BACKGROUND OF THE INVENTION Hydrofluorocarbons (HFCs) in general, and pentafluoroethane (herein referred to as HFC-125) in particular, are of particular interest as replacements for environmentally disadvantageous chlorofluorocarbons (CFCs), which heretofore frequently have been used, for example, in air conditioning and refrigeration applications. Unlike CFCs and hydrochlorofluorocarbons (HCFCs), HFCs do not contain chlorine and therefore do not decompose to form chlorine-containing chemical species, which are suspected of causing depletion of the ozone layer. While HFCs thus avoid the main disadvantage of such chlorine- containing compounds, they nevertheless possess many of the beneficial properties of those compounds. For example, HFCs have been used successfully in place of HCFCs and CFCs as heat transfer agents, blowing agents and propellants. Thus, HFCs are desirable targets of chemical synthesis.

It is known to manufacture pentafluoroethane by reacting perchloroethylene (PCE) and excessive hydrogen fluoride (HF) in the gas phase in the presence of a fluorination catalyst. This reaction generally yields a variety of reaction products in addition to HFC-125, several of which are undesirable chlorinated compounds. Particularly disadvantageous

byproducts include trichlortrifluoroethane (CFC-113 and CFC-113a), dichlortetrafluoroethanes (CFC-114 and CFC-114a) and chloropentafluoroethane (CFC-115).

CFC-115, in particular, has a relatively high ozone degradation potential (ODP) and is therefore even only small amounts of this compound in the HFC-125 product stream can have a severe negative impact on the commercial value of the product stream. As disclosed in U. S. Patent No. 5,346,595, which is incorporated herein by reference, CFC-115 tends to form an azeotrope with HFC-125, which makes it difficult and costly to separate from the HFC-125 product stream.

Certain byproducts of the fluorination reaction, which generally correspond to the formula C2HCll+XF4 x, where x = 0 to 3, can be relatively easily converted to HFC-125, and it is known that such compounds can be recycled to the reactor train from which they were produced. For example, U. S. Patent No. 5, 962,753-Shields et al. discloses a fluorination reaction process in which perchloroethylene and hydrogen fluoride are fed as stream to a fluorination reaction train. The resulting product stream contains, in addition to HFC-125, one or more isomers of dichlorotetrafluoroethane (CFC 114/114a), dichlorotrifluoroethane (HCFC 123/123a), chlorotetrafluoroethane (HCFC 124/124a), chlorotrifluoroethane (HCFC 133/133a), tetrafluoroethane (HFC 134/134a) and chloropentafluoroethane (CFC 115), as well as unreacted hydrogen fluoride and by-product hydrogen chloride. In the process of the Shields patent, unreacted PCE and HF, and HCFC-123, HCFC-133a, HCFC-124 and other intermediate products are separated from the product stream via a series of distillation columns and recycled to the original reaction train.

Although chlorotetrafluoroethane (HCFC-124/124a) is frequently recycled to the reaction train in which it was originally produced, as shown in the Shields patent, it also has independent value as refrigerant and is sometimes included with the HFC-125 product stream

and/or produced as a part of a separate HCFC-124 product stream.

In contrast to HCFC-124/124a, CFC-115 is a compound that is not readily converted to HFC-125 and is not generally recycled to the fluorination reaction. Moreover, compounds such as CFC-114/114a are generally converted to CFC-115 in the fluorination reaction, and accordingly this compound is also not generally recycled. CFC114/114a also creates difficulties in the purification process, as explained in provisional U. S. Patent Application 09/432, 748, which is assigned to the assignee of the present invention and which is incorporated herein by reference.

Because of the above-noted difficulties, there has been a need for a fluorination process which has improved selectivity of HFC-125 and/or which achieves improved ratio of the desired products, such as HFCs (particularly HFC-125) and HCFCs to the undesired (or less desired) products. As explained below, the present inventors have discovered a process that satisfies these and other needs and achieves superior and unexpected results.

One of the other advantages of the present process is that it is capable of providing an improvement in catalyst utilization, and more particularly an improvement in the average life of a production campaign. As is known to those skilled in the art, catalyst utilization factors can be critical to the success of a process used for the production of fluorinated compounds on a commercial basis.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a generalized process flow diagram illustrating one embodiment of the processes of the present invention.

Fig. 2 is a generalized process flow diagram illustrating a second embodiment of the processes of the present invention.

Fig. 3 is a generalized process flow diagram illustrating a third embodiment of the processes of the present invention.

Fig. 4 is a generalized process flow diagram illustrating a fourth embodiment of the processes of the present invention.

Fig. 5 is a generalized process flow diagram illustrating a fifth embodiment of the processes of the present invention.

Fig. 6 is a generalized process flow diagram illustrating a fluorination process which uses a single reaction train.

DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS The present inventors have discovered that it is possible to achieve improved yield, and/or improved selectivity of HFC-125, and/or an improved HFC/CFC ratio (and particularly HFC-125/CFC-115 ratio) by a fluorination process which comprises reacting PCE and HF in a first reaction train to produce a reaction product comprising at least (HCFC- 124), separating from this reaction product at least a portion of the HCFC-124, and reacting the separated HCFC-124 with HF in a second reaction train to produce a second reaction product containing at least HFC-125. Unless indicated otherwise, hereinafter"HCFC-124" shall refer to HCFC-124, its isomer HCFC-124a, and mixtures of these. Similarly,"HCFC- 123"refers to HCFC-123, its isomers HCFC-123a and HCFC-123b, and mixtures of these, while the term"CFC-114"shall refer to CFC-114, its isomer CFC-114a, and mixtures of these. In general, the reaction product of the first train will also include at least (HFC-125) and (CFC-115).

It is contemplated that the first reaction train may be operated in certain embodiments to maximize the production of HFC-125. Furthermore, it is contemplated that

such embodiments will be preferred when it is desired to maximize the yield of HFC-125 and improvements in catalyst utilization are less important. In other embodiments, it is contemplated that the first reaction train will be operated to maximize the production of HCFC-124, thus shifting much of the actual production of HFC-125 to the second reaction train. Such embodiments are expected to be favored for those applications in which improvements in catalyst utilization are considered especially important.

As used herein, the term"reaction train"refers to one or more reaction zones connected in a series arrangement. This term thus encompasses a series of separate reaction zones which are contained within a single reaction vessel, as well as a series of reaction vessels each defining a separate reaction zone, connected by appropriate conduits and valving. A"reaction train"can thus contain a single reaction chamber or zone, but in embodiments where the reaction train contains two or more chambers and/or zones, these are connected in a series arrangement whereby the reaction product from the first chamber or zone is fed, at least partially but preferably substantially completely, to a second reaction chamber or zone.

It has been discovered that substantial and unexpected improvement in yield, selectivity, ratio of desired to undesired products, and/or catalyst utilization can be achieved by processes which involve separating from the reaction product of the first reaction train at least a portion of one or more of the intermediate HCFC compounds in the reaction product and recycling one or more of the separated compound (s) to a second, parallel reaction train.

It will be appreciated by those skilled in the art that the fluorination reaction of the present invention is relatively complex, involving numerous competing and alternative reaction schemes. For example, in the fluorination of PCE with HF, the HF is added across the PCE double bond. As the process proceeds, chlorine atoms are successively replaced by fluorine,

yielding HC1 as a byproduct. At any given time the reaction train will typically contain a mix of unreacted PCE, unreacted HF, HCI, and various ethanes halogenated with different combinations of chlorine and fluorine atoms, depending on the balance of the different possible fluorination schemes. Furthermore, various different fluorination progressions may occur simultaneously. For example, in one fluorination sequence PCE successively converts to HCFC-121, HCFC-122, HCFC-123, HCFC-124 and finally HFC-125, which is a desired end product. Thus, it is seen that since HCFC-124 is an intermediate in the formation of the desired HFC-125, it has heretofore been recycled to the reaction train from which it originated in the expectation that it would be converted therein to HFC-125.

Applicants have found that separating and recycling at least a portion, an preferably at least about 25 percent of the HCFC-124 produced in the first reaction train, to a second reaction train results in a process that is improved in several respects. For example, the overall yield of HFC-125 can be improved, with HFC-125 selectivity of at least about 25 percent on a weight basis being obtainable in certain embodiments, with a selectivity of at least about 35% being even more preferred. Unless otherwise indicated, the term "selectivity"refers to the percentage, on a weight basis, of the indicated product or group of products based on the total of HFCs, HCFCs and CFCs in the product. The present processes can also be operated to obtain improved ratio of desired to undesired (or less desired) components, namely, HCFCs and HFCs's in general and HFC-125 in particular. It is particularly preferred that the HFC-125: CFC-115 ratio (on a weight basis) is at least about 100, more preferably at least about 150, and even more preferably at least about 200.

Furthermore, the processes of the present invention tend to increase, on average, the length of production campaign, which would otherwise be earlier limited by catalyst performance. By preferentially separating and recycling HCFCs in general, and the HCFC-124 in particular, to

one or more different, parallel reaction trains, the overall formation of and/or selectivity to HFCs in general, and HFC-125 in particular, can, be enhanced and catalyst utilization increased.

The mechanisms and conditions which produce the unexpected and superior results described above are not yet fully and completely understood. Nevertheless, without intending to be bound by or to any particular theory of operation, it is believed that the higher reaction temperatures which promote conversion of HCFC-124 to HFC-125 may be disadvantageous when other products in the reaction mixture of the first train, such as PCE, CFC-111, HCFC- 121, HCFC-122 and HCFC-123, are exposed to such high temperatures. For example, the exposure of these other products to such high temperatures may favor undesirable reaction paths and/or may produce increased levels of other by-products and/or cause increased catalyst deactivation. According to preferred embodiments of the present invention, therefore, the average reaction temperatures in the second train of the present invention are higher than the average reaction temperatures in the first train, and even more preferably at least about 20 to about 50°F higher than the average reaction temperatures in the first train.

In general, the preferred embodiments of the present invention utilize a process flow pattern as illustrated schematically in Fig. 1. More particularly, the processes of the present invention preferably utilize a first reaction train RI, a second reaction train R2, and separation means S3. The first reaction train R1 receives PCE and HF, preferably by feed streams 10 and 20, respectively. The first reaction train preferably also receives a recycle of products produced in the first and/or second reaction train, preferably by recycle stream 90. The fresh feed streams 10 and 20 and the recycle stream 90 are fluorinated in the reaction train R1 to produce a final reaction product 30. The second reaction train R2 receives recycled HCFC- 124, preferably via feed stream 50, and fresh or recycled HF, preferably via feed stream 40.

The second reaction train R2 produces a second final reaction product stream 60.

According to preferred embodiments, the first reaction train produces a final reaction product stream containing CFCs, HCFCs and HFCs, and more particularly at least HFC-125, HCFC-124 and CFC-115. The term"final reaction product stream"as used herein means a stream which exits from a reaction chamber or zone and which does not undergo any substantial additional reaction prior to the separation step. The separated HCFCs, and preferably HCFC-124, is then introduced as a reactant, preferably via recycle stream 50, together with HF, preferably via feed stream 40, into a second reaction train where at least a portion of the HCFC-124 is converted to HFC-125. This second reaction train also preferably produces a final reaction product stream containing a relatively high concentration of HFC, and particularly HFC-125.

It is contemplated that each of the reaction trains of the present invention can be operated to advantage under a wide variety of process flow and reaction conditions. In general, the first and second reaction trains each preferably comprise one or more reaction zones which contain fluorination catalyst. The amount of the fluorination catalyst used can vary, but it is generally preferred that for continuous processes in which the catalyst comprises chromium-based catalyst.

It is also generally preferred that the feed of reactants to the first reaction train, on average, has mole ratio of HF : organics of at least about 5: 1, and even more preferably of from about 5: 1 to about 30: 1.

The first final reaction product stream 30, which generally will contain a mixture of unreacted PCE, unreacted HF, HC1, various HFC, HCFC and CFC ethanes, including particularly HFC-125, HCFC-124 and CFC-115, and other unsaturated compounds is preferably introduced into a separation means or step S3. For the purposes of the present

invention, the separation step of the present invention produces at least one stream 50 comprising at least a portion of the HCFC-124 produced in the first reaction train. Preferably stream 50, which is the recycle stream or streams to the second reaction train (s), contains HCFC-124 in an amount that is at least about 25%, more preferably at least about 50 %, and even more preferably at least about 75% by weight of the HCFC-124 contained in the combined final reaction product streams 30 and 60. In highly preferred embodiments substantially all of the HCFC-124 in the combined final product streams (subject to the limitations of the separation step) is recycled to the second reaction train. The separation step S3 also produces one or more streams containing desired products, including an HFC-125 product stream 70, and one or more additional streams 90, which contain products which are generally not desired and will generally either be recycled, included in a product stream, or otherwise disposed of. As explained hereinafter, the streams 90 can be sent for further processing, recycled to the first reaction train, recycled to the second reaction train, or combinationsthereof.

Although it is preferred that the second final reaction stream 60 is introduced into the same separation means S3 as the final reaction stream from the first reaction train, it is within the scope of the present invention for the second final reaction stream to be introduced to different separation means or to bypass the separation step entirely.

THE FIRST REACTION TRAIN As indicated above, the first reaction train can comprise a single reaction zone or multiple zones connected in a series arrangement. Several preferred configurations of the first reaction train RI in accordance with the present invention are illustrated in Figs. 2-5.

The primary reactant feeds to the first reaction train are PCE stream 10 and HF stream 20.

Optionally, but preferably, the first reaction train also receives one or more recycle feed streams 90. The amount and composition of recycle stream 90 can vary widely within the scope hereof, depending upon factors such as the particular desired products, the particular separation process used and other commercially important factors. Its is preferred according certain embodiments, however, that the recycle stream (s) 90 which are introduced to the reaction train RI include only a minor proportion of the HCFC-124 produced by the combined reaction trains, more preferably an amount of HCFC-124 that is less than about 45% of the HCFC-124 that is produced in the combined reaction trains and even more preferably less than about 35 by weight of the HCFC-124 produced in the combined reaction trains. According to especially preferred embodiments, the amount of HCFC-124 recycled to the first reaction train is less than about 10% by weight of the HCFC produced by the combined reaction trains.

Although it is contemplated that numerous methods of introducing the reactant streams 10, 20 and 90 the reaction train can be used, it is preferred that these streams are first introduced to a means for mixing the streams, such a mixing valve V1. The feed streams are preferably introduced to the first reaction zone Zl 1 as vapor streams, and preferably a combined superheated vapor stream 21. Although it is contemplated, as mentioned above, that the first reaction train R1 can comprise a single reaction zone, it is generally preferred that the first reaction train comprise at least two reaction zones, with the first zone Z 1 I in the train receiving the mixed feed from the mixing valve VI and the second zone Z12 receiving at least a portion, and preferably substantially all of the output stream 22 from the first reaction zone.

According to certain embodiment, such as illustrated in Figs. 3-5 for example, the second reaction zone also receives additional fresh feed or additional recycle feed, such a

fresh PCE stream 10, and/or additional fresh HF, which is combined with the effluent 22 from zone Zl 1 in mixing valve V2 and then introduced as a mixed stream 23 into zone Z 12.

The effluent from the second zone can constitute the final reaction product 30 from the first reaction train Rl, as illustrated in Figs 2-4, or some or all of the effluent stream 22 can be utilized as feed stream to one or more additional reaction zone (s) Z13, as illustrated for example in Fig. 5.

As illustrated by the foregoing, numerous variations on reaction train configuration are available and are all within the scope of the present invention.

Preferably the fluorination reaction in the first train comprises a vapor phase reaction, and even more preferably a catalyzed vapor phase reaction. It will be appreciated by those skilled in the art that numerous combinations of reaction conditions can be utilized to advantage in accordance with the present invention, depending upon the particular constraints of each particular process, such as feedstock and catalyst availability, purity requirements and desired production rates. By adjusting various operating parameters it is possible to bias the reactions occurring in the reaction zones to favor the production of certain halogenated ethane products over others. Such operating parameters include temperature, mole ratio of organics, and particularly the mole ratio of organics : HF in the reactor feed, and reaction time, also referred to as contact or residence time. See, for example, Tung, et al., U. S. Patent No.

5, 155, 082, which is incorporated herein by reference. In the process of the present invention, HFC-125 is a desired end product. Therefore, according to many embodiments the various operating parameters, including temperature, mole ratio and contact time, are preferably adjusted to favor production of HFC-125. In such embodiments, the operating parameters of the first reaction train can be adjusted so that HFC-125 preferably comprises at least about 25 weight percent, and even more preferably at least about 35 weight percent of the total of all

organics in the reaction product stream from the first reaction train.

The processes of the present invention can also be used to advantage in embodiments in which the various operating parameters, including temperature, mole ratio and contact time, are preferably adjusted to favor production of HCFC-124. In such embodiments, the operating parameters of the first reaction train can be adjusted so that HFC-124 preferably comprises at least about 40 weight percent, and even more preferably about 75 weight percent of the total of all organics in the reaction product stream from the first reaction train. Since the present invention provides a second, parallel reaction train whose primary purpose is to convert HCFC-124 to HFC-125, such embodiments are illustrative of the flexibility of the present invention to accommodate differing sets of operational priorities.

According to preferred embodiments, the reaction of PCE with HF preferably is carried out in a reaction train containing at least two vapor phase reaction zones. The vapor phase in each reaction zone is preferably conducted at temperatures of from about 550°F to about 750°F, at pressures between atmospheric and about 250 psig, and with a contact time of about 2 to 100 seconds. Preferably, the reaction is carried out over a fluorination catalyst such as, for example, chromium oxyfluoride formed by the partial fluorination of chromium oxide, or other suitable catalysts, such as zinc-promoted chromia, as are well-known in the art. It is also preferred that the mole ratio HF to the reactive organic components in the feed to the first reaction zone is at least about 5: 1.

THE SECOND REACTION TRAIN The second reaction train can comprise a single reaction zone or multiple zones connected in a series arrangement. Several preferred configurations of the second reaction train R2 in accordance with the present invention are illustrated in Figs. 2-5. The primary

reactant feeds to the second reaction train are HF stream 40 and recycle stream 50. The amount and composition of recycle stream 50 can vary widely within the scope hereof, depending upon factors such as the particular desired products, the particular separation process used and other commercially important factors. It is preferred, however, that the recycle stream (s) 50 which are introduced to the reaction train R2 include an amount of recycle HCFC-124 that is greater than a major proportion by weight of the HCFC-124 produced in the first reaction train, and more preferably an amount of HCFC-124 that is at least about 75% by weight of the HCFC-124 that is produced in the first reaction train. Of course, other products from the first reaction train can also be recycled to the second reaction train according to the particular requirements of each individual case.

Although it is contemplated that numerous methods of introducing the reactant streams 40 and 50 to the reaction train can be used, it is preferred that these streams are first introduced to a means for mixing the streams, such a mixing valve V3. The feed streams are preferably introduced to the first reaction zone Z21 as vapor streams, and preferably a combined superheated vapor stream 41. Although it is contemplated, as mentioned above, that the second reaction train R2 can comprise a single reaction zone, it is generally preferred that the second reaction train comprise at least two reaction zones, with the first zone Z21 receiving the mixed feed, and preferably substantially all from the mixing valve V3 and the second zone Z22 receiving at least a portion of the output stream 42 from the first reaction zone. The effluent from the second zone can constitute the final reaction product 60, as illustrated in Figs 2-4, or some or all of the effluent stream 42 can be utilized as feed stream to one or more additional reaction zone (s), which in turn produce the final reaction product stream. In the single reaction zone embodiments, the effluent from this zone comprises the final reaction product 60, as illustrated for example in Fig. 5.

As with the first reaction train, the reaction of HFC-124 with HF in the second train preferably is carried out in a reaction train containing at least two vapor phase reaction zones.

The vapor phase in each reaction zone is preferably conducted at temperatures of from about 550°F to about 750°F, at pressures between atmospheric and about 250 psig, and with a contact time of about 2 to 100 seconds. Preferably, the reaction is carried out over a fluorination catalyst such as, for example, chromium-based or other suitable catalysts, such as zinc-promoted chromia, as are well-known in the art. It is also preferred that the mole ratio HF to the reactive organic components in the feed to the second reaction zone is at least about 5: 1.

THE SEPARATION STEP Numerous means and processes are known and available for separating the desired products, such as HFC-125, from unreacted reactants, byproducts, intermediates and impurities. It will be appreciated that the specific characteristics, parameters and operating conditions of the separation process used do not form a part of the present invention and that all such processes and means can be used within the scope hereof. In general, however, the unreacted reactants, byproducts, intermediates and impurities in the final product stream, which may be present in an appreciable amount, range from low boiling compounds (or lights) to high boiling compounds (or heavies) relative to pentafluoroethane, and their removal from the stream usually is preferentially effected sequentially in a series of distillation columns. In such processes, unreacted HF and PCE, together with higher boiling organics such as HCFC-123, are recycled in one or more streams. In the present process, such recycle streams can-be recycled to one or more zones in the first reaction train and/or to one or more zones in the second reaction train. The HFC 125 is typically withdrawn together

with CFC-115 as a purified product. The HCFC-124 in the product can also be isolated to varying degrees of purity to form an HFC-124 recycle stream. For example, such HFC-124 recycle can include other components according to any of the known separation processes, including widely varying amounts of HCFC-133/133a, HFC 134/134a and CFC 114/114a. A preferred separation process is disclosed in U. S. Patent Application 09/432,748, which is assigned to the assignee of the present invention and which is incorporated herein by reference. Other available separation processes are disclosed in U. S. Patent No. 5,849,160- Hamoto, which is incorporated herein by reference.

EXAMPLES COMPARATIVE EXAMPLE Cl This example illustrates the performance of fluorination processes in accordance with a single train process having the basic flow diagram of Figure 6.

The single reaction train has a first reaction zone comprising a first reaction vessel Z1 and a second reaction vessel Z2 connected in series. Each reaction vessel is charged with a chromium-based catalyst, with each vessel being loaded with the same volume of catalyst. A fresh perchloroethylene feed stream 10 is preheated to a temperature of about 560°F and then introduced into a mixing valve together with fresh HF feed stream 20 preheated to a temperature of about 560°F. A first recycle stream 90a at a temperature of 560°F and a pressure of about 145 psig is also introduced into the first reactor zone Z 1. The mole ratio of HF: organics in the feed (including the recycle stream 90a) is held at about 15: 1. The reaction is conducted at a pressure of about 145 psig. The contact time in the first reaction zone is about 12 seconds.

The effluent from the first reaction zone is heated to a temperature of about 650 °F,

mixed with a second recycle stream 90b and then introduced into the second reaction zone Z2. The second reaction zone is operated at a pressure of about 115 psig. The contact time in the second reaction zone is about 9 seconds. The other conditions and results are reported in Table Cl below.

TABLE Cl Combined Reactant Streams First Recycle Second Recycle Product Stream (#s 10 and 20) Stream (#90a) Stream (#90b) (#30) Components wt% PCE 50.72 1.43 1.43 1.2800 HCFC 123 13.00 13.00 9.7000 HCFC 124 17.59 7.8600 other HCFs 0.02 .02 0.7200 HF 31.69 85.16 85.16 57.9300 HCL 13.1300 HFC 125 8.6400 CFC 114 0.3450 CFC 115 0.0588 other CFCs 0.40 .40 0.3200 Cl2 0.0100 100 10.01 100.01 99.9938 Total Weight, lbs. 33.67 58.92 7.4 100.0000 Total HFC+HCFC+ 27.6400 CFC, lbs. total CFC 2.61% Selectivity, Rel. wt% HCFC + HFC Selectivity - Rel. 97.39% wt% HFC Selectivity - Rel. wt% 32.41%

Combined Reactant Streams First Recycle Second Recycle Product Stream (#s 10 and 20) Stream (#90a) Stream (#90b) (#30) Components wt % HFC/HCFC Ratio 0. 3328 . R125/R115 Ratio 147 (HFC+ HCFC)/CFC Ratio 49. 6800 EXAMPLE 1 Parallel Train Production of HFC-125 This example illustrates the performance of fluorination processes in accordance with the present invention having the basic flow diagram of Figure 3.

The same catalyst as used in Example Cl is used. Moreover, the same amount of catalyst is used, with the catalyst volume being divided substantially equally among the four reaction zones.

The first reaction train RI has a first reaction zone Zl comprising a vessel charged with the above noted catalyst. A fresh PCE and HF are introduced together as a fresh feed stream preheated to a temperature of about 560°F. A first recycle stream at a temperature of 560 °F and a pressure of about 145 psig is also introduced into the first reaction zone Z 11.

The mole ratio of HF : organics in the feed (including the recycle stream) is held at about 15.

The reaction is conducted at a pressure of about 145 psig. The contact time in the first reaction zone is about 12 seconds.

The effluent from the first reaction zone in the first reaction train is heated to a temperature of about 650°F, mixed with a second recycle stream and then introduced into a second reaction zone Z12. The reaction in the second zone is conducted at a pressure of

about 145 psig. The contact time in the second reaction zone is about 12 seconds.

The second reaction train also has a first reaction zone Z21 comprising the first zone Z21 in the second train R2 is feed with a fresh HF stream and HCFC-124 recycled from the separation step. The HF is preheated to a temperature of about 580°F and then introduced into a mixing valve together with the recycle stream, which is heated to a temperature of about 580°F and has a pressure of about 112 psig. The mole ratio of HF : organics in the feed is held at about 15. The reaction is conducted at a pressure of about 112 psig. The contact time in the first reaction zone Z21 is about 65 seconds.

The effluent from the first reaction zone Z21 in the second reaction train R2 is introduced into a second reaction zone Z22. The reaction is conducted at a pressure of about 112 psig. The contact time in the second reaction zone is about 65 seconds.

The other conditions and results are reported in Table 1 below.

TABLE 1 First Second Reactant Recycle Recycle Reactant/ Stream Stream 3 Stream 4 R124 Product Stream Components wt % PCF 47.39 1.4 1.4 1.1000 HCFC 123 13.0 13.0 9.4300 HCFC 124 23.00 35. 21 8.0300 Other HCFCs <0. 01 <0.01 0.7000 HF 29.61 85.16 85. 16 64.79 57. 3800 HCL 11. 7200 HFC 125 10. 9300 CFC 114 0. 2910 CFC 11 S 0. 0488 Other CFCs 0. 40 0.40 0. 3100

First Second Reactant Recycle Recycle Reactant/ Stream Stream 3 Stream 4 R124 Product Stream Components wt % Cl2 0.0200 Total 100 99.97 99.97 100 99. 9598 Total Weight, lbs. 30.24 49.44 6.21 14.11 100.0000 Total HFC+HCFC+ 29.7400 CFC,lbs. Total CFC 2.17% Yield, Rel. wt% HCFC + HFC Yield-Rel. wt% 97.83% HFC Yield- Rel. wt% 37. 70% HFC/HCFC Ratio 0. 3854 R125/R115 Ratio 224 (HFC+ HCFC) HCFC Ratio 62. 4700 A comparison of Examples Cl and 1 reveals that, for the same amount of catalyst and substantially the same amount of fresh feedstock, the process of the present invention is capable of providing a substantial improvement in selectivity and in the ratio of the desired to the less desired components. More particularly, the selectivity of HFC-125 is about 27% (on a relative basis) greater than for a single reaction train embodiment, while the HFC-125/CFC- 115 ratio is improved by about 52%.