一、Process introduction

The traditional ammonium chloride evaporation crystallization technology uses three-effect falling film evaporator and cooling crystallizer. First, in the evaporator, the dilute solution is concentrated to saturation by countercurrent evaporation, then enters the cooling crystallizer to cool to about 40°C for crystallization, and the mother liquor of centrifugal separation returns to the evaporator to continue evaporation. The advantage is that the crystalline particle size of the material is relatively large, and the low-temperature centrifugal separation operation is more convenient. The disadvantage is that the mother liquor has a large circulation volume, the mother liquor is heated after cooling, the heat loss is large, and the investment is large.

When the concentration of ammonium chloride is high, this scheme adopts a three-effect evaporation crystallizer, that is, the solution is concentrated and crystallized in a three-effect evaporator, the crystal suspension is directly fed into a centrifuge for centrifugal separation, and the ammonium chloride crystals are discharged and packaged. The mother liquor is directly returned to the evaporator without cooling. The crystallizer is omitted, the thermal efficiency is improved, and the investment is greatly reduced.

二、Selection scheme

    Take the ammonium chloride concentration of 38% and the incoming liquid processing capacity of 110 tons/day as an example.

    Based on the actual operation of 20 hours per day, it is equivalent to an annual output of 15,000 tons of ammonium chloride, which is equivalent to an hourly production of about 2.1 tons of ammonium chloride, and an hourly evaporation of about 3.4 tons.

    Because of the high temperature and high concentration of the incoming material, 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 not easy to scar and block pipes. According to the corrosion situation of the ammonium chloride solution at the corresponding concentration and temperature, the main material of the equipment is pure titanium, and the one-effect heat exchange tube is a titanium alloy that is more resistant to high temperature corrosion.

    The crystallization chamber of the evaporator is designed to facilitate the growth of crystals and facilitate subsequent centrifugal separation. And reduce the tendency of the material to scatter in the heating tube.

Adopting the multi-stage flash evaporation cooling process of condensed water to discharge the condensed water at low temperature and make full use of the sensible heat of the condensed water.

    In order to increase the solid content of the discharged liquid and reduce the circulation of mother liquor, a salt collecting tank and a cyclone thickener are set at the bottom of the third-effect evaporator. The discharged suspension enters the thickener, and then enters the centrifuge for centrifugal separation, and the centrifugal mother liquor returns to the third-effect evaporator to continue to participate in the evaporation.

三、Technology Process:

  The dilute solution is sent to the first-effect evaporator by the feed pump. After initial concentration, it is sent to the second-effect evaporator by the transfer pump, and finally enters the three-effect evaporator. Discharge into the centrifuge after thickening. The centrifugal mother liquor is returned to the third-effect evaporator, and the crystals are dried in the next step.

   The raw steam enters the shell side of the first-effect evaporator, the second-effect steam enters the heating chamber of the second-effect evaporator, the second-effect second steam enters the third-effect heating chamber, and the third-effect enters the surface condenser for condensation.

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

四、The main process parameters::

1. design condition：

    5.5 tons/hour; the concentration of the original solution is 38%, the temperature is 150 degrees, the evaporation capacity is 3.4 tons/hour, and the steam pressure: 0.4 MPa.

2. operation condition：

五、Detailed description of main equipment

1、Evaporator heating chamber

   The evaporator is a tube-and-tube heat exchanger and is installed vertically. The material flow is single-pass. The material in the medium tube and the shell side are steam. The shell side is equipped with multiple baffles, which serve as a fixed tube and tube in the equipment. Strengthen the heat transfer effect, use a forced circulation pump during heat exchange to perform forced circulation flash evaporation of the material, increase the flow rate of the material to avoid fouling in the heat exchange tube, and the equipment material is based on industrial pure titanium

2、Evaporator

   The one-effect evaporator is a vertical device, which plays the role of steam-water separation and material sedimentation during evaporation. Make the material have a relatively large separation space, reduce the entrainment of foam, but also consider the growth space of the crystal. Set up a two-stage inertial defoaming device on the top of the evaporator, and try not to use a wire mesh defoamer to avoid scarring and blocking the net.

3、Salt chamber

   The crystals circulate in the circulating liquid, and the crystals reaching a certain size are deposited in the salt collecting chamber and discharged to the next stage, which can increase the salt content of the discharged liquid and reduce the discharge of mother liquid.

4、Cyclone thickener

   Using the principle of centrifugal separation to further increase the solid content of the discharged liquid, it can have a significant effect when combined with the salt collection chamber. The solid-liquid ratio of the discharge liquid can reach about 40%.

5、Condenser

   The condenser is of horizontal type, and the equipment adopts four-process tube type, which increases the flow rate of water and improves the heat transfer coefficient. The secondary steam travels through the shell side, and the shell side of the cooler is equipped with multi-stage baffles, so that the cooling water energy can be reasonably used.

6、Forced circulation pump

   It is an axial flow circulating pump, suitable for occasions with large flow and low head. The pump is equipped with a cooler seal, which increases the flow rate of the material in the evaporator, avoids fouling of the material in the tube, increases the running time of the equipment, and the large flow reduces the average supersaturation of the solution and reduces the probability of local explosion and nucleation , Which is conducive to the growth of crystals.

7、Transfer pump and discharge pump

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

8、Condensate pump

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

9、vacuum pump

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