Refrigeration recovery of reactor feed in propane dehydrogenation systems

Abstract

A method of recovering refrigeration capacity from propane fed to a propane dehydrogenation reactor by fully condensing a depropanizer top stream, reducing the pressure of the condensed stream, and evaporating the stream with a process stream at a lower pressure to recover refrigeration capacity.

Description

[0001] This application is a divisional application of an invention patent application with a priority date of September 10, 2019, application number 202080075439.4, and name “Refrigeration Recovery of Reactor Feed in a Propane Dehydrogenation System”. Technical Field

[0002] The present invention relates to a method for recovering propylene from a propane dehydrogenation (PDH) process, and more particularly to recovering refrigeration from the reactor feed in a PDH process. Background Art

[0003] Propane dehydrogenation (PDH) is a process step for producing propylene from propane. PDH is important to the petrochemical industry because propylene is the second most important starting product in the petrochemical industry after ethylene. In the PDH process, the purpose of the deethanizer and cooling train system is to separate the cracked gas into a methane-rich tail gas product, a C2 process stream, and a C3 process stream.

[0004] There is always a benefit in improving the PDH process, for example by reducing refrigeration power requirements and / or reducing the size and cost of the compressor system. In particular, the refrigeration power required to provide process cooling in the range of -5 to 30°C is typically high. Currently, the depropanizer overhead stream is produced as a vapor product that is fed to the reactor system without recovering refrigeration. Process cooling in the range of -5 to 30°C is provided by propylene refrigeration. Summary of the invention

[0005] In one non-limiting embodiment, a method for recovering propylene from a propane dehydrogenation process is provided, wherein the method comprises partially or completely condensing a depropanizer overhead stream with a relatively warm cooling medium at a relatively high pressure; reducing the pressure of the liquid overhead stream; and partially or completely vaporizing the liquid overhead stream with at least one process stream to provide refrigeration.

[0006] In another non-limiting form, a system for recovering propylene from a propane dehydrogenation process is provided, wherein the system includes a depropanizer producing a depropanizer overhead stream, at least one condenser for partially or completely condensing the depropanizer overhead stream, at least one device for reducing the pressure of the depropanizer overhead stream determined by the back pressure of the reactor, and at least one cooler for partially or completely vaporizing the depropanizer overhead stream. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 is a non-limiting schematic diagram of a portion of a PDH process as described herein. DETAILED DESCRIPTION

[0008] In a typical propane dehydrogenation process, a fresh feed rich in propane and a recycle rich in propane from a C3 splitter are passed through a depropanizer to remove C4+ components. A purified propane-rich stream is recovered as a vapor overhead product which is directly fed to the reactor system without refrigeration heat recovery.

[0009] In one non-limiting embodiment, it has been found that the depropanizer overhead stream can be partially or completely condensed at relatively high pressure with a relatively warm cooling medium such as cooling water. In one non-limiting embodiment, "relatively warm" is defined as independently at about 20°C to about 50°C, alternatively, independently at about 28°C to about 34°C; wherein the term "independently" when used with respect to a range means that any threshold value can be used with any other threshold value to give a suitable range for selection. In another non-limiting form, "relatively high pressure" is defined as independently at about 10 to about 20 barg; alternatively, independently at about 12 barg to about 15 barg. Other cooling media besides water may include, but are not necessarily limited to, air cooling.

[0010] The liquid overhead stream is then depressurized (based on the back pressure from the reactor) and vaporized with a process stream to independently provide refrigeration in the temperature range of about -10°C to about +30°C, alternatively, independently about -5°C to about 10°C, and offset the need for refrigeration typically provided by a propylene or propane refrigeration system of the same temperature range.

[0011] For more details, refer to Figure 1 , from the process gas dryer (not shown - from Figure 1 The dried process gas 1 (left side) is cooled with propane in the first deethanizer feed cooler 20 and sent as gas 2 to the first deethanizer feed drum 22. The liquid from the first deethanizer feed drum 22 is fed to the deethanizer 24. The top vapor enters the process gas compressor (PGC) 26. The exhaust gas 3 from the process gas compressor 26 is cooled with propane and partially condensed in the second deethanizer feed cooler 28 and sent as stream 4, then cooled with propylene refrigerant in the third deethanizer feed cooler 30 and sent to the second deethanizer feed drum 32. The liquid from the second deethanizer feed drum 32 is fed to the deethanizer 24. A deethanizer reboiler 38 is also shown.

[0012] Pump circulation on the upper section 5 of the deethanizer 24 by pump 36 uses propane in the deethanizer first side cooler 46 and medium-grade propylene refrigerant 6 in the deethanizer second side cooler 48 to reduce the demand for low-grade refrigerant on the top condenser.

[0013] In one non-limiting embodiment, the overhead stream from the depropanizer (from Figure 1 Liquid propane 7 from the left side (de-propanizer not shown) is depressurized (determined by the reactor back pressure) and completely vaporized in the combination of the second de-ethanizer feed cooler 28, the first de-ethanizer feed cooler 20 and the de-ethanizer first side cooler 46. The vaporized propane 9 from the coolers is sent to the cold box 34 to be reheated and from the cold box 34 to the reactor as propane stream 10.

[0014] Non-limiting examples of logistics information are given in Table I, and non-limiting examples of chiller capacities are given in Table II.

[0015] Table I

[0016] Exemplary logistics information

[0017] Logistics Number 1 2 3 4 5 6 7 9 10 describe unit Gas to reach 20 Gas from 20 Gas to reach 28 Gas from 28 From 36 to 46 From 46 to 48 Propane from the depropanizer Propane from a chiller Propane from cold box 34 Vapor fraction 1.00 0.78 1.00 0.80 0.00 0.00 0.00 1.00 1.00 temperature ℃ 10.0 -1.8 29.7 -1.6 14.2 4.3 42.3 -5.7 38.0 pressure <![CDATA[kg / cm 2 ]]> 13.9 13.7 21.9 21.7 12.8 12.8 15.0 4.0 3.9 Mass density kg / cum 15.8 20.1 18.1 24.0 497.6 513.2 462.6 8.6 6.8 Total mass flow kg / hr 121969 121969 75698 75698 500000 500000 194605 194605 194605

[0018] Table II

[0019] Example power

[0020] power Gcal / h First deethanizer feed cooler 20 4.7 Second deethanizer feed cooler 28 4.0 Deethanizer first pump circulating cooler 46 3.1 Cold box (propane) 34 3.6

[0021] As described herein, the use of propane feed to the reactor for refrigeration recovery significantly reduces the demand for refrigeration from propylene or propane refrigeration system, and can reduce the size and cost of compressor system.In a non-limiting embodiment, refrigeration demand can reduce about 50% or more, alternatively about 30% or more.In different non-limiting forms, the reduction of compressor system cost can be about 30%, alternatively about 20%.

[0022] In the foregoing description, the present invention has been described with reference to specific embodiments thereof. However, this description should be regarded as illustrative rather than restrictive. For example, equipment, processes and operating conditions that fall within the claimed or disclosed parameters but are not specifically determined or attempted in the specific examples are expected to be within the scope of the present invention.

[0023] The invention can be practiced in the absence of undisclosed elements. In addition, the invention can suitably comprise, consist of, or consist essentially of the disclosed elements. For example, a method for recovering propylene from a PDH process can be provided, wherein the method consists essentially of or consists of: partially or completely condensing a depropanizer overhead stream with a relatively warm cooling medium at a relatively high pressure; reducing the pressure of the liquid overhead stream; and partially or completely evaporating the liquid overhead stream with at least one process stream to provide refrigeration.

[0024] A system for recovering propylene from a propane dehydrogenation process may also be provided, wherein the system consists essentially of or consists of: a depropanizer producing a depropanizer overhead stream; at least one condenser for partially or completely condensing the depropanizer overhead stream; at least one device for reducing the pressure of the depropanizer overhead stream as determined by the back pressure of the reactor; and at least one cooler for partially or completely vaporizing the depropanizer overhead stream.

[0025] As used throughout the claims, the words “comprising” and “comprises” should be interpreted to mean “including but not limited to” and “includes but not limited to”, respectively.

[0026] As used herein, the word "substantially" shall mean "generally but not entirely as specified."

[0027] As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0028] As used herein, the term "about" when referring to a given parameter is inclusive of the stated value and has the meaning dictated by the context (eg, it includes the degree of error associated with measurement of the given parameter).

[0029] As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

Claims

1. A system for recovering propylene from a propane dehydrogenation process, the system include: a depropanizer producing a depropanizer overhead stream; at least one condenser for partially or completely condensing the depropanizer overhead stream with a relatively warm cooling medium at a temperature of about 20° C. to about 50° C.; at least one means for reducing the pressure of the depropanizer overhead stream based on the back pressure of the reactor; and At least one cooler for partially or completely vaporizing the depropanizer overhead stream.

2. The system according to claim 1, in, The device is a valve.

3. The system according to claim 1, in, The cooler is configured such that a refrigeration temperature of the cooler is in a range of about -10°C to about 30°C.

4. The system according to claim 1, in, The at least one condenser is configured to partially or fully condense the depropanizer overhead stream at a relatively high pressure of about 10 barg to about 20 barg.

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