James Alan
Upchurch Diamond Shamrock Refining & Marketing Co.
San Antonio
Diamond Shamrock Refining & Marketing Co. is successfully operating a propane/propylene splitter at its underground hydrocarbon storage terminal at Mont Belvieu, Tex.
The unit is designed to produce 600 millon lb/year of polymer-grade propylene (99.5 wt %, minimum) from refinery-grade propylene or propane/propylene (PP) mix. PP mix is typically 65 LV % propylene and 35 LV % C2 and C3 +.
New facilities include storage and movement systems, treating and dehydration, fractionation, and a product pipeline.
With both the McKee refinery in the Texas Panhandle and the Three Rivers refinery in South Texas producing refinery-grade propylene, the PP splitter was a natural extension of Diamond Shamrock's refining business. The company continues to develop its strategy of constructing petrochemical units that use feedstocks from its refinery operations.
Fina Oil & Chemical Co. became a one-third joint venture partner to provide another feedstock source for its polypropylene facility at LaPorte, Tex. Polymer-grade propylene is also supplied to other users in the area.
Fina's portion of the PP mix feedstock comes, at least in part, from its Port Arthur, Tex., refinery. Both Fina and Diamond Shamrock process purchased PP mix as well.
The partners selected the Mont Belvieu site over existing inland refinery sites because large LPG storage wells were available to store feedstock and products, and because several propylene users are located nearby.
With only limited utilities available, it was necessary to construct a high-voltage substation, local control room, warehouse, and flare to support the new unit.
Because vapor recompression or heat pumping was known to fit where fractionation of materials with close boiling points was concerned, the designers chose a heat pump design. It allows the splitter to operate I at lower pressure, where the volatility difference between propane and propylene is greater. Benefits of reduced column size, lower column design pressure, and lower utility costs justified the additional complexity associated with the heat pump compressor.
Utilities for fractionation shifted from fuel gas and cooling water (which were not readily available) to electricity to drive the heat pump compressor. An added benefit of the heat pump design was that on site air emissions normally associated with the reboiler could be eliminated.
PROCESS DESIGN
A typical open-loop heat pump fractionator uses a compressor to pressurize overhead vapors so they can be condensed in the reboiler and trim condenser (Fig. 1).
A trim condenser is needed to condense a small portion of the compressed vapors to remove the heat generated by the compressor and to provide a way of varying condenser duty relative to reboiler duty. The condensed, high-pressure liquid is flashed down to fractionator pressure across a back-pressure control valve.
Liquid is either returned to the fractionator as reflux or drawn off as product. Vapor from the flash is combined with the fractionator overhead vapors and returned to the compressor inlet. Therefore, the compressor must be sized to handle a flowrate that is 10 to 20% more than the fractionator overhead vapor flowrate.
Initial design attempts included a single fractionator to perform both de-ethanizing and PP splitting operations. This was followed by the addition of a high-pressure de-ethanizer, which would operate at temperatures above freezing and eliminate the need for feed dehydration.
The selected design included feed dehydrators, a refrigerated de-ethanizer, and a separate PP splitter.
DE-ETHANIZER
The de-ethanizer contains 60 trays and operates at 210 psia with an overhead condensing temperature of 3 F. With the limited on site utilities available, a motor-driven heat pump design was also needed for the de-ethanizer.
An open-loop heat pump similar to the splitter design was eliminated in favor of a closed-loop system using high-purity propylene as the refrigerant (Fig. 2).
A side condenser and two-stage refrigeration system with an economizer and compressor side load made it possible for much of the condensing to occur at a higher temperature, thus reducing refrigeration horsepower approximately 25%.
SPLITTER
The design consisted of a conventional single-stage vapor recompression system with a parallel condenser-reboiler and trim condenser. Operating pressure was set at 150 psia, and the splitter was divided into two columns containing a total of 240 trays.
The splitter was field fabricated because there were no heavy transport facilities such as a rail siding at the job site. In order to eliminate the high cost of stress relieving in the field, shell thickness for the 16 ft diameter column was limited to a maximum of 1.5 in.
A two-column design was selected to reduce the length-to-diameter ratio and the massive foundation that would have been required for a single column (Fig. 3).
A conservative design philosophy was most evident in the selection of splitter trays. With the exception of heavier liquid loadings below the feed tray, traffic in the splitter column was uniform. Four-pass trays were selected to provide capacity at the expense of a drop of roughly 10% in efficiency.
Tray design was based on 110% of expected column traffic with a maximum of 83% jet flood. Valve trays were selected because of concern about the intermittent operation that typically occurs during start-up.
Along with providing better turndown, it was felt that valve trays would delay dumping of liquid into the bottom of the column.
A recirculating thermosiphon unit with 25% vaporization per pass was selected for the condenser-reboiler, after rejecting, two other designs. Two shells were needed to provide 55,000 sq ft of heat transfer area. Dual return lines from each reboiler reduced the pressure drop in the two-phase reboiler return piping.
COMPRESSORS
Imo Delaval compressors and Westinghouse motors were selected to power the heat pump systems. Although some consideration was given to driving both systems on a common shaft, the only shared items in the final design were the lube and seal-oil system and the control console. The de-ethanizer compressor is a seven-stage machine with a single side load. It is powered by a 1,300-hp motor.
The splitter compressor is a vertical split case, single-stage machine powered by a 9,500-hp motor. Because the motor turns at fixed speed, adjustable guide vanes were included to control head with a minimum loss in efficiency.
TREATING UNITS
Feedstock and product treating were particularly important because of the multiple, tight specifications required by polypropylene manufacturers.
Several refinery PP mix samples were analyzed and polymer-grade propylene specifications were obtained to determine which contaminants would have to be removed, and to what extent.
Because more than one third of the plant feed is supplied by Diamond Shamrock, feedstock specifications (Table 1) were developed based on the company's refinery-grade PP mix after caustic and monoethanolamine (MEA) treating.
Carbonyl sulfide (COS) and arsine were considered the most difficult contaminants to control. In addition, requirements for total sulfur, unsaturates such as methyl acetylene (MA) and propadiene (PD), and oxygenated compounds were of concern. Computer simulations indicated that typical levels of O2, CO, MA, and PD could be suitably removed by the fractionators.
COS REMOVAL
Hydrolysis alumina was selected for COS removal, to make use of existing moisture in the PP mix. Water and COS react to form H2S and CO2, which can then be removed by caustic or amine contactors.
The COS reactors consist of two vessels connected in series with the downstream reactor acting as a guard bed. Vessel alignment can be reversed and either vessel can be shutdown for maintenance while the plant remains on-line. COS content will typically be reduced from several ppm to below 30 ppb (wt).
Two horizontal caustic contactors with recirculating caustic pumps, static mixers, and internal distributors remove H2S, CO2, and mercaptan sulfur. Spent caustic can be replaced with fresh 25 wt % caustic while the unit remains on-line. Either scrubber can be temporarily bypassed for maintenance and the unit will still operate satisfactorily.
Feed from the caustic scrubbers passes through a caustic knockout vessel and a sand coalescer to remove entrained caustic before entering the feed dehydrators. Spent caustic is accumulated in a storage tank and trucked off site for disposal.
Before entering the deethanizer, moisture in the feed is reduced from about 800 ppm (wt) to 1 ppm (wt) in a pair of vessels containing type 3A molecular sieve.
Regeneration gas consists of dry PP mix, which is vaporized and heated to a maximum of 450 F. The gas is passed through the wet dehydrator to strip out moisture, and then condensed and returned to the on-line dehydrator after water is drained off in a coalescer/separator.
Using an unsaturated regeneration gas at higher temperature could lead to a contaminant decomposition reaction with the olefin and loss of mole sieve activity. We have experienced no regeneration problems with either the PP mix or the propylene product mole sieves.
The mole sieve regeneration heaters are small, and they operate intermittently, so emissions are not significant.
ARSINE REMOVAL
Propylene product is treated with copper oxide impregnated on alumina to remove arsine. Although regenerable to some extent, plans call for sorbent disposal when approximately 2 wt % arsine is accumulated in the sorbent bed.
Arsine sorbent will also remove COS and other sulfur compounds. Arsine and H2S are believed to react with the CuO to form CU3As and CuS. Feed to the arsine absorber should be dry, and if the sulfur content is high relative to arsine, the sorbent will remove sulfur at the expense of arsine.
OTHER FEATURES
Feedstock is metered to a 10,000-bbl storage sphere from either a pipeline, storage well, or truck unloading rack (Fig. 4). Any imbalance between feedstock supply and plant demand is automatically adjusted by transferring feedstock to or from the PP mix storage well.
The feed and product meter stations are both designed to meet custody transfer specifications. A continuous sampler on the PP mix feed and propylene product meters helps track feedstock and product qualities.
Treated propylene product enters one of four propylene make tanks. Each tank has a capacity of 6 hr of production at design throughput. Flow is batched through the make tanks and tested to assure that the product meets all specifications before it is pumped into the product pipeline or storage well.
Product flowrates and pipeline demand are balanced by diverting flow to or from the well. Propylene supplied from the storage well is always wet, so a pair of propylene dehydrators was constructed to make dry, on-spec product available at all times.
A Honeywell TDC-3000 distributed control system (DCS) controls the process. Operators can control the plant from either the main control room, which is a quarter mile away or from the DCS building located at the plant site.
There are five on-line gas chromatographic (GC) analyzers in the plant. Sample flows through a vaporizer at the sample point before entering a climate-controlled GC enclosure. Analyzer locations, components analyzed, and analyzer ranges are shown in Table 2.
The C2-to-propylene ratio analyzer in the de-ethanizer bottoms and the C3+ analyzer in the splitter overhead provide trim control for the boil-up rates in the de-ethanizer and splitter, respectively. However, operation of the fractionators has been steady enough that it has not been necessary to use the analyzers for control.
On-line analyzers correlate reasonably well with lab analyses, and operators routinely rely on them.
ENGINEERING AND CONSTRUCTION
Diamond Shamrock awarded Davy McKee-Dresser Engineering Division a contract for detailed engineering and procurement about the time conceptual design for the plant was completed. H. B. Zachry received the primary construction contract 24 weeks after detailed engineering began.
The contractors mechanically completed the project 66 weeks after detailed engineering began and 55 weeks after field construction commenced. Construction of the splitter columns and tray installation took longer than expected. Shell sections were delivered as completed "cans" and welded on top of one another as they arrived (Fig. 5).
START-UP
The unit was commissioned in May of 1990. Startup procedures called for the de-ethanizer refrigeration compressor and the splitter compressor to be stabilized on total recycle before establishing plant throughputs.
The compressor-recycle streams could be cooled by condensing a portion of the recycle gas in the trim condensers and injecting this liquid into the hot recycle gas.
Minor compressor startup problems were caused by faulty temperature and pressure switches, lube oil leaks, and high suction-drum liquid levels. After correcting each problem as it was discovered, the start-up team stabilized the refrigeration compressor and slowly introduced feed to the de-ethanizer.
Compressor discharge was diverted from the recycle loop to the reboiler, and as the boil-up rate increased, refrigerant was introduced to the overhead condenser. Accumulator levels were established, reflux pumps were started, and de-ethanized PP mix was soon produced.
Similar problems occurred at the splitter compressor during start-up. In addition, loss of liquid quench to the splitter recycle stream resulted in high compressor-discharge temperature. An alternate quench supply line re-established quench flow and a low-decibel cartridge installed at the recycle valve reduced noise created by high flowrates.
With the splitter compressor operational, recycle vapors were diverted through the condenser-reboilers. Liquid in the bottom of the column was quickly boiled up to the upper column and overhead accumulator. With most of the controllers on manual, initial operation required close attention, but the column was controllable and surprisingly steady.
After the unit was on-line controllers were tuned more precisely, and other adjustments helped stabilize fractionator operation.
Originally, flow from the compressors to both the deethanizer and splitter reboilers was to be controlled by a pair of split-range control valves on the reboiler and trim condenser outlets. This arrangement provided low pressure drop in the compression loop, but flow control was erratic.
Local differential pressure controllers were installed in the trim condenser loop to provide a fixed pressure drop.
Control was improved at the expense of a slight increase in pressure drop.
Original design called for de-ethanizer pressure to be controlled by varying the overhead product flowrate. But the overhead flowrate was so low that the controller swung it excessively in its attempt to maintain column pressure.
The overhead flowrate was placed on fixed setpoint control, and column pressure was then controlled by adjusting refrigerant flow through the de-ethanizer reboiler.
It took only a few hours after startup to produce polymer-grade propylene from the splitter overhead. But it took 1-2 days of steady operation before it became apparent that the bottom product would also meet specification.
The spotter has operated satisfactory at 110% of design capacity while producing 99.7 wt % propylene and 97.5 LV % propane. Propylene recovery was expected to be 97.7%, but with typical good-quality feed, it has been slightly better than 99%.
Since start-up, capacitors have been installed in the compressor motor and main switchgear to improve the lagging power factor and reduce purchased kv-amp. A spare splitter compressor rotating element has been trimmed and installed, and the resulting improvement in compressor efficiency reduced motor load by more than 1,000 hp.
Additional PP mix feedstock can now be supplied by tankcar through a new rail unloading rack. The propylene pipeline is being extended to additional propylene users, and it will also supply propylene to Olefin Terminal Corp.'s new export facility at Bayport, Tex.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
