Liquid-stream-driven process

Figure 15.8 The liquid-stream-driven process (LSDP) principle.

Liquid fixrm the process stream is sampled by a two position, six port rotary valve with a 5-20 pL sample loop connected between two ports. The sample may be preconditioned to remove any gas bubbles, entrained particulates, and water. Since this part of the instrument is outside the enclosures and could be in an explosive atmosphere, all valves are pneumatically operated, and the sample is driven by compressed air or inert gas. 

Further downstream, in order to concentrate and purify the product more, it is possible to use membrane filtration. Here, some form of semi-permeable membrane is used to separate the components of a liquid stream. In most of the commercially important processes the driving force is pressure, the solvent (usually water) is driven through the membrane while the solute(s) are retained. This type of process includes reverse osmosis, ultrafiltration and microfiltration. 

Extraction from Aqueous Solutions Critical Fluid Technologies, Inc. has developed a continuous countercurrent extraction process based on a 0.5-oy 10-m column to extract residual organic solvents such as trichloroethylene, methylene chloride, benzene, and chloroform from industrial wastewater streams. Typical solvents include supercritical CO9 and near-critical propane. The economics of these processes are largely driven by the hydrophihcity of the product, which has a large influence on the distribution coefficient. For example, at 16°C, the partition coefficient between liquid CO9 and water is 0.4 for methanol, 1.8 for /i-butanol, and 31 for /i-heptanol. 

The lack of selectivity can be circumvented by coupling a postcolumn flow system to a liquid chromatograph. This has promoted the development of a number of efficient liquid chromatography-CL approaches [16, 17]. Eluted analytes are mixed with streams of the substrate and oxidant (in the presence or absence of a catalyst or inhibitor) and the mixed stream is driven to a planar coiled flow cell [18] or sandwich membrane cell [19] in an assembly similar to those of flow injection-CL systems. Many of these postcolumn flow systems are based on an energy-transfer CL process [20], In others, the analytes are mixtures of metal ions and the luminol-hydrogen peroxide system is used to generate the luminescence [21],... 

When a generator is driven by an HPRT on a gas-rich process stream, the generator should be generously sized. The output power of HPRTs can be as much as 20 to 30 percent or more over that predicted by water tests, as a result of the effects of evolved gas or flashed liquid. 

Many processes are heat driven, take place at elevated temperatures, or require product drying. As a result, process heaters and dryers are common equipment in processing facilities. Many of these are fired units fueled by a variety of gas or liquid fuels frequently by natural gas. They may be used to heat a process stream directly, to heat an intermediate heat transfer fluid, or to... 

The need to obtain greater recoveries of the C2, C3, and C/s in natural gas has resulted in the expanded use of low-temperature processing of these streams. The majority of the natural gas processing at low temperatures to recover light hydrocarbons is now accomplished using the turboexpander cycle. Feed gas is normally available from 1 to 10 MPa. The gas is first dehydrated to a dew point of 200 K and lower. After dehydration the feed is cooled with cold residue gas. Liquid produced at this point is separated before entering the expander and sent to the condensate stabilizer. The gas from the separator is expanded in a turboexpander where the exit stream can contain as much as 20 wt % liquid. This two-phase mixture is sent to the top section of the stabilizer which separates the two phases. The liquid is used as reflux in this unit while the cold gas exchanges heat with the fresh feed and is recompressed by the expander-driven compressor. Many variations to this cycle are possible and have been used in actual plants. 

Removal of valuable elements (for example, uranium) from waste streams via liquid membranes - a diffusion mechanism process driven by pH differential. 

Have you ever driven past a refinery and wondered what happens in those tall towers Some of them are distillation towers that are used to separate a mixture of chemicals into two or more streams, each a relatively pure stream of one of the chemicals. The physical process governing that separation is vapor-liquid equilibrium. It has been estimated that 10 percent of the energy used commercially in the United States is used in distillation. Thus, it is important to make this process as efficient as possible. 

The distinction is best made clear by means of an example Take the case of the chemical reaction which occurs between water and sulphuric acid Let us think of an apparatus similar to that indicated in Fig s In one vessel, A, there is a quantity of liquid water, and m contact with it some saturated vapour at pressure p0 The vapour fills the space on the left-hand side of the tap C In the vessel B there is some concentrated sulphuric acid, that is acid containing a little water, and above this acid is some vapour in equilibrium with the water in this sulphuric acid mixture The partial pressure of the water vapour is here p, where p is much less than pa This water vapour at low pressure (along with some sulphuric acid vapour which does not come into the calculation) occupies the space on the right of the tap C If we simply open the tap, water vapour would stream from left to right, that is from the region of high pressure p0 to that of low pressure p If a piston were placed in the tube it would be driven at a speed not by any means infinitely slowly, and the pressure difference on the two sides of the piston would be finite, 1 e (p0 - p ) This process, which is the spontaneous one, is an irreversible one, since the piston is not made to move infinitely slowly with infinitely small pressure difference on the two sides... 

Vapor permeation and pervaporation are membrane separation processes that employ dense, non-porous membranes for the selective separation of dilute solutes from a vapor or liquid bulk, respectively, into a solute-enriched vapor phase. The separation concept of vapor permeation and pervaporation is based on the molecular interaction between the feed components and the dense membrane, unlike some pressure-driven membrane processes such as microfiltration, whose general separation mechanism is primarily based on size-exclusion. Hence, the membrane serves as a selective transport barrier during the permeation of solutes from the feed (upstream) phase to the downstream phase and, in this way, possesses an additional selectivity (permselectivity) compared to evaporative techniques, such as distillation (see Chapter 3.1). This is an advantage when, for example, a feed stream consists of an azeotrope that, by definition, caimot be further separated by distillation. Introducing a permselective membrane barrier through which separation is controlled by solute-membrane interactions rather than those dominating the vapor-liquid equilibrium, such an evaporative separation problem can be overcome without the need for external aids such as entrainers. The most common example for such an application is the dehydration of ethanol. 

Ink-jet describes small drops of ink driven to the substrate, usually by an electrostatic field. The drops are formed by a rapid pressure pulse in a small, nearly enclosed chamber. When the pressure increases, a small amount of liquid is ejected from the chamber through a nozzle (the only exit). When the pressure in the chamber decreases again, that liquid is separated from the bulk of the liquid in the chamber by surface tension in a process called pinch-off. The pressure pulse is commonly generated by a piezoelectric material or by heating the liquid via current through resistive material embedded in the wall of the chamber. The ejection process usually leaves the drop with insufficient velocity to drive it to the substrate, so another force, such as an electrostatic field, is used to complete the delivery. The result is a rapid stream of successive drops, controlled electronically. 

---