MXPA99007194A - Reduction of methanol emissions from a sinte gas unit - Google Patents

Abstract

The present invention relates to the methanol emissions in the CO2 ventilation of a synthesis gas unit in an ammonia or hydrogen plant are reduced by contacting the raw synthesis gas coming from a converter by low displacement. temperature with the spent condensate recycled to absorb methanol. The synthesis gas is treated in a purification unit to form CO2 ventilation with a reduced methanol content. The condensate resulting from the passage of the contact is exhausted with steam to form a process steam stream suitable for feeding to the reformer and a spent condensate stream, suitable for rectification in external installations, from which a recycle is recycled. portion to put it in contact with the synthesis gas in bru

Description

REDUCTION OF METHANOL EMISSIONS FROM A SYNTHESIS GAS UNIT FIELD OF THE INVENTION The present invention relates to the reduction of methanol emissions from the ventilation of the purification unit in synthesis gas generation units using a low temperature displacement catalyst.

BACKGROUND OF THE INVENTION There is a desire to reduce atmospheric emissions from chemical plants and, in particular, the methanol emissions associated with ammonia plants. The reduction of methanol emissions has become critical for both new units and existing units subject to modernization. With reference to Figure 1, in the synthesis gas generation unit 10, such as in an ammonia or hydrogen plant, from a low temperature displacement converter (not shown) a current 12 rich in oxygen. The low temperature shift catalyst of the converter is normally used to improve the conversion of the displacement reaction of carbon monoxide and water into carbon dioxide (C02) and hydrogen. This service P1438 / 99MX normally uses a copper-based catalyst, which under normal operating conditions supports some formation of by-products, such as methanol, from the reagents that are present. Downstream of the displacement section, the process stream 12 is cooled in the cooler 14 to condense the water that is separated from the gas in the trap drum 16 to form the condensate stream 18 and the upper gas stream 20. The condensate of the condensation process, which has a typical methanol content of 500-1000 ppm by weight, is sent to a depleting 22 for the condensate, after heating in the heat exchanger 24 for the feed / effluent of the condensate deplete. On line 26, new steam is supplied to exhaust the pollutants such as ammonia, methanol and higher alcohols and C02 of the condensate from the condensation process in the condensate drain 22. The vapor containing the contaminants is recovered from the upper part by line 28 and is supplied to the steam reformer (not shown) via line 30 together with the steam that does not pass through the condensate drain 22 through line 32. The spent condensate is recovered as bottom stream from the condensate drain 22 by line 34 and can be rectified in external facilities or processed in some other way. The methanol present in the process gas in P1438 / 99MX line 20 is sent to purification unit 36 for the removal of C02 and / or other undesirable components in the synthesis gas product. The purification unit 36 is usually an absorbing-strenuous system or a molecular mesh system, such as a pressure swing adsorption unit (PSA). In line 38, the purified synthesis gas is obtained. The methanol results in an upper product stream rich in C02. In many cases, at least a portion of this C02 stream is vented to the atmosphere along with any amount of methanol that may be present therein. It would be desirable to have available a way to reduce methanol emissions in C02 that comes from purification unit 36. Ideally, the means to reduce methanol emissions would minimize the requirements for additional equipment, would have a smaller impact on consumption energy of the plant and would not produce solid contaminants, which require, its disposal. Conventional technology for the reduction of methanol, for example, catalytic reactors to the end of the tube or alternatively cooling of the crude synthesis gas to increase the separation of the methanol in the trap drum 16, does not satisfy these criteria. The tube end catalytic reactor requires a P1438 / 9MX blower, from a heater (in order to start) and from an oxidation reactor and produces an exhausted catalyst, which must be discarded. The cooling of the crude synthesis gas would require cooling equipment and a high energy consumption. Therefore, there is a need for an acceptable way to reduce methanol emissions.

SUMMARY OF THE INVENTION The present invention removes most of the methanol from the synthesis gas leaving the trap drum, thereby reducing emissions from the upper carbon dioxide product of the purification unit. The stream of tails of the condensate depleting. It usually has a methanol level that is quite low. In accordance with the present invention, a certain part of this spent condensate is recycled to the trap drum, upstream of the purification unit. Also, the trap drum is expanded to incorporate a washing section comprising packaging or trays on top of the main process gas inlet. The recycled spent condensate is then introduced as a scrubbing medium to the upper part of the washing section of the trap drum. The process gas that leaves the washing section will therefore be close to equilibrium with P1438 / 99MX water that has a very low methanol content, instead of the 500 to 1000 ppm by weight methanol that was present in the condensate condensation process, before recycling the exhausted condensate stream. The emissions of methanol into the atmosphere from the C02 ventilation will therefore be reduced accordingly. The additional methanol removed ends up in the steam feed to the reformer, so that it is not released into the atmosphere. Unlike other potential options to treat the C02 ventilated from the exhausting, the proposed design does not add new equipment. The circuit elements of the recycling process will see a certain increase in size, such as, for example, the process condensate pump, the condensate drain, the exhaust / effluent exchanger and the trap drum. However, increasing the size of existing equipment instead of adding new equipment usually results in minimal cost. In addition, the impact on the energy consumption of the plant is very low. There is a slight increase in the charge of the air preheating coil and the reformer mixed feed, due to a slight decrease in the steam feed temperature. However, this is partly compensated by the reduction in process steam extracted from the P1_438 / 99MX steam head. In one aspect, the present invention then provides a method for processing a stream of crude synthesis gas to minimize methanol emissions. The method comprises contacting the stream of crude synthesis gas with the spent condensate to form an upper stream of synthesis gas with a reduced methanol content and a condensate stream rich in methanol. The methanol-rich condensate stream is steam depleted to form a methanol-rich process steam stream and a spent condensate stream with a reduced methanol content. A portion of the exhausted condensate stream is recirculated to the contact passage. The upper stream of synthesis gas is treated in a purification unit to form a C02 rich stream and essentially free of methanol and a synthesis gas stream with reduced C02 content. In another aspect, the present invention provides a unit for processing the crude synthesis gas to produce a synthesis gas stream with reduced water and C02 content, a C02 stream with a low methanol content, a condensate stream exhausted, essentially free of hydrocarbons and other impurities and a process steam stream P1438 / 99MX suitable for feeding to a reformer. The unit has a raw gas separator that includes a methanol wash bed to contact the raw synthesis gas stream with the spent condensate, to form an upper stream of synthesis gas with a reduced methanol content and a condensate stream rich in methanol. The condensate depleting process is provided to contact the methanol-rich condensate stream, with steam to form a higher process steam stream and a bottom stream comprising spent condensate. A line recirculates a portion of the exhausted condensate stream from the condensate drain to the raw gas separator. A purification unit brings the upper synthesis gas stream from the raw gas separator to form a C02 poor synthesis gas stream and a C02 rich stream with low methanol content. In a further aspect, the present invention provides an improvement in a method for processing a stream of crude synthesis gas, comprising the steps of: (1) separating the condensate from the stream of crude synthesis gas to produce a stream of condensate and a stream of synthesis gas with a reduced water content, (2) treat the synthesis gas stream in a P1438A9_9MX purification unit to form a synthesis gas stream poor in C02 and a product stream rich in C02 and (3) steam exhaust the condensate stream from step (1) to form a process steam stream suitable for the reformer and an exhausted condensate stream. The improvement consists in that the step of separation (1) includes contacting, in an effective manner, the stream of crude synthesis gas with a portion of the exhausted condensate stream to significantly reduce the methanol content of the product stream of C02 from step (2).

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified process diagram of the prior art method for processing a stream of crude synthesis gas that produces a stream of synthesis gas with a reduced water content and C02, a C02 current, an exhausted condensate stream essentially free of hydrocarbons and other impurities and a process steam stream suitable for feeding to a reformer. Figure 2 is a simplified process flow diagram, in accordance with the present invention, wherein the process of Figure 1 is modified so that the C02 stream has a greatly reduced methanol content.

P1438 / 99MX Figure 3 is a simplified process flow diagram showing a typical absorber-strenuous unit, suitable as a mode of the purification unit 124 of Figure 2.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES In accordance with one embodiment of the present invention, shown in Figure 2, the unit 100 receives the synthesis gas stream 112 supplied from a conventional low temperature displacement converter, which normally uses a catalyst copper base. The catalyst usually results in the formation of some by-products, such as, for example, ammonia, methanol and higher alcohols. The synthesis gas stream 112 is basically the same as the synthesis gas stream 12 of Figure 1. The synthesis gas stream 112 is cooled in the "cooler 114" by indirect heat exchange, for example, with cooling water. or a process stream The cooled synthesis gas stream from the cooler 114 is a biphasic stream containing some process condensate This biphasic stream is supplied to the separator 116. The condensate is collected at the bottom of the separator 116, while the gas moves towards P1438 / 99MX up through the water washing section 118. The spent condensate is introduced to the top of the washing section 118 via line 120. The exhausted condensate from line 120 is essentially free of methanol, for example, less than 100 ppm by weight, especially less than 25 ppm by weight. The spent condensate introduced via line 120 serves as a scrubbing means in the washing section 118. The process gas leaving the washing section 118 is generally close to equilibrium with the spent condensate containing less than 25 wppm. methanol, instead of about 500 to 1000 ppm by weight of methanol, which are present in the condensate of the condensing process of the biphasic stream of the cooler 114. The methanol content in the upper gas stream 122 is thus reduced by more 90% The upper gas stream 122 coming from the washing section 118 is introduced to the purification unit 124 for the removal of C02, methanol and other impurities. The purification unit 124 can be any conventional purification system used for the removal of C02, such as, for example, a Benfield solution or an MDEA uptake-depletion system or a unit based on molecular meshes, such as a adsorption system by pressure oscillation the current of P1438 / 99MX synthesis gas product 128 treated essentially is free of C02 and methanol. A current of C02 134 is produced which normally contains any amount of methanol transported on line 122. With reference to the absorption-depletion system shown in Figure 3, the upper gas stream 122 of the washing section 118 is introduced to the C02 absorber base 125. The poor absorbent is introduced into the upper part of the absorber 125 via line 126 and pump 127. The absorber passing down through the absorber 125 contacts the gas and absorbs the C02 thereof. . The upper product stream 128 is essentially free of C02 and methanol, which are absorbed in the absorbent medium. An absorbent rich in CO2 is recovered as stream of product from bottoms 130 and is introduced to the top of a stiffener 132, which is normally heated by the reboiler 131 and the vapor or hot synthesis gas supplied by line 133. and it may also operate at a lower pressure than the absorber 125. An upper stream of C02 134 is produced which normally contains any amount of methanol transported on line 130. A poor C02 stream is recovered as bottom product from the exhaust 132 to recycle it by line 126 and pump 127 to absorber 125.

P1438 / 99MX Referring again to Figure 2, the liquid stream of bottoms 136 is supplied by pump 138, through the heat exchanger 140 of feed-effluent / condensate drain and line 142, to the top of the exhaust Condensate 144. Steam, preferably superheated steam, is introduced into line 146 to the bottom of exhaust stubber 144 to exhaust condensate impurities, which are transported to the top in saturated steam line 148. The steam additional required for the reformer (not shown) is supplied in the bypass line 150 of the exhaust. The spent condensate is collected from the lower part of the exhaust in the line 152 and cooled in the heat exchanger 140 to heat the incoming process condensate in the line 142. A portion of the spent condensate is sent to the separator 116 through the line 120, as previously mentioned, and the rest can be sent for further processing via line 154, for example, to rectification in external facilities. In general, 10 to 50 percent of the condensate exhausted from line 152 is recycled via line 120 to the top of the water washing section 118, preferably from 20 to 40 percent. In general, the more exhausted condensate is recycled, the lower the content of P1438 / 99MX methanol in the upper gas line 122; however, an increase in the recycling of the condensate will require more steam through line 146 for the exhaustion. There is a certain small penalty in the energy of the relatively lower temperature of line 149 but, this is more than compensated for by the lower amount of steam coming from the steam head required for a fixed amount of steam from line 149 which will be supplied to the reformer (not shown).

EXAMPLE A synthetic gas conditioning unit for an ammonia plant of 1000 metric tons per day was simulated for comparison with a conventional conditioning unit (with high methanol emissions in C02 ventilation with a synthesis gas conditioning unit based on the principles of the present invention (with reduced methanol emissions in C02 ventilation) The material balance for the simulation of the case "ase (Figure 1), is presented in Table 1.

P1438 / 99MX TABLE 1.

H 4- » As seen in Table 1, the ventilation line of C02 44, contains methanol from upper line 20. The C02 ventilation line has approximately 125 ppm of methanol for a total annual discharge of approximately 115 metric tons by year. Using the principles of the present invention, about 33% of the spent condensate stream 152 was fed to the top of the raw gas separator 116, where the separator was modified to include a water wash bed 118. No new equipment was needed for this configuration. . The height of the separator 116 is, in general, 3.35 times the height of the separator 16 of the base case to include the wash bed with water 118 but, the diameter did not change. The diameter of the condensate drain 144 is, in general, 15% greater than the condensate drain 22 of the base case to admit the largest volume of condensate depletion. Similarly, the heat transfer area of the exchanger 140 is, in general, 31% greater than that of the heat exchanger 24 of the base case and also the capacity of the pump 138 is, in general, 31% greater than the pump 19 of the base case. The cooler 114 has approximately the same size and load as the cooler 14 of the base case (for simplicity of simulation, the crude gas was cooled to 153 ° F against 158 ° F in the base case, for P1438 / 99MX obtain the same upper temperature (158 ° F) on line 122 as on line 20). The results of the simulation are presented in Table 2.

P1438 / 99MX TABLE 2.

As shown in Table 2, the amount of methanol in the ventilation line of C02 134 was reduced to approximately 8 ppm by weight and the total annual discharge was reduced to less than 8 metric tons.

P1438 / 99MX

Claims (18)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following CLAIMS is claimed as property: 1. A method for processing a stream of raw synthesis gas to minimize methanol emissions, comprising the steps of: (a) contacting the stream of crude synthesis gas with the condensate, to form an upper stream of synthesis gas with reduced methanol content and a stream of condensate rich in methanol. methanol; (b) steam exhausting the methanol-rich condensate stream to form a methanol-rich process steam stream and an exhausted condensate stream with reduced methanol content; (c) recirculating a portion of the exhausted condensate stream for the contacting step (a); (d) treating the upper gas stream in a purification unit to form a C02 rich stream, essentially free of methanol, "and a stream of synthesis gas with reduced CO2 content 2. The method according to claim 1 , where the spent condensate contains less than 100 ppm P1438 / 99MX methanol. 3. The method according to claim 1, wherein the spent condensate contains about 25 ppm of methanol or less. The method according to claim 1, comprising the passage of the indirect thermal exchange between the exhausted condensate stream and the methanol-rich condensate stream. The method according to claim 1, wherein the recirculated portion of the condensate exhausted from step (c) comprises from 10 to 50 weight percent of the condensate stream exhausted from step (b). The method according to claim 1, wherein the treatment step (d) comprises the steps of (1) contacting the upper gas stream with a C02 absorbent to form an absorbent stream rich in C02 and ( 2) exhaust the absorbent current rich in C02 to obtain a poor absorbent current in C02 for the recirculation to step (1). The method according to claim 1, wherein the purification unit comprises adsorption by pressure oscillation. 8. A unit to process raw synthesis gas to produce a synthesis gas stream of reduced water content and C02, a stream of C02 essentially free of methanol, P1438 / 99MX an exhausted condensate stream, essentially free of hydrocarbons and other impurities and a process steam stream suitable for feeding to a reformer, comprising: a raw gas separator including a water washing section for contacting the raw synthesis gas stream with the spent condensate to form an upper stream of synthesis gas with reduced methanol content and a condensate stream rich in methanol; a condensate depleting process for contacting the methanol-rich condensate stream with the steam, to form an upper stream of process steam and a bottom stream comprising the spent condensate; a line for recirculating a portion of the exhausted condensate stream from the process condensate deplete to the raw gas separator; a purification unit to treat the upper stream of synthesis gas from the raw gas separator, to form a synthesis gas stream, poor in C02, and a stream rich in C02. The unit according to claim 8, comprising a heat exchanger for indirect heat exchange between the bottom stream of the condensate depleting process and the P1438 / 99MX condensate stream rich in methanol. The unit according to claim 8, wherein the purification unit comprises an absorber-strenuous unit. The unit according to claim 8, wherein the purification unit comprises a molecular mesh unit. 12. In a method for processing a stream of crude synthesis gas comprising the steps of: (1) separating the condensate from the crude synthesis gas stream to produce a condensate stream and a synthesis gas stream with reduced water content, (2) treating the synthesis gas stream in a purification unit to form a C02 poor synthesis gas stream and a C02 rich stream, and (3) steam exhaust the condensate stream from step (1) to form a process steam stream suitable for reforming and a process process run-off stream; the improvement consists in that the current of synthesis gas upstream of the purification unit is brought into contact with a portion of the effective dewatered condensate stream to significantly reduce the methanol content of the C02 stream. from step (2) and to produce a condensate stream rich in methanol. 13. The improvement according to claim 12, P1438 / 99MX where the exhausted process condensate stream comprises less than 100 ppm methanol. The improvement according to claim 12, wherein the spent process condensate stream comprises about 25 ppm of methanol or less. 15. The improvement according to claim 12, wherein the condensate stream. rich in methanol is heated by indirect thermal exchange against the condensate exhausted from step (4). 16. The improvement of claim 12, wherein the portion of the spent condensate stream that comes into contact with the raw synthesis gas stream comprises 10 to 50 weight percent of the spent condensate stream. 17. The improvement according to claim 12, wherein the purification unit comprises an absorber-strenuous unit. 18. The improvement according to claim 12, wherein the purification unit comprises a molecular mesh unit. P1438 / 99MX