Method and plant for preprocessing crude methanol having a paraffin content

The present invention relates to a method for preprocessing crude methanol having a paraffin content and to a plant for preprocessing crude methanol.

Background

A potential raw material for the manufacture of methanol is synthesis gas. Synthesis gas is produced on a large scale by steam reforming natural gas or other lighter hydrocarbon fractions or by partial oxidation from coal.

The principal process for large-scale production of synthesis gas for methanol synthesis is combined reforming or autothermal reforming. These processes involve the use of oxygen-fuelled reactors and catalysis at an elevated temperature. The characteristics of the synthesis gas and the process temperature have been found to give the subsequent methanol synthesis a tendency to catalyse wax as a by-product of methanol. This byproduct causes major production problems in, inter alia, the methanol condensation process as it settles in pipes, heat exchangers and other process equipment and reduces their capacity. The wax may also lead to operational problems further downstream in the process due to deposits and plugging of flow meters and instruments. The wax deposit will result in lost production in the form of non-condensed methanol, reduced production capacity and possible shutdown for removing wax from the heat exchangers and equipment. There are also instances of wax formation causing a breakdown of rotating equipment because liquid separators have malfunctioned due to wax deposit. Methanol synthesis is a continuous process and consequently the preprocessing of the crude methanol is preferably also a continuous process.

The term "wax" is used in this context as a generic term for paraffins that may be formed during the aforementioned processes.

The nature and deposit mechanism of the wax in methanol synthesis has, as far as is known, not been the subject of systematic studies. However, wax formation in connection with the Fischer-Tropsch reaction has been the subject of substantial research and work, made topical by increasing oil prices and a growing demand for middle distillates. Several processes have been developed for dewaxing intermediate distillates and oil fractions, and of these extraction in a solvent such as methylethyl ketone (MEK) and propane, and catalytic dewaxing (chain-breaking) are in use. Such

processes require substantial quantities of wax in order to be profitable. In methanol synthesis, wax is found in such small amounts that unless there are other upgrading plants in an industrial complex it merely represents a problem in the production process. Also, these processes cannot be used directly on the wax-containing crude methanol from the synthesis because methanol has relatively high solubility in the extraction agent and methanol is easily cracked in catalytic processes.

Thus, to find a process for the efficient handling of the wax from methanol synthesis such that shutdowns are avoided is of great economic interest.

From JP61257934 a method is known for condensing and separating methanol which comprising separating the wax-containing methanol from the crude methanol prior to the final condensation of the methanol. The method described requires substantial temperature control and has considerable limitations as regards the cooling media that can be used. Furthermore, there is no disclosure of a method for passing the wax- containing methanol back to the process or preprocessing the methanol prior to such recirculation. Nor is a process for utilising the wax-containing methanol disclosed.

The object of the present invention is to provide a method which efficiently prevents wax deposit and blockage of heat exchangers and similar equipment whilst obtaining a high capacity utilisation of the crude methanol in an energy-efficient manner.

Methanol synthesis:

Methanol synthesis consists primarily of compressors for compressing the synthesis gas to the required reactor pressure and for circulation of the gas in the reaction loop, heat exchangers for heating the synthesis gas to the reaction temperature, a methanol reactor or reactors and lastly heat exchangers for cooling and condensing out the methanol. There exist several processes with different types of reactors. One of the most competitive reaction processes is marketed by Lurgi.

The heat exchanger arrangement for cooling the reaction products in methanol synthesis can be configured differently, but all have the following principles in common: the reaction heat from the reactor is controlled by intermediate cooling (adiabatic reactors) or water baths (Lurgi reactor); - the reaction heat from the reactor is utilised in heat exchange against the inlet gas and/or in the generation of steam. In other cases, different types of heat integration can also be used;

the residual heat (typically from 15O ⁰ C and below) of the reaction products is heat-integrated with the process (e.g., preheating of feed water for steam production) whilst the actual final condensation of the methanol product takes place by use of a final condenser capable of using cooling water or air as cooling medium. The final cooling is terminated typically at 30 - 5O ⁰ C.. _.

Wax is formed in the methanol reactor as a result of the catalyst therein being and/or becoming contaminated with small quantities of iron, cobalt or nickel particles. The wax formation may vary according to the process parameters and the temperature control in the reactor and depending on how many particles over time are introduced into the reactor. Wax formation is thus highly dependent upon the age of the catalyst. However, wax formation is assumed to be a common feature for all types of reactors depending upon the aforementioned parameters.

Experience has shown that the problem of wax in methanol synthesis first becomes manifest in the final cooler where the heat exchanger tubes have slowly lost their efficiency due to wax deposit, and in subsequent separation equipment where the separation efficiency has been reduced because wax has blocked mist eliminators in the separator. Furthermore, wax has been a problem for control valves which control the outflow of crude methanol from the separator and in the water stream from the upgrading/distillation of the crude methanol to a high-grade commodity.

Typically, about 2/3 of the methanol product is condensed out in this final cooler, whilst about 1/3 has already been condensed out in the preceding heat integration (inlet/outlet exchanger and optional preheating of, for example, boiler water). The final cooler normally operates at an inlet temperature of about 100 ⁰ C and an outlet temperature of 25 - 5O ⁰ C. The melting point of wax from FT (Fischer-Tropsch) synthesis is given to be between 45 and 106 ⁰ C (ASTM D87) with hydrocarbon chains of between 20 and 30 carbon atoms.

In tests carried out at Statoil's methanol synthesis plant at Tjeldbergodden, it has surprisingly been found that the wax deposit is reduced to a completely marginal amount at temperatures greater than about 60 ⁰ C.

The present invention is based on the fact that virtually all the wax can be condensed out in liquid form, together with methanol and water, ahead of the final cooler, at a temperature and using a cooling medium that do not cause wax deposit in the heat

exchangers. The wax containing condensate, with dissolved gases, is separated from the product gas and passes to a separate process for wax separation before the methanol is passed back to the process. The separated product gas is passed to the final cooler for condensing out the residual methanol which passes to a high-pressure separator in the usual manner. The present invention also comprises the possibility of allowing the wax containing condensate to bypass the process for wax separation and pass to a processing system common to the whole crude methanol processing and located downstream.

Consequently, the present invention provides a method for preprocessing a crude methanol product gas comprising methanol, water, wax and residues of synthesis gas, wherein the method comprises:

- cooling the product gas;

- partly condensing the product gas to a temperature above the temperature of the melting point of the wax, the method being characterised in that it comprises - separating the condensate from the gas phase so as to obtain a first, wax containing condensate stream;

- condensing the gas phase further by cooling it to a temperature below the boiling point of methanol so as to form a second, substantially wax-free condensate stream;

- optionally separating the wax from the first condensate stream; and - combining the two condensate streams for further processing.

The invention further provides a plant for preprocessing a crude methanol product gas comprising methanol, water, wax and residues of synthesis gas, wherein the plant comprises a first cooling device for cooling and partly condensing the product gas to a temperature above the melting point of the wax, a first condensate separator in fluid communication with the first cooling device, the separator comprising a first condensate outlet and an outlet for the remainder of the product gas, this outlet for the remainder of the product gas being in fluid communication with a second cooling device comprising an outlet in fluid communication with a high-pressure separator, the high-pressure separator comprising a second condensate outlet and a residual gas outlet, characterised in that the plant further comprises a fluid communication device which puts the second condensate outlet of the high-pressure separator in fluid communication with the first condensate outlet of the first condensate separator.

The term "wax" as used here should be understood to comprise all paraffins and paraffin-like compounds that are formed as by-products during the production of methanol from synthesis gas with the aid of one or more catalysts.

The present invention will now be described in more detail with reference to the attached drawings, wherein:

- Figure 1 shows a flow chart outlining first and second preferred embodiments of the present invention;

Figure 2 shows a flow chart indicating first and third preferred embodiments of the invention.

A first embodiment of the present invention is shown in Figure 1. The figure shows a synthesis gas stream 11 that is fed to a methanol reactor 1. The crude methanol stream 12 formed is passed into a first heat exchanger 2. Here, the crude methanol is cooled by heat exchange with a second stream, not shown, which may be the inlet stream of synthesis gas to the methanol synthesis, water for generating steam or any other stream which preferably can make use of the reaction heat from the generation of methanol. The thus obtained partly cooled crude methanol stream is passed via pipeline 21 into heat exchanger 4 for further cooling and partial condensation of the crude methanol. The heat exchange takes place against a stream 13/14 capable of utilising the existing residual heat, for example, preheating of feed water for steam production or the like. The cooled crude methanol is subsequently passed through pipeline 22 into a hot condensate separator 6. Here the gaseous methanol and synthesis gas are separated from the wax containing condensate and the gases are passed through pipeline 23 to a final cooler 8. The cooling in the final cooler 8 is preferably carried out by heat exchange with cooling water or air. The cooling includes condensation of the essentially wax-free methanol. The cooled stream exits the final cooler 8 through pipeline 24 and is passed into a high-pressure separator 10, where non-converted gas and other gaseous by-products are separated out and exit the separator via pipeline 29. This stream of starting materials and by-products may either at least partly be passed back to the methanol synthesis as stream 30, optionally via a compressor 3 and pipeline 32, or a part of this gas stream 29 can be taken out and used as fuel gas, shown as stream 31. The condensate exits the high-pressure separator 10 through pipeline 27 and is passed to a low-pressure separator 51.

Here, the wax containing methanol condensate is, in the first embodiment of the present invention, passed out from the hot condensate separator 6 through pipeline 25 and 26 and is then combined with the condensate stream 27 and forms a single crude methanol stream 28. This crude methanol stream can be treated according to the prior art. In the

embodiment disclosed here the crude methanol 28 is passed into a low-pressure separator 51 for further purification. The purified crude methanol is passed via pipeline 52 to a crude methanol tank 54.

An alternative embodiment is outlined by the broken line 58. In certain cases it will be possible to pass the wax containing condensate 26 into the high-pressure separator 10 for removal of residual gas.

Surprisingly, it has been found that it is possible to bring the two condensate streams together after the cooling/condensation steps. The further processing will consequently also only comprise the preprocessing of one stream. When performing the invention according to the first embodiment, the wax is separated out further downstream in the process as it is passed together with the condensed methanol, often via a storage tank, to a distillation process. In this distillation process, the methanol will be driven off whilst a condensed water/wax stream is formed as a bottom product.

This first embodiment for passing the wax out of the methanol synthesis is therefore highly suitable for use when modifying existing plants which often already have installed one form of wax handling on this condensed stream.

In a second embodiment of the present invention, also shown in Figure 1, the condensate containing essentially all the wax from hot condensate separator 6 is passed via pipeline 25 and 40 into a wax separator 43. Here, the condensate is cooled further, which is illustrated in this figure in that a cooling medium, for example, cooling water, is passed through pipeline 44 into heat exchanger 57 and out again through pipeline 45. By lowering the pressure inside the wax separator 43, residues of synthesis gas and gaseous by-products will be released, and removed through pipeline 42 and optionally used as fuel gas. Upon cooling, the wax precipitates as solid wax on the cooling surface, in the embodiment illustrated here on the outside of the heat exchanger 57. Thus, a purified crude methanol product is obtained which is passed out through pipeline 48 and which is then mixed with the crude methanol product from the final cooler, either in that it is passed through pipeline 50 and mixed with the product stream 27 from the high-pressure separator 10, or in that it is passed through pipeline 53 and mixed with the product stream 52 from the low-pressure separator ahead of or in the crude methanol tank 54. Which solution is chosen will depend on the degree of purity desired for the crude methanol in the crude methanol tank and the selected operating conditions for the wax separator. If the crude methanol product stream from the wax

separator contains substantial amounts of dissolved gases, it will often be desirable to remove these in the low-pressure separator 51, and the choice will be made to mix the product streams ahead of or in the low-pressure separator.

The cooling in the wax separator can take place in different ways and the cooling medium may be air, cooling water or a cooling fluid depending on the purification requirements and preference/availability.

This second embodiment lowers the amount of wax to a level that prevents additional wax from precipitating when the methanol stream is subjected to cooling in the pipe systems.

To clean the heat exchanger tubes 57 and to remove the separated wax, the pipes are heated periodically using a heating medium, for example, steam or water. The steam is supplied via pipeline 47 and valve 46 whilst the supply of cooling medium via pipeline 44 is interrupted. At the same time, the supply of condensate via pipeline 40 is stopped. If such a plant is operated continuously, the condensate from hot condensate separator 6 will instead be passed via pipeline 25 and 26 directly to the low-pressure separator, or the condensate will be passed via pipeline 58 temporarily to the high-pressure separator 10, whilst the heat exchanger 57 is cleaned. By heating the heat exchanger 57, the wax melts and is directed out of the wax separator 43 via pipeline 48 and valve 49 to receptacle 55 for collection and optional additional purification. The valve 49 is only open in connection with the cleaning of the heat exchanger 57.

The design of the heat exchanger 57 is not specifically limited and it may therefore have any design that allows cooling using a cooling medium and heating using a heating medium to the relevant temperatures, and takes into account wax deposit and drainage of melted wax.

The first and the third advantageous embodiments are shown in Figure 2. In these embodiments a synthesis gas 111 is heat exchanged with a crude methanol product stream 120 from a methanol reactor 101 in a heat exchanger 102. A first cooling of the crude methanol is thus obtained. The crude methanol is passed on through pipeline 121 into a second cooler 105. Here, the crude methanol is cooled so that a portion of the methanol including essentially all the wax is condensed out and exits the cooler through pipeline 125. The cooler 105 is shown here as a combined cooler and separator and its function thus corresponds to the cooler 4 and the separator 6 in Figure 1. The cooling

medium that is passed in though pipeline 113 and out through pipeline 114 may be any cooling medium which cools the crude methanol to a temperature above the melting point of the wax, and which at the same time ensures that a substantial portion of the methanol is condensed. The cooling medium is preferably another process stream that can utilise the heat from the methanol synthesis, for example, the cooler can preheat boiler water. The cooling medium may have an inlet temperature of between 4 and 8O ⁰ C, especially 15 - 17 ⁰ C and an outlet temperature of 60 - 12O ⁰ C. The cooling medium may have an inlet temperature below the melting point of the wax provided no inner surfaces are formed in contact with the process stream that have a temperature lower than the melting point of the wax, which is assured by the sizing of the equipment.

The second stream 123 out of the second cooler 105 comprises methanol in gas form, residues of synthesis gas and other gaseous by-products, but is essentially wax-free. The stream 123 is finally cooled in a heat exchanger 108 and fed to a high-pressure separator 110 as described above. Synthesis gas and optionally other gaseous byproducts are passed from the separator through pipeline 129. The synthesis gas may either be passed via pipeline 130 into a compressor 103 and via pipeline 132 into the heat exchanger 102 together with the synthesis gas stream 111, or the synthesis gas may be passed out of the high-pressure separator through pipeline 131 and be used, for example, as fuel gas. The product stream from the high-pressure separator 110 comprising condensed methanol is passed out through pipeline 127 and then into a low-pressure separator 151 for further removal of other gaseous components. The crude methanol is passed from the low-pressure separator 151 via pipeline 152 to a crude methanol storage tank 154.

In the first embodiment of the present invention the condensate containing essentially all the wax from the second cooling device 105 is passed via pipeline 125 and 126 into the low-pressure separator 151. Such a direct passing of the condensate to the low- pressure separator will also normally be a desirable possibility in other embodiments to give increased operational flexibility.

A third embodiment is also illustrated in Figure 2. Here, the condensate containing essentially all .the wax from the second cooling device 105 is passed via pipeline 125 and 160 into a distillation column and/or stripper 161 which is operated above the melting point of the wax. The top fraction that is withdrawn via pipeline 163 consists of a mixture of methanol and water and residues of synthesis gas. This mixture is cooled

in a heat exchanger 164, whereby water and methanol are condensed out. In a separator 169, water and methanol are separated from the synthesis gas and form a wax-free crude methanol stream that is passed to the crude methanol tank 154 via pipeline 167. The synthesis gas is removed via pipeline 168, and can optionally be used as fuel gas or in another process. To maintain the right conditions in the distillation column 161, it may be necessary to pass a portion of the crude methanol stream from the separator 169 back to the distillation column 161 via pipeline 166. In the bottom of the distillation column 161 there is located a fluid/fluid separator, not shown, that separates the liquid wax from water. The wax is passed out through pipeline 162 to receptacle 155 for collection and optional further purification. This embodiment further comprises means 170 for supplying heat to the distillation column 161.

The figures should be understood thus: that a plant and a method according to the present invention will usually only comprise one of the illustrated embodiments, but the first embodiment can be combined with the second or the third embodiment according to the present invention.

In the second and third embodiments, a wax separator is located ahead of where the wax-containing methanol is mixed together with the relatively wax-free methanol from the final cooler. The advantage of placing the wax separator in this stream is, inter alia, that the equipment is smaller (handles a smaller volume flow) and that downstream process equipment in the actual methanol process is not exposed to separated wax due to cooling in the pipe systems.

In some cases it would be economically attractive to direct the wax containing condensate into the high-pressure separator (shown by the pipelines 58 and 158 in the figures) and also to install a top condenser on the gas stream 29/129 from the high- pressure separator (not shown in the figures). Then, virtually all the methanol would be condensed out and lowest possible temperature obtained on this gas stream, which may be desirable.

The design of the individual process components makes use of known principles and may be of varying character according to preferences and the need for functions in connection with operation and maintenance.

After the condensate has been purified of wax, the methanol portion from the purification process is combined with the condensate from the final cooler, which

together form the methanol product from the methanol synthesis. The optimal temperature for condensation of the wax will depend upon its characteristics and the elimination requirement. Thus, there is room for adaptations which may be individual whilst the process is the same.

If the wax amount is not too great, it will in many cases not be necessary to remove it from the condensate. It is then sufficient to cause the wax containing condensate to bypass the final cooler, according to the first embodiment of the invention, in order for it then to be recombined with the rest of the condensate downstream of the final cooler in the methanol synthesis. The wax will then end up in the bottom product from the methanol distillation which is a part of the usual process for processing crude methanol. Adaptations may be made here for handling wax.

When the wax amount is substantial, or for other reasons it is desirable to remove the wax prior to methanol distillation, it can be removed from the condensate by cooling (the second embodiment) or by means of a separate distillation column (the third embodiment).

In what follows some of the elements and processes described will be described in more detail.

The heat exchanger 4 and hot condensate separator 6 shown in Figure 1 can, as illustrated in Figure 2, be combined to give a second cooler 105 with integrated separation of the condensate and remaining product gas. The separator 6 in Figure 1 receives a gas and wax-containing liquid product from the heat exchanger 4. Typical process parameters will be about 80 bars and a temperature of between 80 and 12O ⁰ C. In Figure 2 a similar product is produced in the second cooler 105. The liquid portion of the product will comprise a methanol/water mixture containing dissolved gases (for example, CO ₂ ) and it will contain varying amounts of liquid wax. The separator 6 or the second cooler 105 will separate the gas phase and the liquid phase and allow the gas to move on to the final cooler 8/108 of the methanol synthesis.

A plant according to the invention may further optionally comprise a pump unit for onward transportation of purified condensate (methanol/water) 167 or 50 for further storage and processing (distillation).

The wax separator 43 consists, in one embodiment, of a pressure tank for a level- controlled collection of the condensate. A bundle of pipes is installed in the bottom of the tank for heat exchanging/cooling the condensate. Alternatively, outlets are made to one or two side-mounted bundles of pipe for such cooling of the condensate. This alternative makes it possible to close off between the pressure tank and the heat exchanger when the heat exchanger is to be cleaned of wax. The wax will settle on the cooling surface of the heat exchanger and gradually build up. When the cooling effect has been noticeably reduced, it is necessary to clean the heat exchanger. The cleaning is carried out using a separate cleaning procedure. During this procedure, the pipe bundle(s) is/are heated up using steam or the like so that the wax melts, runs off and collects at the bottom of the wax separator 43, whence it is passed to a collection unit.

The distillation column 161 in the third embodiment is operated above the setting temperature of the wax. A methanol/water mixture is withdrawn from the top of the column and at the bottom wax and water are separated by a liquid/liquid separator. This will remove the wax phase in liquid form and render regular cleaning of equipment to remove deposited wax unnecessary.

The purified condensate will be collected in a condensate collector and transported out of the purifying units by the process pressure or by means of an external pump unit if the process pressure is insufficient. The separated wax will be collected in a wax collector. The collectors can in most cases be made as integrated parts of the cleaning and collecting equipment.

The choice of cleaning method and consequently of the second or third embodiment according to the invention will depend upon anticipated amount of wax and energy considerations. A wax separator in the form of a cooling unit will be the least energy- consuming process and will be well suited where the amounts of wax are small and where the cleaning intervals are not too close together. A distillation column will be the next least energy consuming and will result in the wax phase being removed in liquid form without cleaning the heat exchanger tubes.

Another embodiment of the third embodiment according to the invention comprises further heat integration. In this embodiment the heating of the distillation column is done using the heat in the product gas from the feed/product heat exchanger. The control of the heat amount takes place by means of a bypass line. The distillation column here is a pure stripper in which the methanol is boiled off without undergoing

concentration to any appreciable degree. The concentration takes place in a downstream distillation which normally would be conducted on the whole product volume. This takes place by direct export to the low-pressure separator which may be in the form of a low-pressure methanol distillation column (LP Methanol Distillation Column). It is also possible to choose to condense out this crude methanol and allow it to pass to a holding tank prior to pumping to methanol distillation. The product condensate that is directed into the stripper passes first via a degassing, where dissolved gases are driven off and are directed to the fuel gas system. The heat integration preferably takes place downstream of the stripper reboiler. This heat integration may in addition be carried out upstream of the reboiler. It may also be carried out solely upstream of the reboiler since the high pressure here ensures that the boiling point for the product is substantially higher than the boiling point of the process water in the bottom of the stripper.

Applicability of the invention

The present invention can be used when projecting of new plants and may also be used in existing plants on modification thereof. Since the process does not interrupt the reaction chemistry of the methanol synthesis or other functions, it can easily be adapted for use in any type of methanol synthesis. The wax problem in the methanol synthesis seems to be an increasing problem in new plants, probably caused by the choice of synthesis gas technology (autothermal reformer) and of the catalyst development in the methanol synthesis.