Variant 2 Styrene synthesis with carbon nanotube catalyst

By being able to get a nanometer-scale "look" at events taking place during catalytic styrene synthesis, the actual sequences of the styrene synthesis could be recorded by scientists at the Fritz-Haber-Institute of the MaxPlanck-Society. New research suggests that the coke layer is constantly present during styrene synthesis, even on the active catalyst surface. The coal gasification and coke formation are in a balanced state. Therefore it is assumed that coke is not simply a black layer that promotes deactivation, but rather that there are different types of coke with different properties (Ketteler 2002).

These investigations suggest that the carbon film that forms contains the real catalytically active species and that the potassium iron oxide film is only necessary for the formation of this active carbon species, i.e. it serves only as a "co-catalyst." Moreover, it has been demonstrated that various carbon species (carbon black, graphite, "nanobulbs," and "nanofilaments") all show excellent activity and output (MPG 2002).

The enterprise Nanoscape AG was founded in 2001 with the goal of technically implementing the results of this fundamental research. Their aim is to develop a new method of technical styrene synthesis using a nanotube-based catalytic process. Following the successful production of styrene on a small-scale, this is now being expanded as part of an EU pro ject to a reactor with a 100 g catalytic volume. This will be followed by a correspondingly larger pilot plant.

A new nanostructure catalyst consisting of multi-wall nanotubes will be used. This not only permits increases in styrene output, it also changes the procedure from an energy-intensive endothermic process to a more energy-efficient exothermic process. Additionally the new catalyst makes it possible to run the reaction by adding air instead of water. Moreover, at the same conversion rate selectivity can be increased and the process temperature lowered by 200°C, which significantly lowers the specific expenditure of energy.

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a) reactor; b) heat exchanger; c) condenser

a) reactor; b) heat exchanger; c) condenser

The plant schematic makes clear that this process is also characterised by a simpler plant structure as compared to the traditional production of styrene. The advantages of the new styrene production process on the basis of nanotube catalysts and the associated ecological effects can be summarized as follows.

Table 23. Advantages of the new styrene synthesis using a carbon nanotube catalyst73

Advantages of the new procedure

Ecological impact

Change of reaction type from endothermic (AH600°c = 124.9 kJ/mol) to exothermic (AH400°C < 0 kJ/mol)

Reduction of the reaction temperature by about 200°C from 600°C to 400°C

Change of the reactive medium from superheated steam to nitrogen/oxygen or air

Higher selectivity at same conversion rate

Use of carbon nanotube catalyst

Catalyst production

Reduction of the specific energy consumption by (at least) 1.2 MJ/kg styrene assuming AH = 124.9 kJ/mol,

Dependent on the exothermic conditions, which technologically can be kept low; moreover waste heat from the reactor could be used for other processes, such as preheating of reaction gas, heating of tubes, etc.

Reduction of the specific energy consumption

Reduction of the specific energy consumption, since production and processing of steam is very energy-intensive;

reduction of the plant costs, requirements of reactor construction, heat exchanger (e.g. process water separation is eliminated completely), etc., tube dimensions are reduced due to lower temperature level and different corrosion characteristics of the reactive media

Reduction of the specific energy consumption for less distillation and recycling Replacement for heavy metals, no heavy metal contamination

Easier catalyst management (assumption) Higher (energy) expenditures for the production of the nanotube catalyst are to be expected with the CVD process, knowing that the technical requirements for multi-wall nanotubes cannot be compared to those of single-wall nanotubes.

Detailed life cycle assessment data for the alternative styrene synthesis are not available. However, on the basis of the description of the technology, the following deduction about the energy consumption for this process will be made in order to assess the energy consumption at this process stage:

1. The change of reaction type results in a reduction of the specific energy consumption by 1.2 MJ/kg styrene.

2. Reduction of the reaction temperature by about 200°C from 600°C to 400°C effects a 25% energy savings.

3. With the change of reaction medium from superheated steam to nitrogen/oxygen or air as well as the higher selectivity at the same conversion rate, a further 5% saving of energy can be assumed.

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