Letdown

Reaction times can be as short as 10 minutes in a continuous flow reactor (1). In a typical batch cycle, the slurry is heated to the reaction temperature and held for up to 24 hours, although hold times can be less than an hour for many processes. After reaction is complete, the material is cooled, either by batch cooling or by pumping the product slurry through a double-pipe heat exchanger. Once the temperature is reduced below approximately 100°C, the slurry can be released through a pressure letdown system to ambient pressure. The product is then recovered by filtration (qv). A series of wash steps may be required to remove any salts that are formed as by-products. The clean filter cake is then dried in a tray or tunnel dryer or reslurried with water and spray dried. 

Hydrothermal Synthesis Systems. Of the unit operations depicted in Figure 1, the pressurized sections from reactor inlet to pressure letdown ate key to hydrothermal process design. In consideration of scale-up of a hydrothermal process for high performance materials, several criteria must be considered. First, the mode of operation, which can be either continuous, semicontinuous, or batch, must be determined. Factors to consider ate the operating conditions, the manufacturing demand, the composition of the product mix (single or multiple products), the amount of waste that can be tolerated, and the materials of constmction requirements. Criteria for the selection of hydrothermal reactor design maybe summarized as... 

Other above-ground continuous flow systems have been designed and operated for SCWO processes. A system developed by ModeU Development Corp. (Modec) uses a tubular reactor and can be operated at temperatures above 500°C. It employs a pressure letdown system in which soHd, Hquids, and gases are separated prior to pressure release. This simplifies valve design and material selection on the Hquid leg. 

Is there any pressure letdown without power recovery ... 

Dispersibility. Pigments modified with a dispersion additive take less time and energy to disperse in a coating (12,44). The equipment for blunging is simpler and less expensive than bah. mihs. Color correction is simplified and settling is minimized. Color strength in letdowns is often improved... 

Power Recovery in Other Systems. Steam is by far the biggest opportunity for power recovery from pressure letdown, but others such as tailgas expanders in nitric acid plants (Fig. 1) and on catalytic crackers, also exist. An example of power recovery in Hquid systems, is the letdown of the high pressure, rich absorbent used for H2S/CO2 removal in NH plants. Letdown can occur in a turbine directiy coupled to the pump used to boost the lean absorbent back to the absorber pressure. 

Basic to establishing whether power recovery is even feasible, let alone economical, are considerations of the flowing-fluid capacity available, the differential pressure available for the power recovery, and corrosive or erosive properties of the fluid stream. A further important consideration in feasibihty and economics is the probable physical location, with respect to each other, of fluid source, power-production point, and final fluid destination. In general, the tendency has been to locate the power-recoveiy driver and its driven unit where dictated by the driven-unit requirement and pipe the power-recoveiy fluid to and away from the driver. While early installations were in noncorrosive, nonerosive services such as rich-hydrocarbon absorption oil, the trend has been to put units into mildly severe seiwices such as amine plants, hot-carbonate units, and hydrocracker letdown. 

Substantial energy can be recovered using low-grade waste heat, process gas, or waste gas pressure letdown. 

Figure 1-2. Turboexpander in gas pressure letdown servioe (power reoovery oyole).

Consider a 1,200 kW power reeovery expander-gear-generator designed to be installed in parallel with a natural gas pressure letdown station. The expander shown in Figure 1-2 reeeives the proeess gas at 11 bar and 42°C and expands it to 5 bar. In this ease, the temperature at the diseharge is ealeulated to be 1°C, and sinee the gas eontained water vapor, it will eondense in the expander. This will bring the gas to a suitable dew point, and droplets are removed in a separator downstream of the expander. 

Turboexpanders ean be used for energy reeovery and, in some instanees, their applieation avoids losses in the form of eooling. In other instanees, these maehines reeover energy from waste heat or from pressurized gas streams that would otherwise have to undergo pressure reduetion in meehanieal letdown valves. 

Figure 1-10. Multistage turboexpander in natural gas letdown servioe. (Source GHH-Borsig.)...

Figure 2-10. Atlas Copco expander, rated at 3,731 kW (5,000 hp), used for pressure letdown at a plant in Salionze, Italy.

CASE 15 GENERATING ELECTRICITY FROM EXCESS ENERGY WITH A LETDOWN GAS COMPRESSOR... 

Fuel System. An adequate knockout vessel should be provided for natural gas entering the plant as fuel or feed gas. Hydrocarbon liquids can and will enter the fuel system otherwise. Double-pressure letdown plus heating to preclude hydrates is also typically specified. 

The above criteria are a guide for detecting potential problems with gas letdown systems and apply for the first 90 m of piping downstream of the pressure reducer under concern. Systems with only liquid flow are not considered potential problems and need not be investigated. For systems with two phase flow, use the conservative assumption of the total mass flow rate as gas. Any system exceeding these criteria should be further evaluated. 

The maximum or minimum temperature attainable in a vessel can be limited by properly designed jacket heating systems. If steam heating is used, maximum temperatures can be limited by controlling steam pressure. A steam desuperheater may be needed to avoid excessive temperature of superheated steam from a pressure letdown station. 

When administering oxytocin intranasally to facilitate the letdown of milk, the nurse places the patient in an upright position, and with the squeeze bottle held upright, administers the prescribed number of sprays to one or both nostrils. The patient then waits 2 to 3 minutes before breastfeeding the infant or pumping the breasts. If a breast pump is being used, the nurse records the amount of milk pumped from the breasts. 

Table 8.47 shows the available options for the analysis of polymer processing aids, namely combustion and instrumental methods. The best method is dependent on PPA type, the level to be measured, and the available equipment (see also Section 8.2.1.2). Fluoropolymer processing aid concentrations can be determined by WDXRF configured to measure either fluorine or a tracer, and by EDXRF to analyse a tracer [29]. Calibration curves are required. At present, EDXRF or benchtop XRF units cannot directly measure fluorine. For resin or masterbatch producers who prefer to make on-line XRF measurements of processing aid concentrations (to letdown levels of 50-100 ppm), processing aids that contain a tracer (usually BaS04) are available. The analysis time is less than two minutes. 

After the activation period, the reactor temperature was decreased to 453 K, synthesis gas (H2 CO = 2 1) was introduced to the reactor, and the pressure was increased to 2.03 MPa (20.7 atm). The reactor temperature was increased to 493 K at a rate of 1 K/min, and the space velocity was maintained at 5 SL/h/gcat. The reaction products were continuously removed from the vapor space of the reactor and passed through two traps, a warm trap maintained at 373 K and a cold trap held at 273 K. The uncondensed vapor stream was reduced to atmospheric pressure through a letdown valve. The gas flow was measured using a wet test meter and analyzed by an online GC. The accumulated reactor liquid products were removed every 24 h by passing through a 2 pm sintered metal filter located below the liquid level in the CSTR. The conversions of CO and H2 were obtained by gas chromatography (GC) analysis (micro-GC equipped with thermal conductivity detectors) of the reactor exit gas mixture. The reaction products were collected in three traps maintained at different temperatures a hot trap (200°C), a warm trap (100°C), and a cold trap (0°C). The products were separated into different fractions (rewax, wax, oil, and aqueous) for quantification. However, the oil and wax fractions were mixed prior to GC analysis. 

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