatic hydrocarbon solvent to distil off reaction water at higher temp. (~170-5[degrees]C) under atmospheric pressure. The product is obtained as a solution in solvent which is fractionally distilled to remove solvent. The same process was tried in TFR in absence of any solvent by premixing aniline and Formic acid mixture. Aniline was mixed with slight excess mole (~3-5%) of 85% Formic acid in reactor at 80-90[degrees]C for about 60 min. and the mixture is homogenized and fed into small TFR (0.1 [m.sup.2] heat transfer area) @ 60 kg/hr at ~170[degrees]C (material temp. at the bottom outlet; 10-11 kg/[cm.sup.2] steam pressure in jacket) under reduced pressure of 100 mm of Hg. The residence time of reaction mass in TFR is about 6 to 10 sec. Increase in residence time increases distillation of Aniline as well as carry over of product as distillate. Decrease in residence time results in incomplete conversion with higher Aniline, acidity and moisture content in TFR bottom product. The free and reaction water of feed was distilled of from the TFR and bottom product (Formanilide) was obtained neat in ~97% yield and ~98% purity in a continuous operation.
Acid-base neutralization reaction
In a number of synthetic processes, acidic hydrogen from weak organic acids namely alpha hydrogen adjacent carbonyl group quite often removed by base to generate, carbanions or nitride ion which can then be reacted further as a nucleophile to synthesize many industrially important intermediates & products. To elaborate an example is cited here. Thus to make alkali metal salt of Formanilide, we have used TFR very successfully to develop a continuous process. Potassium Formanilide is normally prepared in batch manner by reacting with a costly base like potassium tertbutoxide in aromatic solvent. Butanol is removed by distillation and slurry of potassium Formanilide is obtained which is isolated by filtration and drying. The purity and yield is poor. To reduce cost, the same process when tried with potassium hydroxide solution dissolved in Butanol, in a xylene solvent, the product although obtained by removal of Butanol--xylene-water under vacuum by maintaining relatively lower temp., but the yield was ~70% with ~30% potassium formate as byproduct which formed due to hydrolysis of Formanilide. To evaluate Potassium Formanilide preparation in continuous manner, we have used TFR by premixing 48% Potassium Hydroxide, aqueous solution & Formanilide and then feeding into TFR under reduced pressure (30 mm Hg) at ~170[degrees]C @ 23 kg/hr. In this process residence time and temp in pre-mixer is crucial as higher residence time (> 30 sec) and temp. (> 60[degrees]C) increases the rate of hydrolysis resulting in higher % of Potassium formate formation. The effect of residence time in TFR is more on the removal of water (higher residence time leads to complete removal of water with low Potassium formate content but more loss of Aniline resulting in concentrated solution of salt leading to handling problems. Whereas less residence time leads to incomplete stripping of water from mass and hence hydrolysis of product to Potassium formate). The excess water and reaction water was stripped of and potassium formanilide is obtained from bottom stream in ~97% yield based on KOH and ~95% purity with only 5% of potassium formate by-product formation. The process was run continuously with continuous removal of water.
Nucleophilic Substitution reaction
In another study, TFR was exploited to carry out nucleophilic aromatic substitution reaction of nitrobenzene by a strong nucleophile like anilide salt of tetramethylammonium ion to give p-nitroso or nitro diphenylamine in a continuous manner. In this reaction, Nitrobenzene (molar basis), excess (2-20 molar excess) aniline, and aqueous tetramethylammonium hydroxide (20-40% w/w solutions, 1-1.5 molar excess) are premixed and fed to TFR at temperature between 70[degrees] and 120[degrees]C under reduced pressure (5-500 mm of Hg) to remove free and reaction water. The tetramethylammonium anilide was formed in-situ and almost instantaneously reacts with nitrobenzene to give p-nitroso & p-nitro diphenylamine as major products along with some Azobenzene, Phenazine and phenylazodiphenylamine as by-products. The distinct feature is that this reaction is very much dependent on water removal rate and tetramethylammonium hydroxide (TMAH) was found to be very selective catalyst which gets decomposed at > 75[degrees]C.. The feed rate was varied in a way so that residence time in TFR can be changed from 5 sec to 50 sec. The change in residence time has an effect on rate of water distillation. Lower residence time resulted in less distillation causing lesser conversion of nitrobenzene to products whereas higher residence time resulted in complete conversion of nitrobenzene to products but resulted in lower selectivity of desired products. Increase in residence time also resulted in increase in decomposition of base indicating less stability of base at higher temp for longer duration. Decrease in absolute pressure has an effect on both conversion and selectivity of products. Low absolute pressure (5 to 50 mm of Hg) leads to complete conversion of Nitrobenzene, but decrease in selectivity to p-Nitroso & p-Nitrodiphenylamine. Also the loss of Nitrobenzene in distillate is more due to carry over in distillate. On the other hand, absolute pressure of about 50 to 500 mm of Hg leads to incomplete conversion of Nitrobenzene with increased selectivity to desirable products, with reduction of impurities (like Phenazine, Azobenzene etc.) & less decomposition of base. The selectivity towards p-Nitroso- & p-Nitrodiphenylamine can also be increased by incorporating a suitable oxidizing agent namely oxygen/air, tertiary amine N-oxide, organic peroxide and hydroperoxides, inorganic oxidizing agents viz. NiO2, (Pt/C or Pd/C)+ O2, Transitional metal complexes (e.g., Fe, Co based co-ordination complexes used as Oxidizing agents) etc. alone or in combination. In addition, a combination of air/oxygen under varying pressure 50 mm-760 mm, in a suitable temp ranging from 60 -1200 C & respective suitable residence time in TFR also improved selectivity to p-Nitroso- & p-Nitrodiphenylamine. Thus removal of about 50% of free water from a mixture of Aniline & 35% TMAH (10 :1.05 mole ratio) in TFR in continuous manner at ~80 0C under 50 mm vacuum, followed by mixing Nitrobenzene with the dehydrated mixture in 1: 10 mole ratio with respect to aniline and passing through two stage TFR under atmospheric pressure with passing 50% excess of hot (80[degrees]C) air over its water vapour saturation value based on total free and reaction water to complete nucleophilic substitution on Nitrobenzene by anilide nucleophile . This reactive evaporation approach in TFR by air has resulted improved conversion (~99% of Nitrobenzene) & selectivity (~96% based on Nitrobenzene) to useful compounds viz. 4-Nitroso- (82%) & 4-Nitro-diphenylamine TMA salt (14%) with formation ~3.5% Azobenzene and 0.5% Phenazine as by-products.
Hence removal of substantial quantity of water from reaction mass by exposing the susceptible base to a very short time at higher temp., helped to accelerate the reaction without much decomposition of base. This enabled to achieve intensification of the nucleophilic substitution reaction with high volume productivity and critical control of process condition which in batch manner takes more than 3 hr. of residence time under reduced pressure and provides very poor volume productivity.
The base used in this process was isolated as aqueous solution from reaction mass after reduction (catalytic hydrogenation) of nitroso and nitrodiphenylamine components. The organic impurities present in isolated base were extracted in organic non polar solvents like Toluene in continuous manner using liquid--liquid extraction in batch or continuous manner. The base solution was then concentrated to desired concentration using TFR in continuous manner and then the concentrated base was recycled in TFR along with fresh base to make-up for losses. The product composition of TFR mainly depends upon the temp., rate of addition, molar ratio of Aniline / Nitrobenzene, mole ratio of Nitrobenzene / base, pressure, air/ oxygen feed rate etc. One can manipulate these parameters to get the desired results. TFR can be used either singly or in series with one other with the same temp, pressure conditions to get the desired results i.e. complete conversion of nitrobenzene, improved selectivity of product, with or without supplemental addition of reactants ,e.g. base (recycled or pure), in the downstream TFR's. The working conditions of temp and pressure for TFR can be different for different units with suitable hardware modifications. The composition of final reaction mass can be varied by variation in parameters in TFR in ways known to a skilled person.
Results & Discussion
From the experimental results in all the aforementioned reactions, adoption of continuous approach in TFR showed a significant improvement over batch processes. In case of Condensation reaction of aniline with Formic acid, besides achieving similar yield and selectivity, no auxiliary solvent was used as entrainer for water removal & hence this process is more efficient both in terms of energy and throughput. Also there is no further purification required at the end of TFR process. In a similar manner, other such condensation reactions such as acid amides, imides, specific esterification etc. can be done to prepare many commercially useful intermediates and products.
In case of making Potassium formanilide, TFR approach not only helped to prepare a difficult and unstable product in high yield from cheap raw material very easily, without any auxiliary solvent. Because, back-mixing was almost negligible which helped to give better yield as in batch process the product also get hydrolyzed, by the reagent water due to back-mixing. Hence this approach is not only economical but also complied Green chemistry principles by using neat raw materials and avoiding any other solvent. This is an important intermediate which can be used in nucleophilic substitution reaction to produce many valuable products, e.g. when reacted with o- & p-chloronitrobenzene, a nearly quantitative yield of corresponding Nitrodiphenylamines was obtained. Similarly, in case of Nucleophilic substitution reactions involving Nitrobenzene and aniline involving a very unstable base like Tetramethylammonium Hydroxide, reaction can occur very fast due to ease of removal of water from process at relatively low temperature, providing good conversion and selectivity to useful intermediates. This concept can be extrapolated to other suitable Nucleophilic substitution reactions.
Thus, from the aforementioned study in TFR system of various organic reactions to produce valuable intermediates namely Formanilide, Potassium Formanilide and p-Nitoso- & p-Nitro-diphenylamine, the results when compared with respective batch process normal agitated batch reactor, we found better selectivity, yield & productivity. Although all these process needed precise control of addition of reagents and suitable equipments to perform, the operation of the process was much simpler and most importantly without incorporation of any solvent. Since most of the reagents were neat and easily available, productivity and overall process economics are better.
Based on our studies, a comparative view between batch reactor and TFR is given below to highlight its advantages in intensification of processes as given in Table 1.
From the detail elaboration, TFR can be exploited in many reactions to carry out fast reaction in continuous manner and can be easily scaled up from laboratory stage to commercial scale via pilot scale by employing multiscale approach, due to linearity in various design aspects and also employing safety criteria. Hence TFR can be used in developing various new processes with distinct advantages in terms of product quality, safety, high throughput in a continuous manner with high flexibility.
 W. B. Glover; Chemical Engineering, April 2004, 55-58.
 W.B. Glover; Chemical Engineering Progress, Dec. 2004,26-33
 A. Green; Chemical Engineering, Dec. 1999, 66-73
Nikhilesh G. Dhuldhoya
Shri HAP Chemical Enterprises Pvt. Ltd, Mumbai, India
Table 1: Comparison between Batch reactor and Thin-Film Reactor. Criteria Batch Reactor Remarks * Operation Discontinuous 1 Hold up 100% 1 Product throughput 100% 1 Mixing Satisfying 1 Efficiency Satisfying 1 Influence on residence good for long 1 time residence time Temperature Uneven 1 distribution Exchange surface/ very small 1 effective reaction volume Handle high specific Weak 1 heats of reaction Note: * Relative advantages in Chemical Process point of view is Highlighted in Remarks column as 1-4 scale in increasing order of Advantages.
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|Author:||Dhuldhoya, Nikhilesh G.|
|Publication:||International Journal of Applied Chemistry|
|Date:||Jan 1, 2009|
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