Use of spray dryer

Project Description

During the period of 19902006, a sharp reduction in the number of cattle was carried out in Ukraine, which led to a decrease in milk harvests.

In turn, this affected the economic performance of the vast majority of enterprises that reduced milk processing, which led to a decrease in the profitability of production.

Along with this, energy tariffs were significantly increased, which further increased the cost of production.

Especially relevant is the issue of drying milk, which in the summer does not find sale, and in winter it is not enough. Given the current economic situation in Ukraine, enterprises are divided into relatively small farms. Thus, the drying units should be designed for a moisture capacity of 10-100 kg/h.

The milk powder line shall consist of the following main components:

separator - for separation of whole milk into cream and defatted milk;

a drying chamber with a spray nozzle;

fan - for air injection into the apparatus;

calorifer - heating air to the temperature required for drying;

exhaust air purification filter - to prevent the ready product (milk powder) from entering the atmosphere

transporting devices.

To reduce power consumption, an air recuperator can be included in the line, in which heat exchange occurs between the spent hot air and cold plenum air.

Liquid, solid or gaseous organic fuel, steam or electric power can be used as heat transfer agents for heating air.

Depending on the type of spray nozzle used, an air compressor (pneumatic nozzle), a high-speed drive (disk atomizer) or a high-pressure pump (mechanical nozzle) may be needed to ensure its operability.

Fabric filters, wet filters or air cyclones may be used to clean the exhaust air.

Introduction

In the food processing industry, spray dryers are used mainly in cases where short-term contact of the product with coolant-air is desired. For example, spray dryers are used to dry liquid products (milk, blood, broth, melange, etc.) to convert the liquid product into a dry soluble product.

Using this method, the liquid product is sprayed in the drying chamber through which the heated heat transfer gas (air) passes, which is supplied for drying by special means (nozzles and centrifugal discs).

By spraying is meant the dispersion of liquid jets, accompanied by the formation of a large number of polydisperse droplets. Due to the developed surface of the dispersed particles, intensive heat and mass exchange with the drying agent takes place, while the sprayed particles quickly release moisture. The whole drying process takes only a few seconds, and the maximum temperature of the particles during evaporation of moisture in the zone of elevated temperatures does not exceed the temperature at which the product retains the basic physicochemical properties, which is especially valuable when drying materials sensitive to high temperatures.

During spray drying, some indicators of the obtained powders can be changed within certain limits: particle size, humidity, bulk mass. By using spray drying, a finished product is obtained which does not require further grinding. The dry product production process cycle can be shortened and fully mechanized.

The good solubility resulting from the drying of the fine powder is important in some cases (e.g., by diluting milk powder with water).

The disadvantages of spray dryers include: the large size of the drying chamber due to the low speed of the drying agent and, accordingly, the low moisture stress of the chamber; significant energy and heat consumption; complex equipment of the drying plant (spray and dust collecting devices).

The purpose of this project is to develop a continuous spray dryer for dry skim milk

Process Technology and Description of Machine and Hardware Design

The machine and hardware diagram of the milk powder production line is given in the graphic part of the project (SRB 02.06.01).

Whole milk is fed through line 1 to separator 4 through filter for milk cleaning 2. In the separator, milk is divided into cream and reversed (defatted milk). Cream is finished product and is drained through branch pipe 3.

Return via branch pipe (6) through cleaning filter (7) is drained into reservoir for collection of return (8). from this vessel the return is supplied to the suction line 9 of the plunger pump 10. The plunger pump provides pressure in the delivery line 11 up to 160 atm, which is necessary for normal operation of the spray nozzle. In order to smooth the pressure pulsations caused by the operation of the plunger pump, the high pressure return is supplied to the receiver 13. Pressure pulsations are dampened by the receiver air cushion, the pressure gauge 12 serves to monitor the pressure in the receiver, and can also serve as an automatic control device for the plunger pump.

From the receiver the high pressure return is supplied to the spray nozzle 14, by means of which it is sprayed onto the dispersed droplets and ejected into the casing of the drying chamber 15.

Hot air (150 ° C) is tangentially supplied to the drying chamber from the heater. Inside the chamber, hot air moves in a downward spiral and washes droplets of sprayed liquid. Due to the large contact surface of the droplets with hot air, moisture evaporates almost instantly, and the air cools to 80 ° C.

The dry particle is, by gravity, lowered to the bottom of the chamber where it is transported to the discharge bin 24 by the scrapers 20 and the screw conveyor 23.

The exhaust air, together with the entrained milk powder particles, enters the fabric filter 16, where these particles are released from the air and supplied to the discharge hopper. The cleaned exhaust air is supplied to the plate recuperator 17 where most of the heat is supplied to the cold outside air supplied by the fan 25, the outside air is accordingly heated thereby.

From the recuperator, the purified and cooled exhaust air is discharged into the atmosphere through the nozzle 26.

The external air heated in the recuperator is finally heated to the desired temperature (150 ° C) in the heater 18, from where it enters the drying chamber.

Current state and development prospects of spray dryers

2.1. Power consumption.

Currently, energy tariffs have been significantly increased, which has led to an increase in the cost of production. This was especially acute in the enterprises of the dairy and canning industry, operating evaporation and drying equipment - one of the most energy-intensive in the industry.

Drying plants are most energy-consuming, where the specific energy consumption is 810 times higher than its consumption during evaporation.

The most effective way to reduce energy consumption for the drying process is to replace the steam or electric heater with a fire heat generator, for example indirect heating, running on both gaseous and liquid fuel, which reduces fuel consumption (gas, fuel oil) by up to 30%.

However, electric heaters remain preferred in terms of simplicity of design, automation capabilities, ease of maintenance and safe operation.

A significant disadvantage of drying plants is the release of exhaust air into the atmosphere having a heat content of only 1015% less than the hot air supplied to the drying chamber.

It is possible to reduce the power consumption of the dryer due to an increase in the initial temperature of the air before the calorifer, for example, when it is heated due to the heat of the coolant used in the dryer (a mixture of air and steam formed during the evaporation of water from milk during its drying).

Currently, different types of recuperators are used in drying plants:

Plate recuperators

The removed and plenum air passes on both sides of a number of plates. The removed and plenum air usually do not contact each other, but practice has shown that some leakage can still occur. In plate recuperators, some condensate can form on the plates, and therefore they must be equipped with condensate branches. Condensate collectors must have a water gate that does not allow the fan to capture and supply water to the channel. There are no movable parts in the plate recuperator. It is characterized by high efficiency (50-90%).

Recuperators with intermediate coolant

Water or water glycol solution circulates between two heat exchangers, one of which is located in the exhaust channel, and the other in the supply channel. Heat carrier is heated by removed air and then transfers heat to supply air. Heat carrier circulates in closed system. The heat transfer can be controlled by changing the circulation rate of the coolant. These recuperators do not have movable parts, but have low efficiency (4560%).

Heat pipes

This recuperator consists of a closed system of tubes filled with freon, which evaporates when heated by the removed air. As the supply air passes along the tubes, the steam condenses and is converted back to liquid. The recuperator does not have movable parts, but has a low efficiency (5070%).

Comparative evaluation of different methods of heat recovery from spent heat carrier to air before its heating showed that the most effective method is the use of thermosyphons capable of transmitting heat with a specific power of 3-6 kW/m2 of the heat transfer surface.

Industrial thermosyphon is a 1.55 m high finned pipe with a diameter of a cylindrical part of 3035 mm, filled with a light boiling coolant. Lower part of thermosyphon is placed in air duct with spent heat carrier, and upper part - in air duct with plenum atmospheric air.

2.3. Exhaust air cleaning.

The air exiting the drying chamber contains a large amount of dry product in the suspended state. Various aspiration devices are used to clean it: fabric filters, cyclones, wet dust collection systems.

Fabric filters are capable of capturing particles with a diameter of 1 μm, but the filter fabric requires periodic cleaning (shaking or blowing in the opposite direction).

Cyclones do not require maintenance, can operate continuously, but provide retention of particles with a diameter of at least 150 μm, which is unacceptable for the production of milk powder.

The most effective for air cleaning are wet dust collection systems (scrubbers), which provide cleaning to a residual content of dust particles in air of 515 mg/mZ. Cleaning of spent air in a wet dust collector (scrubber) provides production of milk powder in amount of 1015 t/year or more. Additional savings are also achieved by evaporation from milk during wet dust collection of water up to 200400 kg/h. The design of wet dust collection systems is developed taking into account the specifics of each type of dryer and the conditions for placing equipment in the workshop.

Design and operation principle of the designed design

The developed spray dryer contains the following elements: drying chamber (body) with transporting mechanisms, spray nozzle, air cleaning filter, recuperator and calorifer.

Drying chamber has cylindrical shape with flat bottom. From the bottom of the drying chamber, the finished dry product (milk powder) is transferred by transport scrapers to the loading hole of the screw conveyor. Transport scrapers are located on horizontal rotating rod and perform rotational motion relative to axis of symmetry of chamber. The attachment angles and radial arrangement of the scrapers are selected to guide the finished product to the screw conveyor without leaving "dead" areas at the bottom of the drying chamber. The screw conveyor moves the milk powder to the discharge bin.

Drive of transport devices (scrapers and conveyor) is series-connected reduction motor and open reduction gear consisting of two conical gears. Since the rotation speeds of the shafts of the transport device and the open reduction gear are insignificant, I choose dry friction sliding bearings that do not require lubrication as bearings.

For reasons of simplicity of construction and economy, I choose a spray nozzle of the mechanical type.

Since the milk powder particles have a size of 20 μm. fabric bag filter is used as air cleaning filter. For periodic cleaning of filter tissue I accept shaking device.

As a recuperator, I accept a plate recuperator with a cross-flow of air flows. Since the exhaust air is discharged horizontally downwards, there is no need to use a condensate sink, but it must be provided on the air-to-air pipeline if the pipeline is to have lifting sections.

Conclusion

As a result of the project, a spray dryer was developed to provide 100 kg/h of raw defatted milk.

The total power consumption of the plant (air heating, fan, mechanical devices, plunger pump) is 140 kW. Energy - electricity.

Operating costs of the plant amount to UAH 257055 per year.

Calculation of the main units and mechanisms of the installation was performed, which is given in the explanatory note of 31 pages.

The graphic part of the project consists of three sheets of A1 format, which show: machine and hardware diagram of the milk powder production line, general view of the drying unit, working drawings of the spray nozzle and the drying chamber.