Miscellaneous processes

Introduction

This chapter embraces a few minor processes each with their own sphere of applicability. The first is vapour compression distillation where the energy input is supplied by a compressor instead of a heat exchanger. Next is solar distillation which relies on solar energy for its operation thus entailing different design concepts from conventional methods. The opportunity will also be taken to discuss within the umbrella of this chapter one of the fringe developments in the distillation field, namely low temperature difference distillation which may grow in importance as energy costs increase.

Vapour compression distillation

Figure 6.1 shows schematically the principle of vapour compression distillation. The inlet feed water is heated initially in liquid/liquid heat exchangers by the blowdown and product streams respectively then passes to a brine heater where it is heated by vapour which has been discharged by the evaporator compressor at a temperature (Tso) greater than Ts the evaporator vapour temperature. The heated feed passes into the evaporator where flashing takes place. The vapour released is compressed and used as the steam supply for the brine heater and the condensate discharged as the product stream. Figure 6.1 shows the compressor coupled to a flash chamber, it can also be linked to a distillation effect. Multiple-effect operation is possible as the steam generated in the last effect may be compressed and used as the feed steam to the first effect as shown in Fig. 6.2.

The advantage of the vapour compression process is that the latent heat of the vapour released by evaporation is in a sense 're-cycled'. The compressor supplies the energy input necessary for the flash drop plus losses, i.e. it may have to compress the vapour over a 55°-67°C (10/12°F) temperature rise. Thus the energy input per unit mass of product is low and very high performance ratios (in excess of 15.1) can be obtained. The only large-scale vapour compression plant is an 3.750 m3/day

De-superheater Compressor

■ Saline water Distillate

Brine heater

Start-up heater

Flash evaporator

■ Saline water Distillate

Flash evaporator

Brine heater

Start-up heater

Brine blow down

Raw water

Fig. 6.1. Vapour compression distillation.

Brine blow down

Raw water

Fig. 6.1. Vapour compression distillation.

(0.83 mg.d.) installation at Rosewell, New Mexico. The energy requirement of the Rosewell installation is around 16.6 kWh/m3 (75 kWh/1 000 gal) but it must be stressed that this is a one-off installation and the use of improved heat exchange methods have projected c. 11.1 kWh/m3 (50 kWh/1 000 gal) as being attainable by the vapour compression process. Silver [11 has drawn attention to the need to compare energy requirements on a common basis, as the production of 1 kWh electrical means the consumption of roughly 3 kWh thermal. The only proper basis for comparison is what is the energy cost per m3 (or 1 000 gal) product? In the example used in Chapter 4 the energy cost for multi-stage plant with a performance ratio of 10 using fuel oil at £13.5 per ton was 11 p/m3 (50 p/1 000 gal). For

Seawater feed

Fig. 6.2. Multiple-effect vapour compression.

Seawater feed

Fig. 6.2. Multiple-effect vapour compression.

electricity at an off-peak price of 0.6 p/kWh the energy cost for vapour compression at a consumption of 16.6 kWh/m3 (75 kWh/1 000 gal) is 10p/m3 (45 p/1 000 gal) which is a potential saving on this component of the water cost but these conjectural savings are obviously totally dependent on the power cost. However, the vapour compression process has never been developed on a large scale

Fig. 6.3. Packaged 4 000 g.p.d. vapour compression plant.

with the exception of the Rosewell plant. It is capital intensive due to the high cost of compressors and liquid/liquid heat exchangers which has tended to keep its application confined to the 4.5-90 m3/day (1 000-20 000 gal/day) range.

Various proposals have been made to improve the efficiency of the VCE process and reduce its high capital costs. These centre round the use of vapour compressor multi-stage flash systems as proposed by Starmer and Lowes [2] and Wood and

Herbert [3]. The combination of vapour compressors and multi-stage flash allows the use of a compressor which handles as vapour a small proportion of the product output and the use of higher compression ratios. This enables a true compressor to be used as distinct from the blower designs commonly employed for the small compression ratios in single-stage or single-effect plants. Efficiencies are increased as losses are minimised due to the reduced vapour flow rates compared with a single-stage plant.

To date there is no indication that these proposals will be taken up and at the moment the main market for vapour compression plants lies in the 4.5-90 m3/day (1 000-20 000 gal/day) class as illustrated in Fig. 6.3.

Vapour compression plants are often specified for those duties not covered by the range of larger installations which means that they may service, say, a Western Australia whaling station or some remote island where supplies in the above range are required. The maintenance accorded may be indifferent and feed-water treatment may be neglected and perhaps for these reasons the process has not had high marks for reliability so far, with the notable exception of the US Rosewell installation which has performed successfully for several years. An Australian Government report [4] gives a detailed breakdown of the problems encountered with vapour compression installations and itemises among others that one plant required its heat exchanger tubes to be replaced after three months due to faulty scale-control techniques. Another plant required tube replacement after six months due to scale formation in the evaporator tubes. The selection of this process requires careful consideration of the raw feed composition and the quality of labour available for maintenance. It is often employed inland on borehole waters with high concentration of Ca, Mg, S02 and HC03. In such cases the process choice may be very restricted and solar distillation may be specified instead as was done at the Australian opal mining town of Coober Pedy where the raw feed is virtually saturated at borehole conditions with scale-forming constituents.

Solar distillation

Solar distillation utilises, in common with all distillation processes, the evaporation and condensation modes but the resemblance ends there. It is a technique with no capital resource energy requirements needing only an adequate supply of solar energy. It can tackle raw feed of any composition and can be a viable proposition for outputs up to roughly 45 m3/day (10 000 gal/day) or greater depending on location. Its use is mainly confined to areas with high solar radiation intensities (which embraces all the world's arid zones), where fuel is expensive and skilled labour scarce or non-existent. The extreme simplicity and reliability renders it suitable for use in many developing countries.

Principle of solar distillation

Solar distillation is based on the 'greenhouse effect' whereby glass and other transparent materials have the property of transmitting incident short-wave solar

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