ROTEX/ROTARTICA AN EARLY EXAMPLE OF MULTIFUNCTIONAL PROCESS INTENSIFICATION C Ramshaw - Protensive Ltd

INTRODUCTION It was recognised during the early phase of PI development that a multifunctional design approach was beneficial. This entails the grouping of several process modules in one unit in order to achieve a given objective. For example we can, in principle combine a re-boiler/condenser/distillation unit in one rotor or alternatively a reactor/precipitator/cross flow filter/drier in a composite spinning disc unit. This strategy is routinely adopted in nature e.g. the human digestive system.

INTRODUCTION During the 1980’s ICI was interested in extending its refrigerant fluid business into machine development (Rotex). In collaboration with Caradon Mira and British Gas the intention was to create a replacement for the UK domestic gas boiler which was to have an efficiency of 140%. This was to operate with an absorption heat pump cycle. While more complex than the conventional vapour compression cycle, an absorption heat pump has the advantage that it is driven by high grade heat rather than electric power. This improves the overall primary energy efficiency of the unit.

THE ABSORPTION CYCLE The vapour compressor used in the vapour compression cycle is replaced by a generator-condenser combination. In addition the simple working fluid used in the vapour compression cycle is replaced by a two component system having a high mutual affinity e.g. NH3/H2O LiBr/H20 NaOH/H20 etc.

Figure 1. The Vapour Compression Cycle CONDENSER PUMP EVAPORATOR POWER MEDIUM GRADE HEAT LOW Figure 1. The Vapour Compression Cycle

Figure 2. The Single Effect Absorption Heat Pump Cycle MEDIUM GRADE HEAT LOW Figure 2. The Single Effect Absorption Heat Pump Cycle CONDENSER ABSORBER GENERATOR S.H.E. P EVAPORATOR HIGH LIQUID PUMP

THE ABSORPTION CYCLE The fluid pair choice is governed by the affinity strength and corrosion considerations. Rotex initially used an aqueous solution of alkali metal hydroxides but subsequently switched to an LiBr solution.

ROTEX/ROTARICA With the exception of the solution heat exchanger all the cycle operations involve multiphase systems. These can all be intensified by operating in an elevated acceleration field within a single hermetically sealed rotor. Accordingly the Rotex machine comprises a series of spinning discs or elements which perform the various cycle component operations. In addition the rotor is used to circulate cold air and hot flue gas over the evaporator and generator respectively.

ROTEX/ROTARICA Given the desired cycle temperatures and the vapour pressure characteristics of water, the pressure differential between the “high” and “low” pressure cycle components is only 0.3 bar. Thus with a rotor tip speed of about 12m/sec liquid circulation can be achieved using Pitot pumps as shown, provided that the machine rotates about a horizontal axis.

DISCUSSION Although the prototype achieved its target performance around 1990, gas prices at this point gave an unattractive pay-back of over 5 years. Interest then switched to air conditioning duties since gas distribution systems have ample spare capacity when air conditioning is most needed. It was then discovered that over the long-term some of the plastic seal components were degrading in the strong hydroxide mixture to give non condensable gases within the hermetic rotor. This resulted in a switch to the less aggressive but lower performance LiBr/H2O fluid system.

DISCUSSION During the long development programme the original partners were extensively restructured and changed their commercial objectives. The project was effectively handed over to the Spanish firms Fagor and Gas Natural who took the lead and transferred development work to Spain where full scale production is now under way.

CONCLUSIONS A multifunctional approach to Process Intensification can provide an elegant solution to many process problems. Development “details” may take significant time and effort to be resolved. Process Intensification requires sustained commitment over the “long haul” if it is to be fully exploited.