Abstract:
Removal of water in a vinyl chloride monomer purification system is achieved through (1) providing a distillation column for separation of a liquid admixture of vinyl chloride, hydrogen chloride, and water into (a) an essentially pure vinyl chloride product stream and (b) a hydrogen chloride distillate stream; and (2) placing a drying system in fluid communication with the distillation column midsection at a connection point where the water is at sufficient concentration to provide a useful mass transfer flux of water from a withdrawn midsection stream into a drying agent.

Description:
CROSS-REFERENCE TO PRIOR APPLICATION 
     This Application is a divisional of U.S. Ser. No. 09/553,509 filed on Apr. 20, 2000 now U.S. Pat. No. 6,323,380 which claims the benefit of U.S. Ser. No. 60/130,787 filed on Apr. 23, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the purification of Vinyl Chloride Monomer and the removal of water in Vinyl Chloride Monomer finishing. 
     BACKGROUND OF THE INVENTION 
     In purifying Vinyl Chloride Monomer (VCM) produced by the cracking of 1,2 dichloroethane (EDC) according to well-known commercial manufacturing processes, trace amounts of water must be handled. This trace water either (a) is formed in the cracking process, (b) results from small amounts of water present in the EDC fed to the cracking furnaces used in the cracking process, or (c) is formed in-situ within the distillation process. Hydrochloric acid (HCl) is formed as a by-product or co-product in the production of VCM from EDC; and this HCl, when mixed with water, forms a mildly corrosive mixture. However, when the overall water concentration exceeds the solubility limit of water in VCM, the VCM becomes saturated and the water enters into a free phase state in HCl; this separate free-water phase is highly corrosive in comparison to the phase where the water concentration is below the saturation limit for VCM. 
     A drying operation can be used to remove water from an admixture of vinyl chloride, HCl, and water where the vinyl chloride is present either in substance or in trace quantity. One such drying system is described in U.S. Pat. No. 5,507,920 entitled “Process And Apparatus For Purifying Vinyl Chloride” which issued to P. Schwarznaier, P. Kammerhofer, M. Stöger, H. Kalliwoda, and I. Mielke on Apr. 16, 1996. This patent describes both the use of an evaporator and an optional molecular sieve or silica gel desiccant in drying water from a stream of HCl, water, and entrained vinyl chloride which has been distilled away (as an overhead vapor stream from a HCl/VCM distillation column, the third distillation column in a three column vinyl chloride separation system) from a feed stream rich in vinyl chloride and also containing HCl and entrained water. The patent describes that “the greatest water concentration prevails at the top of” the third HCl/VCM distillation column in that three column vinyl chloride separation system and that, accordingly, the “drying” system is installed at the beginning of the vapor line recycling HCl and entrained vinyl chloride to the feed stream of the first distillation column of that three column vinyl chloride separation system. 
     The insertion of a drying system in the output vapor stream of a process line has some drawbacks, however. Any breakdown or plugging of such a drying system can rapidly affect the fluid dynamics in the HCl/VCM distillation column generating the vapor stream. Also, vapor streams tend to need physically larger equipment than liquid streams where the same mass of material is being handled; and, respective to the larger equipment, it requires more capital to install a vapor handling system than a liquid system respective to handling of the same mass of material. The use of a liquefaction system for the vapor stream can effectively solve some of the above issues, but this also requires capital and a cooling system to remove heats of vaporization and superheating. A true solution to the issue of water removal, therefore, is to provide a drying system which (1) removes water rapidly and efficiently from the VCM purification system at a location having a relatively high water concentration, (2) does not impact or potentially adversely affect the fluid dynamics in the HCl/VCM distillation column, (3) provides for a safe operating environment, and (4) minimizes the amount of capital needed to effect acceptable water removal from the vinyl chloride monomer purification system. The present invention provides a solution to these needs. 
     SUMMARY OF THE INVENTION 
     The invention in summary provides a method of removing water in a vinyl chloride monomer purification system by steps comprising 
     (1) providing a distillation column (having a top, a bottom, and a midsection) for separation of a liquid admixture of vinyl chloride, hydrogen chloride, and water into (a) an essentially pure vinyl chloride product stream and (b) a hydrogen chloride distillate stream; and 
     (2) placing a drying system in fluid communication with the distillation column midsection at a location where the water is at sufficient concentration to provide a useful mass transfer flux of water from a withdrawn midsection stream into a drying agent. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a vinyl chloride purifying system equipped with a drier, in accord with a preferred embodiment of the present invention. 
     FIG. 2 outlines key process unit steps respective to the drying system process unit of FIG.  1 . 
     FIG. 3 shows pilot plant data for a silica gel regeneration instance. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The physical properties of VCM/HCl/water mixtures are non-ideal and are difficult to model by conventional modeling techniques with commercially available databases; this difficulty has precluded accurate prediction and simulation of stagewise composition within a HCl/VCM distillation column, and it has accordingly been difficult to undertake any effective, focused measures to remedy corrosion problems associated with trace water from EDC cracking. The present invention derives from a surprising discovery that a certain amount of water is effectively concentrated and retained (dynamically “trapped”) to define a “high water concentration zone” in the midsection of a HCl/VCM distillation column used to separate HCl from VCM in the purifying section of the Vinyl Chloride manufacturing facility according to FIG.  1 . It is believed that the concentration of water is sufficiently high to establish conditions leading to at least occasional existence, in some parts of the high water concentration zone, of a first liquid phase having water in vinyl chloride (with dissolved HCl) and a second liquid phase of vinyl chloride and HCl in water. It is further believed that the second liquid phase of vinyl chloride and HCl in water effects relatively rapid corrosion of the metallic components used in the HCl/VCM distillation column and ultimately effects failure of the HCl/VCM distillation column in performing according to design. HCl/VCM distillation column metallic components are constructed of either iron (carbon steel) or nickel/copper alloy (where the nickel/copper alloy has a small percentage of carbon, manganese, iron, sulfur, and silicon); Monel™ (trademark of Huntington Alloys, Inco Allys International, Inc.) nickel/copper alloys are of preferable consideration for use in the corrosive environment in the HCl/VCM distillation column. The various corrosion products accumulate on distillation unit trays and plug openings in those trays, deteriorating operational characteristics of the unit to a point where shutdown and cleaning of the VCM purifying system is needed; such a shutdown represents a loss of productivity. 
     The term “dry” can function as a verb and as an adjective. In rigorous use as an adjective, “dry” references a material free of water (or, in some contexts, free of liquid). As a verb, “dry” references removal of moisture from a material toward a “dry” or “dryer” state; as such, “drying” of a material references a process for removing water (or some other liquid if contextually appropriate) from a material even though a relatively benign amount of water might still be present in the “dried” material at the conclusion of the “drying” process. This latter meaning is the intended meaning of the terms “dry” and “drying” as used herein, so that the VCM product after “drying” by the process of the present invention can still be considered “dry” or “dried”, though some of the trace water remains in the VCM product in relatively benign amounts. (The levels of water that can be considered “benign” from a corrosion perspective in a given set of circumstances will vary from one set of circumstances to another, depending on the design and materials of construction for equipment or apparatus with which the “dried” VCM product will come into contact, the prevailing temperatures and pressures in the apparatus or equipment, the length of time during which the VCM product will contact the apparatus or equipment at such temperatures and pressures, and so forth; but, as a general rule, those water contents characteristic of hydrogen chloride used in the oxychlorination step of the various known overall EDC/VCM manufacturing processes are to be considered “benign”. Thus, for example, VCM product produced by the process of the present invention should certainly be considered as “dry” or “dried” with water contents, on a hydrogen chloride and water only basis, on the order of about 100 parts per million or less by weight or less, but the overall objective of the invention is that the corrosion effects of the VCM product and internal streams within the VCM purifying system should be materially and markedly decreased. In this regard, the VCM product should be dried to a sufficient extent whereby a corrosion rate below 10 mm/year is achieved in the HCV/VCM distillation column tray components.) 
     From the standpoint of a drying operation, it is generally easier and more economical to remove water from a first mixture having water at a “higher” concentration rather than from a second mixture having water at a “lower” concentration because of the higher mass transfer driving force in the case of the first mixture as compared to the second mixture. In the HCl/VCM distillation column used respective to the present invention in separating HCl from VCM in the purifying section of the Vinyl Chloride manufacturing facility, the mid-section of the HCl/VCM distillation column affords, via the surprising discovery of the “high water concentration zone” as noted earlier, an opportunity for economically and effectively removing water from the VCM product without at the same time incurring the capital outlay and operational concerns discussed respective to the system described in the &#39;920, Schwarzmaier et al. patent referenced above. The discovered “high water concentration zone” therefore provides a basis for a useful mass transfer flux of water from a withdrawn midsection stream into a drying agent. 
     It has been determined that silica gel is a suitable drying agent in this application. While silica gel has only limited water adsorption capability at process temperatures above 25 degrees C., silica gel is effective at drying VCM where the temperature is below about 25 degrees C. The mid-section of the HCl/VCM distillation column (where the water is present at a beneficially “high” concentration level) operates at process temperatures of between around 0 degrees C. to 10 degrees C. and a pressure of about 150 psig. This is convenient to the implementation of the preferred embodiment since silica gel water loading capacity increases dramatically as the temperature of silica gel declines below about 25 degrees C. 
     A liquid sidestream is withdrawn from the midsection, and dried (preferably with silica gel) to form an essentially dry liquid sidestream; and the essentially dry liquid sidestream is reprocessed through the vinyl chloride monomer purification system and ultimately returned in the feed stream to the distillation column. 
     The silica gel is readily regenerated using EDC which is ramped from a temperatures of less than about 30 degrees C. to a temperature of about 125 degrees C. during the process of regeneration; this temperature range provides temperatures which are significantly lower than those required for molecular sieve regeneration, which typically uses hot inert gas at temperatures above 200 degrees C. Accordingly, the lower temperature provides some safety benefit in the use of silica gel when compared to molecular sieves. The silica gel is also more resistant to HCl attack and less prone to provide active sites for ebyproduct reactions than molecular sieves. These characteristics, when added to the lower required regeneration temperature, further indicate silica gel as the preferred drying agent since it is also a less reactive media as well as a safer (lower temperature of operation) media. The EDC used in the preferred embodiment is forwarded to an EDC manufacturing facility after use in regenerating the silica gel. In one embodiment, when relatively cool and also relatively hot EDC are available, use of both cold and hot EDC is advantageous in minimizing energy requirements in regeneration. 
     FIG. 1 shows a vinyl chloride monomer (VCM) purifying system  100  modified according to the present invention, in a preferred embodiment. Ethylene Dichloride (EDC) is fed to cracking furnace system  103  via line  101 . Furnace product is conveyed via furnace output line  105  into primary distillation unit  107  which separates the furnace product feed into (a) a VCM and Hydrogen Chloride (HCl) overhead stream which is conveyed via line  109  into HCl distillation unit  115  and (b) a VCM and EDC bottoms stream which is conveyed via line  117  to EDC purification distillation unit  111 . About 50% of the VCM fed to primary distillation unit  107  is further conveyed via line  109  with the other 50% being further conveyed via line  117 . EDC purification distillation unit  111  separates the VCM and EDC bottoms stream from primary distillation unit  107  into (a) purified EDC which exits via line  121  and (b) crude VCM which is conveyed via line  119  into VCM purification distillation unit  113 . VCM purification distillation unit  113  separates crude VCM from EDC purification distillation unit  111  into (a) purified VCM which exits via line  129  and (b) lights which are conveyed via line  123  into HCl distillation unit  115 . 
     In this regard, the composition of water in the midsection of the HCl/VCM distillation column measures between about 100 and 200 PPM water concentration when the feed stream (of vinyl chloride, HCl, and water) to the HCl/VCM distillation column demonstrates a water concentration of about 10 PPM during normal operation. The solubility limit of water in the material being processed at the usual operational conditions of the midsection of the HCl/VCM distillation column is between 50 and 200 ppm depending on the temperature, reflux, split in composition between HCl and VCM, and other tower operating parameters. It should be noted that, in a further surprising discovery respective to operation, a high reflux on the column beneficially increases midsection water composition for enabling mass transfer to a drying agent; accordingly, reflux is situationally used in water profile control. 
     HCl distillation unit  115  is fed with both (a) the VCM and HCl overhead stream conveyed via line  109  and (b) lights conveyed via line  123 . HCl distillation unit  115  separates its feed streams into (a) a crude HCl steam (containing any light impurities generated in cracking furnace system  103 ) which is conveyed via line  131  as recycle to an EDC manufacturing unit and (b) a VCM raffinate stream which is usually conveyed via line  125  as a second VCM product stream or which may optionally be returned to EDC purification distillation unit  111  for byproduct removal. The mid-section of HCl distillation unit  115  (where the water is present at a beneficially “high” concentration level for efficient drying purposes) operates at process temperatures of between around 0 degrees C. to 10 degrees C. at an operating pressure of about 150 psig. This is convenient to the implementation of the preferred embodiment using silica gel since silica gel water loading capacity increases dramatically as the temperature of silica gel declines below about 15 degrees C. Table 1 shows a representative profile of temperatures at various trays in one such HCl distillation unit  115 . 
     
       
         
               
               
               
             
               
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 HCl distillation 
                   
               
               
                   
                 unit 115 
                 Tray temperature 
               
               
                   
                 tray number 
                 (degrees C.) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 50 
                 (top) 
                 −30.7 
               
               
                   
                 42 
                   
                 −30.0 
               
               
                   
                 38 
                   
                 −24.0 
               
               
                   
                 30 
                   
                 −7.4 
               
               
                   
                 23 
                   
                 2.8 
               
               
                   
                 21 
                   
                 3.4 
               
               
                   
                 19 
                   
                 14.1 
               
               
                   
                 17 
                   
                 46.1 
               
               
                   
                 13 
                   
                 61.5 
               
             
          
           
               
                   
                 Bottoms 
                 61.6 
               
               
                   
                   
               
             
          
         
       
     
     The FIG. 1 depiction of vinyl chloride monomer (VCM) purifying system  100  does not show reflux lines, pumps, valves instrumentation, safety relief and rupture devices, environmental safeguarding measures, and a control system which are generally used in the construction and operation of such unit operations and unified systems; except as further detailed herein, the incorporation, sizing, installation, and use of these components are apparent to those of skill. A VCM side-draw is taken off of HCl distillation unit  115  via line  127 , boosted with pump  175 , and conveyed to dryers  113   a,b . Dryer  133   a  has a bed of silica gel  157   a , and dryer  133   b  has a bed of silica gel  157   b . The VCM side-draw is sourced from any one of four take-offs (not shown) which connected to line  127  from trays twenty-two, twenty, seventeen and fifteen of HCl distillation unit  115  (HCl distillation unit  115  has fifty total trays). Normal VCM side-draw flow is usually about 0.25 kg/s, with a maximum VCM side-draw of about 1.0 kg/s, which represents a percentage of about 1.4% to 5.6% of the sum of the input from lines  109  and  123  to HCl distillation unit  115 . In a preferred embodiment, sidedraw is effected as needed to control water consistent with stable operation of HCl distillation unit  115 . 
     Restating the surprising discovery, it has been noted that the mid-section of an HCl distillation column, namely comprising from trays fifteen to twenty-two in a fifty-tray HCl distillation unit  115 , accumulates the highest concentration of water in VCM purifying system  100 . Accordingly, a side stream taken from this section of HCl distillation unit  115  has the most preferable mass transfer concentration gradient of water in VCM purifying lo system  100  when the mass transfer concentration gradient is defined respective to a water absorbent medium. As should be apparent, the vertical water profile in HCl distillation unit  115  shifts somewhat in operation with modifications in compositions of feed streams  109  and  123  and in general operating conditions with reflux effecting control of the water profile in the column. 
     Since the water accumulates to a high concentration at the mid-section, there are certain efficiencies in removing the water from the material at this particular point in the purifying process (i.e., it is essentially “easier” to remove water from about 150 PPM down to about 10 PPM by weight in the mid-section than it is to remove water from about 10 PPM down to about 1 PPM by weight in another stream). The cycle time of silica gel  157   a,b  is further improved dramatically when used to dry VCM side-stream from (e.g.) 150 PPM to 10 PPM instead of drying it from 10 PPM to 1 PPM since (a) silica gel loading capacity increases with inlet water concentration and (b) silica gel regeneration is facilitated with a higher acceptable residual post-regeneration water loading level in the dried silica gel in the 150 PPM to 10 PPM case. 
     Dryers  133   a,b  operate as a dual dryer set and as a virtual drying system process unit in the preferred embodiment-while one dryer (e.g. dryer  133   a ) is adsorbing water from the VCM side-draw stream of line  127 , the other dryer (e.g. dryer  133   b ) is either in a regeneration procedure or in a waiting mode. Hence, water removal from HCl distillation unit  115  is continuously enabled. 
     The dried VCM side-draw stream of line  167  is returned to line  105 . EDC is used in the preferred embodiment as the regeneration media for dryers  133   a,b ; in this regard, EDC is passed through a temperature profile of from less than 30 degrees C. to about 125 degrees C. in a regenerative cycle. Gradual temperature increases are required to (a) control the evolution of acid across the regenerative cycle and thereby (b) minimize the corrosive attributes of the discharged EDC and recycled water conveyed in line  135  to EDC manufacturing. In this regard, even as HCl is adsorbed along with water onto the silica gel during the drying operation, HCl and water in the silica gel are also desorbed from the silica gel during regeneration; this ongoing presence of both HCl and water creates a need for corrosion management in both the absorption and desorption operations. BDC temperature is adjusted (as further explained herein) in heating unit  161  (a hot oil exchanger) prior to entry into either dryer  133   a  or dryer  133   b  in a time/temperature profile generally in accordance with that shown in FIG.  3 . 
     The purpose of dryers  133   a,b  is to remove water from VCM purifying system  100 . If not removed, water mixes with HCl in HCl distillation unit  115  to form a corrosive mixture; the corrosive mixture then reacts with iron and Monel in the unit to form corrosion products. These corrosion products accumulate on HCl distillation unit  115  trays and plug openings in those trays, deteriorating the operational characteristics of the unit. The accumulation of corrosion products eventually requires shutdown and cleaning of VCM purifying system  100 , and such a shutdown represents a loss of productivity for the unit. Dryers  133   a  and  133   b , heating unit  161 , analyzers  171  and  173  for water content analysis, and valves  139 ,  141 ,  143 ,  145 ,  147 ,  149 ,  151 , and  153  function with lines  127 ,  137 ,  135 , and  167  as a continuously operating drying system process unit two bed drying system which is managed as a drying system process unit for control purposes. While one dryer (e.g. dryer  133   a ) is absorbing water from VCM side-draw, the other dryer (e.g. dryer  133   b ) is either being regenerated and emptied or is in process wait mode. In the preferred embodiment, the maximum flow rate through dryers  133   a,b  is 2.0 kg/s, double the maximum side draw off of 1.0 kg/s so that both the maximum side draw from HCl distillation unit  115  and a return of any off-spec material can passed through the dryer prior to return to primary distillation unit  107  for reprocessing. Once a dryer has been regenerated, it remains idle until the on-line dryer becomes saturated with water. At this time, the VCM side-draw flow in line  127  is switched by use of valves  143 ,  145 ,  151  and  153  to the regenerated and waiting dryer. In facilitating measurements of real-time water composition in VCM side-draw and dried VCM side-draw, (a) a first water analyzer  171  is installed to measure the composition of water in VCM side-draw in line  127  and (b) a second water analyzer  173  is installed to measure the composition of water in dried VCM side-draw in line  167 . Dryers  133   a,b  are each constructed of carbon steel; each has an internal volume of about 95 cubic feet, and each is loaded with 3900 pounds of silica gel  157   a,b  having a grade designation of  40 . 
     Under normal operation, the dried VCM side-draw is discharged to line  105 . However, after a dryer has become saturated, the VCM side-draw within it must be first drained at the beginning of the regeneration cycle. In this regard, the VCM side-draw is drained to a recycle tank (not shown) using a nitrogen purge as an assisting propellant and evaporative gas. 
     The discharged EDC from the regeneration process is recycled to an EDC manufacturing facility. VCM side-draw in line  127  typically has a temperature of between 0 and 10 degrees C. and a composition of 80-92% VCM, 8-20% HCl, and 50 PPM-200 PPM water. The dried VCM side-draw leaving the discharge of dryer  133   a,b  is dried to no more than 50 PPM water in normal operation; when 50 PPM water is measured at the discharge, the bed is considered to be saturated and the VCM side-draw is switched to the other dryer. Line  109  has a flow of 16.5 kg/s, and a composition of (a) 73.5 mol %/62.0 mass % of HCl and (b) 26.5 mol %/38.0 mass % of VCM. Line  123  has a flow of 1.5 kg/s and a composition of (a) 4.0 mol %/6.7 mass % HCl and (b) 96 mol %/93.3 mass % VCM. Water overheads from HCl distillation unit  115  are less that 1 ppm and the VCM product specification (line  129 ) is for water at or below 50 ppm and for HCl at or below 0.2 ppm. 
     In operation, a computerized control system is preferably used to control real-time configuration of VCM purifying system  100  for that portion of the system which is beneficially controlled via automation. In this regard, unit operations in dryers  133   a  and  133   b , heating unit  161 , analyzers  171  and  173 , and valves  139 ,  141 ,  143 ,  145 ,  147 ,  149 ,  151 , and  153  function with lines  127 ,  137 ,  135 , and  167  as a continuously operating drying system process unit. FIG. 2 outlines the key process unit steps  200  respective to real-time operation of the drying system process unit as shown in part of FIG.  1 . In this regard, FIG. 2 shows the key process unit steps  200  which apply to dryer  133   a  and dryer  133   b  individually; in this regard, dryer  133   a  is in only one process unit step of FIG. 2 at any particular moment, and dryer  133   b  is in only one process unit step of FIG. 2 at any particular moment. In collectively operating (a) dryers  133   a  and  133   b , (b) heating unit  161 , (c) analyzers  171  and  173 , and (d) the valves ( 139 ,  141 ,  143 ,  145 ,  147 ,  149 ,  151 , and  153 ) as a unified drying system process unit, actions in certain steps respective to one dryer respective to the application of the method of FIG. 2 will be conditional on the active step respective to the other dryer; for example, entry of dryer  133   a  into Regeneration Step  209  is normally not permitted if dryer  133   b  is not in Feed VCM Side-draw Step  205  because such a situation would deprive VCM purifying system  100  of use of the benefits of the drying system process unit. 
     Turning now to FIG.  2  and Maintenance Wait Step  201 , valves ( 139 ,  141 ,  143 ,  145 ,  147 ,  149 ,  151 , and  153 ) are closed, and pump  175  is off. Designation of the Maintenance Wait Step  201  as the active process unit step for real-time control coordination is usually entered (a) if the operating technician deems that the drying system process unit should halt its normal operational methodology for purposes related to repair or (b) if either HCl distillation unit  115 , furnace system  103 , or primary distillation unit  107  are recognized by the control system as in a mode establishing an unsuitable basis for continued operation of the drying system process unit. 
     In the Process Wait Step  203  for dryer  133   b , valves  141 ,  143 ,  149 , and  153  are all closed. 
     In the Process Wait Step  203  for dryer  133   a , valves  139 ,  145 ,  147 , and  151  are all closed. 
     When HCl distillation unit  115 , cracking furnace system  103 , and primary distillation unit  107  are functioning in a stable operational mode in real-time and temperature measurements (not shown) of the dryer are verified to be below 25 degrees C., the process control system (a) defines the status of dryer  133   a  as being in Feed VCM side-draw Step  205  and (b) opens valves  145  and  151 . The control system energizes pump  175  and flow is forwarded to dryer  133   a  until water analyzer  173  detects a high reading. 
     An example of performance data in Feed VCM side-draw Step  205  is shown in Table 2. Table 2 shows pilot plant drying data for VCM side draw taken from HCl distillation unit  115  over a period of four days. Data in Table 2 demonstrates removal of a large differential concentration of water from the HCl/VCM mixture for an extended period of time, with water concentration in dryer discharge being maintained below the saturation limit of 50 ppm (in respect to the highly corrosive aqueous HCl phase). Another performance statistic apparent in the data is that silica gel is capable of a substantial weight % loading with water. 
     Note that the silica gel was loaded to 6.2% water on the third day of operation. 
     
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 HCl col 
                   
                 Dried 
                   
                   
                 Accumulated 
                 Accu- 
                   
               
               
                   
                 Online 
                 Flow- 
                 mid- 
                 HCl col 
                 VCM 
                   
                 Water 
                 water 
                 mulated 
               
               
                   
                 Time 
                 rate 
                 section H 2 O 
                 bottom water 
                 (ppm 
                 HCl 
                 loaded 
                 loaded 
                 wt % 
                 % water 
               
               
                 Day 
                 (hours 
                 (lph) 
                 (ppm w/w) 
                 (ppm w/w) 
                 w/w) 
                 wt % 
                 gms 
                 (gms) 
                 loaded 
                 removed 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Day 1 
                 2.5 
                 4.0 
                 130.3 
                 8.2 
                 13.4 
                 8.1 
                 1.1 
                 1.1 
                 0.3 
                 89.7 
               
               
                 Day 2 
                 24.0 
                 4.0 
                 164.3 
                 8.7 
                 29.5 
                   
                 10.7 
                 11.7 
                 3.2 
                 82.0 
               
               
                 Day 3 
                 48.0 
                 4.0 
                 172.3 
                 15.1 
                 48.5 
                 8.1 
                 10.9 
                 22.7 
                 6.2 
                 71.9 
               
               
                 Day 4 
                 75.0 
                 4.0 
                 108.0 
                 7.7 
                 59.7 
                 7.3 
                 4.8 
                 27.5 
                 7.5 
                 44.8 
               
               
                   
               
             
          
         
       
     
     When a high reading in analyzer  173  (above 50 PPM) in Feed VCM side-draw Step  205  is measured by the process control system, the process control system defines the status of dryer  133   b  as being in Feed VCM side-draw Step  205  and opens valves  143  and  153  to enable flow through dryer  133   b . The process control system then (a) defines the status of dryer  133   a  as being in Drain VCM side-draw Step  207 , (b) closes valves  145  and  151 , and (c) opens a drain valve (not shown) and activates a nitrogen purge (not shown) to enable VCM side-draw to drain from dryer  133   a  into a recycle tank (not shown) until a low level switch (not shown) in dryer  133   a  indicates that dryer  133   a  is essentially emptied of VCM side-draw. When the low level switch activates, the nitrogen purge is discontinued and the drain valve is closed. The recycle tank is periodically recycled into purifying system  100 . 
     When the low level switch in dryer  133   a  indicates that VCM side-draw has been emptied, the process control system defines the status of dryer  133   a  as being in Regeneration Step  209  and opens valves  139  and  147  to convey EDC through dryer  133   a . Heating unit  161  is controlled to provide EDC at a temperature profile as shown in FIG.  3 . Essentially, heating unit  161  begins at a temperature of less than 30 degrees C. and then ramps the temperature of the EDC up at a rate of about 2 degrees C. per hour until a temperature of 125 degrees C. is attained. 
     FIG. 3 illustrates the temperature ramping process by showing pilot plant data for a silica gel  157   a,b  regeneration instance. The silica gel  157   a,b  was loaded with water to a weight percentage of 13.5% by drying VCM side-draw from HCl distillation unit  115 . A loading of 11.6% HCl was also measured in the gel. FIG. 3 shows the regeneration temperature used over a 45 hour period, the rate of water and HCl removal from the gel, and the amount of EDC used. 
     Returning to FIG. 2, after executing completion of the temperature profile in regeneration Step  209 , the process control system (a) defines the status of dryer  133   a  as being in Drain EDC Step  211 , (b) closes valves  147  and  139 , and (c) opens a drain valve (not shown) to enable EDC to drain from dryer  133   a  into the recycle tank (not shown) until the low level switch (not shown) in dryer  133   a  indicates that dryer  133   a  is essentially emptied of EDC. 
     When the low level switch in dryer  133   a  indicates that dryer  133   a  is essentially emptied of EDC, the process control system then defines the status of dryer  133   a  as being in Process Wait Step  203 . 
     As should be appreciated by those of skill, dryer  133   a  can be substituted for dryer  133   b  and dryer  133   b  can be substituted for dryer  133   a  in the foregoing discussion respective to FIG. 2 (with valves  143 ,  141 ,  149 , and  153  also being mutually cross-substituted with valves  145 ,  139 ,  147 , and  151 ) to describe complementary operation of the dryers in the case where dryer  133   b  is regenerated. As each dryer ( 133   a ,  133   b ) is directed by the process control system through its operational procedure according to method  200 , the drying system process unit provides a continuous water removal subsystem within VCM purifying system  100  for treating water rich VCM side-draw taken from HCl distillation unit  115 . In the course of real-time operation, dryer  133   a  and dryer  133   b  alternatively act as the “on-line” dryer in VCM purifying system  100 . 
     The present invention has been described in an illustrative manner. In this regard, it is evident that those skilled in the art, once given he benefit of the foregoing disclosure, may now make modifications to the specific embodiments described herein without departing from the spirit of the present invention. Such modifications are to be considered within the scope of the present invention which is limited solely by the scope and spirit of the appended claims.