Reject pressure

Hence, a theoretical reject pressure can be calculated from pressures measured in the field, together with a constant input from a ratio-cootrol unit. The theoretical pressure proportional to the reject flow rate is then used as the set point in the control loop. The measured variable pn, in the field can then be adjusted automatically by trimming a pressure-control valve in the reject line until the calculated pn) equals the measured pnj. 

Fig. 4 illustrates the two previously discussed pressure drops across the 35-mm (1.4-in.] hydrocyclone. The reject curve is based on a reject ratio of -1 %. A family of reject-pressure-drop curves can be generated for a range of reject ratios. Fig 5 illustrates a family of reject curves ranging from 0 to 3% for the 60-mm (2.4-in.] cyclones used on the Hutton TLP. 

When pressure is the driving force of the process, the gaseous molecules will be transported from the high-pressure side to the low-pressure side of the membrane (see Figure 10.2 [18,19]). The permeation cell (Figure 10.2) is coupled with two (or more) pressure transducers to measure the reject pressure, Ph and the permeate pressure, P2, and a mass flow meter (/ ) to measure the flux (./) passing through the membrane [18,19],... 

The P7T ratio is a process variable or parameter to be affixed by the operator. Furthermore, it can be assumed that Pl and Py are set by back-pressure controllers on gas streams L and V. The feed rate Emay be increased or reduced by a valve in the line (e.g., by a flow controller), where the upstream feed pressure is sufficiently high. Ordinarily, it would be set at a constant rate, at a fixed reject pressure Pj. ... 

Ancillary equipment Energy recovery for SWRO for systems larger than 40 m /h product water capacity reduces energy consumption by more than 40%. Energy recovery is not economical unless the reject pressure is >20 bar g and product water recovery is <80%. 

Therefore, P stands for the higher or upstream pressure (or reject pressure) at the membrane and Py, for the lower or downstream pressure (or permeate pressure), with the difference - P, denoting the pressure-drop across the membrane proper. The analogy is akin to that used for phase separations. Furthermore, the usage in the main appears self-evident in context. 

As an illustration of the conversion calculations involved, consider the information contained in Tables 2.1 and 2.2 for the n-heptane-isooctane system using a membrane of 1 mil thickness, for operating conditions of 100°C or 212°F (with liquid feed and vaporized permeate, constituting the composite behavior called pervaporation). The permeate flux was apparently found to be independent of the feed-reject pressure. This may or may not be contradictive. 

Interestingly, the same result for the diffusivity D- is obtained for inlet feed-reject pressures of 15 and 115 psig. 

These expressions are of the same form as those erived in Chapter 3, save G = V" and the mean pseudo reduced pressure (P ), in the numerator, is the feed-reject pressure P and, in the denominatoii is the permeate pressure... 

Since the experimental data show that K, = 75/50 = 1.5, there is obviously a contradiction with the previously derived relation, which requires that X, < 1. The inference is that the feed-reject pressure should be used in the numerator and the permeate pressure should be used in the denominator, where... 

Accordingly, the permeation pressures may be adjusted to accommodate this restriction, which becomes a trade-off between the degree of conversion and the upstream membrane feed-reject pressure P, for instance, which is also the pressure of the reacting system. (Note that, for the purposes here, a primitive numerical integration is used.)... 

Reverse osmosis is a high-pressure membrane separation process (20 to 100 bar) which can be used to reject dissolved inorganic salt or heavy metals. The concentrated waste material produced by membrane process should be recycled if possible but might require further treatment or disposal. 

The heavy vacuum bottoms stream is fed to a Flexicoking unit. This is a commercial (125,126) petroleum process that employs circulating fluidized beds at low (0.3 MPa (50 psi)) pressures and intermediate temperatures, ie, 480—650°C in the coker and 815—980°C in the gasifier, to produce high yields of hquids or gases from organic material present in the feed. Residual carbon is rejected with the ash from the gasifier fluidized bed. The total Hquid product is a blend of streams from Hquefaction and the Flexicoker. 

It foUows from these two equations that the water flux is proportional to the appHed pressure, but the salt flux is iadependent of pressure. This means the membrane becomes more selective as the pressure increases. Selectivity can be measured ia a number of ways, but conventionally, it is measured as the salt rejection coefficient, R, defined ia equation 6. 

Some data iEustrating the effect of pressure on the water and salt fluxes and the salt rejection of a good quaUty reverse osmosis membrane are shown ia Figure 34 (76). 

Propane and light ends are rejected by touting a portion of the compressor discharge to the depropanizer column. The reactor effluent is treated prior to debutanization to remove residual esters by means of acid and alkaline water washes. The deisobutanizer is designed to provide a high purity isobutane stream for recycle to the reactor, a sidecut normal butane stream, and a low vapor pressure alkylate product. 

Memhra.nes. Liquid separation via membranes, ie, reverse osmosis (qv), is used in production of pure water from seawater. The chief limit to broader use of reverse osmosis is the high pressure required as the concentration of reject rises. 

Mechanical Gleaning. A cleaner is a hydrocyclone device utilizing fluid pressure to create rotational fluid motion (20). Pulp is introduced tangentially near the top of the cleaner. Contaminants denser than water such as chemically treated toner inks and sand migrate toward the outer wall of the cleaner and exit in a separate (reject) stream. For most forward cleaners, optimal ink removal efficiency is obtained at a pulp consistency of 0.2—0.3%. Most forward cleaners deinking efficiency declines at pulp feed consistencies greater than 0.4%. However, a cleaner said to be efficient at 1.2% pulp consistency has been reported (39). 

The pressure difference between the high and low pressure sides of the membrane is denoted as AP the osmotic pressure difference across the membrane is defined as Att the net driving force for water transport across the membrane is AP — (tAtt, where O is the Staverman reflection coefficient and a = 1 means 100% solute rejection. The standardized terminology recommended for use to describe pressure-driven membrane processes, including that for reverse osmosis, has been reviewed (24). 

When it was recognized (31) that the SD model does not explain the negative solute rejections found for some organics, the extended solution—diffusion model was formulated. The SD model does not take into account possible pressure dependence of the solute chemical potential which, although negligible for inorganic salt solutions, can be important for organic solutes (28,29). 

Using this simplified model, CP simulations can be performed easily as a function of solution and such operating variables as pressure, temperature, and flow rate, usiag software packages such as Mathcad. Solution of the CP equation (eq. 8) along with the solution—diffusion transport equations (eqs. 5 and 6) allow the prediction of CP, rejection, and permeate flux as a function of the Reynolds number, Ke. To faciUtate these calculations, the foUowiag data and correlations can be used (/) for mass-transfer correlation, the Sherwood number, Sb, is defined as Sh = 0.04 S c , where Sc is the Schmidt... 

---