T. E. (Thomas Edward) Thorpe.

A dictionary of applied chemistry online

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(c) Hop powder. To every 75 lbs. of snuff or
tobacco are added 28 lbs. of sulphur, 6 lbs. of
asafcetida, and 3 lbs. of sago flour.

(d) Fumigant for horticultural purposes. To
every 100 lbs. of tobacco or snuff are added
10 lbs. of ground hellebore, 18 lbs. of saltpetre,
6 lbs. of asafcetida, 4 lbs. of cayenne, 2 lbs. of
lampblack, 10 lbs. of sago flour.

Vol. II.— T.



Wine which has become unsound and sour
(owing to the formation of acetic acid) so as to
be no longer suitable for use as » beverage, is
released from revenue custody after being
denatured with a view to its conversion into
wine vinegar. The denaturant used in this case
is 20 p.c. of commercial vinegar, or an' equivalent
quantity of acetic acid.

(For methods of denaturing alcohol, v.
Methylated spirits and Industrial alcohol, art.
Alcohol.) j. C.

DENITRIFICATION v. Febmentatiok and
Son.s.

DENSIMETER v. Specific gbavity.

DERMATOL. Bismuth subgallate (w.
Bismuth, Obgakic compounds of; also Syn-
thetic drugs).

DERMOL. Bismuth chrysophamie. A
yellowish - brown powder, said to have the
composition Bi(Ci5H50i)3Bi20.. According to
Merck, it is a mixture of impure chrysaro-
bin and bismuth hy(Jroxide {v. Bismuth,
Oeoanic compounds of ; and Synthetic

DBUQS).

DERRIS ULIGINOSA (Benth.). The stem of
this species of derris, used in the Far East as a
fish poison, contains a poisonous resin (Proc.
Amer. Pharm. Assoc. 1902, 60, 296).

DESCLOIZITE. A basic lead and zinc
vanadate (Pb,Zn)2(0H)V0i, crystallising in the
orthorhombic system, and isomorphous with
olivenite (basic copper arsenate). A small
amount of copper is sometimes present. The
small crystals are deep reddish-brown in colour,
and the streak is a paler yellowish-brown. It
occurs in veins of lead ore in association with
vanadinite, &o., and has been mined to a
limited extent in New Mexico and Arizona, as
a source of vanadium. Recently the mineral
has been found in considerable quantities at
Broken Hill in North- Western Rhodesia.

L. J. S.

DESICCATION AND DRYING. The removal
of mechanically admixed water from substances.
The removal of chemically combined water is
usually termed dehydration, but it is not always
possible to draw a sharp distinction between
the two processes.

A substance may be termed " dry,' even
though it still contains mechanically admixed
water, provided that this water is not apparent
to the touch ; the term ' desiccation ' refers,
strictly speaking, to the complete removal of
admixed water.

The importance of drying and desiccating
will be appreciated by a consideration of the
numerous reasons for which these processes
have to be employed, among which the following
may be mentioned : —

1. To preserve material from undergoing
physical and chemical alterations, e.g. leather,
tobacco, fruit.

2. To remove water needed in preceding
operations, e.g. drying coal after washing,
woollen and cotton goods after dyeing, &c.

3. To facilitate accurate sampling, since
substances are much more readily and uniformly
mixed in the dry state than in the moist con-
dition.

4. To diminish the weight of an unnecessary
ingredient and thereby diminish cost of packing,
carriage, and freight; e.g. timber, peat, &c.

p



210



DESICCATION AND DRYING.



5. To retain desirable moisture in a sub-
stance by drying its surface, thereby rendering
the outer layer impervious to moisture, e.g. the
drying of soaps and chocolates.

6. To prevent injury to health or goods, as
in the case of the moisture-laden air of textile
factories, evaporating rooms, &o.

7. To prevent waste of heat in vapourising
water when combustibles are burnt.

A slight consideration of the various pur-
poses served by drying processes is sufficient
to bring out the wide range of degrees of dry-
ness, and the various conditions to which the
term ' dry ' is applied. This vagueness arises
partly from the difficulty of stating quantita-
tively the amount of water present in a given
instance, and partly owing to the varying
capacity that substances possess for absorbing
moisture. Thus it is that in ordinary conversa-
tion substances are variously described as
'soaked,' 'wet,' 'moist,' 'damp,' 'dry,' 'dried,'
' desiccated ' or ' dehydrated,' in order to indicate
qualitatively their degrees of dryness.

The drying processes in use may be classified
under the following headings, corresponding
with the degrees of drying to which they
lead : —

A. Gravitational drying, by draining, filter-
ing, absorbing, and condensing.

B. Mechanical drying, by pressure and centri-
fugal force.

C. Drying by evaporation.

A. Gravitational drying. The processes in-
cluded under this heading are usually only
preliminary to the employment of method C
above.

Drainage is a natural process occasionally
used for the purpose of removing surface water,
e.g. from washed coal. The procedure requires
' no special description.

Decantaiion is a process by which solid sub-
stances mixed with a large amount of water are
separated from the greater portion of that
liquid by allowing them to settle, and running
ofi the liquid from above the sediment. In
practice it is often the custom to arrange
' settling tanks ' in series, stepwise, and gradually
feed the mixture of liquid plus suspended solid
into the upper vessel by means of a pipe leading
to the bottom. The overflow runs into the
second vessel, and so on throughout the series.
Arrangements are provided for emptying each
tank as required. China clay, chalk, and barytes
are separated in this manner.

Filtration is a process of accelerated drainage.
Thus wet pigments prepared by precipitation
are drained on calico sheets stretched loosely
over a skeleton framework. Filtration is greatly
accelerated by partially exhausting the air from
the chamber into which the liquid drains. A
vacuum filter has been designed to work con-
tinuously, and consists of a hollow drum,
mounted with its axis horizontal, and partly
submerged in a tank containing the suspension
to be filtered. The periphery is covered with
filter cloth, and the water is drawn by suction,
applied to the interior of the drum, through
this cloth, leaving the solid matter as an outside
coating. By slowly revolving the drum, a con-
tinuous coating of solid is thus formed, and,
after it reaches a certain thickness, an adjust-
able skimmer removes the surplus. This



arrangement can be adapted to many drying
operations, and consumes very little power.

Absorption is a drying process in which the
material is dried by causing it to yield its
moisture to other substances which are capable
of annexing it. This process is not often applied
on a large scale, although it is commonly em-
ployed in the laboratory. Instances are fur-
nished, however, in the preliminary drying of
large clay ware in dried plaster of Paris moulds,
and in the drying of the highly water-laden
atmospheres of evaporating rooms, dye houses,
&c., by pumping warm, fresh air into them.
Warm air having a #nuch higher capacity for
holding water vapour than cold air, the result
is naturally to effect a drying of the atmo-
sphere, besides warming up the room and
fittings.

In the laboratory, substances are frequently
dried by placing them in a, desiccator. This is
simply a closed glass vessel containing in a
suitable receptacle some chemical compound,
such as anhydrous calcium chloride, concen-
trated sulphuric acid, or phosphoric anhydride,
which has a remarkable affinity for water, so
that it will, by absorption, remove water vapour
from the interior even when the tension of the
aqueous vapour therein is exceedingly small.
The substance to be dried being placed in tbe
desiccator, in the interior of which the tension
of aqueous vapour is almost nil, it gives off its
moisture, and this is continuously absorbed by
the desiccating agent employed. The latter
must expose a large surface for rapid absorption
to occur. The velocity of drying in a desiccator
may be greatly increased by exiausting the air
from the interior (vacuum desiccator), since the
rate at which moisture can diffuse from the
material to the absorbing agent is thereby largely
augmented. The degree of drying attained
cannot, however, be increased, since that is
limited merely by the pressure of the aqueous
vapour when it has reached eqi;ilibrium with
the absorbing agent. The rapidity of drying is
also increased by placing the material to be
dried at the bottom and suitably arranging the
desiccating material above it. This arises from
the fact that moist air has a smaller density
than dry air at the same temperature and
pressure ; consequently, the natural process
inside the desiccator is for the moist air to rise.
The error in design exhibited
in most laboratory desiccators,
in which the above arrange-
ment is reversed, was pointed
out by Hempel (Ber. 1890,
23, 3566), to whom the
desiccator shown in Fig. 1 is
due. In this desiccator, the '
drying agent is placed in the
trough of the lid.

It is usual to evacuate
laboratory desiccators by
means of the water-pump,
but it may be easily ac-
complished by simple chemical
means (J. Amer. Chem. Soc.
1906, 28, 834; Amer. Chem.
340).

Condensation. — This is employed as a means
of drying materials which must be handled at
low temperatures, and is a method adopted




Fia. I.
J. 1902, 27,



DESICCATION AND DRYING.



211



in cold storages. The principle of the process
is, briefly, that the moist air of the rooms is
drawn out and passed into chambers main-
tained at a very low temperature, where it is
made to deposit the greater part of its moisture
as a liquid. The dry air is then returned
to the drying rooms, either cold, when it warms
itself at the expense of the materials drying, or
after being previously heated. This circulation
of air goes on continuously. The method
adopted in order to maintain a low temperature
in the cooling rooms is to allow liquid ammonia,
carbon dioxide, or ether to evaporate at the
requisite rate through lorig coils of metal pipes
over the outer surfaces of which the moist air
is drawn. The latent heat of the ammonia (or
other) vapour is obtained at the expense of the
heat of the moist air, which is thereby cooled ;
the vapour condenses on the pipes and the
water trickles down into troughs and flows
away. The ammonia (or other) gas is repeatedly
used in a cycle of operations, being successively
liquefied and evaporated. This process is highly
efficient, but requires careful handling for
economical working.

B. Meehanieal drying. These methods are
efficient when water forms a large proportion of
the total weight, and when the solid material
to be separated is porous in texture. They
leave the solid damp or moist.

Drying by pressure. — There are, very few
cases in which direct mechanical pressure is
used for removing water from substances, the
most important of which is the direct squeezing
of peat in specially designed machines, in order
partly to remove the surplus water.

Filter presses are used for partly drying
semi-solids, and leave them still containing about
20 p.c. of moisture. They are efficient in re-
moving the large bulk of the water from granular
substances in a state of thick mud. Essentially
a filter press consists of a number of flat sections
or chambers formed of iron plates covered with
filtering cloths, and supported in a strong
horizontal frame, in which they can be squeezed
up tightly together. The chambers are made
either round or square, and usually each has
its own outlet cock, while the washing arrange-
ment can be made reversible. The sludge to be
filtered is fed in under moderate pressure, and
all the outlet cocks opened. After water has
ceased to flow out, the solid in the press is
washed, if desirable, and the press then taken to
pieces to remove the moist solid cakes from its
interior. This process is used for filtering,
among other things, sewage sludge, gold slimes,
and precipitated pigments.

Drying by centrifugal force. — This method is
largely employed for drying clothes in laundries,
when the centrifugal machines are termed
hydro-extractors. It is also used for drying
wool, cotton yarns, piece goods, &o., after they
have left the dye houses, and for separating and
drying sugar, gun-cotton, and starch.

The process consists in placing the material
to be dried in a basket containing numerous
holes, and rotating it in a horizontal plane at a
considerable speed. The water is thrown off
from the material very effectually, much more
effectually than when simple filtration or filtra-
tion under moderate pressure is adopted.

A centrifugal machine consists essentially of



a circular basket, rotated on a vertical central
shaft and enclosed in an outer stationary casing.
The circular basket is drum shaped, having an*
open top to introduce the material and some-
times a valve at the bottom to form an outlet
for dried crystals. The periphery is perforated
with holes of a suitable size. These baskets are
made of steel, bronze, copper, aluminium, silver,
or china, as may be suitable, and are lined with
rubber, enamel, or lead when dealing with
acid liquids. The outer stationary casing
serves as a guard in case of accident and as a
collector for the water. The successful working
of a centrifugal machine depends upon the skill
with which the vertical shaft is mounted and
driven, since the apparatus must have sufficient
freedom when muning for the axis to take up
the position it requires ; otherwise vibrations
will rapidly wreck the machine.

C. Drying by evaporation. This is by far
the most efficient method of drying ; indeed,
several of the preceding processes are only
employed as preliminary steps to the application
of this more powerful method.

Drying by evaporation is a process in which
the water is removed by first converting it into
vapour and then conveying the vapour away.
Water always has a tendency to pass into the
gaseous state, and is constantly giving off
vapour from its free surface. If this process
occurs in a closed space, some of the vapour
finds its way back into the liquid, and a state
of equilibrium is set up when the rate at which
vapour is leaving the surface has become equal
to the rate at which it is passing back into the
liquid. To the state of equilibrium at any
particular temperature there corresponds a
definite vapour pressure, which is a measure of
the tendency of water at that temperature to
pass into the gaseous state, and this vapour
pressure rapidly increases with rise of tem-
perature. When evaporation takes place in
free - space, no condition of equilibrium is
reached, since, owing to diffusion of the vapour,
the rate at which it passes back into the liquid
can never equal that at which vapour leaves it.
Evaporation therefore goes on imtU no more
liquid remains. Erom these considerations, it
follows that the two main practical principles in
drying by evaporation are; (i.) to warm the
material and thereby increase the tendency
of its contained water to vapourise; and (ii.) to
remove the vapour as it is produced from the
neighbourhood of the substance, in order that
the evaporation shall proceed continuously.

In applying these principles it is necessary
to bear in mind that the rite of drying per-
missible in any given instance is restricted by
(a) the physical and chemical nature of the
material, which put a limit to the rate at
which heat may be safely applied; and (6) by
the cost of drying, since the plant mechanically
most sound may not be most economical in
practice. It must always be remembered that
the nature of the material go^'erns the method
of drying to be employed, and must be con-
sidered before the question of mechanical
efficiency.

The various methods for applying heat
and for removing vapour are indicated below,
and in order to explain more definitely how
these methods are employed in practice, brief



212



DESICCATION AND DRYING.



descriptions follow of various types of drying
instaUations in actual use, and of the purposes
for which they are used.

Application ol heat. In the majority of
oases hot air is employed for conveying heat
to the material, and its use is attended with
many advantages. Air, although an extremely
poor conductor, is an excellent absorber of
heat ; the increase in volume resulting from
this process naturally tends to disperse this
heat and to warm surrounding materials by
convection. The mobility of the air enables
it to come into intimate contact with the
surfaces of materials, thereby supplying the
heat necessary to vapourise the moisture. Most
important of aU, the heated air can be thus
supplied in large or small amounts, at any
desired rate and at uniform pressure, while any
of these factors may be rapidly and easily
altered at will.

Of the various sources of heat, the cheapest
is, of course, solar heat. Owing to its un-
reliability so far as quantity and oontiuuity are
concerned, its direct use is attended by consider-
able risks. Indirectly, however, it is of great
value, in being the primary cause of natural
air currents. Drying directly by furnace heat
obtained by fuel combustion is economical, but
the method is restricted iu use to drying those
substances which are not sensibly affected by
high temperatures. For the commonest method
of drying, by means of a warm blast of air, heat
is usually supplied by passing the air over the
surfaces of radiators. These are simply iron
vessels, built to expose as much surface as
possible, and through the insides of which
steam, hot water, or hot gases are passed. Heat
is conducted through the iron to the outer
surface, and warms the surrounding air mainly
by radiation.

The dangers encountered in the application
of heat result from the nature of the material,
and the factors involved, which are briefly
discussed below, naturally affect various materials
to very different degrees.

(o) Rate of application of heat. Although
the amount of heat required to vapourise a
definite quantity of water is fixed, yet if this
heat be applied at a rate faster than that to
which the material can accommodate itself,
injury will result, e.g. wet bricks will crack
unless the greater part of their water is first
driven off at a slow and uniform rate.

(6) Change in power of absorbing heat with
loss of water. As a material dries, its heat
capacity usually diminishes, and hence the
risk of overheating increases as drying pro-
gresses.

(c) Uncertainty as to degree of dryness,
which is often unavoidable.

{d) Local application of heat. This is the
usual cause of distortion and cracking, since
most materials shrink as they dry, and local
shrinking sets up strain. This makes it advisable
to keep wet material from actual contact with
the source of heat, and to agitate the material
to continually expose fresh surfaces, since
evaporation is a purely surfa;ce phenomenon.

Methods for removal ot vapour. The natural
method by which vapour is removed is by means
of air currents, and, excepting iu the case of
the vacuum process, this is the method upon



which aU artificial processes for vapour removal
are based.

The ordinary atmospheric process for drying
utilises the natural niovements of the air, e.g.
by utilising the action of the wind on floors,
pans, gantries, sheds, &c. It requires no motive
power, but, being dependent upon varying
atmospheric conditions, it is not reliable.

In the exhaust process the moisture-laden air
is drawn away from the wet material quicker
than gravity would perform the removal.
For this purpose a propellor or fan is usually
employed. Since the operation is carried out
under slightly reduced pressure, vapourisation
proceeds quicker than it otherwise would; on
the other hand, the exhaust carries away the
heat-bearing medium from the material to be
heated, and unless proper control is exercised,
this defect may more than counterbalance the
increased rate of removal of vapour.

In the vacwwm process the material is
enclosed in an air-tight receptacle, from which
the air may be exhausted to any desired extent.
Heat is then applied, and the vapour pumped oii
as fast as it is produced. Considerable experi-
ence is necessary in the application of this
process, which, properly applied, is highly
efficient and rapid.

In the 'plenum process the vapour is removed
by supplying sufficient warm air to create an
augmentation in air pressure. A fan is usually
employed for this purpose, and drying must,
of course, take place iu an enclosed room with
regulated egress for the air. This process is
usually the best to employ, since it can be
maintained strictly under opntrol.

Drying installations. Drying floors. Natural
floors are not often used, since serious difficulties
arise as to heating, handling the material, and
removing the vapour. Haymaking affords a
good instance of its employment.

Artifir.ial floors warrtted by the products of
combustion from a furnace, by hot gases, or
by steam passing in flues or pipes beneath, are
a considerable improvement on natural floors.
Advantage is taken in constructing them of
any convenient sandy or porous ground floor ;
sand is put down if the natural ground is clayey.

These floors are used for drying china clay,
calcium phosphate, barytes, chalk, cement, and
calcined quartz, materials which are not readily
injured by excessive or irregular heating.

Drymg gantries or racks. These are em-
ployed in order that as much surface as possible
may be exposed to air currents. Their con-
struction assumes various forms. According to
circumstances, they may be either skeleton
shelves, or racks, or props, &c. Occasionally
hot water or steam pipes are used as supporting
racks, but their use is to be avoided as uneco-
nomical, and tending to produce uneven drying
and distortion.

Such substances as damp corn, peat, raw
rubber, chalk, bricks, and clayware are fre-
quently dried on open racks, while pigments
in colour works are often dried in open gantries
of skeleton shelves.

Drying sheds. These are merely drying
floors or gantries provided with roofs to keep
off rain and snow. These sheds are best con-
structed of light open framing in order to facili-
tate the removal of vapour, and the more open



DESICCATION AND DRYING.



213



it is on all sides the better, provided that the
suhatancc is not such that direct sunlight must
be excluded from it, e.g. rubber. Such sheds
are highly efficient in producing ' air-dried '
materials. Bricks are often dried in sheds
erected on drying floors, while Chile saltpetre is
dried in sheds without the application of artificial
heat, as also are china clay and ochre in the
quarry state.

Drying pans and troughs. Drying pans,
correctly designed and handled, are very useful
appliances. The oldest commercial use of such
pans is in the evaporation of salt brine, which,
as usually carried out, is not at all an eco-
nomical process. The pans are usually rect-
angular, and each stands in its own shed.
They are directly heated, three or four fires
being beneath each pan.

Shallow earthenware pans are of great use
in the colour trade, the pigments being readily
dried if the pans are carefully, arranged on
gantries in well-heated and ventilated chambers.

Semi-cylindrical iron pans heated from
below by gas or steam give good results in drying
soft materials, especially powders. They are
most efficient when provided with revolving
grids for mechanically stirring the wet material
and with valve outlets at the bottom for auto-
matically discharging their dried contents.

Granulated sugar is dried in a type of
troughed pan, through which passes a hoUow
' conveyer ' carrying hollow paddles. Steam is
blown through the conveyer and paddles.
The trough is jacketed on one side, and through
this jacket air is blown into the bottom of the
trough. The sugar is fed into the hot end, and
automatically carried by the heated paddles to
the outlet at the cool end of the trough.

Kilns. These are buildings containing a
furnace, the direct heat of which is used for
drying materials by allowing the products of
combustion to pass through them. For sub-
stances which may be strongly heated without
damage, Iriln-drying is an economical method of
procedure, since the walls of the kiln are very
effective in saving heat that would otherwise be
lost ; but care must be taken to provide proper
exits for the vapour.

A primitive type of kUn is one in which
calcium phosphate is dried. It consists of a
plain rectangular brick building with a, wooden



Online LibraryT. E. (Thomas Edward) ThorpeA dictionary of applied chemistry → online text (page 50 of 183)