Reactant Ratio Control Process

Abstract

The ratio of the reactants fed to a continuous exothermic or endothermic reaction is controlled by measuring the thermodynamic effect caused in a second reaction zone by the addition of a small additional amount of any one reactant to the reaction products. The amount of any reaction, and subsequent temperature changes, in this second reaction zone corresponds to the deficiency of the added reactant in the initial reaction zone. The difference between the temperature of the material in this reaction zone before and after the reactant addition is converted to an electrical signal supplied to a controller which regulates the rate of flow of the added reactant to the main reaction zone.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the art to which the claimed invention pertains is the control of the ratio of reactants in a continuous reaction. More specifically, the invention provides a means for the continuous control of the ratio of a fuel to an oxygen containing gas in a combustion process. In a specific application the ratio of a fuel gas to air in a premixture to a burner is controlled to maximize the flame temperature and the efficiency of the combustion process.

2. Description of the Prior Art

The prior art in the determination of premixture compositions has called for both primary flame temperature measurement and optical observation of a secondary flame produced by the addition of air to the reaction products.

The primary flame temperature is used in the art as a guide to the efficiency of the combustion. This flame temperature is dependent on the composition of fuel being burned and on whether or not the ratio of air to fuel is at an optimum. If air is in an excess, this excess must be heated resulting in the waste of available energy and the lowering of the flame temperature. If the fuel is in excess, incomplete combustion occurs resulting in an inefficient operation, lower flame temperature, a reduction in the available heat, and wasted fuel. Combustion products normally proceed directly to an exhaust system and incomplete combustion would therefore produce an air pollution problem unless the gases are properly treated.

The prior art, as exemplified by U. S. Pat. No. 2,603,085, teaches the monitoring of the primary flame temperature as a guide to the efficiency of the combustion. If a deviation from the normal temperature is noticed the gas to air ratio is adjusted. To determine which reactant is in excess, the hot flue gases are allowed to contact air at a second combustion zone and the presence or absence of a flame at this second zone is taken as an indication of the corresponding presence or absence of excess fuel. The second combustion zone is observed visually during this manual adjustment process until the combustion ratio is again optimized and the flame is not present.

My invention resides in the use of the temperature change caused by the addition of a small amount of one reactant to a reaction zone product stream as an indication of the initial reactant ratio. If the addition of hot air to equally hot flue gas promotes combustion, the heat generated will raise the temperature of the flue gas at this point above that present before the point of air addition. There is no necessity to view the second zone manually. This process allows the continuous optimization of the reactant ratio independently of changes in fuel gas composition or pressure and also makes it possible to locate the control system in places where visual observation is difficult due to high temperatures or incon-venience. This process provides for continuous prevention or reg-ulation of excesses of either fuel gas or air whereas the prior art shows only a point in time optimization.

SUMMARY OF THE INVENTION

The method by which the process of the present invention controls the ratio of reactants fed to a first reaction zone, con-taining either a continuous exothermic or endothermic reaction, is by converting into an electrical signal any temperature change caused by a chemical reaction in a second reaction zone, wherein the reaction products while at reaction supporting conditions are contacted with a small amount of one reactant. This electrical signal is fed to a con-troller which compares the signal to a predetermined optimum value and which in turn adjusts the flow rate of the added reactant to the first reaction zone. The presence of a reaction in the second reaction zone displaces the normal energy balance that is in effect when the desired reactant ratio is present in said first reaction zone.

DESCRIPTION OF THE DRAWING

In the simpler application of the invention shown in FIG. 1 of the drawing, fuel entering by line 1 is mixed with air from line 2 to form a premixture in line 3 which is burned at burner section 4 contained in furnace 5. Combustion products flow upward through flue 6 with a small portion of them passing through flow isolation means 7, over heat transfer means 8, past thermocouple 10, mixing with a small amount of air at nozzle 9 on air line 17 and past thermocouple 11 before leaving the flow isolation means 7. Voltages produced in thermocouples 10 and 11 are carried by means 12 and 13 to controller 14 which sends a signal by means 15 to a control valve 16 on the air line 2. A small line 17 carries an air flow preset by valve 18 and monitored at rate measuring device 19 to the nozzle 9. This embodiment of the invention regulates the amount of air added to the premixture to maintain complete combustion.

A more sophisticated application of the invention shown in FIG. 2 provides a process for regulation of the extent of the combustion by the addition of either air or fuel to the temperature measuring zone. Air flow through line 20 to a combustion zone not shown is controlled by main valve 21 driven in response to a signal carried by means 33 from controller 32. The flue gases from the combustion zone pass through chimney 22 and a small portion enters flow isolation zone 23. This portion is controlled by valve 24, monitored by rate indicator 25, mixed with any reactant added through line 36, temperature equilibrated with a second entering reactant stream by heat transfer means 26, passed over thermocouple 27, mixed with said second reactant stream at nozzle 28 on line 38, passed over thermocouple 29 and discharged from the flow isolation zone 23. Signals from thermocouples 27 and 29 are passed to controller 32 by means 30 and 31 for use in adjusting the main air flow. This flow is common to all variations of this application of the invention.

There are 3 basic variations to this embodiment which consist of; (1) adding air at nozzle 28, (2) adding fuel at nozzle 28, and (3) adding air through line 36 simultaneously with air addition at nozzle 28, within each variation the rate of addition being monitored by the controller. The first variation has a small air flow from line 34 pass into line 35 and through valve 39 to travel by line 38 through rate indicator 40 to nozzle 28. Signals from rate indicator 40 and to valve 39 respectively are carried by means 41 and 42. The purpose of this arrangement, like that of the previous simple embodiment, is to insure that sufficient air is being fed to the combustion zone. The controller 32 adjusts main valve 21 by means 33 according to the temperature change detected.

The second variation allows the detection of an insufficient fuel flow to the combustion zone to perform the optimization using fuel as the added reactant. Fuel flows by line 43 through valve 37, which is controlled through means 44, line 38 and flow indicator 40 to nozzle 28 for mixture with the flue gases. As in the first variation, the controller 32 then adjusts the combustion mixture according to signals from thermocouples 27 and 29.

The third variation permits a controlled amount of air from line 34 to flow through line 36, valve 45 controlled by means 46, flow rate indicator 47 and heat exchange means 48 to contact and complete the combustion of the flue gases in isolation zone 23 prior to the first temperature measurement. Simultaneously, the effect of air addition at nozzle 28 by the method of the first variation is determined by thermocouples 27 and 29. The ratio of flue gas to air added by line 36 is monitored by the controller 32 through means 49 and 50 and adjusted with control valves 24 and 45 controlled through means 51 and 46. The air addition ratio is combined with readings from thermocouples 27 and 29 to enable the controller 32 to adjust main control valve 21 for any desired air deficiency ratio.

The drawing of preferred embodiments of the invention and the description are not intended to place any limitation on the invention and are meant as examples only. Modifications and additions as are obvious to those skilled in the art may be beneficial to the utilization of this invention are included within the scope of this description. Pneumatic, electrical or fluidic control means are applicable for use with this invention, as is any suitable type of valve or other means to control the various gas flows. The controller may be either analog or digital and may receive its signals from thermocouples as suggested or from thermistors, expansion tubes or pyrometers.

DETAILED DESCRIPTION OF THE INVENTION

This method of controlling a reactant ratio is applicable to any reaction that is either exothermic or endothermic and that is suitable for a continuous operation. The reactant ratio will normally be at a preset value established to ensure some desired reactant concentration in the reaction products, which may be zero. The examples given so far have been concerned with the combustion of a fuel gas which could be a natural gas, hydrogen, carbon monoxide or a vaporized liquid fuel. Hydrogenation of a diene or halogenation of an alkane are two other common reactions having a net heat of reaction and which are therefore suitable for application of this method. The regulation of the amount of air fed into a catalytic muffler or CO.sub.2 --CO--O.sub.2 ratios in a fluidized catalytic cracking process are other possible uses. The process of this invention is not restricted to combustion or to a chemical reaction. It is adaptable to any situation in which contacting two fluid streams creates a positive or negative heat generation such as the mixing of a strong acid and water where a standard solution would be used as the added reactant. The place at which the controlled reactant flow is consumed is referred to as the initial, or first, reaction or combustion zone.

The most direct application of this invention is the maintenance of the exact reactant ratio necessary for complete reaction of all material fed to the reaction one. The extent of a reaction can also be controlled by this process by the proper selection of the desired temperature change that occurs in a second reaction zone. The second reaction or combustion zone comprises a zone separated from the primary reaction zone and containing a temperature measuring means located upstream and downstream of the point at which the controlled reactant is added to the primary reaction zone products. If it is desired to maintain a 1:1 CO to CO.sub.2 ratio in a furnace flue gas, a specific positive temperature rise in the second reaction zone would be desired when adding air to the flue gas in this second zone whereas for complete combustion to CO.sub.2 no temperature rise would be desired. Incomplete reactions may be desirable for economic reasons in chemical conversions where there is a decrease in selectivity with increased conversion.

There are two methods in which this process can be operated to provide a less than complete chemical reaction. The first method is to calibrate the amount of temperature change in the second reaction zone caused by the addition of a known amount of additional reactant versus the degree of reaction, and then to set the control device to maintain the temperature relationship corresponding to the desired degree of reaction and preset reactant ratio value. The second method is to add to the reaction products a proportioned amount of the missing reactant before the first temperature measurement and to set the control device to maintain no temperature change in the second reaction zone. In this second method, this first amount of the reactant added must be regulated to be that which will bring the reaction products to be totally reacted before they enter the two temperature measurement zones. This amount may be determined most easily by the control process itself by having the flow of this first added reactant stream regulated by a flow control valve which is operated by the controller in response to the signals it receives from the two temperature measuring points. This may be described as an internal control loop contained within the control loop for the main reaction zone. The amount of the first controlled flow of reactant required to produce no temperature change in the temperature measurement zone, where an additional amount of the same reactant is added, is a direct measurement of the deficiency of the reactant in the main reaction zone and can therefore be used to regulate this deficiency. Both the first and the second method require that the rate of flow of reaction products into the control zone is known.

In those cases where complete reaction of all reactants is desired, it is best to have a separate temperature measurement zone and reactant addition point for each reactant. The process can then easily adjust the flow rates to avoid either an excess or deficiency in any reactant. For example, optimum combustion efficiency occurs when there is no excess of either air or fuel, and at separate points a small amount of fuel and air should be contacted with the flue gas. The complete usage of both available fuel and air in the main burner would be shown by no combustion being present at either point of reactant addition.

This method of control is independent of any changes in fuel gas composition or rate of flow to the main burners. The process is versatile, continuous and automatic. These features are all improvements over the prior art.

As previously mentioned, the optimum control of a reaction in which it is desired that there be no excess of any reactant necessitates a separate temperature indication for each reactant. In the advanced control system of the drawing, only one temperature measurement zone is required if the controller is preset to switch between reactants entering the zone and is properly programmed to interpret the results. The different reactants may be fed to the single zone through a header system with automatic flow control valves.

The amount of reactant added to the secondary reaction zone should be kept small enough to avoid attenuating the thermodynamic effect caused by its addition by increasing the total reactant stream to more than required. The amount of this addition can be either preset manually or determined by the controller. An advantage of the second method is that the controller may be programmed to occasionally shut off the flow of the added reactant and to compare the two temperature readings. In this way the system can rezero itself and check for a malfunction automatically. The additional reactant should be heat exchanged wtih the incoming reaction products prior to the first temperature measurement. This is done to prevent any extraneous thermodynamic effect in the secondary reaction zone caused by an added reactant stream's temperature. The added reactant may be cooler than the flue gas, but the controller would require programming for this arrangement. Complete reaction would be indicated by a temperature drop, but a deficiency of the added reactant in the feed to the main reaction zone would cause a change from this normal temperature difference.

The drawing shows the secondary reaction zones located in the flue of a furnace. It is assumed that at this location the temperature of the flue gas would be sufficient to cause spontaneous ignition of any combustible mixture that occurs with the addition of either more air or fuel. The invention as it relates to this application is not limited to this location. A portion of the flue gas may be diverted into an insulated or heated secondary reaction zone located at quite a distance from the flue. There is no compelling reason to maintain the reaction products at an elevated temperature if this is inconvenient. Rather, a source of ignition such as an electrically heated wire would be sufficient to make the invention operable. The placement of the secondary reaction zones in contact with the hot reactant flow, in this example a flue gas, does have the advantage of maintaining an even temperature distribution. The flue gas flow acts as a temperature bath which prevents uneven temperature gradients. The inclusion of an ignition source introduces a secondary heat source and must be accounted for in the operation of the temperature monitoring apparatus.

The use of catalytically promoted reactions is universal among the chemical process industries. The method of this invention is applicable to those reaction vessels which contain catalyst, but it is required that the reaction zones contain identical catalyst which has had equal usage. This is necessary to assure that effects caused by a difference in catalytic activity are not confused with effects caused by the addition of more reactant.

PREFERRED EMBODIMENT

The preferred embodiment of this invention is the regulation of the amount of air or fuel being fed to a combustion zone, to maintain a desired ratio between the fuel and air, by a process which comprises the steps of: (a) passing reaction products from an initial combustion zone to a second separate combustion zone; (b) measuring the temperature of the reaction products as they enter the second zone; (c) adding to the reaction products in the second zone, a small stream of one of the reactants; (d) measuring the temperature of the stream formed by the reaction products and the additional reactant as it leaves the second combustion zone; (e) converting the two temperatures into electrical signals fed to a control device; and, (f) adjusting the ratio of the added reactant to the other reactant to maintain the relationship between the two measured temperatures at a preset value corresponding to a desired degree of combustion in the initial combustion zone.

Claims

I claim as my invention:

1. A process for controlling at a preset value the ratio of the reactants in a continuous reaction possessing a net heat of reaction, which comprises the steps of:

a. passing at least a portion of the reaction products from an initial reaction zone into a separate second reaction zone which second zone is maintained at conditions suitable for initiation of the reaction being monitored;

b. measuring the temperature of said portion of said first reaction zone products;

c. adding to said portion of said first reaction zone products a stream of one of the reactants;

d. measuring the temperature of the resultant mixture formed by said portion of said first reaction zone products and the added reactant stream; and,

e. regulating the flow of at least one of the reactants fed to the initial reaction zone to maintain the two measured temperatures at a predetermined temperature relationship corresponding to said preset value of the reactant ratio.

2. The process of claim 1 wherein the reactants comprise an oxygen containing gas and a fuel selected from the group consisting of CO, H.sub.2, CH.sub.4, C.sub.2 H.sub.6, C.sub.2 H.sub.4, C.sub.3 H.sub.8, C.sub.3 H.sub.6, C.sub.4 H.sub.10 and C.sub.5 H.sub.12.

3. The process of claim 1 wherein the initial reaction zone and said second reaction zone contain catalyst.

4. The process of claim 3 wherein at least the initial reaction zone is located in a reaction vessel used in the conversion of hydrocarbon material.

5. A process for adjusting the ratio of an oxygen containing gas to fuel being fed to a primary combustion zone as reactants, which process comprises the steps of:

a. passing at least a portion of the flue gas from the primary combustion zone to a second combustion zone maintained at conditions suitable for the initiation of combustion;

b. measuring the temperature of the flue gas entering said second combustion zone;

c. adding to the flue gas in said second combustion zone a quantity of one of the reactants that is equal in temperature to the flue gas at the point of addition;

d. measuring the temperature of the gas mixture formed by the additions made in step (c) as said gas mixture leaves said second combustion zone;

e. comparing the temperature determined in step (d) to the temperature determined in step (b); and,

f. regulating the ratio of fuel and air fed to the primary reaction zone to maintain the relationship between the compared temperatures of step (e) at a value corresponding to the desired ratio of air to fuel fed to the primary combustion zone.

6. The process of claim 5 wherein the added reactant is the fuel.

7. The process of claim 5 wherein the added reactant is air.

8. The process of claim 5 wherein the added reactants are alternately switched so to control the total reactant ratio.