(82d) A Small-Scale Pilot Plant Study of the Synthesis of Tert-Amyl-Ethyl-Ether

 
A small-scale pilot plant study of the synthesis of tert-amyl-ethyl-ether

Aarne T. Sundberg*, Aalto University, Finland

Petri Uusi-Kyyny, Aalto University, Finland

Ville Alopaeus, Aalto University, Finland

  Abstract

A complete small-scale pilot plant consisting of a tubular reactor and a distillation column was assembled and tested. The preheaters, reboilers, and heat exchangers were designed and manufactured. The total volume of the system, excluding the feed and product containers, was less than 100 cm³. The synthesis of tert-amyl-ethyl-ether was used as a demonstration case. Stability of the miniplant during 40-hour long experimental run was excellent.

  Introduction

When developing a new or modifying an existing process a number features must be clarified before moving on to the commercial scale production. A system which produces the desired component with a good yield in laboratory scale might be difficult or unsafe to operate in larger scale. The mixing efficiency and heat exchange ratio of surface-to-volume are lower when the size of the system is increased which may lead to a runaway reaction. In addition to these challenges the accumulation of impurities caused by the use of industrial grade feed or by side reactions may cause unexpected problems. To detect these phenomena as early as possible the process is typically tested in one or more different scale pilot plants. [Gertenbach 2009, Lowenstein 1985, Mannan 2005].

The delay might be several months before the accumulation of impurities can be detected. The separation efficiency of the distillation column may deteriorate much earlier. Lengthy pilot plant experiments can be required in order to detect such problems. [Wörz 1995]. The cost of these experiments can be quite high if performed on a large pilot plant. Especially, as due to the safety issues constant supervision of the process by well trained staff is required. To reduce these costs we have developed and assembled a small scale pilot plant with reactor and distillation column with reflux and boilup ratio controls. It has already been operated in a stable manner for extended length of time with very little supervision. The total volume of the system was below 100 cm³, excluding feed and product tanks. Materials and methods

Chemicals. Ethanol (Altia) purity was over 99.5 mass-% and it was dried over molecular sieves (Merck, 3A) before use. Amberlyst 16 catalyst (Rohm and Haas) was first rinsed with distilled water, then with ethanol. Finally, it was kept overnight in ethanol. 2-methylbut-2-ene (Aldrich) was over 95 mass-% and it was used as supplied.

 

Analysis. Concentration was measured online using two gas chromatographies (HP 6950 and Agilent 6850) at three points; reactor product, distillate and bottom product. Additionally, when the system had reached steady state the concentration profile of the distillation column was measured from samples taken manually at 3 points along the column.

 

Distillation column. The distillation column was similar to presented in our earlier work [Sundberg 2009]. It was planar in shape and it was operated in a horizontal orientation. The column packing was from a porous, open-cell metal-foam manufactured by Recemat Int (RCM-NC-4753.03). The packing was 290-mm-long, 30-mm-wide and 3-mm-thick and it was placed at the bottom of the 5-mm-high chamber.

 

Column temperature control. The column was insulated with a 2-cm-thick layer of rock-wool and encased between two heated aluminum blocks to reduce heat losses. The aluminum blocks were kept at the average temperature of the column.

Preheaters and heat exchangers were manufactured by a milling machine (High-Z S-720) from a block of brass. Outer dimensions were 110 by 20 by 50 mm³. The process fluid flowed through a tortuous channel with a length of 0.4 m. The depth of the channel was 0.3 mm for the heater, and 0.7 mm for the heat exchanger. Total volume of the process fluid was estimated to be less than 0.5 cm³ for the heater and less than 1.0 cm³ for the heat exchanger. The top and bottom plates were soldered over the channels.

 

Fluidic connections. The distillation column had 8 connections for temperature measurements, 1 connection for feed and 5 spare connection located at the side of the column. 3 sampling connections and 2 connections for vapour flow were located at the top of the column. The bottom product was drawn from a connection at the hot end of the column, located at the bottom of the column. The vapour outlet at the cold end of the column was from a 6-mm diameter tubing. All the other connections were from 1/8” diameter tube. Compression type fittings were used for all connections (Swagelok).

 

Reactor. The reactor was constructed from a 12-mm-thick stainless steel tube with a wall thickness of 1 mm. The length of the reactor was 270 mm. The ends were constructed by a compression fitting type reducer with a filter inside the downstream end (Swagelok). The amount of catalyst used in the experiments was 12 g. The feed was preheated inside a 2-meter long loop of 1/16” OD steel tube. Both the preheating loop and the reactor were immersed in a heated water bath. The temperature of the water bath was kept at a constant temperature (+70 °C) in all of the experiments.

Instrumentation. Temperature inside the distillation column was measured at 8 points using NiCr-Ni thermoelements (Nokeval) and logged with a control system (Labview). The heating and the evaporating devices were controlled with the same control system.

 

Liquid flow. Liquid was pumped using micro gear pumps (HNP Microsysteme, mrz-7205/7255). Part of the flow was removed via mass flow controllers (Bronkhorst, mini-Coriflow) and via an overflow.

 

  Results and discussion

The flow chart of the experimental set-up is presented in the Figure 1. The three-way valves were used to manually toggle the concentration measurement between the reboiler flow and the reactor product flows. The concentration of the reflux flow was measured continuously. The tubing was made out of OD 1/8” Teflon®PFA, with stainless steel used at regions of high temperature. All connections were made using compression fittings (Swagelok). The main difference compared to our earlier work in the operation of the distillation column was the vapour generation and condensation. In the current design this was done using external pumps, heat exchangers, and reboilers.

Figure 1. Flow chart of the tert-amyl-ethyl-ether synthesis pilot plant.

The feed and reflux flows were preheated prior to entering the column. The reboiler was manufactured using the same design, but it was operated at 20 °C above the boiling point of the mixture. Flows from the column were condensed and cooled with heat exchangers using tap water as cooling fluid.

Initially both the bottom and distillate product flow rates were controlled with a mass flow controller. In this work, the distillate was removed from the system. Both product flows were collected and weighted. The weighed result compared well with the bottom product counter but only moderately with the distillate. Difference was propably due to pressure fluctuations caused by pumping or evaporation of the fluid inside the tube.

The stability of the pilot plant was evaluated based on the concentration of the distillate and the bottom products, which are shown in the Figure 2. Time interval between measurements was 15 minutes and more than 250 data points are presented. The time to reach stable operation after a change was at most 3-4 hours. The system was kept running until 40 hours of undisturbed stable operation was observed.

Ratio of ethanol to 2-methylbut-2-ene in the feed was equimolar. Feed flow rate was 0.2 cm³/min (0.14 g/min, 2.4 mmol/min). The conversion was 63% ± 1 %.

Figure 2. Composition of reflux (left) and boilup (right); 2-methylbut-2-ene, p;   ethanol, u; 2-methylbut-1-ene, n; tert-amyl-ethyl-ether, ˜.

  Conclusions

A very small scale pilot plant was designed, assembled and tested. The total volume of the system was below 100 cm³. Synthesis of tert-amyl-ethyl-ether was used as a test case. The plant was operated continuously in a stable manner for over 40 hours. In this first step the distillate was removed from the system. In the next step the distillate will be redirected into the reactor feed to study the accumulation of impurities.

The pilot plant required very little supervision; mainly the refilling of the feed pump and the weighing of the product containers. The plant was running overnight and over the weekends without problems or supervision. The plant required relatively small space and it was fitted inside a fume cupboard. Thanks to the small size and low supervision requirement, the small scale pilot plant offers the possibility to test processes earlier in a process development project. A number of them could even be operated in parallel by a single operator.

  References

Gertenbach, D., Cooper, B., Scaleup issues from bench to pilot, AIChE annual meeting, November, 2009, Nashville, USA

Lowenstein, J., The pilot plant, Chem. Eng. 9(1985), 23, pp 62-75.

Mannan, S.(ed), Lee’s Loss Prevention in the Process Industries, 3^rd ed., Vol. 1,  Elsevier 2005

Sundberg, A., Uusi-Kyyny, P., Alopeus, V., Novel micro-distillation column for process development, Chem. Eng. Res. Des. 87 (2009), pp 705-710.

Wörz, O., Process development via a miniplant, Chem. Eng. Process. 34 (1995), pp 261-268.