Brine saturator method of operation

An apparatus is disclosed for dissolving salt in an undersaturated brine to bring the brine up to saturation conditions. A salt slurry, specifically sodium chloride, is brought into a holding tank and recycled to keep the solids suspended in the liquid. Part of the recycling salt stream is diverted to a saturator unit consisting of an upright column and a slurry feed tube mounted inside and coaxial with the column. The slurry passes downwardly through the feed tube and mixes with an undersaturated brine stream entering through the bottom of the column. This mixture flows upwardly through an annulus section defined between the feed tube and saturator column, with the salt dissolving in the brine during the upward flow. Saturated brine is removed at the top of the column for ultimate use, such as in chlorine cells.

BACKGROUND OF THE INVENTION 
The invention relates to an apparatus for producing a saturated salt brine 
solution. More specifically, the invention relates to an apparatus and 
method for dissolving salt in an undersaturated brine to bring the 
solution up to a saturated condition. 
Salt brine is used in many industrial processes, either as a dilute 
solution, or a saturated solution. One of these processes is the 
production of chlorine by the electrolysis of salt (specifically sodium 
chloride) brine in diaphragm cells or mercury cathode cells. In most 
chlorine processes the brine feed for the cells is pumped out of 
underground wells or salt deposits and treated to remove impurities before 
being passed into the cells. Saturated brine contains about 320 
grams/liter of sodium chloride at ambient temperature and pressure. The 
concentration of sodium chloride in chlorine cell brines is typically 
about 10 to 20 grams/liter below the saturation level. The actual salt 
concentration in the brine depends on various factors, such as mining 
conditions, conditions for treating the brine, and whether or not other 
materials are added to the brine between the treating plant and the 
chlorine cells. 
It is desirable to use saturated brine for the feed material to a chlorine 
cell for several reasons: One reason is that the cell is better able to 
utilize the power input; another reason is that the yield of the chlorine 
product is much better from a saturated brine than a solution which is 
less than saturated. At present, there are two general types of 
apparatuses useful for increasing the concentration of sodium chloride in 
brine solution. One of these devices is referred to as upflow saturator 
(or dissolver), and the other is referred to as a downflow saturator (or 
dissolver). 
In an upflow saturator (bed type) the undersaturated brine and solid salt 
are introduced into the bottom of an upright saturator tank. The salt 
forms a bed in the saturator which is fluidized by the upwardly flowing 
brine, so that the salt dissolves in the brine. If the upward velocity of 
the brine exceeds about 0.45 ft/min, solid salt is carried over the top of 
the saturator tank. A saturator of this type, therefore, is undesirable, 
since it is limited to the fluidizing rate of the solid salt. 
A downflow saturator makes use of the principle of gravity. The 
undersaturated brine enters the top of a tank containing solid salt 
(usually rock salt). The brine is saturated as it flows downwardly through 
the salt bed. A major drawback of downflow saturators is that they tend to 
plug up if fine grain salt is used, such as the type of salt generally 
available at chloralkali production sites where the brine feed for the 
chlorine cells is drawn from underground wells. 
For example, in downflow saturators where fine grain salt is used, the salt 
must be supported by a layer of gravel at the bottom of the saturator. 
Above the gravel layer it is critical to maintain a layer of undisturbed 
salt at least a foot thick. The solid salt which lies above and in the top 
layer of gravel, along with very fine, partly dissolved grains which 
"work" down from above, tend to form a very hard, dense salt cake. In 
moving through this cake, the brine eventually forms channels in the cake 
and emerges unsaturated. In addition, extraneous insoluble material may 
accumulate in the caked mass of salt and gravel. Because of these 
problems, downflow saturators must be periodically cleaned. If they are 
left too long without cleaning, the hard salt cake must be broken up, for 
example, with an air hammer, and then manually removed, along with the 
gravel. 
The use of either of the saturator units described above, in a process for 
producing chlorine which involves saturating brine from underground wells, 
would require huge equipment which is not commercially feasible. The 
upflow brine saturator of this invention eliminates most of the problems 
associated with saturating weak brine solutions, including the plugging 
problems of the downflow saturators and the critical nature of the 
fluidizing rate of the salt in the upflow (bed type) saturators. 
SUMMARY OF THE INVENTION 
In the practice of this invention, salt is dissolved in an undersaturated 
(or weak) brine solution to bring the solution to a saturation condition. 
The apparatus consists primarily of a holding tank and a saturator unit. 
The saturator unit includes an upright saturator column and a salt slurry 
feed tube having a smaller diameter than the saturator column. The feed 
tube is supported inside the column, in coaxial relation to the column, 
such that an annulus section is defined between the feed tube and the 
column. The lower end of the feed tube terminates above the lower end of 
the saturator column. A saturator feed line connects the holding tank into 
the salt slurry feed tube and a slurry recycle line connects the feed line 
into the holding tank. 
In operation, a salt slurry composition is delivered to the holding tank 
where it is stored and recycled to bring it up to a desired condition. 
Part of the recycling salt slurry stream is carried downwardly through the 
feed tube of the saturator column and, as it discharges through the lower 
end of the tube, it flows into the annulus section of the column. 
Undersaturated brine enters the saturator column at the bottom of the 
column and mixes with the salt slurry from the feed tube. This mixture 
moves upwardly in the annulus section of the column and in this upward 
movement the salt dissolves in the brine to produce a saturated brine 
which is carried out of the column through a brine outlet near the top of 
the column.

DESCRIPTION OF THE INVENTION 
Referring to the drawing, the brine saturator apparatus includes a holding 
tank 10, which stores a salt slurry 11, specifically, sodium chloride, 
before the salt is fed to a saturator unit. The salt slurry is brought 
into tank 10 through a delivery line 12, in which the discharge end is 
positioned inside the holding tank above the level of the salt slurry. The 
opposite end of line 12, which is the supply end, connects into a means 
for supplying the salt slurry to the delivery line. This end of the 
delivery line and the means for supplying the salt slurry is not 
illustrated in the drawing. The saturator unit includes an upright 
saturator column 13 and a salt slurry feed tube 14. The feed tube 14 is 
smaller in diameter than column 13, and the lower end of this tube 
terminates a short distance above the cone-shaped bottom end of the 
saturator column. The feed tube is mounted inside of the saturator column 
and in coaxial relation to the column. The supporting structure for the 
feed tube is not shown in the drawing. 
A saturator feed line 15 connects the holding tank 10 into the saturator 
column 13. Holding tank 10 also includes a slurry recycle line 16, which 
connects at one end into the cone-shaped bottom of tank 10, passes through 
a centrifugal pump 17, and connects at the opposite end into the saturator 
feed line 15 near the top of the holding tank. Also, on the opposite side 
of holding tank 10 from the point at which the delivery line 12 discharges 
into the tank, is an overflow line 18, for removing the clear phase of the 
brine solution in the holding tank. Undersaturated brine is directed into 
the saturator column 13 through a brine inlet line 19, which connects into 
the bottom of the column. The saturated brine is carried out of the column 
13 through a brine outlet line 20 at the top of the column. 
OPERATION 
The invention can be illustrated by describing a typical operation in which 
the apparatus is used to prepare saturated brine for use in diaphragm 
cells to make chlorine. In this operation the slurry stream was a 
by-product stream from a sodium hydroxide recovery unit, so that it 
contained a small amount of the caustic product. Following are specific 
examples of two runs which were conducted in the practice of this 
invention: 
EXAMPLE 1 
In the first run, the saturator column 13 was a section of plastic pipe, 19 
feet in length and 5.5 inches inside diameter. A salt slurry feed tube 14, 
which was 1.25 inches outside diameter, was mounted inside the saturator 
column. The bottom end of the slurry feed tube was about 6 inches above 
the cone-shaped bottom of the column 13. To start the operation, a salt 
slurry stream containing from about 5 percent to 60 percent by weight 
solid sodium chloride was passed into the holding tank 10 and recycled 
through the tank until the concentration of solid salt in the slurry was 
about 50 percent by volume (45 percent by weight). 
In practice, the actual concentration of the solids in the salt slurry in 
the holding tank is directly controlled by the discharge rate of the 
recycle pump 17. A part of the slurry stream which is being recycled, 
typically from about 10 percent to about 50 percent by volume, is pulled 
off of the recycle line 16 and pumped through the saturator feed line 15 
into the feed tube 14. The remainder of this slurry stream continues to 
recycle through the holding tank 10. The flow rate of the salt slurry into 
the feed tube 14 was about 332 cm.sup.3 /min, with the solids rate being 
about 166 gm/min. 
As the salt slurry flowed out the lower end of the feed tube 14, it was 
picked up by the undersaturated brine entering column 13 through the brine 
inlet line 19. The flow rate of the undersaturated brine into the 
saturator column 13 was about 2 gal/min. The resulting mixture of brine 
and salt slurry upflows through the annulus section 21 of the saturator 
column, and during this sequence the salt dissolves in the undersaturated 
brine. When the solution reaches the brine outlet line 20, it is a fully 
saturated brine solution. The velocity of the upflowing brine was about 
1.9 ft/min, and the time required for the saturated brine to reach the 
brine outlet line 20, which is referred to as the brine residence time, 
was about 10 minutes. The undersaturated brine entering the bottom of the 
saturator column 13 contains about 290 gm/l sodium chloride, and the 
saturated brine solution at the brine outlet 20 contains about 310 gm/l 
sodium chloride. 
During recycling of the salt slurry in the holding tank 10, the clear phase 
of the brine solution, which contains a small amount of sodium hydroxide, 
accumulates at the top of the holding tank opposite from the point at 
which the salt slurry enters the tank from delivery line 12. This clear 
brine solution is removed through the brine overflow line 18 and 
thereafter added to the brine treating system or added to brine which has 
already been treated to remove undesired materials. The time for this run 
was about 5 hours. In practice, it was found that best conditions for the 
chlorine process were obtained when the sodium chloride content of the 
saturated brine was at a maximum, but did not contain entrained solid salt 
at the point where the brine leaves the saturator column on its way to the 
chlorine cells. 
EXAMPLE 2 
The procedure for the second run was substantially the same as the first 
run, but the size of the salt slurry feed tube 14 was changed to increase 
the upflow velocity of the brine and thus decrease the brine residence 
time in the saturator column 13. The saturator column used in the second 
run was the same size as that used in the first run, but the feed tube 14 
mounted inside the column was 3.69 inches outside diameter. The salt 
slurry stream to the feed tube 14 contained the same amount of solids as 
in the first run, that is, about 50 percent by volume, or 45 percent by 
weight solids. The flow rate of the salt slurry into the feed tube 14 was 
about 199 cm.sup.3 /min and the flow rate of the solids therein was about 
110 g/min. The flow rate of the undersaturated brine into the saturator 
column 13 was the same as that of the first run, that is, about 2 gal/min. 
The upflow velocity of the brine was about 2.7 ft/min and the brine 
residence time was about 7 minutes. The undersaturated brine solution 
entering the bottom of the saturator column 13 contained about 301 gm/l 
sodium chloride and the saturated brine solution at the brine outlet 20 
contained about 313 gm/l sodium chloride. The time required for this run 
was about three hours.