Kinetic Study on Dodecylbenzene Sulfonation in a Mixed Batch Reactor

A mixed batch autoclave reactor was used to investigate the sulfonation reaction kinetics of pure samples of DodecylBenzene (DB). The effect of temperature in the range between 20 and 50 °C, and the effect of the concentrations of Sulfur Trioxde and DodecylBenzene upon the sulfonation reaction rate were studied. Two sets of experiments were performed .The first is applying high SO3/DB molar ratio CSO3 /CDB >>1, the second is with high DB/SO3 molar ratio (CDB/CSO3>> 1). It was shown that the sulfonation reaction is a 0.74 order with respect to DB and a 1.59 order with respect to SO3. The reaction rate constant k was determined for both sets. The values determined for the frequency factor and activation energy were as per the following equation: k = 2.2172x10exp{-5207/RgT} (1). The kinetic model for the sulfonation reaction is RD.B = k* CCSSSS3 1.59* CCDD.BB 0.74 " (2). The progress of the reaction was determined from SO3 concentration vs. time data conducted at different time intervals. Evaluations of SO3, DB concentrations and reaction rates were based on titrations of the inorganic phase with caustic soda standard solutions.


Introduction
The recent developments in detergents industry reflect the worldwide trend in detergent products and quality.
The challenge of the recent decades for detergents industry is to produce cleaning products which are of essential role in our daily lives, which can safely and effectively remove soils, germs and other contaminants, help us to stay healthy, care for our homes and possessions, and make our surroundings more pleasant [1,2].
Dodecylbenzene sulfonate's biodegradability and its high cleaning capability were the reasons for the producers went toward the production of large amounts of this detergent and to focus their attention on the chemical process techniques of the sulfonation of the DB [3].
Recent studies had led to the conclusion that the monomer SO 3 [4,5,6].
In USA, 51 out of 101 working sulfonation plants apply Oleum, 44 of them apply SO 3 while 5 units apply chlorosulfonic acid and one plant applies sulfoxidation [2].
Basic Chemistry supports that Sulfur trioxide (SO 3 ) reacts with the Dodecylbenzene to form a sulfur-carbon bond. One of the characteristics of this process is that the resultant Dodecylebenzene sulfonic acid is a stable molecule [3,4,5,6,7] Studies on the sulfonation of organic compounds were carried by Dresel and Hinshelwood (1944) [8], Stubbs, Williams and Hinshelwood (1948) [9] and more recently by Torres et al.(2008) [10]. Torres accepted a second order model for the reaction rate, first order with respect to both methyl esters and sulfur trioxide, while Dresel and Hinshelwood (1944), Stubbs, Williams and Hinshelwood (1948), performed kinetic studies on different Alkylbenzenes with SO 3 and concluded that the reaction rate constant of this sulfonation reaction depends on the Arylbezene being sulfonated with values ranging between 48.8 to 7.85x10 -6 [liters/gmole.sec] and the activation energy is in the range of [4320 -11400] cal/gmole. Ratcliff (1954) [11], studied the sulfonation of benzene with SO 3 and concluded that the reaction is 0.57 order with respect to Benzene and 1.24 with respect to SO 3 .
The primary aim of this study is to investigate the sulfonation of Dodecylbenzene with Oleum 22 wt%.
The sulfonation is carried out in a well-mixed autoclave reactor ensuring homogeneity.
The kinetic study reported in this investigation was carried out in a stainless steel 316 alloy cylindrical autoclave (Figure. .25 D i ), the diameter of the agitating blade dt is 40.5 mm, the distance between agitator and bottom of the reactor ht is 67.5 mm, the width of the blades w is 10 mm and the width of the baffles wb is equal to 13 mm.

Raw Materials
Dodecylbenzene is an Organic compound, colorless waxy solid consists of a Dodecyl group (C 12 H 25 ) attached to a Phenyl group (C 6 H 5 ). Dodecylbenzene is a precursor for Sodium dodecylbenzenesulfonate [12 ].
Sulfur trioxide is a clear oily colorless liquid. It may contain a haze and be off-white to light brown in appearance. Oleum is normally turbid and off-white in appearance [13].
Some of the properties of Dodecylbenze are given below: 
Dodecylbenzene concentration changes were determined at different times. Samples of the reacting mixture of approximately 4 cm 3 were picked at different time intervals, washed with 10 cm 3 of distilled water and then left for approximately 6 hours for the separation of the organic phase in a burette.
The H 2 SO 4 solution is then titrated with 1 M NaOH solutions. The amount of soda solution needed to neutralize sulfuric acid is used to evaluate the amount of H 2 SO 4 and hence the SO 3 present in the solution and thus the moles of D.B not reacted.
The proposed kinetic model for the sulfonation reaction rate is: Where C DB and C SO3 are the concentrations of DB and SO 3. α 1 and α 2 are the reaction orders with respect to SO 3 and DB respectively.
Since SO 3 is in excess, its concentration is assumed constant (not altered throughout the experiment) and is considered to be equal to: The number of SO 3 moles not reacted are calculated as per the following equation: where Vi (NaOH) is the volume of 1.0 M caustic soda solution needed to neutralize v si cm 3 , 1.0 is the molarity of the caustic soda solution, v si is the volume in cm 3 of the sample picked in the i th sampling, n is the total number of samples . The number of unreacted DB moles is calculated as follows: where Δ(N SO3 )i= N( SO3 )°-N( SO3 ) i The D.B concentrations are estimated by dividing the number of un reacted moles by the remaining volume which is equal to (2666-Σv ) cm 3 .
Four classes of experiments were conducted, the first was at 20, the second at 30 °C, the third at 40°C while the forth was at 50 °C, for each class we estimated the observed reaction rate constant k´T i which is equal to the intrinsic reaction rate constant k multiplied by the average concentration of sulfur trioxide (C SO3 ) ave. raised to the power α 2 which is equal to [( C SO3 ) ave ] a2 .
In the second set of experiments, we performed classes with excess in DB moles against SO 3 .
We used 3000 cm 3 of DB with 95 cm 3 of Oleum 22% in the first three classes and 3000 cm 3 of DB with 65 cm 3 oleum 22% in the fourth class of experiments. In this second set, we initiated our experiments with 10.4512 gmoles of DB against 1.926 moles of SO 3 in the first three classes and 1.295 gmole SO 3 in the fourth class of the second set against 10.4512 gmoles of DB.
Experimental runs were performed at 20, 30, 40, and 50°C. Experimental data was treated and values of the observed rate constant k´´Ti were deduced, Where k´´Tj is equal to k multiplied by (C DB ) ave . raised to the power α1 which is equal to[(C DB ) ave ] a1 .

Batch Sulfonator
A schematic presentation of the sulfonation process is shown in Figure 1. The desired amounts of Oleum and DB are loaded to the sulfonation reactor which is equipped with an agitating system and jacketed for evolved heat removal. The reacting mixture is withdrawn from the reactor through an opening in the bottom with the aid of a pump and returned to the top of the reactor after being cooled properly in the cooler. Sampling is done from the recycling line optionally at different time intervals.

Results and Discussion
Illustration of the Experimental data obtained, is shown in Table 1. The operating conditions are as follows: T 1 = 20 (°C), the pressure is P=1.1 atma, the rotational speed of the agitator RPM= 80, and the initial total volume is V t° =2666 cm 3 Similar treatments are carried for all other experiments for the temperatures applied, 30, 40 and 50 °C.
were obtained.
The slope is equal to the order of reaction ( α1 ) with respect to DB. The intercept is equal to lnk´Ti, wherek´Ti is the observed reaction rate constant = kTi* [( CSO 3 ) ave ]a2, kTi is the intrinsic reaction rate constant.
The results of the first set of experiments are given in Table 2.
Thus the order of the reaction with respect to DB (α1) is taken as the average value of all the obtained values of α1 for the four temperatures applied, i.e (a 1 ) ave = (0.72+0.71+0.68+0.83)/4 = 0. 74 This study showed that an increase in the temperature from 20 to 30 °C led to an increase in the reaction rate constant by 1.74 times and the increase in temperature from 30 to 40 °C increased the rate constant by 1.82 times while the increase in temperature from 40 to 50°C increased the rate constant by 1.31.
In the second set, experiments conducted at Tj =20, 30 and 40°C with (CDB/CSO 3 Table 3. α2 is the reaction order with respect to SO 3 and the observed reaction rate constant k Tj´´= k Tj *[(C DB ) ave ] a1 .
Determined values of k Ti and k Tj are shown in Table 2 and Table 3.  The overall intrinsic reaction rate constant k at 20, 30 ,40 and 50 (°C) is determined from the following relation : Values of k evaluated for each temperature are shown in Table 4. The frequency factor and the activation energy of the specific reaction rate k T = k°.exp(-E/R g T), were determined by plotting ln k versus (1/T).

Conclusions
The mixed batch reactor proved to be a successful research tool for the investigation of the technical kinetics of sulfunation systems, demonstrating a superior performance compared to other types of operating chemical reactors.
It ensures reasonable determinations of Dodecylbenzene sulfonation intrinsic kinetic parameters. Thus for sound intrinsic kinetic parameters evaluation, preference should be given to the mixed reactor .The results obtained in this study showed that reaction orders deduced with respect to both components support that sulfur trioxide is more active than D.B and that the temperature influence on the intrinsic rate constant depends on the range of temperature applied in this sulfonation reaction.
Oleum as a sulfonating agent and reaction technical kinetic parameters (temperature, SO 3 and Dodecylbenzene concentrations) is well determined applying this type of reacting system, ensuring homogeneity in all reaction parameters all over the reaction space with the aid of the agitating operating system.