Hiςrh Solids Black Liquor Of Reduced Viaeoaity and Viaeoaity Reduction Method For Hiςrh Solida Black Liquor

BACKGROUND OF THE INVENTION

The sulfate or kraft process is the dominant

alkaline pulping technique used in the pulp and paper

industry. The spent liquor resulting from the pulping

process is commonly known as black liquor. It contains

various organic materials as well as the inorganic

pulping chemicals in an aqueous medium. The typical

contents of black liquor are: dissolved wood substances

(polymeric lignin and various aromatic organic

compounds) ; carbohydrate derivatives (cellulose and

hemicellulose degradation products) ; extracted light organic compounds (fatty and resinous acids) ; and

inorganic chemicals from cooking liquor (sodium

hydroxide, sodium sulphide, sodium carbonate, sodium

sulphite and sodium chloride) . The black liquor is concentrated by evaporation and fired in a boiler.

It is well established that the regeneration of

the black liquor to fresh white liquor is an economically and environmentally necessary part of the kraft process.

The recovery of the pulping chemicals is obtained by

incinerating the black liquor in a chemical recovery

boiler. The chemical recovery boiler is perhaps the single most important unit in a kraft process. In the

boiler, the organic matter in the black liquor is

combusted hence generating process heat in the form of

steam. Meanwhile, an inorganic smelt is also produced,

recovered from the boiler and dissolved to yield what is

known as green liquor. The green liquor is then treated

by causticizing it with slaked lime. The resultant solution of fresh pulping chemicals also known as white

liquor can be used again in the digester unit. Maximum

recovery of heat and pulping chemicals and the minimum

generation of polluting wastes are crucial to the overall pulping process.

It is well established that energy saving,

enhancement of the chemical recovery boiler throughput

capacity, and abatement of pollution, notably S0 ₂ and H ₂ S

flue gas emissions, are improved by incinerating a more

concentrated black liquor in the recovery boiler. Such

concentrated black liquors are known as high solids

content black liquor. Their concentration is usually

expressed as a weight percentage of solids content. To

illustrate the importance of higher solids concentration

in black liquor, it has been shown that if the solids

content were raised from 60% to 80% before firing, the

energy saving would be about 760xl0 ⁹ J/day for a typical

1000 ton/day mill. Consequently, there is a continuous

trend toward firing black liquor at higher solids

concentrations in the recovery boiler.

However, the single most important drawback to

firing high solids content black liquor relates to its viscosity which dramatically increases with its solids

content. The more the black liquor is concentrated in

evaporators and the like, the more it tends to cause plugging of the evaporators, concentrators, transport

lines, and the boiler firing nozzles. Consequently,

although it is possible to currently obtain solids

content of about 85 %, black liquor solids content is

generally maintained at approximately 60 - 65 % to avoid

plugging and fouling of equipment .

Various attempts have been made to modify the

evaporators, transport lines and nozzles to permit the

handling of higher solids content black liquor. However,

these attempts have gained little acceptance in view of

the substantial capital investments and technical

difficulties.

To avoid large capital expenses and equipment

modifications, viscosity reducing additives to the black

liquor have been proposed. Notably, US Patent 4,776,889

teaches the addition of waxes as viscosity reducing

agents and US Patent 4,734,103 teaches the addition of a

water soluble terpolymer composition. Japanese Patent

59228094 teaches the addition of high molecular weight

poly-ether diols made from mixtures of polypropylene

glycol and polyether oxide or propylene oxide. However,

these techniques have also met with limited market

acceptance.

Consequently, there remains an important need

for the discovery of improved viscosity reducing

additives.

It is therefore an object of the present

invention to provide a black liquor comprising a

viscosity reducing additive which will allow the firing

of black liquors having solids content higher than 65 %

wt .

It is another object of the present invention

to provide a viscosity reducing additive which will

improve the overall economic feasibility, tend to

diminish the obnoxious wastes, and tend to improve the

firing characteristics of the chemical recovery boiler

operation.

Other objects and further scope of

applicability of the present invention will become

apparent from the detailed description given hereinafter.

It should be understood, however, that this detailed

description, while indicating preferred embodiments of

the invention, is given by way of illustration oniy,

since various changes and modifications within the spirit

and scope of the invention will become apparent to those

skilled in the art.

STIMMARY OF THE INVENTION

The invention provides a high solids content

black liquor having the property of reduced viscosity

comprising:

a high solids black liquor which has a solids content of

at least 65 % by weight of liquor

and a viscosity reducing additive comprising a compound

selected from monoethylene glycol, diethylene glycol,

triethylene glycol and mixtures thereof.

One embodiment of the invention provides a

method for reducing the viscosity of high solids content

black liquor comprising treating the black liquor with

about 0.01 to 10 % by weight, and preferably 0.2 to 1 %

by weight, of a viscosity reducing additive selected from

the compounds comprising monoethylene glycol, diethylene glycol, triethylene glycol and mixtures thereof.

Thus, the additive formulation of the present

invention controls and reduces the viscosity of the high

solids content black liquor, thus improving processing,

handling and firing performances of black liquor during

the recovery process.

Other features and advantages of the invention

will become apparent to those of ordinary skill in the

art upon review of the following detailed description,

claims, and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a preferred

embodiment of a system suitable for carrying out the

present invention;

FIG. 2 is a schematic representation of a conventional

kraft pulping chemical recovery system which is shown incorporating the preferred embodiment of the present

invention shown in FIG. 1.

DETATLED DESCRIPTION OF THE INVENTION

As stated in the summary of the invention, the invention comprises providing a modified black liquor

having solids content of at least 50 % and a viscosity

reducing additive selected from monoethylene glycol,

diethyelene glycol, triethylene glycol or mixtures

thereof. These compounds are of course well known as

antifreeze-coolants, brake fluids, paint solvents,

plasticizers, and so on. However, their use as viscosity

reducing additives for alkaline pulping black liquors is

clearly novel.

The chemical representations for mono-, di- and tri-ethylene glycol are as follows:

CH, -OH CH, -OH CH, -O-CH.CH, -OH

I I CH, -OH CH, -O-CH.CH, -OH CH, -O-CH.CH, -OH

mono- di- tri-

When synthesized, mono-, di-, and tri- ethylene glycols are usually separated in distillation towers to

yield separate mono-, di- and tri-ethylene fractions. Due to their higher molecular weights, di- and tri-

ethylene fractions are found in the distillation bottoms.

In accordance with a preferred embodiment of the present

invention, it will be preferable to use the blend of

ethylene glycol fractions which is least expensive on the

open marke . This will tend to maximize the economic

efficiency of the chemical recovery boiler used to burn

the modified black liquor. Also in accordance with a

preferred embodiment of the invention, the amount of ethylene glycol additive is about 0.2 to 0.5 % by weight

of the resulting black liquor. However, concentrations

ranging from about 0.01 to 10 % are acceptable. In any

event, the exact proportion of ethylene glycol additive is not crucial to the present invention. It is surmised

that the proper proportions will depend on the exact

composition of the black liquor to be treated and the

economic feasibility of the overall chemical recovery process. Optimum proportions of ethylene glycol

additives can therefore be easily determined by those

skilled in the art.

Referring now to FIG. 1, there will be

described a preferred embodiment of the present

invention. A mobile storage tank 10 contains a black

liquor additive comprising mono-, di- or tri-ethylene

glycols or blends thereof. The additive composition is

pumped to a heat exchanger 12 via transport line 14 and

pump 16. Heat exchanger 12 heats the additive

composition to approximately 175 to 300°F. The warm

additive composition is then piped to a valve 18 which

introduces the additive to the black liquor.

FIG. 2 schematically illustrates where the

introduction of the warm additive could take place. In

this case, the introduction of the additive is made in to

the black liquor line 20 downstream of the pulp washer

unit 22. The modified black liquor then goes through a

multistage evaporator/concentrator unit 24 to yield a

high solids content black liquor. In a most preferred

embodiment, the concentrated black liquor will have a

solids content of about 80 % wt. The concentrated black

liquor is then fired in the recovery boiler 26 through a

suitable nozzle (not shown) . It is to be understood that

the introduction of the viscosity controlling and

reducing additive can take place anywhere in the black

liquor line between the pulp washer unit 22 and the

recovery boiler 26. However, it appears preferable to

add the mono-, di-, and tri- ethylene glycols early on,

i.e. in unit 24, after the multiple effect evaporators

but before the concentrator, since the black liquor

viscosity reduction increases the process capacity by

providing good heat transfer in concentrators, good

pumping in the transfer lines and good nozzle spraying in

the chemical recovery boiler.

As stated before, the addition of mono-, di-,

and tri- ethylene glycols sufficiently controls the

viscosity of the black liquor to render it easy to

handle, even with solids content greater than 65 % wt,

without plugging and fouling of fluid transport

equipment, evaporators and the like and firing nozzles.

Moreover, it is apparent that the implementation of the

present invention does not require large capital

investments and will favorably improve the overall

economy, capacity and pollution control of the chemical recovery process.

EXPERIMENTAL

For illustrative purposes only, the invention

will now be described in conjunction with the following

example.

Example 1

Viscosity measurements were performed on a 62

% wt solids content black liquor separately treated with

three different viscosity lowering agents. Viscosity was

measured at 80°C using a Brookfield type viscometer. As

in well known to those skilled in the art, the viscometer

measures viscosity by measuring the force required to rotate a spindle in the fluid to be tested. For

reference purposes, the viscosity of the untreated black

liquor was also measured under the same conditions. The

results are compiled in Table 1, below. The three illustrative viscosity lowering agents were as follows:

Viscosity lowering agent A

Monoethylene glycol

Viscosity lowering agent B

Diethylene glycol

Viscosity lowering agent C

Mixture of monoethylene glycol (5% wt) ,

diethylene glycol (70% wt) and triethylene

glycol (25% wt) . This mixture is a typical

by-product of ethylene glycol production by

distillation.

TABLE 1

Concentration 0 0.2 0.5 of Agent, % wt

(Brookfield Viscosity in Cps)

Agent A 1150 1000 900

Agent B 1150 800 600

Agent C 1150 700 500

Although the invention has been described above with respect with one specific form, it will be evident to a person skilled in the art that it may be modified and refined in various ways. It is therefore wished to have it understood that the present invention should not be limited in scope, except by the terms of the following claims.