Flow diagram of continuous PA 66 process

Batch polymerisation is realised in autoclave reactors which operate on a cycle of phases at increased levels of temperature where the time and pressure parameters are carefiilly adjusted in order to gradually convert the monomer into polymer. The design of the plant is realised in order to optimise the sequences of different autoclaves, giving better continuity to production. The low quantity of product produced per each batch operation and the higher flexibility of the process, make the batch process suitable for the production of speciality polyamides. Batch polymerisation is also suitable for producing copolymers of polyamide 66 and polyamide 6, using partially caprolactam as the raw material together with nylon salt. 

Batch polymerisation is realised in autoclaves. There, the nylon salt solution, transferred from the storage tank, is processed at different temperatures and pressures. The sequence of the operating cycle is set at first to evaporate part of the excess water of the solution (e.g. to a concentration of 70 %). Then, as a consequence of the continuous heat supply, the temperature slowly rises causing the pressure to increase as well. Under these conditions the polycondensation starts. To allow the reaction to proceed, the water is bled off from the reactor in the form of steam, by a control valve that maintains a constant pressure. When the temperature of the mass has reached about 275 °C, the pressure is gradually decreased to atmospheric pressure. The depressurisation rate is a critical parameter and it is fully under the control of dedicated software. 

In a modem plant, all process parameters of the cycle and the full set of production lines are under the control and supervision of a DCS (distributed control system). 

The granules of the final polyamide 66, of different grades, are sent to storage silos by means of pneumatic conveyors. 

All water vapour blown off from the autoclave during the polymerisation cycle is condensed and collected in a service tank. This water is sent to the main waste water storage tank for proper homogenisation, before it is sent to a waste water system. 

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LC = level control, PC=pressure control, TC = temperature control, TI = thermal insulation) Figure 9.7 Flow diagram of continuous PA 66 process... 

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