1. Field of the Invention
The invention relates to a process airheater, herein referred to as NQAPH (No Quench Air Pre Heater), located after the reaction quenching water spray and subject to high temperature of the reactor effluent (around 1300 C). The airheater (NQAPH) has higher heat transfer rates and results in a smaller airheater compared to the prior art (APH). Spray water and associated energy loss are minimized. (162 kg/h of spray water and 123 kW of energy per 1000 nm3/h of process air)
2. Description of the Prior Art
In the prior art, mostly water has been used to quench the reactor effluent to al-rest the cracking reaction. Additional water is sprayed to cool the effluent further to protect the downstream equipment, typically process air heater, (APH). Such water has to be treated to reduce the impurities content for carbon black product quality as well as trouble free operation Of downstream equipment (due to the deposition of the impurities on the heating surfaces). This water, an increasingly important commodity, is converted to superheated vapor in this cooling process and the water and energy are irrecoverably lost to the atmosphere.
The prior art process air heater, (APH) is typically a shell and tube counter flow unit with the hot reactor effluent, cooled adequately with additional water spray, flowing inside multiple tubes and the process air flowing over the tubes in multiple passes over the tubes.
In the present invention, the reactor effluent, after the reaction stopping quench, enters at high temperature (typically around 1300 C) and high velocity (typically 80 m/s), the inside of the tubes of the shell and tube type process air heater (NQAPH) and flows vertically. In the present invention,
In the present invention, the heat transfer flux increases by about 34%, the length of the tubes is reduced by about 25%, making the process air heater 25% shorter and making the usually vertically oriented airheater, structurally more stable.
In the present invention, the water quench in the prior art to control the hot air temperature of the process air heater, is eliminated saving water and energy. This energy can be recovered downstream of the airheater for useful purposes, instead of being lost to the atmosphere.
In the present invention, unlike the prior art (APH) where the hot exiting air is in contact with the double plate bottom tube sheet, cold incoming air of stream 1 is in contact with the double plate bottom tube sheet, keeping the tube sheet plates cooler and therefore, stronger.
In the present invention, the tube to bottom tube sheet weld, (
In the present invention, the shell, due to full length internal and external insulation, operates at lower temperature than in the prior art (APH) where only part of the shell is internally and externally insulated. (
In the present invention, due to the shorter length of the unit, as in (005) above, and lower temperature, as in (009) above, the shell thermally expands less than the shell in the prior art (APH). The differential expansion between the tube and shell is also lower than in the prior art (APH). (
In one configuration, the shell of this airheater (NQAPH) is fully externally and internally insulated. The internal insulation is retained in place by means of metal pins and metallic liner. The process air flows over this metallic liner.
In another configuration, the shell is only externally insulated and is provided with an internal metallic liner. The second stream of air will enter the airheater at the hot end and flows to the colder end through the annulus between the shell and the inner metallic liner. Typically turbulators are provided in the annulus to improve heat transfer and structural stability of the inner liner.
In one configuration, the internal baffles to create multiple passes for the airheater (NQAPH) are of the segmented type.
In another configuration, the internal baffles to create multiple passes for the airheater (NQAPH) are of the disc and donut type or any other method to create multiple passes of the process air.
In one configuration, the tubes are connected to the top tube sheet at the colder end of the airheater (NQAPH) by means of packing seals, with the tubes free to slide inside the seals.
In another configuration, the tubes are connected to the top tube sheet at the colder end of the airheater (NQAPH) by means of metallic bellow type seals welded to the sleeves in the top tube sheet or by any other method of sealing the process air from mixing with the hot reactor effluent, allowing for the hot tubes to thermally expand freely.
The process air for the CB production may be ambient air, Oxygen enriched air or 100% Oxygen.
The invention is pictorially depicted in
A is the entry of the Carbon black furnace effluent into the apparatus.
B is the exit of the effluent after being cooled in the apparatus
1 is the first stream of process air, entering at the hot end of the air heater through nozzle 12.
2 is the second stream of process air, entering at the colder end of the air heater through nozzle 13.
3 is the third stream of process air, entering from the cooling air header 7 into the double plate bottom tube sheet, 4, entering through cooling air nozzles 15. The bottom plate of this tube sheet 4 is refractory lined to protect this plate from the high temperature of the reactor effluent.
5 are the multiple tubes of the airheater inside which the hot reactor effluent flows at high velocity.
These tubes are connected to the bottom tube sheet 4 and the top tube sheet 6. This top tube sheet 6 is generally refractory lined
8 is the metallic shell of the airheater, fitted with internal insulation 10, retained by metallic pins and metallic plate 9. External insulation on the shell is shown by 11.
14 is the nozzle through which the heated process air stream leaves the airheater.
16 is the return pipe for the cooling air from the top plate of the bottom tube sheet 4 to the colder section of the airheater. This can be a single large pipe or multiple smaller pipes.
17 are the set of baffles inside the airheater which makes the process air to flow in multiple passes over the outside of the tubes 5. These baffles may of the segmented type (shown) or the disc and donut type.
The tubes 5 may be connected to the top tube sheet 6 with packing seals 21 (
Constant load hangers 19 or counter weights are provided to keep the shell always in tension and prevent shell buckling under adverse operating conditions.
Sway brackets 20 are provided on the shell of the airheater to minimize the lateral movement of the airheater due to wind loads, uneven heating of the tubes 5 or any other causes.
In a carbon black furnace, 17,000 nm3/h of hot air at 920 C is admitted along with adequate fuel (oil or natural gas) to raise the flame to a temperature around 1925 C. Hot carbonaceous feed stock is sprayed into this excess Oxygen rich hot flame and the ensuing chemical reactions convert the feed stock to carbon black and other gases. The reaction is stopped by a water spray at around 1300 C, which results in 29,130 nm3/h of gases and solid carbon black. In the prior art, this temperature is too high for the downstream airheater, (Reactor Air Preheater, APH), the gases are further cooled down by additional water spray of 2,750 kg/h to cool the gases to 1,050 C before entering this APH. The volume of gases entering the APH is 32,550 nm3/h. This water (2,750 kg/h) is not recovered and will be lost into the atmosphere. This additional water also causes problems in the downstream equipment like the bag filter with wetness.
With the proposed invention of airheater (NQAPH), all of this water is saved. A single airheater (NQAPH) will cool the gases from 1,300 C to 865 C. Further heat recovery from the CB containing reactor effluent down to the safe temperature for the Carbon Black collector can be achieved with additional heat exchangers for feed stock and fuel preheating, high and low pressure steam generation, tail gas preheating etc . . .
Table 1 compares the airheater (NQAPH) and the prior art airheater (APH), both of them designed to preheat the process air to 950 C.