Patent Grant
8734728
The present technology relates to ammonia (NH3) oxidizers, and specifically to gas distributors for an ammonia (NH3) oxidizers.
Hydroxylamine sulfate can be used in caprolactam production. Caprolactum is an organic compound having the formula (CH2)5C(O)NH, and is widely used as a raw material in making Nylon-6. Hydroxylamine sulfate production can be carried out in a number of parallel trains consisting, for example, of the following operations: ammonium carbonate production, ammonium nitrite production, ammonium hydroxide production, hydroxylamine disulfonate production, and hydrolysis.
The ammonium nitrite production processes can include of a series of steps that begins with burning, or oxidizing, of ammonia in the presence of air. The oxidization of ammonia can occur across a cobalt catalyst bed contained within a vessel referred to as an ammonia (NH3) oxidizer. The catalyst bed tends to be shallow, having a depth of only a few inches, due to the kinetics of the ammonia oxidation reaction.
The present technology relates to gas distributors for ammonia (NH3) oxidizers.
In one aspect, an ammonia oxidizer is provided that includes a vessel having an inner wall, a catalyst bed contained within the vessel, a gas inlet in the vessel, and a gas distributor installed in a gas inlet. The ammonia oxidizer can also include a distribution ring attached to the inner wall of the vessel at a predetermined location below the gas distributor, the distribution ring having a width.
In another aspect, ammonia oxidizer is provided that includes a vessel having an inner wall and a gas inlet, a catalyst bed contained within the vessel, a gas inlet in the vessel, a gas distributor installed in the gas inlet, and a distribution ring attached to the inner wall of the vessel at a predetermined location below the gas distributor, the distribution ring having a width. The gas distributor can comprise a circular gas diffuser plate having a thickness and a center point, and a plurality of holes arranged in a series of concentric rings, each ring having a center at the center point of the gas diffuser plate.
In further aspect, an ammonia oxidizer is provided that includes a vessel having an inner wall and a gas inlet, a catalyst bed contained within the vessel, a gas inlet in the vessel, and a gas distributor installed in the gas inlet. The gas distributor can include a circular gas diffuser plate having a thickness and a center point, a plurality of holes arranged in a series of concentric rings, each ring having a center at the center point of the gas diffuser plate, and a gas distributor sidewall connected to the gas diffuser plate at a lower end of the gas distributor sidewall and connected to the gas inlet of the vessel at an upper end of the gas distributor sidewall.
Specific examples have been chosen for purposes of illustration and description, and are shown in the accompanying drawings, forming a part of the specification.
Ammonia oxidation can include providing an ammonia gas feed stream 106, which can be combined with an air feed stream 108 in a mixer 126, such as a static mixer, to form gas mixture feed stream 110. The ammonia in the ammonia gas feed stream 106 can be combined with the air feed stream 108 in any suitable ratio to form the gas mixture feed stream 110, such as for example at about a 10% mole ratio of ammonia to air. The gas mixture feed stream 110 can be introduced into the ammonia oxidizer 100 through a gas distributor 112 installed in a gas inlet 114 in the vessel 102 of the of the ammonia oxidizer 100.
At higher flow rates, the gas mixture feed stream 110 can tend to cause catalyst in the catalyst bed 104 to shift, which may result in bare spots and allow unreacted ammonia to pass through the catalyst bed 104. Efficient distribution of the gas mixture feed stream 110 across the cobalt catalyst bed 104 can reduce shifting of the catalyst in the catalyst bed 104. For example, current catalyst conversion in ammonia oxidation processes can be about 96% to about 98%, with the average ammonia consumption for a single ammonia oxidizer being about 4,500 lbs/hr. Improving the distribution of the gas mixture feed stream 110 across the cobalt catalyst bed 104 can result in an improvement of catalyst conversion. A catalyst conversion improvement of 1% would result in an ammonia consumption decrease of 45 lbs/hr.
In the example illustrated in
A gas distributor 112 can comprise, or consist of a circular gas diffuser plate. One example of a circular gas diffuser plate is illustrated in
With reference to
The arrangement of the holes 202, 204, 206, 208, and 210 can be referred to as being the hole schedule of the gas diffuser plate 200. For example, the gas diffuser plate 200 of the illustrated examples of
The holes can have any suitable diameter, and the diameter of any one hole can be the same or different as the diameter of any other hole, although preferably the holes all have the same diameter.
The holes 202, 204, 206, 208, and 210 can also each have an orientation angle with respect to the vertical center line C, illustrated in
In one example, the orientation angle of each of the holes 202, 204, 206, 208, and 210 is about 0° with respect to the vertical center line C, meaning that all of the holes have a vertical orientation through the gas diffuser plate 200. In another example, as illustrated in
In some examples, a gas distributor of the present technology can include a sidewall in addition to a gas diffuser plate 200.
In practice, the gas mixture feed stream 110 is provided into the vessel 102 by entering the gas distributor 300 at the upper end 312 of the gas distributor sidewall 308 and exiting the gas distributor 300 at the lower end 310 of the gas distributor sidewall 308 through the holes 202, 204, 206, 208, and 210 of the gas diffuser plate 200.
A Computational Fluid Dynamic (CFD) computer model was used to evaluate Design 1 of a gas distributor of the present technology, which was designed in accordance with
The results indicated that the orientation angles of holes in the gas diffuser plate have a significant impact on the gas flow distribution within the vessel and across the catalyst bed. The results for Case 13 were the best with respect to providing the most efficient gas distribution across the catalyst bed.
Performance of the Design 1 gas distributor of the present technology as described in Example 1 above, having orientation angles in accordance with Case 13 in Example 1 above was compared to performance of a currently known gas distributor having a triangular gas diffuser plate, and no sidewall, at the gas inlet of the ammonia oxidizer. The triangular gas diffuser plate had 45 holes, each hole having a diameter of about 1 inch.
Quantitative comparisons of the two distributors are shown in Table 3 below, where mass percentage of gas through catalyst bed vs. inlet total, area weighted average gas velocity, and standard deviation of velocity magnitude are given side by side.
The pressure drop across the Design 1 and Triangular Plate gas diffusers was also determined, and is set forth in Table 4 below for an operating pressure of about 12 psig. The pressure drop across the distributor for Design 1 gas distributor is larger than the pressure drop across the Triangular Plate gas diffuser. Without being bound by any particular theory, it is believed that this is due to the sidewall of the Example 1 gas distributor. However, the pressure drop across the Design 1 gas distributor at the operating pressure of about 12 psig and a normal gas feed rate (4600 Pa/0.667 psi) was only 5.5% of the operating pressure, which is not deemed to be a significant amount.
The effect of having a distribution ring attached to the inner wall of the vessel of an ammonia oxidizer was tested for an ammonia oxidizer having a currently known gas distributor having a triangular gas diffuser plate, and no sidewall, at the gas inlet of the ammonia oxidizer. The triangular gas diffuser plate had 45 holes, each hole having a diameter of about 1 inch. The distribution ring had a width of about 3 inches and was located about 45 inches below the gas distributor.
Quantitative comparisons of the two ammonia oxidizers are shown in Table 5 below, where area weighted average gas velocity, and standard deviation of velocity magnitude are given side by side.
Based on the quantitative comparison, the addition of the distribution ring reduced the standard deviation of the gas velocity at the catalyst bed to 1.63 from 2.02, which is a 19% improvement of gas distribution uniformity.
A Computational Fluid Dynamic (CFD) computer model was used to evaluate examples of an alternative design of a gas distributor of the present technology in combination with distribution rings of the present technology. The gas distributor was designed in accordance with
Designs 3 through 5 in Table 6 showed the best results for standard deviation of gas velocity at catalyst bed, each being less than the standard velocity at catalyst bed of 2.02 that was exhibited by the Triangular Plate in Table 3 above. With Example 3 from the existing triangular plate in Hopewell NH3 oxidizer. Therefore, 39% improvement of gas distribution uniformity is achieved by installing the new design of Simplified Design #2.
The Example 4 showed less pressure drop cross the gas diffuser than the Example 1, which, without being bound by any particular theory, is believed to be due to the removal of the distributor sidewall. At normal flow rate, the pressure drop for the Example 4 is about 0.16 psi or 1.3% of an operating pressure of 12 psi, compared with the pressure drop of 0.667 psi or 5.5% of an operating pressure of 12 psi that resulted with the Example 1.
From the foregoing, it will be appreciated that although specific examples have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit or scope of this disclosure. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to particularly point out and distinctly claim the claimed subject matter.
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