COIL WOUND HEAT EXCHANGER WITH REDUCED PRESSURE DROP BACKGROUND

[0001] This application relates to a feed-effluent heat exchanger for warming the feed and cooling the effluent of a hydrocarbon reaction that is conducted at a low pressure.

[0002] The present disclosure generally relates to thermal transfer in refinery and petrochemical processes. The exemplary embodiments disclosed herein are particularly advantageous when applied to a dehydrogenation process, where one of two or more bundles warms a tubeside reactor feed stream while cooling a shell side reactor effluent stream, with a second bundle further cooling the reactor effluent by a tubeside aqueous coolant stream. The following description will focus on propane dehydrogenation (“PDH”) processes as a non-limiting example.

[0003] A prior art PDH system 100 is illustrated in Figure 1. Propane dehydrogenation is an endothermic chemical reaction that converts propane to propylene and hydrogen. A propanecontaining feed gas stream 103 is warmed in a feed-effluent heat exchanger 102 to produce a first warmed propane stream 126. The first warmed propane stream 126 is then further warmed in a charge heater 104 using fuel or electricity to produce a second warmed propane stream 105. The second warmed propane stream 105 is then sent to a reactor 106 where propane is reacted to form an effluent stream 107 consisting essentially of propylene, hydrogen and unreacted propane. The effluent stream 107 is withdrawn from the reactor 106 and then cooled in a steam generating heat exchanger 108 against a first water cooling stream 109 to produce steam 119 and a first cooled effluent stream 132. The first cooled effluent stream 132 is then further cooled in the feedeffluent heat exchanger 102 against the feed gas 103 (as described above) to produce a second cooled effluent stream 136. The second cooled effluent stream 136 is then further cooled in a water cooler 110 against a second water cooling stream 128 to produce a third cooled effluent stream 134 and a warmed cooling water stream 130. The third cooled effluent stream 134 is compressed in a compressor 112 to produce a compressed effluent stream 117.

[0004] It should be noted that only one heat exchanger is shown for each of the steam generator 108, the feed-effluent heat exchanger 102 and the water cooler 110. In most commercial-scale plants using conventional shell and tube heat exchanger technology, multiple heat exchangers must be provided in parallel in order to meet the required heating and cooling duties. For example, in a typical PDH plant producing 100 tonne/hr of propylene, the water cooler 110 would require four shell and tube exchangers operating in parallel. An example of a shell and tube heat exchanger can be found in US10527357, entitled, “Feed Effluent Heat Exchanger”.

[0005] The compressed effluent stream 117 is sent to a separation section 114. The separation section 114 may comprise distillation columns, phase separators and/or adsorption-based separators to produce a fuel stream 116, a product hydrogen stream 118, a product propylene stream 120 and a recycle stream 122 containing unreacted propane. The recycle stream may be combined with a fresh feed stream 124 to form the feed stream 103.

[0006] Petrochemical and thermal cracking processes, including PDH processes generate better product yields when carried out at relatively low absolute pressures, typically in the range of 0.2 - 3.0 bara. PDH reactions are preferably carried out under vacuum, typically between 0.2 - 1.0 bara and preferably below 0.4 bara. Due to the low operating pressure, it is important to minimize pressure drop between the reactor 106 and the compressor 112. Lower pressure on the suction side of the compressor 112 results in a greater power requirement (and therefore, increased cost) to adequately compress the third cooled effluent stream 134. Keeping pressure drop within desirable limits is particularly difficult when the effluent stream 132 passes through multiple heat exchangers (both in parallel and series), other components, and connecting piping. As a result, flow is separated, distributed to each cooler, then recombined and collected before being redistributed to the next heat exchange section. Multiple changes in the direction of flow lead to a high pressure drop as well as potential for maldistribution between individual units or within individual flow channels or layers.

[0007] For large plants, in addition to the challenge of preventing pressure drop in the effluent circuit, there are other operational challenges, such as the need to balance effluent and water flow through multiple parallel coolers. Parallel cooling units also increase fabrication costs associated with fittings, structural supports, and piping.

[0008] Accordingly, there exists an unmet need for an improved method and system for effluent cooling that allows for reduced pressure drop, is cost effective, efficient, and is simple to design and operate.

SUMMARY

[0009] In the embodiments described herein, use of a uniquely-structured coil-wound heat exchanger as the feed-effluent heat exchanger provides reduced effluent stream pressure drop and simplifies the construction. In some embodiments, functionality of the steam generator 108 and water cooler 110 are also provided in additional bundles within a single coil-wound heat exchanger, which further reduces pressure drop. The reduction in pressure drop allows for a reduction in compressor power requirement and enables the reactor to operate at a lower pressure, thereby increasing the reactor yield. Additionally, the plant layout can be simplified, construction costs reduced, and plant operation is simplified by eliminating the need to balance effluent and water flow between multiple heat exchangers operating in parallel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a schematic drawing of a prior art PDH process.

[0011] Figure 2 is a schematic drawing of a PDH process according to a first exemplary embodiment. [0012] Figure 2A is longitudinal sectional view of a coil-wound heat exchanger according to a first exemplary embodiment.

[0013] Figure 2B is a sectional view taken along line 2B-2B of Figure 2A.

[0014] Figure 2C is a longitudinal sectional view of an alternative construction of the coil-wound heat exchanger of Figure 2A.

[0015] Figure 3 is a partial view of area 3-3 of Figure 2A, showing an alternate embodiment of the outlet end of the CWHE. In the interest of simplicity, the tubesheet and tubes shown within area 3-3 of Figure 2A are not shown in Figure 3.

[0016] Figure 4 is schematic sectional view showing a representative arrangement of tubes and a spacer within the coil-wound heat exchanger of Figures 2A & 2C.

[0017] Figure 5 is a schematic drawing of a PDH process according to a second exemplary embodiment.

[0018] Figure 6 is a schematic drawing of a PDH process according to a third exemplary embodiment.

[0019] Figure 6A is a longitudinal sectional view of a coil-wound heat exchanger according to the embodiment of Figure 6.

[0020] Figure 7 is a schematic drawing of a PDH process according to a fourth exemplary embodiment.

DETAILED DESCRIPTION

[0021] The ensuing detailed description provides exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the ensuing detailed description of the exemplary embodiments will provide those skilled in the art with an enabling description for implementing the exemplary embodiments of the invention. It is understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention.

[0022] In the claims, letters are used to identify claimed steps (e.g. (a), (b), and (c)). These letters are used to aid in referring to the method steps and are not intended to indicate the order in which claimed steps are performed, unless and only to the extent that such order is specifically recited in the claims.

[0023] In order to aid in describing the invention, directional terms may be used in the specification and claims to describe portions of the present invention (e.g., upper, lower, left, right, etc.). These directional terms are merely intended to assist in describing and claiming the invention and are not intended to limit the invention in any way. In addition, reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures without additional description in the specification in order to provide context for other features. [0024] Unless otherwise indicated, the articles “a” and “an” as used herein mean one or more when applied to any feature in embodiments of the present invention described in the specification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated. The article “the” preceding singular or plural nouns or noun phrases denotes a particular specified feature or particular specified features and may have a singular or plural connotation depending upon the context in which it is used.

[0025] Unless otherwise stated herein, introducing a stream at a location is intended to mean introducing substantially all of said stream at the location. All streams discussed in the specification and shown in the drawings (typically represented by a line with an arrow showing the overall direction of fluid flow during normal operation) should be understood to be contained within a corresponding conduit. Each conduit should be understood to have at least one inlet and at least one outlet. Further, each piece of equipment should be understood to have at least one inlet and at least one outlet.

[0026] The term “conduit,” as used in the specification and claims, refers to one or more structures through which fluids can be transported between two or more components of a system. For example, conduits can include pipes, ducts, passageways, and combinations thereof that transport liquids, vapors, and/or gases.

[0027] As used in the specification and claims, the term “flow communication” is intended to mean that two or more elements are connected (either directly or indirectly) in a manner that enables fluids to flow between the elements, including connections that may contain valves, gates, tees, or other devices that may selectively restrict, merge, or separate fluid flow.

[0028] The terms “hydrocarbon”, “hydrocarbon gas”, or “hydrocarbon fluid”, as used in the specification and claims, mean a gas/fluid comprising at least one hydrocarbon and for which hydrocarbons comprise at least 70% and, more preferably, at least 90%, of the overall composition of the gas/fluid.

[0029] As used in the specification and claims, the term “substantial change in the overall direction of flow” means that there are no elbows or substantial bends in the conduit that contains the fluid in question. For purposes of this definition, a bend of more than 10 degrees would be considered substantial. Structures located within the conduit, such as cleaning devices, nozzle protrusions, fluid distribution devices, tube bundles, spacers, or mandrels, may result in small, localized changes in flow direction but are not considered to substantially change the overall direction of flow, as this term is defined herein. Similarly, expansions or contractions of the conduit are not considered to substantially change the overall direction of flow, as this term is defined herein.

[0030] For purposes of this application, the term “in steady state operation within design operating conditions” means when a heat exchanger is in steady operation within the design parameters for that heat exchanger and does not include startup operation, shut down operation, or when a cleaning process is being performed (either online or offline). Exemplary descriptions of startup and shutdown operations can be found in the Standards of the Tubular Exchanger Manufacturers Association, Tenth Edition.

[0031] Unless otherwise stated herein, any and all percentages identified in the specification, drawings and claims should be understood to be on a mass percentage basis. Unless otherwise stated herein, any and all pressures identified in the specification, drawings and claims should be understood to mean absolute pressure.

[0032] As used in the specification and claims, the term “compressor” or “compression system” is defined as one or more compression stages. For example, a compression system may comprise multiple compression stages within a single compressor. In an alternative example, a compression system may comprise multiple compressors, which may be arranged in series and/or in parallel.

[0033] Figure 2 shows an exemplary embodiment of an improved PDH system 200. Elements of Figure 2 that are substantially identical to corresponding elements in Figure 1 are represented in Figure 2 with reference numerals having the same two last digits as reference numerals shown in Figure 1, but may not be separately discussed in the specification. For example, the reactor 106 of Figure 1 corresponds to reactor 206 of Figure 2.

[0034] In this embodiment, a single coil-wound heat exchanger (“CWHE”) 202 is used instead of the multiple feed-effluent heat exchangers 102 operating in parallel in Figure 1. The feed stream 203 is warmed in a feed bundle 211 of the CWHE 202 to produce the first warmed propane stream 226. As in the prior art, the first warmed propane stream 226 is then further warmed in the charge heater 204 to produce the second warmed propane stream 205. The second warmed propane stream 205 is then sent to the reactor 206. The effluent stream 207 is withdrawn from the reactor 206 and cooled in the steam generating heat exchanger 208, to produce steam 219 and the first cooled effluent stream 232. Optionally, the effluent stream 207 could bypass the steam generating heat exchanger 208 through a bypass 213.

[0035] The first cooled effluent stream 232 is further cooled in the shell side of the CWHE 202, warming the incoming feed stream 203 to produce a second cooled effluent stream 236. In this embodiment, second cooled effluent stream 236 is further cooled against the first coolant stream 228 flowing through a first coolant bundle 210 of the CWHE 202 (instead of in a separate cooler) to produce the third cooled effluent stream 234. The third cooled effluent stream 234 is compressed in a compressor 212 to produce a compressed effluent stream 217.

[0036] The compressed effluent stream 217 is sent to a separation section 214. The separation section 214 may comprise distillation columns, phase separators and/or adsorption-based separators to produce a fuel stream 216, a product hydrogen stream 218, a product propylene stream 220 and a recycle stream 222 containing unreacted propane which is combined with a fresh feed stream 224 to form the feed stream 203.

[0037] Figures 2A and 2B show additional details of an exemplary CWHE 202 used in the system 200 of Figure 2. The CWHE 202 comprises a shell 201 having an inlet end 254 and an outlet end 256, a body 258 that extends between the inlet end 254 and the outlet end 256, and a longitudinal axis L that extends parallel to the major dimension of the shell. In this embodiment, the body 258 is cylindrical in shape, meaning that it has a constant shell diameter Ds. In other embodiments, the body 258 may have sections having different diameters. For purposes of this application, the term shell diameter Ds means the maximum diameter of the body 258. The longitudinal axis, L, bisects the shell diameter Ds along its length.

[0038] The CWHE 202 also comprises a feed bundle 211 and first coolant bundle 210, each comprising one or more bundles of tubes wound in concentric layers around a central mandrel 264, 266 (also referred to herein as tube bundles). A shroud 275, 276 may surround each bundle, and each shroud 275, 276 may include an upper lip 277, 278 that extends outwardly to the shell 201, thereby reducing the ability of the first cooled effluent stream 232 to bypass the tube bundles in the space between the shell 201 and the tube bundles. In addition, each mandrel 264, 266 may optionally include a flow divertor 279, 280 that directs shell side flow around the mandrels 264, 266, thereby preventing the ability of the first cooled effluent stream 232 to bypass the tube bundles in the interior space of the mandrels 264, 266. In some embodiments, the flow divertors 279, 280 may be shaped to minimize flow resistance. In this embodiment, the flow divertors 279, 280 are both dome-shaped. Other shapes, such as conical shapes, could be used.

[0039] The inlet end 254 has an inlet port 260 having an inlet diameter Di and the outlet end 256 has an outlet port 262 having an outlet diameter Do. In some embodiments, the inlet diameter Di and the outlet diameter Do are both at least 0.35 times the shell diameter Ds. The inlet and outlet port diameters Di, Do are unusually large in this embodiment relative to the shell diameter Ds for the purpose of reducing pressure drop of the effluent stream 232 as it flows through the CWHE 202. In addition, both the inlet port 260 and the outlet port 262 may be axially aligned with the longitudinal axis L, meaning that the longitudinal axis L passes through a portion of each of the inlet port 260 and the outlet port 262. In some embodiments, both the inlet port 260 and the outlet port 262 are centered on the longitudinal axis L, meaning that the longitudinal axis L bisects the inlet diameter Di and the outlet diameter Do. In other embodiments, the inlet port 260 and the outlet port 262 are axially aligned with longitudinal axis L, but the longitudinal axis is offset from the midpoint of the inlet diameter Di or the outlet diameter Do by no more than 10% of the diameter, by no more than 15% of the diameter, or by no more than 25% of the diameter.

[0040] This arrangement enables the effluent stream 232 to flow through the CWHE 202 in a direction parallel to the longitudinal axis L and without a substantial change in the overall direction of flow of the effluent stream 232 through the body 258 of CWHE 202. In this case, the conduit for the effluent stream 232 is the shell space, which does not have any bends or elbows. As noted above, it should be understood that structures in the shell space (such as cleaning devices, nozzle protrusions, fluid distribution devices, tube bundles, spacers, and mandrels) may result in small localized diversions of flow around these structures, but do not change the overall direction of flow through the body 258 of CWHE 202. It should be understood that an expansion or contraction in the flow parallel to the longitudinal axis, L, such as at the inlet and outlet transitions 281 , 282, does not constitute a “substantial change in the overall direction of flow”.

[0041] An inlet transition 281 provides a gradual transition from the inlet port 260 to the body 258. Similarly, an outlet transition 282 provides a gradual transition from the body 258 to the outlet port 262. The inlet transition 281 gradually tapers outwardly from the inlet port 260 to the body 258.

The outlet transition 282 gradually tapers inwardly from the body 258 to the outlet port 262. In this embodiment, both the inlet transition 281 and outlet transition 282 are conical in shape. Other shapes that provide low fluid flow resistance could be provided at the inlet and outlet transitions 281, 282, such as a bell-mouth shape or other toriconical shapes. In certain embodiments of the invention, both the inlet and outlet transitions 281 , 282 have a transition ratio that is greater than 0.7, greater than 0.85, greater than 1.0, or greater than 1.33. The transition ratio is defined as the length of the transition divided by the difference between the shell diameter Ds and the diameter of the inlet with which is it associated (i.e., the inlet diameter Di for the inlet transition 281 and the outlet diameter Do for the outlet transition 282). It should also be understood that the surfaces that provide the desired low fluid flow resistance in the transitions could be part of the shell 201 or could be provided by separate structures internal to the shell 201.

[0042] At both ends of each bundle 211, 210 the tubes terminate at tubesheets, which transition to a conduit that passes through the shell 201. For example, tubes 272a and 272b on the inlet side of the feed bundle 211 terminate at a tubesheet 273, which transitions to the conduit 274, into which the feed stream 203 flows.

[0043] In this embodiment, all of the heating duty to the tube bundles of the CWHE 202 (the feed bundle 211 and the first coolant bundle 210) is provided by the effluent stream. In addition the inlet port 260 is the sole port through which the effluent stream (at that point, the first cooled effluent stream 232) flows into the shell space of the shell 201 when the CWHE 202 in operation at design operating conditions. Similarly, the outlet port 262 is the sole port through which the effluent stream (at that point, the third cooled effluent stream 234) flows out of the shell space when the CWHE 202 in steady state operation within design operating conditions.

[0044] Turning to Figure 2B and Figure 4, spacers 270a-d are provided to maintain spacing between layers of tubes 268a-d and to prevent shell side fluid flow from bypassing the tubes in the space occupied by the spacers 270a-d. Each of the tubes has an outside tube diameter d. Tubes of adjacent layers of tubes (e.g., 268a and 268b, or 268b and 268c) are spaced apart from one another by a first tube center-to-center distance Dt in a direction perpendicular to the longitudinal axis L of the shell 201 (called “radial pitch”) and by a second tube center-to-center distance Dp in a direction parallel to the longitudinal axis L (also referred to as “longitudinal pitch”). A dimensionless transverse tube spacing value is defined as the ratio of Dt/d and reflects the fraction of shell side cross-sectional area that is available for shell side flow. In this embodiment, the dimensionless transverse tube spacing is greater than 1.75, greater than 2.00, or greater than 2.25. To maintain a very low pressure drop while providing a large surface area per unit exchanger volume, the ratio of Dt to Dp is greater than 1.40, greater than 1.70, or greater than 2.00.

[0045] Because the PDH reaction occurs under vacuum conditions, it is important to minimize pressure drop in the effluent circuits. The work required by the effluent compressor 212 is inversely proportional to the absolute pressure of the third cooled effluent stream 234, so the effect of the pressure drop is magnified at low operating pressure. Using a first coolant bundle 210 in the coil-wound heat exchanger 202 as the effluent cooler substantially reduces the pressure drop of the interconnecting pipe and fittings between the feed-effluent exchanger 102 and the water cooler 110. The benefit of the lower effluent pressure drop for the current invention may also be a reduction in the reactor operating pressure assuming the same effluent compressor suction pressure. This reduction increases the reactor yield which in turn reduces the flow of the recycle stream 222.

[0046] The arrangement of the heat exchanger in Figure 2 has a number of advantages over the configurations of the prior art. In particular, the bundles 211, 210, can cool the reactor effluent along the flow path (in the direction indicated by the arrow showing the flow of the second cooled effluent stream 236) to a desirable temperature without separate downstream cooling units and without substantially changing the overall direction of flow of the effluent through the body 258 of CWHE 202 along longitudinal axis L. As a result, there is a significantly lower pressure drop in the reactor effluent through heat exchanger 202. In one or more embodiments, the pressure drop is no greater than 0.1 bara when operated at a shell side pressure that is less than ambient pressure. Stated another way, the pressure at the suction side of the compressor 212 is at least 65%, at least 70%, or at least 80% of the shell side pressure at the inlet port 260. The reduction in pressure drop minimizes compression costs and also improves reactor yields for chemical reaction selectivity that favors lower pressures. The pressure drop reduction is attributable, at least in part, to combining effluent cooling processes in the same shell 201 , which eliminates piping to pass cooled effluent from the heat exchanger 202 to effluent coolers and reduces the pressure drop associated with entrance loss, bends, and tees for distributing the flow.

[0047] Figure 3 shows an exemplary alternate configuration for the outlet end 456 of the shell 401. In this embodiment, the outlet port 462 is positioned in a side wall of the shell 401 instead of being axially aligned with the inlet port (not shown in this figure) and the longitudinal axis L. The outlet end 456 includes a closed lower end 488. The outlet port 462 may have the same diameter Do as the outlet port 262 of Figure 2A. In order to reduce pressure drop, the outlet port 462 may include a bell-mouth shaped transition 486 located within the shell 401.

[0048] For some applications, it may be desirable to form the shell in a manner that enables it to be manufactured and/or transported to the plant site in multiple pieces. Figure 2C shows an exemplary alternate configuration for the CWHE 302. In this example, the shell 301 is comprised of an upper portion 394 and a lower portion 395 that are joined at a neck 396. The feed bundle 311 is contained within the upper portion 394 and the first coolant bundle 310 is contained within the lower portion 395. This configuration enables the upper portion 394 and a lower portion 395 assembled and transported to the plant site, then joined at the neck 396 (using any suitable means, such as bolts and/or welds) to form the fully assembled shell 301. In other examples, the neck portion could be omitted and/or the upper and lower portions could be different diameters. For purposes of this application, multiple shell portions (such as the upper portion 395 and lower portion 394) that are, prior to being put into service, joined together to define a single, continuous shell-space are considered to be a “single shell”.

[0049] Elements of Figure 2C that are substantially identical to corresponding elements in Figure 2A are represented in Figure 2C with reference numerals having the same two last digits as reference numerals shown in Figure 2A, but may not be separately discussed in the specification. For example, the inlet end 254 of Figure 2A corresponds to the inlet end 354 of Figure 2C.

[0050] Figure 5 shows a second exemplary PDH system 500, in which the steam generating heat exchanger 108, 208 of Figures 1 and 2 is replaced by a second coolant bundle 508 that is part of the CWHE 502. In other words, the second coolant bundle 508 (which generates steam), the feed bundle 511 , and the water cooler 510 are all contained within the same shell 501. This exemplary embodiment further reduces effluent pressure drop between the reactor 506 and the compressor 512.

[0051] Elements of Figure 5 that are substantially identical to corresponding elements in Figures 1 and/or 2 are represented in Figure 5 with reference numerals having the same two last digits as reference numerals shown in Figure 1 and/or 2, but may not be separately discussed in the specification. For example, the reactor 106 of Figure 1 corresponds to a reactor 506 of Figure 5. [0052] Figure 6 is a schematic diagram of another exemplary embodiment of a PDH system 600. The system 600 is similar to the system 500 of Figure 5, in that it comprises an integrated CWHE 602 where the second coolant bundle 608 (acting as a steam generator), CWHE 602, and a first coolant bundle 610 are contained within the same shell 601. Unlike the embodiment of Figure 5, the bypass 615 for the reactor effluent 607 is fully contained within the CWHE 602 with a bypass valve 650. [0053] Any of the bundles in the CWHE 602 could be split into two or more bundles. For example, as shown in Figure 6, the feed bundle could be split into a second portion 611b that is in downstream fluid flow communication with a first portion 611a and joined by a conduit 695. Providing separate bundle portions 611a, 611b allows first portion 611a to be formed from a different material than the second portion 611b. For purposes of this application a "different material” includes, but is not limited to, different grades or compositions of the same alloy (such as 304H stainless steel and 304L stainless steel). For example, in one embodiment the first portion 611a could be made of 304H stainless steel and the second portion 611b could be made of 304L stainless steel. In another embodiment, the first portion 611a could be made of stainless steel and the second portion 611b could be made of carbon steel or a nickel-chromium alloy (such as Inconel®). This allows each portion 611a, 611b to be better matched to the tube side and/or shell side temperature, as well as other desired thermodynamic characteristics, in portions of the bundle. Optionally, the portion of the shell that overlaps each portion of the bundle could be changed to better match the thermodynamic characteristics of that portion. Similarly, separate bundles could be formed from different materials (i.e. , the feed bundle 611 could be formed from different material than the first coolant bundle 610).

[0054] Other elements of Figure 6 that are substantially identical to corresponding elements in Figures 1 and/or 2 are represented in Figure 6 with reference numerals having the same two last digits as reference numerals shown in Figure 1 and/or 2, but may not be separately discussed in the specification. For example, the reactor 106 of Figure 1 corresponds to a reactor 606 of Figure 6.

[0055] Figure 6A shows additional detail of the bypass 615. Reactor effluent 607 enters the shell side of the CWHE 602. A bypass valve 650 is located at a lower end of the mandrel 672 around which the second coolant bundle 608 is wound. When the bypass valve 650 is opened, shell side flow in the region of the second coolant bundle 608 is split into two paths. One flow path 605 flows through the second coolant bundle 608 and the other path, the bypass 615, flows through the mandrel 672, thereby bypassing the second coolant bundle 608. Flow split between path 605 and the bypass 615 is controlled by the bypass valve 650. At the bottom of the second coolant bundle 608, the two paths 605, 615 rejoin into a combined path 632, which then flows over the feed bundle 611. The other two mandrels 682, 692 are capped, which prevents shell side flow from passing through these mandrels 682, 692.

[0056] Figure 7 is a schematic diagram of another exemplary embodiment of a PDH system 700. Other elements of Figure 7 that are substantially identical to corresponding elements in Figures 1 and/or 2 are represented in Figure 7 with reference numerals having the same two last digits as reference numerals shown in Figure 1 and/or 2, but may not be separately discussed in the specification. For example, the reactor 106 of Figure 1 corresponds to a reactor 706 of Figure 7. [0057] The system 700 is very similar to the system 200 of Figure 2, with the exception of the elimination of the steam generating heat exchanger 208 and the provision of a charge heater 704 having substantially reduced emissions (such as electrical) when compared to a fired charger heater 204. In this embodiment, the effluent stream 707 from the reactor 706 is fed directly to the feed bundle 702. Because the effluent stream 707 is not cooled prior to entering the feed bundle 702, the temperature of the effluent stream 707 is higher as it enters the feed bundle 702. As a result, the feed stream 703 (against which the effluent stream 707 is cooled in the feed bundle 702) will be heated to a higher temperature when it exits the feed bundle 702 as the first warmed propane stream 726. The higher temperature of the first warmed propane stream 726 reduces the required heating duty of the charge heater 704 and enables use of a reduced emissions heater instead of a fired heater. This enables the system 700 to operate with lower emissions of carbon- containing gas.

[0058] EXAMPLE

[0059] In this example, a system similar to that shown in Figure 2 was modeled, in which a single cooling water bundle (the first coolant bundle 210) replaced four parallel shell and tube exchangers. Pressure at the first cooled effluent stream 232 (measured at the CWHE 202 inlet) was 0.39 bara and pressure at the third cooled effluent stream 234 (measured on the suction side of the compressor 212) was 0.33 bara representing a pressure drop of 0.06 bara. This means that the 85% of the effluent pressure at the inlet is maintained at the outlet.

[0060] The present invention is not to be limited in scope by the specific aspects or embodiments disclosed in the examples, which are intended as illustrations of a few aspects of the invention and any embodiments that are functionally equivalent are intended to be within the scope of this invention. Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art and are intended to fall within the scope of the appended claims.