Low temperature reheat distilling plant

Desalination, 31(1979) 145-151

0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

LOW TEMPERATURE REHEAT DISTILLING PLANT

D . K. Forsyth,

M . Takada

Sasakura Engineering Co ., Ltd .

Abstract Is high temperature necessary in a distilling process? True? No! Technical progress with low temperature Reheat Distilling Plant over the last few years has made possible a much higher performance ratio, larger unit capacity, lower plant cost and higher reliability .

Introduction Reheat Type Distilling plant was introduced by the authors company in a paper in 1976 at the IDEA congress in Mexico, in which a high temperature process and a low temperature one were introduced . There is almost no difference between these two processes in the basic principle which is vapour compression by a steam jet ejector combined with a multi - effect evaporator . Both has been proven in performance . However, unfavourable points such as the requirement for high grade materials and high plant cost have obstructed the development of the high temperature process . To counter this, the low temperature process known as reheat plant has gained a greater share owing to its superior features . 145



1 46

FORSYTH AND TAKADA Here, in this paper, every advantage of the low temperature reheat type distilling

plant is reported, but as an introduction, an outline of the process is illustrated first in Fig-1 .

IST EFFECT

VENT EJECTOR

21C

EFFECT

MAIN EJECTOR

sa

W

T. O W W 4

C COOLING WATER

Fig-1 Principle of Low Temperature Reheat Distilling Plant

As shown in the figure, a two effect evaporator is combined with a steam jet ejector / vapour compressor (main ejector) . Steam supplied to the main ejector pumps out the vapour generated in the 2nd effect evaporator . The mixed vapour from the main ejector is introduced into the 1st effect . The mixed vapour introduced in this way works as heating steam for the multi - effect distilling plant, so that the performance ratio, distilling rate / supply steam consumption . can be calculated roughly by the formula (Number of effect) X (Suction ratio of main ejector) . Where the suction ratio is defined as the rate of vapour pumped per supply

steam

consumption . Recently, the performance of main ejectous has been dramatically improved even those working at low temperature, high specific vapour volume, and although low pressure

steam supply is used . The performance ratio has thus increased to 8 .

As shown in figure, a maximum brine temperature of S0° c occurs which means no scaling problems and a high brine concentration possibility .



FORSYTH AND TAKADA

1 47

Materials Requirement Refering to Fig-1, seawater is introduced into the 2nd effect evaporator is maintained at high vacuum . The seawater is distributed over a spray tray above the evaporative tube bundle and discharges almost all dissolved gas such as oxygen or nitrogen by vacuum deaeration . It then falls onto evaporative tubes as a thin film . The thin film and slow downward flow, and the evaporation taking place on the tubes completely releases all gases from the seawater . The complete deaeration, lower than 5 ppb oxygen level contributes to a reduction of the corrosion rate to an extrimely low level as shown Fig-2, so that it becomes possible to use low grade, economical materials . That this low corrosion effect is a fact has been shown by commercial operation for 3 years, the results of which are shown in Table-1 . As is well known, a very small amount of CO2 would be released from low temperature seawater depending on the thermal cracking reaction of dissolved hydro

- carbonates .

Hence carbon steel materials can be used for the distillate well or vapour chamber without tendency to corrosion . No corrosion has been discovered from the thickness measurement of carbon steel plates in contact with distillate and/or vapour after 3 years of operation, as shown in Table-2 .

I i I e 20

40

60

Bo

100

Dissolved Oxygen (PPb) Fig .-2 Effect of dissolved oxygen concentration on corrosion rate of Al-Brass, Cu-Ni 90-10 tube By Mr . S . Yamada of IHI Co .

Nov . 1973

1 48

FORSYTH AND TAKADA

Effect Samples from

1st effect

2nd effect

1 .03 mm 1 .00

0 .94 mm 1 .06

Middle row

1 .00 1 .00

1 .00 1 .00

Bottom row

1 .00 0 .97

1 .00 1 .03

Top row of tube bundle

Nominal thickness 1 .0 mm Tolerance ±0 .1 mm

Table-1 Aluminum - Brass tube thickness after 3 years operation .

Measuring Point

Thickness

Nominal

Distillate well

1 2 3 4

12 .0 12 .0 12 .0 12 .0

mm mm mm mm

Thickness 12 mm Tolerance ±0 .48 mm

Vapour Chamber

1 2 3 4

12 .2 12 .1 12 .1 12 .1

mm mm mm mm

Thickness 12 mm Tolerance ±0 .48 mm

Vapour Duct

1 2 3

9 .0 mm 8 .9 mm 8 .9 mm

Thickness 9 mm Tolerance ±0 .4 mm

Table-2 Carbon-Steel plate thickness after 3 years operation .

Also, an analysis of distillate from a low temperature reheat plant with a carbon steel distillate well, shown in Table-3, reveals very little Fe content and rather high PH . Thus carbon steel can be seen to be a satisfactory material for low temperature distilling plant without any acid injection system . Conversely, when carbon steel is used in high temperature distilling plant, in contact with distillate or vapour, it tends to be dissolved into the distillate by the reaction of dissolved C02 which has the effect of lowering the PH of fresh water, so that it causes much trouble from the coloured water by oxidized iron, which is not suitable for drinking regulations of the W .H .O . or it pollutes the ion exchange resin of a polisher for boiler feed .

(Refer Table-3)

Low temperature also makes possible the application of organic materials . Chlorinated rubber (Neoprene) lining inside the evaporator in contact with salt water assures no corrosion of the shell plate, and synthetic rubber gives a good performance when used at ambient temperature conditions such as a lined seawater pipe or a lined condenser for turbine steam . As Fig-3 shows, good performance of neoprene rubber at low temperature can be expected i .e . it would maintain good



FORSYTH AND TARADA

1 49

conditions for 15 years or more at below 50°c .

Item

Unit

PH Conductivity T .D .S Total Hardness Mg2+ Ca 2 + C1 Na+ -K+ Total Fe Total Cu

- *T,'cm ppm ppm ppm ppm ppm ppm PPM ppm ppm

Content

W .H .O . Regulation

7 .0 6 .7 less than 5 .0 0 .3 0 .06 0 .02 2 .2 0 .45 0 .04 0 .044 0 .015

7 .0 - 8 .5 500 100 - 500 s0 75 200 --0 .3 1 .0

Distillate from a Low Temp . Reheat Distiller .

Table-3

HEAT RESISTIBLE CHARACTER, OF NEOPRENE,

130

110

0

90 E v a 0

70

w

0 6

50

30 (•C )

I

,

,

Woo

,,

1

1

,

10,000

100.000

t Exposeing

,

a , 1

I

I

5

t0

(Hr)

I

r

20 30

c

3

(Year)

term required in which elongation performance has been reduced to 100% .

Fig-3 From a brochure of Showa Neoprene Co ., Ltd .

Technical Fr Econ

omparison

The principle features of the low temperature reheat plant are summarised in the comparison in Table-4 . (Evaporators only are considered, without boiler) . study was carried out by comparing the processes of MSF, MES, and Reheat (RHS) . The MSF plant considered is a conventional multi

- stage flash type with cross

tube design . MES is the multi - effect stack type having o

ntal tubes and thin

1 50

FORSYTH AND TAKADA

film evaporation, which was developed by the authors . The table clearly shows that reheat plant has distinct advantages over MSF in lower shipment cost, lower civil works cost, and in operating costs, but the most striking feature is the savings in the initial price of the plant .

Multi Stage Flash MSF

Multi Effect MES

Reheat RHS

1000T/D 6

1000T/D 6

1000T/D 6

2 . Bar 6944 kg/Hr

2 . Bar 6944 kg/Hr

4 . Bar 6944 kg/Hr

45 kw 100 kw 7 .5 kw none needed 11 kw 163 .5 kw 100%

45 kw none neede 7 .5 kw 22 kw

15 kw none needed 30 kw none needed

11 kw

11 kw

85 .5 kw 52%

56 kw 34%

Installation Area

1OX30m=300m 2

9X13=117m 2

lomXlS=lSOm2

Plant Weight Operation Weight

110 Ton 137 Ton

67 Ton 81 Ton

100 Ton 103 Ton

Plant Price

100 %

75 80 %

SO 60

Capacity performance Ratio Heating Steam Pressure Consumption Electric Power Seawater pump Brine Circulation P Brine Blow-Down P Feed water Pump Product Water Pump Total

Table-4 . Comparison of typical Evaporators .

Applications The development of the low temperature reheat plant commenced with smaller capacities from 150T/D . During commercial running for several years it has been clear that the character of the plant is suitable for application to all types of fresh water demand, so that technical development aimed at scaling up capacity has been carried out . Recently, units of up to 150OT/D capacity have been offered, in standard range of units . The most economical materials have been chosen for long life and automatic controllers giving maximum performance have been kept to a minimun Two new applications are shown in Fig-4 . In the first the plant is combined with a cooling tower which economises on raw feed water from an outside source and also chemicals to treat the feed water . In a case study of this type of application to well water containing 14000 ppm T .D .S ., it was possible to recover 60% of feed water, because of high concentration in low temperature evaporation . This fact indicates that for refining well water or river water into pure water such as



FORSYTH AND TAKADA

151

boiler feed at inland locations, this plant has much to be recommended . Low temperature evaporation means high vacuum evaporation and hence there is a potential difference between the inside and the outside of the evaporator_ It would thus be possible to drive the evaporator using low pressure steam such as saturated steam at 100 °c . This is the second application illustrated in Fig-4 where low potential energy (heat), such as hot water from a petro

- chemical plant,

flue gas waste heat from a furnace or an engine, or even solar energy from a vacuum tube type collector could be recovered to be used to drive the evaporator .

Vent

f

FLASH TANK

I t HEAT COLLECTOR

w Product Water

I

COOLING TOWER

Concentrated Brine Fig-4

Feed Water