Letdown valve

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. 

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. 

The pressure Pq before the pressure letdown valve is high enough to prevent any vaporization of feed at its temperature Tg and composition Xqj (mole fraction jth component). The forcing functions in this system are the feed temperature Tq, feed rate F, and feed composition x j. Adiabatic conditions (no heat losses) are assumed. The density of the liquid in the tank, is assumed to be a known function of temperature and composition. 

An equilibrium-flash calculation (using the same equations as in case A above) is made at each point in time to find the vapor and liquid flow rates and properties immediately after the pressure letdown valve (the variables with the primes F , F l, y], x j,.. . shown in Fig. 3.8). These two streams are then fed into the vapor and liquid phases. The equations describing the two phases will be similar to Eqs. (3.40) to (3.42) and (3.44) to (3.46) with the addition of (1) a multi-component vapor-liquid equilibrium equation to calculate Pi and (2) NC — 1 component continuity equations for each phase. Controller equations relating 1 to Fi and P to F complete the model. 

A, Find the optimum liquid concentration of the propane isobutane mixture in an auto lefrigerated alkylation reactor. The exothermic heat (10 Btu/h) of the alkylation reaction is removed by vaporization of the liquid in the reactor. The vapor is com pressed, condensed, and flashed back into the reactor through a pressure letdown valve. The reactor must operate at 50°F, and the compressed vapors must be condensed at 110°F. 

The system that experienced the failure had been modified within the previous decade. Instrumentation and control changes made it difficult for field crews to diagnose and troubleshoot operating problems. Process safety information had not been properly documented and there was litde evidence that the changes had been formally evaluated. A few days before the incident, a tag was installed on a faulty letdown valve instructing that a bypass must not be used. No explanation was given for this instruction. 

The refrigerant liquid partially flashes to a vapor as it flows through the letdown valve. The flashing represents the conversion of the sensible heat of the refrigerant to latent heat of vaporization. In Fig. 22.1, the refrigerant is chilled from 100 to 40°F. Approximately 25 percent of the liquid flashes to a vapor to provide this autorefrigeration. 

The compressor, motor, condenser, receiver, and letdown valve are all components of your home central air-conditioning unit. They are installed as a package, surrounded by the condenser, outside your house. The evaporator is located in your attic. To continue our description of the process flow ... 

The partially vaporized refrigerant flows into the evaporator. In Fig. 22.1, the evaporator shown is similar to a kettle-type reboiler (see Chap. 5). The process fluid flows through the tube side of the kettle evaporator. The refrigerant liquid level is maintained by the letdown valve. The refrigerant vapor flows from the top of the kettle, to the compressor suction. 

Now let s assume that there is a refrigerant receiver between the condenser and letdown valve. This traps any uncondensed vapors. The accumulation of these vapors raises the pressure in the receiver. This puts backpressure on the condenser. The higher condenser pressure promotes more complete condensation of the refrigerant vapors. 

If, in the heat pumps, the energy of compression is not recovered but is wasted in letdown valves (as the pressure of the working fluid is reduced to the low pressure of the evaporator (Joule-Thomson cycle), the liquefaction efficiency will be low (35-60%). This range of efficiencies is a function of the liquefier size and refrigerant used. If the letdown valves are replaced by turbo expanders (Brayton cycle), which recover some of the compression energy during pressure letdown, and if helium or neon refrigerants are used, the efficiency can theoretically reach 80-90%. 

A number of other optimization strategies will be discussed later in this chapter. The savings resulting from replacing the letdown valves with expander turbine generators will be discussed in Section 2.13. The optimization of multistage chillers will be covered in connection with hydrogen liquefaction in Section 2.15.3, and coolant distribution controls will be covered under pump optimization in Section 2.17.2. 

Experience indicates that an important part of a normal process development is definition of solutions to operability and reliability problems that have been identified. The EDS process development is no exception. Potential mechanical problems associated with feed slurry preheat, slurry pumping, high pressure letdown valves and vacuum bottoms pumping have been identified and will be addressed in the 250 T/D pilot plant program. In addition, several process problems associated with the variety of coals processed have been identified and solutions defined. The status of both pilot plant construction and definition of solutions to process problems is presented in this paper. 

It is important to correctly inject the inhibitor. Injecting the inhibitor upstream of a point of high turbulence such as a pump suction or ahead of a pump letdown valve has been recommended (373). An injection point a long distance upstream of the column should be avoided whenever possible (328). It is also important to disperse the inhibitor correctly. One case has been reported (50)... 

Liquid letdown valve (Badger) repair 11574 58 N 2 Mechanical Liquid letdown... 

No major changes have been made to the SCWO system, but there are some differences between the SCWO unit previously tested and the SCWO units proposed for EDS and for full-scale operation. Changes have also been made to equipment downstream of the SCWO reactor to facilitate processing of the suspended solids in the reactor effluent, especially for aluminum-rich feeds. The effluent flows from the pressure letdown valves to a knockout drum that contains a venturi scrubber, which separates liquid and suspended solids from the uncondensed vapor. The slurry is pumped to an evaporator/crystallizer system that replaces the flash separator in the original design. 

Path 2 21 klb/h of the letdown valve, FE j = FEhp = 1.56MMBtu/klb because a letdown is an adiabatic process and thus FE does not change through the letdown valve. 

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