Cast iron retorts are now being successfully employed on a large scale in New York for the final concentration of sulphuric acid. It is fouud that acid of 65° Be. does not attack the stills, and no injury appears to result from the deposition of arsenic or other substances. The concentration is carried as far as possible in platinum; the acid is then run into the cast iron stills and concentrated to 98 per cent. H2S04. It is stated by Mr. Adams, who has recently published a report on the progress made in twenty years in the concentration of sulphuric acid, that a set of iron stills, used in connection with platinum, have been in use in New York for some time with very satisfactory results, as much as 30,000 lb. per day of 98 per cent, acid being turned out.
Volume 37,200 gal.
Strength. 3° Tw. = l.015 Sp. Gr.
Temperature 82° C. = 180° F.
Volume 3,360 gal.
Strength. 45° Tw. = 1.225 Sp. Gr.
Temperature 71° C. = 160° F. Distilled water:-
Volume 31,290 gal.
Temperature 89° C. = 192° F. Hot water from vacuum pump:-
Vol. of water used = 174,000 gal.
Temp, of injection = 11° C. = 52° F. Do. of ejection = 43° C. = 110° F.
Weight of feed liquor= 37,200 X 10.15 lb. :
= 377,580 lb.
Weight of feed liquor=
3,360 X 12 25 lb. :
= 41,160 1b.
Lb. of water evaporated
The amount of steam required in the apparatus to evaporate this weight of water from the temperature of the feed liquor, 82° C. (180° F.), was found to be 5045 lb. per hour, by actual measurement, which included steam for pumps. The exhaust steam from the pumps connected with evaporators may, and indeed is, now exhausted into the first evaporating cylinder to do useful evaporative work. The evaporative efficiency, expressed in lb. of water evaporated from 100° C. (212° F.), and also from the temperature of the feed liquor, may be fairly ascertained as follows: a. Evaporative efficiency from 100° C. (212° F.). Per hour.
Total steam used including pumps
= 5045 lb.
Less steam required to heat feed liquor from 82° C. to 100° C. .
= 506.5 lb.
Total steam used to evaporate water from 100° C. = 4538.5 lb.
Water evaporated per hour = 336,20
24 = 14,013lb, and
4538.5 = 3-08 lb. of water evaporated per lb. of steam condensed in first cylinder.
6. Evaporative efficiency from temperature of feed liquor.
14,013/5.045 = 2.772 lb. of water per lb. of steam.
These results have been expressed in lb. of water evaporated per lb. of steam used, because before we can arrive at the amount of water evaporated per lb, of coaly it is necessary to know the weight of steam generated by 1 lb. of coal in the steam boilers which supply the evaporator.
It would appear, then, from the above results, that the efficiency of any multiple evaporator depends on the temperature of the feed liquor, and that by a quadruple effect apparatus, slightly over 3 lb. of water may easily be evaporated for every lb. of steam used, provided the feed liquor be at or very near 100° C. - the boiling point of water. When the temperature falls below this, the efficiency of the apparatus rapidly diminishes also. As above stated, in order to express this efficiency in terms of the coal it is necessary to know how much water 1 lb. of this coal evaporates in the steam boilers, and from this figure the efficiency is easily obtained. Thus, for example, if 1 lb. of coal evaporates 8 1/4lb. water in steam boilers, we have
(1) 3.08x8.25 = 25.41 lb. of water evaporated from 100° C. by the multiple evaporator per lb. of coal
(2) 2.772 x 8.25 = 22.87 lb. of water evaporated from temperature of feed (82° C.) per lb. of coal.
It will be admitted by all practical men that heat may be measured and reckoned with accuracy if adequate means are adopted for this purpose. The distribution of the heat, therefore, during say 1 hour's work of a multiple evaporator may be arrived at with tolerable certainty. From our own experience the constancy of such results is undoubted, for when once a multiple evaporator is set to work under its proper working condition with respect to the pressures and volumes and temperature of liquid passed through it, the distribution of the heat which enters the apparatus amongst the various liquids flowing from it, and steam condensed in the condenser, are all practically constant. To quote one case may serve to show the nature of this heat distribution for the individual apparatus under consideration, and in a measure for all similar pieces of apparatus. The distribution or analysis of the heat in the liquids leaving a multiple evaporator has an interest beyond the mere theory of the subject, for by this practical men may arrive at a fair understanding as to how, and to what extent they may make multiple evaporators a valuable adjunct to their necessities in connection with its services as a concentrator of liquids.
They yield two abundant supplies of hot water - one distilled, the other natural water used for producing the vacuum. These supplies are inseparable from systems of multiple evaporation. In many cases they are extremely valuable to the manufacturer, more especially the distilled water.' Both supplies may for instance be used for feeding steam boilers, in which case the heat they contain is utilised, and may be considered to have a commercial value.
The distilled water, if used for this purpose alone, has the further recommendation of producing no scale. Its Volume also is comparatively large, amounting, in fact, to about 85 per cent. of the volume of the weak liquor concentrated, or to that quantity of water required for a set of boilers burning 15-16 tons of coal per 24 hours. On the other hand, the volume of the water used for the condenser is comparatively speaking, very large, and its total consumption for steam raising purposes, excepting in the very largest factories, is quite out of the question. Turning our attention to the distribution of the heat which enters the evaporator among the liquids which leave it, we may employ the following method, which will answer all practical purposes: -
Cent lb. units per hour. Heat entering the apparatus: - • Weak liquor (feed)
377,580 x0.9 x 82
24 = 1,161,021
Steam (60 lb. press.)
5,054x659 = 3,330,586
Total = 4,491,607 Heat leaving apparatus: - Concentrated liquor
41,160x0.6x71 = 73,059 24
Condenser water 174,000 x 10x 32 = 2,320,00
Heat lost by radiation, etc.= 973,121
Expressing these results in plainer fashion we find of the total amount of heat entering the apparatus there came out in the
Lost by radiation and errors of observation
These percentages show clearly that about 75% of the heat entering the apparatus is obtained again in an available form as hot water; that 1/3 of the heat entered the apparatus in the feed liquor, and 2/3 as steam. Such calculations inevitably direct the practical roan to some of the most telling points in systems of multiple evaporation, and guide the manufacturer in arriving at a correct conclusion as to their real commercial value to him. They also serve to guide the engineer in adapting multiple evaporation to certain manufactures, so as to obtain the maximum economical effect.
The foregoing percentages must not, however, be confounded with the percentage amount of heat contained in the distilled and condensed waters expressed in terms of the steam actually-used in the evaporator. The percentage recovery on this basis is greater than the foregoing. For example, the amount of heat contained in the distilled water from the above apparatus represented 33.7% of the steam, and that contained in the hot water from the condenser represented 69• 6% of the steam used. It is also evident that in either case these percentages may be taken as a direct measure of the fuel saved by employing either of these sources of hot water for steam raising purposes. The amount of work done, or in other words, the quantity of water evaporated per hour by multiple evaporators will vary according to their construction, and other conditions as to the pressures under which they work. The heating surface of the tubes, through which the liquor passes, has doubtless the most important bearing on this part of the subject.
An apparatus containing 2000 sq. ft. of heating surface in the form of thin tubes will, under those working conditions laid down as being essential to successful result, evaporate 6-7 lb. of water per sq. ft. (Chem. Trade Jl.)