WO1992019360A1

WO1992019360A1 - Membrane process for treating pump exhausts

Info

Publication number: WO1992019360A1
Application number: PCT/US1992/003807
Authority: WO
Inventors: Richard W. Baker; Jürgen Kaschemekat
Original assignee: Membrane Technology And Research, Inc.
Priority date: 1991-05-06
Filing date: 1992-05-01
Publication date: 1992-11-12
Also published as: CA2102304A1; US5127926A

Abstract

A process for removing an undesirable vapor from a pump exhaust (5) by means of a membrane separation process (6). The process takes advantage of the pressure difference between the inlet (2) and outlet (3) sides of the pump (1) to provide the driving force for membrane permeation. The process of the invention can reduce the emission of pollutants or impurities from pumps to a very low level, such as 10 % or less of its previous value, with little extra expenditure of energy.

Description

MEMBRANE PROCESS FOR TREATING PUMP EXHAUSTS

FIELD OF THE INVENTION

The invention ^elates to removing undesirable components from pump exhausts.

BACKGROUND OF THE INVENTION

Pumps pervade industry. They are used to transfer fluids for every imaginable

use, and to raise or lower the fluid pressure as appropriate. The common factor in all

pumping is that there is a suction, or low-pressure, zone on the inlet side and a

compression, or high-pressure, zone on the outlet side. On the outlet side, the fluid

may pass on to another destination, or it may be a waste stream that is discharged.

The fluid stream passing through the pump may be gas or liquid. Gas emissions

from pumps are often contaminated with vapors whose release to the atmosphere is

environmentally undesirable or unacceptable, such as hydrocarbons, aromatic

hydrocarbons, chlorinated hydrocarbons, fluorinated hydrocarbons, ammonia, sulfur

dioxide or hydrogen sulfide. Likewise, when the outlet stream from the pump is to

be passed to some other operation, not vented, it may contain vapor impurities the

removal of which from the stream would be advantageous.

An undesirable component may be present in the gas on the inlet side of the

pump, or it may be picked up as the gas goes through the pump. One specific

instance of this is when the pump is a liquid-ring pump. In liquid-ring pumps, the

pumping action is provided by a ring of liquid sealant or compressant. This ring may be water or some other liquid. The choice of liquid sealant is vast, including 'oils,

hydrocarbons, chlorinated hydrocarbons, alcohols and inorganic liquids. Depending

on vapor pressure and other factors, some sealant vapor will always be present in the

outlet gas.

No cheap, reliable technology exists to treat pump outlet streams to an adequate

level. Many pumps are already equipped with an external condenser that treats the

exit stream to remove pollutants. Nevertheless, there is a practical limit, usually set

by the process economics, on the amount of the pollutant that can be removed. There

remains a need for economical ways of reducing the pollution caused by pump

emissions, and for treating internal process streams passing through pumps.

SUMMARY OF THE INVENTION

The invention is a process for removing undesirable components from gas streams

that pass through pumps. The process takes advantage of the one characteristic

common to all pumps: the pressure difference between the inlet and outlet sides.

The invention involves removing the undesirable component by a membrane

separation process. In membrane separation, a feed gas mixture is passed across the

surface of a membrane through which two components of the mixture permeate at

different rates, enabling a separation between the components to be made. Gas flow

through the membrane is maintained by providing a driving force, often in the form of a pressure difference between the feed and permeate sides. As part of the

membrane separation apparatus, it is known to include a vacuum pump to lower the

pressure on the permeate side, or a compressor to raise the pressure on the feed side,

or both.

What is recognized in the present invention is that where a gas stream containing

an undesirable vapor passes, for whatever reason, through a pump, this presents an

excellent opportunity to remove the vapor cheaply, efficiently and without having to

expend large amounts of extra energy, by simply connecting a membrane unit between

the outlet and inlet sides of the pump to form a pump/membrane loop, and using the

pressure difference as the driving force for the membrane separation process. If the

membrane is preferentially permeable to the undesirable vapor, vapor will be removed

from the feed side and concentrated on the permeate side. The residue stream leaving

the feed side of the membrane may pass on downstream to wherever its destination

would have been absent the membrane system. However, the removal of undesirable

vapor achieved by the membrane means that the need for a separate treatment system

downstream, or for complex pooling of exhaust streams for pollution control, is

obviated or at least substantially reduced.

Vapor can be removed from the pump/membrane loop at any point by

condensation, absorption or any other convenient method. The process of the

invention can reduce the emission of pollutants or impurities from pumps to a low

level, such as 10% or less of its previous value, with little extra expenditure of energy. In cases where the pollutant is a high-value chemical, the operating cost may be

completely offset by the value of the recovered chemical.

It is an object oY the invention to provide a process for removing an undesirable

vapor from a pump exhaust stream.

It is an object of the invention to reduce atmospheric emissions of pollutants in

pump exhausts.

It is an object of the invention to recover organic compounds from pump

exhausts.

It is an object of the invention to recover pump sealant.

Other objects and advantages of the invention will be apparent from the

description of the invention to those of ordinary skill in the art.

It is to be understood that the above summary and the following detailed

description are intended to explain and illustrate the invention without restricting its

scope.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic drawing showing a basic embodiment of the invention with

optional placings for the removal unit.

Figure 2 is a schematic drawing of a xylene removal process.

Figure 3 is a schematic drawing of a perchloroethylene removal process.

Figure 4 is a schematic drawing of a CFC-113 removal process.

DETAILED DESCRIPTION OF THE INVENTION

The process of the invention involves removing vapor from a gas stream that

passes through a pump. The gas stream may contain the vapor before entering the

pump or may pick it up within the pump. The vapor may be an organic compound

or mixture of compounds, such as a hydrocarbon, an aromatic hydrocarbon, a

halogenated hydrocarbon or the like, or an inorganic compound, such as water, sulfur

dioxide, ammonia, etc. The other component or components of the feed gas stream

may be other vapors, nitrogen, air or any other gas.

The removal process involves membrane separation. The membrane used to

perform the separation may be a homogeneous membrane, a membrane incorporating

a gel or liquid layer, or any other type known in the art that can function with a

pressure difference as the driving force. Two types of membrane are preferred. The

first is a composite membrane, comprising a microporous support, onto which the

permselective layer is deposited as an ultrathin coating. The second is an asymmetric

membrane in which the thin, dense skin of the asymmetric membrane is the permselective layer. Both composite and asymmetric membranes are known in the^" art. References that teach the production of such membranes include U.S. Patents

2,243,701; 4,553,983; 4,230,463; and 4,840,646.

The form in which the membranes are used in the invention is not critical. They

may be used, for example, as flat sheets or discs, coated hollow fibers, or spiral-

wound modules, all forms that are known in the art. Spiral-wound modules are a

convenient choice. References that teach the preparation of spiral- wound modules

are S.S. Kremen, "Technology and Engineering of ROGA Spiral Wound Reverse

Osmosis Membrane Modules", in Reverse Osmosis and Synthetic Membranes.

S.Sourirajan (Ed.), National Research Council of Canada, Ottawa, 1977; and U.S.

Patent 4,553,983, column 10, lines 40-60. Alternatively the membranes may be

configured as microporous hollow fibers coated with the permselective polymer

material and then potted into a module.

A basic embodiment of the invention is shown in Figure 1. Referring to this

figure, numeral 1 refers generally to any gas pump, such as a vacuum pump or

compressor of any type: a rotary vane pump, a liquid ring pump, a diaphragm pump,

a piston pump, a jet ejector, etc. Gas stream 4 is sucked into the pump at low-

pressure side 2 and exits as stream 5 at high-pressure side 3. Stream 5 is a gas mixture

containing an undesirable vapor. Stream 5 passes in whole or in part to membrane

separation unit 6, containing one or more membranes. Any portion of stream 5 that

is not passed to the membrane unit may be discharged, or may pass on to any destination downstream or upstream of the pump. The membrane separation step

normally involves running the feed gas stream across a. membrane that is selectively

permeable to the vapor that is to be removed. The vapor is concentrated in permeate

stream 7; residue stream 8 is correspondingly depleted in vapor. Residue stream 8

may be vented to the atmosphere, may pass on as an internal process stream or may

be recirculated. For example, if the pump were drawing a vacuum on a chamber or

container, it may be appropriate to return stream 8 to the container. Numeral 9 refers

to a removal unit for removing the undesirable component from the pump/membrane

loop. The removal unit will most generally be a condenser or an absorption unit and

may be placed before or after the membrane unit as convenient. If the pump is a

liquid-ring pump, condensation or absorption may take place within the pump, so that

no removal unit is needed. Pollutant is withdrawn from the pump/membrane loop as

stream 10 and can be used elsewhere, recycled, subjected to additional treatment or

disposed of as appropriate.

A number of factors have an effect on the performance of the membrane process.

Important parameters include the selectivity of the membrane, the ratio of the

permeate and feed pressures, and the ratio of the permeate and feed flows.

To separate the components of the gas stream requires a permselective membrane

that is preferentially permeable to one component over the others. The mathematical

model used to predict permeation behavior is the solution-diffusion model. In simple

systems, where the rate-limiting step is diffusion through the membrane, Fick 's Law of diffusion leads to the equation

J - DkΔp , (I)

_

where J is the membrane flux (cm^s(STP)/cm² -s-crnHg), D is the diffusion coefficient

of the gas or vapor in the membrane (cm²/sec) and is a measure of the gas mobility,

t is the membrane thickness, k is the Henry's law sorption coefficient linking the

concentration of the gas or vapor in the membrane material to the pressure in the

adjacent gas (cm^s(STP)/cm^s-cmHg), and Δp is the pressure difference across the

membrane. The product Dk can also be expressed as the permeability, P, a measure

of the rate at which a particular gas or vapor moves through a membrane of standard

thickness (1 cm) under a standard pressure difference (1 cmHg).

A measure of the ability of a membrane to separate two components, (1) and (2),

of a feedstream is the ratio of their permeabilities, α, called the membrane selectivity,

_i/l (2)

A second factor affecting the performance of a membrane system is the ratio, φ,

of the feed pressure to the permeate pressure. Transport of a component through the

membrane will stop if the partial pressure of that component on the permeate side of

the membrane exceeds the partial pressure on the feed side. The relationship between

pressure ratio and selectivity can be derived from Fick 's law and is discussed in detail in an article by K.-V. Peinemann et al. in AIChE Symposium Series number 250, Vol.

82 (1986). When the pressure ratio, φ, is much greater than the membrane selectivity,

, the permeate concentration is proportional to the membrane selectivity and is

essentially independent of the pressure ratio. When the pressure ratio is much smaller

than the membrane selectivity, the permeate concentration of a component is

proportional to the pressure ratio and is essentially independent of the membrane

selectivity. In the intermediate range, both the pressure ratio and the membrane

selectivity affect the membrane system performance. Although the separation

achieved always increases as the membrane selectivity increases, there is a point for

any given pressure ratio at which further increases in selectivity are relatively

unimportant.

Pressure ratios that may be encountered in a pumping operation vary enormously,

depending on the type of pump and the use to which it is put. In vacuum operations,

pressure ratios of the order 10 or more are common. In this case a membrane

selectivity of at least 5, more preferably 10 and most preferably 20 is desirable. On

the other hand, if the pressure ratio is as low as 2, 3 or 4, as may be the case in a

compression operation, a membrane selectivity of 5 may be adequate and the benefits

of a higher membrane selectivity, such as 10 or 20, may be negligible.

The process of the invention can be designed to achieve high levels of removal,

such as 70%, 80%, 90% or greater, of the undesirable component of the pump exhaust.

If a small membrane unit is used to treat a large gas stream, the fraction of the pollutant removed from the stream will be small. As the membrane area is increased,

the fractional removal will increase. However, the quantity of permeate gas that must

be recycled through the pump will also increase, increasing the energy consumption

of the pump. In general, it is possible to achieve 90% removal of pollutant while

holding the recycle stream to no more than 10-20% of the feed stream to the

membrane unit. If a greater degree of removal is required, the recycle stream will be

larger; if a lesser degree of removal is adequate, it may be smaller. To achieve 99%

removal, for example, may require recycle of 20%, 30% or 40% of the feed stream.

To achieve 80% may require only 5% recycle. The percentage recycle is also called

the stage cut.

Figure 1 shows one membrane module. It will be apparent, however, that the

process could be carried out with an array of membrane modules, arranged such that

a fraction of the feed gas passes through each individual module or such that the feed

gas passes through the individual modules in series.

Figure 1 shows a removal unit used to remove the undesirable vapor from the

pump/membrane loop. Various means of removing vapor are possible within the

scope of the invention. The removal unit may be a heat exchanger or chiller that

causes condensation of some of the vapor contained in the stream passing through it

It may be a direct contact condenser. It may be a unit that extracts, absorbs or

adsorbs vapor. It may contain a chemical scrubbing agent. For example, an aqueous

alkali solution might be used to absorb acid vapors such as sulfur dioxide or hydrogen sulfide. It may contain an absorbing liquid, such as oil used to capture gasoline

vapors. It may contain activated carbon or a molecular sieve material to remove

vapors by adsorption. A useful benefit of the invention is that vapor of the

absorption medium is not emitted to the atmosphere or passed along in the process

train but is recovered in the permeate stream from the membrane. In some

applications, such as where absorption is already used to treat the pump exhaust

stream, recovery of the absorption medium may be more important than removal of

the pollutant vapor remaining in the absorption unit outlet.

Figure 1 shows the permeate stream mixing with the inlet stream and passing

directly into the pump. However it may be convenient in some situations to mix the

permeate with the inlet gas further upstream of the pump, and this is also within the

scope of the invention. For example, stream 4 may contain a high concentration of

the undesirable vapor. Running stream 4 through a condenser before it enters the

pump may cause condensation of a fraction of the undesirable vapor, so that the

stream that is sucked in at the pump inlet contains both gas and liquid. If the pump

is a liquid-ring pump, this is perfectly acceptable and may often even be very

desirable, because the volume of gas to be pumped decreases substantially. In this

case, it is preferable to return the vapor-enriched permeate stream to the incoming

stream before the condenser, so that a portion of the vapor from the permeate stream

is also condensed.

Figure 1 shows one pump. It will be apparent to one of skill in the art, however, that the process would also be applicable in situations where multiple pumps are uTsed in series. A common example is a combination of one or more ejectors and a liquid-

ring pump, such as is often used where a sufficiently low pressure cannot be achieved

with the vacuum pump alone. The pump could also be used in combination with a

blower to achieve the required pressure change. In these instances, the

pump/membrane loop could be closed by connecting the permeate side of the

membrane upstream of all pumps or between pumps as appropriate to the specific

circumstances. Such configurations are also commonly combined with a condenser as

described in the paragraph above.

The process of the invention could be used for diverse applications, including

vacuum applications, compressor applications and pump sealant recovery applications.

Typical vacuum-driven applications are vapor recovery from vacuum dryers and

evaporators, in which solvent and other vapors are removed from solutions, process

materials, etc. Similar streams are produced by vacuum rotary filters or distillation

tower vent streams. In these applications, the vacuum pump exhaust is often heavily

contaminated with one or more of a wide variety of organic and inorganic vapors.

Compressors that could utilize the process of the invention are found, for

example, throughout the oil, gas and petrochemical industries, where they deal with

hydrocarbon-containing streams of many different types. As the pressure of the gas

stream is raised, condensable components in the stream approach their saturation vapor pressure. This is undesirable, because it means hydrocarbon condensation

within pipes and process equipment is possible. Connecting a membrane unit across

the compressor to decrease the dewpoint of the gas by removing a portion of the

hydrocarbon is very advantageous. A typical use would be in dewpoint adjustment

of C₃, C₄, C₆ and greater hydrocarbons found in natural gas streams.

A third application is sealant vapor recovery. In a liquid-ring pump, the

pumping action is provided by a ring of liquid sealant. This ring may be water or

some other liquid. The choice of liquid sealant is vast, including oils, hydrocarbons,

chlorinated hydrocarbons, alcohols and inorganic liquids. If the gas to be pumped

contains a high vapor concentration, it may be possible to use that vapor, condensed within the pump, as the sealant. Depending on the vapor pressure and other factors,

some sealant vapor will always be present in the pump exhaust stream. Besides

representing a loss of sealant, the presence of this vapor is often a problem in venting

or in downstream use of the pump outlet gas. Connecting a membrane unit across the '

pump could enable sealant vapor to be captured and returned to the pump inlet.

The invention is now further illustrated by the following examples, which are

intended to be illustrative of the invention, but are not intended to limit the scope or

underlying principles in any way. EXAMPLES Example 1. Removal of xvlene vaoor from a vacuum pumo exhaust.

A vacuum pump is used to remove air leaking into a xylene distillation process.

A membrane unit is^* connected between the outlet and inlet sides of the vacuum pump.

Liquid xylene is removed from the pump/membrane loop by condensation. The

example uses a computer calculation, performed using a computer program based on

the membrane gas permeation equations for cross flow conditions described by Shindo

et al., "Calculation Methods for Multicomponent Gas Separation by Permeation," Sep.

Sci. Technol. 20. 445-459 (1985). The selectivity of the membranes for xylene/air is

assumed to be 50. The process is shown schematically in Figure 2. Referring now to

this figure, stream 12 drawn off by the vacuum pump, 11, comprises 247 lb/h of

xylene and 25 lb/h of air. The stream is at 115°F and 50 mmHg absolute pressure.

Exhaust stream 13 from the vacuum pump is at atmospheric pressure. Membrane unit

17 is connected across vacuum pump 11. Gas stream 16 is passed across the surface

of a membrane selectively permeable to xylene. Permeate stream 19 is enriched in

xylene compared with stream 16 and passes to the low-pressure side of the vacuum

pump. Stream 18, containing only 0.06 lb/h xylene, compared with 25 lb/h in the

feed, is vented to the atmosphere. Xylene is removed from the pump/membrane loop

by condenser 14, operating at 115°F. Stream 15 of liquid xylene is withdrawn from

the condenser at a rate of 247 lb/h. In this example, the stage cut is 35-40%, because

of the high level of xylene removal. The recycle stream to the pump inlet is just

under 5% of the stream from the distillation process and increases the energy

requirement of the pump by only a few percent. Example 2. Removal of perchloroethylene vaoor from a vacuum filtration operation.

A liquid-ring vacuum pump is used in a vacuum filtration operation. The air

passing through the pump contains about 10% perchloroethylene and is at a pressure

of 1 psia. A perchloroethylene removal process in accordance with the invention is

shown in Figure 3. The membrane performance is calculated using the same computer

calculations as in Example 1. Referring now to Figure 3, stream 20, having a flow

rate of 20 scfm, is drawn through liquid ring pump 21. Exhaust stream 22 from the

pump passes to separator 23 from which liquid perchloroethylene stream 28 is drawn

off. Gas stream 24 from the separator contains about 5% perchloroethylene vapor.

A membrane unit 25 is connected between the outlet and inlet sides of the liquid ring

pump. Gas stream 24 is passed across the surface of a membrane with a

perchloroethylene/air selectivity of 35. Permeate stream 27 is enriched in

perchloroethylene compared with stream 24 and is mixed with incoming feed stream

20 and passes to the low-pressure side of the pump. Stream 26, containing only 1%

perchloroethylene, is vented to the atmosphere. Stream 27 has a volume of

approximately 3.7 scfm and contains 24.9% perchloroethylene. This stream increases

the size of the vacuum pump required for the vacuum operation by 15-20% and

reduces the perchloroethylene emissions by 80%. Stream 29 is withdrawn from the

gas/liquid separator and passed through heat exchanger 30 and thence, as stream 31,

to the pump to be used as sealant.

Example 3. Removal of sealant vaoor from a compressor exhaust.

A compressor uses CFC-113 as a sealant fluid in a gas compression operation because of its low reactivity. However, CFC- 113 has a significant vapor pressure at the operating temperature, 0°C, of the pump. As a result, the gas leaving the pump

contains about 2.1% CFC- 113. A CFC- 113 removal process in accordance with the

invention is shown in Figure 4. The membrane performance is calculated using the

same computer calculations as in Example 1. Referring now to Figure 4, stream 40,

having a flow rate of 20 scfm and at 5 psig, is drawn through liquid-ring compressor

41. Exhaust stream 42 is at a pressure of 100 psig and contains 2.1% CFC-113. A

membrane unit 43 is connected between the outlet and inlet sides of the compressor.

Gas stream 42 is passed across the surface of a membrane with a CFC-113/air

selectivity of 20. Permeate stream 45 contains about 5.2% CFC- 113 and is mixed with

the incoming stream to the compressor. Streams 46 and 47 withdraw excess sealant

or add additional sealant to the compressor as necessary. Stream 44, containing only

0.21% CFC-113, is vented.

Example 5. Removal of hydrogen sulfide from a liouid-ring vacuum pump exhaust used to provide vacuum required for a filter in a Stretford sulfide abatement process.

A liquid-ring vacuum pump draws off a hydrogen-sulfide-containing stream at

a pressure of 1 psia. The outlet gas from the vacuum pump contains 200 ppm

hydrogen sulfide at 15 psia and is flowing at a rate of 100 scfm. It is desired to lower

the hydrogen sulfide content of the exhaust to 20 ppm.

A membrane unit, containing a polyether-polyamide block copolymer membrane

with a hydrogen sulfide/air selectivity of 100, is connected between the high and low

pressure sides of the vacuum pump. The membrane performance is calculated using

the same computer calculations as in Example 1. The outlet gas from the vacuum pump passes through the membrane unit, producing a residue stream containing^* 20

ppm hydrogen sulfide and a permeate stream containing 1,400 ppm hydrogen sulfide.

The permeate stream is returned to the pump inlet so that the hydrogen sulfide can

be removed by the absorbent fluid used as the pump sealant.

Claims

I claim: 1. A pumping process, comprising:

(a) drawing a feed gas stream into a pump on a first, inlet side;

(b) exhausting an exhaust gas stream containing a vapor from said pump on a second,

outlet side;

(c) providing a membrane having a feed side and a permeate side, said membrane

being connected between said outlet and inlet sides to form a pump/membrane loop,

such that at least a portion of said exhaust gas passes to said feed side;

(d) withdrawing from said feed side a residue stream depleted in said vapor compared

with said portion of said exhaust gas;

(e) withdrawing from said permeate side a permeate stream enriched in said vapor

compared with said portion of said exhaust stream;

(f ) passing said permeate stream to said inlet side and drawing it into said pump with

said feed gas stream;

(g) removing at least a portion of said vapor from said loop.

2. The process of claim 1, wherein said membrane is a composite membrane

comprising a microporous support layer and a thin permselective layer.

3. The process of claim 1, wherein at least 90% of said vapor is removed from said

portion of said exhaust stream.

4. The process of claim 1, wherein the whole of said exhaust stream is passed to said

feed side.

5. The process of claim 1, wherein said pump is a vacuum pump.

6. The process of claim 1, wherein said pump is a liquid-ring pump.

7. The process of claim 1, wherein said vapor is a hydrocarbon.

8. The process of claim 1, wherein said vapor is an aromatic hydrocarbon.

9. The process of claim 1, wherein said vapor is a halogenated hydrocarbon.

10. The process of claim 1, wherein said vapor is an alcohol.

11. The process of claim 1, wherein said vapor is inorganic.

12. The process of claim 1, wherein removing step is carried out by condensation.

13. The process of claim 1, wherein said removing step is carried out by absorption.

14. The process of claim 1, wherein said removing step is carried out by chemical

scrubbing.

15. The process of claim 1, wherein said removing step is carried out by adsorption.

16. The process of claim 6, wherein said removing step is carried out within said

pump.

17. The process of claim 12, wherein said removing step is carried out on said exhaust

stream.

18. The process of claim 13, wherein said removing step is carried out on said exhaust

stream.

19. The process of claim 14, wherein said removing step is carried out on said exhaust

stream.

20. The process of claim 15, wherein said removing step is carried out on said exhaust

stream.

21. The process of claim 12, wherein said removing step is carried out on said

permeate stream.

22. The process of claim 13, wherein said removing step is carried out on said permeate stream.

23. The process of claim 14, wherein said removing step is carried out on said

permeate stream.

24. The process of claim 15, wherein said removing step is carried out on said

permeate stream.

25. The process of claim 1, wherein said vapor is produced within said pump.