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Cpbl __1 _ DS__Cpw _ DS _ CpDS _ CpE _6_

where Cpbl is the heat capacity of black liquor (in J kg–1 °C–1); Cpw is the heat capacity

of water (4216 J kg–1 °C–1); CpDS is the heat capacity of black liquor solids (in

J kg–1 °C–1); and CpE is the excess heat capacity (in J kg–1 °C–1).

The temperature-dependence of the heat capacity of dry black liquor solids is

expressed by Eq. (7):

CpDS _ 1684 _ 4_47 _ T _7_

9.2 Chemical Recovery Processes

The dependence of excess heat capacity on temperature and dry solids content

is described by the empirical equation:

CpE __4930 _ 29 _ T___1 _ DS___DS_3_2 _8_

The dependency of the heat capacity of black liquor, Cp,bl, for different dry solids

contents is illustrated graphically in Fig. 9.2 (cp,w), as shown in Fig. 9.2.

50 100 150

2,0

2,5

3,0

3,5

4,0

4,5

18% DS 50% DS 75% DS

C

p

[kJ/kg.K]

Temperature [°C]

Fig. 9.2 Heat capacity of black liquor for different dry solids

contents as a function of temperature.

9.2

Chemical Recovery Processes

9.2.1

Overview

The chemical recovery processes contribute substantially to the economy of pulp

manufacture. On the one hand, chemicals are separated from dissolved wood substances

and recycled for repeated use in the fiberline. This limits the chemical

consumption to a make-up in the amount of losses from the cycle. On the other

hand, the organic material contained in the spent cooking liquor releases energy

for the generation of steam and electrical power when incinerated. A modern

pulp mill can, in fact, be self-sufficient in steam and electrical power.

The main stations in the cooking chemical cycle are illustrated in Fig. 9.3. The

digester plant is provided with fresh cooking chemicals, which are consumed during

the course of the pulping process. Spent cooking liquor contains, besides

9 Recovery

chemicals, the organic material dissolved from wood. The spent liquor proceeds

to the evaporation plant, where it is concentrated to a level suitable for combustion.

The thick liquor goes on to the chemical recovery system, which comprises a

recovery boiler and a number of installations for the preparation of fresh cooking

liquor. The recovery boiler separates the inorganic cooking chemicals from the

totality of spent liquor solids, and in parallel generates steam by combustion of

the organic matter in the spent liquor. The inorganics proceed to the cooking

liquor preparation system, which in the kraft industry consists of the causticizing

and lime reburning areas. Fresh kraft cooking liquor is referred to as white liquor,

and spent kraft liquor is called black liquor.

COOKING

CHEMICAL

CYCLE

EVAPORATION

CHEMICAL

RECOVERY

COOKING /

WASHING

Fresh

cooking liquor

(white liquor)

Spent

cooking liquor

(black liquor)

Thick liquor

Fig. 9.3 The cooking chemical cycle.

Besides serving the purposes of chemical recovery and energy generation, the

combustion units in the chemical recovery areas are used for the disposal of odorous

vent gases from all areas of the pulp mill. From an environmental perspective,

these combustion units represent the major sources of a pulp mill’s emissions to

air.

The following sections provide a brief overview over the main processes

employed in evaporation and chemical recovery, with particular focus on the kraft

process. Those readers requiring further detail are referred to the relevant (information

on recovery in the alkaline pulping processes e.g. Refs. [12–14]), and to

Ref. [15] for sulfite recovery.

9.2.2

Black Liquor Evaporation

9.2.2.1 Introduction

Spent cooking liquor coming from the digester or wash plant typically contains

13–18% dry solids, the remainder being water. In order to recover the energy

bound in the black liquor organics, it is necessary to remove most of the water

from the weak liquor. This is done by evaporation, and this in turn raises the dry

solids concentration in the black liquor for the purpose of firing the thick liquor

9.2 Chemical Recovery Processes

in a recovery boiler. Depending on the process used, a final solids concentration

in thick liquor from kraft pulping up to 85% is achievable. Most commonly, kraft

thick liquor concentrations are in the range of 65% to 80% dry solids. Thick

liquors from sulfite pulping reach 50–65% dry solids.

Either steam or electrical power can be used to provide the energy to evaporate

water from the black liquor. As there are usually sufficient quantities of low-pressure

steam available in a pulp mill, the most economic solution in almost all cases

is multi-effect evaporation with steam as the energy source. The use of mechanical

vapor recompression (an electrical power-consuming process) is economically

restricted to the evaporation of liquors with a low boiling point rise. Vapor recompression

is therefore viable mainly for the pre-thickening of kraft black liquors at low

dry solids concentrations, or for the evaporation of liquors from sulfite pulping.

9.2.2.2 Evaporators

Today’s evaporators are mainly of the falling film type, with plates or tubes as

heating elements. For applications involving high-viscosity liquor, or liquor with a

strong tendency to scaling, forced circulation evaporators are also employed. Evaporators

are usually constructed from stainless steel.

A schematic diagram of a falling film evaporator equipped with plate (lamella)

heating elements is shown in Fig. 9.4. Thin liquor is fed to the suction side of the

circulation liquor pump, which lifts the black liquor up to the liquor distribution.

The liquor distribution ensures uniform wetting of the heating element surfaces.

As the liquor flows downwards on the hot surface by gravity, the water is evaporated

and the concentration of the liquor increases. The concentrated liquor is collected

in the sump at the bottom of the evaporator. The generated vapor escapes

from between the lamellas to the outer sections of the evaporator body, and then

proceeds to the droplet separator, where the entrained liquor droplets are

retained.

The steam, which drives the evaporation on the liquor side, is condensed inside

the lamellas. Steam may actually be fresh steam or vapor coming from elsewhere,

for example, another evaporator. In the latter case, the vapor often contains gases

which are not condensable under the given conditions, such as methanol and

reduced sulfur compounds from kraft black liquor or sulfur dioxide from spent

sulfite liquor. If not removed, noncondensable gases (NCG) accumulate on the

steam side of the heating element and adversely affect heat transfer by reducing

both the heat transfer coefficient and the effective heating surface. When NCG

are present, the evaporators require continuous venting from the steam side. The

NCG are odorous and may be inflammable. Kraft NCG are typically forwarded to

incineration, whereas sulphite NCG can be re-used for cooking acid preparation.

Evaporators are usually arranged in groups in order to improve the steam economy,

and to accommodate the large heat exchange surfaces. When black liquor is

transferred from one evaporator body to the other, the thickened liquor may be

separately extracted from the evaporator sump (see Fig. 9.4), or branched off after

the circulation liquor pump. The separate extraction of thick liquor before

9 Recovery

Droplet separator

CIRCULATION

LIQUOR

STEAM

CONDENSATE

VAPOUR

Liquor distribution

Heating elements

THIN LIQUOR

THICK LIQUOR

NCG VENT

Fig. 9.4 Example of a plate-type falling film evaporator.

dilution with thin liquor keeps the concentration level in the evaporator comparatively

low. This is especially helpful at high dry solids concentrations, where the

boiling point rise can considerably reduce the evaporator performance.

The performance of an evaporator is determined by the heat transfer rate, Q

(W). The very basic equation of heat transfer relates the transfer rate to the overall

heat transfer coefficient, U (W m–2 K–1), the surface of the heating elements, A

(m2), and the effective temperature difference, DTeff (°C):

Q _ UADTeff _9_

The effective temperature difference which drives the evaporation is given by

the difference between the steam side condensing temperature and the vapor side

gas temperature, DT, minus the boiling point rise, BPR:

DTeff _ DT _ BPR _10_

The overall heat transfer coefficient U depends on evaporator design, on the

physical properties of the liquor (especially its dry solids concentration and viscosity),

and on potential fouling of heat exchange surfaces. Typical heat transfer coefficients

for falling film evaporators are between 700 and 2000 Wm–2 K–1, with lowend

values related to high dry solids concentrations. The heat transfer rate is pro-

9.2 Chemical Recovery Processes

portional to the evaporation capacity. Thus, more surface area and a higher temperature

difference result in increased capacity.

As concentrations rise during evaporation, fouling of the heat exchanger surfaces

on the liquor side can be caused by the precipitation of inorganic and organics

liquor compounds. Inorganics with a tendency to scaling include calcium carbonate,

sodium salts, gypsum, silicates, or oxalates. Scaling worsens with higher

concentrations and higher temperatures. A high fiber content in the feed liquor,

as well as insufficiently removed soap, also accelerate fouling. Scales reduce the

heat transfer, and by that the capacity of the evaporation plant. Hence, scales must

be removed periodically by, in order of increasing operational disturbance: switching

the evaporator body to liquor of a lower concentration; rinsing with clean condensate;

cleaning with chemicals (mostly acids); or hydroblasting. High-temperature,

high-concentration stages may require daily cleaning, whereas low-temperature

low-concentration stages may continue for several months without cleaning.

9.2.2.3 Multiple-Effect Evaporation

The basic idea of multiple-effect evaporation is the repeated use of vapor to

achieve a given evaporation task. Compared to single-stage evaporation, only a

fraction of the fresh steam is required for the same amount of water evaporated.

The principle is illustrated in Fig. 9.5. Multiple-effect evaporation plants consist

of a number of evaporators connected in series, with countercurrent flow of vapor

and liquor. Live steam is passed to the heating elements of effect I and is condensed

there, evaporating water from the liquor and producing thick liquor. The

liquor temperature in this effect depends on the low-pressure steam level available

at the mill, and is usually in the range of 125–135 °C. The vapor released in the

first effect is condensed in the heating elements of the second effect at a somewhat

lower temperature. The vapor released in turn on the liquor side of the second

effect proceeds to the third effect and so forth, until the vapor from the last

effect is condensed in a surface condenser at 55–65 °C. The vacuum needed at the

last effect is most favorably provided by a liquid-ring vacuum pump.

LIVE STEAM

THIN LIQUOR

THICK LIQUOR

LIVE STEAM

CONDENSATE

CONDENSATE

SURFACE

CONDENSER

COOLING

WATER

CONDENSATE

EFFECT

I

EFFECT

II

EFFECT

III

EFFECT

IV

EFFECT

V

VACUUM

PUMP

WARM

WATER

VAPOUR NCG

Fig. 9.5 The principle of multiple-effect evaporation demonstrated

on a five-effects system.

9 Recovery

Vapor condensates of different degrees of contamination come from the surface

condenser, and from all the effects but the first. The live steam condensate from

the first effect is collected separately from the vapor condensate for re-use as boiler

feedwater.

In Fig. 9.5, the thin liquor is fed to the last effect. The actual feed position of

thin liquor in a multiple-effect system depends on the temperature of the weak

liquor, on the course of temperatures over the effects, and on other unit operations

which may be combined with evaporation such as stripping of foul condensate,

soap skimming or black liquor heat treatment. The vapor from the stage,

into which the thin liquor is fed, contains most of the volatile compounds from

the black liquor. The place where this vapor is condensed delivers the foul condensate.

In Fig. 9.5, this would be the surface condenser. The condensate from the

other effects is less contaminated. Foul condensate is usually subjected to stripping

for the removal of volatile substances such as methanol and organic sulfur

compounds. Cleaner condensates can be used elsewhere in the mill instead of

fresh water, for example for pulp washing or in the causticizing plant.

If the thick liquor concentration needs to be raised to above approximately 75%,

the associated boiling point rise may require the use of medium-pressure steam

in the first effect. The first effect – often called the “concentrator” – usually incorporates

two or three bodies due to the more frequent cleaning required at the

high-end temperature and concentration levels. Other effects may also need to be

cleaned during operation from time to time. Depending on the cleaning procedure,

arrangements may need to be provided for switching feed liquor between

the bodies of an effect, or for by-passing a body when it is in cleaning mode.

In the last stages of a multiple-effect evaporation plant, the dry solids concentration

of the black liquor changes only slightly. This is due to the large quantities of

water to be evaporated at low concentrations. The amount of water in black liquor

as a function of the dry solids concentration, together with calculated concentration

levels in a five-effect plant starting with a 15% feed and thickening to 75%, is

shown graphically in Fig. 9.6. Note that the rise in dry solids concentration is just

7% over effects 4 and 5, but more than 30% over effect 1 alone.

The steam economy of a multiple-effect evaporation plant depends mainly on

the number of effects and on the temperature of the thin liquor. Other factors influencing

the economy are, for example, the use of residual energy contained in

condensates by flashing, venting practices, and cleaning procedures for scale

removal. Typical multiple-effect evaporation plants in the pulp industry comprise

five to seven effects, and have a gross specific steam consumption of between 0.17

and 0.25 tons of steam per ton of water evaporated. The specific consumption is

roughly calculated by dividing 1.2 through the number of effects.

Evaporation plants which deliver high-end thick liquor concentrations usually

have mixing of recovery boiler ash and chemical make-up to an intermediate

liquor before the concentrator. The suspended solids then act as crystallization

seeds for salts precipitating in the concentrator, thus making heating surfaces less

susceptible to fouling. Thick liquors of high dry solids concentrations require a

pressurized tank for storage at temperatures of 125–150 °C.

9.2 Chemical Recovery Processes

Thick liquor:

75%

Effect 2: 42%

Effect 3: 29%

Effect 4: 22%

Feed: 15%

Effect 5: 18%

10% 30% 50% 70% 90%

Water, tons per ton dry solids

Dry solids concentration, wt.-%

Fig. 9.6 Water in black liquor as a function of the dry solids

concentration. _, calculated dry solids concentrations in a

five-effect evaporation plant with 15% dry solids in weak

liquor feed and 75% dry solids in thick liquor.

Increasing the dry solids concentration brings a number of considerable advantages

for subsequent firing in the recovery boiler, including more stable furnace

conditions, higher boiler capacity, and better steam economy.

9.2.2.4 Vapor Recompression

The concept of mechanical vapor recompression is based on a process where evaporation

is driven by electrical power. In general, vapor coming from the liquor

side of an evaporator body is compressed and recycled back to the steam side of

the same body for condensation. The principle is shown schematically in Fig. 9.7.

The vapors from all bodies are collected and fed to a fan-type, centrifugal compressor.

The compressed vapors then return, at an elevated temperature, to the different

bodies and condense at the steam sides, by that evaporating new water on the

liquor sides.

The liquor is pumped from body to body. In contrast to multiple-effect plants,

where the flow rates of condensate from all effects are similar (at the same surface

area), vapor recompression plants have the highest condensate flow rate from the

thin liquor stage and the lowest flow rate from the thick liquor stage. This is

caused by the reduced driving temperature difference due to the increasing boiling

point rise at higher dry solids concentrations. In some applications, it is useful

to install a second fan in series to the first one. The second fan (shown in dotted

style in Fig. 9.7) is dedicated to supplying the higher-concentration bodies with

vapor of more elevated temperature, thus considerably improving their performance.

9 Recovery

THIN LIQUOR

THICK LIQUOR

CONDENSATE

COMPRESSOR

Fig. 9.7 Principle of vapor recompression evaporation

demonstrated on a system with three evaporator bodies.

Compression increases the vapor pressure, but at the same time the vapor is

also superheated. The vapor must be de-superheated by injection of condensate

before feeding it to the steam side of the heating element in order to make the

heat transfer effective. The temperature rise across the fan compressor and desuperheater

is typically around 6 °C. The resulting driving temperature difference

is low, and hence vapor recompression plants require comparatively large heating

surfaces.

Vapor recompression systems need steam from another source for start-up.

Depending on the electrical power input and thin liquor temperature, they may

also need a small amount of steam make-up during continuous operation. The

specific power consumption for evaporation in a vapor recompression plant

depends mainly on the boiling point rise, the heat exchange surface, and the thin

liquor temperature. Typical specific power consumption figures range from 15 to

25 kWh t–1 of water evaporated.

9.2.3

Kraft Chemical Recovery

9.2.3.1 Kraft Recovery Boiler

9.2.3.1.1 Processes and Equipment

A kraft recovery boiler converts the chemical energy of the black liquor solids into

high-pressure steam, recovers the inorganics from the black liquor, and reduces

the inorganic sulfur compounds to sulfides.

9.2 Chemical Recovery Processes

Air pre-heater

BLACK LIQUOR

AIR

FEED

WATER

Smelt spouts

Primary air ports

Secondary air ports

Tertiary air ports

SMELT

Furnace

Forced draft fan

Superheaters

Steam drum

HIGH PRESSURE STEAM

Boiler bank

Economisers

Liquor guns

ASH

Induced draft fan

Electrostatic

precipitator

Stack

FLUE

GAS

Fig. 9.8 Schematic of a kraft recovery boiler with single-drum design.

The recovery boiler consists mainly of the furnace and several heat exchange

units, as illustrated in Fig. 9.8. Pre-heated black liquor is sprayed into the furnace

via a number of nozzles, the liquor guns. The droplets formed by the nozzles are

typically 2–3 mm in diameter. On their way to the bottom of the furnace, the droplets

first dry quickly, and then ignite and burn to form char. After the char particles

reach the char bed situated on top of the smelt, carbon reduces the sulfate to

sulfide, forming carbon monoxide and carbon dioxide gases. Most of the inorganic

black liquor constituents remain in the char and finally form a smelt at the

bottom of the furnace, consisting mainly of sodium carbonate and sodium sulfide.

Some of the inorganic material is also carried away as a fume by the flue gas. The

liquid smelt leaves the furnace through several smelt spouts.

Air is sucked from the boiler house through the forced draft fan and enters the

recovery boiler at three or four levels. The portion of the air going to the primary

and secondary air ports is pre-heated with steam. The oxygen provided to the furnace

with primary and secondary air creates a reducing environment in the lowest

section of the furnace, which is necessary to provoke the formation of sodium sulfide.

The oxygen supplied with tertiary air completes the oxidation of gaseous

reaction products. The hot flue gas then enters the superheater section after passing

the bull nose, which protects the superheaters from the radiation heat of the

9 Recovery

hearth. As the flue gas flows through superheaters, boiler bank and economizers,

its temperature is continuously falling to about 180 °C. After the superheaters,

heat exchanger surfaces are located only in drafts with downward flow in order to

minimize disadvantageous ash caking. After leaving the boiler, the flue gas still

carries a considerable dust load. An electrostatic precipitator ensures dust separation

before the induced draft fan blows the flue gas into the stack.

Ash continuously settles on the heat exchanger surfaces and so reduces the

heat transfer. The most common means of keeping the surfaces clean is by periodical

sootblowing – that is, cleaning with steam of 20–30 bar pressure.

Feed water enters the boiler at the economizer, where it is heated countercurrently

by flue gas up to a temperature close to the boiling point. It enters the boiler

drum and flows by gravity into downcomers supplying the furnace membrane

walls and the boiler bank. Note that most of the evaporation of water takes place

in the furnace walls, and only 10–20% in the boiler bank. As water turns into

steam, the density of the mixture is reduced and the water/steam mixture is

pushed back into the steam drum, where the two phases are separated. The saturated

steam from the drum enters the superheaters, where it is finally heated to a

temperature of 480–500 °C at a pressure of 70–100 bar. The temperature of the

superheated steam leaving the boiler is controlled by attemperation with water

before final superheating. The high-pressure steam proceeds to a steam turbine

for the generation of electrical power and process steam at medium- and low-pressure

levels. Excess steam not needed in the process continues to the condensing

part of the turbine.

9.2.3.1.2 Material Balance

A summary of a simplified calculation of smelt and flue gas constituents from

black liquor solids is provided in Tab. 9.3. An analysis of the black liquor sampled

is required before the boiler ash is mixed. In addition, any chemical make-up

must be considered and the resulting composition is taken as the starting point

for the calculation.

The computation is performed line by line. First, it is assumed that potassium

and chlorine react completely to potassium sulfide and sodium chloride, respectively.

In this simplified model, all the potassium from the black liquor (18 kg t–1

of black liquor solids) turns into K2S in the smelt. Using the molecular weights of

potassium (39 kg kmol–1) and sulfur (32 kg kmol–1), the sulfur bound in K2S is

then 18. 32/(2. 39) = 7 kg t–1 of black liquor solids. The remaining sulfur,

46 – 7 = 39 kg, is distributed between sodium sulfide and sodium sulfate according

to the degree of reduction, DR, also termed the “reduction efficiency”:

DR _

Na2S

Na2S _ Na2SO4 _11_

Values for the chemicals in Eq. (11) can be inserted on a molar basis, equivalent

basis or sulfur weight basis, all of which give the same result. Assuming 95%

9.2 Chemical Recovery Processes

Tab. 9.3 Simplified calculation of smelt and flue gas constituents from black liquor solids.

System

input/output

Composition

[wt.%]

Smelt constituents

[kg ton–1 dry solids]

Flue gas constituents

[kg ton–1 d.s.]

Na2CO3 Na2S K2S Na2

SO4 NaCl N2 H2O CO2

O2

Black liquor solids 100%

Potassium, K 1.8% 18

Chlorine. Cl 0.5% 5

Sulphur. S 4.6% 37 7 2

Sodium. Na 19.6% 137 53 3 3

Carbon. C 35.8% 36 322

Hydrogen. H 3.6% 36

Oxygen. O 34.1% 143 4 288 859 –953

Smelt total 316 89 25 9 8

Air 100.0%

Nitrogen. N 75.6% 3.765

Oxygen. O 23.0% 1.144

Humidity 1.4% 70

Water and steam

Water in black liquor 333

Soot blowing steam 100

Flue gas total 3.765 827 1.181 191

reduction efficiency, 0.95. 39 = 37 kg sulfur are with Na2S, and the remaining

2 kg are with Na2SO4. Next comes sodium, with 37. (2. 23)/32 = 53 kg bound to

Na2S, 2. (2. 23)/32 = 3 kg in Na2SO4 and 5. 23/35.5 = 3 kg in NaCl. The

remaining sodium is converted to sodium carbonate: 196 – 53 – 3 – 3 = 137 kg.

Na2CO3 binds 137. 12/(2. 23) = 36 kg carbon. The rest of the carbon is oxidized

to CO2. Hydrogen from the black liquor is converted to water vapor. Finally, the

oxygen demand can be calculated by summing up oxygen bound in carbonate,

sulfate, water vapor and carbon dioxide:

137. (3. 16)/(2. 23) + 2. (4. 16)/32 + 36. 16/(2. 1) + 322. (2. 16)/

12 = 1294 kg.

9 Recovery

As the black liquor solids contain just 341 kg of oxygen per ton, 953 kg must be

provided with combustion air. Assuming 20% excess air, the oxygen in air is

1.2. 953 = 1144 kg. Nitrogen and humidity follow from the air composition. For

calculating the total flue gas flow, we need to consider the water content of the

black liquor and the steam used for sootblowing. Supposing 75% black liquor solids,

the water coming with 1 ton of solids is 1000/0.75 – 1000 = 333 kg. The final

total is about 450 kg of smelt and 6000 kg of wet flue gas per ton of dry liquor

solids. The flue gas mass is equivalent to a volume of around 4800 standard cubic

meters. Note that the above is a quite rough approach to the boiler mass balance,

as minor streams are neglected, such as dust, sulfur dioxide, reduced sulfur compounds

(TRS), carbon monoxide and nitrogen oxides (NOx) in the flue gas, as well

as other inorganic matter and unburned carbon in the smelt.

9.2.3.1.3 Energy Balance

Once the material balance of the recovery boiler has been calculated, a rough energy

balance is easily obtained (see Tab. 9.4). At first, the enthalpies of input and

output streams to the boiler are listed. Output streams have negative enthalpies.

The reaction enthalpy is then calculated from the higher heating value (HHV) of

the black liquor solids. Since the major part of the sulfur leaves the boiler in a

reduced state, the corresponding energies of reduction must be subtracted from

the HHV. The energy available for steam generation results from summing up all

the stream and reaction enthalpies. In our example, the heat to steam amounts to

9.9 GJ t–1 black liquor solids. We assume a feedwater of 120 °C and 95 bar, as well

as high-pressure steam of 480 °C and 80 bar. Then, the gross amount of steam

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