MX2008003753A - Method of operating a distillation column for purifying 1,2-dichloroethane and for coupled sodium hydroxide solution evaporative concentration - Google Patents

Abstract

Method of operating a distillation column for removing water and components which boil more readily than 1,2-dichloroethane from 1,2-dichloroethane, in which at least some of the heat of condensation of the hydrous vapours of the distillation column is used for the evaporative concentration of sodium hydroxide solution, also at least some of the 1,2-dichloroethane formed from chlorine and ethylene in a direct chlorination can be used for heating that distillation column and then likewise be used as heat-transfer medium for the evaporative concentration of sodium hydroxide solution.

Description

METHOD FOR OPERATING A DISTILLATION COLUMN FOR PURIFY 1, 2-DICHLOROETHANE AND FOR CONCENTRATION EVAPORATIVE COUPLED WITH A HYDROXIDE SOLUTION OF SODIUM DESCRIPTIVE MEMORY The procedure refers to a procedure for the 1, 2-dichloroethane production, which will be referred to hereafter as EDC. The EDC is mainly used as an intermediate product in production of vinyl chloride monomer, to which reference will be made hereinafter as VCM, which in turn is used to produce chloride of polyvinyl (PVC). When the EDC reacts to form VCM, it gets Hydrogen chloride (HCI). Therefore the EDC preferably occurs at from ethylene (C2H) and chlorine (Cl2) in a way that can be maintained a balance between the hydrogen chloride (HCl) produced and the one consumed in the reactions, as shown in the following reaction equations: CI2 + C2H4 - C2H4CI2 (pure EDC) +218 kJ / Mol (1) C2H4CI2 (EDC? C2H3CI (VCM) + HCI -71 U / Mol (2) pyroconsolidated) C2H4 + 2 HCI + 1/2 02? C2H4CI2 (crude EDC) + H20 +238 kJ / Mol (3) The procedure for the production of VCM with a balance of HCI, which will be referred to hereafter as "balanced VCM procedure", comprises: a direct chlorination step, in which a portion of the required EDC is produced from ethylene (C2H4) and chlorine (Cl2) and generated as pure EDC; the use of the heat of the reaction produced in this direct chlorination step is a central aspect of the invention; an oxychlorination step, in which the rest of the EDC is produced from ethylene (C2H), hydrogen chloride (HCI) and oxygen (02), and is generated as crude EDC; an EDC purification step by fractionation, in which raw EDC is released, along with the recycled EDC that returns from the VCM fractionation step, from the by-products formed in the oxychlorination and EDC pyrolysis steps to obtain an EDC released suitable for use in the EDC pyrolysis step; an EDC pyrolysis step, in which pure EDC is combined with the released EDC and the resulting EDC mixture is thermally disrupted; the pyro-dispensed gas that is obtained consists of VCM, hydrogen chloride (HCI) and unreacted EDC, as well as byproducts; a VCM fractionation step, in which the pure VCM preferred as a product is separated from the pyro-released gas, and the other main substances that are contained therein, ie hydrogen chloride (HCI) and unreacted EDC, are recovered by separated as recyclable and return to the balanced VCM procedure as a reusable feed in the form of recycled HCI or recycled EDC.

The chlorine (Cl2) that is required for the direct chlorination step normally occurs in a sodium chloride electrolysis (NaCl) plant, with caustic soda (NaOH) with a concentration of approximately 33% produced as a byproduct. Due to the high toxicity of the chlorine (Cl2) produced, transportation to long distances is avoided as much as possible. Therefore, the chlorine (Cl2) required for the direct chlorination step often occurs in the vicinity of the direct chlorination unit. It is commonly known that the minor constituents that form in the pyroconstructed gas during the EDC pyrolysis step reduce the purity of the VCM product. The removal of these constituents is expensive to improve the purity of the VCM. Therefore, the pyroconsolidated EDC that has been mostly released from said impurities is used in the EDC pyrolysis step. From the large number of techniques that exist to avoid, or if necessary, remove said secondary products and / or harmful constituents, reference is made in particular to the patent specification WO 01/34542 A2, and especially to the state of the art which is described in said document. It shows that the heat released when ethylene (C2H) and chlorine (Cl2) react to form liquid EDC during the direct chlorination process, is sufficient to operate the purification columns for the EDC produced in the "balanced VCM procedure" . However, a disadvantage of the procedure described in that document is that the heat of the reaction that is used to heat The purification columns require the removal of a corresponding amount of heat to condense the vapors. This is done in accordance with a conventional technique of the prior art, usually by means of cooling water, which must be provided in large quantities. However, even if the purification columns are not operated by the leaving heat arising from the reaction heat of the direct chlorination unit, the cooling water must be provided in large quantities to condense the vapors coming from the purification columns. Therefore, the purpose of the invention is to further optimize the utilization of the heat produced in the "balanced VCM process" and to substantially reduce the requirement for cooling water. The invention achieves this purpose by using the heat from the condensation of the vapors arising from the distillative separation of the lower boiling components in EDC at least in part to concentrate by evaporation the caustic soda solution produced as a by-product during the production of chlorine. This type of distillation column, known as a column of lightweight materials, is a normal component of the "balanced VCM procedure". In this process the lower boiling components that will be separated, are mainly reaction water fed to the column of light materials during oxychlorination, which then need to be separated from the EDC, and lower organic boiling components of EDC.

In remote areas, in particular, the transportation costs of the caustic soda solution (NaOH) obtained at the NaCI electrolysis plant are an important factor. These costs can be substantially reduced if approximately 33% of the solution obtained is concentrated in a 50% solution by evaporation. Therefore the plants for evaporating the caustic soda solution (NaOH) can, for example, operate under vacuum at an absolute pressure of 133 mbar and at a temperature of 60 ° C. Even in cases where the EDC production unit and the NaCI electrolysis plant are not located close to one another, it is worth transporting the 33% caustic solution first to the EDC production unit , to concentrate the NaOH solution in a vacuum evaporation unit operated by EDC. Of course, the solution can be evaporated at concentrations different to 50% depending on the requirements of the consumer and the outgoing heat produced. In one embodiment of the invention, at least a part of the EDC formed when chlorine reacts and ethylene is a direct chlorination unit, it is used to heat the distillation column for the removal of water and the lower boiling components of the EDC. In another embodiment of the invention, at least a portion of this EDC that was used to heat the distillation column for the removal of water and the lower boiling components of EDC can also be used as a heat transfer fluid for Concentrate the caustic soda solution by evaporation. This is possible because the level of the temperature to which the column of light materials is heated is higher than that of the caustic soda evaporation unit, and therefore it is possible to further reduce the temperature of condensed EDC vapors during heating from the distillation column by means of the release energy in the caustic soda evaporation unit. The equipment and process apparatus used to transfer the condensation energy consist of the familiar balanced VCM procedure equipment and the apparatus for concentrating the caustic soda solution (NaOH) by evaporation. The last one refers mainly to a roof and tube vertical heat exchanger with two fixed pipe sheets and a NaOH collector, in which the solution of caustic soda (NaOH) flows through the tubes and the lower boiling vapors out of the tubes. The heat transfer between the cover and the side of the tube is carried out in an alternating current flow. The vapors introduced in the upper part of the tube bundle are condensed and can be extracted as a liquid in the lower part. Another suitable method for transferring heat is by means of a heat exchanger tube bundle that is inserted in the caustic soda collector or by means of a kettle, for example, of the teapot type, which is located on the outside of the collector caustic soda. All the methods described above can also be used in additive or in combination. If the procedure that is going to use in combination with other procedures, which also include the evaporation of caustic soda, and in doing so different vapors will be subtilized at the same time, the tube bundle can be located horizontally.

Of course, it must be ensured that the individual streams of the vapors coming from the different distillation columns do not mix with each other. In accordance with the principle that is described in the document WO 01/34542 A2, Figure 1 shows a flow chart of the simplified procedure illustrating a configuration (300) of the purification unit of EDC with a direct chlorination unit (100) and a caustic soda evaporation unit (200) in a plant based on the "balanced VCM process", where • Vapors from the lightweight material column (301) of the EDC purification unit (300) are condensed in a caustic soda evaporation unit (200) according to the main claim, • The column of light materials (301) is heated by the EDC vapors coming from the unit of direct chlorination (100) according to claim 2, and • After heating the column of lightweight materials (301), the condensate from the EDC vapors is used in the caustic soda evaporation unit (200) according to with claim 3.

The direct chlorination unit (100) consists of a circuit filled with a liquid (101), an ethylene feeder (102), a chlorine feeder that has been dissolved in EDC (103), with the previous dissolution of the chlorine gas (104) in liquid EDC (106) in an injector (105) and prior to this, the cooling of liquid EDC to a low temperature in the EDC 107 refrigerant to improve the solubility, and in addition to an evaporation trap (108) , a discharge device for the liquid EDC (109) with an EDC circulation pump (110), a discharge device for the EDC vapors (111) and a feed point to recycle EDC (112), although, for reasons practices, there may be more than one of each of the feeding points and the discharge devices. In the circuit filled with liquid (101) the chlorine reacts with ethylene to form boiling EDC, which is evaporated in the evaporation drum (108) together with the unreacted feed materials and minor inert constituent gases. A part of the EDC vapors (113) is fed to the boiler (302) of the collector of the column of lightweight materials (301). The portion of the total EDC vapors (113) extracted from the direct chlorination unit that will be fed to this kettle, depends on the mode of operation employed by the column of light materials (301), in particular depends on the respective water content of the EDC (303) and the specifications related to the purity of separation, and usually makes from about one fifth to one third the same. The remaining vaporous EDC is normally used in the process of (114) balanced or It can also be used to concentrate the caustic soda solution by evaporation. The temperature of the collector in the column of light materials (301) is usually about 15 ° C, while the vaporous EDC can be extracted from the direct chlorination unit (100) at about 1120-125 ° C. Taking into account this slight difference in temperature, it is hardly possible to remove more heat from the EDC beyond the condensation to operate the column of lightweight materials (301), which is the reason why the EDC condensate (304), which still contains vaporous EDC and inert gas components, is discharged from the collector EDC boiler as a multi-phase mixture at a temperature of approximately 120 ° C. The EDC condenser (304) is sent to the caustic soda evaporation unit (200), if necessary after mixing it with vaporous EDC (115). In the evaporation unit that is designed as a vertical tube bundle with an elongated manifold, is directed to the side of the cover (201) of the shell and tube heat exchanger (202) while a falling film evaporation of the caustic soda solution on the tube side is carried out at about 60 ° C . The non-condensable materials are discharged through an inert gas outlet (203). Here, appropriate technical measures should be taken to avoid the formation of explosive gas mixtures on the deck side (201) of the exchanger heat of cover and tube (202). Such measures are familiar to those skilled in the art and are not an object of the present invention. The light steam (305) from the column of light materials (301) is also sent to the caustic soda evaporation unit (200), where it is condensed in the immersion refrigerant (204) which is installed in the connector ( 205) of the caustic soda evaporation unit (200). A part of the obtained light condensate material (206) is returned to the head of the column of light materials (301) as a light reflux (306) by the condensate pump (207) and a part is discharged as waste water. (208). The large temperature difference between the evaporating unit of caustic soda (200) at about 60 ° C and the head of the column of light materials (301) at about 90 ° C makes a compact design possible. The 33% caustic solution (209) is fed to the collector (205) of the caustic soda evaporation unit (200) and concentrated in vacuo to approximately 50%. The pressure is maintained by means of a vacuum pump (210), which discharges the water vapor as it is released (211). A caustic soda pump (212) discharges a part of the concentrated caustic soda solution as NaOH product (213) and pumps the remaining part to the caustic soda dispenser (214), which distributes the caustic soda solution that will be concentrated to the side. of the cover and tube heat extractor tube (202).

The lower part containing EDC (307) of the column of light materials (301) is sent to the column of heavy materials (308). In this column and in the subsequent vacuum column (309) they are additionally purified in a familiar manner. The heat of the reaction from EDC production can also be beneficially used to heat these columns, but this is not an objective of the present invention. The EDC product (216) is removed from the pure EDC (215) extracted from the side of the cover (201) of the shell and tube heat extractor (202); he Remaining pure EDC is pumped to the circuit in the direct chlorination unit (100) by means of an EDC pump (217) and combined with the EDC (109). At this point, the circuit for the vapors coming from the direct chlorination unit (100) and the column of light materials (301) is only shown as an example. It may also be possible to use an external boiler in the caustic soda evaporation unit (200) or for the cover side division (201). Here, there is some freedom with respect to which steam can be fed to which component as a heating agent, something that one skilled in the art would need to optimize to improve efficiency in each particular case at the same time that, of course, care is taken to ensure that EDC currents of different quality can not be mixed together. Below is a typical example based on a calculation using simulation: it is based on a plant with a capacity of 400,000 tpy of EDC. With this plant size, a thermal output of approximately 3.9 MW can be achieved in the column of lightweight materials at a pressure in the upper part of 1.15 bar abs. and a temperature of about 91 ° C for use in an evaporation of caustic soda, allowing about 6.8t / h of the caustic soda solution (calculated as 100%) to concentrate at about 33 to 50% by weight. 5.2 MW of the direct chlorination unit is introduced into the collector boiler to operate the column of light materials, collecting 30% of the total heat of reaction.

Claims (3)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for operating a distillation column for the removal of water and the lower boiling components of 1,2-dichloroethane, where at least a part of the heat coming from the condensation of the aqueous vapors of the distillation column It is used to concentrate caustic soda by evaporation.

2. The process according to claim 1, further characterized in that at least a part of the 1,2-dichloroethane formed when chlorine and ethylene react in a direct chlorination unit, is used to heat the distillation column to remove the 1,2-dichloroethane water and the lower boiling components of 1,2-dichloroethane.

3. The process according to claim 2, further characterized in that at least a part of the 1,2-dichloroethane which was used to heat the distillation column for the removal of water and boiling components lower than 1, 2-Dichloroethane is subsequently used as a heat transfer fluid to concentrate the caustic soda solution by evaporation.