First, the process introduction

The traditional ammonium chloride evaporation crystallization technique uses a three-effect falling film evaporator and a cooling crystallizer. First, the dilute solution is concentrated in a vaporizer to a saturated state by reflux evaporation, and then cooled to about 40 ° C to be crystallized by cooling the crystallizer, and the mother liquor is centrifuged to return to the evaporator to continue evaporation. The utility model has the advantages that the material crystal grain size is relatively large, and the low temperature centrifugal separation operation is convenient, and the disadvantage is that there is a problem that the mother liquor circulation amount is large, the mother liquid liquid is heated again, the heat loss is large, and the investment is large.

When the concentration of ammonium chloride is high, the solution adopts a three-effect evaporation crystallizer, that is, the solution is concentrated and crystallized in a three-effect evaporator, and the crystal suspension is directly centrifuged in a centrifuge, and the ammonium chloride crystal is discharged and packaged. The mother liquor returns directly to the evaporator without cooling. The crystallizer is eliminated, and the thermal efficiency is improved, and the investment is greatly reduced.

Second, the selection plan

The example is based on the case where the ammonium chloride concentration is 38% and the liquid treatment amount is 110 tons/day.
According to the actual operation of 20 hours per day, it is equivalent to an annual output of 15,000 tons of ammonium chloride, equivalent to about 2.1 tons of ammonium chloride per hour, and the required evaporation per hour is about 3.4 tons.

Because the incoming temperature is high and the concentration is large, this scheme adopts a three-effect forced external circulation evaporator, which is operated downstream, and has the advantages of low energy consumption, large heat transfer coefficient, high evaporation intensity, and easy plugging and blocking. According to the corrosion condition of the ammonium chloride solution at the corresponding concentration and temperature, the main material of the equipment is selected pure titanium, and the heat exchange tube of one effect selects the titanium alloy which is more resistant to high temperature corrosion.

The crystallization chamber of the evaporator is designed to facilitate crystal growth and facilitate subsequent centrifugation. And reduce the tendency of the material to crusting in the heating tube.

The condensed water multi-stage flash cooling process is adopted to make the condensed water discharge at a low temperature, and the sensible heat of the condensed water is fully utilized.

In order to increase the solid content of the discharge liquid and reduce the circulation amount of the mother liquid, a salt collecting tank is arranged at the bottom of the third effect evaporator, and a swirl thickener is arranged. The discharged suspension is fed into the thickener, centrifuged in a centrifuge, and the mother liquor is returned to the third effect evaporator to continue to participate in the evaporation.

Third，Process flow:

The dilute solution is sent to the first-effect evaporator by the feed pump. After the initial concentration, it is sent to the second-effect evaporator by the transfer pump, and finally into the three-effect evaporator. The third-effect evaporation concentrated crystal slurry is discharged by the pump and is increased by the swirling flow. Densely drained into the centrifuge. The centrifuged mother liquid is returned to the third effect evaporator, and the crystal is dried in the next step.

The raw steam enters the shell of the one-effect evaporator, the secondary steam of one effect enters the heating chamber of the second-effect evaporator, the secondary steam of the second effect enters the three-effect heating chamber, and the three-effect enters the surface condenser to condense.

One-effect condensate is returned to the boiler room via a steam trap. The second-effect condensed water is flashed into the third-effect jacket through the steam trap, mixed with the third-effect condensed water, and discharged to the condensed water tank, and mixed with the condensed water of the surface condenser. The non-condensable gas is discharged into the atmosphere by a vacuum pump.

Fourth, the main process parameters:

1. Design conditions:

Feed flow       Set 5.5 tons / hour

Stock concentration             38%

Stock temperature             150℃

Evaporation          3.4 tons / hour

Raw steam pressure         0.4MPa

2. Operating conditions:

Project	Ⅰeffect	Ⅱeffect	Ⅲeffect
Various heating chamber temperatures (°C)	143	113	86
Each effect evaporation chamber solution temperature (°C)	118	93	66
Boiling point rise (°C)	5	6	7
Steaming water volume (kg/h)	1160	1140	1100
Third effect vacuum	-0.09Mpa
Raw steam consumption (t/h)	1.235 (considering 3% heating loss)
Steam consumption ratio (t steam / t water)	0.36
Ammonium chloride production	~2.1 tons / hour
Installed power	~60KW(Without centrifuge)
Cooling water circulation	120 m3/h

Five, the main equipment detailed description

1. Evaporator heating chamber

The evaporator is a tubular heat exchanger, which is installed vertically. The material flow is a single program. The material in the medium pipe and the shell are steam. The shell is equipped with a plurality of baffles, which play a fixed tube in the equipment. Enhance the heat transfer effect, use forced circulation pump in the heat exchange, forced circulation flashing of the material to increase the flow rate of the material to avoid scaling in the heat exchange tube. The material of the equipment is industrial pure titanium.

2, evaporator evaporator

The one-effect evaporator evaporator is a vertical device, which functions as steam-water separation and material sedimentation during evaporation. The material has a relatively large separation space, reducing the entrainment of the material, and also considering the growth space of the crystal. A two-stage inertial defoaming device is placed on the top of the evaporator, and the wire mesh demister is not used as much as possible to avoid clogging.

3. Set salt room

The crystal circulates in the circulating liquid, and the crystal which reaches a certain particle size is deposited in the salt collecting chamber and discharged to the next stage, which can increase the salt content of the discharging liquid and reduce the discharge of the mother liquid.

4, hydrodynamic thickener

By using the principle of centrifugal separation, the solid content of the discharge liquid can be further improved, and the combination with the salt collection chamber can have a significant effect. The solid-liquid ratio of the discharge liquid can reach about 40%.

5, the condenser

The condenser is horizontal, and the equipment adopts four-flow tube type, which improves the water flow rate and improves the heat transfer coefficient. The secondary steam is taken away from the shell, and the shell side of the cooler is equipped with a multi-stage baffle plate, so that the cooling water energy can be rationally utilized.

6, forced circulation pump

It is an axial flow circulating pump, suitable for high flow and low lift. The pump is equipped with a cooling machine seal to increase the flow rate of the material in the evaporator, avoiding the fouling of the material in the pipe, improving the running time of the equipment, reducing the average supersaturation of the solution by a large flow rate, and reducing the probability of local nucleation nucleation. Conducive to the growth of crystallization.

7. Transfer pump and discharge pump

The transfer pump and the discharge pump are AZ type open impeller pumps, which can transport materials with high concentration and solid content. The pump is equipped with a cooling machine seal, which has long service life and high energy output under high vacuum.

8, condensate pump

The condensate pump is a high-lift centrifugal pump. The pump is equipped with a cooling machine seal, which has a long service life and a high lift. It can output distilled water under high vacuum.

9, vacuum pump

The vacuum pump is 2BV series, the ultimate vacuum is high, the installed power is almost half of the SK series, and the operation is stable and small. It has been widely used in the evaporator industry.

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  Ammonium chloride evaporator