Patent Description:
Certain gas streams, for instance tail gas streams from the Claus process, contain elemental sulfur that should be removed as much as possible to reduce overall sulfur emissions. Typically, tail gas processes such as Superclaus or amine-based tail gas treatment units (TGTU) use sulfur condensation at temperatures around <NUM> to remove elemental sulfur vapor. Any sulfur species (e.g. SO<NUM>) in the tail gas may also be converted back to H<NUM>S in a hydrogenation reactor before absorption and/or recycling it back to the front of the process (e.g. Claus process). Alternatively to these conventional processes, elemental sulfur can be solidified (typically sublimed) by cooling the gas stream containing the elemental sulfur (herein also referred to as sulfur-containing gas stream). Examples of such processes and the heat exchanger that can therein be used are described in <CIT> and <CIT>.

<CIT> and <CIT> describe processes that are generally based on leading the gas stream through tubes of a shell and tube heat exchanger to cool the gas stream to a temperature well below the solidification temperature. To facilitate removal of the solidified sulfur from the heat exchanger, the heat exchanger can be placed at a slant or vertically to make use of gravity once the sulfur come falling off. However, the anticipate shrinkage of the solidified sulfur that was believed to result in the sulfur come falling off did not take place.

In addition, the conventional processes were found to suffer from operations below the water dewpoint due to a poor control of the temperature of the cooling air used. This in turn led to high corrosion rates in the presence of Claus tail gas. Moreover, plugging of the inlet of the tubes as a result of cold tube sheet was observed. Typically, more than half of the tube inlets were obstructed by frozen solid sulfur, increasing the overall velocity through the remaining tubes. Furthermore, the high velocity of the gas stream resulted in a thick and dense layer of solid sulfur which was difficult to remove. Finally, regeneration of the heat exchanger (i.e. removal of solidified sulfur from one or more tubes) was performed online via heating of the entire shell. This resulted in a temporary much lower sulfur removal rate.

Shell and tube heat exchangers for the removal of substances capable of subliming are also known in other fields. For example, although not directly applicable and not directly related to the recovery of sulfur, a discontinuously operating desublimator based on a shell and tube configuration has been described in <CIT> for the sublimation of phthalic anhydride. A drawback of the apparatus described in <CIT> is that it must be operated in a discontinuous manner, meaning that it is operated by a loading phase and an unloading phase. During the unloading phase, the loading phase is discontinued and no recovered product can temporarily be recovered. This negatively influences the process efficiency.

<CIT> discloses an apparatus comprising a reactor wherein at least in a first zone a plurality of heat exchange means are present. The heat exchange means comprise a tube formed by at least one duct. Notably, the purpose of <CIT> to minimize the deposition of insulating coke on the heat exchange means is contrasting with the purpose of the present invention to deposit solid elemental sulfur on the heat exchange means.

An object of the present invention is to provide an improved process and an improved apparatus for the removal of elemental sulfur from a sulfur-containing gas stream that addresses at least one or more of the above-mentioned drawbacks.

Surprisingly, the present inventors found that maintaining a laminar flow of the sulfur-containing gas stream through the shell and tube heat exchanger results in improved removal of sulfur. This is particularly surprising because in other recovery processes, laminar flow gives generally poorer results than turbulent flows; in line with the common theory and observations on mass transfer rates in other systems that generally show an increase in mass transfer coefficient and rates by reaching or improving the turbulence of the system. However, for the present process, a laminar flow was found to lead to much higher sulfur removal.

In addition, the present inventors surprisingly found that maintaining the laminar flow of the sulfur-containing gas stream in accordance with the invention results in a more favourable form of the solidified sulfur, namely in a needle-type crystal structure. This favourable form allows easier removal of the solidified sulfur from the heat exchanger by melting of the crystals. In contrast, in the processes using a turbulent gas stream as described in <CIT> and <CIT>, thick and dense solid sulfur cakes are typically formed which are more difficult to remove by melting because an insulating layer is formed during this removal.

Accordingly, in one aspect, the present invention is directed to a process for the recovery of elemental sulfur from a sulfur-containing gas stream comprising leading said sulfur-containing gas stream (herein also referred to as the gas stream) over a flow path through a shell and tube heat exchanger and cooling said stream to a temperature below the solidification temperature of elemental sulfur to obtain solidified elemental sulfur, wherein said sulfur-containing gas stream is lead over the flow path with a flow that is essentially laminar over essentially the entire length of the flow path.

Flow patterns such as laminar and turbulent flow can be described with the Reynolds number. The flow pattern and the Reynolds number can be influenced by various process parameters and equipment choices such as the design of the shell and tube heat exchanger and the flow velocity as further detailed herein.

In a further aspect, the present invention is directed to an apparatus for the recovery of elemental sulfur.

As illustrated in <FIG>, the apparatus comprises a shell and tube heat exchanger (<NUM>) that comprises a shell (<NUM>) defining a recovery chamber (<NUM>) and one or more tubes (<NUM>-<NUM>) for leading a cooling fluid through wherein the tubes (<NUM>-<NUM>) are positioned substantially parallel in the recovery chamber (<NUM>). Further, the shell and tube heat exchanger (<NUM>) comprises a gas inlet (<NUM>) and gas outlet (<NUM>) that allow the respective in- and outflow of the gas stream. In the heat exchanger is further provided a flow path (<NUM>) in between the gas in- and outlets for a sulfur-containing stream. The flow path (<NUM>) is provided around the tubes (<NUM>-<NUM>).

To have a sufficiently large cooling surface on which the elemental sulfur can solidify, the tubes preferably comprise a cooling surface area that is at least in part formed by outwardly extending projections (<NUM>) which are extending away from the tube into the flow path of the of the sulfur-containing gas stream in the recovery chamber. In general, the projections are extending perpendicular from the surface of the tubes. The projections can be given various shapes such as for examples fins (e.g. straight fins, waffled fins and/or curved fins), spikes, rods and the like. The shape of the projections can be selected such to optimize the sulfur recovery yield. For each tube, the projections can independently be absent or present, and if present on a tube the projections can have different shapes over the length of the tube. In a particular embodiment, all projections are essentially similarly shaped. Moreover, the projections can form a bridge between two or more tubes.

Preferably, the cooling surface area increases over the length of the flow path. It was found that by an incremental cooling surface area, an adequate sulfur removal can be provided as well as a good distribution of the sulfur load over the various tubes. By increasing the cooling surface area and concomitantly the removal efficiency, the decreasing concentration of sulfur and thus driving force for mass transfer in the gas flow can be compensated.

Increasing the cooling surface area can be achieved in various means. For example, the density of the cooling tubes in the recovery chamber can be increased or the density or number of the outwardly extending projections per surface area of the tubes can increase over the length of the flow path. The length of the projections can also be increased. In a preferred embodiment, the density of the projections increases with the flow path. For example, the amount of projections (<NUM>) increases for each tube (<NUM>-<NUM>) following the direction of the flow path (<NUM>). In even more concrete terms, a first tube (<NUM>) can have less projections (<NUM>) than the subsequent tubes (<NUM>-<NUM>), as illustrated in <FIG>.

Further, as also illustrated in <FIG>, the apparatus according to the present invention is preferably configured such that the shell and tube heat exchanger comprises two or more sections (<NUM>-<NUM>) in the flow direction of the flow path. Each of these sections comprises a part of the recovery chamber and one or more tubes. The apparatus is further configured such that the tubes in each section can independently from other sections contain and lead a cooling fluid or a heating fluid. If a tube is leading a cooling fluid, this tube is capable of recovering sulfur from the gas stream, while if the tubes is leading a heating fluid, the tube is capable of unloading sulfur from the section in which the tube is positioned. Accordingly, by enabling independently operating the sections, the apparatus can be operated in a continuous manner.

As described herein-above, the sulfur-containing gas stream flows essentially laminar over essentially the entire length of the flow path. Flow patterns of a flowing stream can be expressed with Reynolds number. For the present process, the Reynolds number is preferably less than <NUM>, more preferably less than <NUM>, even more preferably less than <NUM>, even more preferably less than <NUM>, most preferably less than <NUM>. It was surprisingly found that a decreasing Reynolds number corresponds with an increasing yield. This is an uncommon finding in the art.

Maintaining a laminar flow in accordance with the present invention can be achieved by choosing the appropriate process parameters and the appropriate apparatus setup. One of these parameters is the gas velocity of the sulfur-containing gas stream over the flow path. By keeping this velocity sufficiently low, the gas flow pattern can be maintained essentially laminar. However, the gas velocity should also not be too low as to be detrimental to the process efficiency and/or economic viability because the too low gas velocities require a too big and uneconomical shell to maintain a certain process efficiency. Therefore, in preferred embodiments, the sulfur-containing gas stream over the flow path has a gas velocity in the range of <NUM> to <NUM>/s, preferably <NUM> to <NUM>/s.

Moreover, carrying out the process in the apparatus as described herein, has a positive effect on the flow pattern and the process. Accordingly, the process is preferably carried out in the shell and tube heat exchanger that comprises the shell defining a recovery chamber and tubes through which a cooling fluid is lead and wherein said flow path of the said sulfur-containing gas stream is located in said recovery chamber around said tubes. Moreover, the tubes are preferably positioned substantially parallel to each other and which longitudinal direction is essentially perpendicular to the flow path of the sulfur-containing gas stream in the recovery chamber.

To solidify the sulfur in the sulfur-containing gas stream, the stream is typically cooled in the shell and tube heat exchanger to a temperature in between the dew point of water and the solidification temperature of elemental sulfur. This temperature avoids condensation of water, which in typical applications results in excessive corrosion rates, and allows the crystallization of sulfur in high purity. The temperature preferably allows for sufficient mass transfer driving force. In this respect, preferable temperatures at atmospheric pressure generally lie between <NUM> and <NUM>, more preferably between <NUM> and <NUM>, most preferably between <NUM> and <NUM>. It was found that the temperature may directly influence the efficiency of the process. In particular, the difference between the temperature of the entering sulfur-containing gas stream and the temperature to which it is cooled in the shell and tube heat exchanger may influence the efficiency. Generally, a larger temperature difference allows for a higher efficiency. However, care can be taken to keep the cooling temperature from reaching the water dew point of the process gas.

Cooling of the gas stream is achieved by the cooling fluid flowing through the tubes in the shell and tube heat exchanger. The cooling fluid can in principle be any type of suitable fluid, such as water, steam, alcohol-containing liquids, air, and the like. Cooling the gas stream results in loading of the heat exchanger with solidified sulfur. To increase the effectiveness of the cooling liquid, the tubes comprise a cooling surface area that is at least in part formed by outwardly extending projections, preferably fins, that are extending away from the tube into the flow path of the of the sulfur-containing gas stream in the recovery chamber, as described herein-above. Moreover, as detailed above, the cooling surface area preferably increases over the length of the flow path. This can be achieved by increasing the number of the outwardly extending projections over the length of the flow path.

Notwithstanding the embodiments wherein the tubes can also be used to transport a heating fluid, it is preferred that the cooling is carried out at essentially the same temperature throughout the shell and tube heat exchanger. In other words, it is preferred that the cooling fluid in all tubes in the recovery chamber is maintained at essentially the same temperature with a tolerable deviation of ± <NUM>, preferably ± <NUM>.

When a tube or set of tubes is loaded with solidified sulfur, the tubes can be used to unload the sulfur. To this end, the heating fluid can be led through the tubes. The heating fluid can be any type of suitable fluid. Low-pressure steam or hot oil are for example suitable. Accordingly, the process can comprise a loading mode and/or an unloading mode.

In the unloading mode, the heating fluid typically has a temperature above <NUM>, preferably above <NUM> such as about <NUM>. At these temperatures, the solidified sulfur melts into liquid sulfur which detaches from the cooling surface area at which it had solidified. The liquid sulfur can then be led as a liquid sulfur stream out of the recovery chamber. Alternatively or additionally, the solidified sulfur may detach from the cooling surface area at which it had solidified by gravitational forces after which it may be recovered out of the recovery chamber.

To enable carrying out the loading and unloaded modes simultaneously, in preferred embodiments, the said shell and tube heat exchangers comprises two or more sections in the flow direction of the flow path. As such, in each section independently, the cooling fluid or the heating fluid to liquify solidified elemental sulfur flows can be controlled. Every section comprises a part of the recovery chamber and a tube or set of tubes. Advantageously, in preferred embodiments, the temperature of the heating stream when used in one or more sections is not too high in order to not be detrimental to the cooling capability of other sections that are still being in the loading mode. As such, the heating fluid is preferably kept below <NUM>. However, additional measures such as heat insulation between the sections (e.g. by sufficient spacing) can be taken to limit the effect of unloading a section on a section that is being loaded.

In <FIG>, a preferred embodiment of the process in accordance with the present invention is illustrated. In the shell and tube heat exchanger as illustrated in <FIG>, the sulfur-containing gas stream (<NUM>) enters through the inlet (<NUM>) after which it continues over the flow path (<NUM>) through the recovery chamber. In <FIG>, the first section (<NUM>) is in unloading mode, as the heating fluid (<NUM>) is led at <NUM> through the tube (<NUM>). In this section (<NUM>), the sulfur (<NUM>) is liquid and detaching from the tube (<NUM>). The other sections (<NUM>-<NUM>) are in loading mode, as the cooling fluid (<NUM>) is led at <NUM> through the respective tubes (<NUM>-<NUM>). Solidified sulfur (<NUM>) at loaded on the surface area of these tubes (<NUM>-<NUM>). Further, as illustrated in <FIG>, a sulfur-lean gas stream (<NUM>) is exiting the shell and tube heat exchanger through outlet (<NUM>).

The present process provides recovered sulfur and a sulfur-lean gas stream. The amount of sulfur present in the sulfur-containing gas stream is typically in the range of <NUM> to <NUM> ppmv. After the sulfur has been recovered from this gas stream, the sulfur-lean gas stream typically contains <NUM> to <NUM> ppmv sulfur.

The present process can be carried out to remove elemental sulfur from basically any type of vent gasses from the sulfur degassing and/or storage sections. The present process can be particularly advantageously carried out on a tail gas stream originating from said Claus plant. Accordingly, it is preferred to position the shell and tube heat exchanger in accordance with the invention after a Claus plant.

The present invention can be illustrated with the following nonlimiting examples.

In a shell and tube heat exchanger, a preheated inlet gas stream of N<NUM> saturated with elemental sulfur at a temperature between <NUM> - <NUM> was passed over a tube bundle of <NUM> finned tubes constructed in <NUM> rows in series with a total height of <NUM>. The tube wall temperature was kept at <NUM> by cooling oil. The fins had a height of <NUM> and were constructed with <NUM> fins per <NUM> of tube. The gas stream leaving the tube bundle was routed to a number of cold traps to solidify any remaining sulfur present. Total run time of the system was <NUM> hours. The impact of velocity on the systems performance was assessed by changing the nitrogen flow rate.

Via gravimetric analysis the mass balance over the system could be established allowing determination of the sulfur removal. The results are shown in Table <NUM>.

The experiment of Example <NUM> was repeated with a tube bundle containing <NUM> finned tubes constructed in <NUM> rows in series with a total height of <NUM>. The fin density was increased to <NUM> fins per <NUM> the fins extending at a similar height of <NUM> as in Example <NUM>. By lowering the tube count, the heat exchange area was kept identical to the set-up of Example <NUM>.

Via gravimetric analysis the mass balance over the system could be established allowing determination of the sulfur removal.

The test result from Example <NUM> and <NUM> illustrate the positive impact of lowering the velocity with respect to the sulfur removal efficiency.

Reference is made to Example of patent <CIT>. A process gas of <NUM> was cooled to <NUM> inside the <NUM> tubes with a length of <NUM> and inner diameter of <NUM> of a heat exchanger while the tubes were kept at <NUM> with cooling air. The Reynolds number of the gas was <NUM>, at a velocity of <NUM>/s. The sulfur removal was <NUM>%.

A commercial pilot unit of patent <CIT> and <CIT> was constructed. Data of performance test runs on this unit after a few years of operation are listed in Table <NUM>.

It was observed during inspection that the inlet of a number of tubes were blocked by elemental sulfur as a result of the tubesheet temperature being below the sulfur solidification temperature.

Claim 1:
Process for the recovery of elemental sulfur from a sulfur-containing gas stream comprising leading said sulfur-containing gas stream over a flow path through a shell and tube heat exchanger that comprises a shell defining a recovery chamber and tubes through which a cooling fluid is led, and cooling said stream to a temperature below the solidification temperature of elemental sulfur to obtain solidified elemental sulfur, wherein said sulfur-containing gas stream is led over the flow path with a flow that is essentially laminar over essentially the entire length of the flow path, and wherein said flow path of the said sulfur-containing gas stream is located in said recovery chamber around said tubes.