Flow Profile for Reciprocating Pumps

Reciprocating pumps are often used in the chemical process industries (CPI) because of their ability to generate high pressures at low velocities. A subcategory of positive-displacement pumps, reciprocating pumps act through the recipricating motion of a piston, plunger or diaphragm. Such pumps work by way of a connecting-rod-and-crank mechanism with a piston.

By nature, reciprocating pumps generate pulsing flow, which, when plotted as a function of time, or of crank angle, produces a curve that resembles a sine wave to a first approximation. For example, manufacturers of pulsation dampeners and surge suppressors often use sinusoidal curves for piston pumps and compressors in their product literature and sizing formulas. However, a closer examination of the flow profile for a piston-and-crank pump or compressor reveals the curve to be a significantly distorted sine wave because of the interaction between the crank and the connecting rod.

Calculating flowrate

In graphical form, the crank and crankshaft of a reciprocating pump can be visualized by placing the crankshaft center at the 90-deg mark of a 180-deg x-axis, and placing the crank bearing at the origin (see figure). A connecting rod links the crank to the piston.

Determining the position of the piston at any crank angle can be accomplished by measuring on a piston pump, compressor, or piston engine, or it can be calculated using trigonometric relationships.

The degree to which the actual flow profile curve deviates from the sinusoidal curve is determined by the ratio of the connecting rod length to the crankshaft length. Smaller values of the ratio translate into greater levels of distortion. As the connecting rod becomes very long, the flow profile would approach the sine curve .

To calculate the flowrate at a given crank angle, use the following procedure and definitions:

Crank length = OC

Piston rod length = CP

For any angle a, Line AC = OCsin (a)

Line SA = OC – OC cos (a)

Line AP = (CP² – AC²)^0.5

Line SP = AP + SA

1.     Calculate the piston position for two crank angles, perhaps 2 deg apart.

2.   The difference in piston positions equals piston displacement over the time interval between the two crank angles. The value is an average over the span of the two readings, not an instantaneous reading. As the step size approaches zero, displacement nears the true velocity.

3.    This value can be converted into flowrates (gal/min or other units) if the piston diameter and speed (revolutions per minute, rpm) are known.

Observations of the plot

In an illustrative example, plots of piston velocity versus crank angle are shown (see graph). The ratios of the connecting rod length to crank shaft length are 1.05 to 1 (blue line), 2 to 1 (red line) and 5 to 1 (green line). The following observations can be made:

1.    At the beginning of the discharge stroke, flowrate approaches zero asympotically, rather than as a sinusoidal curve

2.     Peak flowrates do not occur at the 90-deg point, but rather at 95–120 deg, depending on the ratio of rod length to crank length

3.     Peak flowrates are higher than would be predicted with a pure sine curve

4.     From 180 to 360 deg (the suction portion of the pump cycle), the curve mirrors the 0-to-180-deg portion

5.     Flowrates during the suction portion of the curve are also higher and occur earlier than the 270-deg point

Effects of distorted sine curve

Within the areas of fluid flow and mechanical pump design, there are a number of aspects that are affected by the deviation of flow profile from a perfect sine curve for pumps and compressors. The effects include the following:

•       Check valves and passages will have higher-than-predicted peak flowrates and pressure drop will be higher, by the square of flowrate

•   The higher flowrates and pressure drops will affect net positive suction head (NPSH) and possibly induce vaporization

•    Maximum crank revolutions per minute will be lower than what would be allowed by the pure (non-distorted) sinusoidal curve

•       Loads experienced by bearings will increase somewhat, especially in high-speed compressors

•       Stress analysis of the connecting rods will be affected

•       Surge dampeners must handle the sharper peak of a bell curve, rather than a smoother sine curve

•       Multi-piston pumps and compressors would have less “smoothing” effect than would be predicted because the bell-shaped curve has a sharper peak

References

1.     McGuire, J.T., “Pumps for Chemical Processing,” Marcel Dekkar, New York, 1990.

2.     Henshaw, T.E., “Reciprocating Pumps,” Van Nostrand Reinhold Co., New York, 1987.

3.     Krugler, A., Piston Pumps and Compressors: Exploring the Flow Profile, Self-published, 2010.

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Evaluation of Crude Oil Quality

Abstract

Fourteen type crude oils originated from USA, Mexico, Africa, Middle East, Russia, Canada, Colombia, Ecuador, and Venezuela having density and sulfur in the range API = 12.1 ÷ 40.8; S =  0.4 ÷ 3.3%  and total acid number varying in the range TAN = 0.1 ÷ 3.72 mg KOH/g oil have been investigated. The studied crude oils have been classified into four groups: I group – light, low sulfur one (30 – 40 ° API; S ≤ 0.5 % mass); ІІ group – light, sulfur one (30-40 °API; S= 0, 5 - 1.5 % mass);  ІІІ group – heavy, high sulfur one (15-30 °API; S=1.5 ÷ 3.1% mass);  ІV group – extra-heavy, high sulfur one (15 °API, S ≥3 % mass). It has been established that extra-heavy crude oils (IV group) are characterized by light fraction low content, diesel fractions low cetane index, vacuum gas oil fractions low K-factor and vacuum residue fractions high Conradson carbon content. It also has been found on the base of crude oil averaged prices for June 2009 (Brent crude oil price = 69 US $/ barrel) that the difference of the I^st  and IV^th  group crude oil prices was about 9 US $/ barrel. This difference amounts up to 22 US $/ barrel, as the crude oil price rises up to 140 US $/ barrel. The high acid crude oil price (such having TAN > 0.5 mg KOH/g oil) may be approximately 9 US $/ barrel lower than one determined on the base of density and sulfur content for the corresponding group.  

Key words: opportunity crudes, crude value, high acid crudes, crude oil quality

Introduction

Crude cost is the single most important determinant for the profitability of an oil company. With crude costs accounting for around 80% of the refinery expenditures, processing cheaper crudes can have a very positive impact on refinery margins.

For refineries that have freedom in crude choice, the selection of an optimum crude package is of vital importance. This requires intense teamwork between the trader and the supply/ manufacturing economist and typically linear-program (LP) models of  individual refineries are routinely used to determine the relative use values among crudes. To achieve optimal crude selection and processing decisions, a refiner must have exact information refer to crude oil quality. This includes: crude oil TBP-curve as main data for correct operation of refinery crude oil atmospheric-vacuum distillation plants; the characteristics of crude oil fraction: 

Naphtha fraction: 

density, naphthenes and arene content, octane number, sulfur and metal as lead and arsenic content, affecting isomerization and reforming plants operation; 

Kerosene fraction: 

density, sulfur, pour point, freezing point, arene content (for aviation fuel); cetane index, low temperature properties (CFPP), pour point and diesel and fuel oil blending viscosity; 

Diesel fraction:

density, sulfur, arene content, cetane index, pour point, low temperature properties (CFPP), diesel and fuel oil blending viscosity; 

Heavy gas oil:

UOP “К” factor or hydrogen content, nitrogen and Conradson carbon quality characteristics of this fraction as feed for Fluid Catalytic Cracking and other conversion processes;  

Atmospheric residue and vacuum gas oils: 

density, pour point, sulfur, viscosity, metals for fuel oil blending; UOP “К” factor or content of hydrogen, nitrogen and Conradson carbon as suitability criteria for conversion processing (Fluid Catalytic Cracking and Hydrocracking) 

Vacuum residue: 

density, pour point, sulfur, viscosity, metals for fuel oil blending; UOP “К” factor or hydrogen, nitrogen  and Conradson carbon content as suitability criteria for their processing by the conversion processes (Catalytic cracking and Hydrocracking).

In practice, data is also needed for additional cuts in order to generate property profiles as a basis for recutting to actual refinery straight-run products. For example: the increase demand of diesel in Europe drives the European refiners to cut naphtha fractions and in this way to increase diesel yield. This information may be obtained by use of standard laboratory test methods normally API or ASTM. One extensive laboratory analysis of crude oil may cost over 20 000 US $ and generally it takes two to four weeks. In practice it is extraordinary difficult and very expensive to carry out full laboratory analysis of every cargo crude oil received at the refinery. This has resulted in development of a number computing methods  that permit prediction of petroleum fractions properties by routine laboratory analyses. These methods require information about petroleum fractions distillation characteristics, density, sulfur content, viscosity and refraction. This information may be obtained only after crude oil TBP distillation, analysis which duration is not less than 24 hours. Another important information of crude oil quality is organic acid content since organic acid presence at high concentration may generate number of problems at equipment operation and especially at crude oil atmospheric-vacuum distillation plants via their acid high corrosion activity. The crude oil total acid number is an indicator of the organic acids content in the crude oil. It is well known that problem crude oils are those which total acid number is over 0.5 mg KOH/g oil. But not always low acid number of crude oil feed means that it is not possible corrosion problem to occur due to the presence of naphthenic acids in definite petroleum fractions. Such cases exist when definite fraction has very low sulfur content and presence of organic acids. Sulfur presence result to formation of protective layer of ferric sulfide that renders difficult organic acid attack of the metal and by that manner the equipment accelerated corrosion is reduced. Because of lower prices of high acid crude oil feeds (total acid  number over 0.5 mg KOH/g crude oil) their processing is one opportunity to increase crude oil processing profit after use of adequate programs for minimizing the  risk of accelerated corrosion and subsequent equipment untimely, unplanned damage.

There are basically four types of crude available to refiners around the world. They are light-sweet (30-40 °API, <0,5 wt% S), light-sour (30-40 °API, 0.5-1.5 wt% S ), heavysour ( 15-30 °API, 1.5-3.1 wt% S) and extra-heavy (<15 °API and >3 wt% S ). High acid crude oils (HACs) represent the fastest-growing segment of global oil production.

California, Brasil, North Sea, Russia, China, India and West Africa are known to supply HACS.  Over half of the world’s oil supply is heavy and sour. Hence, synthetic petroleum feeds derived from bitumen sands are considered as high acid ones. Table 1 represents comprehensive assay of fourteen crudes which belong to the four basic type petroleum feeds. The crude comprehensive assay of all investigated crude oils was obtained in the Lukoil Neftochim Research Laboratory. Price of the crudes was obtained from the Energy Information Agency (EIA) and this price was related to June 2009. These data show that for June month 2009 the average prices of four type basic petroleum feeds are as follow: 

-         І group – light low sulfur ones (30-40 °API; S ≤ 0.5 % mass) = 68.5 US $ / barrel

-         ІІ group – light sulfur ones (30-40 °API; S=0,5  1.5 % mass) = 67.5 US $ / barrel

-         ІІІ group – heavy, high sulfur (15-30 °API S=1.5 ÷ 3.1% mass) = 66.8 US $ / barrel

-         ІV group – extra-heavy high sulfur (15 °API, S ≥3 % mass)= 60.0 US $ / barrel 

Here in, it is seen that the price of light, low sulfur crude oil from Louisiana nevertheless of its low sulfur content and low density (High API) according to which itshould be applied to first group crude oils, is equal to that of the lowest quality crude oils of fourth group. This may be explained by the high acid number of the crude oil that in combination with low sulfur content means high corrosion reactivity and so unfavorable feed for processing. The price of EMERAUDE (origin – Congo) and TIA JUANA PESADO (origin Venezuela) crude oils is not included in Table 1 as they are not available on the market due to their extra high acid number. The crude oil light fraction (distilled up to 343 °C) content decreases in the following order: I group > II group > III group > IV group. Vacuum residue content decreases in reverse sequence. The average characteristics of the fractions derived from crude oil groups included in Table 1 show (Table 2) that IV group crude oil naphtha has the highest octane number and diesel fractions of the same crude oils have the lowest cetane number that corresponds to the conclusions drawn up by other authors.

Table 2 Average characteristics of the fractions derived from crude oils presented in Table 1

Vacuum gas oil fraction from group IV has the lowest K-factor and due to this at conversion processes from them will be produced the lowest yields of valuable products like naphtha, diesel fraction and C3 and C4 alkenes. The IV group crude oil vacuum residues have the highest Conradson carbon and so they are unfavorable feeds for catalytic conversion processes. They are suitable for processes like cocking at which as by product is produced low valuable coke. All of these characteristics show that from IV group crude oils may be produced the lowest  yields of high valuable transport fuels.

Similarly, they require higher costs for processing due to required high degree of upgrading at hydrotreating processes and higher consumption of high prices hydrogen. The difference in prices between group I and group IV crude oils has been 8.5 US $ / barrel on June 2009, but as a whole this difference depends on the crude oil price. The plot of Brent type crude oil price change for period 1997 – 2009 is shown on Figure 1. It may be seen from it that difference of prices between high quality crude oils (low density and low sulfur content) and low quality crude oils (high density and high sulfur content) increases along with the increase of crude oil price. For example at crude oil price of order 30 US $ / barrel the difference between Brent и Maya crude oils is about 30 US $ / barrel and at 140 US $ / barrel it is already 20 US $ / barrel. Hence, as higher is the crude oil price so more advantageous is to process low quality crude oils.

Figure 1 Crude Brent price and difference in prices of crudes Brent and Maya in the period 1997 - 2209 (Source: Energy Information Administration)

Conclusions

The following conclusions may be drawn up as a result of the carried out investigation: 

1.    The high quality crude oils (low density and low sulfur content) are characterized by light fractions high content, diesel fractions high cetane index, vacuum gas oils high Kfactor and vacuum residue fractions low Conradson carbon content.  

2.    Low quality crude oils (high density and high sulfur content) are characterized by light fractions low content, diesel fractions low cetane index, vacuum gas oils low K-factor and vacuum residue fractions high Conradson carbon content. 

3.     The difference between the high quality and low quality crude oils increases with the crude oils price growth. 

4.   At high crude oil prices the most profitable will be refineries having available heavy petroleum residues conversion plants that allow them to process low quality crude oils to high valuable transport fuels. 

5.     The high acid number and low sulfur content of crude oil results to its price decrease by about 9 US $/ barrel at crude oil price about 69 US $/ barrel. 

6.     It is proved the conclusion drawn up by other authors that high acid crude oil diesel fractions have low cetane index. 

D. Stratiev, R.Dinkov, K. Petkov, K. Stanulov

Lukoil Neftochim Bourgas, 8010 Bourgas, Bulgaria University of Chemical Technology and Metallurgy - Sofia, 1756 Sofia, Bulgaria, e-mail stratiev.dicho@neftochim.bg

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Merits of Hydrogen Production/Consumption Optimization for a Hydrocracking-Reforming Complex

For many refiners, the hydrocracking unit is a key component in refinery optimization. Proper selection of catalysts for this unit is critical but not often straightforward.

For hydrocracking units where the most important product is gasoline, catalyst selection decisions are complicated by hydrogen utilization issues. The traditional logic for selecting hydrocracking catalysts based on activity, stability, yield pattern and cost may not provide an optimum solution if the decision-maker’s view is confined only to the hydrocracking unit. If the optimization calculations are based only on this unit, yield improvements achieved through volume swell could incorrectly influence the decisions.

Most hydrocracking units which produce maximum naphtha product slates, operate in conjunction with a reforming unit. UOP has found it very useful to consider the combined reformer/hydrocracking unit performance when designing hydrocracking catalyst configurations for these refiners. The results of this type of optimization are interesting and sometimes surprising.

Our analysis has shown that judgements must include consideration of hydrocracking unit yields and product quality plus reformer severity. If heavy naphtha aromatics saturation increases in the hydrocracking unit, required severity and resulting yields from the reforming unit are impacted. To some extent, hydrogen is being added to this fraction in one unit and the severity in the reformer must be increased to remove the added hydrogen.

Relative to the base case, saturation of the heavy naphtha shows significant negative impacts on octane barrels for cases where the there was no gain in heavy naphtha yield or where the gain in heavy naphtha was offset by decreases in light naphtha. A positive result is seen only when the gain in heavy naphtha is offset by butane and lighter losses.

Optimization for all aspects of refining is becoming more and more critical. As an important component in the overall optimization, catalyst selection for hydrocracking units will achieve the highest value for the refiner if the reformer/hydrocracking complex is taken into consideration as a whole. So, delivering this broad-scope optimized solution requires both catalyst and process expertise.

by Don Ackelson, Catalysts and Advanced Materials

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