PATENT
Patent number: 7981256
Issue date: Jul 19, 2011
Application number: 11/937,796
Inventors: David A. Wegerer, Paul A. Sechrist
Original Assignee: UOP LLC
TECHNICAL FIELD
This disclosure relates generally to hydrocarbon processing. More specifically, this disclosure relates to the initial processing of hydrocarbon-containing materials into an intermediate stream including two materials with similar boiling points, e.g. propylene and propane, and the separation or splitting of those two materials.
BACKGROUND OF THE RELATED ART
Light olefins serve as feed materials for the production of numerous chemicals. Light olefins have traditionally been produced through the processes of steam or catalytic cracking of hydrocarbons such as derived from petroleum sources. Fluidized catalytic cracking (FCC) of heavy hydrocarbon streams is commonly carried out by contacting relatively high boiling hydrocarbons with a catalyst composed of finely divided or particulate solid material. The catalyst is transported in a fluid-like manner by transmitting a gas or vapor through the catalyst at suflicient velocity to produce a desired regime of fluid transport. Contact of the heavy hydrocarbons with the fluidized catalyst results in the cracking reaction.
FCC processing is more fully described in U.S. Pat. Nos. 5,360,533, 5,584,985, 5,858,206 and 6,843,906. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
The FCC reactor serves to crack gas oil or heavier feeds into a broad range of products. Cracked vapors from an FCC unit enter a separation zone, typically in the form of a main colunm, that provides a gas stream, a gasoline cut, light cycle oil (LCO), heavy cycle oil (HCO), and clarified oil (CO) components. The gas stream may include hydrogen and C1 and C2 hydrocarbons, and liquefied petroleum gas (“LPG”), i.e., C3 and C4 hydrocarbons.
There is an increasing need for light olefins such as propylene for the production of polypropylene, propyl benzene, cumene and the like. Research efforts have led to the development of an FCC process that produces or results in greater relative yields of light olefins, such as propylene. Such processing is more fully described in U.S. Pat. No. 6,538,169.
A conventional FCC process produces a combined propylene/propane stream. The recovery and purification of propylene from the combined propylene/propane is accomplished via a sequence of distillation operations. The sequence consists of distillation columns to separate both lower and higher boiling components from propylene and generally includes a distillation operation to separate a mixed stream of propane and propylene into a propylene product or “polymer grade” propylene, which can be used for polymer manufacturing in a downstream operation. The propane/propylene separation by distillation is both energy and capital intensive due to the relative volatility of species to be separated, feed composition, and product purity requirements of “polymer grade” propylene.
Because of the energy consumption requirements of splitter colunms in general, splitter colunm configurations for similar boiling point materials that reduce utility consumption are desirable given increasing energy costs and a general need to reduce CO2 emissions associated with fossil fuel consumption.
SUMMARY OF THE INVENTION
A splitter system is disclosed for separating a first material from a mixture of first and second materials. The system includes a colunm with a feed inlet that introduces a mixture of first and second materials to the colunm. Two reboiler-heat exchangers and two associated circulation loops are utilized. An intennediate loop is connected to the colunm below the feed inlet. The intermediate loop passes through an intennediate reboiler heat exchanger. As an alternative to the intermediate loop, the intennediate reboiler is a stab-in heat exchanger disposed within the colunm below the feed. A bottoms loop is connected to the bottom of the colunm with the bottoms loop passing through a bottoms reboiler heat exchanger. As an alternative to the bottoms loop, a stab-in heat exchanger may be utilized within the bottom of the colunm.
The top of the colunm comprises an overhead outlet connected to a first overhead loop and a second overhead loop. The first overhead loop connects the overhead outlet to a first heat pump compressor and the second overhead loop. The second overhead loop connects the first overhead loop to a second heat pump compressor.
The first heat pump compressor passes vapor from the first overhead loop through the intennediate reboiler heat exchanger at an elevated pressure to heat and at least partially vaporize the intermediate material exposed to the intennediate reboiler. The second heat pump compressor passes material from the second overhead loop at an elevated pressure through the bottoms reboiler heat exchanger to heat and at least partially vaporize the bottoms material exposed to the bottoms reboiler. The first and second heat pump compressors act to increase the pressure of the first and second overhead loop vapor streams, thereby increasing their respective condensation temperatures to enable the first and second overhead loop streams to deliver heat to the intennediate and bottoms reboilers.
The use of two heat pumps, two overhead vapor loops and the additional intennediate reboiler results in substantial energy savings as shown below. The energy savings opens the possibility of using stab-in heat exchanger for the inter-reboiler and bottoms reboiler, depending upon the separation being carried out.
In a refinement, the first heat pump compressor and the second heat pump compressor are first and second stages of a two stage heat pump compressor.
In another refinement, when the colunm is operated at a lower pressure, a small flow third stage of compression can be employed between the second stage of compression and the trim condenser, to decouple the operating temperatures of the colunm from the trim condenser temperature. In such a refinement, a third overhead loop is provided which connects the second overhead loop with a third heat pump compressor disposed upstream of a trim condenser. In such a design, a three stage heat pump compressor may be employed.
In another refinement, receiving drums are disposed upstream of each heat pump compressor or each heat pump stage. More specifically, the first overhead loop passes through the first heat pump compressor and through a second receiving drum that has a vapor outlet connected to second heat pump compressor and a liquid outlet connected to a first receiving drum. The first receiving drtun has a vapor outlet connected to the first heat pump compressor and a liquid outlet connected to a reflux pump that is comlected to the reflux inlet as well as a first product outlet. The second overhead loop passes through the second heat pump compressor to a trim condenser before passing through to the second receiving drum. Liquid from the second receiving drum is passed to the first receiving drum.
The disclosed splitter systems preferably operate at a reflux to feed ratio (R/F) of about 5 or greater. Because of the employment of multi-stage heat pump compressors in preferred embodiments, the disclosed splitter systems are particularly beneficial for separating materials having a difference in boiling points of about 11° C. (20° F.) or less.
In one embodiment, the first material is propylene and the second material is propane. For a propylene/propane splitter, the colunm houses from about 120 to about 220 theoretical stages, more preferably from about 150 to about 190, still more preferably about 170. Further, the feed inlet may be connected to the colunm above about the 120th stage counting from the top of the colunm. Also, the intermediate loop outlet may be connected to the colunm above about the 149th stage counting from the top of the column. The number of stages will vary greatly, depending upon the two materials being separated and the ratio of the two materials in the feed.
For a propylene/propane splitter, the first heat pump compressor pressurizes vapor taken from the first overhead loop to a pressure exceeding 1379 kPaa (200 psia) and the second heat pump compressor pressurizes vapor from the second overhead loop to a pressure exceeding 1725 kPaa (250 psia).
However, this disclosure is not limited to a propylene/ propane splitter or even to a light olefin/paraflin splitter. The apparatuses and techniques disclosed herein are applicable to any two materials with similar boiling points, examples of which are too numerous to list here.
A method for separating a first material from a mixture of the first material and a second material is also disclosed. The disclosed includes introducing the mixture into a column through a feed inlet disposed between a top and a bottom of the column. The method further comprises delivering heat to intermediate and bottoms reboilers via overhead vapor taken from the top of the colunm and circulated at elevated pressures through first and second overhead loops that pass through the intermediate and bottoms reboilers.
Other advantages will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims and drawings.
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