VACUUM EVAPORATOR

3.1 Definition of Vacuum Evaporator

Vacuum Evaporator unit is a process aimed at concentrating the waste in the leaching pulp Pulp Making (Black Liquor) by evaporation of water from the Black Liquor using steam. Black Liquor with a high content of total solid fuel will be used in the Recovery Boiler.

Figure 3.1 Position in Vacuum Evaporator Pulp Factory

3.2 Principle of Evaporator

Production processes in the pulp and paper cycle is a very complicated process and each one of the unit processes depend on each other. To maintain the lowest possible production costs, many of the remnants of a by-product production (by Product) be recovered, for example: Weak Black Liquor is a by-product of wood in the digester at cooking pekatkan the evaporator and the fuel in the Recovery Boiler for the purification of chemical compounds and also the utilization of the combustion heat.

Pulp produced by cooking wood chips (Wood Chip) mixed with a solution cook (Cooking Liquor) in the digester. At the time of cooking, cooking solutions containing inorganic chemical compounds with the main composition of caustic soda (NaOH) and sodium sulphide (Na2S) reacts with the sap wood (lignin) so that the wood fibers (fiber) apart and break down into fine fibers. The sap wood is mixed with a chemical compound is separated from the fine fibers (fibers) and the solution. Cooking the remaining aqueous solution is called "Weak Black Liquor" (WBL).

WBL water contains approximately 85-90% and the remaining 10-15% is a mixture of residual chemicals and organic compounds cooking wood (lignin). Percentage of cooking chemicals and chemical compounds other than water is expressed as "Black Liquor Solid" (% TS).

To allow the purification of this chemical compound return, WBL was concentrated by evaporating most of the water content in the evaporator, to obtain a solution with a greater percentage of their chemical compounds of water content, about 70% TS. Concentrated solution is called "Heavy Black Liquor" (HBL).

HBL is easily burned in recovery boilers, where the organic chemical compounds (lignin) went up in flames and heat release, and inorganic compounds are converted back into shape first compound was then used again as a solution cook.

3.2.1 Evaporation (Evaporation)

The water molecule consists of two hydrogen atoms (ions +) and one oxygen atom (ion -). These ions are strongly bonded to each other and join together with other water molecules. Evaporation happens when water molecules on the surface of the solution receive sufficient heat to escape from the bonds of other water molecules and change the liquid phase to gas phase.

In other words, the primary function of the evaporator is to separate the water from the Black Liquor by evaporation. Basically, the black liquor evaporator is heated until it reaches the boiling point (boiling point) so that the water contained in the Black Liquor will gradually evaporate and turn into steam (vapor) and separated from the Black Liquor is the higher viscosity.

3.2.2 Boiling Point (Boiling Point)

Water in the sea level (P = 1 atm) will boil at a temperature of 100oC. At the boiling point, the water will gradually evaporate. But if the air pressure is increased up to 4.2 bar, the water will evaporate at a temperature of 153oC. Accordingly, the vacuum pressure of water at 0:16 bar (-635 mbar) only requires the boiling point temperature of 56oC. It can be concluded that the water pressure is proportional to the temperature of boiling water, when the pressure off the boiling point of water will drop.

Figure 3.3 The water pressure is proportional to the temperature of the boiling point of water

3.2.3 Boiling Point Rise (BPR)

The boiling point of pure water at varying pressures are shown in the steam table. But to a solution of water containing dissolved solids, the boiling point will be higher due to more strongly bound water molecules with the solid, so it takes a stronger heat by evaporation of water.

Difference between the temperature of the solution to the boiling point temperature of water vapor that is formed (pure water base) is called the boiling point or boiling point elevation rise. The greater the amount of dissolved solids or total solid percentage, the more heat required to raise the temperature in line with the increase of boiling point.

Figure 3.4 Effect of increase in the percentage of total solid of the BPR

Figure 3.5 Comparison of temperature with the temperature Liquor steam / vapor

Figure 3.6 Design Solid (%) and the Boiling Point Rise (BPR) in each effect

3.2.4 Economy

Economic terms or steam economy is used to calculate the amount of water evaporated per ton of steam heating.

We know that by lowering the pressure will cause a decline in liquor boiling point. In this way allows the water vapor produced in a unit can be used as a heater next to the unit throughout the pressure on the next unit is lower than the previous unit.

This principle is applied in a vacuum evaporator with the system and is equipped with a condenser, to allow the Black Liquor gradual release of water vapor by utilizing only water vapor generated in a heating effect as the other effect. With this may be noted that the evaporator system which consists of 6 effect steam economy has a higher than 3 effect.

3.3 Type Evaporator

a. Film Evaporator, divided into two systems:

- Rising Film

- Falling Film

b. Forced Circulation Evaporator

Forced Circulation Evaporator drain operated with a Black Liquor filled piping where evaporation does not occur in the piping, but in a separate room.

- Rising Film Type

A common type of evaporator is of type Long Tube Vertical (LTV). Where Black Liqour evaporate in the heat of condensed steam piping and piping on the outside (shell side) and Black Liqour turbulent flow in wetting the surface of the pipe.

Black Liqour inserted at the bottom of the piping and flows up through the pipeline once. Black liquor received during passage through the piping hot, out on the top of the piping and vapor release. Black Liqour overflow into effect with the higher temperature and the resulting vapor is passed as a heater in a cooler effect.

This type is less profitable, which in the event of interference with the black liquor flow rate will affect all other units and cause unstable operating conditions.

In other words, turn down rate is low, so the reduction in black liquor under the design flow is not possible because it will reduce the thickness of the layer of Black Liquor on the surface of the pipe and out of control, which in the end of evaporation process must be stopped.

- Type of Falling Film

Preheat the operation principle of Falling Film (PF) is similar to the principle of Rising Film where PF is the development and refinement of the Rising film with some striking changes.

PF is still using the type of LTV, which occurs in the vapor piping and liquor sprayed each coat the walls of the piping and the flow of black liquor layer is much faster.

Tables. 3.1 The main difference PF and Rising Film

Figure 3.7 Differences Rising Falling Film with Film

With this circulation system, the flow velocity layer of Black Liqour expected to be higher, wherein the residence time (residence time) Black Liqour in the pipeline would be shorter and the thickness of the coating on the piping Black Liqour is controlled by regulating the circulation flow rate. The advantage is the tendency of scale formation is reduced and can be operated at turn down a lower rate, where the evaporator can operate normally with a low capacity of approximately 25% of the design.

To keep the load flow to Black Liqour Liqour Black coating thickness on the pipe steady and evenly, Black Liqour vapor is circulated from the body back into the distribution box at the top of the piping. Therefore, the operation of the circulation pump plays an important role of the evaporation system, where there is no other possibility to maintain the operating condition when the circulation pump can not run / damaged.

Black Liqour circulated through the piping at the top of the discharge pipe is placed at the center of the pipe heater (heating element) is called a preheat section (preliminary heating section). In this section Black Liqour temperature rose slightly and will release a little steam after arriving at the top of the liquor box. Then through a distribution box, Black liquor is distributed evenly kesetiap heating piping.

Steam or vapor is passed through the heater header and distributed around the outer wall heater through the piping and piping condensate after releasing heat.

Distribution of vapor and condensate separation as well as NCG gas collection takes place with the help of horizontal baffles arranged in such a way in the shell side.

Baffle also serves as a barrier (support) piping on body equipped with a vapor of Regional euroform entrainment separator, which serves to filter the vapor before it flows into the next effect, where if there is black liquor carry-over will be captured in the separator.

Separator consists of the elements in the form of a series of parallel baffles, where the incoming vapor direction vertically then deflected three times before exiting the separator. In this way the Black Liquor is shipped will be attached to the baffle separator, clump together and unite to form larger droplets, and falls by gravity back to the solution of Black Liquor.

Figure 3.8 Cross-section Evaporator

Figure 3.9 Cross-section Evaporator Body

Figure 3.10 Principle of Evaporator

Figure 3.11 Increase in Total Solid in every effect

Figure 3.12 The difference in temperature of heating steam temperature Black Liquor

3.4 Heat Transfer

Basic equation for Heat Transfer (Heat Tansfer) is:

where:

Q = Total heat energy is required (Kcal / h)

U = Heat Transfer Coefficient (Kcal / h.m ². º C)

A = Area of heat transfer area (m2)

 T = temperature difference between steam and vapor heating temperature Black Liquor premises (oC)

Matters affecting the Heat Transfer:

A. Scaling on the tube

2. Low Temperature Black Liquor

3. High levels of chemical in Black Liquor

4. High solid Black Liquor

5. The low velocity in a tube of Black Liquor

6. High viscosity Black Liquor

3.5 Description of Process Weak Black Liquor to Heavy Black Liquor

Weak Black Liquor at a concentration of 15% is pumped into the Feed Tank Flash causing the temperature of the vapor into the PF-6 and then go to PF-5. Of PF-5 PF-4 entered, then the PF-3 and then to the PF-2. Liquor from PF-2 body vapor through the reflux condenser to the PF-1 (A, B, C, D). In every body is equipped with circulating pumps and transfer pumps in effect 6,5,4,3 and 2. In every effect is also equipped with a control valve that regulates the amount of liquor in the transfer. On a three-body effect in operation and a standby body (washing / washing). Only one pump that serves as a transfer and circulation, unlike the other effect. Of the fourth body in effect one connected with each other piping with a program sequence. Piping system is equipped with an automatic on-off valve.see also previous article
"What is the recovery boiler?"
Maybe usefull.