GB2129705A - Process for contacting a gas with atomized liquid - Google Patents

Abstract

A process for contacting a gas with atomized liquid in a contact chamber employing an internal mix atomizer assembly in which process the risk of spray collapse and liquid impingement on the confinement wall of the contact chamber is substantially eliminated by using the following process conditions: N(the no. of exit ports)/sin ???</=15.6, the Sauter mean drop diameter of droplets </=200 ???m, and x1/R where x1 is the distance between an exit port and the point where spray from that port hits the chamber wall and R is the chamber wall radius, is 2.75-5.65. <IMAGE>

Description

SPECIFICATION Process for contacting a gas with atomized liquid The invention relates to a process for contacting a gas with atomized liquid employing an internal mix-type fluid atomizing nozzle assembly, hereinafter called internal mix atomizer.

Atomizers can be subdivided into two main groups, viz. the above-mentioned internal mixatomizersand pressure atomizers. In said latter type of atomizers liquid sprays are formed by passing liquid at high pressure through relatively small nozzle openings.

Internal mix atomizers have the advantage over pressure atomizers that betteratomization rates can be attained in combination with a good control of droplet size.

In the above process liquid is supplied along with an atomizing gas under pressure to a mixing cham berofan internal mix atomizer, having a plurality of exit ports communicating through hollow passages with the mixing chamber. The liquid leaves the atomizer via said exit ports and enters in the form of expanding spray jets into a confined space containing a gaseous stream to be contacted with said spray jets.

Atomizers can suitably be used in a large variety of industrial processes, such as processesforcombust- ing heavy liquid fuels, spray drying processes for incinerating liquid waste streams, wet cleaning processes for removing soluble gaseous components or solid dust particles from a gaseous stream, and processesforquenching hot gaseous streams.

In orderto obtain an efficient gasiliquid contact it is essential thatthe sprays of liquid leaving the atomization ports do not collapse. Sprays may collapse iffor example the distance between two neighbouring atomization parts is too small, so that the expanded liquid sprays can get too close together. In European patent application published under number 0058437 on 25th August 1982, an internal mix atomizer is claimed which atomizer is so formed that it is particularly suited for elemination of spray collapse of the type above described.This previous mix atomizer has its exit ports so positioned that the ratio do)+ ri(o) tij where rj(o) and rj(o) are jet radii directly after expansion ofthe liquid sprays from exit ports, i and j, respectively, and tij is the distance between the center of exit port i and the center of exit port, is less than 0.8.

The above type of collapse phenomenon is to be distinguished from collapse ofthe spray jets at larger distances downstream from he mix atomizer. This latter type of spray collapse can be caused by the exit ports geometry and/or by external conditions, such as gas flowing along the atomizer thereby pushing the jet sprays towards each other, so that the sprays tend to merge into a single jet.

Apartfrom spray collapse, there is another phenomenon which can cause problems in spray contacting processes, viz. that of impingement of liquid on the confinement wall of the mixing chamber, in which gas is contacted with jet sprays. Depending on the type of spray contacting process and the fluids being used, impingement of liquid on the mixing chamber wall might result in damage of the process equipment due to for example erosion, blockage of the process lines due to deposits on the walls and/or reduction of the process efficiency.

The object of the present invention is to provide a process for contacting a gas with atomized liquid employing an internal mix atomizer substantially as claimed in European patent publication number 0058437 in which process the above-mentioned risk of spray collapse and liquid impingement have been substantially eliminated.

Accordingly, the invention provides a process for contacting a gas with atomized liquid, which process comprises injecting droplets of liquid into a contact chamber with a confinement wall, containing said gas, through an internal mix atomizer assembly substantially as claimed in European patent publication number 0058437, wherein the ratio Nisinoc, where N is the number of exit ports of the assembly and a is the angle ofthe internal passages and exit ports to the axis of the assembly, is equal to or less than 15.6, in which process the Sauter mean drop diameter of the droplets entering the contact chamber is equal to or less than 200 cm, and the ratio xj/R, wheres is the downstream distance between an exit port i and the location at which the spray from said exit port impinges to the confinement wall when one exit port is used and R is the radius of the confinement wall, is in the range from 2.75through 5.65.

The present invention has a numberofimportant aspects. Initially, a significant difference between this invention and the prior art is that the nozzles employed in the prior artwere often pressure nozzles, and there was poor atomization of the liquid. These pressurized nozzles did not give good control of droplet size. However, in the present invention, the atomizing gas (e.g. steam) is at high pressure and adds a lot of energyto the system. Accordingly, it is nowpossibletoobtain bettercontrolofdropletsize.

Droplet size is important because the greaterthe droplet size, the longer time (and hence larger quench chamber) is required for exchange of heat or matter between the liquid and the gas.

Asshown by the Examples hereinafter, the process according to the present invention also results in an excellent spray pattern. The spray pattern is of extreme importance to the contacting operation. An excessively collapsed spray does not mix well with the gas while an excessivelywide spray pattern may result in direct impingement of the spray jets on the wall.

The risk of impingement of droplets of liquid to the confinement wall is minimized by choosing the Sauter mean drop diameter equal to or less than 200 um. As shown in the Examples the process according to the invention do not give rise to liquid impingement and spray collapse.

The present invention can advantageously be applied in the SCOT process. The SCOT process involves heating a Claus tail gas stream to an elevated temperature along with a reducing gas comprising H2 ora mixture of H2 and CO. Claus tail gas typically contains H2S, SO2, CO2, CO and H2 with the remainder being mainly N2. The said reducing gas may be supplied eitherfrom an outside source or from sub-stoichiometnccombustion in an in-line burner.

The combined gas stream is then passed into the top of a reactorcontaining a cobalt-molybdate on alumina catalyst. As the gases flow downward overthe catalyst, the sulphur compounds, including 502, CS2 and COS, are converted into H2S. The gas is then cooled and the cooled gas is passed to an amine absorption column where the H2S is absorbed. The loaded absorbent is regenerated and the regenerated absorbent may be used again in the absorption column.The H2S liberated bythe regeneration is recylcled to the Claus unit.

An important aspect ofthe SCOT process is the control ofthe temperature ofthe gases going to the reduction reactor. Typically, the desired temperature in the reactor is around 290 C. However, in certain circumstances where sub-stoichiometric combustion is used to generate the H2, much highertemperatures are encountered. For example, temperatures of 475C are often found in some units. It then becomes necessarytocool the gas stream priorthecontact with the catalyst in the reactor. Previously, such streams were cooled by a pressurized water spray quench (employing standard commercial spray nozzles) directly in the relatively small (e.g. cm) connecting lines between the furnace and the reactor.

Typically, the first portion of the carbon steel connecting line is lined with refractory. To prevent damage to the refractory, impingement of the spray must be minimized. Further, all the spray must be evaporated within the refractory lined section to prevent corro sion of the exposed carbon steel segment ofthe line by the sourwaterformed during the quenching operation. Previous nozzles were not satisfactory in that a significant amount of the spray water reached the unlined pipe, therein presenting serious corrosion concerns.

The application ofthe present invention in the SCOT process is ideally in the connecting line between the furnace and the reactor. The nozzle will typically be located in the line such that it is nearthe center line ofthe connecting line. Typical compositionsofthe hot gaseous stream to be quenched and othertypical operating conditions are disclosed in U.S. Patent Specification numbers 4,153,674 and 4,001,386 and in Canadian Patent Specification 934,937, which disclosures are herein incorporated by reference. Typically, it is desired to quench the reactor feed stream from about 475 C to about 290 C.

The present invention may also be used to quench the hot gaseous overhead stream from a fluid catalytic cracking unit regenerator. Such streams comprise mainly CO, CO2, N2 and H2O and often have a temperature of 660 C or higherthan 660 C. The present invention is usedful in keeping the temperature under control and preventing the occurrence of afterburning. The general conditions ofthe regenerator in a catalytic cracking unit are described in U.S.

Patent Specification numbers, 3,012,962 and 3,137,133 which are herein incorporated by reference.

The quench liquids suitable in a quench process according to the present invention include water, hydrocarbon solvents and heavy hydrocarbons with water being particularly preferred. The atomizing gases include steam, air, N2, 2 and methane with steam being particularly preferred.

The pressure ofthe atomizing gases should be sufficient to create critical or near critical (0.7 critical) two phase velocities for the combined quench liquid-atomizing gas at the exit ports. When steam is employed,thetemperature ofthe steam should be high enough so as not to cause excessive condensation in the mix chamber or passages. Atomizing gas pressuresforsteam used in the SCOT process are typically about 16.5 bar. The weight ratio of liquid to atomizinggasisfroml:1to7:1,preferably2:1to6:1.

In most cases the nozzle assembly is cylindrical in shape, and the passages are also typically straight, cylindrical passages. In a preferred embodiment, a plug or similar device is employed to mix the liquid and atomizing gas in the assembly.

The numberofexit ports (and passages) are usually chosen first (assuming about 6to 12 for each nozzle).

The size of the ports depends then upon the amount of contacting required, and hence the amount of liquid and atomizing gas required.

In orderto prevent liquid impingement on the confinementwall ofthecontactchamberandto prevent spray collapse a number of requirements have to be fulfilled.

Afirst requirementfor preventing liquid impingementto the confinement wall of the contact chamber consists herein thatthe individual droplets in the sprays should have such a sizethatthey do not follow individual trajectories, but remain in the spray jets.

Thereto, the Sauter mean diameter of the droplets leaving the exit ports should be equal to or less than 200pom. Sauter mean diameter (SMD) is defined thus:

where Ni is the number of droplets in the diameter range Dj.

Typical sizes for the droplet diameters are 200 to 40 pm.

As a second requirement for preventing liquid impingementontheconfinementwall ofthe contact chamberthe ratio -i should be equal to or R largerthan a certain minimum value, hereinafterto be discussed. In the expression -, means R the downstream distance between an exit port and the location atwhich the spray from said exit port impingestotheconfinementwall ofthecontact chamber, when said exit port is the only one which is used, and R means the radius ofthe confinement xi wall. - can be estimated from the following R formula: x1 ############# = cotg&alpha;.

R Thevariousterms employed above are defined as follows: Pa = density of ambient gas V2 = velocity of ambient gas G = momentum flew, G"times an, an being the cross sectional area of one exit port G" = momentum flux,

K = ratio of specific heats, 30/50 Pmix= internal mixing chamber pressure Pa = pressure of ambient gas &alpha; = angle of an exit port with the axis ofthe atomizer assembly.

From tests carried out it has been foundthat spray impingement onto the confinementwall ofthe Xi contact chamber is prevented if the ratio - Is R chosen equal to or greater than 2.75. This requirement can be easily deduced from the Examples shown hereinafter.

At a given density of ambient gas and a given atomizer assembly, the ratio Xi# can be influ R enced by varying the ambient gas velocity. An increase of the ambient gas velocity will result in a decrease ofthe rate of spreading outwardly ofthe spraysfrom the exit ports thereby reducing the risk of spray impingement on the confinementwall of the contact chamber.

Apartfrom liquid impingement, there is a further important cause for malfunction of gas/liquid contacting operations, viz. spray collapse. An excessively collapsed spray does not mix well with, for example, a hot gas to be cooled, which adversely affects the rate of cooling obtained. Spray collapse can be caused by an incorrect design of the mix atomizer, or an incorrect contacting operation with a correct mix atomizer. Spray collapse duetothe mix atomizer design, can occurwhen the atomizer is provided with too many exit ports andtorwhen the angles ofthe interior passages and exit ports with the axis of the atomizer assembly are chosen too small.

Spray collapse due to an incorrect design of the N is prevent if the ratio - atomizer is prevented if the ratio Is sina chosen equal to or lessthan 15.6. In this expression N is the number of exit ports and a is the angle of the exit ports with the axis ofthe atomizer assembly. The angle a of the exit ports with the axis of the atomizer assembly is suitably chosen in the range of from 30 degrees to 75 degrees, inclusive. The number of exit ports is suitably chosen in the range from 6through 10, inclusive. A lower number of exit ports results in a less effective use of the area available for liquid/gas contact in the contact chamber.When the number of exit ports is chosen largerthan 10,the risk of spray collapse is relatively high, whereas hardly and improvement of the use ofthe available liquidtgas contact area is achieved.

Spray collapse can alsooccuriftheambientgas pushesthejet sprays from the exit ports towardsthe axis of the atomizer assembly, which in the most extreme case can result in a merging ofthejetsprays into a single jet When this occurs only a very minor part of the ambient gas is contacted with the liquid from the exit portx. In order to prevent spray collapse of the last-mentioned type the ratio Xi# should R be chosen equal to or less than 5.65. The Xj ratio - is estimated in the same manneras R discussed in the above with respecttothe phenomenon of liquid impingementtothe confinement wall ofthe contact chamber.

In orderto describe the invention morefully, reference is made to the accompanying drawings, wherein Figure 1 illustrates a longitudinal section (taken along line 1-1 of Figure 2) of an internal mix-typefluid atomizing nozzle assembly used in the process accordingtothe invention; Figure 2 is atop view of the same nozzle; Figure 3 is a schematic representation of a contact chamber with atomizer according to the invention; and Figure 4 is a graphical representation of test results.

Figures 1 and 2 show an internal mix atomizer used according to the invention comprising a member 1 provided with a mixing chamber2 communicating with a plurality of passages 3, preferably cylindrically shaped. Member 1 is provided with openings4and 5 for introducing atomizing gas and liquid, respectively into chamber 2. The passages 3 are positioned in member 1 in accordance with the principles described herein, that is the angle a of the passages 3to the axis (7-7) is suitably between 30 and 75% The exit orterminal ports 6 debouch into a contact chamber (not shown ). The nozzle assembly may be anchored tothecontactchamberwith use of aflange connection 8.

For cooling a gas with atomized liquid, a quench liquid is introduced atthe opening 5 and an atomizing gas is introduced atthe opening 4 under pressure into the mixing chamber2via a source, such as a supply tube or tubes (not shown). The mixed fluids are forced through the passages 3, and through the exit ports 6wherethey expand because of reduction in pressure. Exit ports 6 are preferably circular in shape, and are preferably, as shown, atan angletothe exterior surfaces of member 1. ln general, the exit ports are spaced around the periphery of the atomizer ata location somewhat disposed from the liquid supply -fluid supply opening. Forexample, if the atomizer is generally cylindrical, as illustrated in the embodiment of the drawing, the exit ports may be spaced, in accordance with the relationship described herein, in the side of afrustoconical section whose smaller baseisthe "base" of the "cylinder" opposite the liquid4luid supply opening ofthe atomizer, the side ofthefrusto-coriical section terminating atthe "cylinder" sides orwall.

Various experimentswere performed to simulate operating conditions (exceptfortemperature) in a SCOT process unit. Referenceismadeto Figure 3, which schematically shows the equipment employed in the experiments. Instead of a hot gaseous reactant stream, ambient air was employed. The air (21) was provided by an air blower (notshown). The airwas injected into a metal duct assembly (22) having a 1.83 m radius miter elbow. The duct assembly (contact chamber) had a 0.96 m inside diameter. The flow of ambient air gas was measured by a flow indicator (23) and is reported in Table 1 as Va (m/s). The various nozzle assemblies (24) were inserted downstream of the elbow. An atomizing airsupply (25) and quench water supply (26) were also provided.

The operating conditions and results are presented below in Table 1. The various nozzles are described bytheangleofthe nozzle passages to the axis of the atomizer and the number of exit ports. The relative amounts ofthe water supply, atomizing air supply and ambient air velocity were varied.

Pa ## Ta @@@@@@@@ Module W/min kg/m (K) (m/s) H2/# 40 -10 15.56 1.25 11.1 6.23 3.24 Ne implugement,no collaps 40 -10 15.56 1.35 11.1 3.40 2.12 Implugement to confirmant well 40 -10 15.56 1.25 23.3 12.5 3.40 We implugement,no collaps 40 -10 15.56 1.28 23.8 7.0 1.99 ,, ,, ,, ,, ,, ,, ,, 40 -10 15.56 1.27 10.3 6.7 2.71 ,, ,, ,, ,, ,, ,, ,, 40 -10 15.56 1.29 10.3 12.8 4.38 ,, ,, ,, ,, ,, ,, ,, 40 - 9 14.00 1.25 11.2 12.5 5.97 No implugement,no collaps 40 - 9 14.00 1.25 12.4 6.4 2.37 Implugement to confirmant well 40 - 9 14.00 1.25 26.4 11.9 7.43 Implugement to confirmant well 40 - 9 14.00 1.26 26.6 7.62 2.02 Implugement to confirmant well 40 - 9 14.00 1.26 26.6 10.36 2.61 No implugement,no collaps 40 - 8 17.43 1.26 13.9 12.50 4.35 No implugement,no collaps 40 - 8 17.43 1.24 12.9 7.31 2.51 Implugement to confirmant well 40 - 8 17.43 1.24 30.5 12.20 2.34 Implugement to confirmant well 40 - 8 17.43 0.65 16.2 23.20 5.65 Spray collaps 50 -10 12.99 1.16 10.9 11.69 3.11 No implugement,no collaps 50 -10 12.99 1.28 10.5 7.32 2.07 Implugement to confirmant well 50 -10 12.99 1.28 24.5 12.82 2.41 Implugement to confirmant well 50 -10 12.99 1.26 22.6 7.02 1.40 Implugement to confirmant well 35 -10 17.54 1.20 27.3 12.80 3.99 Spray collaps 35 -10 17.54 1.16 17.6 7.31 2.72 - - 35 -10 17.54 1.20 8.43 7.01 2.93 - - 35 -10 17.54 1.28 8.41 22.50 8.52 - - 45'-10 14.14 0.65 14.8 22.28 5.70 Spray collapse Module tip with 4" off-nek" so that during operation a #light moment of tangential momentum in added to the atomized liquid The test results mentioned in Table 1 are graphically illustrated in Figure 4. In this Figure 4, the ration xi/R has been plotted on the X-axis and the ration N/sin a on the Y-axis. The symbols used in this figure have the following meaning: o no impingement, no collapse, with nozzles having interior passages perpendicularto the surface ofthe atomizer.

no impingement, no collapse with nozzles having interior passages 40 off-set.

impingement to confinementwall spray collapse due to nozzle design spray collapse due to flow ambient gas.

In the shown diagram four areas can be distinguished, viz. area l: spray impingement to confinementwall, area II: spray collapse due to nozzle design, area III: spray collapse due to influence ambient gas and area IV: no impingement, no collapse.

Claims (11)

1. A process for contacting a gas with atomized liquid,which process comprises injecting droplets of liquid into a contact chamberwith a confinement wall, containing said gas, through an internal mix atomizer assemblysubstantially as claimed in European patent publication number 0058437, wherein the ratio N/sina,where N is the number of exit ports of the assembly and a is the angle of the internal passages and exit ports to the axis ofthe assembly, is equal to or less than 15.6, in which process the Sauter mean drop diameters of the droplets entering the contact chamber is equal to or less than 200 m. and the ratio xi/R where xi is the downstream distance between an exit port and the location at which the spray from said exit port impinges to the confinement wall when one exit port is used and R is the radius of the confinement wall, is in the range from 2.75 through 5.65.

2. A process as claimed in claim 1, in which the exit ports are uniformly arranged in a circularfashion in the surface of said atomizerassembly.

3. A process as claimed in claim 1 or2, in which the droplets of liquid are injected into the contact chamber in a co-current direction with the gas, which is caused to flow through said contact chamber.

4. A process as claimed in any one of the claims 1 -3, in which the velocity of said droplets atthe exit ports is critical or at least 0.7 times critical two phase velocity.

5. A process as claimed in any one of the claims 1-4, in which the weight ratio of liquid to atomizing gas is between 1:1 and7:1.

6. A process as claimed in any one of the claims 1-5, in which the process is applied for quenching a hot gaseous stream by contacting said stream with droplets of a quench liquid.

7. A process as claimed in claim 6, in which said quench liquid is water, wherein steam is used as an atomizing gas.

8. A process as claimed in claim 6 or7, in which said hot gaseous stream comprises the hot regener ator overhead gas stream from a fluid catalytic cracking unit.

9. A process as claimed in claim 6 or7, in which said hot gaseous stream comprises the feed stream to a reduction reactor in a Claus off-gas treating process.

10. A process as claimed in any one of the preceding claims, in which the Sauter mean diameter is in the range of from 40 to 200 m.

11. A process for contacting a gas with atomized liquid substantially as described with particular reference to the accompanying drawings.