Pressure 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 ... 

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. 

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. 

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

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 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. 

This solution is regenerated by pressure letdown and steam stripping of the solution (2 ). 

Gas pressure letdown Redundant control valves and pressure regulators... 

Liquid pressure letdown Redundant capillaries and control valves... 

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 effluent from the reactor contains only 5-20% methanol because the one-pass conversion is very low. After cooling and pressure letdown, the liquid methanol can be removed and further purified by distillation. The unreacted synthesis gas is compressed and recycled to the reactor. 

Inorganic salts and oxides may have corrosive properties that can damage reactor vessels, heat exchangers, pressure letdown devices, and heaters. 

Recommendation 3-la. The Army should develop criteria and a schedule for resolving design and operational issues raised in the 1998 report, Using Supercritical Water Oxidation to Treat Hydrolysate from VX Neutralization, that have not yet been resolved for supercritical water oxidation operation at Newport. These issues include materials of construction, fabrication methods, system plugging, pressure letdown, and the duration of successful continuous pilot-scale operations. 

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%. 

Slurry Mixing and Pumping Slurry Preheater Dissolver Fractionation Heat Exchange Pressure Letdown Gasification Oxygen Compression... 

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. 

Internal vessel heating and agitation add cost, but these capabilities are required to ensure uniform and efBcient contaminant removal. In addition, internal heating is required during pressure letdown to prevent atmospheric condensation on the parts when the vessel is opened. The parts may be below room temperature due to the cooling effect caused by the expansion of the carbon dioxide during depressurization. This is especially important if the cleaning is not performed in climate controlled atmospheres. 

Module 1 (Gas preconditioning) Gas cooling, pressure letdown Nozzle, valves, sampling lines... 

Sodium carbonate is a less costly absorbent, but also less efficient. Other acid gas removal techniques use physical absorption of carbon dioxide in organic solvents, such as the dimethyl ether of polyethylene glycol, or methanol (the Selexol or Rectisol processes). For effective absorption these require high gas pressures. Regeneration of solvent is done by pressure letdown plus some air stripping (sparging of air through the solvent). 

The low distribution coefficients, the attendant requirement of recycling CO2 containing very little ethanol in order to achieve a high recovery of ethanol from the feed stream, and the inability to achieve the separation of ethanol from the extract stream by pressure letdown required the development of this SCF extraction-distillation process. The diagram shown in figure 8.14 pictorially summarizes that an old distillation technique can be combined with new supercritical CO2 extraction to solve the separation problem supercritical CO2 can extract the ethanol from the feed stream, distillation can separate and regenerate the solvent for recycle, and vapor compression can achieve energy efficiency. 

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