Critical Service Valves for Hydroprocessing Units
By Masoneilan/Dresser Valves & Controls Div.
Table of Contents
There are increasing trends in the refining industry towards implementation of bottom-of-the-barrel conversion hydroprocessing units. These conversion units have a number of areas around the reactor and separator sections where control valves are used in critical and severe service applications. Specifiers and end users need to be aware of the unique considerations for successfully applying severe-service control valves in critical sections of hydroprocessing conversion units.
The Growth of Hydroprocessing
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Environmental agencies around the world have continued to tighten regulations on motor fuel specifications, forcing refiners to revamp operations to better manage oxygenate production, benzene allowance, metal removal, and sulfur reduction. Refiners continue to add new units and upgrade existing processes, in order to produce end products to meet these new specifications. The projected worldwide increase in demand for transportation fuels will drive refiners towards converting bottom of the barrel products to more useable middle distillates as shown in Figure 1 below.
Two of the key conversion processes being applied by refiners to meet these new product requirements are:
- Hydrotreating and
- Hydrocracking
The hydroprocessing area has seen a constant growth since its inception in the late 1950's as illustrated Figure in 2. These types of units typically operate at elevated temperature and pressure ranges, and can vary in configuration depending on the particular geographic location and product demands.
Figure 3 shows an example of staged or phased implementation of various conversion units. The fully implemented conversion project provides refiners with the flexibility to produce a variety of products to meet changing market demands. As illustrated, an atmospheric tower bottom can be revamped to produce motor gasoline for the gasoline blending pool by adding a residual hydrotreater in combination with a residual fluid catalytic cracking (RFCC) unit. RFCC operation and efficiency is highly dependent upon this serial type processing for many types of crude.
In certain markets where middle distillates are more desirable, the hydrotreater can be designed up front with provisions for vacuum tower bottoms upgrading capabilities. The vacuum gas oil (VGO) from the vacuum tower can be used to produce high quality middle distillates with the addition of a hydrocracker unit. The vacuum residue can be upgraded to produce naphtha and diesel with the hydrocracker unit, and produce motor gasoline with the hydrotreater and RFCC combination.
Addition of all three units allows a refiner to vary production of gasoline and middle distillates with changing market demands. The cut point on the vacuum tower must be varied in order to maximize production of gasoline or middle distillates.
Residue-upgrading projects are typically installed in phases due to the large amount of capital investment required for a complete conversion. Phase I would consist of adding a residual hydrotreater for upgrading atmospheric tower bottoms. This would reduce the quantity of high sulfur fuel oil production while simultaneously improving the quality of oil produced. Phase II includes the addition of a RFCC to completely eliminate the residuum and produce motor gasoline. Phase III consists of adding a vacuum tower with variable cut point and a hydrocracker unit.
Hydrotreating processes are used by refiners to produce low-sulfur fuel oils, and to prepare feeds for VGO, FCCs, RFCCs, visbreakers, and delayed cokers. Hydrotreaters upgrade residual oils by removing impurities and cracking heavy molecules in the feed to produce lighter product oils. They are also well suited for removing nitrogen, carbon residue, nickel, and vanadium from the oil, and cracking heavy vacuum residue molecules to VGO, distillates and naphtha products. The actual amount of impurities removed depends on the feed and the end product specifications.
Reactions in a hydrotreater reactor occur in the liquid phase. The residual feed is saturated with hydrogen gas, which makes the molecules absorb on the catalyst surface where the reaction takes place. This exothermic reaction allows the breakdown of molecules in the residual feed that contain sulfur, nitrogen, and heavy metals (vanadium and nickel). A typical flow diagram of a hydrotreating unit is shown in Figure 4 below. Some of the key applications for control valves in this process include cold separator letdown, sour water letdown, and hot separator letdown. The hot separator cuts the heavy and light reaction products and recovers the hydrogen flashed off during the letdown from the reactor. The heavy product liquid is let down in pressure and sent to the low-pressure separator where it is then sent to the fractionation section. The light product liquid is cooled and injected with wash water to absorb ammonia (NH3) and hydrogen sulfide (H2S).
The mixture is further cooled before entering the cold-high-pressure separator. The cold separator isolates the vapor, sour water, and light hydrocarbons. The light hydrocarbons are letdown in pressure to the low-pressure separator and then forwarded to the fractionation section. The sour water is sent to a recovery unit for removal of H2S and NH3. The hydrogen rich gas is sent to a high pressure-scrubbing unit for removal of the remaining H2S before entering the recycle compressor. Liquids from the low-pressure separators are fed to the atmospheric fractionator, which splits the hydroprocessed oil from the reactors into the desired final products.
Hydrocracking technology plays a major role in meeting the needs for cleaner-burning fuels, feedstocks for petrochemical operations, and more effective lubricating oils. Hydrocracking is the only process for converting heavy fuel oil components into transportation fuels and lubricating oils with quality levels meeting environmental and market demands. The increasing market demand for middle distillates and cleaner-burning transportation fuels with lower-sulfur, has driven refiners towards increased conversion capacity with hydrocracking. Hydrocracking is a highly flexible process option that can be used to convert virtually any refinery stream into value-added products. It can be integrated synergistically with other conversion technologies such as FCC and delayed coking.
Figure 5 illustrates a single stage hydrocracking process where the feed is a combination of raw stock, recycle oil, and recycle gas. As with a hydrotreating unit, there are various sections within the hydrocracking process where control valves are highly critical to the performance and operation of the unit. The flow process starts with various heating cycles, then mixing and distribution of the hydrogen quench through the reactors, and subsequently into the hot separator.
In the hot separator, conversion products are flashed overhead and cooled before entering the high-pressure-cold separator. The hot, heavy, unconverted product is routed to the fractionation section to prevent fouling and catalyst coking in the reaction section. In the cold-high-pressure separator, the recycle gas is separated and sent to the recycle compressor. The liquid is separated into recoverable products and sour water. The recoverable products are directed to the fractionation train for final end product stripping and separation.
Critical Control-Valve Applications
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Figure 6 shows a typical cold high-pressure separator application where the light products have been separated in the hot high-pressure separator.
The product stream is cooled and proceeds to the cold separator. In a residuum hydrotreating unit, the fluid has wash water injected into the stream before entering the cold separator to remove NH3 and H2S. Level controls are used to let the usable product pressure down to the fractionation section pressure, and the sour water down to the recovery unit pressure. Each application involves high-pressure drops across the control valves.
In addition, the fluid temperatures are sufficiently high so that fluid flashing is present during pressure reduction presenting potential erosion problems within the valve. The process fluid is both erosive and corrosive in nature, and in some cases catalyst fines may also be entrained in the fluid. As a result, the valve design should be trash tolerant and be able to provide multi-step pressure letdown. The valve trim should have expanding stages to compensate for the expansion in volume of the flashing fluid. Expanding stages will also reduce flow velocities in the valve body and trim, and downstream piping. It is highly recommended that the bypass valves be of the same construction, or that a parallel valve configuration be implemented for redundancy. Design & Metallurgical Recommend Both Sour Water Letdown and Cold Separator Letdown applications require a multi-step-expanding trim valve design to handle properly. The number of stages will depend upon the pressure letdown requirements. Required valve sizes will generally range from 2 to 8 in., and body ratings will typically be ANSI 1500 or 2500, based on pressure and temperature requirements. Angle style configurations are typically used because of the flashing and potential cavitation in the final stages of the trim. Figure 7 shows a valve trim construction that has been used successfully in both applications described.
Metallurgical specifications are critical since H2S and NH3 are present in the process media, therefore high alloy trim materials are recommended for the valve trim components. Some typical materials that have been used successfully are Nitronic 50, Nitronic 60, 316 stainless steel with hardfacing, and Ferralium 255 (ASTM A351 Gr CD4MCU).
Figure 8 shows a typical hot-high-pressure separator where the reactor effluent has already been cooled in the feed preheaters. In the hot separator, the light and heavy products are separated as follows. The light products are sent to the cold separator, and the heavy products are letdown in pressure by the level control valve and sent to the fractionation section or low-pressure separator. This level valve is one of the most critical valves in the hydroprocessing unit, and witness's high-pressure-drop and flashing conditions at elevated temperatures. Similar control valve characteristics are recommended for this application as defined for the Cold Water Letdown and Sour Water Letdown applications.
The Hot Water Letdown application requires a multi-step-expanding trim valve design to handle properly. The number of stages specified will depend upon the pressure letdown requirements. Required valve sizes will generally range from 2×3 to 6×10-in., and body ratings will typically be ANSI 1500 or 2500. A trim design with top-and bottom-stem guiding is highly recommended to maintain dynamic stability. Angle style configurations are typically used because of the flashing and potential cavitation in the final stages of the trim. Figure 9 shows a valve trim construction that has been used successfully in this type of application.
High alloy trim materials are also recommended for this application along with a chrome-moly (2¼ Cr-1 Mo), or a stainless steel valve body. Chrome-moly lacks adequate resistance to H2S corrosion at temperatures above 500 °F. Valve manufacturers have used higher chromium content alloys (321 or 347 stainless steel) to overcome this corrosive atmosphere.
These are just a few examples of typical applications where proper control valve specification is critical to the operating efficiency and reliability in hydroprocessing units. Attention to detail is imperative to successful implementation to meet the changing needs in today's marketplace.
For more information: Masoneilan/Dresser Valves & Controls Div. 65 Bodwell St. Avon, MA 02322. Tel: 508-586-4600, fax: 281 871 6569.