Background of the Invention

This invention concerns apparatus and a process for the elimination of hydrogen sulfide from incondensible gases derived from the distillation under vacuum of heavy- fractions of crude oil.

In the majority of cases, these gases are burned directly in the furnaces of- the same distillation column. However, this mode of operation results in problems of atmospheric pollution, owing to sulfur dioxide which is formed during the combustion and which exits the furnace chimney. Moreover, the presence of humidity and of hydrogen sulfide in the gases is often the cause of notable phenomena of corrosion in the equipment concerned.

Increasingly stricter international regulations concerning the control of gaseous wastes and the processing of crude oils, which are ever more rich in sulfur, make it imperative that the gases be treated, with the goal of eliminating the hydrogen sulfide which, in certain cases, can reach even up to fifty percent by weight .

On the other hand, different factors make it difficult to devise an efficient and convenient system for desulfurization. In fact, the use of amine washing

systems, normally used by refineries for that purpose, is in this case particularly hampered and made uneconomic by the fact that oxygen (always present, often in appreciable quantity in the gaseous stream to be desulfurized) causes the degradation of the amines, with consequent formation and entrainment of foams.

Another difficulty is caused by the fact that the gases coming from the vacuum system are available at pressures only slightly above atmospheric, such that a desulfurization system must have extremely low pressure drop in order to allow the gas to be sent for combustion, without the need to resort to the use of compression systems.

The use of ammonia solutions for the elimination of H ₂ S from gaseous mixtures is a well-known process, which is based on the following reactions:

NH ₃ + H ₂ S = NH ₄ HS

2NH ₃ + H ₂ S = (NH ₄ ) ₂ S

(NH ₄ ) ₂ S + H ₂ S = 2NH ₄ HS

Furthermore, the practical realization of an efficient and convenient process has until now been hampered by various operating difficulties. The currently known apparatus and processes for the abatement of H ₂ S to within acceptable limits require the use of quantities of NH ₃ in significant excess with respect to the stoichiometric ratio, the use of very dilute solutions to take into account the volatility of NH ₃ at temperatures normally used, and in any event the need to resort to quite high operating pressures. Additionally,

other processes resort to a high recirculation flow rate of the washing solution, with a consequent increase in the investment cost and energy consumption. Finally, none of the treatment methods so far used is able to guarantee that the pressure drop is compatible with the requirements cited above.

Description of the Invention

The invention now proposed overcomes the previously- cited limitations and problems of the earlier technology, making available desulfurization apparatus and procedure for gas which contains H ₂ S by the use of ammonia solutions. The current invention is able to furnish recovery yields which are considerably higher with respect to those achievable by traditional methods.

An additional object of the invention is that of realizing desulfurization apparatus and process which enable the efficient removal of H ₂ S, even without the use of quantities of ammonia in excess of the stoichiometric amount .

The invention also achieves the goal of realizing desulfurization apparatus and process with the use of ammonia solutions in which the desulfurized gases are completely free of NH ₃ .

Another goal of the invention is that of realizing apparatus and process particularly adapted to obtain efficient desulfurization of gases with an ammonia solution at a pressure that is substantially atmospheric.

The invention has additionally the goal of providing for the efficient desulfurization of gases using a solution of ammonia, making available a process and apparatus which does not cause significant pressure drop in the gas stream that is being subjected to the desulfurization treatment.

It is yet another goal of the invention to minimize the quantity of liquid used in the desulfurization treatment, such that the unit which carries out the successive separation of the gas from the liquid phase is not overloaded.

Finally, an additional goal of the invention is that of realizing an apparatus provided with packing beds which are able to offer high mass transfer and also provided with a distribution device having an improved ability, with respect to traditional devices, to irrigate the packing beds in the column sections, while at the same time having simpler and more economical structure than the already known analogous devices.

These and other goals are achieved in the desulfurization of incondensible gases coming from the vacuum distillation of heavy fractions of crude by providing a desulfurization column having first and second contact sections for the said gases. An ammonia solution is fed between the first and second sections. Water is fed to the top of the column above the second section. Suitable means are provided for promoting the wetting of the packed beds of the said first and second sections of the said column.

According to another characteristic of the apparatus of the invention the said means for wetting take the form of liquid distribution devices in the said sections of the desulfurizing column, each liquid distribution device being provided with a cylindrical body having a perforated base.

The apparatus of the invention is further characterized by the fact that the said distribution device may include a ring that is positioned above the said cylindrical body and that is suitable to collect and distribute the liquid stream fed to the said first section of the column.

Both of the two sections of the desulfurizing column preferably are filled by random or stacked packing material such as sold under the trademark CASCADE MINI- RINGS ^® or by structured packing such as the type sold under the trademark GEMPAK ^® .

The current process provides:

a first stage in which gas containing H ₂ S and which forms the initial gaseous mixture is brought into contact with fresh ammonia solution and with an aqueous solution coming from a second stage, and

said second stage, in which a gaseous stream liberated from the first stage is brought into contact with water, the aqueous solution resulting from the gas liquid contact in the said second stage being sent to the said first stage.

The said aqueous solution resulting from the said second stage is an ammonia solution obtained by contact between the water fed to the second stage and the ammonia entrained by the gas rising from the first stage. The ammonia solution employed in the first stage contains the stoichiometric quantity of ammonia available for the reaction with the H ₂ S contained in the said initial gas mixture.

The process of the invention is characterized further by the fact that the said initial gaseous mixture contains up to 500,000 ppm by weight of H ₂ S, and the desulfurized gas contains from less than 1 to 10 ppm by weight of residual H ₂ S. The said process additionally is carried out at a pressure below 1500 mbar abs, and the pressure drop of the gaseous stream between the inlet and the outlet of the column is not above 15 mbar.

With respect to traditional processes for the removal of H ₂ S from gaseous mixtures, that of the invention offers the advantage of allowing the almost complete absorption of the said sulfur compound by the use of a stoichiometric quantity of ammonia. In this way, as well as avoiding the extra costs due to the traditional use of excess quantities of reactants, the process of the invention furnishes desulfurized gases which are completely free of ammonia. The absence of excess quantities of liquid introduced into the column allows for a liquid stream leaving the desulfurization unit to have a total volume which can be treated by the downstream units of the apparatus, without the need to install new equipment of larger capacity than those originally installed.

In view of the current state of the art of available knowledge, the current invention provides the surprising and unexpected result of allowing the effective application of packed columns (of the type more fully described in the following) as desulfurization columns operating at a pressure slightly above atmospheric. In this way it has also been possible to obtain, downstream of the desulfurization treatment, a gas that has not been subjected to substantial pressure drop and which can be used without the need for additional compression equipment. In such a way, the current invention is particularly adapted to perform the desulfurization of incondensible gases coming from vacuum crude distillation units. These gases, after desulfurization, are reused in the furnaces of the same column.

The current invention, thanks to the presence of the previously-mentioned distribution device, offers the advantage of providing notable irrigation of the packing bed, with 300-400 irrigation points per square meter, while leaving an open area for the passage of the gases equal to twenty to twenty-five percent of the total cross-sectional area of the column. In traditional towers with riser-style distributors, in order to maintain the same area available for the passage of the gases, it has not been possible to obtain more than 60-65 irrigation points per square meter.

The distributor described herein offers in addition the advantage of having a very simple structure, which allows it to be inserted as a single cartridge into the column without requiring the internal supports or

intermediate flanges normally necessary in traditional equipment .

Description of the Drawings

The brief description above, as well as further objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of presently preferred but nonetheless illustrative embodiments in accord with the present invention when taken in conjunction with the accompanying drawings wherein:

Figure 1 illustrates a flow scheme of the invented apparatus.

Figure 2 illustrates in longitudinal cross-section the details of the desulfurization column of the apparatus in Figure 1.

Figure 3 illustrates the detail of the distribution device used in the column in Figure 2.

Description of the Preferred Embodiments

Figure 1 shows a knock out drum 1 for the separation of the liquid phase which generally is entrained in the gas stream 8 coming from a vacuum crude distillation column and from which it is desired to eliminate the H ₂ S.

Above the knock out drum 1, mounted directly on top, is a desulfurization column 2. For the treatment of a refinery gas coming from apparatus processing 3,000,000 tons/year of crude oil, this column would typically have a diameter varying between 200 and 300 mm.

Inside column 2 are two distinct sections, each having a single packed bed. A lower or first section 3 and an upper or second section 4 are positioned in cascade one above the other. Each of the sections is characterized by a portion containing a packing material. The preferred packing material is the metal version of a type shown in patents GB 1 385 672 and GB 1 385 673. It is sold under the trademark CASCADE MINI-RINGS ^® . An alternative but also suitable type of packing material would be structured packing of the type sold under the trademark GEMPAK ^® .

The first or lower section 3 of the column 2 effects the first stage of desulfurization, in which gas stream 14 from the knock out drum 1 is brought into countercurrent contract with a fresh ammonia solution 9. Solution 9 contains a stoichiometric quantity of NH ₃ that reacts with the H ₂ S contained in the initial gas mixture. In this first stage an initial desulfurization of the gas to be treated is effected.

The gas stream 15 liberated by section 3 still contains a certain quantity of H ₂ S. Stream 15 also entrains part of the ammonia with which it entered into contact in the packing material of the first section 3. The gas stream so composed enters the upper section 4 of the column 2, where it enters into countercurrent contact with a stream of water 10, again in a bed of packing material like that in section 3.

The ammonia entrained by the gas 15 which comes from section 3 of the column, by contact with the water stream in the packing of section 4, forms a weak ammonia

solution which in practice contains the stoichiometric quantity of NH ₃ that was not reacted in the first section of the column 2. In this way in the second section 4 the H ₂ S not removed in the first stage is absorbed, with the use of a quantity of ammonia which corresponds stoichiometrically to the composition of H ₂ S in the gas stream 15 liberated from section 3 of the desulfurization column. The sulfur-ammonia solution generated by section 4 of the column, by contact between the water 10 and the gas stream 15 previously described, makes liquid stream 17 which in turn falls back into section 3 below, combining with that produced by the stream of ammonia solution 9. The total sulfur ammonia liquid stream 16 exiting from the bottom of the column 2 falls into the knock out drum 1.

In effect therefore, due to a particular feature of the invention, the upper section 4 of the column realizes the final desulfurization of the initial gas mixture, eliminating the traces of H ₂ S not absorbed by the first stage 3. This absorption is additionally achieved by using a quantity of ammonia (present in gas stream 15) which comes directly from the first stage and which for this reason is that which corresponds exactly, from the stoichiometric point of view, to the concentration of H ₂ S in the gas exiting from the lower section 3 of the desulfurization column.

In this way the desulfurized gas 11 which exits finally from the top of the column 2 has a content of H ₂ S ranging from less than 1 ppm up to a maximum of 10 ppm by weight.

Added to this is the fact that the desulfurized gas exiting from the column 2 does not contain any trace of NH ₃ because it has reacted completely with the H ₂ S in the initial gas mixture. In traditional methods for desulfurization treatment the final gas always contains a certain quantity of NH ₃ , the elimination of which, as well as being necessary, is made particularly difficult due to the simultaneous presence of significant quantities of residual H ₂ S in the treated gas mixture.

Thanks to the use of the packed tower previously described and also due to the contribution of the direct connection between the column 2 and the knock out drum 1, the desulfurized gases 11 still have sufficient pressure to be sent to the vacuum crude tower furnaces. The method of construction indicated above has the advantage of reducing to the minimum the pressure drop (the reduction in pressure is in general less than 15 mbar) so as to make unnecessary the use of auxiliary compressors for the feeding of the desulfurized gas to the burners.

From the bottom of the knock out drum 1 there exits therefore a liquid phase 12 which contains the condensate separated from the gas stream 8 coming from the vacuum column, plus the sulfur-ammonia solution 16 composed of monosulfide and bisulfide of ammonia, exiting from the bottom of the desulfurization column 2. In particular, due to the invention, the quantity of the liquid phase 16 which comes from the desulfurization column 2 does not exceed usually ten percent of the total rate of the liquid 12 exiting the knock out drum. In this way, as already explained, the liquid stream 12 exiting the knock out drum 1 and being sent to the stripper 5 has a volume

which does not exceed the design capacity of the latter, even in apparatus that existed prior to the installation of the column 2. The gas fraction 13 separated by the stripper 5 is finally sent to the Claus unit for the recovery of the sulfur.

As better seen in Figure 2, the desulfurization column 2 has inlets 6 and 7 for the input of ammonia solution 9 to section 3 and the water stream 10 to section 4 respectively. These liquid streams 9 and 10 are in turn received within the respective sections 3 and 4 by a distribution device 18 which is illustrated in Figure 3. It has been emphasized that the distributor illustrated in Figure 3 is designed to be mounted above section 3 of the column. The corresponding distributor 18 positioned above the section 4 of packing material differs from that of section 3 by the absence of a collector ring 19, which will be better described as follows.

The distribution device 18 includes a support 20, which may conveniently be formed of upper and lower cross bars 20a connected by four support rods 20b. A cylindrical body 21, and above it a ring 19, are fixed on the four support rods. The cylindrical body 21 has in particular a base 22 equipped with holes 23. It receives the liquid phases coming from above ( water in the case of section 4 and ammonia solution 9 plus liquid stream 17 in section 3 ) and it distributes the liquid phases uniformly onto the packed beds present in the sections 3 and 4.

With the distributor device 18 installed above section 4 of column 2, the water stream 10 fed to section 4 is received directly by the cylindrical body 21 described above. A portion of the liquid stream fed to section 3 of the column is received above the cylindrical body 21 by the previously-mentioned collector ring 19, which has the function of collecting that portion of the stream of liquid 17 coming from section 4 that is near the wall of the column and distributing it into the body 21 below.

The ring 19 has a surface which is preferentially inclined toward the center of the column so as to favor the collection of the liquid 17 which adheres to the walls of the column and send it into the perforated cylindrical body 21 of the distributor 18. For this purpose the ring 19 advantageously has a larger diameter (for example 260 mm) than that of the body 21 below (for example 230 mm) , the difference representing the surface left open for the passage of gas, which is equivalent to twenty to twenty-five percent of the total available area.

The distributor 18 is mounted on a containment grid 24.

The distribution device 18 illustrated in Figure 3 is particularly well adapted to flanged columns of less than 900 mm diameter.

The following examples are given to further illustrate the invention, without however limiting it to

the details given.

Example 1

A column of diameter 300 mm had a first section 5 m high and a second section 3.3 m high. Both sections were equipped with 25 mm CASCADE MINI-RINGS ^® packing material and were separated by a distributor of the type previously described. This distributor presented a collector ring having an external diameter of 300 mm. The cylindrical body had an external diameter of 265 mm. A second distributor at the top of the column was identical to the former one, but without the collector ring. Into this column was fed 300 kg/h of gases having the following percentage composition by weight.

^H 2 6.6 N ₂ +0 ₂ 11.3

Cl 29.1

C2 20.6

C3 15.4

C4 7.1

C5 1.2 co ₂ 1.9

H ₂ S 6.8

Between the two sections of the column 100 kg/h of water solution were fed containing twenty percent by weight of NH ₃ , while at the top of the column 800 kg/h of water were fed. At the top of the column a gas was obtained with a H ₂ S content less than 1 ppm.

The gas pressure at the entrance of the column was measured at 1300 mbar, and the pressure at the top was 1290 mbar, giving a pressure drop of 10 mbar.

Example 2

A column of diameter 450 mm had an upper section 4 m high and a lower section 5.50 m high. Both these sections were equipped with GEMPAK ^® structured packing. Between them a distributor was placed that was the same as the one described in Example 1, except that it had a cylindrical body having an external diameter of 390 mm and a collector ring having an external diameter of 450 mm.

At the top of the column there was placed a second distributor that was identical to the first one but was not supplied with a collector ring. Into this column was fed 700 kg/h of gas having the following composition by weight :

^H 2 3.8

^N 2 7.3

°2 1.2

Cl 20.8

C2 12.3

C3 11.8

C4 6.5

C5 2.7

C6 0.8 co ₂ 1.1

H ₂ S 31.7

Between the two sections of the column 1300 kg/h of water solution were fed containing nineteen percent by weight of NH ₃ . At the top of the column 2200 kg/h of water were fed.

The gas obtained at the top of the desulfurization column had a H ₂ S content less than 10 ppm.

The gas pressure at the entrance of the column was measured at 1350 mbar, and the pressure at the top was 1335 mbar, yielding a pressure drop of 15 mbar.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be used without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention therein. For example, the desulfurization column 2 could have a completely independent structure from the knock out drum 1.