US3529030A - Vaporizing and superheating the liquid feed in an isomerization process - Google Patents

Description

EATING THE LIQUID FEE IN AN ISOMERIZATION PROCESS 2 Sheets-Sheet 2 Filed April 5, 1967 OOOF on m 02 oN 9 i 0W u r M m 4 7 46 w fi a o 3 o w 5:; o 2:63 zomm uoma x E \m 6.2m

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I4 l/IIAU; ATTORNES United States US. Cl. 260-668 Claims ABSTRACT OF THE DISCLOSURE A process for completely vaporizing and superheating hydrocarbons without going through the dry point in a heater, which comprises passing liquid hydrocarbons through a first heater wherein sufiicient heat is transferred to the liquid hydrocarbons so that, after the hydrocarbons are next passed through a presure-drop valve, the liquid completely vaporizes to a vapor; then passing the vapor through a second heater to obtain a superheated vapor. Pressure drop across the pressure-drop valve may be automatically controlled by a pressure recorder controller. The pressure recorder controller is used in conjunction with a temperature difierential recorder and a presure diiferential recorder to avoid danger of passing through the dry point in either heater.

BACKGROUND Field of invention This invention relates to the heating and vaporization of hydrocarbons. More particularly, it pertains to a proc ess or method whereby a stream may be heated and completely vaporized without going through the dry point in a heater.

Definitions In the description herein, the following terms are used with the indicated meanings.

(1) Heater: Heating means, such as a tubular heat exchanger or a furnace.

(2) Dry point: Point at which a vapor-liquid mixture passes to 100 percent vapor.

(3) Coke: Any carbonaceous deposit, generally pyrolyzed decomposition products of hydrocarbons.

(4) Hydrocarbon: A compound consisting of carbon and hydrogen.

(5) Superheating: Heating a substance or mixture after it has been completely vaporized, i.e. adding further heat after passing through the dry point.

(6) PdR: Pressure differential sensing and recording instrument.

(7) TdR: Temperature differential sensing and recording instrument.

(8) TdRC: Temperature differential sensing, recording and controlling instrument.

(9) TR: Temperature sensing and recording instrument.

(l0) PRC: Pressure sensing and controlling instrument.

(11) Valve: Means for regulating flow capacity.

(12) Enthalpy: Heat content in British thermal units per pound of substance.

Description of the prior art In many different types or processes, such as isomerization, catalytic cracking, thermal cracking and pyrolysis, a hydrocarbon oil vapon'zable by heating is passed through heater tubes in which the oil becomes heated and finally vaporized before leaving the tubes. As the feed atent ice becomes heated to progressively higher temperatures, higher boiling hydrocarbons vaporize until a point is reached at which all of the feed has vaporized and no liquid remains. At or near this point of final vaporization or dry point, coking ordinarly occurs so that after some time such a heater must be taken out of service for tube cleaning. Further, some coking occurs even prior to the dry point, the general tendency being, however, the closer to the point of complete vaporization, the greater is the rate of coke deposition. The generally small cross-sectional area of heater tubes typically results in higher vapor velocity in the heater tubes. Iiquid entrained in this high-velocity vapor stream gives a scrubbing action on the tube walls. This action continues only until a point of complete vaporization is reached, at which point there is no liquid to assist in keeping the Walls washed free of carbon or coke.

The coke deposits on the heater tubes tend to materially decrease the heat transfer across the tube walls as well as restrict hydrocarbon flow, and hence increase operating expenses. These deposits also require eiTort and time to remove and to restore the equipment to its original operating efliciency.

There have been a number of attempts to solve the problem of coke deposits. However, the methods thus far tried have met with only limited success.

Various recycle schemes have been suggested in the prior art for avoiding the dry point in a heater. Basically, the recycle scheme involves passing a liquid through a heater, vaporizing the liquid, passing the vapor on to further heating (superheating) or to processing (such as isomerization or catalytic cracking), and recycling the unvaporized portion of the feed back to the heater. With the recycle there is, of course, more volume of fluid being sent through the equipment. Also, additional equipment is required, specifically, at least a pump and recycle piping to recycle the recycle liquid.

In one example of a recycle method to obtain 100 percent vaporized feed, partially vaporized materials from a heater are sent to a separation drum where equilibrium is obtained between vapor and liquid phases, with all of the liquid phase being recycled back to the feed. The vapor stream is withdrawn from the top of the vaporliquid separation drum. Thus the vapor stream is in equilibrium with the liquid in the vapor-liquid separation drum or, in other words, at its dew point. From 10 to about percent of the feed is vaporized within the heater, depending upon the temperature at which the heater is maintained. The remaining liquid is recycled from the vapor-liquid separation drum via the recycle pump to the furnace inlet. While the system is being brought to equilibrium conditions, the vapor composition from the vapor-liquid separation drum is different from the feed composition. Also, in attempting to bring the system to equilibrium, it may be necessary to withdraw surplus liquid from the bottom of the vapor-liquid separation drum. Further, since there is a continually recycling recycle stream in which heavy ends will tend to build up, it may be necessary to periodically withdraw a liquid bleed stream from the vapor-liquid separation drum.

A somewhat similar type of recycle system of the prior art consists of two heaters in series with a vapor liquid separation drum in between them. Liquid is partially vaporized in the first heater and then passed to the vaporliquid separation drum. Liquid from the vapor-liquid separation drum is recycled to the first heater. Vapor at its dew point from the vapor-liquid separation drum is passed to the second heater to be superheated. Since the vapor from the vapor-liquid separation drum is at its dew point, there is a high likelihood of coking in the second heater due to slight pipeline cooling of the vapor from the vapor-liquid separation drum with resulting slight liquid formation. To avoid liquid formation at this point, in accordance with some current processes, an inert gas is injected to lower the dew point. The disadvantage of such a remedy is that the foreign stream (the inert gas) must eventually be withdrawn and additional piping and controls are required.

Another attempt to solve the problems caused by passing through the dry point involved the partial vaporizing of the feed-stream along with the adding of an extraneous gas stream to promote vaporization. Any limited success achieved by this process was counteracted by difficulty in controlling the process. Furthermore, the additional heat required along with fractionation, which was needed to separate the vapor stream, entailed a great deal of expense.

An extraneous high-boiling liquid may also be added to the feed; this was tried several times.

One of these attempts is worthy of some note. In U.S. Pat. No. 2,472,669 to Mathy, a method for preventing or minimizing coke formation in preheater tubes is described. This patent pertains to a process wherein complete vapori- Zation of a charge oil is desired while coking is to be minimized. This is accomplished by adding,'along with the feedstock to be treated, I to percent by volume of a substantially noncoking, high-boiling oil. The oil is of such boiling range that it remains in the liquid state during passage through the tube heater. Crude oil fractions utilized would have an average boiling point about 1050 F. This invention may be somewhat successful in that the presence of this heavier liquid will serve to wash away coke deposits which Would normally form on the tubes within the furnace. However, this process is not without serious drawbacks. There is some question as to the added liquids own coking tendencies. When utilized in an isomerization or a thermal or catalytic cracking process, the liquid in the heater efliuent would have to be discarded; it could not, for example, be recycled to the heater after each pass. This is because it would contain carbon residue from the added heavy oil as well as the portions of the original charge oil which have greatest coke-forming tendencies.

Furthermore, some of this added heavier liquid will be present in the removed vapor and this will cause problems of product purity as well as possible catalyst poisoning or possible increased corrosion. The product purity of the vaporized portion will naturally be lowered by the presence of this extraneous material. This material could also introduce traces of elements or compounds not found in significant amounts in the charge stock, which would lead to poisoning of catalyst or excessive corrosion of the materials of construction. Expensive separation techniques would be needed to rid the vapor stream completely of the vaporized heavy liquid or to rid the heavy liquid of poisons or corrosive materials before its addition. Also, some of the higher boiling constituents of the charge stock would be found in the separated liquid and thus be lost from the charge stock.

SUMMARY OF THE INVENTION In accordance with the present invention, a hydrocarbon stream is heated in a first heater and then flashed to 100 percent vaporization and the vapor superheated an amount suflicient to insure that no liquid will condense in transit to the next heater by lowering the pressure. The slightly superheated vapor stream thus obtained is further superheated in a second heater. A superheated vapor stream is thus obtained without passing through the dry point in either heater. Preferred instrumentation for effecting the vaporization and superheating of the hydrocarbon stream, including an essentially completely automatic control system for avoiding danger of going through the dry point in either the first or second heater, is shown in FIG. 1 and described herein under Detailed description of the drawings.

There are many advantages obtained from vaporizing the feed in this manner. Of most importance is the reclustion in the coke formation. As is mentioned under Description of the prior art coke is particularly likely to form while heating a hydrocarbon when passing through the dry point; but, under the practice of the invented process, the dry point is not passed through in either of the heaters. Instead, the dry point is passed through when the partially vaporized feed from the first heater is flashed to percent vapor before being fed to the second heater.

Because the coke deposits are substantially reduced, the heat transfer coefficient-that is, the ease with which heat is transferredis substantially higher than it would be with the coke deposits present. This results in saving on operating expenses, such as fuel requirements, if the heater is a fuel-fired furnace; steam requirements, if the heater is an exchanger using steam as the heating fluid; pumping costs that would be higher if the flow were constricted by coke deposits causing increased pressure drop; and maintenance expenses on the heater for cleaning.

In many processes, it is very important to have a uniform vapor feed; and, in almost all vapor feed processes, it is at least helpful to have a uniform vapor feed. For example, in a vapor phase isomerization process or in a vapor phase, catalytic cracking process, variations in the vapor feed will result in the variations in the product which are undesirable.

The invented method of vaporization without passing through the dry point in a heater results in a uniform vapor feed, whereas this is diflicult to attain with a recycle method. In the invented method, the entire feed is passed straight through the heaters and is entirely vaporized. In a typical recycle method, the liquid remaining after passing through the first heater is recycled back to the heater. This frequently results in a nonuniform vapo feed as it is well known to those skilled in the art that the amount of heat transferred by a heat exchanger or even a furnace is subject to variation depending on the heating medium or heating fuel composition, the condition of the tubes in the heat exchanger or furnace, the temperature of the feed to the heater and the ambient atmosphere temperature.

During start-up of a recycle system, there is very decidedly a nonuniform vapor feed, as it takes a certain length of time before the recycle stream will level out to equilibrium. Also, a surplus stream may be required to be withdrawn during the start-up period when the recycle stream is being brought to equilibrium. It may also be necessary to withdraw a small amount of liquid via this surplus stream to remove heavy ends or impurities that tend to build up in the continually recycling recycle stream.

In the present invention not only is a uniform vapor feed obtained, but also uniform temperatures are more easily achieved with the present method of vaporization. This results in a smoother operation that is more easily controlled. This is particularly important, for example, in the reactor effluent heat recovery scheme described hereinafter in the description of FIG. 1.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of a preferred embodiment of the invented vaporization process.

FIG. 2 is a temperature vs. enthalpy vaporization curve for a narrow boiling range hydrocarbon.

DETAILED DESCRIPTION OF THE DRAWINGS will condense in transit to E2. The superheated stream is superheated further in heat exchanger E2. The superheated vapor from E2 passes in line 4 to a furnace 17 for still further superheating if necessary. The effluent from the furnace is passed to the reactor 18 shown in FIG. 1. This reactor may be, for example, a xylene isomerization reactor. Hot efiluent from the reactor is passed in line 5 to the heat exchanger E2. After passing through heat exchanger E2, it is then passed through heat exchanger E-l. Thus heat is recovered from the reactor eflluent by countercurrent heat exchange, and the liquid hydrocarbon feed to the process is heated.

Since the reactor efliuent is used at least in part to heat the feed in this embodiment of the invention it can be appreciated that changes in the reactor effluent temperature will change the amount of vaporization occurring in E1. This is because with a different reactor effluent temperature the amount of heat available to be transferred from the reactor effiuent to the feed passing through E1 will be different. In the present invention this is compensated for by taking a larger or smaller pressure drop in the AP valve shown (or by partially bypassing E2 as described in Example 5) so that all the liquid in the effluent from E1 will be vaporized before passing to E2. The amount of pressure drop taken is thus increased or decreased depending on whether there is a greater or lesser amount of liquid in the effiuent from E1. Similarly, compensation may be made for changes in the feed (line 1) temperature.

But, if a recycle system was employed with a vaporliquid separation drum between the heaters, for example, E1 and E2, it is difficult to correct for changes in the reactor temperature or feed temperature. This is because all the feed stream liquid, i.e. the unvaporized liquid, from E1 would be recycle liquid unless the vaporliquid separation drum were made so large that more or less than the entire liquid stream from E1 could be recycled for sustained periods. With all of the liquid from E1 being recycled, if the reactor efiluent temperature decreases, for example due to a change in the reaction rate or the feed composition, then not only is there less heat available for transfer in E2 but there also is less available for transfer in E1. This results in more liquid in the feed stream efiluent from E1, which in turn means there is more recycle liquid. Thus E1 would need more heat for the increased recycle but instead would rave less. Thus the liquid from E1 will increase still further and, most importantly, the vapor from E1 will decrease meaning less feed will be sent to the reactor and the feed will be of a different composition than it was before the change in reactor effluent temperature. Thus it can be seen that the present invention will have an advantage in easier control and smoother operation.

It can be seen in this processing scheme of FIG. 1 that, since all of the liquid feed is vaporized, the composition of the vapor feed to the reactor will always be exactly the same as the composition of the liquid feed to the first exchanger E1. If one of the recycle systems described above were employed to avoid passing through the dry point in one of the heaters, then the composition of the vapor feed to the reactor would vary by a large amount during start-up and also would vary during post-start-up operations. The variation during post-start-up operations would result from the factors previously mentioned-that is, variations in the heating medium, or heating fuel compositions in the case of furnaces, the condition of the tubes in the heat exchangers or furnaces, the temperature of the feed to the first heat exchanger and the ambient atmosphere temperature.

The bypass around E2, shown as line 12, valve 13, and line 14 provides an additional path for vapor from the pressure-drop valve so that the amount of hydrocarbon passed through E2 may be varied. By passing vapor through the bypass line less heat is removed from the hotter stream (the reactor effluent) passing through E2. Therefore, the temperature of the hotter stream leaving E2 will be higher as the amount of vapor bypass through line 12 is increased. The higher temperature thus obtained in the hot stream to E1 will result in more heat being transferred from the hot stream to the hydrocarbons through E1 countercurrent to the hot stream. Thus the effect of the bypass line around E2 is to allow a shift of part of the heat transferred in E2 to E1, or, of course, vice versa.

Instrumentation shown in FIG. 2 includes a TdR, PdR, PRC and TR. TaR, item 9, senses and records the difference in temperature between the hydrocarbons in line 2 and line 3. PdR, item 10, senses and records the difference in pressure between the hydrocarbons in line 2 and line 3. PRC, item 11, senses and records the pressure of the hydrocarbons in line 2 and controls the opening and closing of the pressure-drop valve depending on the pressure PRC senses in line 2. TR, item 15, senses and records the reactor effiuent temperature as it leaves E2.

The examples, particularly Example 2, should be referred to at this point for visualization of the use and function of the instruments. As the fed hydrocarbons are passed through the first heater or heat exchanger (FIG. 1, E1) it is important that suflicient hea-t be transferred to the hydrocarbons so that they will at least completely vaporize upon assing through pressure-drop valve 16. But it is also important that not so much heat is transferred to the hydrocarbons in E1 that they completely vaporize and thus pass through the dry point in E1.

As explained above, the bypass line around E2 enables shifting between E1 and E2 of part of the heat transferred by either of the heat exchangers E1 and E2. Thus the bypass is used to provide for sufiicient heat transfer in E1 to the hydrocarbons passing through E1 countercurent to the reactor efiluent so that the hydrocarbons will at least completely vaporize (and preferably superheat a small amount) after passing through pressure-drop valve 16. To determine proper functioning of the system, TdR, PdR and the pressure reading of PRC are used. Thus, using similar numbers as in Example 3 and the same hydrocarbon stream as in FIG. 2, if the PRC is controlling to a pressure of 60 p.s.i.g. and the pdR reads 30 p.s.i. with the TdR reading 65 F., no superheat is being obtained by the flash across the pressuredrop valve 16. This is based on the fact that the temperature difference reading of 65 F. it can be seen from FIG. 2 that the hydrocarbons are not being superheated. (65 F. is the temperature difference between the approximately parallel 60 p.s.i.g. vapor-liquid line and the 30 p.s.i.g. vapor-liquid line in FIG. 2. If superheat were obtained, the temperature difference would be less than 65 F. In moving from 60 p.s.i.g. to 30 p.s.i.g. in an adiabatic flash, i.e., constant enthalpy, the difference in temperature is less than 65 P. if the saturated vapor line is intersected above the point where the 30 p.s.i.g. line intersects the saturated vapor line. The amount of superheat is the difference between the temperature where the saturated vapor line is intersected in reducing the pressure and the 30 p.s.i.g. (final pressure) saturated vapor temperature.) To remedy this condition of incomplete vaporization and/or no superheat, more heat is needed to be transfered to the hydrocarbons as they pass through E1. Therefore the bypass around E2 will be increased. If after increasing the bypass to a new higher amount the TdR reading drops to 50 F., with the pressure reading and the PdR reading remaining constant, this means 15 F. superheat is being obtained in the flash across the pressure-drop valve. Thus by observing the TdR, PdR and PRC the invented rocess may be operated to avoid danger of passing through the dry point in E2.

Fluctuations in the pressure in the system may be automatically compensated for by the PRC. If the pressure goes down, then the pressure-drop valve is moved toward a closed position by the PRC thus increasing pressure at the outlet of E1. Conversely, as the pressure increases the PRC causes the pressure-drop valve to move in an open direction so as to lower the pressure at the outlet of E-l. The pressure-drop valve thus operates to keep the pressure at the desired pressure. The process is operated to protect E1 as well as E2 by use of the PRC to avoid low pressure at the outlet of E-1 which would cause too much vaporization in E--1. The TdR, because it senses and records the temperature drop across the pressure drop valve, is also used to avoid danger of passing through the dry point in E-1, as the temperature drop across the pressure-drop valve is indicative of the amount of vaporization across the pressure drop valve. It is important to avoid coming too close to the dry point in E-l, as due to uneven heating some of the tubes in the heater may become dry even though the overall hydrocarbon mass has not been heated past the dry point.

To elaborate, if the TdR showed only a 5 F. drop at an E1 outlet pressure of 60 p.s.i.g. and pressure drop of 30 p.s.i. this would indicate that the hydrocarbon stream (using the same hydrocarbon stream as in FIG. 2) out E-1 was 96% vapor or dangerously near the dry point. This can be seen by referring to FIG. 2. Starting at 60 p.s.i.g., if the TdR shows only a 5 F. drop for a pressure drop of 30 p.s.i., the enthalpy of the hydrocarbon stream is 300 B.t.u.s per pound and the temperature is 415 F. as this is the only point on the 60 p.s.i.g. line of FIG. 2 that would give a 5 F. temperature drop when the pressure is adiabatically lowered 30 p.s.i. At an enthalpy of 300 B.t.u.s per pound, a temperature of 415 F., and pressure of 60 p.s.i.g., the hydrocarbon stream is 96% vapor as it is 96% of the distance along the 60 p.s.i.g. line from the saturated liquid line to the saturated vapor line shown in FIG. 2. In general, temperature differences as low as 5 F. across the pressure-drop valve would indicate too much heat is being added to the hydrocarbon stream as it passes through E1 and that therefore the bypass should be decreased to shift more heat to =E-2. In a situation where the PRC is controlling to 60 p.s.i.g., shifting some of the heat transferred in E-l to E2 can be visualized on FIG. 2 by dropping down along the 60 p.s.i.g. line. The temperature recorder on the outlet of E-2 serves as a guide in shifting heat from E-Z to E-1 or vice versa. This TR is used in conjunction with the TdR. The TR is particularly useful as a guide at ranges on FIG. 2 vaporization curve where the temperature difference across the pressure-drop valve is approximately constant. The temperature difierences are approximately constant in moving from a given pressure line to another given pressure line if both vapor and liquid are presen in the hydrocarbon mixture at each pressure.

FIG. 2 vaporization curve was obtained by calculating the enthalpy (heat content in B.t.u./lb.) of a mixture of hydrocarbons and a small amount of dissolved water at different pressures and temperatures. The lower left hand line labeled saturated liquid shows how the enthalpy in creases with increasing temperature. As the temperature is increased along the saturated liquid line a point is reached where, at a given pressure, vaporization beings to occur. The lower the pressure, the lower is the temperature at which vaporization begins.

Several pressures are shown as parameters in FIG. 2. The lower end of a given pressure parameter of FIG. 2 touches the saturated liquid line and represents the bubble point or point where vaporization is incipient at the given pressure. The upper end of a given pressure parameter touches the saturated vapor line and represents the dew point or point where the vaporization is just completed. It can be seen from FIG. 2 that more heat (enthalpy) is required to move upward along a given pressure line from the bubble point to the dew point. Although the temperature increase in moving from the bubble point to the dew point is only slight in FIG. 2, this temperature increase would be more if the hydrocarbon mixture had a wider boiling range. (Since the invention depends in part on a given amount of liquid hydrocarbon being present at a given temperature and pressure, the only restriction on the hydrocarbon feed stream to the process is that it have a fairly uniform vaporization curve from about to vaporization.)

The saturated vapor line connecting the upper ends of the pressure lines shows how the enthalpy of the vapor increases with increasing temperature. At a given pressure the vapor is at its saturation or dew point when it is at the temperature that meets the upper end of the given pressure line. At higher temperatures at this given pressure the vapor is superheated by an amount equal to the dilference between the vapor temperature and the saturation temperature for the vapor.

EXAMPLES Example 1 As the partly vaporized feed flows from E-1 through the superheat valve, the liquid portion flashes. The valve pressure drop is designed to be suflicient to allow all the unvaporized liquid from E1 to flash and to produce vapor with 10 F. of superheat in line 3. The change in state of the feed as it passes through the superheat valve may best be visualized by plotting a constant enthalpy path (see the vaporization curve, FIG. 2) from E1 outlet pressure and percent vapor to line 3 pressure and temperature. This plot shows how to interpret the superheat valve temperature drop. As the AT increases and reaches the maximum for a particular AP, liquid is not completely flashed across the superheat valve so that undesired liquid would exist in the line to E-2. As

the AT approaches zero, the degree of vaporization at the outlet of E1 approaches 100%, the dry point.

Example 2 Refer to FIG. 1 for flow sequence. A crude xylene liquid feed, at 100 F. as indicated on FIG. 2 by letter A, is heated at 60 p.s.i.g. in E-l along the path A B C. At point B the feed has an enthalpy of B.t.u./lb., temperature of 415 F. and is just starting to vaporize. At point C the feed has an enthalpy of 237 B.t.u./lb., temperature of 415 F. and is about 50% vapor. Next the feed is passed through the pressure reducing valve so that the pressure is reduced from 60 p.s.i.g. to 30 p.s.i.g. along the path C D. At point D the feed has an enthalpy of 237 B.t.u./lb., temperature of 350 F. and is about 73% vapor. Thus the feed to 11-2 would have 27% liquid. Therefore it would pass through the dry point in E-2, since a superheated vapor is desired in the eflluent from E-2. The invention solves this problem as follows:

Referring again to FIG. 1 for the flow sequence, the same crude xylene liquid feed is heated in E-l along the path A B E. At point E the feed has an enthalpy of 280 B.t.u./lb., temperature of 415 F. and is about 82% vapor. Next the feed is passed through the super heating valve so that the pressure is reduced from 60 p.s.i.g. to 30 p.s.i.g. along the path E F. At point F the feed has an enthalpy of 280 B.t.u./lb., temperature of 365 F. and is 100% vapor with 15 F. of superheat .(saturated vapor temperature at 30 p.s.i.g. is 350 F.). It should be noted that in moving from E to F the enthalpy remains constant as the pressure reduction is done adiabatically; i.e. without heat loss. Because the pressure reduction is adiabatic, the temperature does not continue to decrease once the saturated vapor line is intersected, as this would require that the enthalpy decrease.

Example 3 The controls shown in FIG. 1 are provided to maintain a constant degree of vaporization (-90%) at the inlet to the superheat valve, a desired. condition for proper functioning of that valve.

The example will illustrate how the feed instrumentation is used, excluding, for the time being, the automation instrumentation shown by dotted lines in FIG. 1. Assume that the plant has been operating smoothly and that the operator is ordered to decrease feed rate. First he reduces the hydrocarbon feed rate by lowering the set point on a flow controller used to control the How rate of the hydrocarbon feed. Automatically the PRC would also reduce the superheat valve opening enough to maintain the pressure in line 2. If the superheat valve pressure drop increases significantly, this would imply a corresponding increase in superheat and the operator could improve heat recovery by decreasing the bypass rate around E-2 using the recorded temperature on the reactor efiluent to E-1 as a guide.

Example 4 Refer to FIG. 1. The invented method of vaporization in this embodiment provides a constant flow of feed to the furnace and reactor and downstream heat exchange. The value of this constant feed can be illustrated as follows: If the feed system were one in which the liquid was not completely vaporized, the unvaporized feed would have to be withdrawn from the system to avoid the dry point in E-2. Thus, the vapor rate at E-Z and downstream would be variable depending on changes in the system, which in turn would change the amount of vaporization and therefore the amount of vapor. In turn, the variable vapor rate itself would make temperature control of the feed heater difficult upset the temperature and conversion in the reactor, and cause variations in heat transfer in both E2 and E-l. The variations in E-l would, in turn, disturb the degree of vaporization of the feed thus tending to perpetuate the nonuniform flow rate. If the rate to the reactor is variable, the degree of conversion would vary and cause nonuniformity of the reactor products.

Example 5 Refer to FIG. 1. The following illustrates the usefulness of item 9, temperature differential recorder; item 10, pressure differential recorder; and item 11, pressure controller. We assume that the pressure in line 2 is 60 p.s.i.g. and in line 3, 30 p.s.i.g. With this differential pressure of 30 p.s.i. if the temperature difference is 65 degrees, it is maximum. This would indicate insufficient vaporization at the outlet of E-1 and that the flashing across the superheat valve had not produced superheat. On the other hand, if the temperature differential were significantly less than 65 degrees, say 50 degrees, it would indicate the stream was superheated in line 3. In this case, the superheat would be degrees. Correction for insuflicient vaporization or for too much vaporization in E-l, as detected by observing the TdR and the PdR, are made by adjusting the bypass amount around E2 or by resetting the pressure to which the PRC controls.

The value of the PRC in maintaining pressure at point 2 is to stabilize the degree of vaporizatiton at the outlet of E4. This controller also serves to maximize the pressure drop across the superheat valve and, in so doing, maximizes the potential for superheat, thus operating to insure sufiicient heat transfer in E-1 so that flashing across the superheat valve will produce superheat and thereby avoid danger of going through the dry point in E-2. If the superheat valve were not operated to maintain the pressure in line 2, a drop in flow rate would reduce the pressure in line 2. This would tend to increase the degree of vapor in line 2 and increase the danger of going through the dry point in E-1.

AUTOMATION The bypass valve 13 may be automatically controlled by TdRC, item 9a in FIG. 1. As described above, the bypass around E-2 is used to shift heat from E-1 to E-2 or vice versa. The temperature difference across pressure drop valve 16 is indicative of the state of the hydrocarbon stream in both lines 2 and 3 (inlet and outlet of pressure drop valve 16). At a PRC controlled pressure of 60 p.s.i.g. and a PdR pressure differential of 30 p.s.i., a de sired temperature differential across the pressure drop valve 16 for a hydrocarbon feed, such as the mixture of FIG. 2., is F. Higher temperature differences, such as 65 F., would indicate the hydrocarbon was not completely vaporizing across the pressure drop valve or that the hydrocarbon was just barely being flashed to 100% vapor (and therefore might drop out some liquid due to pipeline cooling in transit to E-2). Lower temperature differences, such as 5 F., would indicate the hydrocarbons had been almost completely vaporized in E-l. Thus the higher temperature differences indicate danger of passing through the dry point in E2, and the lower temperature differences indicate danger of passing through the dry point in E-1.

The TdRC controls the opening and closing of bypass valve 13 to maintain a desired temperature differential, for example 50 F., as follows: If the TdRC senses the temperature difference dropping below 50 F. (an indication that the hydrocarbons are being heated too much in E-l), then it causes bypass valve 13 to move toward a closed position. This causes more hydrocarbons to flow through E-Z and thus take up more heat in E-2. Consequently, the hot stream effluent from E-2 (line 6) comes to a lower temperature than formerly. This results in less heat being transferred to the hydrocarbons in 13-1 as, with the lower temperature for the hot stream to E-1, the driving force for heat transfer is less. With less heat being transferred to the hydrocarbons as they pass through E-l along lines 1 and 2, the percent liquid in the hydrocarbon effluent from E-1 (line 2) will increase. The greater amount of liquid in the hydrocarbons out E-l results in a greater temperature drop across pressure-drop valve 16.

If the TdRC senses the temperature difference increasing above 50 F. (an indication that the hydrocarbons are not being heated enough in E-l), then it causes bypass valve 13 to open wider. This causes the temperature drop across pressure drop valve 16 to decrease as it is just the reverse of the chain of happenings when bypass valve 16 is moved toward a closed position.

The automation of the system may stop at this point. However, it has been found that the system is further improved by inserting a function generator 10a so that the TdRC controls automatically to the desired temperature difference for a given pressure drop. Because PRC 11 causes the pressure drop to vary in order to maintain a desired pressure on the hydrocarbons in line 2, it is necessary to adjust the bypass to produce the desired temperature difference across pressure-drop valve 16 that coincides with the new pressure drop. It can readily be seen from FIG. 2 that at lower pressure drops the desired temperature difference is lower. For example, if the pressure drop is p.s.i.g. to 40 p.s.i.g.:ZO p.s.i., the maximum temperature difference is about 30 F. and a desired temperature difference is about 23 F.

In one preferred embodiment of automatically controlling the system, a signal from the PdR, depending on the pressure differential sensed by the PdR, is sent to the function generator 10a. The function generator sends a signal to TdRC 9a which is a function of, or dependent on, the pressure drop signal that the function generator has received from PdR 10. The signal from the function generator is such that it tells the TdRC what the desired temperature difference is for the current pressure drop at a given pressure in line 2. TdRC in turn causes bypass valve 13 to allow more or less bypass in order to adjust the system to the currently desired temperature difference across pressure-drop valve 16.

The desired temperature difference across pressuredrop valve 16 will also vary depending on the inlet pressure to the pressure-drop valve. For example, FIG. 2 shows the maximum temperature difference between 60 p.s.i.g. and 30 p.s.i.g. is about F.; thus a desirable temperature difference is about 50 F.; whereas the maximum temperature difierence between 100 p.s.i.g. and 70 p.s.i.g. (still a 30 p.s.i. pressure drop) is about 27 F.; thus a desirable temperature difference is about 22 F. Therefore, the function generator must also receive a signal from PRC 11 telling the function generator what the pressure is in line 2. Thus the function generator, based on the pressure signal from PRC 11 and the pressure drop signal from PdR 10, tells TdRC 9a what the desired temperature difierence is for the current pressure drop at the current inlet (line 2) pressure.

With the system automated as above described, the chance for human error allowing the hydrocarbons to pass through the dry point in one of the heaters is virtually eliminated. Also, the automation of the system results in less operator attention; therefore allowing savings in labor expense.

It is to be understood that the forms of the invention shown and described herein are to be taken only as preferred embodiments. Various changes may be made in the basic process, such as the manner of taking the pressure drop between heaters or the manner of heating the fluid to be vaporized in the heaters.

I claim:

1. A process for completely vaporizing and superheating feed liquid hydrocarbon comprising mainly xylenes without going through the dry point in a heater which comprises:

(a) heating all of said feed liquid hydrocarbons in a first heat exchange at a first pressure to obtain a liquid-vapor mixture of said hydrocarbons at said first pressure;

(b) passing said mixture from said first heat exchanger through a pressure reducing zone wherein the pressure is reduced on all of said hydrocarbons to a pressure below said first pressure to obtain at least complete vaporization of all said hydrocarbons to a vapor;

(c) superheating the vapor obtained in Step (b) in a second heat exchanger to obtain superheated vapor;

(d) further superheating the superheated vapor in a vapor superheating furnace;

(e) passing the superheated vapor to a xylene isomerization reactor; and

(f) passing at least a part of the efiluent from the reactor through at least one of said heat exchangers.

2. A process in accordance with claim 1 wherein the amount of heating of said feed liquid hydrocarbons in said first heat exchanger is controlled in response to temperature difierence resulting from the reduction pressure on said hydrocarbons from said' first pressure to said pessure below said first pressure.

3. A process in accordance with claim 1 wherein the amount of heat input to said feed liquid hydrocarbons in said first heat exchanger is decreased when said temperature difierence falls below a predetermined amount; and wherein the amount of heat input to said feed liquid hydrocarbons is increased when said temperature dilference rises above a predetermined amount.

4. Process for completely vaporizing and superheating feed liquid hydrocarbons without going through the dry point in a heater which comprises:

(a) heating all of said feed liquid hydrocarbons in a first heat exchanger at a first pressure to obtain a liquid vapor mixture of said hydrocarbons at said first pressure;

(b) passing said mixture from said first heat exchanger through a pressure reducing zone wherein the pres sure is reduced on all of said hydrocarbons, by passing said hydrocarbons through a pressure-drop valve, to a pressure below said first pressure to obtain at least complete vaporization of said hydrocarbons to a vapor;

(c) superheating the vapor obtained in Step (b) in a second heat exchanger to obtain superheated vapor; and

(d) adjusting said pressure-drop valve automatically by means of a PRC which senses pressure on said hydrocarbons between said first heat exchanger and said pressure-drop valve and controls the opening and closing of said pressure-drop valve so that the degree of vaporization ahead of said pressure-drop valve is stabilized at an amount which avoids danger of passing through the dry point in said first and second heat exchangers.

5. Apparatus for completely vaporizing and superheating hydrocarbons without going through the dry point in a heater which comprises:

(a) a first heater, having an inlet for liquid hydrocarbons and an effluent outlet, means for supplying liquid hydrocarbons to be vaporized to said inlet;

(b) a second heater, having an inlet and an efiluent outlet;

(0) conduit means for passing hydrocarbon effluent from said first heater outlet to said second heater inlet;

(d) pressure drop means in said conduit means;

(e) means for determining pressure in said conduit means between said first heater outlet and said pressure drop means;

(f) means for determining pressure drop across said pressure drop means;

(g) means for determining temperature drop across said pressure drop means;

(h) a bypass conduit for passing a portion of the hydrocarbon efiluent of said first heater from downstream of said pressure drop means to the efiluent of said second heater;

(i) means in said bypass conduit for regulating flow through said bypass conduit in response to the temperature drop across said pressure drop means;

(j) means for passing a hotter stream through said second and first heaters in series in countercurrent heat exchange with said hydrocarbons; and

(k) means for regulating the temperature drop across said pressure drop means in response to both the amount of pressure drop across said pressure drop means and the pressure determined by Step (e), whereby there is maintained at least some liquid bydrocarbons throughout said first heater and no liquid hydrocarbons in said second heater.

References Cited UNITED STATES PATENTS 2,113,816 4/1938 Saacke 208-81 2,146,553 2/ 1939 Rembert 208-92 2,428,666 10/1947 Hemminger 208-92 2,207,552 7/1940 Putt 260-668 2,415,998 2/1947 Foster 260683.4 3,288,702 11/1966 Dowd et al. 208-48 DELBERT E. GANTZ, Primary Examiner G. J. CRASANAKIS, Assistant Examiner US. Cl. X.R.

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