Wet combustion of waste liquors

Abstract

Process are provided for the web combustion of waste liquors in which the waste liquor is reacted with oxygenating gas in a reaction vessel at a temperature between 450.degree. F. and 705.degree. F. at superatmospheric pressure, characterized in that superheated water condensed from the exit gaseous stream from the reaction vessel is added to the waste liquor prior to its entry to the reaction vessel.

Description

This invention relates to improvements in the wet combustion of waste liquors containing combustible organic materials, and refers especially to improvements which facilitate the wet combustion of black liquor obtained from the soda process for the pulping of wood, but the invention can be employed with advantage for other waste liquors which in terms of their nature, composition and concentration give rise to operational problems generally similar in nature to those overcome by the application of this invention to the wet combustion of soda process black liquor.

An object of the invention, hereinafter described, is to avoid the necessity to employ the known types of apparatus for indirect heat exchange in the wet combustion process, such apparatus being normally essential for preheating of the waste liquor and air mixture required for the process, and by avoiding the use of such apparatus, eliminating inefficiencies caused by fouling or scaling of heat exchange surfaces caused primarily by the nature of adventitious impurities present in typical waste liquors and also waste material itself. Applicant's invention in addition to achieving this object also achieves several concomitant advantages hereinafter described and these are reduced consumption of water, reduction in power required for pumping and improved precision of temperature control.

In the known method of applying the wet combustion process, the waste liquor which contained combustible organic material in aqueous solution or fine dispersion and also may contain in addition inorganic substances in solution, is mixed with an oxygenating gas such as air under superatmospheric pressure. In this specification and in the claims the term "air" is taken to include oxygen, oxygen-enriched air and other oxygenating gases, since it is only the element oxygen which is requisite for the operation of the process.

The aim of the process is to cause a reaction to occur between the said oxygen and the organic materials of the waste liquor whereby the latter are substantially or completely destroyed, being converted finally to carbon dioxide gas and water by oxidation-type reactions substantially identical to those of normal combustion.

To achieve the aforesaid reaction it is required that the mixture of waste liquor and air be preheated to a predetermined temperature, hereinafter referred to as the preheat temperature, at which temperature the reaction will proceed at a technically significant rate and become self-sustaining as a result of the exothermic heat produced by the oxidation reactions.

When the reaction has been initiated by preheating the reactant mixture to the desired preheat temperature, the mixture is transferred to a suitable pressure vessel, hereinafter referred to as a reactor, where the reactions are permitted to continue to the desired degree of completion. For the purposes of the description which follows, the process is regarded as being one in which the reactant mixture is passed continuously through the system from inlet to outlet, but such description does not exclude operation in a batch or semi-continuous mode for which the applicant's invention may also be used with advantage.

The temperature of the mixture of waste liquor and air within the reactor is normally substantially greater than the temperature of the preheated mixture since once the reaction is initiated the exothermic heat liberated from such reaction raises the temperature proportionately and thereby further increases the rate of reaction in accord with known laws of chemical reaction.

In typical cases a desirable preheat temperature would be in the range 230.degree. to 350.degree.F and a desirable reaction temperature within the reactor would be in the range 550.degree. to 650.degree.F althougn these typical ranges are not exclusive of other temperature ranges in particular circumstances provided that no temperature at any point within the system exceeds the critical temperature of water, namely 705.4.degree.F.

The use of high reaction temperatures of the order of 600.degree.F is requisite to ensure high degrees of oxidation in given reaction time determined by the permitted time of residence of the mixture within the reactor and governed in practice by the need to employ a reactor having technically feasible dimensions in relation to the desired throughput.

The establishment, maintenance and accurate control of the temperature within the reactor is of utmost importance to the operation of the system. It is known that this temperature, when thermal equilibrium has been established, depends on the preheat temperature and on the quantity of organics (i.e., organic substances) in relation to the quantity of water or water vapour within the reactor. The exothermic heat of reaction is transferred quantitatively, excepting for adventitious losses of heat by radiation and conduction, to the aqueous phase which in turn is partly converted to water vapour, the proportion of water vapour to water in the liquid phase being dependent on the temperature and pressure extant in the system and calculable from known thermodynamic relationships. If the quantity of water in relation to organics is excessive the reactor temperature cannot rise to the desired value as the exothermic heat available will be insufficient in relation to the quantity of water. At the other extreme a high concentration of organics and relatively small quantity of water may result in the situation that substantially all water is vapourised as the temperature rises close to the critical temperatures of water. In practice the concentration of organics in the aqueous phase has to be initially adjusted such that the desired reaction temperature is attained while maintaining sufficient water in the reactor at all times to keep the organic substances in true solution or fine dispersion and in particular to keep desired or adventitious inorganic residues in true solution.

When, for a given waste liquor, a desirable initial concentration has been thus established to achieve a satisfactory reaction temperature and thereby a desired degree of oxidation and reaction rate, the waste liquor, together with the requisite quantity of air, has to be preheated to the calculated preheat temperature for the purposes of initiating the reaction. The sum total of the heat added as preheat plus the heat evolved by the exothermic reaction thus controls the maximum reactor temperature attained within the reactor subject only to adventitious heat losses.

In the pre-existing art the water liquor may be concentrated by known means, such as evaporation, or alternatively diluted with water from external sources to obtain the desired initial concentration. The waste liquor suitably adjusted in concentration is then mixed under superatmospheric pressure with the requisite quantity of air and the admixture passed through a suitable indirect heating apparatus such as a shell and tube heat exchanger to obtain the desired preheat temperature prior to entering the reactor. Alternatively the water, waste liquor and air may be separately heated by indirect heat exchange and then mixed in the required proportions. The heat required for preheating may be supplied from external sources or recuperated from the exit liquid or gaseous streams leaving the reactor since these are at a suitably high temperature to exchange their heat to the incoming mixture.

Applicant has found that the known methods of indirect preheating of the incoming mixture of air and waste liquor or alternatively the waste liquor by itself have serious practical and economic disadvantages because the heat exchange surfaces foul or scale readily due to factors inherent in the nature of many waste liquors and in particular caused by adventitious inorganic impurities frequently present in such liquors. Such fouling or scaling of the heat exchange surfaces limits the efficient transfer of heat after short periods of operation and applicant has found it may eventually result in a complete blockage of the waste liquor side of the heat exchanger. Applicant has found further that the scales causing the fouling may be very difficult to remove by physical or chemical means and may cause the process in terms of the known art to be inoperable in practice for many waste liquors. In the case of soda process black liquor applicant has found that the scales are variable in chemical composition but consist essentially of calcium, magnesium, sodium, alumina and silica together with carbonate anions derived from impurities in the waste soda process liquor. Such scales derived from soda process waste liquor greatly limit the efficiency of the prior art of wet combustion for use with such liquors and thereby limit the utilisation of the process for these and similar liquors.

The fact that this nature of scale has been established for soda process waste liquor should not, for the purposes of the invention hereinafter described, be taken to exclude scales of other natures derived either from organic or inorganic constituents of waste liquors in general and which may be similarly disadvantageous. Further, applicant has also established that scales may form from substances in natural waters used to dilute waste liquor whether or not scale forming substances are present in the waste liquor before dilution. Such scales formed from substances in natural waters are generally of the inorganic type described above and may be compared with those known to form in steam boiler tubes when impure water is used. Applicant has also found that where the indirect heat exchange is arranged to derive its heat source from the hot exit liquor after reaction (known hereinafter as oxidised liquor) such liquor has severe fouling and scaling properties since the inorganic impurities orginally present in the waste liquor remain substantially unchanged but become more concentrated in the exit oxidised liquor from the reactor. As a result the fouling problem may occur on both sides of indirect heat exchangers giving rise to a further limitation of the present art which it is desired to recuperate heat from that available after reaction and which procedure is otherwise economically advantageous.

Applicant has discovered that the above disadvantages arising from indirect heat exchange for preheating in association with the complex and variable nature of waste liquors and dilution water, may be substantially avoided by a novel method whereby direct exchange preheating and the requisite dilution of waste liquor may be achieved simultaneously by utilising superheated pure water obtained from within the reaction system itself.

In the novel method of the applicant the waste liquor is preferably not prediluted with water from external sources but if required may be preconcentrated by conventional means. Applicant's invention avoids the use of indirect heat exchangers for the purposes of preheating the admixture of waste liquor and air by providing a condensing surface in the exit gaseous stream from the reactor and other ancillary devices. The exit gaseous stream from the reactor normally consists substantially of carbon dioxide, water vapour and also nitrogen in the case where the oxygenating gas used is air. The water vapour is derived from the water in the reactor and in quantity is substantially that which is required to saturate the non-condensible gases in the exit stream at the equilibrium temperature and pressure of the reactor. The water vapour to gas weight ratio in the exit gaseous stream depends on the composition and nature of non-condensible gases and on the equilibrium temperature and pressure in the gaseous phase.

When such water-saturated gaseous stream is allowed to impinge on a cooler surface water vapour condenses as a consequence of the reduced temperature and such water may be then separated from the gaseous stream by known means such as suitable traps or separation apparatus.

By suitable adjustment of the temperature at which condensation is permitted to occur and with the system pressure being maintained substantially constant the proportion of water vapour condensed can be controlled and furthermore the temperature of such condensed water can be arranged to be substantially higher than the desired preheat temperature but necessarily lower than the maximum reaction temperature.

In the method of the applicant's invention a suitable condensing surface indirectly cooled by external water, external air or other external cooling fluid is arranged in the exit gaseous stream from the reactor. Condensation is permitted to occur such that the temperature of the condensed water phase is substantially greater than the desired preheat temperature. The condensed water is collected by suitable separation apparatus and transferred and if necessary raised in pressure by means of a suitable pump or other device in order that the requisite proportion of this condensed water may be directly injected into and admixed with the inlet stream of waste liquor and air prior to admission of the mixture into the reactor. In this way applicant's invention provides both the necessary preheat and simultaneously provides necessary dilution to the waste liquor/air mixture thus avoiding any necessity for indirect heat exchange and overcoming the limitations of the prior art as previously described.

Applicant's method has several evident concomitant advantages. The condensed water obtained as above for dilution is substantially free of dissolved solids and unlike natural waters cannot contribute further adventitious impurities to the waste liquor stream as may occur with the use of natural waters for dilution. This is of advantage in minimizing scaling tendencies later in the system since the quantity of scale-forming impurities in the oxidised liquor is maintained at the minimum level and cannot exceed the quantity orginally introduced by the waste liquor itself before dilution.

Another advantage is the reduction in power required to pump liquor into the system which necessarily operates at superatmospheric pressures frequently of the order of 200 atmospheres. If for the purposes of description it be assumed that the initial concentrated waste liquor must be diluted at some stage with water in the ratio of one part of waste liquor to one part of water, which is typical of the case of soda process black liquor, then it is evident that if pre-dilution is not employed, as in the applicant's method, the volume of liquor to be raised to the superatmospheric system pressure is reduced by half thereby approximately halving the power used in pumping. In the applicant's invention the dilution water obtained by condensation is approximately at the system pressure and is retained at such pressure by a suitable interim or buffer storage device. In order to pass it into the stream of waste liquor and air the suitable pump has only to overcome the small pressure difference between the inlet and outlet points of the reactor, such pressure difference being only the sum of the hydrostatic head of the reactor plus pipeline and reactor friction losses, and such pressure difference arising from these causes may be typically from one to eight atmospheres.

It is also obvious that many factors in the operation of the applicant's method are controlled by highly predictable and calculable thermodynamic relationships and that the transfer of heat at all points is direct and highly quantitative and unlimited by external influences. Therefore, by its nature applicant's invention offers more precise control of temperature and dilution in the preheating stage than previously possible. Applicant's method is therefore highly amenable to automatic control.

The invention will be further described in the following, taken with the accompanying drawing, in which

FIG. 1 is a simplified flow sheet of the process of the invention, and

FIG. 2 is a diagrammatic representation of a system of apparatus adaptable for use in carrying out the process of the invention.

In the applicant's invention the following equations may be used to describe the balances of heat and materials on a general theoretical basis and without consideration of minor corrections arising from adventitious losses of heat by radiation or conduction or from minor endothermic chemical reactions which may proceed within the system alongside the predominant characteristic exothermic oxidation reactions.

The basic parameters of the system are expressed by the following algebraic symbols and when substituted by numeric values must be in a consistent system of units. For clarity of expression the units are shown below in the British system but any consistent system may be used.

When,

F.sub.S = flow rate of combustible (oxidisable) organic solids in waste liquor expressed as pounds per hour and,

F.sub.1 = flow rate of water in the initial undiluted waste liquor expressed as pounds per hour and,

F.sub.2 = flow rate of water derived from waste liquor as condensate within the process expressed as pounds per hour and,

F.sub.3 = flow rate of water in waste liquor after dilution with condensate represented by F.sub.2 and expressed as pounds per hour and,

T.sub.1 = temperature of undiluted waste liquor expressed in degrees Fahrenheit and,

T.sub.2 = temperature of condensate water derived within the process from waste liquor and expressed in degrees Fahrenheit and,

T.sub.3 = temperature of waste liquor after dilution with condensate derived from within the process and expressed in degrees Fahrenheit and,

c.sub.p = specific heat of solids in waste liquor expressed as British Thermal Units per pound per degree Fahrenheit and,

H.sub.1 = enthalpy of water present in the initial undiluted waste liquor at the temperature (T.sub.1) of such liquor and expressed in British Thermal Units per pound and,

H.sub.2 = enthalpy of condensate water at temperature T.sub.2 derived within the process from waste liquor and expressed in British Thermal Units per pound and,

H.sub.3 = enthalpy of water at temperature T.sub.3 in waste liquor after dilution with condensate derived within the process and expressed in British Thermal Units per pouhd

then, with reference to FIG. 1 of the accompanying drawings, the following equations apply.

and therefore,

above equation (1) defines the water balance and can be used to determine the required flow rate of condensate.

Also,

and therefore,

and substituting for F.sub.2 by equation (1) then,

whereby, ##EQU1##

Above equation (3) can be used to determine condensate temperature when F.sub.1 H.sub.1 T.sub.1, F.sub.S and c.sub.p are predetermined by waste liquor feed conditions and F.sub.3, H.sub.3 and T.sub.3 are predetermined by maximum desired or allowable reaction temperature and maximum desired or allowable concentration of solids in solution in the reactor at the maximum or any reaction temperature.

A practical example of the embodiment of the applicant's invention in a reaction system for the wet combustion of waste liquor derived from the soda process of pulping wood is now described with reference to FIG. 2 of the accompanying drawings. Waste liquor from the soda process of pulping, hereinafter called black liquor, was processed in a suitable wet combustion system the equipment components of which were arranged in accord with the flow sheet shown in FIG. 2 and this arrangement permitted the wet combustion process to be operated on a continuous flow basis.

The composition of the initial black liquor before processing and ignoring minor constituents and expressed on the basis of actual quantity of black liquor constituents per hour of operation was 156,561 pounds of water (per hour) 34,569 pounds of total dissolved solids (per hour) and such 34,569 pounds of solids included 10,897 pounds of sodium in combined form but calculated and expressed as the equivalent weight of sodium hydroxide. The initial supply temperature of the black liquor to the process was 181.degree.F.

The said black liquor was pressurized to just greater than 3,050 pounds per square inch gauge pressure by means of Pump 1 and thereby introduced into the reaction system which was maintained by suitable devices at a controlled average gauge pressure of 3,000 pounds per square inch. The system pressure ranged from 3,050 pounds per square inch at the discharge of Pump 1 to 2,850 pounds per square inch at the high pressure side of the system outlet valves that is: 8, 13, 14. This pressure difference from system inlet to outlet was due to factors such as pipe friction and hydrostatic head in the Reactor 5.

At `T`-junction 2 air at the rate of 151,915 pounds per hour on the dry basis was injected into the black liquor such air being compressed to a suitable desired pressure of 3,050 pounds per square inch gauge and also being at a temperature of 460.degree.F. The high temperature of such air was acquired as a result of partial adiabatic air compression but such temperature of the air is incidental and not relevant to the applicant's invention except that it must be known to compute the thermal balance in conjunction with other given conditions.

At `T`-junction 3 situated close to `T`-junction 2 and prior to the inlet point 4 of Reactor 5 water condensate obtained from within the process by arrangements hereinafter described was injected into the mixture of black liquor and air at the rate of 108,150 pounds of water per hour and the temperature of this water condensate was 460.degree.F. The injection of this condensate into the black liquor-air mixture was achieved by means of Pump 12 which was a suitable device to raise the pressure of the condensate from 2,900 pounds per square inch to 3,050 pounds per square inch. This pressure difference substantially represents small pressure losses through the system attributable to pipe friction and related factors previously mentioned.

As a result of the heat derived by direct contact heat exchange from the preheated air (which was proportionately very small) and in particular the large amount of heat derived by direct contact heat exchange from the water condensate added at `T`-junction 3 the temperature of the total admixture was raised to 310.degree.F. Simultaneously the proportion of water condensate introduced at `T`-junction 3 reduced the concentration of total solids in the liquid phase of the mixture from 2.05 pounds per gallon of liquor existing at the inlet of Pump 1 to 1.254 pounds per gallon of liquid phase after `T`-junction 3 and prior to the inlet point 4 of Reactor 5.

The desired temperature of 310.degree.F was thus achieved in the mixture of air, water condensate and black liquor and the desired concentration of total solids at 1.254 pounds per gallon of liquid phase simultaneously achieved these conditions having been found to be requisite in the case of soda process black liquor to maintain a satisfactory rate of reaction and to maintain all solids in solution when the Reactor 5 was operated at a maximum temperature of 608.degree.F.

At the top of Reactor 5 a suitable Separator 7 was incorporated whereby the steam and non-condensible gas phases were separated continuously from the liquid phase. At the top of Reactor 5 where the temperature was substantially 608.degree.F the proportion of steam to non-condensible gas was substantially greater than the proportion of steam to non-condensible gas existing at the lower temperature of 310.degree.F at the entrance point 4 of Reactor 5 or at any temperature intermediate between 310.degree.F and 608.degree.F, such intermediate temperatures representing the rise in temperature occuring as a result of exothermic reactions during the passage of the reactant mixture from the entrance 4 to the exit 9 of Reactor 5. The steam thus generated within the reactor was substantially free of dissolved solids and demonstrated to form substantially pure water after condensation.

The liquid phase or oxidisable liquor collected at the base of Separator 7 was discharged continuously via a suitable liquid discharge Valve 8 suitably controlled to maintain constant level in Separator 7.

The steam and non-condensible gas phase obtained at the top of Separator 7 consisted of 180,165 pounds of steam (per hour) 152,682 pounds (per hour) of non-condensible gas consisting substantially of nitrogen, carbon dioxide and a small proportion of oxygen, together with 7,634 pounds (per hour) of entrained water, such water being the result of both physically incomplete separation and a small reduction in temperature of the steam/gaseous phase caused by unavoidable thermal losses. At point of discharge 9 of the steam/gaseous phase from the Reactor 5 the temperature was 606.degree.F.

The mixture of steam and non-condensible gas from Reactor 5 was passed through the tubes of a suitable shell and tube Heat Exchanger 10 whereby its temperature was reduced to 470.degree.F thereby condensing 162,683 pounds (per hour) of steam to produce the same weight of water condensate at a temperature of 470.degree.F. The condensation was effected by indirect heat exchange to water at about 460.degree.F or less which was passed at a suitable rate through the shell side of Heat Exchanger 10.

The partially condensed mixture of non-condensible gas, steam and water was then passed to a further Separator 11 in which substantially all the water condensate was separated from the balance of the steam and gaseous phase. The water condensate so separated was withdrawn continuously from the base of Separator 11 at a rate of 108,150 pounds of water per hour at a temperature of 470.degree.F and at a pressure of 2,900 pounds per square inch gauge. By means of Pump 12 this water condensate was elevated in pressure from 2,900 pounds per square inch gauge to 3,050 pounds per square inch gauge and injected at `T`-junction 3 into the mixture of air and initial black liquor. The temperature of the water condensate immediately prior to the point of injection was 460.degree.F the reduction in temperature of 10.degree.F in the temperature of the water condensate being due to adventitious heat loss. The balance of 54,533 pounds (per hour) of water condensate not utilised was passed to waste via outlet valve 14 but it will be obvious to those skilled in the art that more or less condensate can be used in the manner described to vary the preheat temperature and concentration of the mixture prior to admission to Reactor 5 and in a precise manner and which is desirable for the purposes of process control.

In this manner and by the use of the applicant's invention as described the required preheat temperature of 310.degree.F and the required concentration of 1.254 pounds per gallon of total solids in the liquid phase was achieved at the point of entrance of Reactor 5 without the use of other devices such as indirect heat exchangers subject to fouling or scaling for example with soda process black liquor.

Claims

I claim:

1. A continuous process for the wet combustion of a waste liquor containing combustible constituents in which the waste liquor is reacted with oxygenating gas in a reaction space at a temperature between 450.degree. F. and 705.degree. F. at superatmospheric pressure, which comprises

a. establishing a stream of waste liquor and heated oxygenating gas;

b. passing said stream into and through a reaction space to produce an exit stream consisting of steam, non-condensible gas and liquid phase wherein the proportion of steam to non-condensible gas in the exit gaseous stream from the reaction space is substantially greater than in the waste liquor inlet stream at the inlet to the reaction space, the steam thus generated within the reaction space being substantially free of dissolved solids;

c. passing said exit stream, from the reaction space, through a separator to separate the steam and non-condensible gas from the liquor phase;

d. passing the steam and non-condensible gas from the separator through a condenser in which superheated water is condensed at a superatmospheric pressure slightly less than the pressure existing at the inlet of said reaction space;

e. raising the pressure of this superheated water to at least the pressure in the reaction space; and

f. injecting the superheated water under pressure into the stream of waste liquor and heated oxygenating gas at step (a) prior to the entry of said stream into the reaction space,

the temperature of the injected pressurized superheated water being higher than the temperature of the undiluted waste liquor in said stream but lower than the maximum reaction temperature in the reaction space and being sufficient to raise the temperature of the stream of oxygenating gas and diluted waste liquor to not less than 230.degree. F. and simultaneously to dilute the waste liquor to a predetermined extent.

2. A process according to claim 1, wherein the temperature of the injected pressurized superheated water is between 450.degree. F. and 650.degree. F.

3. A process according to claim 1 wherein the exit gaseous stream from the reaction space is passed through a separator to separate the steam and non-condensible gas from the liquid phase, the steam and non-condensible gas are passed through a heat exchanger where water is condensed, the partially condensed mixture of non-condensible gas, steam and water condensate is passed to a second separator where the water condensate is separated from the balance of the steam and gaseous phase, and the water condensate is raised in pressure and injected into the waste liquor and oxygenating gas stream prior to its entry to the reaction space.